Carb Adjustments
 

I finally launched my boat today here in the chilly fall water of New England. I installed the thermostats that everyone on the site suggested and it was nice to see the water temp gauges move and steady at 140 degrees. Thanks for all the information to those who helped me with that.

My port engine is giving me some trouble which I am attributing to carb problems. It has a very rich idle and has been difficult to keep running at idle without cleaning it out every few minutes. It was running somewhat ok until a long no wake zone. After passing, I gave it some power and could not get any power above 2000 rpm. More throttle would reduce rpms and when I pulled it back some the rpms would steady at 2000 and hold. Occasionally it would surge to 2500 and then back down. No amount of cleaning it out would help.

I pulled in to a marina and went to work. I changed both remote fuel filters (no water noticed in either) changed spark plugs, cleaned the strainer to the carb (not dirty), checked distributor for moisture, loose wires etc, and re-set the 4 corner idle screws to Demon factory setting of 1 1/2 turns out. It started fine and idled smooth. I tried adjusting the float level again with no results noticed. Since it was running fine I figured I would try it out on the water under load. Gave it power and the port engine promptly died. I re-started and even in neutral with any power it would die out, although it is idling fine now. (although still noticable rich with fuel in the exhaust and some smoke.) Right engine has a slow bumpy and hesitant pickup to 1500 RPM but smooths out after that and runs great.

Where do I start from? Any base line settings I should go to with the float level adjustment, 4 corner screws, etc? Here are the specific related parts for your help with knowing whats going on:

Gen IV 454, 440 cyclone, BG Speed Demon 850 auto carbs with no choke and mechanical secondaries (pri - #85 sec - #83 jets), Thunderbolt IV ignition (new sensors), NGK BR6FS plugs, hurricane single plane high rise alum. intake, mild cam with no overlap, Holley 130GPH mechanical fuel pump with no pressure regulator.

Any suggestions on where I sould start? Fuel was high octane with an octane booster, but a few years old when I started(only 40 gal or so left over in the tank from the old motors, motors which I am wishing I kept now) I then filled with 150 gal of 93 octane about a month ago. (10% ethanol)

Thanks guys, pretty soon I hope to stop asking so many questions, but I guess thats how I am to learn something......

Re: Carb Adjustments
 

If you ask me that gas has got to go. Put it in the truck or the lawn mower or something but get it out of the boat. 3 year old gas is way past done and month old 10% ethanol is actually passed it's prime too. Gas with Ethanol in is has about a 2 week shelf life and that's about it before it starts to degrade. If your not gona burn it within 2 weeks you need to put fuel stabalizer in it, I actually just recomend putting stabalizer in every time you fill up with todays gas blends. I can't say that it will run perfect with new fuel but there is no point in settign the carb up with that fuel running through it, if you get it to run right it will be all wrong when you get fresh fuel in it anyway.

Re: Carb Adjustments
 

If it was the quality of the fuel causing my problem, would'nt the stbd engine be running terrible as well? They both draw from the same tank. There also was very little of the older fuel in the boat, that was stabalized and did not smell varnishy or even look old (when I removed and sampled 50 gallons into a clear 50 gallon container), and all opinions from those who saw the 50 that I removed said it looked, smelled, and felt fine and why was I removing it from the boat? I figured that what was left when mixed with new 93 would be ok. (this stuff is liquid gold these days right...)

The front sight gauge to the front fuel bowl is showing constantly full. I can't seem to get the level to adjust. I was thinking the needle and seat could be obstructed, or fuel pressure is too high for the needle to seat but would'nt that cause the fuel to run out of the carb? And, the stbd engine has the same fuel pump without a problem.

Everything seems to point to a very rich carb. fuel in the exhaust, dark smokey exhaust (especially when reved), the bogging out when power is advanced under load, black spark plugs (before I changed them). The standard plugs for this motor are BR6FS NGK's. Could a 6 be too cold for the higher power output and should I try a hotter plug? The Barry Grant site calls for a main jet size of 85 and sec of 93. I have 85 pri - 83 secondary which seems too lean. I know that should not come in to play until higher power is reached but could that jet discrepancy make any difference? I'm all confused at this point.

Todd

Re: Carb Adjustments
 

Replaced a mangled main needle and seat assembaly today and cleaned carb metering blocks, bowls, etc. Got it running ok at the dock and idling up the river in gear Idled up to the end of the no wake with no problems, running great. (except for still a noticable fuel slick in the exhaust.) Gave her some power and she stopped at 2000 and would pulsate up and down between 2000 - 2200 RPM but no higher. Hard to keep it running after that and did not start very easily. Tied up on one engine. Noticed what looked like an amount of blue exhaust smoke, but I'm not positive. (sticking oil ring? bad compression?)

Pulled all 8 plugs and all were black and some wet with oil or fuel, or something. (replaced left bank yesterday, right bank a few days ago.) Replaced all 8 with new tonight but have not run yet as I wanted to check the initial timing in the morning.

This is how I have the carbs set as of now: primary and secondary floats at the middle line on the sight gauge, 4 corner screws 3/4 turn out (have tried several settings) I checked the float level when I had the bowl off and the float seated at .5" from the top of the bowl when I turned it upside down. Everything clean and no water or setiment anywhere in the filters or carbs. I'm thinking that blue smoke was not a mirage. 7 hours on motor, only .5 above 1500 rpm.

I'm running low on ideas here, except that I'm going to start shopping for oars pretty soon. It might go faster. Where should I keep looking, vacuum, carb, timing, ignition? Thunderbolt IV, how do you determine the timing advance on that system, and even if it is working?

Todd

Re: Carb Adjustments
 

I think you will really have to narrow it down to whether its a fuel, or ignition problem. Try swapping the carbs between the two motors. (assuming you have twins?) Make sure its firing on all 8 cylinders. Get some insulate pliers and remove the plug wires from the cap and see if the rpm's drop. If you remove a wire and no change, then you can focus on that cylinder. If you have cylinders not firing, it will cause the plugs to be wet and fouled. Since its a new motor, triple check the firing order. Many guys chase similiar problems only to find crossed plug wires. Stick a timing light on it and check the base timing and the advance. My cyclone 454's have a 0 mark and a 30 mark on the crank. Make sure base timing is checked on the zero mark. Run the engnine to 3000 and check the timing on the crank. How does it run in nuetral? Does it pop or backfire at all?

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Sound way rich. My 502's were jetted 78 primary. If you have a sheen on the water and wet plugs...I say too much fuel.

Re: Carb Adjustments
 

blue smoke might be all that fuel washing oil off the cyl walls.either way , way too much fuel is bad :cool:

Re: Carb Adjustments
 

My 420's are running holley 800's with 71p, 84s with a 6.5 PV in the rear.

Re: Carb Adjustments
 

It seems like it does not want to start is slowly comes to life when cranking over, not a nice snap over like the stbd motor. Once running it does seem to pop a little especially when reved. I did see a few occasions when the engine would quit it would spit a mist of fuel like a back pop through the carb. Now, if it looks like it will stall I just hit the key off before it does to keep that back pop from happening. It does not seem to backfire aggressively, but certainly does not sound smooth, especially when reved.

I'm wondering of my Thunderbolt IV module is any good. I'm going to check my timing this morning, I think 8-10 degrees BTDC is right for this engine for base timing, but I'll check the book. If the module is bad I'm assuming I wont see the timing mark move when the engine is reved to 3000? If the module is good (V8-24) what should the timing mark move to, is it 24 degrees or more BTDC?

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Also, my carb is a BG speed demon 850 with 85 main jets, and 83 secondary. I have not checked the other carb for it's jet size, but since they are a matched pair of engines, together since new, I assume they are the same setup. Perhaps I should not assume. When do the jets take over from the idle circuit anyway? I thought they were for higher power running. I'm fairly sure my power valve is 6.5 as well.
One guy mentioned he thought my carbs were a little too small but I can't imagine they are on 454's with only mild mods. If anything I was thinking too big.

Re: Carb Adjustments
 

I honestly dont think your probem is in the carb. I could be wrong, but to me it sounds like an igntion probem. First thing you have to do is get a timing light on it. Check timing, firing order is clockwise on the cap-18436572. Cylinders are as follows, right bank(standing in front of engine) 1,3,5,7 Left 2,4,6,8. The ignition system is fairly simple. The module could be bad, but I doubt it. There's also a sensor in the distributor that I had to replace last month on a friends 454. It caused a no start condition.

Hard starting, missing, popping, sounds like ignition. But, because its a fresh motor, theres also the possibilty of The new cam going flat. Break in is very important, and all that idling will kill a new cam if it wasnt broke in right. Also happened to me before on a 454 mag. Wouldnt hurt to pull the oil filter and look for lots of metal shavings in the oil. Chances are though with all the smoke out the exhaust, the valves are probably opening letting fuel in and out, but its just not igniting. You have plenty of carb, considering guys run 502's and 540's on an 850 carb. Personlally, at this point, I wouldnt waste time playing with jets and stuff, get it running first, and make those adjustments later. Also, when you get it running right, change the oil immedialty. Its probably got a bunch of fuel in it, which will kill your new motor. Good luck. MT

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By the way, if the popping is constant when you revit up, like pop, pop, pop, pop, I'd say a flat lobe on the cam. Does it pop thru the exhaust or intake? Who adjusted the rockers? Set base timing to 10 to start. Have someone crank it and put your ear near the carb and listen for air poofs thru the carb. If a exhaust lobe is flat, you will hear air come up thru the intake because it has no where to go except back up thru the carb.

Re: Carb Adjustments
 

clean all your air bleeds on the carbs they get drity quik

Re: Carb Adjustments
 

Well..... timing was fine, advance working fine (up to 35 degrees) compression fine, everything fine.......

Frustrated I switched carbs guess what...problem gone! No adjustments were made or anything. I thought it was wierd but when I was bolting the port engine carb on the starboard I noticed that the right rear bolt was bottomed out but not seating the gaskets together and there was a space on the intake manifold allowing an air leak under the carb. I pulled the bolt out and added 2 more flat washers to make the bolt head higher up and allow it to tighten fully adainst the intake. If that was the same case when it was on the port engine I could see how that would cause a bad air leak and stumble the carb. I ran it from Old Saybrook CT to Mystic, CT and it ran great the entire way. Still some things to mess with of course but I'll post a new question for that if I need to.

Thanks to everyone that helped with this especially MildThunder.....your idea of swoping carbs worked.....I just wish I had read your response before I went to work on it today....I would have started there......

New thread for the tweaking process soon.

Re: Carb Adjustments
 

Glad to be of help! Good to hear you got it fixed and running better. Now, send me some pics of the 390 stinger. Its basically the same boat as my excalibur Hawk. :drink:

Re: Carb Adjustments
 

Well.....took her out today and the same problem occured now on the right engine. Whatever the problem is it seems to have followed my carb swop from left to right. Ran good on the way over with little idle time. After the run....20 min of idle in a no wake. Started it today, and idled 20 min out into the sound. Gave her power and right engine only got to 2000 RPM. I throttled down and it ran terribly all the way in.

I am still thinking rich running fouling plugs out and not getting me any power. Does a vacuum leak cause an engine to run richer or leaner? How do I get this carb to lean down? 4 corner idle screws only 1/2 turn out on the primarys. Have tried different settings with no effect. How many places can I get a vacuum leak?

I'm going to change the plugs again tomorrow (BR6FS NGK) Clean out the carb again, check the jets on the other carb and match them up....any other ideas, this is getting old on great days like today. I can't seem to get my wife a ride on plane which is not convincing her that this is the best place for our funds.

Todd

Re: Carb Adjustments
 

have a good carb guy go thru it. i had a problem with 1 of my dominators three shops looked at it until a guy that really understood how all the carb cuicuts intreract saw my problem as soon as he looked the carb over. cost 60 bucks for him to go thru it. my boat rons and idels sweet now.

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I have 1" spacers between the carbs and the power plus hurricane high rise single plane intakes installed. Is there any need for the spacers? Are they really doing anything especially with an intake that has a high rise to it? Is this just another place for a vacuum leak?

Thanks

Re: Carb Adjustments
 

It does not sound like a vacuum leak. A vac leak will make the engine lean. It will only idle at high rpms and will stall when put in gear.

Take off the flame arrestors and look down into the carbs. Look for fuel dripping. If it is dripping then you have a problem with the needle seats sticking and or the floats. If not chances are that a powervalve is blown. Also, you need to set the float levels with the boat in the water or at least on a hill so it is the at the same attitude as it would be sitting in the water at idle.

Also 850's are more carb than you need, but they should still work ok.

Re: Carb Adjustments
 

Is the power valve on the Demon the same as a Holley generally? I replaced the needle and seat last week, and set the float level mid way up the sight glass with the boat in the water. I did not notice fuel dripping into the carb but I'll look again. Would a even slight backfire cause the power valve to blow out?

I have been thinking the 850 is too big. I have had some people telling me it is too small which I could not believe. I was thinking a 750 or 800 would be more appropriate and fuel efficient than an 850. The engines came with these carbs though and I am reluctant to spend $800 - $1000 on new carbs right now.

I'll replace the power valve today and see what happens.

Thank you,
Todd

Re: Carb Adjustments
 

Those carbs should have power valve blowout protection, but I would still check it. Dont forget to change the oil after you get it fixed!

I know how you feel. I had similiar problems when I bought my Hawk this summer. I sold my good running Four Winns Liberator and stepped into a 40ft boat with twin 454's. I had some trouble getting it running right and was spending a lot of dough on parts, not to mention the frustration. I was so excited to take my fiance out on a ride. We took it out the first time and the port motor kept stalling. I couldnt even keep it running. I dont think my fiance was to confident in the boat, not to mention the amount of money I was spending. But, after time I got it running much better, and once it was running I took her out again. She loved it, and now tells me to keep it. You'll get it running, you got a great looking classic miami vice boat. Hold on to it.

Re: Carb Adjustments
 

I changed the power valve in the carb, and changed my jetting slightly from 85 front to 83 and 83 rear to 88 in both carbs. Changed plugs again (they were thickly crusted black) and adjusted the idle. I did not see the same amount of smoke or fuel in the exhaust water. Went for a ride.....

Opened power after 20 min idle and right engine (problem carb) stumbled a little but accelerated. Set at 2500 RPM and trimmed out a little and got 2800 RPM. Poured some more power on slowly and it stopped at 3000 RPM. The RPM's would fluctuate from 2700 - 3100 occasionally but go no higher. Both engines this time. Went around a small island and back in. Dropped to idle. Tried to get going again and the right engine would not come back up to power outside of idle. It was hesitating and popping but would not power up. Pulled to idle and it ran fine all the way to the dock with a good smooth idle.

In this short ride my fuel gauge went from 1/4 to 1/8 in a 200 gallon tank. If the gauge is accurate I used 20 gallons for a 30 minute idle and 5 minute ride on plane at only 2700 - 3000 rpm. That just can't be right but I'm not that familliar with how the gauge reads yet.

Any more ideas? Water in the fuel? Vacuum leak from somewhere? Mis-adjustments? Float level off? Jetting still off? Any chance the carbs are just too big for these engines? (Demon 850's) It seems that for 454's pushing only 420 - 450 hp that a 750 would be the right size. I want to make this thing work correctly, but also be as economical as possible.

It is fun to tinker with, but I have been working on this thing long enough and really want to get a nice ride for me and my wife without having to open my toolbox. Can everyone forward what setups they are running on their 454's to give me some ideas on which way to go? (" I thought this was going to go fast" she said)

Thanks for the information everyone.....hopefully I'll get enough feedback on your setups to lead my next move.

Re: Carb Adjustments
 

Assuming a BG is similar to holley, set the float level so fuel barely trickles out the sight hole with the engine running and in the water. After that, surging for me is usually a vacuum leak or a fuel pressure issue. Check both of those next starting with fuel pressure. You could also spray some wd40 around the base of the carb and intake with engine running to try and find a vacuum leak. If rpm changes you found one.

BT :cool:

Re: Carb Adjustments
 

DEMON IS NOT LIKE A HOLLEY ON THE FLOAT SETTING SET
THE LEVEL ALL THE WAY TO THE TOP OF THE SITE GLASS
AND IT SHOULD HAVE (2) IDLE SCREWS ONE IN THE FRONT AND ONE IN THE BACK THEY SHOULD BE SET AT THE SAME TIME.

Re: Carb Adjustments
 

I'll try raising the float level. Actually there are 4 idle screws on my speed demon just like the holley. If this carb is too big for this engine, is there anything I can do to set it up as if it were smaller to make the engines happy for now? Should the jetting be smaller to compensate? Would the fact that it wont pick up over 3000 RPM indicate too much fuel and flooding out, or too little fuel? At first I thought it was running out of fuel at higher RPM but now I'm not so sure.

Right now: primary jets 83, secondary 88, 6.5"Hg power valve on the primary only. I'm not sure what I'm getting for power total as the engine was not dynoed. It has a mild cam with no overlap but a higher lift, the hurricane single plane high rise intake, Thunderbolt IV Ign with V8-HP module, Gil (Mercruiser) exhaust manifolds with stainless risers, and this big 850 carb. Timing was set at 11-12 degrees initial and the v8-HP module box advances by 20 degrees. Perhaps I'm not getting the 420 - 450 HP that I thought I was. This engine was orriginally a Mercruiser 440 Cyclone but who knows what it is now. I certainly am starting to think it does not need an 850 CFM carb.

Mercruiser used a Holley 4150 850 CFM on the 420, 440, and 460 in the 80's leading me to think it was correct to use such a big carb.

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The carbs aren't too big. They may not be optimum but they should work just fine. You say the other engine works good?

Re: Carb Adjustments
 

Can you give us more cam info? Generally, small cams make good low rpm vacuum and require less jet. Large cams require more jet. It seems that you're running rich enough that you're fouling plugs but only on one engine.

Re: Carb Adjustments
 

I wish I could provide more cam details. Right now the guy who put them together has yet to get me the build sheets or any details about the engines profile. I'm kind of working blind.

History: Running rich and smokey with fuel in exhaust. Several adjustments later running ok. Changed plugs, no time in no wake zone and it picked right up and ran fine. No power over 3800 - 4000 RPM's. Lots of throttle left with no power!
Idled 20 min to my slip. Next day took my wife for a ride, 20 min out of slip to the sound. Gave it power nothing about 2000 RPM's on the stbd engine and breaking up.

Changed power valve on stbd engine, changed plugs, jetted both carbs 83 front, 88 Rear thinking it was not getting fuel to run higher than 4000 RPM. Idled out 20 min. Both engines ran up to 3000 RPM and would surge from there up and down 200 RPM. Dropped to idle after a few minutes hoping it would clear out. Tried to get on plane again, and right engine would not produce power under load. Just spit and cough. Return to idle and it would run fine until I tried to go above 1000 RPM again, then spit and cough. I am assuming the plugs are fouled again.

Perhaps I moved the jetting in the rear too far rich when I sould have gone leaner. Perhaps it would not run above 4000 RPM because of too much fuel and that was with 83 rear jets.
The low power seems to have improved with 83 jets but wont go above 3000 RPM now. I think that is still on the primary's and the secondaries would not have opened at that RPM yet.

I only have a power valve in the front which should mean richer jetting in the back.

Confused beyond belief, and my set up time quickly coming to an end here in Connecticut. I was hoping to have it dialed in so spring would be more boating and not as much wrenching, and avoiding a winter worth of wondering whats going on.

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Also, the intake is a single plane aluminum high rise, rectangular port which I understand is better for high rpm running. The low / no overlap cam with a slightly higher lift should give better low end torque. It seems I also have a mis-match in the cam / intake performance, coupled with a big carb throwing tons of fuel. My type of boating is mostly cruise with occasional WOT. If this thing won't run well at the low / mid range then I will never be happy. I also don't think I need it to run over 5000 RPM or so. Basically MCM 420 specs are what I am looking for, and asked for but did not seem to recieve. Now I need to clean this job up and make it what I want, and run efficiently and reliably.

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You have the correct plugs for your engine. However they are fouling quickly. Once fouled they aren't good for anything. Lean out the idle srews a little and lean out the primaries about 4 jet sizes and see if you can get them to stop fouling. Once you get it running steady, you can start doing plug checks. Do you know if you're carb is model# 1563020?

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Have you put a vaccuum gauge on it? Try that, and see what your numbers are. I would check the distributor caps. This time of year, cold nights, warming up, you may have some corrosion and or moisture in them. Ive had a brand new cap malfuction and it would only fire 7 out of 8 cylinders.

It seems like you have pretty much gone thru the carbs, power valves, jets, etc. At this point, I may try looking into the ignition. When I first bought my hawk, it had the old points distributors. One motor would cough, die, as soon as i put it in gear. Also would not go over 3000. Swapped it for a new mallory breakerless and ran geat. Might not be a bad idea to put new coils in, you can get a msd blaster 2 for about 30 bucks at pep boys or advance auto. Weak spark will foul plugs too.

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Make sure you are setting the idle mixture screws when the carb is on its idle circuit. Meaning, if you have the idle speed screw set too high, the idle mixture screws will have little affect. Back out the screws about 3 full turns, and lower the speed screw, turn screws in till idle lowers(or highest vacuum on a gauge), then out a 1/4 turn. Then if you need to, turn the speed scew so it idles in nuetral about 900 and about 700 in gear.

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Why Don't You Jet It Like A Hp 500 Lf-81 Rf-75
Lr-93 Rr-89 And This Maybe To Big What Plugs Are You Using?

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Have you contacted BG to see what they recommend as an initial setup (Jets, Idle screws, etc)? Everything you're saying sounds like it's running way rich. I would bring the carbs back to where they were when you first started, and then start making your jet changes from there, otherwise you'll be chasing your tail! Also, don't make big jumps when changing jet sizes, maybe only two sizes at a time (like 83 to 81). You'll also have to make sure everything else in the carb is functioning properly. Changing jets all day won't help if there's another underlying problem. It's frustrating, and time consuming, but you'll get there. I personally don't think it's an ignition problem. I think it's all a result of the carbs.
I've been there. You say you run in the sound. Where are you located?
I run out of Hempstead Harbor, and Manhasset Bay.

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Running out of Mystic, CT (actually idleing out of Mystic right now :D )

Changed jets to 79 front, 83 rear. (83 was what was in there from the start in the rear.) Changed gaskets on stbd carb, raised float level to the upper mark on the sight gauge, checked #7 plug, way black from yesterday. Plugged in my vacuum gauge started and warmed for a while. Put the boat in gear at the dock and adjusted idle screws to vacuum. In gear, max vacuum was about 11 - 12. Set each screw for max vacuum then 1/4 turn rich. Reved good, idled good at 700 in gear, and about 800 to 850 in neutral. Adjusted both engines exactly the same.

Put stbd engine in gear again, tied to the dock, advanced the power slowly, motor started stumbling and coughing and sounding like it was missing badly, same thing it did on the water the last several times out. When returned to idle it will idle right where I set it perfectly, try to advance the power and she stumbles and coughs but will run up to about 1300 rpm and no more. If I go back to neutral I can rev it right up and it sounds smooth.

Shut it down, closed the hatch, here I am home with the first of many beers gone!

It is only on this carb. The other is fine. It must be the transition circuit or something. Perhaps a crack somewhere or bad metering blocks. I have not done anything with the squirters or the venturi's. I'm going to swop them again engine to engine to make absolutely certain the problem will follow the carb, (and keep the right eng. as the good one for steering's sake) I'm also thinking of connecting a remote fuel tank to rule out fuel, but if it were fuel the other engine would do the same. Perhaps the fuel pressure is a problem, both engines have the same pump though, Holley 130 GPH with no regulators. No flooding is occuring with the carb and the bowl level is holding the same so I doubt it is that.

I feel like yanking off these older Demons and changing to a 750 holley or 800 - 9022 holley or somthing, out of sheer frustration. Who is to say that a new or rebuilt carb will work any better though.

What about smaller power valves? Perhaps a 5.5 (if they make it) or 4.5. Since idle vac was around 10-11 average perhaps that would lean the engine out enough to stop costing me $23.00 in plugs every day, cut the smoke, and the transom soot, and get a little more economy out of the liquid gold? What are the thoughts on this idea?

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I would take the carb completely apart. I would clean out every pasage, and ensure that the all passages are not blocked in any way. Make sure all the correct gaskets are used, and are installed correctly. Get an assembly diagram / manual for the carb just to make sure.

Do both carbs have a 6.5 PV? Can you hook up a vacuum gage to the good engine under load and see how much vacuum you have? Your PV should be about 2" less than whatever that steady engine speed/steady load vacuum is.

You said earlier you have the float set at the highest mark? I'm not familiar with your carbs, but raising the level of gas in a Holley bowl will enrichen, if anything.

Just curious - have you sprayed any carb cleaner down the carb while idling? Did the idle speed go up or down? What about around the carb base?

Lastly, you should be able to salvage those almost new plugs with a plug sandblaster. Just make sure you get all the sand out after you blast them.

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It sounds like you have it isolated to the carb. Have you tried swapping the metering blocks one at a time?

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I did spray carb cleaner around the base of the carb, around gaskets, everywhere that I thought there could be a leak and found no leaks. Idle vacuum is the same on both engines.

Perhaps it's getting all the fuel it needs but not enough air. I doubt the flame arrestor is clogged enough to cause this but I'll clean it anyway. I have not tried swoping metering blocks but I will this morning.

What is the best power valve for my idle vacuum of 10-11? 6, 5, 4 ? I thought power valve 1/2 the idle vacuum is the right formula for figuring what size. I would think that 2" less than idle vacuum would cause the valve to be opening constantly making it really really rich at the low end. Would a smaller power valve than the 6.5 that is in there make any difference?

Todd

Re: Carb Adjustments
 

I guess you could try a smaller pv, but if that was causing the problem, why wouldnt the other carb do the same considering they are the same now? I would sell those bg carbs, and look for some holley 9022 800cfm double pumpers. Jet them 79f, 90r, 6.5 pv, then you will have "OE" 420Hp carbs.

When you are on plane, say at 2500 rpm, and you mash the throttles, does the one good engine gain rpm while the other bogs, or do both only max out at the same rpm?

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With your vacuum, those power valves are fine. Changing them would be a waste of time and money. Dropping jet size didn't help. The problem follows the carb from engine to engine so the problem has to be in the carb. Send it back to BG for a rebuild.