MILD THUNDER

iTrader: (3)

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.

stinger390

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.

blue thunder

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

baja36ft

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.

stinger390

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.

PatriYacht

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

PatriYacht

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.

stinger390

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.

stinger390

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.

PatriYacht

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?