Carb help....
 

Hi guys. Have some issues tuning my carbs and wondered if I could bet some ideas on jetting. ENGINE SEEMS TO RUN OK AT Cruise (3500rpm). When I go WOT, it starts to pop thru the carb and won't pull any rpms. I have a 500HP with a 256 blower and a set of Holley 750 blower carbs that are boost referenced from Holley. Jetting at the moment is 74 pri. 84 sec. with a 6.5 p.v. Plugs are black at idle and pure white at WOT. What do you guys think I should do as far as jetting goes? I have been screwing with this thing for a month and am at my ends wit! Can you lean down the primary jets and go bigger on the secondaries or do you have to stay with an 8-10 spread when running a pv???

You need to ck fuel pressure at wot make sure the system is keeping up, some parts of a factory system really restrictive, jetting does not seem all that off, what timing are you running ? you should have a max of 28 total, and would like about 18 initial to clean the idle up, you may even lock the advance as starting usually not a issue with the low compression...Rob

Fuel pressure at wot sets right at 6psi. distributor locked out at 34 deg. taking 1 deg. of timing out per 1lb of boost (taking a total of 6 deg. out).

Jetting has nothing to do with idle mixture.

Are you running primary and secondary PV's????

This search may help.
http://www.offshoreonly.com/forums/s...=2198798&pp=25

Popping is usually lean and jetting doesn't affect idle.
You may try 78's and 84's.
Check fuel pressure at WOT, must be over 4 psi.
Run a compression check, You may have damage already.
Timing should be 30 or less (total).
What boost are you running?

JB

Running a 6.5 PV in the primary side only.

Fuel pressure at WOT is at 6psi.. Did a compression test and all cylinders are good. running 6lbs. of boost.

Where did you connect the carburetor PV hose to?
Have you measured the PVCR openings yet?
JB

I am picking vacuum up from the intake. Basically ran each hose off of the carbs to a tee then one line into a tee off of the intake where my boost gauge runs off of. Not sure as to what you are referring to when you say PVCR. It's pretty sad when I can tune a 7 sec. race car and can't tune a damn boat lol!!!

A spread of 8-14 jet sizes is fine then.

at this point I would call Tom at weiand, he is the blower guy there and is very sharp, has helped me a lot ! Rob

Idle mixture is controlled by the air bleed screws on the side of the float bowls. Check your primary jets by running just below the point where the secondaries start to open, typically about 3800 rpm. I bet they won't be black there. You've already checked wot and that's lean. If you have 6 lbs. of fuel pressure at the carbs and your ignition is functioning properly, start jetting up.

Does it pop through the carbs when you hit the throttle from 3500 RPM to go to WOT or are you already at WOT when it starts to pop through the carbs?

How much higher have you gone on the main jets so far?

if your flat tappet still,have you checked the exhaust lobes?

It seems to start popping at about 5000rpm. Motor should spin 5800. If I keep it about 3/4 throttle, it runs faster. As soon as I open it up that last 1/4, it gets to around 5000 and starts popping thru the carb. Right now, I haven't rejetted as of yet. Currently running 74/84 with 6.5 pv on the primary side.

Its sounds to me like you are leaning out in the upper rpms. I would jet it richer in the primaries and secondaries by about 4 sizes and go from there.

You really can't go by the color of the porcelain for plug reading any more. Fuel burns too clean to leave the nice light brown and they are almost always white at WOT. You need to look at the base circle of the plugs.

Agreed 4 sizes would be a good start. I would keep going until there was a little color on the insulater and definate color on the ring. Another thing you can use as an indicator is the oil temp. If the oil heats quickly under boost you may need more jet.

PVCR= power valve channel restriction. These are the holes in the metering blocks that are open to the fuel bowl when the power valve actually opens. They are drilled passages which control the amount of fuel that is bypassed around the main jets under low manifold vacuum [enrichment]=(open power valve). Unscrew the power valve and measure the hole size. From this determine the area of the hole and compare this to the area of the jet. (see Holley jet tables).
The area of the secondary jet should be similar to the area of the primary jet + the area of the PVCR when the primary and secondaries are the same size, like a 750 or an 850 cfm

Also you should check the needle and seat, inlet to the float bowl, size. For your combination, the bigger the better, like .140" or .150".

Check Holley for this information. If this still isn't fixing it, call Dean Nickerson. He does these carb conversions all the time.

Forget about the seats, you have two carburetors= 350 hp each is OK.

Are you running a different cam? Many aftermarket hi-lift cams (eat) valve springs [loose seat pressure], in marine applications!

JB

Some more information on PVCC and 750's.
http://www.hangar18fabrication.com/blowthru.html

I would check for a broken valve spring. Moroso sells a spring pressure checker for less than $100.

Ok. Took out the boat lastnight to do some testing. For some reason I think that it is an electrical issue. I ohmed all of the plug wires to start and noticed that plug wire off of the #7 hole was abit higher then the rest of the wires but plug seems to be burning the same as all the others. I had a new msd coil so I gave it a shot....seemed to run a touch better. Checked plugs and still lean. Jetted up 4 sizes on the pri/sec. to 78/88 and it ran worse but the plugs had good color to them (light brown). I am now thinking that it has to be electrical. I gave it more fuel which is what it indicated that it wanted and ran worse which brings me to the conclusion that something electrical is going on in either the box or the boost retard box. I am going to switch out MSd boxes and throw the boost retard box out and set the timing at 30 degs. and see what happens. I am also going to pull the valve covers of and check for a broken spring. I will let you guys know if this solves the problem. Thanks for all the input!

I've run into problems where the ignition wires were arcing and causing a pop in the carb on the top end.... One time it was the wires, the second time it was a coil....

thought for sure it was either leaning out, springs or valve job at the time but it actually had nothing to do with with any of that.....

Go figure...

A few thoughts:

With dual carburetors in the HP Range you're making there is no need to use anything larger than .110" N&S assembly.

You can "Read" spark plugs even with todays fuels, plugs, and good ignition systems, however you have to know what you're looking for. I do this on a weekly basis at race tracks around the country.

Usually an ignition related problem will happen at a specific RPM each time. If changing the amount of fuel made the engine run worse, but didn't change the problem split the difference and go down 2 sizes to be safe. If the problem changed when you changed the coil you should look at the ignition system.

First thing I would do would be to lock out your timing, and remove the retard box. They have been known to cause issues from time to time. When you run the boat next set up to run "Heavy" drag your tabs, and tuck your drive so the engine has a lot of load on it. When you get to the RPM that is having the problem back the throttle off to just below the RPM when it's happening. Now free the boat up with the drive and tabs. If the problem is fuel related the amount of load will make a difference, but if it's ignition related it should happen at the same RPM again.

RumRunner. I think you have narrowed down my problem. The "pop" is at the same rpm every time no matter if I am trimmed up or dragging tab. I am pulling out the retard box tonight when I get home from work. I also have an extra ig.box I may swap out to see if that solves the problem. Going to run a load test on the batteries as well. I have run 84 on gps with this issue. I cant wait to see what she'll do once I get this all sorted out!

If it were me I'd pull out the retard box, and run it that way. Set up your second ignition box so you can swap them while you're out on the water.

I can do that as well. One other question for ya. What kind of resistance should you see on a good set of wires like Taylor or MSD? I ohm'd mine and the seem to be high.

It depends on the wires, check with the manufactures. They should be able to give a ± range for their different series, and lengths.

My taylor spirolpro wires made both tachs go crazy when they got to 400-800 ohms. I was suprised with thier limited life. I did not have any popping though.