Carburetor set up help - engine stumbling while planing
 

I’m looking for some guidance on getting my carb to function correctly.

Background: 27’ Fountain Fever, 509CI BBS, FAST dual wide band, Pro Systems Venom II carb, HP500 intake, AFR heads etc. Bravo 1 with 27pitch 5 blade.

Currently when I try to get on plane the boat coughs and stumbles. The problem occurs around 2800-3000 PRM when the boat is at maximum bow angle. The AFRs are reading ~10.7 (I’ll verify the number, going from memory here). I can push the throttle more with no increase in engine power.

Once I finally get the boat to plane out, no further problems with power. I can run to 5000 rpm, AFRs look safe. It’s only getting on plane that is a problem.



I swapped on a Holley 800 set up for an HP500 clone engine and the problem completely went away. No issues getting on plane, able to increase throttle with the bow in the air without any stumbling.



Where should I start looking in the carb? My thought is too much fuel combined with a severe angle the carb is sitting when the bow is in the air is contributing to the problem?

No carb expert here but here is my thoughts.
1) On the step (not quite on plane) is when you have the most load = least amount of vacuum. Look at the power valve.
2) Is it coughing from carb or exhaust? That will tell you if you are lean or rich. The afr gauge has a hard time reading momentary fluxuations..
Backfire thru carb = Lean. Backfire thru exhaust = Rich.
3) recheck the float adjustment.
Good luck.

i agree,make sure floats are set correctly and that the fuel pressure is not above 8 lbs of pressure.also,10.7 afr is pretty rich.

Make sure the float bowl baffle is installed on the primary side as well.

Thanks for the suggestions. What am I looking for float wise - fuel to be in the window when the boat is bow high?



Here are photos of the carb and the sheet that came with it - I've not changed anything inside the carb yet. Does the carbon or black discoloration help the diagnosis?

https://live.staticflickr.com/65535/...850904b2_h.jpg

https://live.staticflickr.com/65535/...0dc6cc0d_h.jpg

https://live.staticflickr.com/65535/...b8b73361_b.jpg

Ryan

how close does your exhaust get to the water when getting on plane?could be reversion.

You might need longer vent tubes.

If all else fails you might want to do a wedge to move the carb closer to horizontal. Maybe just enough to reduce the nose high attitude a bit. You want it basically level at speed.

Sutphen30 - The exhaust is pretty low - it's a Fountain. I don't have any typical telltales of reversion - but I'll get a gopro camera on the transom and see what's going on when trying to plane.

NHGuy - a wedge may work for planing when the bow is at the extreme angle, but would it then put the carb out of level for normal cruise? I currently have no issues once on plane from 2700 to 5000 rpm. As far as i can tell the engine is relatively level while on plane.

Great thoughts everyone, keep them coming.

I'll be water testing later this week and hopefully fix the problem
Ryan

I'd adjust the floats first. Reversion off idle doesn't seem possible. But I'm no expert. However I am a person who has had some problems with it!
My understanding is you want somewhere around 12.4 AFR during acceleration and under load. Which in boats is most of the time.

The wedge idea came from some early straight drive engines i have seen. And I agree, it can't tilt you any more than running level or you will get a different problem.
Question, are these carbs normally used in a boat?

Buy the 800, or maybe match everything to it.

The carb (and whole engine build) was spec'd by a once very reputable engine builder on OSO - and my build was specifically a marine build. Beyond that I don't know.

That's what I'm leaning towards just to be done with this (WTB already placed) - I hate to fire the parts cannon, but I'd like to use the boat without all the drama this year.


I'll be attending the Sacandaga Fun Run this weekend, I'm sure there will be enough time to swap back to the pro systems carb and do some troubleshooting.
Ryan

Is it coughing back thru the carb? Sounds like it might be lean. Try richening up the primary idle screws. It might be having trouble getting good fuel flow from the boosters at such a high load / low speed situation

if the AFR is 10:1 then its running way rich....sounds to me like the fuel is sloshing through the metering block vent and enriching the engine...maybe a vent extension would help in blocking the sloshing fuel from getting in when in the bow up position.......

Afr meters work great when the engine is running in a good state. Once they start coughing, sputtering, backfiring, i wouldnt even bother turning the wideband on

During alot of trials and tabulations doing on the water carb tuning with widebands, ive had a few engines where ive seen afrs in the 10s. While yes , its rich, it has never made the engines sputter, lose tons of power, fall on their face, etc. Heck, some hot oem engines are actually tuned so they are in the 10s at wide open throttle..

i see the lean idle circuit thing alot. The carb people tell you to adjust screws for highest vacuum reading, or basically leanest setting you can get. Works great on cars, where you have an accelerator pump, or two in holleys case, to help give that burst of gasoline to get the engine revving to a speed where the airflow makes gas able to be drawn from the boosters. In a boat, the accelerator pumps become nonexistent in a getting on plane scenerio. The engine either needs gas from the boosters, or idle circuit.

He may need to adjust the high speed air bleeds if in fact its a lean condition and richening the screws dont help.

#32 is kinda big for a high speed air bleed. Id prob step down to a 28 to start

10.7 is flat out rich for a N/A application and not helping the situation. Get it closer to 11.8 at that same break point and see how it runs.

I had a similar issue years ago. I did not see any post about setting the floats while the boat is in the water. The Vent tubes do no look like J tubes in the picture. The J are supposed to put gas down the carb throat if float sticks. My problem - the float settings was too high and extra fuel was coming out of the boosters. Reversion is not the problem at 3,000RPM and the 800 carb worked fine. You might be surprised if you check the float level on the water. My 2 cents :cool:

yes the carbon is from intake backfire.

I would check with prosystems where he wants the float levels installed in the boat. Most carbs with windows that I've set, were at the bottom 1/4 - not inch but the actual sight glass.

Short boats that can and do go sky high when planing don't have issues with the floats set this way.

I agree with MildThunder on how idle mixture screw should be set. If too lean, can cause backfire thru carb when planing. I set carb for best idle in neutral and then usually 1/8-1/4 turn richer(counter clockwise) will make engine idle better in gear and get rid of planing backfire if caused by too lean idle mixture screw setting. heated intakes don't have as much of this issue,but who runs those. :)