Carburetor set up help - engine stumbling while planing
06-17-2019, 07:41 AM
#1
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?
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?
06-17-2019, 08:31 AM
#3
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.
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.
Last edited by GLENAMY 242SS; 06-17-2019 at 08:40 AM.
06-17-2019, 08:18 PM
#6
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?
Ryan
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?
Ryan
06-17-2019, 10:10 PM
#7
Registered
how close does your exhaust get to the water when getting on plane?could be reversion.
06-18-2019, 10:23 AM
#9
Registered
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.
06-18-2019, 10:35 AM
#10
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
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
