Any (Rochester) carb experts here?

[wellcraft guy]

I've got an '88 Wellcraft with an OMC 4.3, and up until yesterday it ran and idled like butter. Launched for the day yesterday with no issues, idled out the river, and took off. When I came to our first stop, the engine wouldn't idle worth a crap. Anything under 2200 RPM and the engine dies immediately. Anything over 2200 RPM, and it runs perfectly.

I know the basics of how a carb works, but I've never torn into one. I'm guessing I have an issue with the low speed jets? Anyone have any experience with this issue and/or repairing them?

It's a 4.3 with a Rochester 2bbl carb, 1988 vintage (I'm pretty sure it's original). Thanks!

 

[wellcraft guy]

Re: Any (Rochester) carb experts here?

Here's a quick vid of the issue...

 

[airdvr1227]

Re: Any (Rochester) carb experts here?

I wouldn't be so quick to assume it's the carb. Filters? Tank vent? Fuel pump? Tune up? I've see electrical issues do that to a motor when there is a large drag that the alternator can't overcome at idle. There is a ton of possibilities.

 

[wellcraft guy]

Re: Any (Rochester) carb experts here?

I wouldn't be so quick to assume it's the carb. Filters? Tank vent? Fuel pump? Tune up? I've see electrical issues do that to a motor when there is a large drag that the alternator can't overcome at idle. There is a ton of possibilities.

Fuel filter is new, gas is new (and the same gas I used last weekend without issue), and it runs perfectly at anything above 2200 RPM.

It goes from running perfectly fine at 2200+ RPM, to dying like the key was switched off below. I know the basics about carbs, and am under the assumption that different jets are used for lower RPMs, and they might switch over around that point. The issue also arose immediately... It wasn't a gradual thing like a bad filter or bad gas.

I guess I'm hoping someone will say "yep, that's when the high speed jets kick in, your low speed jets are clogged", and confirm my hypothesis. If that's the case, I'll have someone rebuild the carb.

 

[kidmet]

Re: Any (Rochester) carb experts here?

Fuel filter is new, gas is new (and the same gas I used last weekend without issue), and it runs perfectly at anything above 2200 RPM.

It goes from running perfectly fine at 2200+ RPM, to dying like the key was switched off below. I know the basics about carbs, and am under the assumption that different jets are used for lower RPMs, and they might switch over around that point. The issue also arose immediately... It wasn't a gradual thing like a bad filter or bad gas.

I guess I'm hoping someone will say "yep, that's when the high speed jets kick in, your low speed jets are clogged", and confirm my hypothesis. If that's the case, I'll have someone rebuild the carb.

wondering about the alternator at this point. also, have you jumped the positive terminal of your coil to your positive on the battery? That is a cheap try you can do right away.

 

[Renken1]

Re: Any (Rochester) carb experts here?

Sorry to say there is no " low" jets in the carb. It only has 2 located in the bottom of the bowl. Engine vaccum controls how much fuel that flows thru them at a given rpm, up to the jet's flow limit. At wide open throttle there is a power valve that opens and lets additional fuel into the booster circuit that doesn't pass thru the jets. You have an "idle circuit" that comes into play when the throttle blades are nearly closed, but that fuel also passes thru the same jets.

If you have a multimeter, hook it up to your battery and watch how much your alternator is charging, then slowly idle it down til the instant you detect its gonna quit and see if there is any drastic voltage change. The way its shutting off almost makes me think its electrical instead of fuel related

 

[Renken1]

Re: Any (Rochester) carb experts here?

Thinking about it more, when it goes to shut off, can you give it throttle back and it refire or does it go ahead and die no matter what you do with the throttle?

 

[PaJon]

Re: Any (Rochester) carb experts here?

I would check the gasket at the base of the carb or possibly an intake gasket. Get it to idle as low as possible and spray starting fluid around the carb gasket. If you get an increase in rpm's, it's sucking air at that gasket.

 

[Celtichawk]

Re: Any (Rochester) carb experts here?

Air sucking in just might be a issue. Alongside all that everyone mentioned I would highly suggest making sure your flame arrester is also in right and not causing a air suction leak.

 

[kidmet]

Re: Any (Rochester) carb experts here?

Just fixed my boat with the prob that sounds just like yours. I'd try to take the carb apart and boil it in white vinegar. I know it sounds weird but did it with mine and she runs like a champ. I had tried to soak it in a carb cleaner with no luck. I think my idle circuit was clogged and the boil cleaned it up.

 

[bonzoscott]

Re: Any (Rochester) carb experts here?

Just fixed my boat with the prob that sounds just like yours. I'd try to take the carb apart and boil it in white vinegar. I know it sounds weird but did it with mine and she runs like a champ. I had tried to soak it in a carb cleaner with no luck. I think my idle circuit was clogged and the boil cleaned it up.

Now, THAT is the snits shizzle! Creative I must say :lol:
I have boiled carbs before but just water.
Was the new Filter the one in the carb where the line enters? Pretty easy check. I agree that a carb rebuild is in order.

 

[yknotgosailing]

Re: Any (Rochester) carb experts here?

I've got an '88 Wellcraft with an OMC 4.3, and up until yesterday it ran and idled like butter. Launched for the day yesterday with no issues, idled out the river, and took off. When I came to our first stop, the engine wouldn't idle worth a crap. Anything under 2200 RPM and the engine dies immediately. Anything over 2200 RPM, and it runs perfectly.

I know the basics of how a carb works, but I've never torn into one. I'm guessing I have an issue with the low speed jets? Anyone have any experience with this issue and/or repairing them?

It's a 4.3 with a Rochester 2bbl carb, 1988 vintage (I'm pretty sure it's original). Thanks!

Have worked on many carbs being it happened so sudden I would say you have something lodged at the needle and seat .This will allow enormous amounts of gas to flood the engine at lower r.p.ms and immediately stall the engine. When you open the throttle and engine runs above 2200 your allowing enough air to compensate for the rich mixture. A trick I used which worked 80% of the time is to use a means to cut off fuel supply with engine running ( rubber supply line), yes you will have to run the engine at a high rpm and allow the engine to stall from lack of fuel this might take 1 to 2 minutes then immediately open the fuel supply and start engine, this procedure hopefully will flush anything stuck in float needle .You may have to do this 2 or three times ,when it clears engine should start to run fine. If this does not work then I would remove the top carb air horn and physically remove float needle and check for any debris . It takes very little to hold that needle open. If this does the trick you may want to have a look at the gas tank for contamination. You may also want to try blocking off the top of the air horn when engine is running this causes a great deal of vacuum in the throttle body clearing out the pull over tubes. Stall engine when revving by slowly cupping your hand over carb intake , it won't hurt!