Surging RPM,

[ughmo2000]

A friend is heading down the ICW and having problems with his A4. Apparently the RPM will suddenly increase then decrease, etc. He's already bypassed the inboard fuel tank with a outboard tank so it's not dirt in the fuel. I'm thinking carburetion. Anyone care to offer their opinion?

Thanks in advance for any assistance.

[pschoonveld]

I'd be curious whether the RPMs are dropping or climbing from the throttle position.

If they're dropping, and it seems to go away when the choke is applied, I would guess water in the carb. On some model carbs, there is a bolt that can be removed on the back, bottom port side. It's a good way to do a quick check. Either way, I'd start with the carb.

[jrd22]

I agree, sounds like a fuel or carb problem. Since he has changed fuel sources I'm guessing carb. Did this just come on suddenly or has it slowly been getting worse? Sometimes an air leak can cause a change in rpm, like from a worn throttle shaft.

[Omatako]

Don't know much about Atomic whatevers but I have been around petrol engines for several decades so maybe I can help.

RPM surge is almost certainly a fueling issue and conmtrary to popular belief, it is not always caused by the engine "running out of fuel" only. It is often caused by a mixture momentarily too rich for proper combustion.

I don't think it will be caused by a choke unless the butterfly is allowed to fluctuate on and off. Chokes are generally permanently positioned and the engine reacts more permanently to the condition.

It is more likely to be a float level issue in which the float is either too low (insufficient fuel supply, blocked lines or filter, etc) or too high (stuck needle'n'seat, float contains fuel, sides of chamber dirty causing float to stick, dirt under needle, etc).

As said above, there are possible issues with the fuel pump as well so check the flow at the incoming line.

Have the carb serviced by someone who (really) knows how.

[timebandit]

I would check for intake leaks using wd40 on all the conections, it's cheap and easy.

[ughmo2000]

Thanks everyone.

The A4 was rebuilt before going on the hard for several years. The carb was taken off and cleaned. The motor ran fine for several days before problems began. I too am steering him toward carburetion.

[COOL]

I would also check for blockage in the exhaust elbow,
or any constriction of the exhaust system.

[ughmo2000]

Thanks for everyone's suggestions. It was indeed the carb. Junk in the passages from sitting for so long. She's apparently running like a top now.

[QuickMick]

I just rebuilt my carb, and seems that overall idle is fine, yet i too am getting the occasional random surge. i was inclined to think that it was related to the fact that my engine had sat for 4-6?? yrs with no tlc and might just be blowing out some old crapola as the MMO works its magic... as it seems that my 'surge' will come then go, as if it is passing a kidney stone...

[degreeoff]

Yep I'd change the float needle and seat. Get it through Moyer Marine. Mine looked good felt good but still wouldnt seat.