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O-360-A1A RPM Oscillation

FlyFastJP

FlyFastJP

Well Known Member
My new-to-me RV-4 has the above engine. Not sure of which carb it has...I didn't take note of the part number when I had the cowling off. Winter upgrades are about to start tho.

Last flight, I noticed an rpm oscillation at about 1400. Goes up and down 50-75 rpm. No stumbling or stuttering...just steady up/down constantly. No fuel pressure gauge (guess builder didn't think one was needed...but I bought one), but it does it with and without boost pump. I don't recall it doing it on my first flight when I was checking stall, slow speed handling, etc.., but that doesn't mean it didn't. I may have just missed it.

Any ideas?
 
DO you hear an rpm change (pitch change)? If not, I would guess a gauge issue. Any chance you have a constant speed prop? I see your signature says warnke prop. I have seen oscillations like this caused by a prop governor.
 
What Gil said

I had similar event on my -4, and found my bowl screws had loosened enough to allow air leakage. The tight fit of the cowl/snorkle on the -4 can cause alot of left/right shift of the carb and loosen the screws even if the locking tabs are tight. I actually reworked the rubber seal at the inlet to allow a little more freedom of movement. In my case, at full/high throttle settings, I couldnt notice any RPM fluctuation.. just mid-range.
 
fixnflyguy said:
I had similar event on my -4, and found my bowl screws had loosened enough to allow air leakage. The tight fit of the cowl/snorkle on the -4 can cause alot of left/right shift of the carb and loosen the screws even if the locking tabs are tight. I actually reworked the rubber seal at the inlet to allow a little more freedom of movement. In my case, at full/high throttle settings, I couldnt notice any RPM fluctuation.. just mid-range.
Click to expand...


That's exactly what I was thinking today, and will be the first thing I look at this weekend when I pull the cowl off. Thanks.
 
Does it have Pmags or other EI? The only likely causes are:

Changing ignition timing (quite unlikely with mags and your symptoms, but have heard plenty of wierd stuff about Pmags - anything with electronic control is suspect with these symptoms)
Changing mixture (lean conditions can create surging at lower RPM's)
Changing airflow
Combination of the two above

I would start with the carb, as it is the highest probability. The loose carb bowl is a logical first place to start, but you'll want to look for other loose or missing parts as well. It would be worth looking at the venturi. Older model carbs had a separate venturi that could come loose. Not sure it would cause your symptoms.

I'll leave ignition ideas aside until you report, but I would focus my energies on the carb. A loose throttle linkage could also possibly present with these type of symptoms, but you would likely notice the slop in other areas.
 
If you determine that it is the carb and not just loose screws on it and it is a
MSA carb, they have several SB's on them including the float, I had them do a rebuild that brought everything up to date on the SB's and also stamped the carb as done. Done it in a few days for around $150 and run great after. Note you do have to do the idle/mix adjustment after a reinstall. I'd recommend the MSA company rather than any other unless you know a good carb A/P.
 
It's running standard Bendix Mags...nothing fancy.

This weekend I'm planning on pulling the cowl and checking it out. Will also add to my list (ever growing) the model and part number of the carb so I can research it for SB's. Will also be adding a fuel pressure gauge, red cube transducer, and FC-10 fuel computer. Can't wait for that!

Thanks for the help. I love this forum. Will send some cash to DR soon.

I'll report back when I know what's going on.
 
BobRV7 said:
If you determine that it is the carb and not just loose screws on it and it is a
MSA carb, they have several SB's on them including the float, I had them do a rebuild that brought everything up to date on the SB's and also stamped the carb as done. Done it in a few days for around $150 and run great after. Note you do have to do the idle/mix adjustment after a reinstall. I'd recommend the MSA company rather than any other unless you know a good carb A/P.
Click to expand...

Who did you have do the rebuild / SBs? $150 is a good deal.
 
Ok...it's an MSA 4-5L 10-3878.

Bowl screws and seals looked fine. Butterfly arm is a bit sloppy, which could be allowing a variable amount of air into it.

I contacted MSA about the RPM issue and asked about just rebuilding it. We'll see what they say.

Thanks for the help.
 
10-3878.

10-3878.
The 10-3878 is a known lean carb on the o-360 RV's.
It is not as lean on FP props as CS....but you might consider a richer jet or use as a core for the richer carb.
 
BobRV7 said:
If you determine that it is the carb and not just loose screws on it and it is a
MSA carb, they have several SB's on them including the float, I had them do a rebuild that brought everything up to date on the SB's and also stamped the carb as done. Done it in a few days for around $150 and run great after. Note you do have to do the idle/mix adjustment after a reinstall. I'd recommend the MSA company rather than any other unless you know a good carb A/P.
Click to expand...

Hey Bob, can you tell me who exactly you contacted for the rebuild? I just emailed MSA and they told me to buy a rebuilt carb for $958.
 
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