rough engine at low power

[3flyer]

So my father and I have been dealing with a very frustrating engine problem for about a year now and are looking for others to chime in with opinions and/or ideas.

THE FACTS;The aircraft is an RV3B with an O-320-E2D. Standard Marvel carb and Vettermans exhaust. The propellor is a Sensenich fixed pitch wood prop of 68" diameter and 76" pitch

THE PROBLEM; anytime in cruise or descent when the throttle is pulled back to almost anything under square (we have a manifold press gauge) the engine hiccups and sputters to the point of running quiet rough. With the prop thats on it this is almost always when in cruise as the engine easily winds up the RPM with the nose level. In order to keep the RPMs below 2600 when level the throttle MUST be pulled back, which obviously results in the stated condition above. This leads to quickly fouling plugs, somewhere in the realm of 10 hours. Fuel burn flights have been conducted and are consistent with the power settings being flown?.actually a bit lower than expected (more on that later) but within normal ranges.

OUR TROUBLE SHOOTING AND RESULTS; when it first began happening we figured it was a weak cylinder or a plug not firing. It turned out the front two cylinders (#1 and #2) did not break in as well as the rear two cylinders. as they were fouling faster and had definite evidence of oil seep and rings not having seated properly. Originally the engine had used a propellor pitched at 72" and we opted to up it slightly to obtain more MP also new jugs (#1, and#2) were put on as well. This new prop with pitch of 76" as stated above proved effective in breaking in the new cylinders well and we were able to obtain a few more hours before the plugs began fouling?again. Mag timing was checked and rechecked, the hiccup problem was always the same. We then upgraded the mags to a Emag Pmag setup with automotive plugs (this is experimental after all). We now run Iridium plugs and the fouling issue rears its head a lot less frequently and the fuel burns reduced slightly, however when the airplane is leveled out and the power pulled back, we still get the hiccup and sputter.
In further experimenting with power settings and flight scenarios we have observed that if we fly SLOW enough with the power at 16-17" and RPM at 1600-1700 RPM the engine purrs like a kitten and never has any hiccup issues?ever (ran through almost a full tank of fuel flying this way) also if the engine is ran very hard, MP around 25"or greater depending on altitude and RPM at 2600-2700, the engine also runs great. Smooth and predictable, fuel burns consistent with power settings once again. As we'd never had the issue happen during climb we also tried leaving power settings alone in cruise and once they produced engine hiccup we would increase pitch to climb, almost immediately as the aircraft slows or loads up the prop, the engine begins running smooth once again.
These observations have us believing that the Lycoming engine does not like to run lightly loaded. Not necessarily low power, but low LOAD. Our theory we've come to is that with a clean airplane (easily pulled through the sky) and a fixed pitch prop, that under pitching for climb leaves you with an un-flyable engine in cruise. We aren't completely ruling out that it could be some sort of carb issue as well, as it could probably be rebuilt anyway, but right now we are trying to decide whether to drop the money on a new Carb or a new Prop (again).

Looking for help, thanks.

[BobRv4]

Hi,
I had this almost same problem for years, actually found the fix on google!
The carb can have two different main jets, one has more fuel holes than the other. This jet lets the fuel distribute better. What happens is fuel pools in the manifold under a steady state, once you move the throttle the pooled fuel enters the cly making a very rich mixture, then the stumble. Take the carb apart and check out the jet. Also I'm not 100% sure if the correct name is jet, or ?
Let me know what you find out. Bob

[GeneL]

Also check for a float getting heavy. Nothing but the latest ones are reliable. If I remember correctly they are blue plastic

[3flyer]

Thanks for the reply's. Will tear it down and check it out.

[JoeLofton]

I have the same engine in my RV-3 turning a fixed-pitch Prince composite prop, and it runs fine across the RPM spectrum. I do sometimes get a slight stumble on application of power from idle, but I think that is a common carb characteristic.

[170 driver]

Have you seen this? I copied it a while back from another thread,,,

It seems that there was a Lycoming SB258 (which I having trouble finding) that specified need to swap out the 10-3678-12 for a 10-3678-32 carb. But due to difference in throat diameter - a tapered insert needs to be put in the sump riser.

My sump hole diameter is about 2 1/4", but the carb throat diameter is much less at 1 15/16" creating a step. This according to one post causes the rear cylinders to run lean (which just happens to be the issue I have).

The solution is apparently this insert:

OS-5372-1 Textron Lycoming SLEEVE INSERT $56.48 EA
the dowel pin that holds it in is
STD 798 and the gasket is 66224


Which smooths the transition from carb to sump. This may also mean that I do not have to drill the jet - not sure.

[krw5927]

See this post:
http://www.vansairforce.com/communit...4&postcount=12