Engine Rough at low speed

Hi all,

I have an RV 8 with a TMX IO360 M1B built by Mahlon and the Mattituck folks about five years ago. 9.5:1 pistons, Red-Gold package. First engine start was last month (I'm not a quick builder, it seems).

The engine is set up with dual Lightspeed Plasma III ignitions and an Airflow Performance FM-200 horizontal induction.

The basic problem is low speed operation. Above 1200 RPM, the engine is smooth as silk. Powerful, quiet. It idles pretty well at 650-700 RPM. When you advance the throttle, it bucks and spits and then accelerates. In the RPM range of 850-1100 or so, it is really rough.

I spent a bunch of time on the ignition. It clearly is firing on all cylinders, both upper and lower ignitions. Timing was verified with a timing light. Advance works correctly. I think that has pretty much ruled out ignition as the problem. Klaus asked me what my idle MP and fuel flow were: 3.0 and 14. Definitely not right.

Spoke at length with Don Rivera at Airflow performance. He was quite helpful and had me check the throttle plate clearance and adjust the idle mixture. He opined that this was an induction leak. For my limited understanding of engines, that made sense.

I started by checking my sniffle valve. It was clogged in the open position. Cleaned it and re-installed. No change. I rigged up a hair dryer in the non-heat position to blow air into the filter (taped up to get some pressure). Put soapy water over all the induction fittings. No bubbles. Started the engine and ran it to about 900 RPM. Sprayed ether over all the induction fittings. No RPM change. Plugged off the sniffle valve. No change. Checked the manifold sensor hose. No leak.

The only anomaly I have found in the engine readings is that the #4 EGT is about 150 cooler than the other three at idle. CHT's are pretty similar.

I am stumped. Anyone have any ideas about where to go from here?

Regards,

Michael Wynn
RV 8 Phase 1
Livermore, CA

Couple of things?are you idling at Full Rich? If so stop doing it. Idle by adjusting to peak RPM and then as the RPM starts dropping, leave it there. Taxi this way.

Have you a light weight propellor ? With 9.5:1 and a light prop you may just be feeling the power pulses.

mlwynn said:

Hi all,


Spoke at length with Don Rivera at Airflow performance. He was quite helpful and had me check the throttle plate clearance and adjust the idle mixture. He opined that this was an induction leak. For my limited understanding of engines, that made sense.

I started by checking my sniffle valve. It was clogged in the open position. Cleaned it and re-installed. No change. I rigged up a hair dryer in the non-heat position to blow air into the filter (taped up to get some pressure). A

Click to expand...

First off Don is likely correct so I don't see why you should be stumped. A hair dryer cannot create an air pressure difference. Bottom line: remove all intake tubes and put new gaskets on but do not put them on dry. I rub them with titeseal so they are easily removable in the future.

Pull back slowly on the mixture control when the engine is running badly. If operation improves engine setting is too rich, If it gets worse setting is too lean. If it has no effect it isn't a mixture thing. Normally, a induction leak that is common to all cylinders will cause fast idle that can't be slowed down by adjusting idle speed. Induction leak common to one cylinder will cause the same, with roughness. Induction leak common to one cylinder should also cause a cold or cool cylinder when the engine is left to run, for a while, at a setting that the engine runs badly at.
Try this out and we will go from there.
Good Luck,
Mahlon