O300A right front cylinder not firing at idle

[gwillford]

Need some help diagnosing an engine problem with my O300A. Problem is the right front cylinder is cold (not firing) when at idle. The cylinder comes on line after about 1500 rpm. High RPM operation is normal when flying around. Upon return to low rpm the cylinder again drops out. Additional info:

Engine is newly overhauled (me)
Carb was overhauled by a shop and it has been off and rechecked
Idle mixture has been adjusted for cold weather, 100 rpm rise
No intake leaks (pressure checked, soap spray). All new gaskets
New plugs
New Slick mags
New harness
Confirmed all plugs firing
New mag switch
New P leads
Cylinder off and reinspected. All OK
Before removing compression was 80/80
Valves are not sticking, springs are ok
Valves opening as far as other valves do
Pushrods same length as others
No looseness in valve train
Exhaust removed and inspected for any plugging (is new anyways)

I am fresh out of ideas. Any assistance would be greatly appreciated. Thanks!

George

[davevramp]

If what you say is true, insure that the plugs fire on the compression stroke for that cylinder. New harness leads going to the right plugs??
dave

[gwillford]

Plug wires verified correct from each mag to each cylinder. Top and bottom,

The only other wild idea is can a lifter pump up ok to give the impression all is well at static but actually not hold pressure at lower rpms and lower oil pressure but be ok at higher rpms? This would give the effect of delayed valve timing. Really running out of ideas here.

[blueldr]

Did this problem exist prior to the all new ignition components? If not, try installing a couple of spark plugs with the gap closed down to about half of the normal setting. This will allow a lower voltage to fire the plugs. However, it does not seem likely that both mags would be doing the same thing. It has been my experience that Bendix mags will normally have a considerably lower coming in speed than will Slicks.

[gwillford]

Same problem with the harness, plugs, and mags that were originally on it. Mags were overhauled by a shop previous. They went out of business since then. I sent these mags recently to a new shop and it turns out that there were many problems and ADs not complied with. Was cost prohibitive to repair so I went with the new Slick kit. problem still persisted. Thanks anyways. Keep thinking!

[johneeb]

George,
How about checking the dry tappet clearance. Do a search of these forums on this subject.

[blueldr]

The hydraulic tappet units do not have to be punped up for the valves to operate adequately for the engine to run. Dry tappet clearance is .030 to .110 inches, which is considerably less than tappet lift.

If the cam lobe was worn down to the point that the lifter would not lift the valve until the haudraulic unit was pumped up and the hydraulic unit leaked down rapidly, this might be the problem. That is a lot of things to go wrong in a given sequence. To check for this , I would open the rocker box and check for valve action with the engine cold and pulling the prop through by hand.

[Bruce Fenstermacher]

BL this is a newly overhauled engine. I'd think the cam wouldn't have worn that much.

[hilltop170]

If it is not ignition, it sounds like an intake leak to me, running too lean until higher flowrate overrides the lean mixture from the leak. If it was checked with pressure, it might seal itself. I'm not suggesting doing this but if you squirted a little gasoline on that intake at all the joints while the engine is at idle and the cylinder fired, that would show you where the problem is.

[c170b53]

I like BL's idea,a worn cam. I had a VW bus with an engine that did the same thing. But the wear would have been found when the cam was measured at OH.

[blueldr]

This engine apparently had a field overhaul. Was the camshaft checked by a facility authorized to yellow tag it?

The intake leak didn't occur to me but, it would certainly be a good place to look. An intake leak is maximized at idle when manifold vacuum is greatest and fuel flow is the lowest.

[gwillford]

First of all, thanks a lot for the suggestions! Yes the cam was inspected, processed and tagged by a reputable shop so it is not worn. Also verified again when I had this cylinder back off looking for this very thing and anything else I could find. I will check the dry tappet clearance. It has all the appearance to me of being a fuel problem also. Intake leak is where I started this journey. When I had the intake system apart on that side I even touched up the sealing surfaces that mate up to a gasket and checked all on a flat surface plate. I may have stated earlier, the intake hoses are new. Question. Fuel distribution issue? Carb has a one piece venturi. Does not have the new fuel nozzle. Would help?

[sloPoke]

We had that happen on my racer with an O200.Dead but comes right in as you push up the power. It was a leak in an intake gasket.
I'd check that first. Edit - I went back and reread your first post - looks like you already checked. It sure sounds like it otherwise.
Good luck,
Charlie

[blueldr]

My experience with the one piece venturi was extremely discouraging, and I finally went back to my two peice venturi which I had fortunately saved.
I tried everything to get the engine to run smoothly and never was able to do it. The one piece venturi was made by Precision and their carburetor guru said I needed a new nozzle and since I had a Precision part (I sent them a copy of my receipt for the part from Chief Acft.), they supplied the new nozzle at no cost.
It didn't do a bit of good, and when I checked the hole sizes in it with a pin gauge I found them identical to the original. After I reinstalled the original two piece venturi, everything work beautifully. I don't have a clue to why the problem existed, but I do know that I was not alone with it. The roughness problem was reported by many others at the time.

[sloPoke]

Since it existed before the rebuild it might be a porous casting on the intake elbow that's leaking through the elbow itself.
We had that once. We had opened up the elbow to try to get more flow on the racer and it was leaking through a thin spot that was porous. Just an idea. I thought the same thing about the lifters not pumping up or something to do with the cam, but at idle with a bit of intake leak the mixture can be too lean to fire. As the flow increases with rpm the mixture richens and it runs okay, just leaner than the others. Unless you have a 4pt egt system you can't tell.
Charlie