I can tell you a story about this issue that burned up about 2 months and $2000.00. When I bought my first RV-4 I didn't like the "carb heat" set up because it didn't really have carb-heat, just the ram air door pulled closed and that was it. So we (my trusty AP IA friend and I) installed a muff on the exhaust, put in some scat and a diverter box, etc. etc. I don't remember every detail of what we did, but it also ended up improving ram air flow in cruise over the initial setup when the carb heat wasn't on.
Fast forward a few weeks and I'm out flying and sense the engine running a little rough; was that me, or did it really stutter? Yep, there it goes again, definitely rough. Did a mag check, Woah! 500 RPM drop on the right mag. (Ground mag checks were perfect.)
Got back, cleaned and gapped the plugs -- they were pretty dirty, and somewat worn -- flew again, same thing. Bought all new plugs -- man these little dudes are expensive when you buy all eight -- fixed it though!
Well, until the third flight, anyway, then rough again. Seemed more likely to happen when it was hot. Tried the mag sw again -- and now the engine just about quit completely when put on the right mag only. Scared the *:&# out of my friend when I did it in the pattern at his grass strip.
Pulled the ole Bendix mag -- had it overhauled, guy who overhauled it found several problems. Put it back on, and then she was fixed.
Well, maybe not, problem came back -- intermittent but getting more frequent. Getting into hotter part of summer seemed to make it worse. Maybe a problem with the mag sw -- chased every possibility with a meter -- all good. Just to make sure, put in two temporary mag toggle switches and isolated out the key switch -- no joy.
Well, it has to be the mag -- the in-flight mag check proves that unequivocally. So screw it, ordered a whole new slick mag kit and harness. Put them on, still same problem.
Okay, let's look at the fuel system, induction system, and carb: carb float level, checked, needle checked, etc, etc. no problem, idle mixture fine--put it back together. Checked for induction leaks -- none. Checked for primer leaks -- none. Went all through the fuel system, no debris or restrictions. I give up, my AP IA friend gives up we both wish we had never seen this plane. (The plane had never, ever run rough for the previous owner. I know this because it was based at my home field and I watched him fly it all the time and flew with him myself several times.)
By now the plane was running rough on every flight and I was getting scared to even fly it. Exept for one flight I took it up to 11,500 feet, much higher than I had been flying -- and it ran great! In fact it ran great at full rich mixture at eleven-five! Hmmmm..
Finally, in desperation I search the furthest crevice of the internet and find a post on the ole' Matronics list about drilling out carb jets -- I seem to remember 41 to 39, but that may not be right, so don't quote me on it. Took the main jet out, took to the workshop of my buddy who's building an RV-8 and has a really good drill press, grit my teeth, we drill it out. And walla -- problem solved. Ran beautifully and never another hiccup.
To me it was a reminder of why we use the word, "experimental". An MA4/O-320 combo designed for use on a slow moving C-172 can have quite a different personality when housed in a tight RV-4 cowling and heated up with fast ram air being forced down its throat. Apparently, the subtle modification to the carb heat/ram air setup combined with warmer temps (may seem counter-intuitive when we're used to thinking about carb heat causing richer mixtures but a master motorcycle mechanic explained to me how heat can also cause leaning with a carb, but I don't remember the specifics) caused the change. Spent a lot of time and money chasing the mag when it never was the problem. (I wonder if Unison had fun destroying my perfectly good Bendix mags I sent them for trade-in?)
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Originally Posted by aerial
I have been chasing a rough running engine now for a while. Mag timing, and now jetting. Unfortunately I do not have a EGT or CHT guage, only Oil temp and feel. But it ran really smooth for 40 hours in the spring. I can tell it's rich when it runs smoother by leaning, and my fuel burn goes from 7.6 gph to 10.2 gph in cruise.
I talked to the guys at Precision Airmotive and they are very helpful. On their website they have a link to MSA carburetor applications, then you can go to their catalog and see the part number. Here's the scoop on the two jets I have.
813 - 47 Jet 715cc-725cc/min. flow
773 - 47 Jet 950cc-970cc/min. flow
I was informed the numbers aren't necessarily sequencial, but both jets are for an 0-320 both at each extreme of fuel flow. Thats over a 30% fuel flow increase between jets.
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Marvel Schebler Jet Size
Both Mahlon and Bart are suggesting drilling the jets however so I'm not sweating the fact that I did mine. If either of these guys would do it it's good enough for me.
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