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3406a misfire

321 Views 5 Replies 3 Participants Last post by  sname5978
I'm having problems with a 400 hp 3406a DI engine. It runs fine when engine is cool but at about 150 degrees it starts to misfire at idle. By the time it reaches 180 degrees, it misfires at 1500 rpm. It only misfires at a very narrow range of rpm. Speed up or slow down a few rpm and it smooths out. It's not like a miss on one cylinder, but like all cylinders are short of fuel.

Under load it runs fine, good power 30 pounds of boost. The only time it misfires is at no or minimum load. Fuel pressure is steady at 30 pounds. Valves have been lashed, I had the nozzles tested, checked the timing, no air in bypass fuel, plungers appear to have been replaced. I purchased this as a used engine and put it in my 1980 W900.
It has run like this since installation probably 400 miles ago. Any suggestions would be appreciated.
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I've done some more work on the 3406 replaced all the fuel lines and filters. New fuel pressure gauge shows 40 psi. Test drove it, no change still misfires when warmed up.
Thanks for the reply Techmech. I pulled the automatic timing advance apart yesterday, it looks like new, no broken springs or worn parts. I'm pretty sure the governer is the problem, gonna pull it apart today.
I got it fixed!!! After reading a post by Ynot on rebuilding A-model governers and how the torsion drive gets slack in it and causes loping, Ipulled mine apart and the torsion spring, yoke and drive pin were wore. Replaced those pieces now it runs like a top. Thanks Ynot and everyone else. I doubt i would have fugured it out if it wasn't for this forum.
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