Fuel leak in TBI 350

[CamMan]

Hopefully I don't perform any big sins since I don't have truck, but engine is from one. It's -91 350 TBI that is controlled by Megasquirt2 and has also wasted spark ignition.

Problem is that when starting hot and engine has been sitting short while (maybe 5-15mins) the engine seems to back fire/pop to intake. If engine has been sitting long enough maybe 30mins+ or overnight starting is no problem. I've been running for 8 years and seems that this summer it's getting worse so I started finally inspecting more.

I suspected that fuel is leaking to intake when turning engine off and due wasted spark the wrong cylinder has combustible mix and that cause the pop/backfire when starting. I thought that maybe the injectors maybe leaking. I ran the fuel pump today when engine wasn't running, but I could not see any fuel leak either from injectors or anywhere else, but when I opened butterflies completely I could see fuel pooled at the bottom of the intake under both butterflies.. I tested it with q-tip and it smelled like gas not antifreeze.

Now I'm puzzled where that gas is leaking from. Is there a path under the TBI where it could leak? I've taken it apart few years back but can't remember anymore. I don't think there should be any path under for gas to leak?

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[Schurkey]

ALL the fuel has to enter from above.

I suppose there could be a gasket leak at the injector pod...but you'd see the "clean" trail where the gasoline has solvented all the varnish and dirt away.

I think you're on the right track with waste-spark ignition and available fuel from the "wet" intake manifold. Any TBI system will have some fuel in the manifold. Seems to me most waste-spark systems are used on port fuel injection vehicles that would not have "wet" intake manifolds. Maybe there are some waste spark/wet manifold engines, but I can't think of any right now.

What is your low-speed (initial) ignition timing?

 

[CamMan]

Originally I think the advance was 10deg btdc and now it's been 0deg btdc. I don't think it made any difference.

But yeah, it is possible that there is always enough gas there that can fire the 'wrong' cylinder with hot engine and when fuel evaporates more. I'm guessing with the cold engine that isn't that much of a problem since I've never had that problem with cold engine.

Had I known almost 9 years ago when built the system I might have gone with normal ignition setup, but seemed easier to do the wasted spark and had MS control spark as well since I didn't have the original ecu with the motor. Maybe I'll have to bite the bulled and get HEI - distributor and setup MS to work with it. There was a reason why I didn't do that originally. I just can't remember it anymore.

 

[CamMan]

Just to return here. I swapped GM HEI distributor back with Megasquirt controlled and as for good measure I also replaced started (old one happened to break at the same time) and I can say car has never started that nice. Only took me few years to realize that wasted spark was wrong style ignition for TBI.