Bah! Engine trouble! Timing? Fuel?

[wotan2525]

I've been chasing an oil leak this year that seems to keep getting worse. I had a mechanic friend over and his best guess is lots of small leaks from lots of small places.... Frustrating though it was to not find one spot to start with, we attacked a bunch of places that seemed to be leaking or seemed like they could start leaking. Tightened the oil pan bolts, removed and sealed the oil galley plugs, but probably the biggest one was removing and replacing the intake manifold gaskets. Since then, things have not been running like they should be.

The symptom: boat runs fine at idle and in neutral through the whole range of RPMs. In the water it runs perfectly up to 2000rpms and then it begins to backfire through the carb (I assume this is whats happening... it POPs and smoke comes out carb.) This only happens when it is in gear but (oddly) it seems to happen worse in forward than in reverse (I suspect this is only because it has reduced load on the engine when in reverse, the tranny seems fine?)

I've got the engine timed to 34degrees @ 3200RPMs. I've advanced and retarded this and the backfire remains always around 2000rpms. It seems to start, idle, and rev (in neutral) best at ~34degrees.

We sprayed carb cleaner around the carb gaskets and intake to head surfaces to check for vaccuum leak but nothing was apparent.

Two things that my mechanic friend noticed when we reassembled everything was that the advance mechanism in the distributor seemed really loose/old. The entire rotor wobbled around far more than what he thought was normal. I've already ordered a new distributor.

The other thing was that he didn't think my fuel pump was putting out enough flow. I have an electric in-line fuel pump and it seems to put out the 6-7psi required but (and we haven't tried with the engine running) it doesn't seem to flow very much. Could fuel starvation cause my issue? I've run out of gas before and it never backfired.... just died or experienced surging. I'm thinking the fuel pump is (hopefully) fine... but... anyone recommend a good marine in-line fuel pump? Does it have to be marine?

For reference -- it's a 454 with an Edelbrock intake and carb.

[rludtke]

A lean mixture can cause backfiring because the lean mixture burns more slowly, and may still be burning in the compression chamber when the intake valve begins to open.

Carburator mixture is not really dependant upon fuel pressure, as the carb requires a steady flow of fuel (not pressure) into the atmospherically vented float chamber (it is not pressurized), it is possible that low fuel flow could result in a lean mixture and backfire. I actually rather doubt that the pump would be the cuplrit of a overly lean mixture, its a long shot. Much more likely is an intake leak, but you have already checked for that. I suppose the intake leak could be internal to the engine, but I have never encountered that.

I am surprised that you don't have an engine driven mechanical fuel pump. The mechanical pump will likely out-flow any electric pump, and the big block needs its gas. Electric pumps are typical of fuel injected engines.

I also worry a little bit about a bad cam. A cam lobe going "flat" or changing shape could do this as well ( I have experienced this).

[saltare inverts]

I want a direct tap to rludtkes knowledge of motors. Will you be my mechanic friend?

[rludtke]

I want a direct tap to rludtkes knowledge of motors. Will you be my mechanic friend?

Thanks Salty!

My background before I got into aviation engineering was aviation maintenance. I was formally trained as both an FAA airframe and powerplant mechanic, and a FAA commercial pilot. Much (but not all) of what I know translates ok to boats.

My Supra is my first boat, and this has been quite a learning experience, let me tell you...

[warek1814]

Well if it's lean you can pull a spark plug and it will be white. You want a light brown/ish color on the plug. A fuel injection pump will feed plenty of fuel to a carb motor but would require a regulator(I have done it before). This is assuming the pump is working right. I have had electric pumps that would work but not supply the correct pressure(low). Just had a walbro electric pump on my brothers street car do this exact thing.

[rludtke]

Well if it's lean you can pull a spark plug and it will be white. You want a light brown/ish color on the plug. A fuel injection pump will feed plenty of fuel to a carb motor but would require a regulator(I have done it before). This is assuming the pump is working right. I have had electric pumps that would work but not supply the correct pressure(low). Just had a walbro electric pump on my brothers street car do this exact thing.

Excellent, this puts the lean mixture caused by faulty fuel pump theory back on the table in my mind. I think you will want to confirm that the output of your pump is correct. Yes, your spark plugs will tell the story about your mixture.

[wotan2525]

It's actually not a fuel injetion fuel pump -- it's this one:

http://www.ecrater.com/p/4056430/mar...pump-universal

I don't know who put it on or why but it's had that pump on there since I've had the boat. (7 years now.) The line goes from the tank -> fuel/water seperator -> pump -> inline filter -> (some strange silver fitting... I'm really not sure if it's a pressure regulator or a check-valve. it doesn't have a return line so??) -> carb.

What's interesting to me is that this problem started when I had the intake manifold gaskets replaced... so.... what parts were touched (nothing in the fuel delivery system) and what changes when the engine is under load (this is one I still have no idea about!)

How should I test or check the flow of the fuel pump?

[rludtke]

This little pump bothers me. 30 gph should be enough, but i sorta doubt it's actually capable. I may be biased though. My motorhome generator has a pump like this...

I agree, the pump and fuel system isn't related at all to your intake manifold and/or oil leaks, but seems to be the most likely cuplrit for the behaviour. I am not sure how you would test the pump. If you were to convert back to a new mechanical pump, you could rule it out, but it's probably only a 60/40 chance that those dollars will fix the problem. I would be interested in what your spark plugs look like, we might be able to glean something from the information. If all are dusty and white, than pump becomes more likely.

If only one or maybe two plugs are white, then perhaps it possible that something could have fallen into the intake manifold, or into the cylinder head ports while it was disassembled. Perhaps an intake valve is being held open, or there is a partial obstruction to one of the cylinders in the intake manifold.

If the plugs are nice and brown/dry, then all bets are off ;0)

[wotan2525]

Ok... thanks for your thoughts... new distributor should be here tomorrow. I'll pull plugs at the same time I swap that out and post some pictures if my phone cooperates.....

It's interesting the knowledge that your background has given you. Oddly enough my former marine mechanic (a friend that recently moved out of state) was the chief engineer on the USS John F Kennedy before he retired. His experience with "tiny" boats was limited to those in his support fleet but he was always impressed and delighted when I would call him up to come troubleshoot something for me. Last year he diagnosed a leaky transmission cooler and then told me he'd only worked on coolers about 10xs that size!

[edgewater1]

ELECTRONIC IGNITION!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!http://www.skidim.com/prodinfo.asp?number=RK107025A