Bet you can't figure this one out..... I CAN'T!

berth control

berth control

New Member
Nov 16, 2008
886
Saugatuck, MI
Boat Info
1985 270 Sundancer
Engines
3.7 liter Mercruisers
I had engine trouble the other day, the motor lost power until it eventually died. I diagnosed it as a faulty check valve on the fuel tank, but not before I replaced the carb. and fuel pump. The new check valve got the motor running, but it does not run right. It idles rough and when I go to rev it up it backfires flames and black smoke out the carb. I thought maybe the rebuilt carb. was to fault, so I swaped it with the one from the other motor that runs perfect, same thing. Then I thought about the check valve again and swapped fuel lines between motors, no change. Plugs look black but not fouled. I cleaned them and put them back in, no change. I changed coils, again no change. I pulled the valve cover off, all the springs and rockers appear to be in tact and working fine. I don't see any coolant leak in the intake manifold and the motor is not losing coolant, so the exhaust must be sealed up too. New plug wires. Cap and rotor are not new, but look like new.

A buddy suspects the little timing advance thing in the distributor, that is the next thing I am going to check.

Any ideas?? A motor only needs so many things to run :smt101
 
I'm thinking you are on the right track with the timing or ignition guess. We had timing issues lately, and it caused the plugs to be blacked up with carbon and eventually foul. Only thing I can think of that would make it backfire through the carb like that is an ignition pulse getting there at the wrong time causing ignition through an open intake valve into the carb. Just my $0.02
 
Backfire through the carb can be traced sometimes to a leanout condition. Recheck the gasket under the carb and start it up so it runs. Then spray a few short bursts of carb cleaner around the base and then the intake where it meets the heads. (assuming you can do this in an environment where it's ventiliated and safe.) see if rpm changes. If no change then that's ok. If it does change then you have a vacuum leak bad enough to make a leanout backfire. Other than that, you're on to the advance in the distributor as mentioned by kpetry. Check the static timing and see if you can disconnect the advance before you set it. It's possible the distributor hold down may have let loose and moved. Also check the cap for carbon tracking. G/l, and let us know what you find.


EDIT: Just reread your post about the cap and rotor, so I'd go right to the centrifugal advance and if it has a vacuum advance check that too. Plugs are black? Maybe it's so advanced that it's firing before the intake valve closes...? :huh:
 
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Set the timing per the manual. Rev the engine with the light connected and check the total advance to see if the distributer is advancing how it should. The manual should have an advance curve in it. If you removed the wires and reinstalled them double check that they are connected properly, I screwed mine up last time, imagine that. Run the boat in the dark and check for lightening. Sometimes wires leak and cause another cylinder to fire. Timing chains get worn and cause an engine to hunt timing. Just some stuff to do in your spare time. Good luck Eric.
 
Thanks for all the ideas guys, I appreciate it. I think I will switch distributors from the good motor to the bad and see if that does any good. I didn't think about the possibility of the timing chain jumping or being worn and sloppy, that may be it also.
 
I talked to a mechanic today at the marina, he said the problem is likely a leaking gasket from my exhaust elbow. The back spark plug was a bit wet when I pulled it out, so I am going to check that. I said that if that was it the problem would go away as soon as the motor ran that water out of the cylinder, he said no, it will do it the whole time. I guess it's one more thing to check. any thoughts?
 
Wet with fuel or water?
 
Check your wire harnes and that you are getting a consistant charge to the coil. Easy way is to put a test light on the coil wire and a if it goes dim when reving then the current is not steady and will cause all of the problems you listed.
 
berth control said:
I talked to a mechanic today at the marina, he said the problem is likely a leaking gasket from my exhaust elbow. The back spark plug was a bit wet when I pulled it out, so I am going to check that. I said that if that was it the problem would go away as soon as the motor ran that water out of the cylinder, he said no, it will do it the whole time. I guess it's one more thing to check. any thoughts?
Click to expand...
:smt038 same thing happen last season too...the motor sounds as if valve ping @ 4300rpms backed down and went away.. water from exhaust was fouling plug at hi rpms raw water had rusted thru head seal...new motor this season:smt038
 
Water in the cylinder is a no-brainer. Could be a leaking gasket, could be a hole in the riser. Not losing coolant has nothing to do with it. When was the last time you pulled and inspected the manifolds and risers?

Water in the cylinder= pull the risers ASAP and inspect before you hydrolock the engine.
 
Okay, another update. And thank you to all you guys for the continued help. This is the most mysterious problem I have EVER had.

I pulled the riser, looked good. The manifold looked good and dry, no traces of water whatsoever. Then I pulled the exhaust boots and checked the little flapper down in there. It is not doing any good keeping water out, but it is also defanitely not restricting the exhaust. I put a new gasket on and re-assembled.

Still backfires.

I switched distributor caps, rotors, and plug wires between engines.

Still backfires.

I adjusted the timing and experienced with all different settings.

Still backfires.

I'm thinking the plug must have been wet with fuel, that was my initial thought when I pulled it but I wish I had smelled and tasted it at the time. So to recap, the motor backfires through the carb at certain RPM's and loads, but it will run almost smooth. It is NOT the distrubutor cap, rotor, exhaust, carb, fuel pump, filters, timing, or fuel supply. Or lack of effort!

The marina's mechanic is coming by today to look at it, I gave up:smt089
 
Did you ever switch the ignition module under the dist cap? I saw you switched caps, but was unsure about the module. Make sure to use heat transfer compound.
 
I never did switch the module under the cap, just the rotor. It could be that thing. It looked a little intimidating in there so I didn't get that done.
 
Assuming :grin: you did all the other stuff you mentioned. it sounds like a like an exhaust valve mite not be opening as it should and when your intake opens to suck fuel and air, it's letting the compression out through the intake and up through the carb. Or the intake is tight and not closing all the way and letting compression out the carb. Typically if a valve is not too tight, but out of spec it will float and backfire only at higher rpms. Just some more food for thought.
 
Did you ever try spraying carb cleaner around the carb flange to test for vacuum leaks? Are you positive that the plug wires are in the right order? If these aren't an issue than you may have a sticky valve like posted above. Have it compression tested? Is this an HEI ignition? (Coil in the cap?)
 
stevea661 said:
Assuming :grin: you did all the other stuff you mentioned. it sounds like a like an exhaust valve mite not be opening as it should and when your intake opens to suck fuel and air, it's letting the compression out through the intake and up through the carb. Or the intake is tight and not closing all the way and letting compression out the carb. Typically if a valve is not too tight, but out of spec it will float and backfire only at higher rpms. Just some more food for thought.
Click to expand...


I pulled the valve cover, they are all opening and closing like they should. Mine only backfires around 2000 to 3000 rpm's
 
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