Force 85hp Backfiring Out Exhaust

[rodsailor]

Model 858F8A, the motor is backfiring and stopping the engine dead every five or ten seconds. The backfire is coming out the exhaust, not the carbs.

Checked spark on all three with spark checker, all are firing strongly. Pulled wire from each plug one by one, grounded that coil wire to the spark tester, and ran. Each time, backfire out the exhaust stopped the motor after five or ten seconds. The motor ran the same on any combination of two cylinders.

New gas, gas tank, and oil. Water pump is running fine, the motor starts fine on two or three cylinders. On the water, the motor bogs down as throttle is opened and dies much past 1/2 throttle.

Plug from No. 1 cylinder is looking a little oily and black.

New plugs, water pump. Could the fuel pump be leaking into the exhaust through the crank case, causing this type of problem? I am rebuilding the fuel pump diaphragm today, along with replacing the fuel hose.

 

[Tim Wagner]

Re: Force 85hp Backfiring Out Exhaust

A compression check is first.
Sounds like the timing is out of wack to me.
I think you have to take that big "jesus" nut off the flywheel to see it but check and see if the key is sheared on the flywheel.
With that big old nut off, you should see if the two slots (one on the crank and the other on the flywheel) are aligned. If not, theres your problem.

 

[rodsailor]

Re: Force 85hp Backfiring Out Exhaust

Tim,

Thanks for the suggestions.

Short of pulling the flywheel nut, I visualed the key slot and it looks like the key is still in its seat. I marked the flywheel and nut to see if there was any creep, but none shows.

I checked the compression today, all three cylinders were 130 to 132 PSIG, which is good enough for me. This is prior to decarbing, and without squirting oil into the cylinders through the sparkplug hole before the test.

I did a static test of the timing using my dial calipers and the No. 1 piston. When I stopped the piston at TDC, the timing mark lined up with the indicator mounted above the carbs. As a double check, I turned the motor over with the plugs out, and the timing light showed an advance of 6 degrees or so. I pulled the connecting link between the throttle and the tower and did the check over. Without the link, the timing at crank speed was maybe 4 degrees BTDC at most.

Recalling my misspent youth, I remember that cross arcing between the plug wires can cause a backfire, so once it gets good and dark around here, I am going to go out and start up the motor and let it run, while checking for arcing between the spark wires and any other elements of the ignition system.

I checked the idle screws, and they were set between 1 and 1 1/4 turns out, which seems a bit rich but is preferable to leaning it out too much. I will wait to set the idles when I have the motor running and in the water.

I ordered carb rebuild kits which are due on Wednesday, so I will do that when the parts come in. Other than that, I am starting to run out of ideas.

Rod

 

[JB]

Re: Force 85hp Backfiring Out Exhaust

I agree with your suspicion of crossfire, but think it is more likely before the coils, in the stator.

 

[rodsailor]

Re: Force 85hp Backfiring Out Exhaust

Short of putting the leads on an oscilloscope, I am not sure how to diagnose a problem past the CD units. I don't think it could be the stator, as it should only feed power to the CD units, not determine timing. It is possible that the trigger is cross firing, but as the voltage is very low on the triggers, this seems unlikely that they would be jumping across or to ground.

Best guess is that a cross fire would occur at the high voltage points in the system, either at the CD module outputs or at the coils/wires.

Keep in mind, however, that it may still be a fuel system problem.

Rod

 

[MikDee]

Re: Force 85hp Backfiring Out Exhaust

One more thing, make sure your plug wires are not crossed.

 

[rodsailor]

Re: Force 85hp Backfiring Out Exhaust

I am pretty sure that the wires are not crossed, though that remains a possibility. As a check, I marked the flywheel with the approximate fire points for each of the three cylinders, and when it gets dark, I will fire the timing light off each wire to make sure they correspond to the appropriate cylinder.

Rod

 

[rodsailor]

Re: Force 85hp Backfiring Out Exhaust

Well, the midnight diagnosis session went well, even though no source of my problem was revealed.

After it got very dark, I ran the motor at idle and there were no sparks indicating cross firing or jumping from any of the electrical components. Just to check, and because I had a couple of glasses of wine with dinner, I ran my fingers over the spark wires and coils...no sudden revelations there, either. The motor starts up with a single crank, and idles until it backfires out the exhaust, which brings it to a sudden stop.

I put the timing light on plug 1, and the light flashed consistently on the #1 I had marked on the flywheel with a magic marker at TDC. Plug wire 2 flashed on the #2 marked on the flywheel 120 degrees CCW from TDC (looking down), and plug 3 flashed on the #3 marked 240 degrees CCW from TDC. That should indicate that the spark wire connections are correct.

When I was playing with the motor yesterday afternoon, I screwed in the idle jet screws to see where they were set. The setting was between 1 and 1 1/4 turns out. I left them at a full turn out until I can get the boat in the water. I noticed during the midnight test that the motor ran a little better, not backfiring as frequently and idling a bit smoother. Any ideas?

Next up, rebuild the fuel pump and all three carburetors. when the parts arrive on Wednesday If that doesn't do the job, then the motor is heading for the boat mechanic on Thursday.

Rod

 

[Leinenkugel#1]

Re: Force 85hp Backfiring Out Exhaust

I had the same problem, would start up and idle ok, but when I would give it some throttle it would back fire, cough and then quit. I was puzzled too because the last time I had it out it ran fine.
Then I remembered that I kind-of guessed on my mixture Fuel/oil when I was at a gas station because I had forgotten my marked bottle that I use for mixture, 6gal to 15.4 oz of oil.
( used a 16 oz pop bottle that was in garbage) and guessed. Well, I did the correct mixture when i got home and took care of the problem. It would be nice if it was that easy!!!

end of story.

 

[rodsailor]

Re: Force 85hp Backfiring Out Exhaust

Leinenekugetl #1,

I run two separate tanks, one a 12 gallon and the other a 6 gallon. When I fill them, I use one pint of oil, and pump 6 gallons into each tank, no matter how much more the 12 gallon needs.

I have run the motor off each tank with the same results. I am leaning towards a fuel system problem, since I have checked out everything but the reed valves thus far. I will check the reed valves when I pull the carbs tomorrow. Just ordered the gaskets for the manifold so I can do this, as it makes sense once I have the carbs off.

I am going through the fuel pump tomorrow as well, having ordered the complete gasket set and the fuel metering valve. Gaskets are the cheapest element of the repair, except my labor cost (zippo thus far).

Any ideas are always welcome.

Rod

 

[Skychasen]

Did you find the problem? I’m having the same problem with my 20HP Chrysler

 

[jerryjerry05]

Post is 15 years old.
Maybe start another with good details??
Backfiring can be:
too much gas an air leak
timing off
and much more