Volvo penta AQ 125 A backfiring and plugs fouling

[reptileguy]

I have a 1982 AQ 125 A . engine has ran great for the last 5 days morning and night ( fishing derby) one morning after pulling rods and just as I hit 3000 rpm it backfires every 5 secs . I throttle down and it runs fine up to 2000 rpm so I limped it home pulled plugs and cyl 4 is black sooty looking and rest are perfect light brown color. ( this was regular gas as that's all previous owner put in) I cleaned the plug, put in 8 gals premium fuel . next morning ran great cruse 4000rpm then trolled for 5 hrs packed up and started to backfire again. I pulled plugs and all 4 are black and sooty. I checked carb and mixture screw is perfect.I replaced cap and rotor and it ran worse. Checked plug wire resistance all 4 were .1.01 to .98. The wire from coil to cap was 2.4 but noticed on the coil side of wire it was black and smelled burnt from arcing . I replaced coil this am and it ran great again after a few hours it backfired again and plugs fouled I tried leaning out carb but it just stalled out on the slightest adjustment. Carb was rebuilt 4 months ago

 

[reptileguy]

Also rpms bounce a lot , around 200 rpm they bounce up or down at idle I think a plug is missfiring but I have spark at all 4

 

[alldodge]

Also rpms bounce a lot , around 200 rpm they bounce up or down at idle I think a plug is missfiring but I have spark at all 4

Go to the coil on the negative side you will find two wires on it, one goes to the distributer and the other goes to the tach. Remove the one going to the tach, I think it should be gray. If this fixes your issue then your tach has gone bad

 

[reptileguy]

I can clearly hear the engine rpms going up and down when the tach does. Almost like a plug kicking on and off. Can a bad tach cause this ?

 

[alldodge]

I can clearly hear the engine rpms going up and down when the tach does. Almost like a plug kicking on and off. Can a bad tach cause this ?

Yes it sure can. The distributer causes the coil to fire by grounding the neg side of the coil. This same grounding causes the tach to count and displays a reading. If the tach is causing the same grounding then its just like a miss, and if it gets bad enough it will kill the engine

 

[jerryjerry05]

The coil: What voltage into the coil? I think it should be 9v
How hot is it getting?
You should be able to touch it without burning yourself.

Points: make sure they are opening and closing and the dwell is set right.
Can't remember the setting

The carboned plugs sounds like it's sucking air somewhere.

Start it and spray some starting fluid around the intake.
If it makes any changes in the way it runs then it's sucking air.

 

[reptileguy]

Not hot just warm. The only other symptom I seen was after backfiring there was engine oil sitting on carb surface and flame arestor . When I pulled hose from PCv valve there was a lot of smoke from it. More than usual and hose was covered in oil as well

 

[jerryjerry05]

Do a compression test.

 

[reptileguy]

Did a compression test
Cyl 1 - 149 psi
2- 147 psi
3- 145 psi
4- 145 psi
I cleaned and gapped plugs to 0.030
Checked and cleaned points they were at spec 0.016"
Coil has 10.5v with key turned engine off.
I set mixture screw 1.5 turns out which is pretty much where it has been the whole time. I pulled tach wire off coil and it ran on all cyl at idle then once engine temp got up to 175 (has never gone above 180 even on plane) then it sounded like it dropped a cyl started running like crap valve cover becam hot ( can only hold hand on it for 3 secs) . I shut it off and pulled plugs and all black sooty dry chalk looking with the smell of gas. I also sprayed some carb cleaner around intake manifold gaskets and carb base but no change in rpm

 

[nylyon]

I would replace the condenser, might as well replace the points too, a bad condenser can do what your describing.

 

[reptileguy]

sorry think i forgot to say that i replaced it when i did the cap and rotor

 

[reptileguy]

This engine had been run on regular gas and once the first issue happened I put 10 gals of premium in and my problem has gotten worse since. The valve cover says minimum 90 oct or USA 87 oct .

 

[alldodge]

I'm starting to lean toward a mechanical or carb problem. Your plugs are black and sooty, so your getting to much fuel or all the fuel is not being burned correctly. Now if your getting so much fuel that it can not burn it all, when the intake valves open there is a bit of unburned fuel which ignites and you have a back fire.

Or you have some valve problems, either springs, guides or carbon build up

 

[reptileguy]

Ok so the carb mixture screw got turned out 5 turns now and tach smoothed out a little but still backfired. But when I hit 2500 rpm and the throttle plate opens up it lags a bit until I throttle a little more then gets to 2800 rpm then backfires. So I advanced the setting for the accelerator pump and tried again. Backfires at 3000 rpm but then it got up on plain and ran fine. I throttle down no prob then throttle up and backfires 3-4 times before it starts to plane out. Checked the plugs and they are still fouled .