1986 Johnson 40 VRO compression low

[tetienne]

This is my first post...this forum is great! I have an 86 Johnson 40hp VRO with low compression in bottom cyl. Upper is 120 lower 95. So far I have replaced plugs, new gas, new oil, wires and coils are good. I replaced head gasket but when I cracked her open, the walls of the cyl are smooth. The top of each piston was covered in carbon, bottom one the worst. Can timing effect compression? Any ideas?

 

[ob]

Re: 1986 Johnson 40 VRO compression low

Rings probably carboned causing weak compression on bottom cylinder.Might have some luck with some of the products sold for this purpose.I was going to say because the motor is over 100 years old , but I'm sure that's a typo.Timing will only indirectly effect compression by cylinders not firing at proper time.Too rich of fuel to air mix and or oil to fuel mix.Also , trolling extended periods and lugging motor.

 

[tetienne]

Re: 1986 Johnson 40 VRO compression low

Correct...make that 1986. Although it runs like it's an 1886.

 

[JB]

Re: 1986 Johnson 40 VRO compression low

Ahoy, Walleye.<br /><br />I agree with OB. You probably only have carbon build-up sticking rings in the bottom cyl. Try one of the commercial decarbonizing products. That should put you back in business.<br /><br />Good luck.

 

[tetienne]

Re: 1986 Johnson 40 VRO compression low

Thanks a bunch! I'll give OMC engine tuner a try. I have seen posts saying to leave it sit for anywhere from 30 minutes to 12 hours...directions on the can say 3 to 16 hours. Is the high end of that range too long?

 

[ob]

Re: 1986 Johnson 40 VRO compression low

I wouldn't think so Wallyeye , since you suspect enough build-up to effect compression.Post back and let us know if it had some positive effects.

 

[tetienne]

Re: 1986 Johnson 40 VRO compression low

One thing that I forgot to mention, when its running and I pull the bottom wire, there is no change in the motor's performance... Would that be consistent with low compression / carbon on rings? I will post if the OMC tuner works.

 

[ob]

Re: 1986 Johnson 40 VRO compression low

Definately.Check for spark on lower cylinder by removing plugs .Plug wire should jump 7/16 gap while cranking.Be careful.

 

[tetienne]

Re: 1986 Johnson 40 VRO compression low

Thanks again...I'll let you know how it comes out.

 

[tetienne]

Re: 1986 Johnson 40 VRO compression low

Well, I used the OMC Engine Tuner and the good news is that compression in my lower cyl increased to 118# vs. 120# or so on upper. The bad news is, I can still pull the wire off the lower plug with no change when the motor is running??? I decided to give it another tuner spray treatment but this time took the shield off the carbs and sprayed it right in. When I sprayed in the upper carb the engine would bog down, but when I sprayed it in the lower carb the engine rev'd up. I also pulled lower plug and held it near the block. The spark jumped close to an inch with bright blue spark? Any ideas what to try next? Help...

 

[Hooty]

Re: 1986 Johnson 40 VRO compression low

Back when you replaced the plugs, did you happen to notice if the bottom plug looked different then the rest?<br /><br />c/6<br /><br />Hooty

 

[tetienne]

Re: 1986 Johnson 40 VRO compression low

Hi Hooty - when I replaced the plugs the top one was dry and blackened. The bottom one was more oily. Both seemed to have carbon built up on them.

 

[ob]

Re: 1986 Johnson 40 VRO compression low

Wallyeye , you fixed the symptom ,now we need to adress the cure.Initially we figured that cylinder was not firing is why the heavy carbon build-up to lower cylinder.Now we need to determine why lower plug was not firing.If it is now firing and jumping 1/2" gap it appears to be working.HMMMM.

 

[tetienne]

Re: 1986 Johnson 40 VRO compression low

Ok, sounds reasonable...what is the powerpack?

 

[Hooty]

Re: 1986 Johnson 40 VRO compression low

If you've got spark at nr. 4 and it's not hitting (producing power) the plug is shorted either inside or out. It almost sounds like the bottom cylinder is flooding and shorting the plug. To much oil/fuel?? Carb?? VRO??<br />Lettus know what ya find.<br /><br />c/6<br /><br />Hooty

 

[tetienne]

Re: 1986 Johnson 40 VRO compression low

The motor bogged when I sprayed tuner into the upper carb and revved up when I sprayed into the lower carb? Would that mean its fuel related?

 

[ob]

Re: 1986 Johnson 40 VRO compression low

Like hooty says if it's not oil rich for some reason on that cylinder causing plug to wet foul then perhaps coil or powerpack is intermittingly firing that one cylinder .We're trying to decide which came first ,the chicken or the egg.I would think that if it was oil rich fuel condition that all cylinders would be effected.I"m also not sure if running on hose will show significant change .

 

[ob]

Re: 1986 Johnson 40 VRO compression low

wallyeye,Try to inspect lower carb for sticky float valve.Maybe it's sticking causing rich mix to lower cylinder.

 

[tetienne]

Re: 1986 Johnson 40 VRO compression low

Thanks...I'll try to inspect the lower carb. Also, I just noticed a small crack in the fuel line leading to the fuel filter...would a leak in the line cause more oil to be added? But then it would effect both cyls I suppose... I'll let you know what I find...thanks again

 

[ob]

Re: 1986 Johnson 40 VRO compression low

From what I hear air in fuel line will cause vro pump to occilate and inject more oil in relation to fuel volume.Since I defeated mine that's history on my motor.