1976 850 merc

[circle hook]

I have a 1976 85H.P. merc and it idles ruff, when I spray 2+2 carb cleaner into the top carb the motor races up in RPM, but with the bottom carb it does nothing, I have done a spark test on all 4 cylinders and I have a lighting blue spark on all 4 cylinders, and 150 PSI on all 4 cylinders also on a comperssion test, could i have a bad lower reed valve and how can I test that before going any further with this engine any help Thanks Bob

 

[green4themoney]

Re: 1976 850 merc

clean the carbs first.... after that... if its still idling rough... pull off the lower spark plugs and see if they are wet.... if not i believe you have a problem with your reed valves possibly stuck

 

[circle hook]

Re: 1976 850 merc

Yea the reason I thought its the reed is even a dirty carb after spraying 2+2 carb cleaner into the bottom it should have raced up in RPM like the top carb does, but again she does nothing, and I know with the spark test & compression test @ 150 PSI, there is no reason the RPM does not increase, I think I need to test the reeds, but not like the reed blocks behind the carbs these are on the crank shaft and the power head has to come apart to see them, any other Idea on how I can determan if it is a reed with out a total over haul Thanks Bob

 

[Moody Blue]

Re: 1976 850 merc

Just guessing here so don't take it as being correct.

If, when running, you placed your thumb over the carb throat you should feel a vacuum if everything is working properly, yes? I would think that if the reeds were stuck you would not achieve any vacuum?

Try that with each carb and compare the results between the two.

 

[circle hook]

Re: 1976 850 merc

question are there 2 sepreate crank cases, like where I can co a vacum test on the top 2 cylinders and then the 2 bottom cylinders and then compare the 2 Thanks Bob

 

[daveswaves]

Re: 1976 850 merc

question are there 2 sepreate crank cases, like where I can co a vacum test on the top 2 cylinders and then the 2 bottom cylinders and then compare the 2 Thanks Bob

Yes Bob, the crank cases are separated enough that you should be able to compare. There are bleed circuits between each cyl and the next but they are very small. I suspect the reeds are ok, Did you check the spark with a spark gap type tester or just using a plug? The spark must be able to jump a 1/4 inch gap to be considered OK. Under compression, and it seems you have compression at 150 psi, the spark will blow out. No amount of extra fuel in the cyl will make it run faster.

 

[circle hook]

Re: 1976 850 merc

I used a inline adjustable spark tester and she jumped a 1/2 to 3/4" gap bright blue spark, but what puzzles me is why will the engine will speed up at idle when spraying 2+2 in one carb but not the other, seems to me that the bottom carb, the carb cleaner ain't getting into the cylinder like the top cylinder in turn then reving up from the extra spray.

 

[daveswaves]

Re: 1976 850 merc

I used a inline adjustable spark tester and she jumped a 1/2 to 3/4" gap bright blue spark, but what puzzles me is why will the engine will speed up at idle when spraying 2+2 in one carb but not the other, seems to me that the bottom carb, the carb cleaner ain't getting into the cylinder like the top cylinder in turn then reving up from the extra spray.

I hear ya, sounds like the spark is good, any chance the plug wires are reversed?

 

[circle hook]

Re: 1976 850 merc

I don't think so the 4 coils are in line from top to bottom and the wire harness for the spark plug wires are bolted to the block also in line from top to bottom, the only thing I haven't tryed is to get new plugs, but what is the chance of just one plug going bad, I will bet you with my luck thats the problem. even if plug #4 was bad wouldn't plug #3 ingite the carb cleaner,

 

[arcrat]

Re: 1976 850 merc

top two cyl run at idle .. after 1800 rpm bottom two start io fire

 

[Chris1956]

Re: 1976 850 merc

That motor is not a 2+2, all cylinders should always fire. First check the fuel pump diagram If it leaks it will foul the lower cylinders. Next check the #4 spark plug for water intrusion. If it looks brand new, it is likely being steam cleaned by water. If that checks our, lean the idle mixture (clockwise) on the bottom carb a bit and see if it picks up. Normally the bottom carbs run richer. Is fuel leaking from the carb? If so, rebuild with new gaskets and inlet needle and seat.

 

[circle hook]

Re: 1976 850 merc

Chris what I ment was that when I used 2+2 carb cleaner at Idle on the top carb the engine would pick up rpm's and when sprayed into the bottom carb she won't change at all thats in rpm's, this don't make much sence shouldn't it be the same either top or bottom carbs, I will try what you wrote but with 150 PSI compression on all 4 cylinders would that rule water leaking into the lower cylinders Bob

 

[Chris1956]

Re: 1976 850 merc

Bob, Arcat suggested that your motor was a merc 2+2, which it is not.

Water intrusion into the cylinders does not affect compression. It can leak in thru the crank seals or the exhaust cover or baffle. If you have spark on all cylinders, it is likely the bottom carb is too rich from fuel or water is putting out the fire.

 

[circle hook]

Re: 1976 850 merc

Chris what I will do this week is adjust both carbs and see what happens then, I will let you know what happens next week, first I need to get my tach back sent it out for repair. Chris Thanks Bob

 

[circle hook]

Re: 1976 850 merc

Chris that was the problem the carbs, readjusted them and she purrs like a kitten, can't wait to see what she will do on a 16 StarCraft