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Mercury 150hp straight 6 stalling

J

jaseman

New member
"I am a young marine mechanic,

"I am a young marine mechanic, i have a straight six 1976 150hp mercury. It is clearly an old engine, but it starts and idles great, but when put into gear it drops 500rpm and if idle is set too low stalls. Problem is, idle has to be set at 1500rpm to stop it from stalling. at this speed it go into gear with a hell of a crunch. My boss tells me its becouse the inner seals between the cylinders are worn. I didnt believe him, i have striped the carbs, removed the power head, replaced all gaskets, etc. put back together, still does it! Any help greatly apprciated. I can handle being wrong, i just want another opinion."
 
Check if it is firing on all c

Check if it is firing on all cylinders. Remove one spark plug boot one by one and listen if there is a difference in rpm. The cylinder with no difference when removing plug boot is faulty.
Also do a compression check.
Robert
 
"all cylinders firing, at adle

"all cylinders firing, at adle, in and out of gear, compressions all 120, except #5 at 110. I looked at the sides of the pistons when the exhaust was off, no marks on any of the pistons or bores"
 
"Lets assume it is fuel relate

"Lets assume it is fuel related.
When there is no decrease in performance (with pulled boot), that cylinder has a bad function. Clean the carburator feeding that cylinder.
With one cyl not working, the others have to work more and the motor stalls when going into gear.
Robert"
 
"Thanks Robert
I checked pull

"Thanks Robert
I checked pulling the boots off again, and there is definitely something funny going on, when I pull 3 or 4 off, the engine runs worse, but rpm increases. I cleaned out the middle card (twice now) But how do you set the float? number 3 plug is dry also, but 4 is wet!"
 
"that was ment to say carb! op

"that was ment to say carb! opps. When i said that the engine runs worse with leads off, i mean it doesnt run smooth, where as it does run smooth with all six leads on."
 
"Is that caburator feeding bot

"Is that caburator feeding both cyl 3 and 4, and only no 4 is wet?
They should be equally wet.
A. Too wet = unburnt fuel = ignition problem on no 4.
B. Too dry = bad fuel feed = reed valve on no 3.
check if you get "air puffs" from the air inlet of the faulty carb.

Float setting: The float should be horisontal when the needle valve is closed.
Robert"
 
"Jaseman, First check to make

"Jaseman, First check to make sure firing order is correct (spark plug leads are on correct plugs). There are numbers cast into dist. cap. If that is OK, make sure fuel pump diaphragm doesn't leak raw fuel into crankcase. This could affect cylinder 4/3. Other than that, make sure synch and link are correct, and adjust carbs for best idle, and then richen idle mixture to allow smooth acceleration"
 
"You can't put an old head

"You can't put an old head on new shoulders!
Your boss is right, if you've tried all the basics fuel spark compression then the problem will be fixed with new rings and reed cages.
You see in order for a 2S to run you must have good crank case compression, lack of crankcase comp will cause poor low speed running but will not affect high speed. These engines are renowned for flogging out the labrynth seals (reed cages) ie. lossing crankcase comp, always replace the endcap seals.

Regards Brad."
 
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