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1978 75hp evinrude

DANIELTB

New member
Checking pressure on engine with use of blank off plate pressure and vaccum on all three cylinders same. Install backer plate and manifold
no vaccum on center cylinder.Looking at parts diagrams noted newer 1988 models have reed valve bolting directly to manifold. Were their issues with older set up?
 
Blank Off Plate, Backer Plate? Sorry, I have no idea of what you speak.

Explain the problem you're having with the engine.
 
Blank off plate is something I made to check pressures. Backer plate is factory that reed valves bolt too. No suction on center carb.
 
Engine will not run at lower speed get it up to 3-4 mph and runs like new. Have had this at two different shops and both came up with no suction in center cylinder. No real reason but is something inside of motorand not worth fixing. second guy at first said carbs but no fix. He did say that while carbs were off that all reed valves were openibg.Removed carbs and intake made plate to test pressure and vaccum and all seamed normal 30psi and 5 vaccum. Replaced with new gaskets and before installing carbs turned motor over no vaccum on center carb. Went to parts diagrams and found that manifold setup on 1988 was changed but same manifold part number, leading me to belive there was a issue with manifold . Sorry this is so long a storry but the bottom line is no vaccum on center cylinder.
 
There were no issues with the earlier intake manifold set-up.--------Results of a compression test are ?-------You may bave a bad bearing that wiped out the labarynth seals ( part of the block ) and that would mean a lack of vacuum on center cylinder.
 
if it were bearing and or seal would you still have 30_ pounds pressure in crank case? Cylinder pressure 125 - 130 all cylinders
 
If (at the problem low end rpm) by spraying a little premix into the #2 carburetor OR by blocking off the carburetor throat somewhat of the #2 carburetor, the center cylinder starts firing... you're overlooking something with that carburetor.

You have compression and crankcase vacuum/pressure... what about the spark? At cranking rpm, with the spark plugs removed, the spark should jump a 7/16" gap with a strong blue lightning like flame... a real SNAP! Does it? Note that the 7/16" gap is important.

Using Champion QL77JC4 spark plugs, gapped at either .040 or .030?

With the proper spark, compression, and crankcase pressure... that would lead me right into that #2 carburetor.
 
What exactly is your test method for " no vacuum " on #2---------You say the piston and rings are good and the crankcase holds pressure.--------Maybe the wrong gasket on the manifold ?----I have rebuilt more than one of the 70 HP triple cylinder motors.Never needed any fancy trouble shooting on these motors.
 
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