78 Merc 90 hp 6cyl not running on all cylinders

[cbcmarine]

Hi this motor was running good now all of a sudden, it does start all be it hard, then only at a high idle. Replaced the plugs and good compression in all cylinders. It does have a distributor cap. Do these normally cause problems??
When it is running you can here it missing and every once and a while the other cylinders catch and all is good but that is becoming less often
Ideas

Help

 

[emckelvy]

Re: 78 Merc 90 hp 6cyl not running on all cylinders

Fire it up in at night and see if you get any spark leakage or "glow" from the spark plug wires or boots. If so, replace as required.

Also you might pull the distributor cap and check for debris/trash in the cap. This'll cause a miss. Flush the cap out with carb cleaner. Check the carbon spring-loaded center contact is free to move in its hole in the cap. Pull it out and clean if it isn't. Be careful with that contact when reinstalling the cap, as it's easy to break if you don't push it back in the hole when sticking the cap onto the distributor.

Check spark on all (6) cylinders with a timing lite. This'll tell you if the ign is strong enough to fire each cyl under compression pressure.

HTH..........ed

 

[CWKboat]

Re: 78 Merc 90 hp 6cyl not running on all cylinders

I think the caps generally last a long time as the one on my 1976 1500 has lasted a while so far. You can take it off and wash it in the dishwasher and reinstall and see if that cleans up the contact points. Also, you may want to run it at night in the dark with the cowling off and see it the spark is arcing anywhere? I also got some new wires from OldMercs.com I believe it was, so that might be an option also. Good luck!

 

[cbcmarine]

Re: 78 Merc 90 hp 6cyl not running on all cylinders

Thanks guys I will try this tonight and check the cap as well.

Will let you know.

Also in the 80's merc's the power pack sometimes went and the same condition would show do these have a power pack and are they problematic ?



Thanks Shaun

 

[cbcmarine]

Re: 78 Merc 90 hp 6cyl not running on all cylinders

So looked in the distributor cap and there was water in it and the center cap as dirty. This seemed to fix the problem. Is there a moisture problem in that style or is a seal maybe shot ??

Thanks

 

[emckelvy]

Re: 78 Merc 90 hp 6cyl not running on all cylinders

There is a very thin gasket (cork if I recall or maybe just some kind of gasket paper material) that has adhesive on one side, and sticks itself to the distributor body on the machined lip where the cap sits.

This does provide a minimal seal to keep water from splashing into the cap.

Likely time for a new gasket! Your local Merc dealer should have this part in stock, # 27-55896 ($2.58)........ed

 

[cbcmarine]

Re: 78 Merc 90 hp 6cyl not running on all cylinders

Thanks Ed for the help has been ordered.

Sorry for the late reply

Shaun