Distributor trigger?

[57thompson]

1978 Merc 900 inline six

A guy who supposedly knows such things, diagnoses my up/down/up/down power fluctuations as a possible bad DISTRIBUTOR ASSEMBLY HOUSING or "trigger". He also says there is no way to test it, and that it will fight me like a b*tch trying to remove and replace it.

Oh, another thing. This is the same guy who will gladly sell me a new one for about $400.

Anybody agree that this may by the problem? Motor idles fine, drops power 30-50% every couple of minutes under way. Power returns full for couple of minutes. Drops again. Repeat, repeat.

Carbs are clean as whistles. Rebuilt fuel pump yesterday. Replaced gas tank fuel line yesterday. No help. Plugs aren't fouled. Help!!

All input greatly appreciated.

 

[DHPMARINE]

Re: Distributor trigger?

Can't help too much,but some of those old inline distributor motors were real challenges.

The 'trigger' seems to be part of the distributor body,and is $400.00.And the distributor has to be taken off the engine to work on.

I'd also pay attention to the disc.Had one once someone installed wrong,broke the tab.Timing was everywhere but where needed.

DHP

 

[Chris1956]

Re: Distributor trigger?

57, Does the power drop off at near full throttle? If so, have someone pump primer bulb whne it slows down. If that fixes it, you likely have a weak or bad fuel pump or a fuel restriction. If not fixed by that trick, check the plugs to see if it is overheating (white or green deposits) . Also, check cylinder 5 and 6 for water ingestion. Spark plugs will look steam cleaned.

 

[wbblc87]

Re: Distributor trigger?

1978 Merc 900 inline six

A guy who supposedly knows such things, diagnoses my up/down/up/down power fluctuations as a possible bad DISTRIBUTOR ASSEMBLY HOUSING or "trigger". He also says there is no way to test it, and that it will fight me like a b*tch trying to remove and replace it.

Oh, another thing. This is the same guy who will gladly sell me a new one for about $400.

Anybody agree that this may by the problem? Motor idles fine, drops power 30-50% every couple of minutes under way. Power returns full for couple of minutes. Drops again. Repeat, repeat.

Carbs are clean as whistles. Rebuilt fuel pump yesterday. Replaced gas tank fuel line yesterday. No help. Plugs aren't fouled. Help!!

All input greatly appreciated.

The guy is right about a few things. But, before you go throwing a trigger at it, there are alot of things that need to be tested first. Every part of the ignition can be tested except the trigger (and the trigger also) with the right test equipment. Has anyone tested the coil primary voltage when this engine is acting up. If you do have a primary ignition problem it will show up in the coil voltage. The parts are too expensive for this engine to just throw something at it and hope for the best.

 

[emckelvy]

Re: Distributor trigger?

One thing to try that certainly can't hurt, pull the dist cap down and check for "trash" inside, carbon tracks, cracks, etc. Also check to see that the center carbon contact is free to move and is sticking out enough to contact the rotor.

It's quite common for the above problems to cause a misfire or crossfire inside the cap and could be contributing to your problem.

Cheap to check, anyway, just loosen the screw/nut assy holding the clamp in place and drop the cap down.

You can clean out the inside of the cap by 'flushing' it out with carb cleaner or other suitable solvent. Spray can usually works better since it gets into the nooks & crannys better, but you could do it with a stiff-bristled cleaning brush too.

Another thing you can try that won't cost anything is to run the motor at night and see if you have any 'fireworks' coming off the plug wires. A bad set of plug wires can cause all kinds of havoc.

And something you could try underway, with a 2nd person in the boat; if you have an inductive timing lite, when you start experiencing the problem slap the timing lite around each plug wire to see if it's sparking. If all have indication of strong spark, it's not likely an ign issue.

You can also use a rag to choke-off one carb at a time; if the motor dies out you know that carb's probably working OK and is going rich when you cover it with the rag.

If the motor picks up with the carb blocked, it's got a fuel issue and needs attention.

Don't run for any great periods of time with a known lean-carb issue, you're starving not only fuel but lubrication from the (2) cyl's supplied by that particular carb.

HTH and let us know how it turns out..........ed

 

[57thompson]

Re: Distributor trigger?

Many thanks to all who responded. Clearly I have some poking around to do. I'll update.

 

[57thompson]

Re: Distributor trigger?

UPDATE: Spark test found that my #3 plug was not firing, and the wire was bad. Had to order a new wire from NAPA (who knew they carried marine parts?). Also found the distributor cap trashy and the contacts black. Ran a glass bead cleaner in the cap (instead of paying $140 for a new one), replaced the #3 wire, and I'm back in business. Runs fine now.

Many, many thanks to all who responded. Love this forum.

Cheers, all.

 

[Texasmark]

Re: Distributor trigger?

Napa has a marine catalog. Have used them for years maintaining my Mercs as a dealer is not handy. Have to order parts, but get them in a day or two.

Glad you got out for a few bucks and got your engine back. d

Mark