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a friend of mine has a 24' cabin cruiser with a 250 chevy six (year unknown) for propulsion. he said he changed the points a week ago and it ran fine as usual. he then changed out some gauges and ever since it runs like crap. he said went through all the usual stuff looking for the issue. since he said it was a chevy six i thought id go have a look at it to see if i could help him figure it out.

he fired it up for me and at idle was very rough and could only keep it going if he increased the throttle which then it ran fine. when he brought it back down to idle it would back fire throught the carb and stall. i said it was either a vacuum leak or timing.

after looking the motor over a bit i was sort of perplexed. first it has a non conventional intake and exhaust manifold on it. it has no vac. advance on it and no place to hook up a vacuum gage. the carb is a funky rochester ??? barrel carb. and mounting base. the timing scale is located as facing the front of the engine is located around two o clock position on the crank pulley (im not familiar with anything other than 235's and gmc sixes so this is new to me)

we hooked up the time light and couldnt see the timing marks while the engine was running but then we couldnt slow it down to be able to see it without the engine stalling. after a few more attempts i found it spitting out air at the carb insulator when he was bumping the starter. pulled the carb off and the gasket was shot. he had another gasket and we put it on. at the same time i paint the mark on the crank pully to make it visible.

when we fired it back up the engine ran without spittin and sputtering unlike before but it still ran like crap. we still couldnt get it to idle down to stay running and find where the timing mark was with the light. i found the timeing mark with the light over at the ten o clock position. no matter how i turned the dist. i couldnt get it to come around either by advancing or retarding the timing and could only find a happy medium to keep it running but its still like crap like it had a miss. so while it was running i pulled one wire off at a time and we heard no change what so ever in the engine. we just looked at each other with a dumb founded look.

based on my knowledge which is limited i decided to check static time. let me tell you its really difficult to spin an engine with no fan on it but i managed to get it at top dead center on the timing scale and the rotor pointed to # 1 plug. scratched my balding head and fired it up again with the same results. at that time i said i was going home becasue i had to walk away and present this to real pros as to what were missing or how to get this thing ready for the water.

as he said prior to all this the motor ran fine. what are we missing here?

Is there any possibility he's got the firing order wrong? It's the same as a 235, 1-5-3-6-2-4, with the distributor turning clockwise. "Changed some gauges"- - - - -which ones? Also, check to be sure the centrifugal advance hasn't locked up in the full-advance position - - - - -the Mercruiser distributor does have centrifugal advance, just no vacuum. The rotor should turn clockwise by hand, then spring back to its original position when you turn the rotor loose.
Jerry

Weird deal with changing gauges causing his problems. With mysterious timing mark, are you sure it's a standard rotation engine? I have seen other Chevys with counter rotation for marine applications. Just a wild guess on my part. Good luck

SimS

jerry, i check the wiring and its correct. when i had the cap off i could feel the springs on the dist shaft when i turned the rotor back and forth slightly just like on my stuff. he said he changed out the tach, and some other dash gage but i forget which one. he even disconnected the tach wiring as a possibility but didnt think touching the wiring had any effect which when disconnect didnt change a thing. yeah it's a mercruiser his manuel was thicker then our maintenance manuels. i thought that was funny for a book that thick just for a boat.

what i did find was that with the dist hold down bolt loose the dist was hard to turn more than i would expect.

i went back to the boat and pulled the dist cap and removed the plate to look over the the mechanical advance. we found the weights were sticky and hanging up to move back to thier static position. we cleaned them up so they were free to move without hanging up, put it all back together and fired it up. it still was the same nothing changed so we called it a night.

Maybe corrosion between the distributor housing and the engine block, interfering with the ground? Try running a ground wire from one of the cap hold-down screws to a side cover bolt, or some other known good ground. Not likely, but possible.
Jerry

I'd suspect a bad tach lead. From experience with a mercruiser V8.

Good point, Larry! Did he disconnect the wiring at the tach head, or at the distributor? There's a definite possibility that the tach trigger lead might have an intermittent short or a high-resistance connection to ground somewhere between the engine and the gauge console.
Jerry