Ideas on Mercruiser 260, 1980 - Good idle under load, high in neutral, and shift prob

Recently bought 1980 Hammond Challenger with 260 Mercruiser. Lots of unknowns obviously... Supposedly engine was "re-built" just before prior owner bought it a year and half ago.... and he only added maybe 5 hours to the 15 on the re-build. With no substantiating paperwork on the re-build, I have to take that with a grain of salt. but I do know the disty was converted to electronic from points just before I got it. Starts cold and stays in the 600 range on muffs sitting in driveway. Originally would stall badly when putting in and out of gear and idle under load was very low... maybe 3-400. Set mixture by ear (which I am sure is not perfect... can't seem to find where I would hook up a vacuum gauge to do it better) at about 3.25 - 3.5 turns on either side. Set idle in the water, engine warmed up, in forward with no extra throttle... to 650-700 according to dash tach. But when pulling into neutral, idles soars to double that amount at 1300...... No idea where to start chasing this. Apparently no vacuum advance to check, but not being a mechanic, I am not really versed enough to know about checking disty springs if that would affect vacuum maybe... and I have no experience in checking timing/dwell etc. if that is an area I need to explore. Pre-buy check by local marina showed cold engine compression around 125 - 130 on all cylinders - engine cold, and they said plugs looked good. Have yet to shoot starting fluid around manifold to check for vacuum leaks... Engine seems to run fine otherwise. Smooth acceleration and tops out around 4400-4500 and will happily stay at that. Turns a thru-hub, cleaver style 23 pitch, about 14.5 dia from what I measure.

Still have somewhat if a problem with engine dying when docking or coming up to trailer at end of day. during shifting it conks out occasionally. I am sure the cables are worn, as there is play in throttle after putting into gear. Checked distance on leg-shift cable and it measures 6" on the nose... and just last night played with shift interrupt switch in driveway. Don't think it was really bumping as it should going back to neutral, but then again, if it weren't, I don't think it would have come out of gear...? And as I understand it, if it were the neutral safety switch in the ancient panel-mount control box, that would only affect starting....?

So, two issues in question... the huge disparity in idling and what causes I might check myself with limited mechanical background.... and also diagnosing the occasional dying while shifting.

I did put in a fresh fuel /water separator in it and cleaned the filter at the carb, which was actually quite clean. Have also added some seafoam to fresh, premium gas, and I replaced short vacuum hose from carb to choke.

Any help from you guys would be immensely appreciated.


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The stalling going in and out of gear sounds like a sticking transom shift cable time to replace it. Carb could be issue with the carb. JMO


Dave

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Originally Posted by Dave Strong

The stalling going in and out of gear sounds like a sticking transom shift cable time to replace it. Carb could be issue with the carb. JMO


Dave

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I appreciate the quick reply. Any way to diagnose the sticky cable prior to tackling a replacement? Could probably use it, but being tax time, trying to minimize any throwing parts at it without a solid diagnosis... although I would like to replace a LOT of things in due tome. Really, all cables and the control box for starters.

Get some one to shift while you watch the shift interupter, probably find it is hanging up. You will need some special tools to change it your self. Job best done by the experienced.

Dave

agree with the cable or cables on their way out
typically the cable for the drive would go first , 1980 most likely ( also a good time to do all bellows )
it is possible to milk it for awhile by adjusting the shift interrupter switch
https://www.youtube.com/watch?v=_xDXZv1yHQo
the best way to adjust this is to have pressure on the prop (boat in water)

idle problem I would lean to a carb rebuild very typical

^^^THAT....plus don't rule out a vacuum leak.....It's JUST a small block chebby!!!!

put a timing light on it to see if the timing is changing when going in and out of gear

Have you fired it up and take a can of carb cleaner and squirt it around the carb, intake, hose ends, etc to see if it changes the idle. That is one of the simplest ways to find a vacuum leak and quick.

When shifting into neutral and it idles way high, Have you pushed the neutral button on the throttle/shifter in forward and reverse with button pressed. It could be sticky in there with old grease and thats a pain in the ass to work on too. I would try this just to verify if something is loose in there. Like a loose post that the throttle cable attaches to or the thin plastic bushing that goes between the post and cable. Thats enough to make it idle high coming out of gear to neutral.

Just disconnect the cable at the carb, make sure you have the throttle on the idle stop and shift.

Dave

Thanks for all the feedback. I'll check for vacuum leaks. Going to check timing as well. Seems that if too retarded it would cause mixture screws to be less effective and idle high. Ultimately will replace controls and rebuild carb just because... But one step at a time, especially with tax man knocking on the door. What is interesting to note on the idle issue, it is consistent. And I can dial down the idle speed screw, but it goes much lower under load. Something is consistently affecting it. Shift interrupt was not quite going in all the way and completing connection, so worked on arm til it just touched enough to complete circuit (used multi tester to verify). That might help a bit, but will not account for huge difference in idle adjustment. Must be some reason for needing so much idle adjustment to get rpm' under load.

If the carb is not working properly and you have the throttle plates open to the point the transfer circuit is pulling vacuume it can cause that. Vacuum leak or high overlap cam will also cause it. When you adjust idle speed make sure you adjust the threaded barrel on the cable as well.

Dave

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Originally Posted by Dave Strong

If the carb is not working properly and you have the throttle plates open to the point the transfer circuit is pulling vacuume it can cause that. Vacuum leak or high overlap cam will also cause it. When you adjust idle speed make sure you adjust the threaded barrel on the cable as well.

Dave

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O.K., some great ideas and input here. Had a feeling you guys would be a big help. Crappy weather today but will be working through all these ideas as best i can and will advise on what I find. Thanks very much for everyone's time and help on this.

- Doug

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Originally Posted by Krazymaan

Have you fired it up and take a can of carb cleaner and squirt it around the carb, intake, hose ends, etc to see if it changes the idle. That is one of the simplest ways to find a vacuum leak and quick.

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Well, shot some carb cleaner all around the manifold and carb base plate and hoses.... no change. Thinking it may be internal... or maybe choke adjustment? Choke on this thing baffles me. Got Cliff Ruggles book today, but guess it is more geared towards a general understanding and re-building, rather than trouble shooting and tuning. Will likely send it off to him for his magic. After looking in the book, WAYYYY too much for me to get involved with. Sure would like to understand what is failing though... hate when I can't grasp or diagnose something.

Thanks for the suggestion; at least I feel a bit more confident there is no vacuum issue around the manifold or carb. Looked at online manual and setting choke coil rod and vacuum break choke lever... but not really getting it. Back to the drawing board...

If ya lived closer, I'd let you use my UltraSonic Cleaning tank, those carbs would come out like N.O.S.