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RPM Fluctuations

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48tony

Regular Contributor
On my 1970 60HP ESL 70D, I have tried numerous approaches to solve my RPM's from fluctuating at idle speed. I rebuilt carbs, new floats, adjusted correctly, new needle valves, new fuel pump and timed and set points. On a cold start, the choke works correctly, it will idle smooth only after a few laps around the lake. I will put back in neutral and the RPM's seem to fluctuate. Sometimes between 100 RPM's to 50 RPM's. Watching the tack you can see the needle bouncing up and down. At take off there is a slight stumble and after 2,000 RPM's the high fuel circuit seems to work fine -- no stumbles no misses -- holds steady at RPM's of 5,000 smoothly. When I drop down to neutral then the problem occurs. Again it seems to be after the motor has warmed up.

At home on the muffs it holds steady at 10,000 RPM's smoothly. I am kind of at a loss as to where to go with this issue. Although after a long time trying to get my carbs set correctly at slow speed idle adjustment, and learning from this site to get these carbs tuned correctly, sometimes you have to run them on the rich side as I have done, altering a smooth idle. But is the sacrifice of me willing to accept this RPM fluctuation? Or is there something I am missing.

Thank you,


Tony
 
10,000 rpms? I hope that's a typo!

Assuming the compression and spark is as it should be.

Make sure that the linkages between the carburetors are as they should be.... NOT having one slightly tight so as to have one throttle slightly held open. This alone will cause the rpms to travel up and down constantly.

Also, on that model, it is important that the carburetor slow speed needle valve adjustment begins with the center carburetor, as follows.

(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)
Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.

Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.

Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

Note 1: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.

Note 2: If the engine should be a three (3) cylinder engine with three (3) carburetors, start the adjustment sequence with the center carburetor.
When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.

Thousands of parts in my remaining stock. Not able to list them all. Let me know what you need and I'll look it up for you. Visit my eBay auction at:

http://shop.ebay.com/Joe_OMC32/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
 
Initial slow speed setting is --------------------5/8 turn out -----------------------Straight from the factory manual !!!!
 
ya 10,000 i would just need a broom to sweep up the parts that blow up.I will look at the carb linkage once again.But i am pretty sure they are correct.I am wondering if it could be a reed valve issue.? I put the boysoen dual stage power reeds in .But it runs great exept for the rpm,s at low speed.Would reeds do this?The 5/8 adj is what I have been doing But at take off i get a slite flutter so i have them at one full turn out off seat a bit on the rich side.I will mess with the adj again to find the right spot.
 
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