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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 11-28-2011
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Forward idle speed

My Bristol 27 with an Atomic 4 (don't know prop dimensions) goes about 4.2 ktn in forward at idle (flat water little wind). Is that too fast? Do most have to pop in and out of fwd to keep from speeding through mooring fields etc.?
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Old 11-28-2011
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Atomic 4 carburetor shows some pictures of the carburetor. Look for that long lever that pivots on side of it. Forward and downward of the lever at about the 7:00 oclock position from where it pivots, is a screw used to change the idle speed. The Atomic 4 has an idle speed with the transmission in neutral of 700 RPM. If higher than that, check to make sure throttle controls are not binding. You want to throttle to come all the way closed against its stop on the carburetor. This is an idle speed adjustment on the carburetor, which is just a screw that rotates on a shaft that controls idle speed. The end of the screw comes up against a stationary piece of metal on the carburetor body and that is the stop I mention above. Make sure the screw can touch the stationary piece of metal when throttle is in idle position. If it does, then back the screw off to get the 700 RPM. When you shift into forward the idle will drop some and the boat should barely move.

Last edited by LakeSuperiorGeezer; 11-28-2011 at 04:39 PM.
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Old 11-28-2011
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A friend happened to describe the exact same symptom on his Cheoy Lee this weekend. Turned out he was way over propped. I don't recall the original prop, but had it re-pitched as much as possible which brought it down to a 9" pitch, which is still too high, but at least he can navigate the marina in gear. In researching props for my Beta 14, which is similar in RPM range, 7,7.4 and 7.5 seemed to be the predominate pitches for Atomic 4's.
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Old 11-28-2011
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My A4 is set up correctly and with a 12 X 8 prop (original size 41 years) it goes 6 Knots at 1400 RPM and i cant recall exactly but at 700 RPM its sure NOT dead slow
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Old 11-28-2011
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Barquito,
After you adjust the throttle linkage to get lower RPMs you may need to also adjust the low rpm jets in the carburetor. You may find they are clogged or improperly adjusted and that the throttle linkage was adjusted to stop low rpm stalling. You should be able to go from dead slow to hull speed. Popping in and out of gear should not be required.
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Old 11-28-2011
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Quote:
Originally Posted by tommays View Post
My A4 is set up correctly and with a 12 X 8 prop (original size 41 years) it goes 6 Knots at 1400 RPM and i cant recall exactly but at 700 RPM its sure NOT dead slow
Yes, 700 RPM at idle and not in gear, but put in in gear the RPM is going to drop, my guess is about to 500 RPM because that is the RPM that gives smooth running on a gasoline 4 cylinder engine. Bring it back up to 700 RPM and the boat will not be dead slow.

Last edited by LakeSuperiorGeezer; 11-28-2011 at 11:55 PM.
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Old 11-30-2011
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Thanks for the responses. Next time I'm out at the boat I will check the prop dimensions. Next spring I will put a tach on the engine. My A4 runs well, but maybe that is b/c it is spinning too fast. Will be interesting to see if it still runs at the proper rpm. Afterall, it is hard to cruise serenely through the mooring field if I am madly dodging boats at nearly hull speed.
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Old 12-01-2011
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This may be useful information, I had the same symptoms but with a diesel.

Morse single lever control, boat in gear went 5k. It also went from neutral @600 rpm to in gear @1000. From 1100 to 1500 it would return to 1000 rpm.

I talked to a lot of people, had two professionals on board to check it out. No one got it, but the most common suggestion was get a new cable.

Eventually I did, and tried to set everything up the way it was, but now the transmission engaged @1200, talk about jack rabbit starts and prop wash in reverse!

I finally determine the entire problem was a poorly adjusted cable. it now goes into gear while the engine idles @600 and we move quietly along about 2.7k.

From 600 to 1000 the engine returns to idle when the throttle is released, great for low speed maneuvering.

From 1000 up I can select any RPM I wish and it stays put.

maybe you have a cable adjustment issue.
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