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Surging Idle

R

rojo

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"My 1968 Merc 20hp starts fine

"My 1968 Merc 20hp starts fine in neutral, but the rpms increase and drop in rhythm about every second. I don't seem to have this problem when I shift to forward and increase the throttle. Adjusting the trottle idle screw has no effect. Does this sound like a fuel problem or a problem with ignition?"
 
"It does sound more like a fue

"It does sound more like a fuel problem, more specifically a carb problem. The 68's used possibly one of 2 different side bowl carbs, one being a back drag. Each are adjusted slightly differently. If you don't have a manual, one that explains how to adjust each type of carb would probably go a long way to solving your problem, or at least determining if it needs something a little more than adjustment."
 
"One more note, you mention ad

"One more note, you mention adjusting the throttle idle screw, there is also and an idle mixture screw. Alot of times a motor that "surges" at idle is due to a poor fuel/air balance. This screw is located (looking from the front of the carb face on) to the left above the throat of the carb. You could try adjusting it minutely clockwise/counter-clockwise to see if you can steady up your idle. Get the motor up to operating temperature first."
 
"Thanks Graham, I'll focus

"Thanks Graham, I'll focus on the carb. The Seloc manual identifies my carb as "Type A". The throttle idle screw I was mentioning is the mixture screw and I'm surprised adjusting it has little effect. Perhaps I have an obstruction here.

Ron"
 
"Yes, type A carb - but two &#

"Yes, type A carb - but two "different" type A's - very similar though. It may not be an obstruction, the needle valve may just be worn, because it definately should make a significant difference if you adjust it - too far either way should completely stall the motor. On this vintage, it's referred to as a mixture screw - on later models it's called the "low speed jet". In any case, given what you have stated, I would change this and see what it does for you. Even if this isn't your entire problem, this is definately not working properly."
 
"I checked the parts list and

"I checked the parts list and of course, the needle valve is no longer available (neither is the kit for that matter) from Merc. Although, I suspect that a number of parts dealers may have old stock laying around, these were popular motors. The idle adjustment screw (needle) part number is 1399-1713 and the kit is 1399-3637. Hope this helps... Graham"
 
"Graham, the problem is fixed!

"Graham, the problem is fixed! I replaced the idle mixture screw with another from a "parts" engine I have. It was slightly scorn. I also backed off the screw on the trottle linkage so the butterfly is only open a crack while idling. The combination fixed the problem.

Thanks,

Ron"
 
"That's great - a surging

"That's great - a surging (spelling?) engine is almost always an air/fuel mix problem. Enjoy your engine
"
 
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