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Idle Problem

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Big Sam

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The starboard engine in my 1975 Marinette has decided not to idle properly. It was tuned up and ran great for about 6 hours (3 trips) then started to dog at about 2800 RPM when trying to come back up to speed after idling for a while. It cleared up after a minute or so but the problem persisted the next time out. When I got to the dock (almost) it decided not to idle either. It now starts fine and will idle over 1500 RPM or stumble along at 500 RPM but nothing in between. I have checked the alternator, prop shaft and water pump for drag (none), swapped carbs and coils (no difference), and checked the advance springs (a little play at the beginning but them seem healthy). It has been suggested I swap the distributors but this is getting a bit beyond my comfort zone.
 
Fuel filters (canister/water-separator) have been replaced to no effect and the screens in the carbs were clean.

Mechanical ignition; no ICM. Fuel line replaced this spring along with fuel pump and gas is fresh (ran out the week before tune up).
 
Curious....

I see where you replaced the fuel lines tis spring. Is it possible that some Teflon tape got into the shut off valve? I just went through this with my own Marinette--took me weeks of filter changing and parts swapping to stumble onto the problem.

If that's not it, here's something you could try: Tee in a pressure gage at the carb (0 to 15 psi) and see what fuel pressure it mainatins. It should stay around 5 to 7 psi at all times. (If not, that's a problem.) If that reads okay, then tee in a vacuum gage just before the fuel pump. If it stays low at all times that's okay. If the motor passes both of these tests, then your fuel system is not the problem; and, since you've already done the carb swap, it's not that either.

Jeff

PS: E-mail me direct and I'll give you my phone number so we can talk.
 
Had idle problems for about 4 tanks of 10% ethanol gas. Found small particles in both carbs that had to be cleaned out. Last trip out all idle problems are now gone. I think it just took that many tanks of fuel to flush out all the dirt that the ethanol fuel takes off the tank walls. Bad news is that I haven't flushed out my forward tank yet just the back tank.
Jeff
 
That stuff should NOT be getting into your cartbs--that's what filters are for. I hope you're not relying on those tiny filters built into the carb body? Hose are too small to contain any crud, which bypasses right into the carb. A decent sized METAL in line filter is MUCH better.

Jeff
 
Fuel was last year's problem - the filters got overwhelmed and ended up tearing down both carbs and cleaning out what looked like tapioca. I now use fuel treatments and monitor the filters.
 
Swapped distributors to no effect but got improvement when I replaced the original breather on the PVC system. Now it will idle with the choke on so I seem to be down to a vacuum leak. Will have to find it in the spring.
 
Vacuum Leak?? Are you referring to a leak around the base of the carb? There are no vacuum lines on our engines...
 
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