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i just bought a 98 airboat 13.5'hul with an lycoming 0540 it wasnt running when i bought it it hadnt been run in over a year i drained the gas replased the fule pump fule water sep. and took the carb. apart and cleaned it also had to replase all the intake tube gaskets ive got the boat running but it woulnt idle . i had to rap a rag around the air cleaner . who ever had it before me had drilled holes in it so i think it is geting to much air if i take the rag off it stalls out and wolnt start but if i put the rag on it runs fine it just woulnt idle the previus owner drilled holes in the bottem of the air cleaner im wondering if thats it or if any one has any thing better that i could do
Sounds like it could be a leak in the intake system. Check all of the connections on the intake shrouds or tubes. They have rubber hose connecting them to the intake manifold. They can get old and crack, developing leaks. Look closely at the intake tubes as well. Lots of them get a little rusty and can get a pin size hole in them from the rust. If they are rusty, a good round on a wire wheel (or a bead blaster) will let you see holes clearly.

A few small holes in the bottom of the breather cover shouldn't hurt to much. If the carb is not set up well, they are known to leak fuel like the devil and will leak like the devil if you sit and pump on the throttle alot before starting. Several of the older Lycoming flame arrestors (metal covered breather) had holes in the bottom to keep leaking fuel from pooling up in the breather. My old arrestor had a 3/8 to 1/2 inch hole in the center of the bottom plate. Ran fine like that.

The throttle adjustment may be set wrong or the idle adjustment could be set wrong. Don't have my lycoming manual handy, will try to grab it back from a friend this weekend. I think it says how to reset those (assuming everything else is good).

On the marvel carburators, there is an accelerator pump plunger and check valve in the bottom of the shaft the plunger sits in. The accelerator pump plunger has a leather gasket with a spring underneath, and the valve has a little steel ball inside with a spring clip holding that in place. We had problems with some bad fuel issues(water/rust in tank) a few years ago. That spring in the plunger rusted out twice (boat hard to start, no idle, and starts/runs only at high RPMs). Had trash in the check valve as well that caused some bad idling problems. Beware, if the spring clip on the check valve has rusted out, the BB-sized steel ball will fall out as you remove it. (Ooops, catch it, damn another 20-30 dollars down the drain. I learned you don't take them apart on the tailgate in a gravel driveway.) The last time we had a problem, I took the carb apart and cleaned it all up with B-12 carb cleaner and flushed all the jets and passages real good. (P.S. - don't look at the jet when you push the accelerator pump down, thank goodness I had glasses on.) Had some tiny rust pieces in there. Installed a pipcock on the carb drain to try to stop the problem in the future.

All the Lycomings around here have pip-cock valves on the back of the carburators. When you put the boat up, open the valve for about 3 seconds and let some fuel drain out. Water that is in the fuel tank or carb bowl from condensation and collected in the carb bowl will drain out. Stops the rusting up spring problem.

A good contact is Jr. Jurnigan. Has lots of those parts on hand, does a great job with overhauling all those lycoming parts (mags, carbs, engines), and is honest.

Marks Airboats in Louisiana has a really nice air breather/flame arrestor assembly that I recommend if you are running cattails. Has about twice the screen size as the standard metal breather on most boats so it doesn't get choked down as easy. Another good source of lycoming parts, but doesn't have the inventory on hand that Jurnigan does.

Hope this might help, let us know.

Big Daddy pretty much summed it up. Holes in the air cleaner can only give you unfiltered air and drain out excess fuel. If your idle goes up when you plug up your air inlet (carb) 90% of the time it’s because you have a vacuum leak. You can use an unlit propane torch or a can of carb cleaner. I prefer the carb cleaner you just spray the carb cleaner or move the torch around the intake tube seals, manifold and carb base when you hear the idle go up you found your vacuum leak. It sounds like it’s pretty substantial leak.

QUOTE “A good contact is Jr. Jurnigan. Has lots of those parts on hand, does a great job with overhauling all those lycoming parts (mags, carbs, engines), and is honest.�

I agree to that I have dealt with Jurnigan a few times, great guy him and Paul Gast seam to be some of the few airboat aircraft engine builders that don't pedal snake oil. I tell everybody who asks me about another engine builder. (Run from any engine builder who bad mouths another engine builder to get your business). Neither of these guy’s will bad mouth a competitor to get someone’s business. They rest on their laurels and I respect both of their abilities!!
im located in kiss. fl. where is jurnigam located and/or how do i contack him to get parts for my lycoming i know alot about car moters but nothing about aircraft moters its greek to me so any one or any place that might be of help i would greatly apreciate
:D Hey guys thanks for all of the in put. This boat know belongs to a real good friend of mine and I have spent a few hours working on it to try to get it running.Like Kenny and his father Randy, I have no clue on aircraft motors but would like to learn as much as i can.Any futher help on this project is greatly appreciated.

Look foward to meeting some of you guys when we get this thing to idel.