Irratic idle

ALAG3

Seaman
Joined
May 22, 2005
Messages
71
Hello All...I posted a few weeks ago about an idling problem along with an overheating problem. I found that my manifold and riser were extremely rusted. I replaced those and took the boat out this afternoon for some on the water tuning of the carburetor. After getting it up to normal temperatures, it seemed to adjust okay and ran fine, but after a while, started to run rough. Upon inspecting the carburetor, I found what seems to be an air leak right at the base of the carb, below and just behind the air/fuel mixture screw. It is hard to tell if air is coming out or sucking in, because it is minor. However, I can hear the air even with the engine running and now that it is doing this, the air/fuel mixture screw has little to no effect on the motor. What could be wrong????Thanks in advance for the help! You guys were great with all advice on overheating problem, no longer overheating and job was a breeze from articles here!!
 

rbezdon

Senior Chief Petty Officer
Joined
Jun 20, 2004
Messages
689
Re: Irratic idle

Sounds like a classic vacuum leak. You can try putting a little heavy motor oil over the leak and see if the idle dropps back for a few seconds. If it does you need to find what the leak is from. Maybe the carb bolts are loose. Happens sometimes after an overheat, the gaskets shrink a bit. I hope that is all the damage you have from the over heat. Likely the best way is to pul the carb if tightening doesnt solve it and check for cracks or other damage to.
 

KRS

Banned
Joined
May 15, 2004
Messages
2,383
Re: Irratic idle

I am sure others would not agree with this method of diagnosis.... and maybe you shouldn't either!!!<br /><br />In my opinion and at my own risk, I believe the best way to diagnose an intake leak is to use a propane torch with the gas on but NO FLAME, and move the tip to the area of the suspected leak while the engine is idling.<br /><br />If the idle increases then the propane is being sucked into the crack/hole and is being burned during combustion. This means there is a hole that needs to be fixed.<br /><br />I have used this with success. Good luck.<br /><br />By the way, is your fuel pump overflow tube empty at idle or is fuel running through it? If it has fuel, then the rough idle could be a bad fuel pump that needs to be replaced a.s.a.p.
 

ALAG3

Seaman
Joined
May 22, 2005
Messages
71
Re: Irratic idle

vatter,<br /> thanks for the input...I had a minor overheating problem and replaced the manifold and gasket between the new manifold and carb. The exhaust and intake manifold are one in the same on this boat. I wonder if something may have happened to the new gasket after it heated up?<br /><br />azfyrfyter,<br /> would I be able to do the same with some type of aerosol spray (ie wd/40, etc.) I read on here that the propane is heavier than air and would sink into the bilge and be very dangerous. As far as the fuel sight tube, I had that problem before diagnosing the overheating, so my fuel pump is new and the sight tube is empty, but thanks...that was a problem !!
 

rbezdon

Senior Chief Petty Officer
Joined
Jun 20, 2004
Messages
689
Re: Irratic idle

ALAG, I am pretty sure the resilts with other stuuf besides propane are not as good, try the oil trick. It was recommended by Don S several times in the past. I also read the propane post (by Don too I think) and propane in the bilge would not be a good thing. I actually dont think it is necessary as you can hear the leak so you know it is there. Only reason to do the tests is if you cant tell where it is coming from. If you know, pull the carb again and re-replace the gasket. Could the carb mounting have gotten cracked/damaged during the previous R&R, especially if you had an issue removing the old gasket (and I am only being half sarcastic)
 

ALAG3

Seaman
Joined
May 22, 2005
Messages
71
Re: Irratic idle

I guess anything is possible....boy, I sure hoped this thing was gonna run fine when I put it back together!
 

ALAG3

Seaman
Joined
May 22, 2005
Messages
71
Re: Irratic idle

I used ether and sprayed it around the base of the carb....immediately revved the engine up a little. Why, with a new intake and gasket, would it still be leaking? I heard this same noise with the old intake, so I don't think it is an intake problem. What can I do to remedy it? Permatex? Try another new gasket? Maybe the gasket that was sent with the manifold was not the right one? Thanks for any input.
 

lowshovel1

Petty Officer 2nd Class
Joined
Dec 11, 2004
Messages
135
Re: Irratic idle

the base of carb may be warped or cracked. Inspect CLOSLY for cracks if crack is present replace carb. If you have a spacer inbetween carb and manafold try replaceing it with 2 carb gaskets.if you have a warped base try the two gasket,it has sealed base leaks on a carb to manafold for me.
 

ALAG3

Seaman
Joined
May 22, 2005
Messages
71
Re: Irratic idle

Well, I finally sprung for the carb rebuild kit and had it rebuilt. Put it all back together tonight and it runs like a top....what a relief. I still need to put it in the water and fine tune the carb, but I was thrilled to hear it purr at idle!!!! Thanks for the input.
 
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