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Carb Vacuum Leak
Posted by: Steve ()
Date: February 15, 2005 04:01PM

110 Engine

My engine has begun to idle rough. It pings when accelerating. I pulled the filters, cranked the engine, and put my hand over the suspected offending carb. I can hear a sucking sound coming from the carb. I'm suspecting that it has a vacuum leak. Does this sound like a typical leak at the base. What's the best way to narrow down my problem?

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Some air leak finding ideas
Posted by: Larry Forman ()
Date: February 15, 2005 05:13PM

Hi Steve,
There are several ideas:
1. To find it, I like to place a sheet of plastic sheeting held in place with rubber bands or something else. You could rig up a gasket and metal plate if you were adventurous. Otherwise just a baggie and rubber bands works fine. Idle it and "snoop" for the air leak. Some alternatives are a rubber hose about 3 feet long used as a stethescope, or spray WD-40, propane, or Chemtool B-12 around the carb and see if it speeds up, then you have isolated the source. Being VERY careful spraying the flammable stuff! Fire extinguishers nearby are always a good bet.
2. I found a pesky air leak when I had a mixture of a late carb top that was modified for a FCs early carb air cleaner. There were holes cross drilled to hold the hold-downs and in drilling it, it went into an air vent passage. Check if this might be your problem. The hole would be mounted high on the carb and horizontal. Just place your finger over each side of the web that is drilled and that stopped it in my case.
3. Replace the plastic carb spacer and paper gaskets. They often crack when old and heat soaked combined with excessive bolt tightening.
4. Cracks in the vacuum line to the choke pulloff are frequent. Replace the tubing with silicone for a near permanent cure.
5. I had a choke pulloff that popped apart when I placed the tubing on it. It leaked big-time after I snapped it back together. Once apart it is destined for the trash.

Please remember to let us know what you find.
-- Larry

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Re: Carb Vacuum Leak
Posted by: "UNSAFE" ()
Date: February 15, 2005 06:02PM

>>>>My engine has begun to idle rough. It pings when accelerating<<<

Could be something besides an air (vacume) leak or carb problem.

Points, distributor advance, bad gas, etc could cause what you described.

>>I can hear a sucking sound coming from the carb.<<<

I think you should "hear" a sucking sound if you put your hand over the carb.

To find an air (vacume) leak with the motor running, I use the WD 40 method Larry Foreman mentioned. I throw "caution to the wind" and use whatever is handy, Wd40, starting fluid, carb/brake cleaner etc.

If you are concerned about a fire you could probably just use a spray bottle with water in it instead .

A common leak is the short rubber hoses that connect the balance tubes.


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Re: Carb Vacuum Leak
Posted by: Steve ()
Date: February 17, 2005 08:06AM

I may have jumped the gun.
Long story short, the engine fixed itself

The sucking sound was the air vent at the base thats sealed by the rubber vent cap. It was just doing its job.

I had an appointment to keep and ended up having to drive it through the city and down the highway. It the city, it definately felt like one of its cylinders wasnt firing. On the highway it felt like some of the power had come back. When I exited the highway and stopped at the light, the lumpiness was gone and the power was back.

I guess I'm not driving it enough.

Any ideas on what may have happened? I know it could be a thousand things.

Anybody experience this when they let their engine sit extended periods?

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