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Finding An Intake Leak


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*Finding An Intake Leak

This is a very common happening on Harley engines and a rather difficult thing to diagnose for some...

Your engine had been running fine but recently it has started running poorly, maybe when its warm, maybe when its cold or sometimes the problem just seems to come and go. The problem shows signs of both a rich and lean running carburetor. You have done the obvious like change jets if it's carbed, replace spark plugs, check plug wires and replace your air filter element, but the problem persists. You can't seem to identify the problem or what's causing it.

Erratic symptoms like these can be the sign of an air leak at the intake manifold at the joint to the head or the carburetor seal.

The design of the Harley-Davidson engine and the intake manifold make it prone to air leaks. Improper installation and age are two major contributors to the condition. Symptoms usually develop just after working on or around the carburetor usually.

Finding an air leak at the manifold is simple. All you need is a can of penetrating oil like WD-40 with the extended straw type nozzle attached. With the engine idling, direct a heavy spray of the penetrating oil at the manifold gasket area and base of the carburetor and also where the manifold joins onto the head. When the spray hits the area leaking, there will be a distinct change in the sound of the engine. If the air leak is major, the change in sound will be quite obvious. Smaller leaks may only create minor changes in the sound.

The engine temperature need to match the times the problem most frequently occurs. If you accidentally spray the exhaust system, you may get a little smoke. The penetrating oil can be cleaned up by washing the bike after the engine cools.

Most carburetors have vacuum lines or fittings attached. The vacuum lines normally run to the VOES and on late model bikes, the fuel petcock. If you suspect an air leak, just replace these hoses. You can get vacuum hose at your local auto parts store. Replacement of the hoses is the best way to eliminate vacuum leaks.

The CV carburetor has one additional area that can create problems. The slide has a neoprene diaphragm at the top. If the diaphragm is installed improperly or develops cracks, an air leak will occur. Problems are most likely to develop just after re-jetting or making modifications to the carburetor. Make sure you properly install this diaphragm after upgrading your CV carburetor. The diaphragm must be properly seated in the land that is provided for it or an air leak will develop.

Using this information, you now have the means to identify the cause of may baffling performance problems on your engine.
yeah, that's what has me puzzled....check this out....

recently (about a week ago), I was trying to fire up the bike to go for a spin (the bike hadn't run for a few days at this point, obviously engine was cold)...pulled out the choke, gave the throttle a quick twist as always, hit the start cranked for a few seconds, nothing....tried this again a couple of times, fired up (just about) for a second or two, then died. After a few frustrating minutes of repeating this procedure, it finally fired up....normally, when the engine is good and warmed up after a decent ride, not usually any starting problems. This problem is when the engine is cold.
I pulled out the spark plugs (the rear one was kind of on the black side, other one didn't look too bad...cleaned them up....I have one of the recleanable air filters, tried cleaning it up a little bit, looks a little better.....for the record, my bike is a 1986 Heritage Softail. Any thoughts???
I used to use spray carb cleaner (like 2+2) on cars to find intake leaks. With motor running spray it around where the intake bolts to the heads and carb to the intake, if you have a intake leak the RPMs will go up when you spray where the leak is.
If you find a leak make sure you align the intake when you install new gaskets:s
Here's what I did with my 1986 Tour Glide....assuming that the engine is supposed to be in tune, I used a propane torch valve with a flexible plastic tube purchased from HomeDepot's bulk roll. I took the propane torch pipe off the valve and pushed the tube onto the valve. I started the engine and moved the end of the tube around the intake manifold and the carburetor. The engine running changed when the directed propane was at the top of the carburetor. What I found was the bottom of the rubber plug sealing the unused vacuum port had split because of age. That was the only problem. The alternative would have been the hassle of pulling the carburetor for a close inspection. I simply stuck a new rubber plug on the port to correct the rough idle.