I think I have an air leak , but cant pin it down

thegnu

thegnu

New Member
My bike acted up 2nd time on a long ride yesterday night was cruizing at about 30 in top gear pulled in the clutch preparing for a stop made sure I released the throttle an the motor began to race , I have already checked throttle move ment an cables carb bore an the barrel for any obstructions all of these move free , went so far as at idle took some carb cleaner an sprayed around all the joints while at idle an no change , an been looking for anything leaking out of the cases. nada. it seems to occur after the motor is good an hot after pulling a steep grade . chasing this ghost problem is gettin frustrating anyone if ya think I am missing something throw it at me an I will give it a shot .
 
and which carb do you have?

...it could be the seal between the carb and the manifold, could be crank seals, or it just kinda sounds like a cns! ...could be an internal glitch in the cdi unit...

Fun little bikes, aren't they? :-)
rc
 
the carb is an NT, both cases mag side an clutch side are dry already checked the manifold to carb joint with carb cleaner sprayed the joints while running , no change at idle
Gary
 
Gary,
How are you sealing the carb to the intake? The NT has those slots in the thoat that allow it to be clamped. I've found that on some NTs those slots can cause some problems (leak air) if not filled and sealed. The best sealer I've found so far is SealAll. It's gasoline resistant, unlike most silicone based sealers. It makes a good air tight seal and if you need to remove the carburetor it will break loose with a little twisting action.

As far as not finding evidence of a leak by spraying the area, my experience is that engine temperature will effect how much air gets in. Expansion and contraction probably explains that. Also at idle speeds you might not be pulling enough air in to hear a difference in rpm when spraying carb cleaner, or whatever on the area. I had the exact same problem you're having and I too spent some time searching for what I knew was a leak. It wasn't until I sealed the carb to the intake that the problem went away. Just my thoughts.
Let us know what you find.
Tom
 
You can try pressure testing the crankcase if the carb socket proves to not be the problem.

Here's the way we checked for leaks on the ultralight engines.
You'll need an air compressor with a good regulator.

Block the exhaust port with a plate and gasket, or you may be able to just keep the piston at TDC for the test.
First, get your air regulator adjusted down to 5 psi or less. more than this might blow out a seal.
Figure a way to adapt an air hose to the intake manifold. Might be able to use a blowgun with the rubber nozzle or tape wrapped around a standard one. and just hold it firmly in the intake pipe.
Apply the air pressure, and spray soapy water anywhere you want to check for leaks, and watch for bubbles. Be sure to check crankshaft seals and crankcase gasket joint.
 
I made two plates, one for the intake, the other for the exhaust. Took a ole spark plug (broke the electrode out) and welded a air fitting. Like said, pressure low, piston at BDC.

The plug I've had for over 30 years, I'd use it if I had to replace valve guide seals in my vehicles.
 
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A trick on saw on the other forum in it's early days was cutting a strip out of a soda can and using that as a carb gasket.
I used that trick for a few thousand miles until I got an O-ringed intake! lol
 
2door said:
Gary,
As far as not finding evidence of a leak by spraying the area, my experience is that engine temperature will effect how much air gets in. Expansion and contraction probably explains that. Also at idle speeds you might not be pulling enough air in to hear a difference in rpm when spraying carb cleaner, or whatever on the area. I had the exact same problem you're having and I too spent some time searching for what I knew was a leak. It wasn't until I sealed the carb to the intake that the problem went away. Just my thoughts.
Let us know what you find.
Tom
Click to expand...

Well heres what I found , I made some quick an dirty cover plates for the intake an exhaust an took an old plug that was laying around the shop welded a fitting to it an instead of pressurizing my case I drew vacume on it with a old vacume pump I have quickly pulled 3 negative psi an let it set , 15 mins later no change , I have to asume my prob was exactly as tom said above sealed up the top of the barrel its got slots too, an carb to manifold slots an seat once its dry gonna go for a ride tomorrow an see if its fixed .
Thanks all
 
Al.Fisherman said:
A "O" ring like this also works well. NAPA has a assortment, that's where I buy them.
Click to expand...
Ron,
I've found a few NT carbs with the slots machined deeper than they need to be and actually deep enough that the 'O' ring won't seal them. They go all the way past the flange where the 'O' ring would normally seat. This is why I always suggest using a sealant on the intake manifold and to make sure the slots are filled with whatever you choose to use as a sealer. As you've heard me say before, most silicone based sealers are not compatible with gasoline. It clearly states this on the label. RTV and others will work for a time but will eventually be attacked and turn to a leaking goo where it comes into contact with fuel. To date, Seal All is the only product I've found that is completely impervious to gasoline.
Tom
 
Luckily in 6 engines that all have a NT carburetor, none of them can't be sealed with just the "O" ring...just lucky I guess. But it's good to know about the properties of Seal All, which I remember you stating the fact.
 
My NT carb needs to be pushed all the way down so that the manifold goes past the slots and I'm fairly certain that adding an oring would actually create the leak. I am using Permatex RTV sillicone at the moment and it seems to last.
 
I have an o-ring, and I also have a steel bolt that is probably twice as long as the crappy stock carb manifold bolt. Where he suggested putting RTV is where the slots in the carb flange are, not inside the carb flange itself, gasoline shouldnt be coming in contact with the outside of the carb flange anyway. if you put rtv on the outside of the flange to seal it and it turns to goo, then you got other problems.
 
motor_bike_fanatic said:
I have an o-ring, and I also have a steel bolt that is probably twice as long as the crappy stock carb manifold bolt. Where he suggested putting RTV is where the slots in the carb flange are, not inside the carb flange itself, gasoline shouldnt be coming in contact with the outside of the carb flange anyway. if you put rtv on the outside of the flange to seal it and it turns to goo, then you got other problems.
Click to expand...



Although I only have a "O" ring, I'm going to keep the Seal All in mind. I feel if needed, that you need a sealer impervious to fuel, as that these engines spit unburned fuel out into the intake, and other sealers can be effected from fuel mix in to intake.
 
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put 17 miles on it an it didnt act up not even once , now if i can just get it to idle a lil better I will be much happier .
Thanks guys
 
thegnu said:
put 17 miles on it an it didnt act up not even once , now if i can just get it to idle a lil better I will be much happier .
Thanks guys
Click to expand...
Probably that lumpy cam you put in it. They never idle very well :)
Tom
 
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