How to properly chase down air leaks

[sam000lee]

I noticed a hanging idle on my GS450 and hit it with some carb spray around the intake boots - I can get the idle to stagger and with enough of it in a certain place I can get it to stumble enough to kill the engine.

My intake boots are around 3 years old with new O-rings. I pulled them off and the O-rings are still nice and round. I switched the O-rings with an extra pair I had and no difference.
I cut out a couple gaskets in the shape of the boots (I know it's not supposed to have a gasket) and put them between the head/boots, with the O-rings still on and it didn't make a difference.

I've tried to `pinpoint' the leak with the little straw and have had varying results with this very inexact method but am somewhat confused by the results. Pretty much if I spray in position #1 (right above the top fin) it gets the most engine stumbling and everywhere else nothing much happens, even in position #2 (right below the top fin) and #3 (head/boot seam) nothing really happens when I try to restrict the carb spray to that area. Results are the same for both carbs.

Any tips on how to proceed? It really feels to me that it's not getting past the intake O-ring but I'm a little stumped as to where else there might be a leak.
I have an extra pair of boots that are in pretty good condition that I'm going to try and I ordered new ones anyway as soon as I noticed it but am anticipating having to do some more testing.
I figured maybe the weight of the carbs without the support of the airbox was causing a gap but if I prop the carbs up while spraying the carb spray it doesn't make a difference.

 

[mvalenti]

When were the carbs last cleaned? What do the orings look like in the carbs? Is that phillips head screw under the #3 tight? I believe that is a sync port..

 

[roeme]

Wait, if I read this correctly, getting a result at all at #1 would mean the engine head (or gasket) is leaky? :k

Anyway, I can't imagine the boots and rings deteriorate so much within only three years to get a hanging idle. I'm not familiar with the 450, but does it have a vacuum petcock? If so, I'd have a thorough look at both the tube, clamps if present, and the petcock.

I traced a high idle on some other bike down to a weak clamp combined with a tired vaccum tube.

 

[sam000lee]

Carbs cleaned a year and a half ago to GSR specs (dipped, new Orings, etc.). Checked the screw under #3 and it's tight. I hadn't considered the valve cover not being tight enough but I will.
The 450 does have a vacuum petcock with the vacuum port being on the left carb. I havn't checked it `thoroughly' so will take a look later.

 

[SVSooke]

Try putting proper clamps on the boots.Those rad hose clamps can distort the boots. Probably not the problem but it could be.

 

[Steve]

Try putting proper clamps on the boots.Those rad hose clamps can distort the boots. Probably not the problem but it could be.

That would be my first line of attack, as well. The proper clamps are narrow enough to fit into the groove in the boot. Those clamps you have on there are gripping outside the clamping area, which can cause some strange problems.

I don't see how spraying anything at the #1 area will have any effect at all. You are spraying below the valve cover gasket (which would not make a difference anyway) and you are WAY above the head gasket area, which is below the boot. Based on the numbers in your picture, I would expect results only at locations 4 and 5, the rest of them don't make any sense. :-k

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