Very Persistent air leak

[pablohoney]

I have an 1979 xs650 special, and it has an air leak. The plugs are white and when I spray starter fluid at the carbs the RPMs drop. It also dies as soon as the choke is flipped on. I have replaced the manifolds, air box gaskets and choke mechanism gaskets and it is still leaking just the same. I have also triple checked the diaphragms for pin holes or anything else and they are perfect. If someone could just list the rest of the places that it could be leaking from, I could check them, but I don't know where else. Thanks

 

[grizld1]

First off you should be able to pinpoint the source of the leak when you direct the spray. Often overtorquing deforms the flanges on the carb boots so that they seal under the screws and break loose elsewhere (BTW they're not manifolds--only one tract). Overtorquing can also extrude the screw holes in the head even if boots are new, gaskets are thick, and a tough sealant is applied; if that's the case, give the flanges a few strokes with a known-flat file and follow up with 320 or 400 grit paper on the file. One thing you didn't mention was throttle shaft seals. Procedures for replacement are in the 650 Garage Carb Guide. Don't mess with them unless you get a response when you spray test the ends of the throttle shafts; it can be a touchy job. Finally, check the vacuum lines to your petcocks and the petcocks themselves. The easiest way to do that is to put blind plugs on the vacuum tubes on the boots, set the petcocks on "Prime" to run, and see if the problem stops. Also try a shot of spray where the vacuum tubes join the boots.

 

[inxs]

- some things to look at

inlet manifold cracked, worn, not sealing to carb​

inlet manifold gasket​

vacuum barb bung​

butterfly shaft seals​

diaphragm​

air filter manifold​

air mix screw o-ring​

choke gasket​

 

[pablohoney]

Thanks guys, that was exactly what I needed. I have a good feeling about the air mix screw o ring

 

[XSLeo]

If the carb holder bolts have been over tightened a ring of metal can form around the bolt hole, this can keep the holder from seating on the head. This can cause a leak no matter how tight you get the bolts.

 

[xjwmx]

I have replaced the manifolds, air box gaskets and choke mechanism gaskets

To be a leak the air box doesn't count. It's on the wrong side. Another thing you can do is smear grease where you think it might be leaking. If that doesn't fix it it's not leaking there. Also, check the clearance of your exhaust valves. I bought an old Yamaha bike where the exhaust valve was so tight it was an air leak during the intake stroke.

 

[pablohoney]

I will check that too XSleo, thanks

Thanks for the grease tip, definitely try that and I have triple checked the valve clearances, another good tip though.

 

[pablohoney]

I am 100% positive that the butterfly shaft seals are leaking horribly, I spray a tiny bit of fluid at them and the bike dies! They carb holders are also not leaking. So thanks a bunch! Finally.

 

[chopitdropit]

My diaphrams looked good holding them to bright light also but on testing them I found a bad one. Here is the test that XSLeo shared with me. According to him, you can raise the slide and put your finger over the kidney shaped hole on the front of the carb bell, then release the slide. It should come back down slowly with the hole covered. If it falls back down fast, the diaphram is bad.

 

[grizld1]

xjwmx, you stand corrected. Any condition that contributes to leaning out the fuel-air mixture "counts" regardless of which side of the venturi it's on, and a leak at an air box joint will do that very nicely.

 

[xjwmx]

Ha! You cut a hole in the air box and cut a hole in the intake and if the results are even remotely comparable, I'll even buy YOU a beer. So you stand corrected your own self until you weasel word your way out

 

[grizld1]

I don't have to weasel my way out of anything, you ignorant jackass. I've tuned enough motorcycles in the last 45 years to know the effects of relieving the intake.

 

[xjwmx]

Apparently you haven't.