carb boot(s) bad?

[endlessGSgeek]

Please forgive me for extended absence. No excuses (I have plenty!).

My buddy and I found a 1980 550E(T) at a local shop that the owner thought had bent valves because he thought it was 'over-revved'. Compression was fine, good spark, and it REALLY wanted to run with some ether, and after lots of messing around, all spark plugs were DRY...so into the Carbs, as usual.

I gave them a thorough cleaning, returned the airscrews to about 1 and a third and re mounted them.

Not bad ! Clearly almost there (lots of power), but the engine didn't want to stop revving, and kept creeping up. My buddy reminded that this is classic boot o-ring fail. Well, YES, and I found the lower screw on #2 LOOSE!

I replaced all, but some of the rubber around the metal on the flange cracked off during the job (more than one of them). One re-mounted, the problem is just as bad or worse!

Can the air be venting through the boots (NOT at the head, and not at the carb clamp). Flanges are well sealed to the head (with new allen bolts), and the carbs are nicely secured in the boots.

[chef1366]

Did you replace the o-ring behind the boots?

[endlessGSgeek]

Yes, boot o-rings new

I found perfect fit at ACE, 3mm thick, and I think it was 48mm OD. They fill the recess and protrude about a mm. Clearly sealed (new allen bolts, across).

[chef1366]

Spray some ether on the boots and see if the rpms rise.

[bwringer]

The hardware store o-rings will quickly crumble -- they can't withstand the heat of the cylinder head. You'll need Viton o-rings.

That may not be the current problem, since it'll take a little while for them to go.

What's on the other side of the carbs? In other words, how are the airbox boots and the airbox?

[Big T]

Those O rings probably won't last because they should be the Viton material.

I'd just get the correct O rings from Robert -cycleorings.com (plus the ones for the carbs if you didn't do that)

and some new intake boots and be done with it.

I know, it's not cheap, but how much fiddling around do you want to do?? Those O rings will deteriorate and you'll have more problems.

[endlessGSgeek]

That may not be the current problem, since it'll take a little while for them to go.
What's on the other side of the carbs? In other words, how are the airbox boots and the airbox?

The rubber boots that are clamped onto both sides of the carbs are supple, probably OK (no cracks either). I think you are right about the new o-rings between the head and boots (OK for now, but will need to be replaced).

Thefact that the revs are creeping upward and won't settle down after rev must mean air is getting in on the high vacuum side, rather than the low vacuum side (airbox). But I think the carbs are snug on those boots as well. About ~$100 for a set of boots, though I'd like to make it work with a sealant before making the diagnosis. Must be a way to seal them from the outside for testing purposes. Any ideas for the right stuff? Silicon?

[endlessGSgeek]

...some new intake boots and be done with it.

I know, it's not cheap, but how much fiddling around do you want to do?? Those O rings will deteriorate and you'll have more problems.

Yes, that would probably fix it, but I don't want to put much money into this before we are sure it is worth the investment. It is pretty close.

[Eli69]

i made a temporary repair on intake boots with an industrial grade clear silicone. basically caulk. then i covered that with black liquid tape mostly for looks. it works but i really don't expect it to last. should be enough to get any other problems figured out.

[themess]

Some silicones can easily handle the temperature, and a few will even be OK with a bit of exposure to gasoline. They fail under physical stress. In other words, you'll probably be fine for the test, but you want to go to fluorocarbon rubber (trademarks are Fluorel and Viton) as soon as reasonably possible.