Topic: Damned be the leaks!

[550Resto]

So I've got a 78 550 with PD46C Carbs. Been chasing a high idle for what seems like forever. Only mod is a 4 into 1. Carbs have been gone through, bench synced and vacuum verified good. I've got 100 mains and stock slows. I though it was a tune issue and today when I got home I thought I'd see if I indeed had an air leak, so I lifted up on the right side of the carbs and the idle dropped. I'll be damned that I tried that before and got no response. Is there a way to ensure the boots are seated and tight without taking them off again?

I don't mind taking them off again, but felt they were snug the first go around. I've got new boots on the engine side with correct new clamps. Would wd40 be reccomened to figure out exactly which carb is the issue? Any help is appreciated.

[550Resto]

Ended up pulling my carbs and trying to get the boots from the carb side to the engine tightened down to what seems like to avail. I still seem to be getting a leak from the #3/4 boot area. I am able to pull the carbs up from the bottom and the idle which seems to be high comes down to a more normal idle. Haven't done a wd40 on the boots to see if that indeed is the location of the air leak. Anyone have suggestions on a way to stop this leak? I've got the band pretty much tightened all the way down. Unsure where to go at this point. Any suggestions would be appreciated.

[eigenvector]

My suggestion - do a test to see where it's coming from.  These carbs aren't super sensitive to air leaks like the vacuum operated Keihin models - so if you have a high idle, it's got a pretty large air leak and should be easy to locate.

Something else to consider - the bands themselves get worn out over time.  After 40 years they're stretched out and even after tightening them down all the way they aren't nearly as snug as they were when new.

But firsts things first - figure out where it's coming from, assuming you have one at all that is.

[DaveBarbier]

Did you replace the o-rings between the manifolds and the head? I use propane with no flame to check for air leaks. No mess.


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[Scott S]

 Are you sure that when you're pulling it up you're not just putting slack in the cables?

[550Resto]

Manifold o rings were replaced as well as new boots from the carb to engine. I did notice when I tried to install a new push pull cable on the carbs that I was able to get one and not the other even though they were direct replacements. The new one seemed slightly shorter and I could not get it to slide over the carb pull. I ended up sticking with one old and one new cable. Perhaps that could also be an issue?

[Gene]

Manifold o rings were replaced as well as new boots from the carb to engine. I did notice when I tried to install a new push pull cable on the carbs that I was able to get one and not the other even though they were direct replacements. The new one seemed slightly shorter and I could not get it to slide over the carb pull. I ended up sticking with one old and one new cable. Perhaps that could also be an issue?

That could explain the high idle. If the cable is a wee bit shorter it is constantly being pulled.

[RAFster122s]

You could have installed the intake boots backwards, the intake manifold and carb sides are different.  Pull the rack to remove the boots and reinstall to same orientation on all four.  You should be able to figure out the correct direction comparing the moldings to the mounting profiles for the carbs vs intakes.
i would bet this is the source of your leak.

[550Resto]

I thought that was an issue which is why I pulled them again. I notice that one side is tight when put on to the manifolds and when flipped around it's loose. Vice versa on the carbs. I imagine when it's tight fitting I'm good vs the loosened, yes?