nasty varnish

I just pulled the carbs off my '82 750 to replace the intake boots, and noticed from the butterfly forward to the valves on all four are covered in nasty reddish brown varnish. The bike runs great, and idles fine, but I know that should not be there.. Is that because of the float levels, or air fuel mixture screw, possibly re-jet is needed?

I have seen that on just about every old carburetor I have looked at, and fuel injectors as well. I even see it on the vacuum pistons (slides). I think gas vapor just accumulates deposits over time as the engine sits.

If it is excessive, it's possible the petcock is not sealing well and some gas continues to seep and ends up in the intake bore due to gravity. It can oxidize there and form that brown varnish.

As long as the carbs are clean where they need to be, I don't think it hurts anything. If I have the carbs apart, I usually hose them with gumout spray to take it off.

If the carbs are off the engine and still together on the brackets, a good boil in distilled water should disolve the varnish.

Some people have used Lemon Juice for the boil but I've read the carb bodies get a dull finnish from this.

MFolks wrote:

Some people have used Lemon Juice for the boil but I've read the carb bodies get a dull finnish from this.

Yessir.

So I have everything all cleaned up again. Sound like it is running better now. I have been fighting cylinder 3, it doesn't like to fire all the time. I spray water on the head pipes and it takes a minute to evaporate, while the other three are instant. I have good spark, but noticed that fuel was sitting at the boot on my pod and the filter was soaked with fuel after it was warm and run around the block a few times. Could it be blowing back through the intake valve a little bit?

Seen172 wrote:

So I have everything all cleaned up again. Sound like it is running better now. I have been fighting cylinder 3, it doesn't like to fire all the time. I spray water on the head pipes and it takes a minute to evaporate, while the other three are instant. I have good spark, but noticed that fuel was sitting at the boot on my pod and the filter was soaked with fuel after it was warm and run around the block a few times. Could it be blowing back through the intake valve a little bit?

Might be leaking float valve and/or too high fuel level.

Would sniff-test crankcase oil to detect possibility of fuel contamination.

Good Fortune!

I rebuilt the carbs, and set the float levels. Everything is all nice and clean too. I even dropped the float a tiny bit more to see if it helps any, and it still does it..

I have my carbs off for cleaning on my project GPZ750. I like the idea of the boil in distilled water. The carbs are still on the rack right now with all of the guts removed. Will the boil do anything to the rubber choke boots?
Thanks
Nick

Seen172 wrote:

I rebuilt the carbs, and set the float levels. Everything is all nice and clean too. I even dropped the float a tiny bit more to see if it helps any, and it still does it..

If not already done, would verify fuel level inside float bowl by performing the clear tube test, which test is imo the best way to determine for sure the actual true fuel level.

The important consideration is not "float height."
The important consideration is "fuel level."

Good Fortune!

Sorry that's what I meant by setting the floats. The clear tube test was good, everything is sitting where it should be. I'm stumped...

Seen172 wrote:

So I have everything all cleaned up again. Sound like it is running better now. I have been fighting cylinder 3, it doesn't like to fire all the time. I spray water on the head pipes and it takes a minute to evaporate, while the other three are instant. I have good spark, but noticed that fuel was sitting at the boot on my pod and the filter was soaked with fuel after it was warm and run around the block a few times. Could it be blowing back through the intake valve a little bit?

First thing is to always check compression and valve clearance. A valve with no clearance will cause blowback.

Seen172 wrote:

...The clear tube test was good, everything is sitting where it should be....

When fuel is rising up into the carb throat, the clear tube test will indicate a fuel level higher than the requisite 2~4mm below the float bowl gasket.

Am thinking problems are due to yet unresolved issue(s) with the float valve and/or float. And that the non-combusting cylinder is "flooding out."

Good Fortune!