One more thing to check when you rebuid VMs

[raistian77]

If you have a 79 or older bike you most likely have the Mikuni 26mm VM carbs. When rebuilding them there is one thing you need to check before you are done.

If you remove the fuel pilot screw (mix) and look inside you will see a tiny hole. It has been mentioned many times here to check this hole in order to be sure the mix needle has not broken off in it. However if you look inside the throat of the carb on the engine side you will see there are 2 holes there. Blow compressed air through where the mix needle goes and ensure that both holes have air coming through them. On old carbs that sit up it is easy for the secondary hole to be plugged up.

As has been mentioned here 1 million times, do not think a can of carb cleaner or sea foam is going to clean 30yr old carbs that have sat for a long time. Tear the carbs completely down and soak em, for at least 30 hrs on the first go. Then wash the carbs in hot soapy water and blow dry with compressed air. Carb cleaner only works if the passage is not blocked, Seafoam only works if the carbs are 95% right and run pretty good on the road.

As I have learned the hard way if the bike is 30+years and original, when you rebuild the carbs just go and replace the petcock and intake boots/o'rings. Working around bad boots and a leaking petcock is more headache than it is worth.

 

[KEITH KRAUSE]

Yup. People always mention the "little hole" but I've also mentioned over the years there are two.
One is for the pilot jet itself and the other is regulated by the adjustment of the pilot fuel screw which assists the pilot jet and fine tunes the pilot circuit jetting.

 

[raistian77]

Its easy to miss and if its plugged up will cause crazy running issues that will drive a sane man insane.

 

[Steve]

The same principle applies to the CV carbs, too, but there are three holes.

The mixture adjustment screw is obvious, the other two are back at the throttle blade and are progressively uncovered as the throttle is opened.
If they are not open, the transition from idle to part-throttle will be rough.

Here is a cutaway of the VM carb. You can see the pilot fuel screw on the right side (#3 in the drawing).
Just to the left of that, under the edge of the throttle slide, you can see the second hole.

Here is a cutaway of the CV carb. You can see the idle mixture adjustment screw on the top of the outlet of the carb on the left (near the #5).
Just to the right of that are the two other holes at the top of the throttle blade.

BOTTOM LINE: No matter what carb you have, for a thorough cleaning, you can NOT just drop the bowls and spray some cleaners. :-\\\

.

 

[raistian77]

BOTTOM LINE: No matter what carb you have, for a thorough cleaning, you can NOT just drop the bowls and spray some cleaners. :-\\\

.

yup, but everyday new members claim that a can of seafoam or can of carb cleaner fixed their bike, but they still have a flat spot or problem thats "unrealted" to the carb.

 

[Steve]

yup, but everyday new members claim ...

You have only been noticing that for five months. :-k

I have been noticing that for five years.

.

 

[tejasmud]

I don't believe it is a new members fault, as if they deliberately just don't clean these little holes out. Or, use one can of carb cleaner per carb, and think they have cleaned carbs.

They are NEW to bikes and they just don't know.

Even if a new member reads through the ba-zillion threads on carbs, if they don't know the language and can't mentally picture how carburetors function it can be hard to grasp the concept sometimes.

Tearing a set of carburetors down for the first time can be intimidating. After doing it once or twice it can become old habit pretty quickly.

Not saying it can be done blindfolded after one or two times, but those who can do it blindfolded don't write carb threads though. Have to learn somehow.

E

 

[raistian77]

I don't believe it is a new members fault, as if they deliberately just don't clean these little holes out. Or, use one can of carb cleaner per carb, and think they have cleaned carbs.

They are NEW to bikes and they just don't know.

Even if a new member reads through the ba-zillion threads on carbs, if they don't know the language and can't mentally picture how carburetors function it can be hard to grasp the concept sometimes.

Tearing a set of carburetors down for the first time can be intimidating. After doing it once or twice it can become old habit pretty quickly.

Not saying it can be done blindfolded after one or two times, but those who can do it blindfolded don't write carb threads though. Have to learn somehow.

E

very true,

What drives me up the wall is when someone asks what to do to a new (to them) bike. Then they are told to check valves, replace intake orings and strip and dip carbs,t hen they are given the links and "welcome package" that has ALL the info needed to do it right.

Then they are back on stating that they don't need to do such things cause the carbs look GREAT and the PO already did the valves (probably 20,000 miles ago) but the bike has a hard time starting cold and only run to 3/4 throttle etc. what can that be?

Before I posted first thing I did was follow the welcome package, eliminated 99% of my problem with the bike. Just wish I had replaced the intake boots and petcock while I was at it. Thinking they could still be serviceable after 32 years was just silly on my part.