Main jet

It never ceases to amaze me how these stories get started and keep going. I've rebuilt hundreds of VM series carbs and very rarely have to replace the needles or needle jet. My personal bike has the original carbs VM24 with over 150,000 miles I've never had to replace those items. If the needles and needle jets are the original DO NOT replace them with aftermarket ones they are not the same.

650ed wrote:

hardrockminer wrote: You may want to also consider replacing the needle jets and the jet needles. Most people don't realize that they wear over time. A new set will bring your carburetors back to original function. You can probably buy complete new kits (I like Keyster) from Z1 enterprises.

I guess I'm one of those folks. My bike still has the original Mikuni needle jets and jet needles and with 62,000 miles they still work like they did when the bike was new. Ed

Each to his own I guess. I never used to replace them either....until I did, and saw a significant difference in performance. Better fuel economy and better acceleration.

The needle jets wear from action of the needles going up and down. There is contact between the needles and jets so the needles also wear. This affects the amount of fuel flowing into the venturi and as well, affects how it diffuses upon entry. It actually puzzles me why someone would replace a non-wear part like a main jet or a pilot jet but would not replace a wear part like a needle or needle jet.

I don't like most of the aftermarket parts either. Except for Keyster.

One thing I would ask of those who have high mileage with original jets....how many have you measured to see if they're still round and within spec? They are cheap and easy to replace. If you're going to replace mains and pilots why not do them all?

baldy110 wrote: It never ceases to amaze me how these stories get started and keep going. I've rebuilt hundreds of VM series carbs and very rarely have to replace the needles or needle jet. My personal bike has the original carbs VM24 with over 150,000 miles I've never had to replace those items. If the needles and needle jets are the original DO NOT replace them with aftermarket ones they are not the same.

This is copied from Kawasaki's carburetor training manual. It's one of a series of manuals on vintage Kawasaki motorcycles used to train mechanics in maintenance and repair.

" Check the jet needle for signs of rubbing. If the needle has been rubbing, it would be against the needle jet, and this too would have to be replaced. A worn jet or needle will allow too much fuel to enter the engine, resulting in a rich mixture. The main jet and pilot jet rarely "wear out" but should be checked for clogging, deformation, or stripped threads."

hardrockminer wrote: ..................................

One thing I would ask of those who have high mileage with original jets....how many have you measured to see if they're still round and within spec? They are cheap and easy to replace. If you're going to replace mains and pilots why not do them all?

I don't know if 62,000 miles is considered high mileage, but I have never replaced mains, pilots, jets, needles, or any similar parts. I haven't replaced any of those things and the carbs still work like new, so I guess my view is why replace something that is working perfectly fine? Maybe I've been lucky, but I bought my 1977 KZ650 new more than 42 years ago and other than needing the clean the carbs once after the bike sat unused with fuel in it for about a year I've never had any carb issues at all. The only thing I have replaced related to the carbs are the carb holders (the originals developed hairline cracks that affected idle), and the bowl gaskets and chamber top gaskets when I cleaned the carbs. Ed

650ed wrote:

hardrockminer wrote: ..................................

One thing I would ask of those who have high mileage with original jets....how many have you measured to see if they're still round and within spec? They are cheap and easy to replace. If you're going to replace mains and pilots why not do them all?

I don't know if 62,000 miles is considered high mileage, but I have never replaced mains, pilots, jets, needles, or any similar parts. I haven't replaced any of those things and the carbs still work like new, so I guess my view is why replace something that is working perfectly fine? Maybe I've been lucky, but I bought my 1977 KZ650 new more than 42 years ago and other than needing the clean the carbs once after the bike sat unused with fuel in it for about a year I've never had any carb issues at all. The only thing I have replaced related to the carbs are the carb holders (the originals developed hairline cracks that affected idle), and the bowl gaskets and chamber top gaskets when I cleaned the carbs. Ed

I agree...if everything works fine then why replace? But almost all vintage bikes have an unknown history. Yours is far and away an exception....a one owner bike since new. When I get a new (to me) bike where I don't know the history I want to make sure it's going to function correctly. I spent over $20k on my last rebuild. Why would I not replace carb internals for an extra hundred bucks and be certain they are as per original?

And yes Ed, I would consider 62k miles to be high for a vintage bike. My last one had 45k miles and I thought that was high.

Good point. When buying a bike that's been owned by others there's a very good chance jets and other carb parts have been changed so it makes sense to start with fresh, known carb parts. Ed

I've had trouble with new pilot screws before, they seemed to be made out of softer brass and got screwed up probably when I bottomed them out before backing them off.
I have no worrys about good new jets though