Saturday, July 18, 2015

New Jet Blocks for the VM29 Carbies

I've been spending a lot of time on these carbs recently. This is mainly because they were beat to death before they ended up in my hands. I got the rack for 75 bucks, which turned out to be about all they were worth. Damage inside and out.

Luckily over the years, these carbs have become more and more desirable and the once obsolete parts are now coming back. Gaskets, jets, screws, etc. Have been available but hard to find on occassion. But nothing was worse than the jet block.

The jet block is a cast piece that rests in the throat of the carb and makes the transition in the slide region smooth. It is obviously critical to the function and performance of this carb, but is also the weakest link. The area of failure is where the needle jet threads into the bottom of the jet block. The cast aluminum is soft and porous, making for fragile threads. Many people who have worked with these carbs have dealt with stripped jet block threads by boring the boss open, pressing a brass slug in, and retapping. This makes a better block than the original but it must be perfectly straight.  Others have just used epoxy to glue in the needle jet. This can also be tedious and it's not permanent. I ran a set with an epoxy-ed needle jet for two years and it was ok. Eventually it began to disintegrate even though I used PC-9, which is approved for fuel.  All these bandaids aside, there is a much easier way.

Z1 Enterprises has been making new billet jet blocks for a few years and they are quite nice. For about $40, you get a block that is nearly drop-in with stronger threads. Don't  forget to replace the small paper gasket underneath and clean the area well. The jet block must be centered well in the slide bore to avoid rubbing.

That being said, when I installed a new Z1 block, I used the small screw and the jet needle to get the new block seated as well as I could. With a few iterations it only has a minor bit of contact on the inside of the slide. It slides smoothly but witb a bit more friction than the others old blocks. I would expect that the interface will clearance itself a bit after some short use. I may mark the contact areas and polish the block where needed to prevent scoring on the slide.

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