DIY: 32 34 PDSIT Throttle Shaft Bushing Replacement

[averr789]

Hey everyone,

I picked up a set of PDSIT Carbs recently for a very reasonable price. I started reading into them and I found that they commonly leak air through worn throttle shaft bushings.


http://www.youtube.com/watch?v=xT7Q93xipCk

In this link the guy shows what he suspects is a leaky throttle plate. He describes the play as "subtle."

The thing is the bushings are very narrow (a few mm at most). So if they start wear out there is not much extra surface area to help seal air out.

The bushing replacement is not rocket science. But there are a few things that have to be done very slowly and carefully so that you don't ruin your carb.

Also two specialty tools are needed. The first is a set of calipers. Second is a drill press.

Next I warn you...the aluminum the throttle plates are made out of is very very soft. You can easily nick it or do other damage. Also it would be easy to drill an off center hole and ruin them because the bits go through very easy.

Soft aluminum is also advantageous because it makes the carbs easy to "precisely machine."

Lets get into it.

Here is the throttle plate.

Here is the throttle shaft. Be careful not to bend it.

A note for disassembling : Make sure you get a strong flat head screw driver that only fits inside the screw on the butterfly. A larger one could strip out the screw and make this job a whole lot less enjoyable.

The top bushing is the old one. Look at how small and narrow it is. I sourced a new bronze bushing from the sears hardware section. Notice that the inner diameter is much smaller. Also the outer diameter was a hair smaller but I had to drill out more of the throttle plate to fit it so it went in snugly.

Next check out this extremely complex jig. That's right; it's a piece of wood with some screws on it. Basically I took a section of 4x4 and lined the carb up as close to level as I could then I drilled holes to mount it. Then each time I drilled into the throttle plate I would check the level and adjust if necessary. It didn't move much. Also I used a screw driver to make adjustments.

Here is our drill press. I think my dad said it's from some time in the 1960's. It is a single speed deal and it works great. The one thing I did before I went at it was leveled the drill press with shims at the legs.

The only flaw with those bushings is the raised lip on them. Basically I worked out a system after learning on the first one. Calipers are essential here (at least they make things much easier). I drilled out the throttle shaft hole with a bit just smaller than the bushing. I measured and made sure that the raised lip was sticking out enough to get a saw between it and the throttle plate.

Then I proceeded to drill out the throttle shaft hole. I slowly stepped up in bit sizes. I started smaller. Then I drilled a hole the exact same diameter as the throttle plate. I thought that would be perfect but it was way too tight. Then I drilled the hole out 1/32'' bigger. That gave it mobility and tightness.

During this whole drilling the inside process a few things must be noticed. First you have to make sure you drill right in the middle of the holes. It may seem obvious but with a drill press it is easy to force the hole off to one side or another. Next it will take several slow passes with the same bit once at the final inner diameter to get everything fully smooth and easy. You will also probably have to run the bit through both holes at the same time to help line them up a bit.


Finally I cut off the lip and filed it back to the throttle plate.

Finished

Cheers!

[Wildthings]

We had a similar drill press when I was a kid, brings back memories. It actually worked pretty well.

Good job, thanks for the post. BTW, You can get a variety of bushings at any place that rebuilds small electric motors. My local automotive electric shop has dozens of sizes.

[averr789]

Thanks. I really enjoyed it.

Here is a link to another DIY: This guy had different bushings.

http://shoptalkforums.com/viewtopic.php?f=28&t=129722&hilit=throttle+shaft+bushing