Rapidly fluctuating idle

I just finished installing pazon EI on my 72 commando last night. The bike starts first kick, but the idle fluctuates between 800 and 2000 rpms. The carbs are stock twin amal 32s that were supposedly rebuilt by the previous owner before I purchased the bike. When I rev the bike up, the rpms drop very slowly. The bike also stalls occasionally during the fluctuations. Would worn out slides be this drastic? Thanks

Bowtoy70 said:

I just finished installing pazon EI on my 72 commando last night. The bike starts first kick, but the idle fluctuates between 800 and 2000 rpms. The carbs are stock twin amal 32s that were supposedly rebuilt by the previous owner before I purchased the bike. When I rev the bike up, the rpms drop very slowly. The bike also stalls occasionally during the fluctuations. Would worn out slides be this drastic? Thanks

Click to expand...

This generally signifies an air or sync issue. Assuming what the PO may or may have not done is not a good thing.
Remember, these carb were run on another motorcycle, if I assume correctly by your post, and for that reason you should at least get back to ground zero by check the idle circuits and resyncing these to your bike. Check the balancing hose for leaks along with good flush mounting surfaces throughtout.

Also check the carb numbers. 850Mk2A and Mk3 would not be appropriate for a 750 engine as they have cut away spray tubes.
Mk2A are R932/33 and L932/34 or R932/31 and L932/32
Mk3 are R932/33 and L932/34

If you have a '72 750 combat engine the original carbs are R932/19 and L932/20, however these are more related to jetting from new and any 932 carb without the cut away spray tubes will work assuming you use the appropriate jets.

These were the carbs that came on the bike. I will check the numbers though.

You might want to look at the carburetor float bowls to make sure they're cinched up properly--the bowls on my Combat loosen up so regularly I carry a modified Allen wrench in my jacket pocket--good luck



Tim Kraakevik
kraakevik@voyager.net

I tried everything on my Amals except the re-sleeving including new anodized slides. The re-sleeving did the trick, but it's certainly worth going through the carbs totally and checking everything before doing the re-sleeving. My carbs had maybe 12K on them when they were worn enough it wouldn't hold an idle. I wouldn't say it changed rapidly, just was erratic at what speed it wanted to idle. I'm sure I spent more than a new set in the end since I put all new parts in them. The pazon helped and the anodized slides helped but perhaps only 60% total.

I would think it's related to your floats since your idle is cycling. If your float is sticking in the up position (closed) until your fuel level falls to the point where the weight of your float , no longer floating on fuel, finally releases and drops to refill your bowl, then you could get the effect you discribe...

The other thing I might think you have an air leak on your intake manifold. Start the bike and warm it up, then spray some carb cleaner at the intake manifold to see if you have an air leak. If you spray it on the manifold and the bike stumbles when it sucks in the liquid, you know where your leak is... If there's no effect, you've eliminated an air leak as your issue

o0norton0o said:

The other thing I might think you have an air leak on your intake manifold. Start the bike and warm it up, then spray some carb cleaner at the intake manifold to see if you have an air leak. If you spray it on the manifold and the bike stumbles when it sucks in the liquid, you know where your leak is... If there's no effect, you've eliminated an air leak as your issue

Click to expand...

+1 Try spraying on and around the slides as well to test for worn slides .... a light film of grease on the slides is also a test for worn slides, but don't ride it with grease.

Slick

" When I rev the bike up, the rpms drop very slowly. "

Also check your cable runs and smoothness of action.

And check or just replace your balance hose between the carb manifolds for the hell of it. These have a tough life with the heat. Sounds like an air leak.