Carb question?

L

lhemrick

Active Member
I have a 1970 cb350k twin with a fresh engine rebuild that I cant get to run "idle" correctly.
First of all I know to check compresion with the bores open and both cylinders are close to 180.

With the bores closed I have 60 out of the left and 120 on the right.
The left cylinder exhaust runs cold while the right is hot on idle.
All of the jets are the same and mixture screws are set the same.
When I swap the carbs from one cylinder to the other the symptoms change from one cylinder to the other.

Something is wrong with one of the carbs either allowing to much air through at idle or not enough. What is my problem and which carb needs attention?
 
Ignore your compression numbers for now..... but have you synced your carbs?
 
Yes, Somethings wrong with one carb (I don't know which) Because one cylinder is pulling the other along at idle. you can pull off the plug wire on the cylinder with the lowest compression at idle and it will quit. If you pull off the other it continues to run. Its pulling more air and fuel on one than the other. correct?
 
I want to say your carbs are just needing a good cleaning / rebuild. Clean carbs, with proper float height, good condition internals, and synced, should have you back up and running properly.
 
+1 for syncing
but yeah as mentioned make sure they are clean too and have the floats set
 
I've had them apart three times. They are clean and all passages are open. New diaphrams installed. Now I did notice the second time that the floats weren't set to the right height. I had them first set to a figure I found in an old clymer manual but found a differnet numert number in several other locations. I now have them set to 26mm. As I stated when I swap the carbs the symptoms also follow them from cylinder to cylinder. I have a spare carb and I'm going to change the jets install it and see what happens.
 
This is going to sound rediculous, but are you 100% sure the carbs/parts were not mismatched during cleaning?
I had a similar problem once, wich drove me mad. Solution was to simply switch the gas-slides from one carb to another. 2 minute job solved a problem that persisted hardcore for weeks. I found out by proces of eliminating. must have switched the slides over somewhere along the line...
 
I had a equal problem. You have to make sure, that mixture screw and idle screw are exactly on the same position. The slider must open up at the same time. If not, there is no idle.
 
Here's what I have discovered: I got to thinking that the air being pulled into the cylinders needs to be as balanced as possible form one side to the other. The right side that was reading 120lbs of compression with the bores closed must be wore out or cracked somewhere allowing that cylinder to pull more air than normal into the system at idle. I got my hands on another set of carbs and started switching and swapping stuff around and got it running correctly (I think) by not using the carb that was on that right side. It's also the reason I've had trouble trying to tune them because I couldn't get the air screws to make a difference in the running of the engine before. I can now! I suspect maybe the throttle shaft is worn and allowing air to be pulled through. This weekend I'm gonna get the air cleaners back on and tune them up as they should be, take it for a spin and see if I've fixed it!

Thanks!
 
Brand new. And I swapped all the internals from the carb in question to a different one and it seems to function properly now. I will know more tomorrow when I can get time to mess with it some more.
 
Big Rich said:
Ignore your compression numbers for now..... but have you synced your carbs?
Click to expand...
The fact the open numbers are similar and closed numbers vary so much means carb sync is massively out
Even with jetting it won't run right, without re-jetting you will break something go look at the pods thread in 1-800 section
 
Don't think so because I didn't even have the throttle cables hooked up and the screws were backed completely off. Both butterflies were completely closed and there was still that big of a difference in the air being pulled in to the cylinders. The carbs are now pulling the same air so tomorrow I'm gonna reattach the cables set the mixture and idle and I believe I have it fixed.
 
Check intake to head, you may have broken 'O' ring or section of gasket missing (I forget which type of manifold you have?)
 
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