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My approach to diagnosing and fixing Hard Cold Starting and 1 Cylinder Running.

ALBeix

Veteran Member
Joined
Sep 7, 2021
Total Posts
170
Total likes
2
Location
Kaslo, BC, Canada
After I cleaned and rebuilt my carbs I found the bike was hard to start when cold, intermittently ran on a single cyl running when cold and was general unhappy at idle and just above idle. So given that it is raining and cool today I decided to tackle and try to resolve this problem once and for all.

My approach, once carbs were on the bench was as follows -

1 - Blow some compressed air into the inlet side (air goes into the small holes at top of throttle bore) to ensure that the vacuum operated slides were functioning as per spec - All Good!

2 - Put some gas in the bowls and ensured that the accelerator pump circuit was working on both Carbs - OK

3 -Pulled the number 1 float bowl and checked the float height. - OK'

4 - Pulled the idle jet and gave it a squirt of carb cleaner. Flowed clear. then plugged the end of the jet and gave it another shot the check the emulsion holes on the sides of the jet - All Good
5 - Ran a shot of Carb cleaner through the port the idle jet screws into to check Carb idle passages - All Good Replace Idle jet.

6 - Pulled main Jet (my 83 does not have an intermediate jet) repeated steps 4 and 5 on main jet and related carb circuits. On the main jet I opened the throttle and gave a shot of carb cleaner down the needle to ensure the needle jet flowed properly. - All Good

7 - Removed the idle mixture screw, pulled out the little o-ring and gave a shot of carb cleaner to the circuit to confirm its flow. Had to open the throttle plate to see the results.

8 - Reinstall float bowl.

Repeated steps 3-8 on the # 2 Carb. Discovered that the Idle jet on # 2 was blocked so cleared it out and tested carb circuits - now all good.

That done I set the carbs in a pan on a fresh piece of blue shop towel and filled the bowls. I left it like that overnight to ensure I have no leaks or weeping of fuel. Do not skip this step unless you really enjoy having to remove and reinstall the carbs after having fixed a leak.

Next morning I reinstalled the carbs and behold a bike that starts, idles and runs as one would expect.

I know many on here will see this as basic stuff, but my hope is it will assist some of the folks new to bike mechanics and the mysteries of Carburation.
 
As a double check on float bowl levels I drained each bowl into a small measuring cup and got 2 ounces out of each bowl. There was a very small difference between them but you had to look real close to see it. Not sure that is a bench mark, but it does tell me that both carbs are running on virtually identical fluid levels in their respective float bowls.
 
As a double check on float bowl levels I drained each bowl into a small measuring cup and got 2 ounces out of each bowl. There was a very small difference between them but you had to look real close to see it. Not sure that is a bench mark, but it does tell me that both carbs are running on virtually identical fluid levels in their respective float bowls.

If the carbs have drain screws you can attach the clear overflow line and hold it up against the carb body and loosen the drain screw. It will fill with gas and self level somewhere near the gasket surface (typically about 2-3mm below this) when set properly. You can do this to see how much gas is in each carb to see how close they are.
 
... It will fill with gas and self level somewhere near the gasket surface (typically about 2-3mm below this) when set properly. ...

Would this test apply to any model with a bowl drain? I imagine that if the float had been set too high, excess gas would run out of the tube before leveling with the gasket line, and if the float were set too low, perhaps barely fill the tube.
 
Would this test apply to any model with a bowl drain? I imagine that if the float had been set too high, excess gas would run out of the tube before leveling with the gasket line, and if the float were set too low, perhaps barely fill the tube.

Yes. With that said I've never seen it get so high that it literally flows out of the tube. If it goes ABOVE the gasket surface that does mean it's way too high.

I just walked outside and did it on my CM400A so you can see how it's done. Turn the fuel on, then attach a drain hose to the bottom and open up the drain screw. If you hold it up, as in the picture, it will level out and show you the float level inside the carb. You may get a couple of air bubbles when it first comes out, just flick it with your fingers and they will settle down. Once you're done, close the drain screw, empty the gas out of the tube, then turn the petcock off.

It's hard to tell in the picture where the fuel level is, so I've put a green arrow next to it in the picture.

20220816_134610.jpg
 
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