Vacuum leak

[Robert Dell Aquila]

Hi everyone, hope all is well. So I have been working on my 78' through the winter so I can ride for spring, shes almost there I can feel it. However, I noticed something strange. The LH cylinder is running a lot hotter than the RH. When I spray the RH with carb clean the rpms shoot up. And when I spray the LH the bike dies. I have already replaced the throttle shaft seals. I tried spraying the carbs with two different manifolds I have (they're not in the best condition) the results were the same. New gaskets for the choke plunger would also be a good idea. Wanted to get everyones opinions before I started dropping more money. I just don't know why the rpms would shoot up on one side and then completely die on the other? Any insight would be appreciated. Thanks

 

[Bushyeyed1]

Time to tear down the carbs and see what's up. Besides the old o-rings and gaskets the floats probably need adjusted too. If there is old gas residue then it needs the coking cleaned out. And lastly you may want to check the slide diaphrams. Your carb sync may be off if one of the muffler is flowing exhaust more then the other. There is two good articles in the tech section to help you out. Then again I can tell you with my build I fix one thing and the next shows itself.

 

[xjwmx]

One side running a lot hotter than the other, if you're going by touch at least, makes me think the cold side isn't running at all, if that's a clue. Yanking the wire off the hot one would confirm that.... Make sure both carbs are seated in the boots all the way. Check compression on the cold side if need be.

 

[Robert Dell Aquila]

Time to tear down the carbs and see what's up. Besides the old o-rings and gaskets the floats probably need adjusted too. If there is old gas residue then it needs the coking cleaned out. And lastly you may want to check the slide diaphrams. Your carb sync may be off if one of the muffler is flowing exhaust more then the other. There is two good articles in the tech section to help you out. Then again I can tell you with my build I fix one thing and the next shows itself.

I've torn down the carbs already. Removed all gaskets and dipped them in carb cleaner. I replaced the needle jet o-rings and idle screw o rings. Slides move down slow15 seconds with thumb over the hole. I do need to sync the carbs better. Ive just done a bench sync.

 

[Robert Dell Aquila]

One side running a lot hotter than the other, if you're going by touch at least, makes me think the cold side isn't running at all, if that's a clue. Yanking the wire off the hot one would confirm that.... Make sure both carbs are seated in the boots all the way. Check compression on the cold side if need be.

Yea I had a problem with my RH carb over flowing and just dieing out. Turned out to be the float valve needle. I replaced and the over flow stopped. I checked the spark on RH plug and it looks good. I'll do compression test again. Last time I checked it wasent too good. About 120-130 both sides.

 

[Robert Dell Aquila]

One side running a lot hotter than the other, if you're going by touch at least, makes me think the cold side isn't running at all, if that's a clue. Yanking the wire off the hot one would confirm that.... Make sure both carbs are seated in the boots all the way. Check compression on the cold side if need be.

Just checked compression on both sides. About 135-140 for both cylinders. I'm leaning torwards a carb issue. Maybe something is blocking fuel from getting in. The thing is when I start up the bike the RH exhaust pipe feels hot when I touch it and then eventually it gets cold to the point where I can grab my exhaust and not get burned. Meanwhile the LH is blazing hot (probably running lean).

 

[xjwmx]

The thing is when I start up the bike the RH exhaust pipe feels hot when I touch it and then eventually it gets cold to the point where I can grab my exhaust and not get burned.

See if this works for ya. When you start it up you're revving the motor which brings the main jet into the picture. While you let it sit running at zero throttle, it's all pilot jet. So the cyl runs and warms up when you're using its main jet, but when you let off the throttle and let it sit the cyl doesn't run, and it cools off. That says the pilot jet is clogged.

Could also be something electrical though, maybe related to charging voltage when revved, or points setting or something else. Could try switching sides with the wires and plugs.
P.S. could also be a weak coil if you have two coils. Switching plugs and wires will tell you if the prob is carb or electric though. Or both. LOL

 

[mrtwowheel]

Intermittent ignition, coil, TCI, plug wire/cap, plug.

Scott

 

[Robert Dell Aquila]

See if this works for ya. When you start it up you're revving the motor which brings the main jet into the picture. While you let it sit running at zero throttle, it's all pilot jet. So the cyl runs and warms up when you're using its main jet, but when you let off the throttle and let it sit the cyl doesn't run, and it cools off. That says the pilot jet is clogged.

Could also be something electrical though, maybe related to charging voltage when revved, or points setting or something else. Could try switching sides with the wires and plugs.
P.S. could also be a weak coil if you have two coils. Switching plugs and wires will tell you if the prob is carb or electric though. Or both. LOL

Yea I'm leaning torwards pilots as well. I actually noticed that my pilots look different and the RH carbs pilot was a little difficult to remove. I have a spare set I'll throw in and see if that changes anything. Thanks for the insight

 

[xjwmx]

If the problem is the pilot circuit, could also be the pilot screw way out of adjustment rather than the pilot jet or passage. Not a carb expert at all though. In fact have never seen a BS38 (up close).