Cafe Racer Forum banner

1 - 19 of 19 Posts

·
Registered
Joined
·
51 Posts
Discussion Starter #1
I'm working on a cb500t and am having a few issues with the left cyl only firing when the throttle is applied, or the bike is above 2500 rpm. Picked the bike up with 12k on the clock. Was an okay runner but had an issue with firing on the left cyl. I picked it up pretty cheap so I figured I'd be fine spending a bit to get things like I want them. Swapped the carbs with mikuni vm 34s with 180 mains. The carbs were built specially for the Cb450t and 500t. That being said...I know the carbs aren't the issue. It has a new battery, new rectifier, and the harness is completely intact. At idle the engine not only sounds like it is only firing the right side, but it also vibrates more, and the left header is cold. At around quarter throttle and above the right starts to fire like it should. Should I suspect the coil, the plugs, or the points? Any ideas are helpful.
 

·
Registered
Joined
·
1,537 Posts
Have you checked valve clearance?
 

·
Registered
Joined
·
659 Posts
I wouldn't rule out the carbs.

If you pull the plugs and turn the engine over do you have spark on both sides?

You may have to adjust the idle mixture screw. Just because they are set up for the 450/500 doesn't mean they are ready to run.
Idle speed might need adjustment. Are the slides synched as in when you open the throttle are they opening at the same time or is one lagging behind the other?
 

·
Registered
Joined
·
51 Posts
Discussion Starter #5
The original carbs did the same thing. Went through an ultrasonic bath, cleaned all orifices, and synchronized.
 

·
Registered
Joined
·
1,497 Posts
My 350 was doing the same thing, and it turned out to be a loose connection on the small wires going to the coils.
Squeezed the connector with pliers and plugged it back up, and it runs like a top.
It was intermittent because it was touching most of the time, but just barely.
 

·
Registered
Joined
·
840 Posts
Often times the wire on the coil / spark plug cap will corrode and cause this. If they are NGK hard plastic type unscrew the cap cut the wire back 1/4 of an inch and screw the cap back on.making sure the point of the screw goes in to the wire.
 

·
Registered
Joined
·
1,731 Posts
Often times the wire on the coil / spark plug cap will corrode and cause this. If they are NGK hard plastic type unscrew the cap cut the wire back 1/4 of an inch and screw the cap back on.making sure the point of the screw goes in to the wire.
My CB500T would not run at all. Turned out the plug caps were no good under load. A pair of new NGK caps later and it was running great.
 

·
Registered
Joined
·
573 Posts
Another issue with the 450/500T is that the points can not be timed individually, except by adjusting the point gap. Not sure how many other twins are like that. I would check the static timing on both sets of points (after of course making sure the plug on the cold side actually had any spark at all at idle). Then check the point gap on both sets. If I am remembering points correctly, a smaller gap will open later. I would adjust the gap on which ever was smaller (make it bigger) till the timing was synced, since the gap will get smaller with use, then time it by rotating the plate till it is at the F mark
 

·
Registered
Joined
·
2,072 Posts
Often times the wire on the coil / spark plug cap will corrode and cause this. If they are NGK hard plastic type unscrew the cap cut the wire back 1/4 of an inch and screw the cap back on.making sure the point of the screw goes in to the wire.
44s correct. I would nearly guarantee this is the culprit. Every honda I've owned that was older had this issue at on point or another...........usually caps or wires but also like the Chief said.......connectors, coils, etc. Also just for good measure..........make sure those carbs are getting fuel. They may be being starved on the left.
 

·
Registered
Joined
·
51 Posts
Discussion Starter #11
So the valves are set vm32 carbs have been set up and bench sync has been done. Both spark plugs have spark. Cold compression is 15 pounds off of what the specifications say it should be warm. Still doesn't run on left cyl. Would the caps do this? Starting to get pretty frustrated. Haha. Any tips or ideas at this point?
 

·
Registered
Joined
·
2,107 Posts
Do both plugs have a nice fat spark? There are cases where there is a spark, but not enough to light things under compression. Caps are worth a shot. At this point, if it were mine, I would introduce some propane into the left side at idle and see if it starts firing on that cylinder. If it starts firing, then you have a fuel problem. Something like maybe a bit of old o-ring jammed up between the idle mixture screw and the low speed circuit. I can't remember if that bike has a wasted spark and I'm too worn out from working on my own shit to look it up. If it does have a wasted spark, you could switch the plug leads and see if the problem goes to the right side.
 

·
Registered
Joined
·
573 Posts
I can't remember if that bike has a wasted spark and I'm too worn out from working on my own shit to look it up. If it does have a wasted spark, you could switch the plug leads and see if the problem goes to the right side.
No, almost all Honda twins are 180 degree with a separate coil and points for each cylinder
 

·
Registered
Joined
·
573 Posts
Have you checked the timing for each set of points? Just because they have the same gap, and are on the same plate does not mean they are both timed correctly.

Also, when you swapped the carbs did you use new fuel lines? If I recall correctly this bike has 2 outlets on the petcock, and a line to each carb. It is possible the line is kinked, or one outlet is clogged or partially clogged. Make sure you have the same fuel level in each carb
 

·
Registered
Joined
·
1,231 Posts
Have you checked the timing for each set of points? Just because they have the same gap, and are on the same plate does not mean they are both timed correctly.

Also, when you swapped the carbs did you use new fuel lines? If I recall correctly this bike has 2 outlets on the petcock, and a line to each carb. It is possible the line is kinked, or one outlet is clogged or partially clogged. Make sure you have the same fuel level in each carb
switch the fuel lines at the
petcock and see if the problem moved to the other side.
 

·
Registered
Joined
·
51 Posts
Discussion Starter #17
New lines and petcock was rebuilt when bike was redone. I will however try the swapping of the coils. Thanks a bunch.
 

·
Registered
Joined
·
2,740 Posts
A couple of clues you stated - above 2500rpm's the ignition is fully advanced. Check for spark with another plug in the side that's not running and turn the engine over. If your getting a spark - then problem is coming from the low speed side of the carb where there is no gas flowing through. Gas start flowing through the mains after 1/4 throttle.
 

·
Registered
Joined
·
51 Posts
Discussion Starter #19
Brand new carbs, both flowing fuel through idle circuit and at full throttle. Thinking it's an electrical issue. Coils, points, or plug caps. Valve clearance is at .0035. A hair over spec. But just as a saftey precaution to take the chance of premature wear out of the picture.
 
1 - 19 of 19 Posts
Top