One cyclinder doesn't fire below 3000 RPM

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colehooey

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I have an 81 XS650, pods and Mikes Reverse Cone mufflers. Jets are as follows: Main jet 142.5, Pilot Jet of 45, Air Jet 27.5

The bike starts easily enough but the right cylinder won't fire unless reved above 3K (and even so not every time). When I do this, that pipe will start to backfire a bit and eventually start going.

My spark is good (running a Pamco). I have pulled the carbs and cleaned out the jets and what everything else I can see (I have not done a full soak and rebuild yet). I have not played with the floats either. Could either of those things be causing this issue or does anyone have any other ideas I should try first?

:banghead:
 
Check the carb sync, it could be way off. It may also be a bad spark plug or spark plug cap. Switch them side to side and see if the problem follows. The Pamco fires both plugs at the same time so no need to switch the plug wires at the coil, just swap them at the plugs.
 
www.amckayltd.com/carbguide.pdf will help you get your carbs clean. Don't soak the carbs in carb dip unless you remove ALL the rubber parts. This requires removing the throttle shafts to remove the seals.
On the side that is not running at idle means the idle passageways in that sides carbs are NOT clean.
The carb guide will help you identify your carbs to determine the year. With out knowing the year of the carbs you can't set then right. Carb adjustments are carb year related not bike year.
Leo
 
The syncs should be good and I've swapped the plugs already. It is possible that plug caps are faulty. Any ideas why the RPMs would affect any of this?
 
Yes, at idle the engine gets fuel through the idle circuits. When you rev it up the engine gets fuel through the mid range needle and needle jet.
With plugged idle passages in one carb that cylinder won't get fuel until you rev it high enough to get into the midrange.
Leo
 
XSLeo said:
Yes, at idle the engine gets fuel through the idle circuits. When you rev it up the engine gets fuel through the mid range needle and needle jet.
With plugged idle passages in one carb that cylinder won't get fuel until you rev it high enough to get into the midrange.
Leo
Click to expand...
XS Leo, are you are far too quick on the draw for me! :)
My response was directed at 5twins who had mentioned various electrical issues. That being said, how would electrical issues be affected by the RPMs?
 
If you have a plug cap going bad, which usually means it's resistance is climbing, it may not fire at lower RPMs. Higher RPMs just might push the spark through it. It's a simple matter to measure the resistance through the caps. Were they new NGKs with the Pamco? Usually, they are very good but it's always possible to get a bad component right out of the box, or shortly there after.
 
5twins said:
If you have a plug cap going bad, which usually means it's resistance is climbing, it may not fire at lower RPMs. Higher RPMs just might push the spark through it. It's a simple matter to measure the resistance through the caps. Were they new NGKs with the Pamco? Usually, they are very good but it's always possible to get a bad component right out of the box, or shortly there after.
Click to expand...
Ya they were the NGK components I got with the Pamco.
 
Your problem may be electrical but most of the time when one cylinder won't run at idle but run at higher rpms it's a carb problem, as I described earlier.
Leo
 
Hey there gents would love your imput. Trying to get the carbs tuned in and question how well right cyclinder is firing.
I am getting spark. I pulled both plugs and grounded them to the frame.
Carbs have been rebuilt and cleans again last night.
Video should explain the rest.
 
Thanks guys for the messages they have helped. I have checked all the jets for the third Tim and pushed carb clean through all the pilot hole. Problem is still present. The only thing I haven't been able to figure out is the fuel pick up on the bottom of the bowl. I cleaned it out and pushed carb clean with no problem. Then it gets to the gasket and meets this whole. It's about 5mm deep and doesn't appear to go anywhere. Could some one clearify for me. Thanks.
 

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That's for the choke circuit but it's blanked off on your right carb (as it should be). You only have one choke assembly and it is on the left carb. It shares it's rich mix with your right carb through a crossover hose. The choke feed passageways in your right carb were never drilled completely through. They didn't need to be because you have no choke assembly on that carb to use them.
 
I may have caused you to the gooses wild be chased. Some years use a choke feed from the left to the right so the right doesn't have a pickup tube or any other choke circuits, it gets a rich mixture from the LH carb choke circuit. I guess you can check to be sure the choke cross over tube is in place and air/fuel tight. 2m has an interesting idle circuit cleaning tool and and how to somewhere around here. What air cleaners filters are you running?
 
After idling for a bit shut it off and pull the right plug. wet or dry? did you hot water dip test the floats? Do float bowl fuel level tests?
 
It may be as simple as having the idle speed adjustment turned down too low on that right carb. You have two separate carbs and two throttle cables. You have to set the idle speed independently on each carb.

You also appear to still have the original plug caps? They can and do go bad. When that happens, their resistance climbs and that can choke off the spark. I would test them, measure the resistance through them. Yamaha used a rather oddball spec of like 8 or 9K ohms. Most of us replace them with 5K ohm NGK caps.
 
gggGary said:
I may have caused you to the gooses wild be chased. Some years use a choke feed from the left to the right so the right doesn't have a pickup tube or any other choke circuits, it gets a rich mixture from the LH carb choke circuit. I guess you can check to be sure the choke cross over tube is in place and air/fuel tight. 2m has an interesting idle circuit cleaning tool and and how to somewhere around here. What air cleaners filters are you running?
Click to expand...
New stock filters from mikes
Leveled the floats at 25mm
 
5twins said:
It may be as simple as having the idle speed adjustment turned down too low on that right carb. You have two separate carbs and two throttle cables. You have to set the idle speed independently on each carb.

You also appear to still have the original plug caps? They can and do go bad. When that happens, their resistance climbs and that can choke off the spark. I would test them, measure the resistance through them. Yamaha used a rather oddball spec of like 8 or 9K ohms. Most of us replace them with 5K ohm NGK caps.
Click to expand...
Even with the idle screw all the way in. I can pull the left side and engine dies. When I pull the plug and ground it on the gram it sparks well. The is jerky till I hit 3k and it takes off too. Not sure if that helps?
 
do the float dip test I find 75% or more of the brass floats are bad or soon will be (cracks) miles don't matter much.

What did you see when you removed that plug?
 

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