cb360 Backfiring from one cylinder

boykinjw

boykinjw

Active Member
I know there are probably hundreds of posts on this same question, and I have read almost all of them...So Im just looking for any last minute insight.


I cleaned and set the points and timing. Im on my second set of carbs and have probably had them off and on a dozen times in the past couple months. the last time I had them off I replaced the needle and seat that the float is attached to on the cylinder in question. put everything back together and adjusted both carbs to 1.5 turns out. The bike started up and ran great. I dont have the carb sync adapters from PJ yet so Im not sure where the carbs are. I drove up and down the street about 10 times and then took off around the neighborhood. I got about a mile and it started backfiring out of the left pipe. I took some back roads and limped home. Pulled the plug and it looked ok, it was black but not built up and not wet. I cleaned it any way and checked the gap. Put it back in and started the bike and its still backfiring on that one side. It really backfires through the exhaust when accelerating and its popping back through the carb sometimes. When its idling it pops through the exhaust. adjusting the air screw either way doest seem to help. Im thinking ignition since its doing it at idle and acceleration?


Thanks!
 
no go on the valve adjustment, I was looking in the clymers manual and I dont think my feeler gauge goes that small. Is a special set required, I dont see how I can get the feeler gauge under the rocker arm without it having a 90 degree bend on it.


Here is why I dont think its the valves but I do agree they should be adjusted. When I first bought the bike a couple months ago it was only running on the cylinder in question(left), and it ran pretty well.


The right cylinder looked like it hadn't fired in years. It seems like the roles have been reversed and now its running off of the right cylinder perfectly and the left is backfiring. I really hope it isnt a carb issue, im so tired of taking those things off.


Why would it run fine for a mile and then start backfiring if it was valves?


Just as a guess I hooked the battery tender up to it and will try it in the AM. Maybe it isnt charging the battery and the spark is getting weak after riding for a little bit.
 
Because the metal is cold and as you drive the engine gets hotter. If it needs a valve job then the clearances will get worse as the engine heats up.

You can bend and modify feeler gauges to do what you need them to if you're careful.
 
Ill see if one of the local cycle shops has a feeler gauge that will work, Ill try and adjust the valves tomorrow and update.
 
You don't need special feeler gauges, standard is fine, just roll the ends up.
Clearance is two thousandths intake and three thousandths exhaust.(0.002" & 0.003")
Easiest way to set them is watch (or feel) the intake valve go down, come up, look for TDC mark on cylinder your working on.
Use go/no-go (0.002 fits, 0.003 wont go for intake, 0/004 wont go for exhaust)
I got a real good set from Wal-Mart (checked near perfect with micrometer)
There is a good chance there was some corrosion on one side valve face/seat, as it wears off the clearance goes away
BTW, I'll have adapters done ASAP and PM you
 
I didn't see you mention how many miles are on the engine and I hope you get an easy fix here but if it still happens it may need a valve job.

Just another thought.
 
Thanks PJ, Ill stop by walmart today and see if I can pick up a set. I was wondering if it was the coil, Ive been looking at the mikesxs coils, but I wasnt sure if there were better options to stay with stock style mounting coils. I havent seen any aftermarket coils for the 360. Im hoping that if I charge the batt and update the coils it will clear up. My spark isnt fat and blue, its kinda skinny and white.
 
Pull your plug wires off the plugs and twist the ends off. Get a very sharp knife/scissors and trim about 1/4" off of the wire than replace the plug ends with new ones from the parts store. Theyll twist back in. See if that helps at all. You should still adjust the valves by the way, Im sure theyre out. And get some very fine sandpaper and gently scuff the points, and reset them to spec.
 
Also, clean and make sure all wire end are tight between the charging system/points/coils. When you do this, put a tiny dab of dielectric grease on the ends to help prevent corrosion.
 
If the parts store has this stuff Ill try and get this knocked out tonight and let you know, thanks for all of the comments.
 
Ok, I went through the valves last night and they were all tight. I guess that could be from crud build up on the valves and/or warped valves...Set them to spec, cleaned the plug with some carb cleaner and a rag. I put some dielectric grease in the connections between the points and coils. Also the battery was trickle charged the night before.


Started it up and the exhaust popped a little at first but then cleaned up. I had it choked pretty good while it was cold and could rev it without backfiring. the left pipe was noticeably colder at idle though. Once I revved it up a couple times that pipe started to get some heat into it. And after running for about 5 min it started backfiring again out of the left pipe. I tried different choke positions and idle adjustments. It was popping a little at idle and would backfire when I hit the gas. The right side runs beautifully...It was late about 11pm so I didnt want to lay on the gas but with a volt meter on the battery, bumping the throttle a little did not produce a change in voltage. I didnt get to 4000 rpm or anything like that but thought I should see some change in voltage. It was just hanging around 12.3 or so...I may have a charging problem, but Ive heard the bike wont charge until 3000-4000 rpm?


I guess my next step is to take the carbs off AGAIN, clean everything, and recheck the points and timing. The screws on the Points are pretty bad from the previous owner. I might have to look for a replacement set.


Should I get a set of Mikesxs coils and condenser?
 
If you take the carbs off again, check the float height on the carbs.. you said you replaced the float valve on the problem side which could have changed the float height.. it sounds like that left side is running rich because of the black plug and the fact that its ok when cold.. you can try running the bike till it backfires then turn the petcock off letting to let the gas level in the carb drop, if it cleans up then you will know if the float lets too much gas in the carb

If you are questioning the coil.. i would try switching the coils to see if it causes the right to backfire
 
Voltage should rise from just over tickover, so it sounds like it isn't charging. Check/Fix that first
 
Is it worth doing that radioshack rectifier swap even if mine is ok? I was thinking about going by the store today to see if they have that part in stock..
 
First things first
you didn't do any ignition work
it worked before you did the carb work
Your answer lies there (ok, 98% chance)
 
It ain't necessarily so. Most times when we do work on carbs, we knock a connection off or tweak a cable or leave a vacuum hose off. I would check that charging issue and look at all the connections along the way. If/When it's charging then see if it still missfires and THEN tear into the fuel system again.

You can't very well get the carburation right if the motor is dying with a flat battery is what I'm getting at.

BTW, did it run fine before you swapped carbs or is this the first time you have had it running?
 
When I bought the bike it ran on the left cylinder only (currently the problem cylinder). I don't think the right side had fired in quite some time. The plug was just plain bad, cleaned and couldn't get it to spark. Replacement sparked right away. The right carb was pitted really bad inside, I think the gas with ethanol was just sitting in it since there was any real vacuum on that cylinder.
I tried to clean the pitting up on the carb and actually had it decent but couldn't get it to run on that cylinder. That's when I noticed the idle air screw has a needle thing on the end and it was broken off in the carb. So that carb was toast, bought some rebuilt carbs off ebay that were in good condition. Only thing was that the left carb was leaking gas from the overflow. I checked the float height and it looked ok so that's why I ended up replacing that needle and seat. Float height looked good when I put it back together. I hold the carbs sideways and just let the float tab touch the top of the needle and measure from the highest point of the float to where the gasket hits the bottom of the carb if that makes sense. I think its supposed to be 3/4 inch.

No sides have flipped and the right side is running good and left is cold...
 
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