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72 CB 350 not idling all of a sudden

IOMTT

IOMTT

Veteran Member
Joined
Jun 24, 2020
Total Posts
169
Total likes
7
Location
Colorado
Sorry for the length of this.
Last week I went out and started my 72 CB 350 to get some fluids running through it and since it hasn't been ridden in about 2-3 months because it's winter here in Colorado. The bike once warmed always idles at 1200 rpm as it should and was idling at that speed for about 10-15 minutes after being started when all of a sudden it jumped to 2,000 rpm for no reason ( I'm sure there's a reason ) and I couldn't figure out what was going on so I finally just shut it off.
Today, I go out to the garage and try to start it the usual way, chokes on, fuels on and gas is in the tank, kill switch to the "on" position, keys on and I hit the starter. The bike fires up and I need to give it just a bit of throttle ( as usual ) for about 20-30 seconds to keep the revs up and then I can shut the chokes off and the bike idles fine..............until today !
When I went to shut the choke off, the bike wouldn't idle and even trying to give it throttle, the engine was laboring and finally died. I tried starting it up again without the chokes and I started getting backfire out the exhaust and still it wouldn't idle. Tried the choke on again but still would not run unless I gave it a good twist of the throttle. Shut it off and discovered that the left exhaust pipe was red hot and the right exhaust pipe was barely luke warm.
This sounds to me like a possible timing issue ????? The bike has been running great for the last couple of years up until now (today). Could the timing just go off all on it's own without anyone messing with it ?
Any help or advice would be greatly appreciated.
Thanks in advance.
 
Sounds like you got something in a passageway in one of the carbs. I would pull the carbs, clean them out with some carb cleaner spray, blow all the passageways out, and see what happens.

You are correct that the timing wouldn’t just out of the blue get wonky for no reason. Is your battery on a trickle charger?


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A few things occur to me... first, I'm hoping you shut off the fuel at the petcock when you put the bike up. Second, to go along with that for planned inactivity for more than a few weeks, you should drain the carb bowls or run thew bike until the carbs run dry (better to drain them though). And before you start the bike after the layoff, you should check the oil to be sure the level is still correct and not overfull due to fuel leaking past a shut off petcock, and smell the fuel as well to be sure. Even if the level isn't over normal, it could still have some fuel in it and that's bad news for top end lubrication. As far as points are concerned, if the bike was stored at ambient temperatures (pole barn or other storage with open sides or no insulation) and the temperatures varied a fair amount, the points could get corroded a bit and the advancer could get sticky from light rust if not lubricated well enough prior to being parked. Even if the advancer is okay, if the points are corroded (even on one side) that will affect spark and could easily be what is keeping the one cylinder from firing. Points gaps don't close on their own, but rust or corrosion between the set that was closed all that time (and there's usually one set open almost every time the engine is turned off due to the design of the breaker cam lobe on the advancer) would cause that cylinder to either not fire at all (cold exhaust pipe) or fire poorly and the timing would be affected by that as well. So, you have a few possibilities to check out. And yes, as mentioned above, these engines need top battery voltage to run properly so if the battery wasn't on a maintainer it could have gone bad due to slow discharge and staying that way.
 
The battery is new (replaced in September with a new Motobatt Gel ) voltage was at 12.5 when this startup issue starting occurring and went to 12.9 after it starting running ( though it didn't run for very long ). It hasn't been on a tender and started up and ran fine last week until the issue with the rpm's going to 2,000 at the end. I've heard from others on this site that you don't need to keep a Gel battery constantly on a tender.
The carbs are Mikuni's , but I ALWAYS turn the petcock off and let whats left in the lines run through until the engine dies after I'm done riding.

Tom - I guess I'll take the cover off and check the points for any corrosion issues and also check the advance as well. What should I use to lubricate the advance with and where should I lubricate it at ?
 
Don't know exact numbers for the Motobatt Gel, BUT, a normal lead acid battery reading anywhere below 12.6 needs charged or is dying.......
Defective batteries are somewhat common, with perhaps a 10 to 15% failure rate from new with the lead/acid ones........
 
IOMTT said:
Tom - I guess I'll take the cover off and check the points for any corrosion issues and also check the advance as well. What should I use to lubricate the advance with and where should I lubricate it at ?
Click to expand...

Use a little bit of motor oil to lube the breaker cam lobe on the center shaft and the pivot points for the weights, if you use grease on the center shaft it can cause slow reactions as the engine rpms drop. To service the advancer you'll need to remove the points plate (meaning you'll have to set the timing again afterward as there's enough looseness in the tolerances between the plate and base that the points gaps will not be the same when you put things back together), then take the bolt out of the end of the camshaft to remove the advancer unit from the cam. If it's sticky on the shaft you might end up pulling the breaker cam off the center shaft, and know that it can only go on one way for the bike to run afterward. If you put it on 180° out it will backfire and not start, so take note of the position of the lobe. There's a dot in the top edge of the breaker cam that aligns with something on the base of the advancer, can't recall the arrangement as I haven't seen the picture in quite a while but others here know what it is. As for the points, you can sand them lightly with fine sandpaper or emery cloth and then clean the contacts with a clean piece of paper run between them a couple times, or wash them with contact spray
 
While not necessary to keep those batteries on a tender it's not a bad idea. At least connect one monthly to bring it back to full charge. Also it is a good idea to keep the battery warm, freezing temps are really bad for batteries.
 
So, I threw a battery tender on it a few hours ago and we'll see how the bike responds tomorrow with a ( hopefully
) fully charged battery and if I'm still having issues I'll take Ancient Dads advice and look at the points and advance untit.
 
OK, well though the tender may have charged up the battery a bit it didn't help with the engine issue. I checked the points and they were fine, no rust or crud in them when I ran a piece of paper through them. I didn't check the advance.
Now, here's the weird part...
I tried starting the engine again and like the previous times it would somewhat run when choked and then as soon as I take the chokes off the engine labors and dies even while trying to give it throttle to run. SO,...just for grins and giggles I opened up the gas cap and started it up (engine choked) and it seemed to run a bit better and after about 20 seconds I shut the chokes off and give it a bit of throttle for a few seconds and WALAA, the engine stars idling on it's own at 1,200 rpms ( as it should ). Now you may be thinking that the issue was the gas cap vent being plugged but that isn't the issue as the gas cap is brand new !!
So I continued to let the bike idle and as it continued to idle the idle began to creep upward, first to 1,400 rpm then 1,500 then 1,700 then would come back down to 1,400 and then work back up to 1,700 again. I closed the gas cap thinking maybe that had something to do with the erratic idle but it made no difference. I shut the bike down completely and came back an hour later to try again and once again I choked the bike ( it was very cold in the garage ) and it started up and again after about 20-30 seconds I shut the chokes off and the bike idled at 1,200 rpm ( as it should !! ) but after a minute or so the idle began to creep up again as it did before and trying to blip the throttle to get it back to 1,200 was fruitless.
So I believe that the increase in idle would be the sign of an air leak somewhere ??? But I have no idea where, as the bike idled fine a week ago and I have not touched a thing on it between then and now.
Thoughts ??
Again,thanks for any insight anyone has to offer.
 
While the bike is idling, spray carb cleaner around the carbs, manifolds, insulators, etc. If there is an air leak the idle will increase when the spray hits it. Some folks use an unlit propane torch.
 
IOMTT said:
OK, well though the tender may have charged up the battery a bit it didn't help with the engine issue. I checked the points and they were fine, no rust or crud in them when I ran a piece of paper through them. I didn't check the advance.
Now, here's the weird part...
I tried starting the engine again and like the previous times it would somewhat run when choked and then as soon as I take the chokes off the engine labors and dies even while trying to give it throttle to run. SO,...just for grins and giggles I opened up the gas cap and started it up (engine choked) and it seemed to run a bit better and after about 20 seconds I shut the chokes off and give it a bit of throttle for a few seconds and WALAA, the engine stars idling on it's own at 1,200 rpms ( as it should ). Now you may be thinking that the issue was the gas cap vent being plugged but that isn't the issue as the gas cap is brand new !!
So I continued to let the bike idle and as it continued to idle the idle began to creep upward, first to 1,400 rpm then 1,500 then 1,700 then would come back down to 1,400 and then work back up to 1,700 again. I closed the gas cap thinking maybe that had something to do with the erratic idle but it made no difference. I shut the bike down completely and came back an hour later to try again and once again I choked the bike ( it was very cold in the garage ) and it started up and again after about 20-30 seconds I shut the chokes off and the bike idled at 1,200 rpm ( as it should !! ) but after a minute or so the idle began to creep up again as it did before and trying to blip the throttle to get it back to 1,200 was fruitless.
So I believe that the increase in idle would be the sign of an air leak somewhere ??? But I have no idea where, as the bike idled fine a week ago and I have not touched a thing on it between then and now.
Thoughts ??
Again,thanks for any insight anyone has to offer.
Click to expand...

If after you attempt to find an air leak if you come up empty handed, as I think you may, I would check the carbs by pulling them off, check the floats, float needles, jets, idle/fuel screws. I would also check the vacuum cylinders to make sure they move freely and don’t stick as well as the air cutoff valves to make sure they move freely and aren’t ruptured or torn. Spray all the brass bits with carb cleaner and then blow the passages out with compressed air from whatever source you have for that.


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The carbs on the bike are brand new Mikuni's which have only been on the bike 3 years along with new boots/insulators and running ethanol free gas and have less than 300 miles on them.
 
66Sprint said:
Don't know exact numbers for the Motobatt Gel, BUT, a normal lead acid battery reading anywhere below 12.6 needs charged or is dying.......
Defective batteries are somewhat common, with perhaps a 10 to 15% failure rate from new with the lead/acid ones........
Click to expand...

agree, I have a new lead acid battery, a month old and it reads 12.52-12.58 after a few days sitting in sud-35 degrees F.
 
So you've got Mikuni carbs, a brand new gas cap and what shape are the coils, plugs and plug caps?

i would first check the spark advance mech isn't sticky. Just quickly remove the points cover and jiggle it with your fingers

next I would pull the plugs and inspect them, maybe replace them. Same with the ignition wire... trim it not replace it.

Check the primary and secondary on the coils and make sure they're in spec. I had a brand new coil that wasn't firing, the give away was the poor idle and drivability, along with spitting fuel out the pipe and fouled plug.

Last balance and sync the carbs. This would actually be my guess to the issue. The other stuff is quick and easy to check. Ride the bike til hot and get the carbs dialed.
 
Anyone consider moisture (water) in the fuel?.... Doesn't take much ice to hamper fuel flow in cold temps.....Just a Thought.....
 
IOMTT said:
OK, well though the tender may have charged up the battery a bit it didn't help with the engine issue. I checked the points and they were fine, no rust or crud in them when I ran a piece of paper through them. I didn't check the advance.
Click to expand...

if your getting backfiring/popping like you originally described...I would sure check the advance. Just a thought....
 
Do a quick cylinder balance test with a screwdriver at the points. With the engine running, use the screwdriver to ground the points wire connection to ground for a few seconds. Do one side and then do the other and compare the rpm drop between sides. If the carbs are balanced there should be an equal drop in rpms on either side, the dead/low cylinder will drop fewer rpms.
 
So today I start the bike and it idles at 1200 rpm ( as it should ) for about 5 minutes. Then after 5 minutes the rpm starts climbing to 1400 then 1500 then 1700 then drops back to around 1500 and stays that way.
If I spray starter fluid in the left carb it kills the engine, and if I spray it in the right carb nothing really happens and the engine keeps running. What would that mean ?
Also, would outside air temperature effect the idle ? It was around 30 degrees.
 
My first thought is that except when these bikes were new, they generally don't idle right away when cold (especially in that temperature) so setting the idle speed so it will idle cold generally results in a higher than desired idle once it warms up. J-T's suggestion was to spray around the intake boots at the carbs and head to check for vacuum leaks, not directly into the carb.
 
If nothing at all happens when you spray starter fluid in the carb while it’s running then there’s no spark on that side. The RPM’s should rise. If it kills the engine then I don’t know. I’ve never had that response to starter fluid before.


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ancientdad said:
My first thought is that except when these bikes were new, they generally don't idle right away when cold (especially in that temperature) so setting the idle speed so it will idle cold generally results in a higher than desired idle once it warms up. J-T's suggestion was to spray around the intake boots at the carbs and head to check for vacuum leaks, not directly into the carb.
Click to expand...
Tom, I haven't touched the idle on the carbs since they were set 300 miles ago ( in warm weather I believe ).
And, oh dopey me !!!!! I'll try spraying the starter fluid AROUND the intake boots at the carbs instead of what I did !!
 
When I put my hand over the right carb intake, the idle slowed down and the right exhaust was red hot.
 
There’s an issue with the right carb getting too much air and running lean. Pull the carbs and clean them out. I’m willing to bet that the right one has something lodged in a passageway somewhere keeping something open. Just my guess. I had the same issue but with the stock carbs.


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I think there's more than one issue here, prob carbs and vacuum leak. Also, I'm not convinced the new cap is venting properly. The more fuel there is in the tank the quicker this will be noticed. A low amount of fuel in the tank may take some time to build enough vacuum to affect running. Manifold connections are suspect
 
slofut said:
I think there's more than one issue here, prob carbs and vacuum leak. Also, I'm not convinced the new cap is venting properly. The more fuel there is in the tank the quicker this will be noticed. A low amount of fuel in the tank may take some time to build enough vacuum to affect running. Manifold connections are suspect
Click to expand...

I was having issues on my 72 CB350 and finally narrowed it down to gas tank venting issues. The original cap was completely clogged. I bought a new gas cap and still had issues - I about lost it! Then I noticed something on the new gas cap underside - the small hole in the vent assembly itself seemed clogged. There was some type of fiber-mesh material in there. I grabbed a thin finishing nail and proceeded to clear it out. Worked like a charm.
If these gas tanks aren't venting properly, there can be a number of different symptoms that can definitely send you down a rabbit hole.
 
Well, today was finally a warmer day so decided to put the battery back in and fire it up and see what happens today.
Started up but as soon as I took the choke off it died. Pulled the plugs and they looked pretty black so I replaced them with better plugs and fired the bike back up and finally got it to idle though am still having issues with it having a bit of erratic idle. Idle goes from 1,200 to 1,400 to 1,700 back down to 1,500 rpm as it had been doing previously. Took the bike out for a short ride and it runs great, no hesitation or backfiring or anything,....... just this weird idle issue. And as stated earlier in the thread , have not touched a thing on the carbs or anything else on the bike. One day it was fine the next wonky !! Don't know if the awful cold had anything to do with it or not but the warmer weather still hasn't helped the idle issue.
 
I feel your pain, mine does the same thing. I have changed everything In the ignition system, Rebuilt and checked the carbs twice. Made sure the throttle cables are synced, rechecked the valve clearance, and checked ignition timing three times. Changed the intake boots and gaskets, new plugs, and spark plug wire caps, checked compression, no vent cap restrictions, new OEM air filters and boots. The only thing I haven’t changed is the carb diaphragms, but they were pliable and had no tears or holes in them. I think they are the original diaphragms installed when the Carbs were new. I will probably change those next week and re-check the slides again to make sure they are moving smoothly. I wish there was a way to check the vacuum on these old carbs.
 
Old carbs???
I've got brand new Mikunis and you can't check the vacum on them either !
 
Jetmech said:
...The only thing I haven’t changed is the carb diaphragms, but they were pliable and had no tears or holes in them. I think they are the original diaphragms installed when the Carbs were new.
Click to expand...

From my experience, it's not necessarily immediately apparent visually that your diaphragms need replacing. Mine were pliable with no cracks, splits, etc. and seemed 100% until I held them up to my shop light. 50 years is a long time for rubber, especially thin rubber that sees so much action.

a0f0a237fabfadf95798842927b07157.jpg
 
I don't think the diaphragms will affect idling. Intake boots... were they reproductions? They have the indented ring inside to accept the carb? Use spray carb cleaner when its idling to find your vacuum leak.
 
slofut said:
I don't think the diaphragms will affect idling. Intake boots... were they reproductions? They have the indented ring inside to accept the carb? Use spray carb cleaner when its idling to find your vacuum leak.
Click to expand...

yes, reproductions intake boots. I will check for leaks around them. And yes if the diagrams are bad it wouldn’t effect idle, but for sure acceleration.
 
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