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Discussion Starter · #1 ·
So I got brave and rode the CB350 around for about an hour last night. I was cruising with a friend through town. I wasn't really revving it super high, not past 6 or 7k.

I had the idle set really well, maybe 2k rpm just to be safe. It'd fall from throttle back to idle really nicely like it should. However, after about 45 minutes of riding it died on me at a stop sign. I had no trouble kick starting it again, but it'd die soon after I let off the gas.

I turned each idle stop screw in 1/4 turn which earlier in the day would have meant a really fast idle. No such luck. It would take longer to fall back to idle, but eventually it'd die on me. Again, kick starting was no trouble and it fired right up. At speed, it always ran about the same as it has since I got it running. So I turned in the idle screws even more.

Now it was almost like it wanted to cruise at 2k RPM when in gear and it took even longer to fall off to idle at throttle speed. But in neutral, eventually it'd just idle down and die.

I live on a highway, so after riding home I pulled the plugs to check them out. They both looked normal. Not rich, not lean. And both cylinders were firing. I've got what looks to be clean gas going to the carbs. I have clear inline fuel filters (brand new) installed on each fuel line. So I doubt my carburetors have plugged up with anything.

I pulled the points cover off since it was night time so I could check to see if there was any arcing across the points. Nope. No arcing even at high rpm.

I'm going to try to start it cold this morning and see if it dies or if it idles super high. It should idle really fast with the screw stops set where they are now. I'm stumped. Could it be heat related? Valve issues? I haven't adjusted the valves yet. Cold, the bike is quiet, but the hotter it got the more valve clatter I was hearing. Obviously it's something I need to give attention to, and I'll get around to it shortly.

I'm just curious what would cause the bike to die off only at idle like that. I readjusted the pilot jets last night to 3/4 turn from bottomed and like I said when the bike was cold, it was running great. My friend said my right hand cylinder was puffing blue or gray smoke when I got on it. The right cylinder was the one that wasn't firing at all when I first got the bike going, but then came on board on its own. However, like I said, the spark plug looked fine when I pulled it. No sign of sooty rich or oil burning. I'll probably throw a tablespoon of marvel mystery oil in each barrel tonight to see if it can help loosen up the piston rings. Now that I've ridden it I should pull a new compression test.
 

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Hows the battery? These bikes run worse with low battery charge...
 

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Discussion Starter · #3 ·
The battery seems good. At speed, the turn signals flash quickly and slow down at idle, so I know the battery is charging. The battery itself is new. The bike started and ran great even when I first put the battery in after it sat all winter in my basement and the battery has more of a charge on it now than it did then. The battery is a brand new one I had bought last fall for my XS650 project.
 

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Have you checked the advance springs regarding inconsistent idle? Could be contributing factor (as we XS650 riders all know) ;)
 

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Discussion Starter · #5 ·
I have not. I haven't looked much at the points yet other than to clean them with emery cloth and grease the cam. Well, and to make sure the condensers were doing their thing to suppress arcing.

To be honest, I haven't studied the mechanical advance on the CB350 yet because it's different from an XS :)
 

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since your issue seems to come when the bike is hot I'd be curious about the state of the engine.
have you done a compession test when hot?
adjusted valves? (they get tighter when hot so if they are too tight cold could be getting blowby when hot.)
Also, have you rebuilt the carbs with new orings lately? if the orings are 30 years or older you may be leaking air or fuel past the jets,
float hight ok?
any holes in the mufflers?
more information about the general state of the bike in terms of tuning and condition is helpful
 

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Discussion Starter · #7 ·
I've probably been a bit over anxious to get the bike on the road. I have not adjusted or checked:

-the valves
-the points
-hot compression

I did completely rebuild the carbs including new rubber o-rings for the jets and needle seat. Carbs are completely clean and don't leak gas even with the petcock never shut off, a first for me! ;) Float heights are good, measured with my cut up credit card.

The mufflers are stock 1971 jobs I bought on ebay. They both seem to be devoid of baffles. The right one rattles around a bit when you're riding so something inside is loose. I'm using factory air boxes and air filters. Those look great and seal up well.



The state of the bike is that I got it for 100 bucks and it hadn't been run since the 80s. Electrically I was surprised that it seems to be working fine. Every electrical item on the bike is working. When I got the bike it had a CB350 engine with only 20 psi in the right cylinder, so I bought a 72 CL350 engine for the bike that had 5200 miles on the clock and was stated to have been 'running a week ago' before the guy parted out the bike. The engine had cold, unrun compression of 160psi in each cylinder. I need to check it now that I've ridden the bike for an hour.

I'm going to check cold valve clearances at lunch time. Like I said, it was quiet (other than the cam chain noise) until it got hot and then it started to get valve clatter in both cylinders. Perhaps the valves are too tight and you're right about losing compression with a heated engine.
 

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Idle jets are clogged? Tank vent clogged, timing slipped, air leaks at carb?

does it idle with choke on?
 

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Discussion Starter · #9 ·
Without taking the carbs apart I can't guarantee the idle passages are not clogged, but I cleaned them out with piano wire last week and had everything apart. It does die off even with the choke engaged. I did clean the gas cap vent when I cleaned out the tank. It was hissing when I was shaking gas around in there, so I am pretty sure it vents OK. Also, my fuel filters both showed gas in them as did the float bowls when I turned out the drain screws to make sure they weren't dry.

Slipped timing chain was my first fear when it happened. I was goosing the engine in 2nd gear screwing around immediately before this symptom presented itself. And this is with me having no idea about timing belt tension adjustment or the condition of the tensioner system. Since I'll have the cover off of the stator cover off to check the valves, is it easy to verify that your cam chain has slipped by using a timing light? If so, what should I be looking for?
 

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from what you've described a couple things come to mind:
if your mufflers have had the baffles deteriorated, that means less back preassure which could require more fuel. this would require bumping up a size or so on the pilot and possibly even the mains.
before I do that though I'd rule out other possible causes first (don't fix the symptom, fix the problem).
the only way to do this is by methodically eliminating other tuning areas

Points adjusted properly with timing set
good compression hot
proper tension on the cam chain
good spark
ignition advance functioning properly
valves adjusted properly
carbs properly sealed (rubber intake boots in good shape, vacume slide diaphrams in good shape with no holes)
Once you can check off each of these items as being properly done then you can go into the carbs and rejet for the missing baffles (assuming the issue isn't fixed by any of the above.)
 

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Discussion Starter · #11 ·
:lol:

That's what I knew you'd say :) And yes, since it's an old engine with unknown history, who knows what the last guy set the stuff up like.

I hate messing with points even though I understand how to adjust them. I'm going to focus on the valves and hot compression numbers first. If it runs cold the same way it ran hot last night, I'm going to focus on the cam timing too. Because things went from one state to the next all of a sudden.

As far as the mufflers go, the bike seems to be running rich if anything. The plugs were a dark brown color at the electrode with soot on the outer rings after running home at highway speeds at 5,500 RPM. Both plugs looked identical too, which was nice.

My first inclination was that my right cylinder was acting up like it had when I first got it running and I was expecting to find that plug either wet with gas or black with burned oil, but neither presented themselves thankfully.
 

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Discussion Starter · #12 ·
I tried to check valve clearance over the lunch hour, but I realized spec for the CB350 is 0.05mm and 0.10mm. I don't have feeler gauges that thin. Do auto parts stores typically carry something that thin?

I think all three sets of feeler gauges I own start at about 0.25mm in thickness.
 

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Yep.

My cheapie feeler gauges (3 different sets I've accumulated over the years) go to .001". .05mm is .002" and .10mm is .004". I've never even seen a set that didn't go down that low.
 

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Discussion Starter · #14 ·
I got a set of feelers at Napa today that have the paper thin ones necessary to do the job.

I got the valves adjusted. Hot compression is only 115psi in the left cylinder and about 90psi in the right one. So out comes the snake oil. I put a squirt of Marvel Mystery Oil in the jugs over night to see if it helps unstick the rings. I drained about 3/4 quart of oil out of the stator opening today and it looked like black shampoo with silver sparkles everywhere. I just put in fresh Rotella T. So it's eating itself at the moment. I suppose that's to be expected when bringing back old stuff from the dead.

I made a running video showing myself operating the throttles independently of one another. The left cylinder with the slightly higher compression is smoother. The right cylinder is kind of laggy. At the end of the video, I flip on the choke which kills out the warmed up engine pretty quickly.

[youtube:2kdoyleh]z9eDpxYTnGs[/youtube:2kdoyleh]

The plugs after this somewhat meaningless test looked fine to me. The electrodes are the sort of color I expect in something that's running right.




What's the verdict? Too low on the compression numbers. I don't really want to mess with the engine, so I'm going to keep trying old tricks to see if it just needs to get its legs back. I've got too many other projects at the moment to be tearing down the engine and boring out the jugs. I also have that non-running CB350 engine for it that I'd probably do the rebuild on.

Oh, I forgot to mention, tonight it wasn't dying on me like it was after riding it for a long time. I couldn't ride it since we had a thunderstorm here though. It felt doggy when I rode it around the block though. Not all is well mechanically.
 

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It may not need boring. Sometimes a hone and new rings will do a lot.. Depends on the condition of internal parts really..


GB :mrgreen:
 

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The "doggy' feel is probably the bad compression.....170 is normal, and it might be as simple as cleaning up, a quick hone and new rings.....
 

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Discussion Starter · #17 ·
What I'll probably do is pull apart the CB350 engine I got with it. It only had 20psi of compression in the right cylinder. Since it's out of the bike it'll be a good lesson on the internal workings of the engine. I can get familiar with the tensioning system and whatnot. I can do some head work with new valve seats and valves if it needs it. I have a feeling that engine has a stuck valve because it was frozen when I got it and I had to dump ATF in the jug over night to set it free by rocking the bike back and forth in 2nd gear.

This is where most of my projects officially slow way down ;) If I can't just get on and ride stuff hard, I usually lose interest. I have mechanical ADD, ha!
 

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one quick question...when you did your hot compression test did you have the throttle all the way open? a closed throttle will have a big impact on compression.
I run VM roundslide carbs on mine and with the throttles closed I get 120 PSI but with them open I get 190PSI
 

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Discussion Starter · #19 ·
I had the throttles closed all the way. Also, I was using the kick starter since I don't have the electric starter hooked up. I forgot to keep them open to get a good draw of air into the jugs.

I will re-attempt it tonight.

The right cylinder is missing about half of its spark plug threads. As you guys probably know you can't simply use a spark plug socket to put the plugs in without removing the valve adjuster cover. Looks like someone tried that and gauled half of the threads before they gave up and backed the plug out. Fortunately that bottom half is in tact, but I wonder about compression. I'm not putting any more than about 8 foot pounds on the plugs because I don't want to strip the right one out. I might pull the heads and helicoil it if I get really ambitious.

I read the service manual this morning and the engine is simple enough that I think I'm going to tear into the CB350 engine tonight to examine the valves and cylinders on that engine. If it's only in need of a hone and a valve lap, I will probably do that work this weekend.
 

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Discussion Starter · #20 ·
Over the lunch break I didn't have time to put the plugs in and fire it, but cold compression numbers are way up when you hold the throttle open.

190psi at both cylinders. This is a cheapie Advance Auto compression tester. The more important fact: Both cylinders had the same reading. I will try tonight with a hot engine.

Perhaps my Marvel Mystery Oil in the cylinders overnight have loosened up some sticky piston rings? We can hope.

 
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