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Discussion Starter · #1 · (Edited)
I read LWRider's thread (So Close and Yet So Far - CB 350) and saw lots of great troubleshooting advice. I am hoping there is some good advice for me.

I am bringing back to life a 1972 Honda CB 350, which was my father's bike (I had an identical one back when I was 18). This bike has not run since mid 1990's. I rescured it from his garage after he passed in 2001, and it sat in my garage until this summer, where I finally went to work on it.

The fuel system was pretty gunked up. Acid bathed the tank and cream lined it. Cleaned the petcock. Fortunately the petcock was off so the carbs were nice and clean - but dry The gaskets had shrunk. Found carb kits - replaced parts, re-installed the carbs plus a new throttle cable. Threw on the tank, put in some gas and guess what - third kick - she sputtered to life. I was impressed and feeling heady.

So of course it can't be that easy for me and there is a problem. The bike starts in a single kick. It idles well, It revs well, runs real well, accelerates strong - everything you can ask for.... for about 10 minutes. At around that time the bike dies out. It will not start again until everything you can touch is almost cold. In fact while letting it cool down, about half through the cool down time I can start it and it will run idle for a few seconds. The longer you let it cool the longer it will idle each time before it sputters out. But until it complety cools down if you try to apply any fuel (rev it) it sputters off, as if I had hit the Kill Switch. When it is finally room temperature cool again the performance pattern as described in this paragraph starts all over.

Time (and therefore maybe temperature) seems to be consistent. Unlike LWRider's problem where he stalls out at around a certain speed on the highway, I can be standing at idle, or going at varous speeds. It will simply shut down when I assume I reach a certain engine termperature, in what feels like around 10 minutes. I have not actually timed it, I should.

I also replaced the petcock gasket and spring with original OEM parts. Fuel easily flows from each petcok barb. I don't think the petcock is blocked because I get easy RPM's, until time has passed.

I feel it is being starved for fuel and not flooded. I do not smell any fuel when the engine dies. This reminds me of what I used to hear about in auto engines of old - perhaps a vapor lock of some sort?

I did replace one float with a new one - the old one having a hole in it. I found a brand new ORM float online, and I did not adjust the float tab, but it was pretty darn close to the tab from the other carb which I had not touched.

Also, I have not put the air filters back on yet nor the covers. I don't think their being absent would be a problem - maybe so.

Any ideas. This problem has happened almost the same way each of the three times that I have kicked her over. Like LWRider, I am close to having a smooth running machine, if I can take past this problem.

Thanks in advance - Jerry
 

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It is possible that it is a fuel delivery issue rather than a temperature-related one. The gas tank may not be venting properly, causing a vacuum in the tank that takes time to equalize again. A clear plastic hose used as a fuel line, or clear in-line filter can let you see visibly if you are getting gas to the carbs at any one moment. It's a cheap test anyway.

The biggest clue is the 10-minute time window. That's about how long it will take to run out of fuel if the tank isn't venting properly. Running fine after sitting again is also suspicious.

It's a theory anyway.
 

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You could also do a quick test for venting problems by timing how long it takes to die just idling, then doing the same test with the gas cap off.
 

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Discussion Starter · #4 · (Edited)
Dodsfall...

Thanks for your response.

1. I love the idea of the clear fuel line. Will go and get some (and report back).

2. The vented tank issue - I almost want to jump on this as my "aha, that must be it!", because when I (Kreem) lined the tank after the acid wash, I (probably in error) used the cap to help keep the liner chemical inside the tank as I turned it and let it sit at different positions. So, the underside of the cap is "lined" too, perhaps now a barrier to any venting.

BUT... In the first two startup scenarios, having just brought this bike to life, I had only a little bit of gas in the tank, filling it from my lawnmower can. Thinking that I simply ran out of gas, I opened the tank to check, which would have vented it. It should have started again I would think? On the third time I actually was on my way to get some gas (having added a little more to the tank to get there) when the engine shut off as I was rolling into the station. I of course thought I had just run out of gas, and how lucky to have made it it the station. In this case, the lid came off and 2.85 gallons of fresh fuel went into the tank. I would think with it should have started here too - new gas, vented tank. It did not.

It was at this point at the gas station when I finlly thought through all three scenarious and started to connect the dots. I moved the bike to some shade and proceeded to sit. Little by little as the engine cooled down, I could get 3 seconds of idle - (sit some more), 5 seconds of idle (sit some more) maybe 10 seconds of idle but could not yet rev it (sit some more) and it was finally ready for me to make a mad dash back to my house, which I did. It ran fine all the way, shutting off in a few more minutes after returning, in my driveway.

I am going to try the lid off theory and see what happens. I am assuming it doesn't have to vent to some precise pressure would it? -Jerry
 

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The venting should be to normal atmospheric pressure, so no worries there. There could be an additional restriction to the fuel as well. Try the gas cap test and see what results you get. If it stops running after sitting with the cap off it will eliminate venting as a problem.

Once you try to visually see if there is fuel supply with a clear hose or filter, it will eliminate fuel blockage to the carb as a problem.

Have you checked to make sure the fuel filter is not gunked up?

Edit: Some tanks may use the cap to vent and others may use a vent tube attached to a hose.
 

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Spark testing

The same sort of common sense testing applies to the spark as it does to the fuel. Time and heat can also change how or if a spark happens. Again,use some logic.In LW's case, it was one cylinder in your case,it is two. Since the 350 has two coils and one condenser [if it is like my 360] it then seems likely that a single part is shutting it down. The likelyhood is that if it is an electrical part,it 'could' be the condenser,or for all you know,it could be hidden in the wiring to the condenser or even the wiring somewhere else. Of course,you want to test it to see,so a spark checker is the answer. Many people simply lay a spark plug against the block,but that is not a fair test of spark. To simulate the environment of a cylinder,you need a known 'good' plug with the electrode bent straight. The straightened electrode is harder to fire and so is a sparkplug in a pressurized and fueled cylinder . Of course,an inline spark checker allows the bike to fire and for you to see the spark. Possibly,an inline would show a weaker spark as the bike heats up and that would you help to diagnose the problem.
To show how weird a thermal problem can be, the point on my single cylinder CB100 was almost touching [grounding out] at the base. It started and ran fine for about five miles until the heat expanded the point slipper spring,and the bike quit running. It took pure luck to see the problem,but a multimeter reading 3 volts at the point showed an "almost grounded " condition
In any case,isolating the problem as definately electrical or definately fuel will surely save you a lot of grief. Please excuse my poorly written post,but I had a very tiring day
 

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As far as tank venting; I found it as simple as blowing compressed air through the cap and making sure it escapes out the other side. I don't know on your year bike, but my '68 has a simple cap with a little hole in the middle of the underside. If you blow though that hole it should come out along the edges under the rim.

Also, easy to check the coil primary windings anyway. Put a multi-tester across the leads to the primary windings (the little black and blue or black and yellow wires) and you should get 4-5 ohms. If it checks good cold, try it when the engine cause you fits.

Also, the condenser is still available from Honda and are nto very expensive, so you could try that first. If you end up needing coils (I did, but it is not common) I may have a fix. Still have to test it (see the other thread).

Good luck,

Mike
 

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Discussion Starter · #8 ·
The venting should be to normal atmospheric pressure, so no worries there. There could be an additional restriction to the fuel as well. Try the gas cap test and see what results you get. If it stops running after sitting with the cap off it will eliminate venting as a problem.

Once you try to visually see if there is fuel supply with a clear hose or filter, it will eliminate fuel blockage to the carb as a problem.

Have you checked to make sure the fuel filter is not gunked up?

Edit: Some tanks may use the cap to vent and others may use a vent tube attached to a hose.
Dodsfall - Thanks for your response. I have had some overtime this week so I have not been back to the lab (garage). On one trip to take out the trash did deviate from said purpose to inspect my gas cap. You may recall I had used the cap as a retainer of Kreem liner liquid when chemically lining the tank. There was a hole in the center of the cap that was completely sealed over. I reemed that out and then also picked at some liner material that was hainging around the two spring loaded sealing clamps. I have not had the chance as of this evening to run a test with the cap off or with this vent hole unsealed. I am hopeful. Will report back when I can get some garage time. Work is harmful to biking.

Jerry
 

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Discussion Starter · #9 · (Edited)
The same sort of common sense testing applies to the spark as it does to the fuel. Time and heat can also change how or if a spark happens. Again,use some logic.In LW's case, it was one cylinder in your case,it is two. Since the 350 has two coils and one condenser [if it is like my 360] it then seems likely that a single part is shutting it down. The likelyhood is that if it is an electrical part,it 'could' be the condenser,or for all you know,it could be hidden in the wiring to the condenser or even the wiring somewhere else. Of course,you want to test it to see,so a spark checker is the answer. Many people simply lay a spark plug against the block,but that is not a fair test of spark. To simulate the environment of a cylinder,you need a known 'good' plug with the electrode bent straight. The straightened electrode is harder to fire and so is a sparkplug in a pressurized and fueled cylinder . Of course,an inline spark checker allows the bike to fire and for you to see the spark. Possibly,an inline would show a weaker spark as the bike heats up and that would you help to diagnose the problem.
To show how weird a thermal problem can be, the point on my single cylinder CB100 was almost touching [grounding out] at the base. It started and ran fine for about five miles until the heat expanded the point slipper spring,and the bike quit running. It took pure luck to see the problem,but a multimeter reading 3 volts at the point showed an "almost grounded " condition
In any case, isolating the problem as definately electrical or definately fuel will surely save you a lot of grief. Please excuse my poorly written post,but I had a very tiring day
Slumord - Thanks for chiming in. I feel the brotherhood. I have not had a chance to get back to work on this since last weekend (the job and honeydos). But you are the second person to suggest a heat related electrical problem. One of the reasons I like this old Frankenstein bike (keeping it alive) is for something you said - fuel or electrical?. That is an old troubleshooting "Y" in the road that we used to pursue on car engines pre computer. Is it fuel, or is it electrical? If you could answer that question you were more then halfway on track with the eventual answer.

I don't know the answer to that question until this weekend, but will report back. Thanks for your very detailed suggestions, which I will follow. Jerry
 

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Discussion Starter · #10 · (Edited)
LWRider (Mike) - what can I say, but thanks. Will take your suggestion and run with it. Two of you now have suggested a heat related electrical problem, which I don't think I would have entertained on my own. I am anxious to delve into this as a possible cause\cure.

You and I are in the same boat, trying to nurse a good machine back ot life. You kind of started the whole thing for me as I came to this forum thinking I had no where else to go. I read your thread and decided to jump in and ask for help. There are some good peeps here on the forum. - Hope you and Gary get your ride fixed as soon or sooner then mine. Jerry
 

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Thanks! I am afraid I have had another setback. But a new, used set of coils is on its way. I will have them by Friday. The worst part is the wait between trying things because everything has to be ordered from some far away land. I guess that is what seduced me into trying the aftermarket coils--they were here in town and I could simply walk in, pick them up, and get back to the bike.

Cheers and good luck,

Mike
 
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