'73 CB350F - Running issues/need help

imseamless

imseamless

Been Around the Block
Hey Guys,


I've got a 1973 CB350F that ran like a charm last summer... Until one day. I thought it was a bad tank of gas, but the problem has persisted. Here is the deal:


Starts first or second kick, as always. Warms up and rides like a champ. Smooth acceleration and full power through all the gears. About 7 - 10 minutes into riding, it will start popping and backfiring. Once that begins, it will lose power and proceed to die. I can force it to start by choking it, but it is very colorful- Backfiring all over the place, won't idle steady, just all over the place. At that point I'm lucky to get it off the road.


As this is my first bike and I have no mechanical background, I'm at a bit of a loss here. I have two manuals and have read through them but have found no relevant troubleshooting help and am not sure where to begin. I took it to a shop and basically gave them 100 dollars to tell me what I already know: That is is running rich and backfires under load once its hot.


So far I have ran a few tanks of gas through it, to ensure it wasn't bad gas, and changed all 4 spark plugs. I'm not sure where to go from here so any advice is much appreciated. Let me know if photos/video/etc would help, as I will do anything I can to get this bike back in action.




Thanks in advance,
Ben
 
Sounds like it might be fuel stavation. Check your tank cap is venting properly and allowing fuel to flow.
 
Maybe the coils are overheating causing an incomplete burn?

Next time it craps out on you, let it sit for about 30 minutes and then try to fire it up. If it starts easy and runs great, it's probably the coils.
 
Thanks for the responses. Rich, I think you're onto it. Every time it dies I let it sit for about, you nailed it 30 mins, and it will get me another 10 mins of ride time. So from here, can you recommend any easy and relatively cheap fixes? I've looked into coils before and they can be quite expensive. Aside from checking the local bike shops for used ones, I've heard you can use auto coils?


Hillsy, I will check the cap. I know if I fill it up too full it will leak a bit when riding, so that tells me there is some air flow. Any other ideas for testing?


In the meantime, I'll grab a multimeter from work and see if I can get some readings off the coils for good measure.


Ben
 
Aside from running them out in the open, replacing them is really your best bet. I don't know about auto coils though......

You can find some cheap new ones, but even those aren't the best alternative. Dyna coils are top notch- available from Z1 Enterprise or Dime City Cycles.
 
I'll pull them out and zip-tie them to the frame and see if that fixes it. If so, I'll just close my eyes and hand over the card for those Dyna coils. I'll let you know how it goes in the next few days.


Thanks again for the help. I'm glad there is a community of helpful people and hopefully I can start contributing back. I'll start a build thread as well.


Cheers
 
The reason I said it might be fuel starvation is that you said you could coax the bike back to life (to some degree) with some choke.


To check your tank, grab a length of fuel hose and a suitably sized container. Put the hose on your petcock, the container on the floor and drain the tank into your container with the fuel cap shut. If the fuel stops flowing after a few minutes, your tank cap is not venting properly.



I'd try this first because it will cost you next to nothing to check compared to replacing coils / chasing electrical gremlins.
 
It's either electrical overheating as Rich suggested or a blocked filler cap vent. I'd put my money on Hillsy's suggestion. or ride it til it stops and then open the filler cap. If you hear air rushing in it's the cap.
 
If it is your coils, let me know, I have a set of coils I just pulled off my 73 CB350f I will let go cheap to ya!Mine are good, I just upgraded to dyna coils and a dyna CDI!
 
Teazer- I'll check it out tomorrow night and report my findings.
TwistedWheelz- I appreciate the offer and may be in touch, depending on the above results.


Thanks guys!
 
All right, just finished some troubleshooting. I removed the tank, opened the petcock and let it drain until I had filled two coffee cans:
9dc5QP

(Image is also here: http://flic.kr/p/9dc5QP)


The entire time the gas was draining, it did not slow down or trail off, so this leads me to conclude that the gas tank is venting correctly. At the end, I popped the cap and it did not increase the flow of fuel from the petcock.


Next I checked the numbers on the coils and determined they were the same as the ones on my CB750, so I swapped them out. Turns out, the spark plug cable that goes to cylinder 3 was broke, so I reverted back to the original coil for cylinders 1 and 3. That leaves me with a replacement coil on 1/3 and the original on 2/4. The bike fired ride up and I want to think it ran longer than before, but it could be wishful thinking. Regardless, sooner than later it started its backfiring. As soon as I felt the loss of power I pulled over, ripped off my helmet, put my ear to the gas tank and popped the cap. Unfortunately, this did not help the bike and it continued to sputter until it died.


So my thoughts are it is the coils, as suggested. Rather than dump money into used and/or old stock replacements, I think I'll fork over the extra and buy Dyna's. If you guys' are still thinking it is the coils, I'll go ahead and order them. Could anyone please confirm the following links would be the applicable coils/wires for my bike, a 1973 CB350F? The coils I pulled off my bike read "FL703 12V."


Coils: http://www.z1enterprises.com/detail.aspx?ID=1306
Wires: http://www.z1enterprises.com/detail.aspx?ID=209




Thanks,
Ben
 
Ben, I suggest if you are buying Dyna coils to buy the 5ohm coils and not the 3ohm, as both will work fine with the stock points system, but only the 5 ohms will work with a dyna "S" ignition system if you ever decide to upgrade!
 
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