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Discussion Starter · #1 ·
A year ago I bought a beautiful 1971 cb350. Was running great when I bought it, honestly not a single issue.

Then mid september I got hit by a truck, luckily nobody was hurt. The bike miraculously came out with little to no aesthetic damage, but I was stuck trying to get it started for about 2 hours on the side of the road. After I got it started I rode it home, and everything seemed to be fine. Although the bike never felt the same after that.

Gradually, the bike got more and more sluggish until one day it completely seized up at a stop light. Wanted to die idling so I had to keep the throttle going, couldnt top 20mph- it was a slow cruise back into the garage. Now the tank had some rust issues that the previous owner treated pretty well, but no tank ever fully comes back from that. My guess is when I crashed I unsettled some debris that clogged up my carbs. After that day it was much harder whenever I tried to get it started, and the sluggishness persisted. I gave up as the weather was turning here in Washington State, and I was getting busy with work...

And there it sat for about 5 months living out the winter.

I give this back story in the hopes that it helps in figuring out a diagnosis because now I am back to working on it, and I am at a loss-

When I came back to the bike it was no surprise it didnt want to start, let alone turn over. Since then I have:

  • replaced the battery
  • replaced throttle cable (old one got pretty messed up in the accident)
  • rebuilt the carbs
  • replaced fuel lines



All that and a little starter fluid and I managed to get the bike started back up, but it was having that same issue of sluggishness, so then I:

  • cleaned air filters
  • cleaned out the petcock (which was damn near clogged, confirming that some rust had broke loose in the tank)
  • installed inline fuel filters
  • replaced/gapped the spark plugs
  • And then for good measure, I cleaned the carbs again because I hadn't caught that the petcock was clogged up until after I had gotten it started and ran it for a bit.

After all that and some more starter fluid I managed to get the bike going and to my glee it was running back up to about 80% of what I expect out of the machine! Torque has returned, wants to die while idling, doesn't like accelerating at around 50mph and tops out at 70mph. Last summer I topped out at about 90mph, she was running really clean. On the test ride the sluggishness slowly returned.

Back in my garage and I am scratching my head. The bike doesnt want to start again...I open up the drain plugs on the carbs and sure enough they seem to be flooded. I give the bike a go and she running at about 80% again, and again after some time the sluggishness returns. I drain the plugs again, get her started up, by now the bike is warmed up and I adjust the idle mixture screws, so where I am at now:

After adjusting the idle mixture screws I got the bike to idle at about 1200, but it jumps up and down pretty sporadically. After about 30seconds-1minute without touching the throttle though rpms start to drop and it eventually dies. I thought I might have fixed the problem of the carbs flooding with the idle mixture screws but the flooding is still an issue, sluggishness still an issue... even after draining plugs the bike isn't running quite up to par with how it used to...

I am about as far into the rabbit hole as I can get without someone else showing up with a flashlight- I am a novice motorcycle mechanic and have reached the end of my knowledge. Anybody out there know what the heck is going on?
 

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Good job on the back story - it offers clues. For a well-running bike to stop running well after a crash, we have to consider what happens in a crash . . .
Is it possible that the impact caused the carbs to shift in the rubber intake bots? Or for those boots to be displaced or cracked? That would cause air leaks and a poor mixture (although, it would be lean, not rich).
Another thing to consider - especially if your bowls are flooding would be that the floats got jarred or bent. Did any wiring connections get pinched, crimped or displaced? Poor connections at the coils could cause weal spark and might appear to be a too-rich condition.

I would recommend going to basics. What do the plugs look like? They will tell you the story of what's happening in the combustion chamber. Black/sooty? Too much fuel (or not enough spark?)
Do a compression test, just to rule out any major mechanical issues - if compression is low or uneven, you'll never be able to tune properly.
Make sure valves are adjusted, although unlikely, a crash could have shifted things and maybe you're out of adjustment.

When a machine takes a huge blow, you never know what might have been affected. Don't assume anything, double-check everything.

Good luck!
 

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Even better that you were okay after being hit by a truck! Crazy story, indeed. Good luck with checking out everything, hope you get it back to normal soon - riding weather ahead :D
 

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Discussion Starter · #4 · (Edited)
Is it possible that the impact caused the carbs to shift in the rubber intake bots? Or for those boots to be displaced or cracked? That would cause air leaks and a poor mixture (although, it would be lean, not rich).
Another thing to consider - especially if your bowls are flooding would be that the floats got jarred or bent. Did any wiring connections get pinched, crimped or displaced? Poor connections at the coils could cause weal spark and might appear to be a too-rich condition.

I would recommend going to basics. What do the plugs look like? They will tell you the story of what's happening in the combustion chamber. Black/sooty? Too much fuel (or not enough spark?)
Do a compression test, just to rule out any major mechanical issues - if compression is low or uneven, you'll never be able to tune properly.
Make sure valves are adjusted, although unlikely, a crash could have shifted things and maybe you're out of adjustment.

When a machine takes a huge blow, you never know what might have been affected. Don't assume anything, double-check everything.
I sprayed wd40 around the boots and didn't hear any leaks, also didn't see anything with a good ol fashion visual inspection. The bike was/is exhibiting symptoms of running lean. When I adjusted the idle screws I made it more rich and that seemed to help with how long the bike is able to idle and overall sound of the bike while its running but the carbs are still flooding...

I haven't done a full survey of the wiring but plan to soon, just gotta get a buddy over here with a voltage meter, I've never done much in the way of electrical maintenance.

The plugs seem fine to me, on one the end of the threads is singed black but the insulator tip itself is still white and the electrode looks fine. This makes me thing that side could almooost be running too rich but again, its only the thread that looks singed. Also if I drop off the idle mixture screw at all the bike dies without constant throttle. The other side is the opposite, no singing at all. Looks a lil greasy but thats it!

As for the floats- they seemed fine to me when rebuilt the carbs, but really other than completely replacing the floats how can I tell if they weren't bent a little?

I don't have a compression tester but I will either go out and buy one or see if I can find a friend/shop that will let me use one.

this is where I stopped writing my post and went out to try running the bike again now that its been sitting a couple days since last testing

Here is the real trouble- today goin out and giving it another go and I cant get the bike to run well at all. It will start up but only after laying on the electric start for longer than one would like to. I drained the carbs and the sluggishness isn't going away. So it now appears that draining the carbs has no effect on the problem which would imply that maybe the carbs were never flooding in the first place.

Keep in mind any starter fluid I sprayed in there when the air filters were off is now gone, so maybe the engine running better was just a facade all this time?

Praying that my compression is good, it would be a huge bummer if there is any major mechanical failures going on in the engine. At this point it would seem the next thing to check out would be all of my electrical, starting at the coils? Any advice here is welcome I have officially traversed into the realm of the unknown.
 

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Have you changed the oil since the accident? Is it possible that there may be gas in your oil?
 

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Discussion Starter · #7 ·
Ive got a friend coming by this weekend that has a larger tool/skillset than I do. I did figure out one of my cylinders isn't firing as often as it should be though- so with any luck that diagnosis will show me the rest of the way to a clean running motorcycle.
 

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I would double CHECK all your electrical connections.
They need to be refreshed after 40 years of sitting.
The Drop may have knocked it just a little out for that cylinder.
 

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are you running on both cylinders? sounds like one is cutting in and out.

basically I'd do a full tune up at this point. these bikes need regular tune ups anyhow...Honda recommends to do one every 2000 miles.

start with a compression test. 170 is good 150 is minimum...below that and you've got issues.
adjust valves
adjust cam chain
adjust inspect/service ignition point/timing
inspect air filters...are you running new clean stock filters? good filters will show light through them when held up to the sun
adjust carbs...inspect/adjust floats, make sure fuel inlet valves fully close
synch throttle cables, synch carbs butterflies, synch idle, synch idle mixture
do you have a good battery, fully charged and a properly functioning charging system?
 
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