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Honda CB500T // Runs for 30 mins, dies, then starts but won't rev past 2000 rpm

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engine dies engine failure fuel line ignition
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E

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Discussion Starter · #1 ·
Hi there

I have a 1975 Honda CB500T and recently I have been having a problem where after running for 30 minutes the bike begins to putter out (especially when moving at low speeds i.e. traffic). When I try to start it, the engine turns over but will not rev past 1000-2000 rpm even with the throttle wide open.

If I let the bike sit for 30 mins it seems to be fine and will start up great. I am unsure if it is a problem with the mixture clogging up the fuel lines or if something electrical is happening with the ignition. Maybe bad ground?

Any advice would be much appreciated!
 
jd50i

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#2 ·
run it until it quits, pull the bowl drain screw and see if bowls are full or empty, if empty open the tank cap and listen for a hiss. Try cranking with tank cap open, if it runs fine tank vent is clogged. If not, next is to check fuel flow to carbs
 
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Yendor

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#6 ·
Really good chance the Vent in the Tank Cap is Clogged and it reaches a point where it is in vacuum.

Sitting allows it to relieve the vacuum and fuel starts to flow again.
You can try riding with the cap not latched, but only with a 1/2 full tank.

OR:

I had a similar issue with a 1948 Buick.
It would idle forever, but die 10 minutes into a drive.

Turned out to be RUST in the tank would get stirred up when in motion and clog the screen on the pickup tube.
You could have that issue too.
There is a screen in the petcock.
It may allow just enough fuel to flow to start but not enough for the long run.
BUT... I'd still bet on the tank cap vent.
 
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E

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Discussion Starter · #7 ·
Hey thanks for all the suggestions--couple updates:

1. Tried opening the tank and starting it with it open and it did not make a difference.
2. Once the issue has started, with the fuel petcock off, the engine will eventually rev (20 seconds or so), but it will not do this with the petcock on.
3. The window of it "working" seems to have shrunk as the most recent time that I started it, it had the same issue once the engine was warm.

Someone suggested they thought the voltage regulator was getting too hot, potentially needed to be replaced. Any thoughts?
 
J

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#8 ·
When you close the petcock it shuts off the supply of fuel to the carbs. As the engine burns fuel the level in the bowls goes down causing a leaner mixture which in turn leads to a faster idle.

If your problem is electrical, I would be more likely to suspect that the coils are breaking down as they heat up. If you happen to have another set of coils, that could be worth a try.
 
jd50i

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#9 ·
I suggest reading back over the posts and doing what was suggested to start narrowing down the problem. You can also ride it until it quits, use starting fluid to see if it fires off. If it does then you have a fuel issue, if it does not you have an ignition issue.
 
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jd50i

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#13 ·
dsmith181 said:
Yeah but these carb seem to be extra finicky.
I have a cb360 and a cb350 and the 500t carbs don’t seem to want to stay as consistent that those other ones for some reason.. at least in my experience


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Click to expand...
When properly cleaned and adjusted the 500T carbs will give years of service without being constantly messed with, same as my 450K6.
 
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D

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#14 ·
jd50i said:
When properly cleaned and adjusted the 500T carbs will give years of service without being constantly messed with, same as my 450K6.
Click to expand...
Weird. I don’t know what I’m doing wrong then.
I’ve had to readjust 4 or so times in the last 6 months.
Even then after riding WOT and coming to a stop sometimes the idle drops below 1000 rpm and the bike shuts off. But when you start it back up the idles constantly 1200-1400. I can’t figure it out.

Sent from my iPhone using Tapatalk
 
Yendor

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#15 ·
If you have correctly rebuilt and adjusted the carbs as many times as you say, and still have the problem, it is NOT the Carbs.

You need to start looking elsewhere.

Air/Vacum Leaks.
Timing
Etc.
 
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76TWIN

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#16 ·
Did you check your coils? A bad coil could start to act up after the engine is warmed up.
Next time it does this while you're out riding, pull over and check that you are getting sparks from both cylinders.
 
D

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#20 ·
I picked up one of those to test it. In the meantime I went for a 30 mile ride and it ran great. 2 hours later went out and all the electrics were totally dead. The key didn’t turn the headlight , neutral light anything. Battery was at 13 according to a small meter I keep with me.
I was able to kick start the bike but it was only firing on one cylinder, occasionally backfiring the second cylinder.
Any idea why all the electrics would be dead after a healthy ride?



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jd50i
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