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Discussion Starter #1
Hello all! I've got a 1986 Rebel 250 with an intermittent problem.

I'll be riding with everything working like it should and then, like flipping a light switch, the bike will lose power. The motor will start bogging down and I'll lose a good 10mph at least from my top speed.

And then, like a light switch, it'll kick me in the seat of the pants and come back to life.

Does this sound like an electrical or fuel delivery issue? I recently had the carb ultrasonically cleaned but this issue existed both before and after the cleaning. I've also recently replaced the spark plugs. The issue existed both before and after.

I hate that it is intermittent. Sometimes it'll go days/weeks without acting up. Other times it'll act up every time I ride. Sometimes it only lasts a few seconds and other times it acts up the entire trip.

I'd love some troubleshooting advice from the group!

Thanks in advance!
 

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I’m currently dealing with the same issue. Happens when running around 60 mph for an extended time. Bike will just start to bog and lose power at times. Once it clears up, it just comes back to life.

I thought it was the carb. I cleaned and tried different jetting combos. I thought it was the tank creating vacuum, so I ran without a fuel cap for a test and it did it also.

Mine will only do this on a high rpm extended run. Hoping to find the answer. You can ride around the problem now, especially around town, but it sometimes shows up on a high speed run.
 

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sounds like fuel starvation,, I assume you have installed an inline fuel filter as Rebel is noted for rusted tank..
Switch fuel off, remove settlement bowl from petcock check for contamination.. You may need to remove petcock and clean tank screen and internal base of petcock..
 

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No fuel starvation in my case. Not sure about OP. The petock on my tank flows well. That was tested as a possible source of fuel starvation.
 

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Discussion Starter #5
In November of last year I had the carb cleaned. I was surprised to find that it was already very clean. No sediment in the bowl. No clogged jets that I could find. I also pulled the petcock apart and found the sediment bowl was also very clean. I noticed there was no screen so I bought and installed one before threading the bowl back on.

There is no inline filter. My uncle who owned the bike before me didn't have an inline filter on it either.

The one thing I haven't done yet is to pull the petcock from the tank to see in what kind of shape that screen is in as well as the bottom of the tank. From what I can see looking in from the top it seems to be spotless. I'll try to do this between now and Sunday night.

So, is there any electrical things I can be looking at? Given how suddenly this problem comes and goes (like flipping a light switch) I'm still wondering if it's electrical.

Thanks!
 

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Discussion Starter #6
Rode the bike to work this morning. I usually start the bike and let it warm up while I put on all my gear, check the tires, put my lunch in the saddle bag, etc.... Today I noticed exhaust coming from the left pipe and not the right (it was 35 degrees this morning so you can see the exhaust). I put my hand up to the left side and felt a nice strong exhaust. The exhaust coming from the right pipe was very faint. So I got my infrared thermometer and checked the pipes up at the cylinder head. The right side was running about 50 degrees cooler than the left.

On the way to work the bike did what I described in the posts above one time and it was only for about 10 seconds then it came back to life and made the rest of the 18 mile ride with no problems.

I checked the exhaust temperature again when I got to work and they were both the same at around 350 degrees.

Should any of that be leading me in a particular direction? Thinking about swapping some components left to right to see what happens.
 

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With one cylinder putting out less exhaust, I'd check compression. There's a how-to on it in the Wrenching section of the forum. Let us know what you find.
 

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Discussion Starter #8 (Edited)
I ran cold compression back in March of last year. Got 110 on the right side and about 117 on the left. I should probably take the time and check it the right way...warm and at wide open throttle.
 

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Rode the bike to work this morning. I usually start the bike and let it warm up while I put on all my gear, check the tires, put my lunch in the saddle bag, etc.... Today I noticed exhaust coming from the left pipe and not the right (it was 35 degrees this morning so you can see the exhaust). I put my hand up to the left side and felt a nice strong exhaust. The exhaust coming from the right pipe was very faint. So I got my infrared thermometer and checked the pipes up at the cylinder head. The right side was running about 50 degrees cooler than the left.

On the way to work the bike did what I described in the posts above one time and it was only for about 10 seconds then it came back to life and made the rest of the 18 mile ride with no problems.

I checked the exhaust temperature again when I got to work and they were both the same at around 350 degrees.

Should any of that be leading me in a particular direction? Thinking about swapping some components left to right to see what happens.
If that happens again, immediately check for spark and compression. Or if you can't check right then, leave the bike until you can. The problem with intermittent issues is that you can never be sure if the test you're running is giving accurate results. You could find one cylinder not firing and in getting ready to run the test it fixes itself. Your issue could be either electrical, fuel, or compression. Maybe sticky valve, maybe loose electrical connection, it sounds less likely to be a carb or fuel issue because that would affect both cylinders.
 

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Discussion Starter #10 (Edited)
Ran compression the right way when I got home from work (warm engine, wide open throttle) and got 160psi on the left cylinder and 150psi on the right cylinder. So unless a valve is hanging open randomly I don't think the problem is there.

Does anyone have any ingenious way to monitor spark while your riding down the road? It's such a random and intermittent problem I almost need a way to troubleshoot while I'm riding.
 

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To me, it sounds like an electrical problem. Like the ignition in at least the right cylinder simply stops at some point. Then maybe your bike is being propelled by only 1 cylinder, or 0 cylinders, then comes back alive when the ignition begins working again. Something like that.
 

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7milesout and other Rebel people:

I had the same intermittent issue (full power then sudden down to 60-70% power for a period of time and then back to full power; fairly unpredictable) with 1987 Honda Rebel CMX450.

1. NOT vacuum lock with fuel tank or clog in fuel line / petcock or vacuum leak.
2. NOT ignition coils (I replaced original coil (which works OK) with cheaper coils for scooter; but intermittent problem only moderately improved). After messing with ignition and coil wires the bike almost always went back to 100% power; then problem came back later.
3. One cylinder gets hotter than the other. Switched ignition wires and other cylinder got hotter indicating ignition issue. Strange since both cylinders fire together every revolution of the crankshaft.
4. Did not rebuild carbs but did not appear to be fuel issue.
5. May be poor grounding of ignition coil(s). Check all wiring for broken wires/connectors, grounding, etc.
6. May be CDI unit. But I don't understand how it could be CDI since both cylinders fire together every revolution of the crankshaft (as mentioned above). Someone needs to try a cheap scooter CDI that fits a Rebel (NOS CDIs are expensive).

Suggestion: Try running a single conductor wire from the negative side of the coils back to the negative side of the battery and clean ALL electrical connections.

This problem looks like a funky electrical/grounding issue that may be associated with older bikes that have corroded electrical connections/grounding and are impacted by humidity, vibration, etc.

The CMX450 was traded before it was fixed. Somewhat frustrated since I like to solve these types of problems and did not get much $$ for the trade due to the problem (I put > 15,000 miles on the bike over 8 years so it served me well).

I have several parts and a manual for the bike that am planning to put on Ebay soon. Let me know if you want parts / manual for a 1987 CMX450.

Best Regards and Happy Riding!
 
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