Honda Rebel Forum banner
  • Hey everyone! Enter your ride HERE to be a part of November's Ride of the Month Challenge!
1 - 9 of 9 Posts

·
Registered
1985 Honda Rebel CMX250C A
Joined
·
3 Posts
Discussion Starter · #1 · (Edited)
New to me 85 CMX250 won't budge over 40/42... maybe 45 headed downwind. Have a bad feeling I may have inherited the problem without knowing; being that my pre-buy test ride was the first time being on a bike, I took it down the neighborhood block and back and called it good. Regardless, it started getting boggy on me a few months ago and I decided to give it some attention. Said "attention" should be found under my profile's About Me, detailing the service I gave it if it can help any troubleshooting for you guys...

I'm looking at a few things currently:
1. Spark plug picture attached (L & R respectively*)
2. Compression tests; three tests yielded results of:
-Left (dark): 75, 72, and 70 psi. Also noticed this plug was warmer to the touch when removing than the right.
-Right (white/dry): 68 psi all 3 tests.
both are well under the 198 psi required...
3. Exhaust headers: I noticed the right header joint was loose, and found the innermost stud snapped in the cylinder head. I hadn't touched the headers prior, but they need replacing and I have the parts ready to install.
4. Carburetor: The carb seems to operate much better after cleaning twice, but I got my hands on an ultrasonic cleaner to clean a third time for peace of mind. Also see pilot screw picture attached, threads seem a little long but I don't think that'd cause my issue.?
5. Oil: Seems I overfilled it a tick on the change and I am going to extract it with a turkey baster until the level is where it should be. Bike being level and stick dipped with the threads sitting atop one another.

Now, I'm stuck wondering whether the loss of compression was caused by the loose joint, the oil overfill (I've only ran it periodically since the change), or was part of my inheritance. Seems to me to be the main issue however. The bike starts and idles fine... maybe idles a little slow. Has been backfiring on throttle release. I have a tachometer I'll use to tune but it seems like I have bigger issues in the meantime. Want to get this bike running fast and smooth but I could use some advice from you guys. Hope I'm thorough, but ask away for info I missed... thanks for reading.

*my lefts and rights are as if I'm sitting on the bike looking forward, not sure if I have that backwards...
Edit: added a few more thoughts
 

Attachments

·
Premium Member
85 CMX 250C, 82 GW Remember that you are invisible, treat all others accordingly. Avoids Road RAGE!
Joined
·
2,113 Posts
I had a similar problem a while back with my 85. 1st I replaced the CDI and it improved performance. After that I replaced the spark wires and it resolved the issue completely. The problem was the sparkwires were arcing out to-ground. And the higher speed the engine goes, the higher the voltage is going through those cables, and the less power goes to the spark plugs. It was worse especially during a rainstorm.
I hope this helps.
Normally though I would have focused on a carburetor issue.
 

·
Super Moderator
1987 CMX250C AC
Joined
·
2,737 Posts
Very low compression w/oil fouled spark plug would indicate to me your rings/cylinders are shot.. I'm assuming you preformed compression tests with throttle wide open..
if you have an air compressor rent or buy a leak down tester to confirm location for loss of compression, rings or valves or both...
 

·
Premium Member
85 CMX 250C, 82 GW Remember that you are invisible, treat all others accordingly. Avoids Road RAGE!
Joined
·
2,113 Posts
I'm just wondering if your battery was fully charged during the compression test. I think that can big make a difference, but not absolutely certain. For a proper test back in 2011 GUBA said,
"-Warm the engine
-Remove both spark plugs and install the gage in place of a spark plug.
-Turn the petcock to off.
-Open the throttle ALL the way.
-Then press the starter button.
-Watch the gage until it stops rising. Compression should be 159psi or 11.2 kg/square cm or 1,100kPa for the second generation and for the first generation, 198 psi give or take 28 psi, 1,400/200 kPa, 14/2kg/cm2.
-Repeat for the other side."
Was the engine warm during the test?
 

·
Premium Member
2003 rebel 2004 sportster
Joined
·
196 Posts
It seems to me like you may have two problems. If that was my bike I would do the compression check that Emil outlined with a different tester. Then I would squirt some oil in the cylinders and repeat. Finally it looks like you are not firing on one side. You could try to isolate the problem by swapping plugs, wires, etc. I hope you have good luck and get it fixed 😀
 

·
Premium Member
85 CMX 250C, 82 GW Remember that you are invisible, treat all others accordingly. Avoids Road RAGE!
Joined
·
2,113 Posts
Did this start happening shortly after riding in the rain? That again would point to the spark wires. You could possibly take I heat gun carefully to the wires to dry them out, (if they were wet), to see if this improves performance. If this does, it kind of verifies the wire issue. Also make sure the wires aren't touching to anything. Normally that shouldn't be an issue unless the wires are cracked. Being a 1985, that means they are probably 37 years old & in dire need of replacement.
 
  • Like
Reactions: Markrkolb

·
Registered
1985 Honda Rebel CMX250C A
Joined
·
3 Posts
Discussion Starter · #7 ·
...replaced the CDI... ...replaced the spark wires... ...It was worse especially during a rainstorm.
I'm just wondering if your battery was fully charged during the compression test... ...For a proper test back in 2011 GUBA said... ...Was the engine warm during the test?
Did this start happening shortly after riding in the rain?... ...Being a 1985, that means they are probably 37 years old & in dire need of replacement.
Emil, thanks for mentioning the CDI and spark wires. Per the same GUBA post I followed during the test, the engine was warm. I caught your battery reply before going to sleep and hooked up the battery to charge on Tender Jr. overnight. I will repeat the same test today before moving forward with Karma's leak-down test. I'm not sure by looking at the CDI, but the spark wires do look like they've been replaced sometime between the years 1985-2022...
It's funny, I noticed the power loss issue when I was trying to outrun a storm... and got caught in it. So to answer your question, no it was shortly before :LOL:... I can't recall performance changes in the rain but we have a front coming in tomorrow and I'll test the theory then. I assume by worse you mean slower top speed... anything else I can take note on?

Very low compression w/oil fouled spark plug would indicate to me your rings/cylinders are shot.. I'm assuming you preformed compression tests with throttle wide open..
if you have an air compressor rent or buy a leak down tester to confirm location for loss of compression, rings or valves or both...
This is more along the lines of what I was thinking. Yep, WOT per GUBA. Thanks for the leak down tester, that's my next step. Assuming the test comes back as I think it will... time for replacement?

...I would do the compression check that Emil outlined with a different tester. Then I would squirt some oil in the cylinders and repeat. Finally it looks like you are not firing on one side. You could try to isolate the problem by swapping plugs, wires, etc...
If tester and test kit are the same, the one I used last night was new off the shelves of harbor freight. Thanks for the tricks, I'll try the oil/repeat today after initial retest. Then plugs, then wires per you and Emil...

I have new spark plugs ready to go, but I'm trying to save them for when the more prominent issues are solved.
 

·
Super Moderator
Joined
·
31,333 Posts
If you decide to repeat compression tests with different equipment, most auto parts stores loan compression testers.
 

·
Registered
2002 Honda rebel 250
Joined
·
177 Posts
If you decide to repeat compression tests with different equipment, most auto parts stores loan compression testers.
I found that all the auto parts stores loaners don't have the correct spark plug size. I bought one on Amazon for about 18.-, with a variety of sizes. Including correct size for rebels. (10mm?) And the 'push' type.
 
1 - 9 of 9 Posts
Top