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Discussion Starter · #1 ·
When I first got the bike, I instantly knew something was wrong when at idle and warm the rpms sounded high. It idled at 1600 and sounded rough(this lead me to my other post about the vacuum, was making sure I didn't have a vacuum leak.). It also dies while warm during idle and did occasionally make popping sounds. I checked the plugs an they were black and sooty. Of course this means its running rich. My first guess was the fuel mixture. While following the manual on adjusting the mixture, It was very difficult to get the high rpm sweet spot. When I finally did, I went to lower the rpm to 1200, it ran but it would eventually die at idle. Next I find that I can close the mixture screw all the way closed and it will still run. Also the bike cold starts without the choke.

This leaves me to believe its either, bad float, needle adjusted too high or bad valve adjustment. I might try the valve adjustment first before I start tinkering with the carb. I have some experience with carbs and have rebuilt a cb350 and cb750 carb and both motorcycles ran fine after.

I am posting this so I can get some suggestions that I might not have thought off and most importantly to document my problem for future owners. Thank you.
 

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Hey Macross!

Cleaning your carbs would identify if floats are bad or just stuck. The needles should be factory unless you have a modified intake or exhaust, then the PO may bave been in there. Checking the valves is easy and an overall tune up also is always a good idea. Do you have a fuel filter? If not, I would highly recommend, especially after carb cleaning. I also have found cracked hoses and other rubber parts, so recommend checking those as well.

I think you should clean the carb, reseting adjustment screws to factory settings, check valves, and maybe replace spark plugs (if you haven't) and you should be able to fire it and tune from there. I always recommend 450 owners check stator and coil resistances along with replacing the CDI.

Are you using an OEM petcock? I used to rebuild them, but find capping the left cylinder port and running a 250 petcock is way more reliable.

Have you checked cylinder compression?

Hope this helps a little.
 

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Discussion Starter · #3 ·
It’s all appears to be bone stock. Changed oil, spark plugs and air filter. It’s does have a fuel filter. Mine is non Ca set up so I checked all the vacuum and they seem good.

Will check compression today. Never thought about the ignition parts. How do I check if coils are bad? I can search it as well. I might just replace them if they are cheap enough. Yes it’s OEM petcock with the vacuum.

Along with ignition parts I will check valve clearance and tackle non carb stuff first. After scouring through the site it seems like a pain lol.

Thank you for responding.

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Discussion Starter · #4 ·
Ran a compression test on a cold engine full throttle and I go an even 120psi on each side. I know the manual says its should be 170 but I have a cheap Harbour Freight tester and I think it never read correctly. I basically use to check if i have even compression. I might have to splurge on a good tester.
 

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Discussion Starter · #5 ·
I finally got the valve cover off and ready to do the valve adjustment. Reading the manual it says I just need to make sure the T mark is lined to the marker. Then check my clearance. I've work on other bikes and usually you have a marker for each cylinder. This one seem like you set it to the T and then feel which one valves are loose. Is there a way to see markings on the top area to actually see if you TDC on the left cylinder? I'll try searching the forum for more info as well.
 

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With a multimeter (and YouTube), you can check for proper resistance of coils, stator, rectifier, wiring, etc.

For the compression check, drop a couple of cc's of oil in each cylinder and recheck. If it reads 160+ w/the oil, it is not the compression gauge, you likely have bad rings.

For the valves, as you discovered, the 450 cover only comes off one way, no negotiationg there :). Setting the t mark allows you to adjust one side. After adjusting rotate the engine oen turn to adjust the other side, The valves will be loose on the side that is TDC, if I am correct. Does that help?

Do you have a service manual?
 

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Discussion Starter · #7 ·
Thanks for the advise, added oil to the cylinders and the test shot up to 180 for both sides. Alot of white smoke came out of the exhaust when i fired it up. Is that from the oil I added?
 

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Yup. Run it for a sec, till smoke clears. Check compression again. If it's back down to ~120, you likely have bad rings and possibly pitted cylinders.

Bike may run, but won't 'efficiently'
 

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While in civilized areas, "if you hear hoofbeats.. don´t think of zebras..."

It´l probably just dirty carbs ! Especially the float-needle valves.
 
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Yes, the fuel mixture, comes from the carbs. The burning in the cylinders is after.

Clean those carbs.

I would suggest getting a close look inside those cylinders while the bike is down as well.
 

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85,86,87 CMX250C, 81 CM200T, 74 CL360, Invisible to cages, treat them accordingly. Avoids Road RAGE!
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Harbor Freight has a cheap scope that'll work to look inside without a teardown. Just a suggestion. It's helped me in a few situations.
 

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Discussion Starter · #13 ·
I finally go the carbs out. The float level is up to spec. going to give them a clean. I cant seem to find the numbers on the jets. I wanted to make sure they were stock. I also noticed the slow jet doesnt look like the one on the manual when its seated. It protrudes higher than the image.

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Discussion Starter · #14 ·
Harbor Freight has a cheap scope that'll work to look inside without a teardown. Just a suggestion. It's helped me in a few situations.
Thanks, will look next time at Harbor Freight
 

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Two carburetor circuits I'd check closely are the enricher/choke and air cutoff diaphragms..
Both circuits are can be responsible for an overly rich AF mixture..
assure that the enricher/choke valves are fully closing. I know that the 450 shop manual calls them choke which makes me choke on the term as all years of Honda Common Service Manual provides a complete description of the bystarter/enrichment system..
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Secondly the Air Cutoff circuits are known to have problems when diaphragm valve fails..
there is no replacement diaphragm available for the CMX450..
Following Link explains it's function and how to defeat the diaphragm valve if it fails..
 

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Discussion Starter · #18 ·
Well I just got done re-installing my carbs. It is a pain to back together lol.

Soak you might be right, I was already ready to write about the choke/enricher before I read your comment because they looked bad. Also when I got the bike running again I noticed that the choke/enrichers didn't go all the way down. I had to press them down. Now I am not sure if I caused this and it was ok before. I will check my cable adjustment later to see if I just didn't put the cable correctly.
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I followed the manual on setting balance screw to set the flaps to the edge of the hole. Instead of 2.5 I opened the air/fuel screw to 2. After several turns to get the gas back in the bike it started. I had the chock on and after that I turned off the choke and that's when I noticed the choke valves not going all the way down.

I rode the bike around to warm it up and then proceeded with the air/fuel mixture procedure. It seems to be happy at 1.75 turns. The bike idled better from what I can hear and I was able to get the idle to 1250 and any lower it struggles and dies eventually. I will look for a better rpm gauge, I am using a cheap one that wraps around the spark plug wire. For all I know I am not reading the rpm correctly. Pretty tired for the day and I will probably run more test rides and observations next week. Thanks for the help so far and I'll keep updating my post.
 
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