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Discussion Starter · #1 · (Edited)
Hi! New rider here. Just bought this baby (83 CM250) and everything runs fine except when I am going at an excess of 45+ mph and coming to a stop, mainly when downshifting from 4th to 3rd I think, my bike loses power and by time I reach the light, its dead. Basically, whenever I downshift from top gear coming to a stop, the bike loses power. Its incredibly dangerous to me at least, makes me scared to ride it.

I am not sure what’s wrong?! I am clutched in fully when braking. Could it be user error? I keep seeing that my idle could be too low, but she starts up most days without the choke? my idle sounds… fine not the greatest but I assume that could be from it being like 50 years old. The dealership I got it from said they cleaned the carb too before I got it. Its only been like 2 weeks since I’ve got it.
 

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Welcome to the forum

Could be your idle setting is too low.. I don't have specification for CM250
85-87 cmx250 spec is 1300 ± 100 rpm
Later models spec is 1400 ± 100 rpm
While the CB250 spec is 1500 ± 100 rpm
turn the black plastic idle stop screw on right side of carb to increase idle a bit.
Human body Organism Font Line Parallel



I'd take it to the dealer if you bought it 2 weeks ago..
A good shop/dealer should be willing to at least take a look at no charge of used (as is) vehicle they sold..
You are a potential future buyer to those who customer service is important..
 

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Discussion Starter · #3 ·
Welcome to the forum

Could be your idle setting is too low.. I don't have specification for CM250
85-87 cmx250 spec is 1300 ± 100 rpm
Later models spec is 1400 ± 100 rpm
While the CB250 spec is 1500 ± 100 rpm
turn the black plastic idle stop screw on right side of carb to increase idle a bit.
View attachment 112677


I'd take it to the dealer if you bought it 2 weeks ago..
A good shop/dealer should be willing to at least take a look at no charge of used (as is) vehicle they sold..
You are a potential future buyer to those who customer service is important..
Thank you. I’ll increase my idle speed and see if that helps. Is there anything else it could be?
 

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Discussion Starter · #6 ·
do you see any fuel starvation behavior at higher speeds?

downshifting has nothing to do with it - but pulling in the clutch does
I increased my idle speed and havent had any problems… yet. I was riding it pretty hard on the way back from the dealership (15-20 miles) to try and see if I could replicate the problem. nothing yet.
 

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Discussion Starter · #7 ·
do you see any fuel starvation behavior at higher speeds?

downshifting has nothing to do with it - but pulling in the clutch does
it doesnt sputter at high speeds to be honest. I do hear some sloshing in the gas tank when coming to a hard stop. but it would be like it turning off the motorcycle while rolling into a stop.
 

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Discussion Starter · #8 ·
Welcome to the forum

Could be your idle setting is too low.. I don't have specification for CM250
85-87 cmx250 spec is 1300 ± 100 rpm
Later models spec is 1400 ± 100 rpm
While the CB250 spec is 1500 ± 100 rpm
turn the black plastic idle stop screw on right side of carb to increase idle a bit.
View attachment 112677


I'd take it to the dealer if you bought it 2 weeks ago..
A good shop/dealer should be willing to at least take a look at no charge of used (as is) vehicle they sold..
You are a potential future buyer to those who customer service is important..
hey man I increased the idle speed and it helped for a little bit but now the motorcycle dies everytime I slow down from 40+ mph. I tossed in half a can of seafoam and rode it around today, no dice. I am still having this problem.
 

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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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The dealership has been known time & again to not do a thorough job when cleaning the carb. You might want to pull the float bowl & look in the bottom for starters. If it's not completely clean, the rest isn't.
 
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Discussion Starter · #12 ·
The dealership has been known time & again to not do a thorough job when cleaning the carb. You might want to pull the float bowl & look in the bottom for starters. If it's not completely clean, the rest isn't.
you think its a carb issue? I talked to a couple of guys and they said that since I dont have stock exhaust that could be the issue. (I bought the bike like that) So now I’m thinking about buying the stock exhaust and putting it on myself since no one in my area wants to touch a bike this old.

since dumping the rest of the can of seafoam and filling up, I have been speeding up and down the street trying to recreate the situation in which it dies (slowing down at 40+ mph) and it only died once. I messed with the idle speed again and got a nice idle sound, although it sounds like its struggling a bit. It still starts up everytime without the choke. I smell a faint smell of fuel coming from the exhaust, so maybe I am running a bit too rich? Would that cause it to die while slowing down?
 

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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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Too much or too little fuel can often produce similar effects. When it dies, have you ever tried immediately pulling the plugs to look at them? I'm wondering if they're fuel fouled.
 

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seafoam is snake oil imho

Seafoam's 2015 MSDS listed ingredients
40-60% Pale Oil (CAS 64742-54-7),
25-35% Naphtha,
10-20% Isopropyl Alcohol "IPA".

Pale oil, a light machine oil sometimes used in electric grid transformers for cooling.. simply a lubricant
Naphtha aka white gas or Coleman lantern fuel, a solvent to cut through dried gas lacquer
Isopropyl alcohol, a good gas dryer..

Newest SDS leaves ingredients up to buyers imagination.
Hydrocarbon blend* <95%
Isopropanol (CAS 67-63-0) <25%
 

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Discussion Starter · #16 ·
Too much or too little fuel can often produce similar effects. When it dies, have you ever tried immediately pulling the plugs to look at them? I'm wondering if they're fuel fouled.
hey man, still having the same issue. messing with the air fuel mix helped out for a little bit. I was referring to the screw with the tab on it on the carburetor. I twisted it a little bit and adjusted the idle again. It just loses power going down from 4th down. I can chill in third gear all day. but when I go to a complete stop while in 4th, it dies.
 

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I would hope an inline fuel filter was installed after dealership cleaned carburetor..
It's not unusual for gas tank rust or lacquer flakes to migrate into float bowl and plug idle or main jet..

Makes no since to me engine would idle in 3rd but not in 4th, or am I misinterpreting your post?
 

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Why are you in fourth gear with it stopped? Does it die if you downshift into a gear matching the bike's speed? The bike should be in first gear or neutral when stopped.

Have you taken a basic riding course? I ask because we were taught to shift into the gear appropriate for the speed the bike is travelling. When stopped in traffic, bike should be in first gear, left foot down, right foot on brake, right hand on throttle, left hand pulling clutch lever, and eyes periodically checking for vehicles approaching from the rear that may not stop.
 
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Discussion Starter · #19 ·
Why are you in fourth gear with it stopped? Does it die if you downshift into a gear matching the bike's speed? The bike should be in first gear or neutral when stopped.

Have you taken a basic riding course? I ask because we were taught to shift into the gear appropriate for the speed the bike is travelling. When stopped in traffic, bike should be in first gear, left foot down, right foot on brake, right hand on throttle, left hand pulling clutch lever, and eyes periodically checking for vehicles approaching from the rear that may not stop.
Yes I have taken the basic riding course. I have my license. When I am in 4th gear and downshifting into 3rd, I lose power. It only happens when I go into 4th and go down in the gears. I can cruise around town in 3rd, no problem. But when I try to go beyond 45 mph its risky cuz its a 50/50 chance of it dying. I can keep it alive by reving the engine when slowing down. But even then, I’ll go to give it some gas and its dead.
 

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Discussion Starter · #20 ·
I would hope an inline fuel filter was installed after dealership cleaned carburetor..
It's not unusual for gas tank rust or lacquer flakes to migrate into float bowl and plug idle or main jet..

Makes no since to me engine would idle in 3rd but not in 4th, or am I misinterpreting your post?
The situation is that when I am cruising 45 mph in 4th and downshifting into 3rd I am loosing power. I can cycle through gears 1-3 without it dying but when I try to push the bike to go faster, it dies. I can keep it alive by reving the engine as I am downshifting but even then I can go to give it gas and its dead. I gave it 2 cans of seafoam about a week apart and adjusted the fuel/air mixture and idle speed.
 
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