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Discussion Starter · #1 ·
Hello, I recently bought a cheap CBR250RR MC22 1993.

It wasn't working at first and I found this to be a bad connection in the wiring which has now been fixed.

Now i've managed to get it to start a few times, the idle will start out normal but the rev's will quickly climb without me touching the throttle at all.

If I stop the engine and start it again right after the revs will start from where it was last time. So for example the first time I started it it revved up to 5k rev's. I then killed the engine and started it again and it started from 5k and revved up to 7-8k, where it seems to stop going up. When it was doing this revving thing I tried to give it some throttle and the revs actually went down... Also there was a bit of white smoke coming out the exhaust, and the first time I started it there was petrol (or oil?) inside the tip of the exhaust.

I'm pretty sure this has nothing to do with the throttle cable or idle screw as I took the carbs off and gave them a clean and all that seemed to be working as it should.

One thing I noticed when cleaning the carbs was one pilot screw was different from the rest, and also the O ring seems a bit worn. Apart from this everything looked okay (to me).

I'm not really sure what to try next. The guy who sold me the bike said he went overseas for a month and came back and it wouldn't start.
He also said that recently it has had new spark plugs, oil filter, fork seals & rear tyre fitted.

I have checked the spark plugs and they looked normal.
I'll probably change the oil next, when I checked it the dip-stick smells a bit like petrol... is that normal?

Other than that all I can think of is to buy new rubber boots connecting the carbs to the manifold. When I removed the carbs I actually only got one boot off the manifold and the rest I couldn't remove. I couldn't see any obvious damage but some were a little frayed.

Also I will probably order new o-rings and gaskets for the carbs as well. But I though i'd post here first and maybe I can find out what's wrong and if it can even be fixed before I start ordering parts from overseas. (I live in New Zealand)

Hopefully someone can tell me what's wrong. I can also try start it again and upload a video if that will help, but i'll have to jump start it as the battery has gone flat now.
Also I have pictures of the pilot screws and how one is different which I could upload if it helps, also have pics of the spark plugs from when I removed them.

Thanks!

Edit: Also I had some trouble getting it to start after cleaning the carbs, I guessed this is probably from incorrect pilot screw settings? I set them to 1 and 3/4 turns. (I stupidly didn't note what they were originally). It did eventually start and was doing the same revving thing.
 

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Carbs are probably way out of balance. Idle screw is probably wrong too. Try setting the idle screw when the bike is warm. You also might have messed up the float heights and causing it to run rich. If there's fuel in the oil then that is a sign of worn rings, or that your carbs are leaking fuel into the cylinders while the engine is off and it's seeping past the rings and into the oil.
 

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Discussion Starter · #3 ·
Jashdown said:
Carbs are probably way out of balance. Idle screw is probably wrong too. Try setting the idle screw when the bike is warm. You also might have messed up the float heights and causing it to run rich. If there's fuel in the oil then that is a sign of worn rings, or that your carbs are leaking fuel into the cylinders while the engine is off and it's seeping past the rings and into the oil.
Thanks for the reply.
I have tried changing the idle screw even setting it to the very minimum and the bike is still revving.
I'm not even sure how to go about balancing carbs when bike is doing this. I had planned to do it after I stop the revving issue.
 

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Discussion Starter · #6 ·
Jashdown said:
The revving is also a sign of an air leak. Are you sure you put the carbs back on correctly with the boots fitting properly? I would say there's an issue with the boots.

Well the revving problem was actually the reason I cleaned the carbs, and was happening before. I couldn't see any obvious damage to the boots but like I said I plan on ordering some new boots next, just thought someone might have another suggestion.

Cheers
 

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Oil should not smell like petrol.
If its contaminated you should change it.
Check the coolant and make sure its normal, anything resembling a milkshake colour is bad.
Certainly get new carb boots, a bike running lean due to an air leak will rev high on its own. what did you set the pilot screws to?
 

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Discussion Starter · #9 ·
didjabringabeer said:
Running to lean?, recheck your pilot screws are correct.
My 2 cents, not worth much but might be right though:D
Thanks. Would I turn the pilot screw in or out to make the mixture less lean?

Also one of my pilot screws was different from the other 3. Could this be the problem? I have attached a picture. The pilot screw at the top was the different one.

Aduro said:
Oil should not smell like petrol.
If its contaminated you should change it.
Check the coolant and make sure its normal, anything resembling a milkshake colour is bad.
Certainly get new carb boots, a bike running lean due to an air leak will rev high on its own. what did you set the pilot screws to?
I set the pilot screws to 1 and 3/4 turns out.
 

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Don't worry about the screw head end looking different, the bit that matters is the tapered end that protrudes into the carb, and from that pic, they look the same.

When the bike is off, does the throttle snap back nice and fast without resistance?

Are the throttle cables routed properly and not twisted etc?

It's a good bet the boots are rooted and the cause of your troubles but check these other things too while you're at it.
 

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Discussion Starter · #11 ·
Lane said:
Don't worry about the screw head end looking different, the bit that matters is the tapered end that protrudes into the carb, and from that pic, they look the same.

When the bike is off, does the throttle snap back nice and fast without resistance?

Are the throttle cables routed properly and not twisted etc?

It's a good bet the boots are rooted and the cause of your troubles but check these other things too while you're at it.

Okay thanks.

When I first put the carbs back on and re attached the throttle cable it wasn't snapping back, but I sprayed it with some crc (at the end where it attaches to the carbs) and it loosened up and springs back fine now.

I'm pretty sure it's not an issue with the throttle, I've even checked the butterfly valve with the airbox off looking down the trumpet things, and it seems to be working as it should.

Any suggestions of where I can buy the rubber boots? I live in New Zealand and assume I'll have to order them online from overseas.
 

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You can rule out throttle cable/ routement issues altogether by looking down the carb with the airbox off and ensuring the butterflys are closing when the throttle is not engaged. I doubt this is your problem. idle screw could be causing you grief but I'm much more inclined to suggest its your carby settings/ carby boots causing your issues
 

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Spray carby cleaner around the carby boots to check for leaks, you will hear the engine rev up if theres a leak.

I would've replaced the carby boots unless i knew they were new or still good. 20yrs who knows whats gone wrong
 

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Discussion Starter · #16 ·
Have replaced the rubber carb boots with genuine honda ones from japan and it still idles at about 6k. (sometimes it climbs very slowly) It is nothing to do with throttle cable / idle screw. Any other ideas? Could incorrect pilot screw settings or carbs way out of balance do that?

There is also a lot of white / slightly blue smoke coming from the exhaust.

I also changed the oil and it was really black and looked like it had a fair bit more than 2.4L in there, also had a slight petrol smell. I'm guessing this may have been because it was sitting for a few months and the fuel tap had been left on and possibly leaking a bit?
 

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Hey Mate, Probably a long shot, hoping by some miracle you are still active. Fast forward 10 years, I've picked up an old MC22 and am having the same issue. Wanted to see if you every sorted your bike out, if so, what was the fix?
 

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Engine speed is determined by how much air engine is sucking in. ONLY way a bike can rev to 6K-rpm is if there's enough air going in to support that much speed.

OP never reported synching carbs. One or more of them probably has butterfly open more than others when throttle is closed. Easiest way to set sync is when carbs are off for rebuild. Use feeler gauge between butterfly and side of carb-throat to set them all same opening amount. This gets you roughly in ballpark.

Then with carbs back on bike, use sync gauges to set identical vacuum on all cylinders. This fine-tunes butterfly of each carb to be identical.

Had to do this on both my MC19 and MC22 when I put them together.
 
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