CB250N Won't rev beyond 5000 revs

[MadDogMcQ]

I have a 1980 CB250N which I have refurbished over the last year or so. Having virtually completed the work, I fired it up and Vroom!! Engine started first time and ticks over superbly. BUT, it won't rev beyond 5k. It splutters and dies until the revs drop back down.

Fuel supply is okay, vent in tank okay, plugs okay, timing okay, compression okay, carb inlet-boots okay, alternator okay, valve clearances okay, cuddly toy okay!

Having read a similar thread by ANT123 on here, I was overjoyed to read his final post and rushed out to remove the air filter to see if it made any difference - and it doesn't! So I'm guessing that Ant's air filter had bugger all to do with it and his Dad has more likely cured it (inadvertently) some other way.

Anyone any ideas?? Damn, its soooo frustrating!! PLEASE help!

https://www.mcqart.com/mcqweb/images/bikes/cb250n/b4_96.jpg

[temeluchus]

drained the carbs? carb diaphragms ok?
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Some shite cruiser. Now with guns and FREEDOM!

[TQ]

Check the jets aren't blocked.
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First Bike :Gilera DNA 50 | Second: Imported 1989 NSR125
Third: 1998 Yamaha XJ600N | Currently: 1999 Yamaha FZS600

[MadDogMcQ]

Carbs have been completely overhauled, ultrasonically cleaned, new Keyster Carb Kits fitted (jets, needles, washers, etc) and the float heights have been set.

Go figure!

[temeluchus]

if the carburettors are ok as you say and all functional Then youll have to check the timing again. Is it electronic ignition or points?

Id also recheck the float heights as this can starve the engine if set incorrectly.
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Some shite cruiser. Now with guns and FREEDOM!

[pepperami]

Does it have an advance/retard part to the ignition? is that stuck/not working?
did it used to be easier to get it to fire on one cylinder and harder to get it to fire on the other cylinder,..... that can be a sign that the advance/retard is not working right?

i am having similar problems with an old XS400 i am putting back together.

just an idea!
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I am the sum total of my own existence, what went before makes me who I am now!

[temeluchus]

[temeluchus]

this is you i take it

https://cb250n.blogspot.com/
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Some shite cruiser. Now with guns and FREEDOM!

[Odie]

Check your alternator.
It can be that your alternator isnt kicking out enough voltage to allow the ignition to advance the timing at 5,000 rpm, same problem that the CX500 used to get.
Also, your carb to cylinder head rubbers, if they are not split, have they gone hard, if so they will not seal properly and let air in and cause problems.
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You are so ugly when you were born the doctor said "I?m gonna drop it, if it falls is a rat, if it flies is a bat."

[temeluchus]

The pickups appear to to be integrated with the generator stator. Do you have another stator to try?
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Some shite cruiser. Now with guns and FREEDOM!

[Ditto]

Is the fuel new and good?

I had a similar problem because of water in the tank.
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CG 125 > Bandit 600 > CBR 600 FY > Street Triple R

[MadDogMcQ]

Hi Guys,
The fuel is fresh. The alternator voltage is nice and high and increases with revs, the inlet rubbers are new. The advancing of the ignition is done electronically by the CDI and is working fine.

It has me VERY puzzled up to now. I'm away for the weekend and then the carbs are coming off (I'm not sure why, but I've looked at everything else!).

[temeluchus]

well assuming the ignition is 100% it must be fuel related....

Float heights, hunction of the tap, state of the filters, breather in the fuel cap, condition of the vacuum diaphragms etc etc
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Some shite cruiser. Now with guns and FREEDOM!

[Ant132]

[Blue_SV650S]

This may sound a bit stupid, but have you actually tried riding it or is all this done on the stand???

My wetdream always needed a bit of a run before it would rev cleanly ....
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The purpose of life is to fight maturity.

[MaybeGuy]

cam timing. does it start without the choke? almost too easily?
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Blue_SV650S wrote: it was a sh1te wheelie, but it proves that he can get it up in 3rd and can do angles. In summery, mattsprattuk is a gobby little sh1tebag, dopehead tw4t, but sadly for all of us, he probably isn't THAT full of sh1te!!
Kickstart wrote: Hi I tend to agree with Matt. All the best Keith

[MadDogMcQ]

Sorry Ant, but I just cannot accept that it was because your airfilter plate was upside down. It is, after all, just a plate full of holes and whether it's upside down or not, the holes are still there. My air-filter is fitted correctly anyway and it makes no difference to the problem whether it's installed or not. I assume you mean the plate indicated in the arrow below btw.

Ant, your bike was running perfectly before you changed the rocker-gasket and the oil, so how has the air filter suddenly effected it? Come on, let's be sensible. I'm REALLY glad your bike's running fine, but I reckon you must have had a dodgy connection somewhere and your Dad's "fiddling" has sorted it.

And yes, the problem on my bike exists on and off the road and the cam-timing's spot on. I didn't even look at the bike yesterday - it's peeing me off.

[27cows]

It's got to be something simple. Something so ridiculous you probably wouldn't think of it, like the ignition wiring block not fully pushed home, leading to a weak spark under compression (happened to me on one of my 400 Superdreams...and took me days to suss ).
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The RXS100: vehicle of choice for Chuck Norris

[MadDogMcQ]

I think you may be right 27cows. Getting all those cable and connectors into the headlamp shell is a total pig(!) and has all the potential for problems. I have, however, checked mine and everthing looks sound. I've even tried changing the ignition switch itself and that made no difference either.

[27cows]

Is the crankcase breather clear of obstructions, pipe not kinked/trapped?
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The RXS100: vehicle of choice for Chuck Norris

[nightshaddow]

i had the same problem with my 125 tdc wetdream turned ot the 24 year old bike(at the time) needed new coils, not sure if you can have them tested but it would start and run up to 4500 rpm but almost choking out any higher, i checked loads of things and turned out under load it wouldent fire not sure if its the same for you but maybe something to check
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"WHAT DO YOU MEAN 100 MPH THATS IMPOSSIBLE Its chinese OFFICER."
Current rides CB 125 TDE(the next project) NS125RK ( now on the road ) CB125TDC (soon to be project 17) melody ( the nephews )

[MadDogMcQ]

Yes, KnightShadow, the coil is a likely culprit. In fact, I was on wemoto.co.uk earlier, pricing one up - £27. It's just such a bugger having to spend money on this stuff to find out if it's the problem item. My CDI arrived today and I know now that it's not that, so there's £15 wasted!

[nightshaddow]

generally to my knowledge the cdi normally works or not and theirs no in between (but i could be wrong) a coil may brake down and voltage and heat build up could make it worse,

i hope the coils work for you but keeping my thinking cap on

good luck
____________________
"WHAT DO YOU MEAN 100 MPH THATS IMPOSSIBLE Its chinese OFFICER."
Current rides CB 125 TDE(the next project) NS125RK ( now on the road ) CB125TDC (soon to be project 17) melody ( the nephews )

[temeluchus]

do you have another bike the you could grab a coil from?

any twin lead japanese coil should do the trick, like a coil from a bandit, early GSXR, GPzs and probably many others.
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Some shite cruiser. Now with guns and FREEDOM!

[big lad]

Hello, I have exactly the same problem... Did you ever find a cure??? I've had my carb off about 20 times thinking its something to do with that..I'm ready for putting a flame to the bike!!!

Please help