HELP!! rev problems

Author
Discussion

roders89

Original Poster:

20 posts

96 months

Wednesday 29th June 2016
quotequote all
My bike is a 125 4stroke when is idling for about 10 15 mins and warm if I rev it up to 7 thou rpm and let go of the throttle it drops to 3thou rpm stays there for 1 min then slowly drops to idling speed of 1500 rpm.
Has anyone had this problem and hows it fixed?? My bike has a cv carb iv striped and checked it and all seems ok
I cant get the bike throw m.o.t because they keep saying sticky throttle cable but is not iv new one installed to be sure

bimsb6

8,034 posts

220 months

Wednesday 29th June 2016
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Normally caused by an air leak between carb and cylinder head or a dodgy carb diaphragm.

Private Pile

754 posts

194 months

Wednesday 29th June 2016
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Was the problem exactly the same before and after you stripped the carb? If you disconnect the throttle cable at the carb , that'll confirm if it's a sticky cable. Try spraying wd40/carb cleaner around the carb to see if it is drawing air. The revs wil rise if it is.

crofty1984

15,830 posts

203 months

Wednesday 29th June 2016
quotequote all
Get some easy start and when it's idling spray it at the joints in the carb rubbers between the carb exit and manifold. If the revs rise you have an air leak.
Be careful not to get it in the air filter/carb INLET. You won't break it, but you might get a false reading as the easy start gets into the engine the "normal" way.
I hear you can use WD40 if you don't have ready start.

roders89

Original Poster:

20 posts

96 months

Wednesday 29th June 2016
quotequote all
Iv tryed all that and nothing realy has me stumped

crofty1984

15,830 posts

203 months

Wednesday 29th June 2016
quotequote all
Sticky throttle slide in the carb/damaged diaphragm maybe?

bimsb6

8,034 posts

220 months

Wednesday 29th June 2016
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Is the slide the right way in ?

s3fella

10,524 posts

186 months

Wednesday 29th June 2016
quotequote all
Sounds like the mixture may be wrong, is the idle mix too week and you or someone has compensated by upping the idle speed? The idle speed sounds quite high. When you stripped the carb, you didn't do the fuel screw up tight did you, thinking it was a jet?

Try turning the idle right down and opening up the fuel screw a full turn. Rough rule of thumb, fuel screw (idle mix screw) would be about 2.5 to 3 turns out on a typical mikuni style carb.

BTW, I just bought a 450 enduro bike cheap as the carb was "knackered" "been rebuilt and it is kaput!"
Symptoms like you describe, and this was the simple fix, took me 5 mins! And three of that was modding a screw driver bit to make the adjustment!

Edited by s3fella on Wednesday 29th June 19:39

roders89

Original Poster:

20 posts

96 months

Wednesday 29th June 2016
quotequote all
My carb has only one screw on it its mikuni bs26 doesnt matter wat way I screw it does nothing???

roders89

Original Poster:

20 posts

96 months

Wednesday 29th June 2016
quotequote all

s3fella

10,524 posts

186 months

Thursday 30th June 2016
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Its number 11 on that diagram

Yazza54

18,464 posts

180 months

Friday 1st July 2016
quotequote all
roders89 said:
My carb has only one screw on it its mikuni bs26 doesnt matter wat way I screw it does nothing???
As above try turning it all the way in then about 2 full turns out. It certainly does matter, this is your air screw not to be confused with a simple idle adjuster. The more turns in the richer the mixture I.e less air, more turns out it becomes weaker. If it's not that I really would be surprised if you didn't have an air leak somewhere. The leak doesn't necessarily have to be at the carb, could be a base gasket for example.

Edited by Yazza54 on Friday 1st July 06:00

bimsb6

8,034 posts

220 months

Friday 1st July 2016
quotequote all
Yazza54 said:
. The leak doesn't necessarily have to be at the carb, could be a base gasket for example.

Edited by Yazza54 on Friday 1st July 06:00
On a four stroke ?

Yazza54

18,464 posts

180 months

Friday 1st July 2016
quotequote all
bimsb6 said:
Yazza54 said:
. The leak doesn't necessarily have to be at the carb, could be a base gasket for example.

Edited by Yazza54 on Friday 1st July 06:00
On a four stroke ?
It's Got a removable cylinder, it's only a little 125 it's not part of the crank cases like a big bike.

Mr2Mike

20,143 posts

254 months

Friday 1st July 2016
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Yazza54 said:
It's Got a removable cylinder, it's only a little 125 it's not part of the crank cases like a big bike.
I think the point is that a bad base gasket on a four stroke would just leak oil out, it wouldn't cause it to run lean.

Yazza54

18,464 posts

180 months

Friday 1st July 2016
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Doh you're right. Too early for my puny brain.

roders89

Original Poster:

20 posts

96 months

Sunday 3rd July 2016
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S3fella you were right bikes purring like a kitten thank you this prob really toke the piss

s3fella

10,524 posts

186 months

Monday 4th July 2016
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roders89 said:
S3fella you were right bikes purring like a kitten thank you this prob really toke the piss
Nice one! It's normally something simple!!