Another carb problem

Hi all,

I have recently rebuilt my ST70 and am now at the stage of getting it to run correctly but have come across a problem, when the bike is running it coughs and splutters when I am trying to accelerate, it is great on tickover and revs freely when stopped but I have to give it 3/4 choke to get the bike to run above 20 mph. I have checked the needle jet and the circlip is on the middle setting, the main jet is clean and is a 58 jet. I want to get it running so I can put it up for sale to finance the next project so any help would be appreciated.

Thanks
Dom. (yelloperil)

Hi dom, my problem was running on idle too rich and cutting out! ive had my carb ultra-sonic cleaned 4times and had it in bits 10 or more times. in the end i bought carb of ebay BUT!!! its jetted different so you will have to change them over and fuel tap on wrong side and was fitted wrong but it now runs ok touch wood. if you look on ebay hes goes under name jimsixy and was £39.00 goodluck dom ps you carnt get rid of it!!
regards lee

When the problem occurs at
0/3 - 1/3 throttle opening = idle jet
1/3 - 2/3 = needle height
2/3 - 3/3 = main jet

0/3 - 3/3 = wrong mixture settings, broken parts or worn gaskets.
Since you need to use the choke, it's obvious you're getting too much air, or not enough fuel.

Things to check:
- gaskets, always replace them, even if they look like new
- spark plug, the color gives a lot of info about the mixture. Check with a new one and make sure the gap width is correct
- mixture settings, reset your carb correctly.

Hi All,

I too have the same running problem and I can not seem to get to the bottom of it.
The bike runs lovely at tick-over but will not pull above 25mph with out a little choke (Same as Yelloperil). I have set the bottom end pilot jet up with a colour tune kit, all the other settings on the carb are standard and the spark plug is a lovely lite chocolate brown.

Not wishing to spend too much time with the old carb I got a new one of eBay from Jimmysixty (same as Monkey Bike UK sell), I haven't fitted it yet though as I notice the manifold mounting holes are a different spacing. How have others got around this problem? Should I give up or perceiver with the original carb?

Cheers David

hi dav, i didnt have that problem with mine the inlet manifold and the carb was the same but the bolts wasnt long enough so i used longer bolts! but watch out because mine run alright with new carb on but wouldnt revv up properly? ive took old jets out of old carb and put in new carb and fingers crossed. i think new carbs are jetted like c90 because it looks spits. when you strip them look at the main jets you can see difference, goodluck regards lee

Hi Guys, I have got mine running great now. I purchased a keyster carb kit and replaced all the jets , gaskets and the needle. The bike is a different bike, I am reluctant to sell it now.

Does anyone know if both jets are suppossed to have a hole all the way through, as it would appear to me that the smaller of the two doesn't have a hole all the way through. It would suggest, by the hole at the centre of it, that it should.

Any ideas?
Cheers

just to confirm my carb is this one and it is part 19 that I am confused about:

http://images.cmsnl.com/img/partslists/ ... 3_73ba.gif

Cheers

Yeah, I'm pretty sure it should.
The main jet has a hole all the way from bottom to top, and some holes in the side.

I'm not 100% sure, but I thought the original idle jet has just a hole from top to bottom.
Try to poke in it with a small piece of copper wire (from an electricity wire) or something, or try to blow the dirt out of the jet.

It has a four holes coming out of the side and these were a little dirty when I took it apart. I tried to blow through it with no luck, will try the small piece of wire idea. So it is an idle jet, this would probably explain why the idle can be a little irregular.

Will let you know how I get on.
Cheers