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Engine knocking when under load and cutting out when going flat out across an field


dazzagreener
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Hello

Iv noticed that my bike makes a very faint engine knock when doing climbs ect and also if i go flat out across a feild and as soon as i let off to slow down the bike stalls and takes a while to start agian.

my fuel mixture is 64ml for 4 litres

the carb is set up as standard for a 2002 this is from the service manual

33 pilot

126 main

3.5 turns on the air mixture

3rd notch on the pin

i noticed that on the 2009 service manual that all the settings are the same but they use 122 main jet instead can anyone explain should i change mine to this, would it help with my problems

hope someone can help me

cheers

Darren

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Hello

Iv noticed that my bike makes a very faint engine knock when doing climbs ect and also if i go flat out across a feild and as soon as i let off to slow down the bike stalls and takes a while to start agian.

my fuel mixture is 64ml for 4 litres

the carb is set up as standard for a 2002 this is from the service manual

33 pilot

126 main

3.5 turns on the air mixture

3rd notch on the pin

i noticed that on the 2009 service manual that all the settings are the same but they use 122 main jet instead can anyone explain should i change mine to this, would it help with my problems

hope someone can help me

cheers

Darren

the smaller main jet would make it worse,it sounds like its running lean.

if it ran fine before then it could be fuel starvation, this could be caused by the follwing, blocked fuel tap or filter, sticking float or float needle, tank not venting or carb vent tubes blocked.

check these first then try raising the needle to middle clip to run a little richer, (you can also give it a little choke once warmed up as this will enrichen the mixture a little to see what difference it makes) if its better then the jettings needs richening if worse then the oposite

Edited by stump magnet
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Hi

Some Sherco came with a 200 float valve this would cause the problem you are experiencing, so check to make sure you have a 250 or even a 270 (if you like going flat out a lot) float valve in your carb.

You can purchase a dellorto float needle valve here.

Other things to check are, make sure the breather pipe is clear and nothing is causing it to block.

Hope this helps.

Chris

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Check the gas tap -Stand pipe in the gas tank - you need to remove the gas tap - sorry petrol tap - on some ealier bikes

this plastic tube was too long and caused lean conditions. - before doing this try running the bike with the tap on the reserve position. - you can check all the service data by clicking on www.shercousa.com

Note ! We always fit a larger pilot jet - 35 or 36.

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I was on a #38 pilot befor I ever got my '07 to stop pinging on accel. Seems from feedback, many have not need to go that high, specially in UK with their fuel, yet I never though I would need it either, yet seems some of this varies with the carb and so on. The alcohol contents of most fuels seem to influence this I think, which may also vary. All point being, never presume a 36 will fix this, based upon my experience!

Mix set accordingly, was sweet after that with the Dellorto, till I changed, then start over with the OKO, which ironically enough ended on the #38 as well. :thumbup:

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It can be a natural feature of two stroke carburation (due to pulsing through the carb at low / medium RPM which does not occur flat out) that they run weak at the top end.

From you symptions you certainly need to richen the bike across the whole rev range (assuming timing and plug correct). You plug colour should give an indication of rich / lean. 60:1 (68:4000)is very little oil for sustained high speed blasts whatever type of oil you are using.

Cheers

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  • 2 weeks later...
 
  • 2 months later...

iv still got the problem!!!

i think the bike runs lean because a way to start it again when it cuts out going acoss a feild it to put the choke on. also when it starts it revs up very high like its lean.

i but a new plug in hoping this would solve the problem

cheers

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iv still got the problem!!!

i think the bike runs lean because a way to start it again when it cuts out going acoss a feild it to put the choke on. also when it starts it revs up very high like its lean.

i but a new plug in hoping this would solve the problem

cheers

If it does this just running accross a field, yet ok normally or just short run, one would want to insure fuel is flowing properly. A pinchet vent tube or stuck vent to the tank cap can cause a vacume in the tank. A blocked screen on the petcock valve could restrict proper flow. With a Dellorto the banjo filter on the carb could do the same. Then there is the float valve as Chris stated. A #250 min.

The fuel must flow reliably, lest it runs out. It is easy enough to ck these things. :thumbup:

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If it does this just running accross a field, yet ok normally or just short run, one would want to insure fuel is flowing properly. A pinchet vent tube or stuck vent to the tank cap can cause a vacume in the tank. A blocked screen on the petcock valve could restrict proper flow. With a Dellorto the banjo filter on the carb could do the same. Then there is the float valve as Chris stated. A #250 min.

The fuel must flow reliably, lest it runs out. It is easy enough to ck these things. :thumbup:

Thanks mate i will look in to this. i did check the vlave on the tank thats ok. also iv checked the float valve thats ok to and cleaned the banjo filter. i will look at the fuel tap filter tonight hopfully.

also what mixture should i run i normally do 60:1 but a mate of mine with the same bike does 75:1

cheers

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