Montesa 348 Engine Bog

[texley]

I have a 348 that when I start the engine runs really badly and bogs on rev. The eventually dies. The fuel is at 33:1 the scew has been at 1.5 turns and in lots of other places. Carb has been cleaned. New Spark Plug used. Any ideas, more the better so I can keep trying things. Thanks

[feetupfun]

More info needed

What do you call bogging

What carby

Air filter state/condition

What jetting

Is float valve passing

Is it different behaviour hot to cold

Has it run well before

Is it new to you

Are there any changes to inlet tract or exhaust system from standard

Whatever other clues you can give

[texley]

Hi Thanks for coming back, answers below.

What do you call bogging- The engine die's as after you begin to apply the revs on throttle. So can't get it to rev out.

What carb- Standard Orginal

Air filter state/condition- Air Filter is brand new

What jetting- Not sure

Is float valve passing- May need help, not sure what this means

Is it different behaviour hot to cold- Can't get it hot

Has it run well before- Yes, I rebuilt it, though I didn't touch the carb or engine much, just a good service. Ran like a dream last time out!!

Are there any changes to inlet tract or exhaust system from standard- Exhaust is completely standard with a new end pipe but that was there when it ran well.

Whatever other clues you can give

It does seems to be spitting out a thin black substance, from the end of the pipe, but I assume this is just unburnt/poorly burnt fuelmix.

It also does not like the choke at all to start it?

Any help appreciated, it's been about 15 years since I messed with bikes so a little stumped, given it ran like a dream a few weeks ago and nothings change. I have started it and just run round the garden a few times, could I have coked it up, but I did add a new plug with no difference?

[heavywrecker]

Are you sure the points are in good shape and the condenser hasn't failed?

Run the bike with the cover off and check for arc-ing across the points.

[texley]

I'll try that and come back to you. Thanks

Tim

[feetupfun]

Excellent info there. It sounds like the float needle valve is stuck open. Inside the bottom of the carby is a plastic float that opens and closes a valve to control the fuel level inside the bottom half of the carby. The mechanism can sometimes stick with the valve open if the fuel dries out as it would have while you had the bike apart.

Diagnosis:

Try starting the motor again but as soon as it starts, close the fuel tap and see if the running improves as the fuel in the float bowl is used up. If it does that then you will need to free up the float valve mechanism. On that bike you can do that by taking off one of the frame braces and the top of the carby then rotating the carby so you can take the bottom (float bowl) off.

Do the fuel-tap-shut testing first before working on it.

[tayld]

When cleaning or checking the carburettor have the pilot and cold start jets been swapped round?

The 20 pilot jet is screwed into the main body on the face where the float bowl fits. The 40 cold start jet is found at the bottom of a hole in the float bowl. Both jets look identical at first glance.

The 40 jet in the pilot position will make the mixture rich.

Hope this helps.

Edited November 30, 2010 by Tayld

[texley]

I've bought a Carb refurb kit and will try that when it arrives. Thanks all for help and advice. I'll update progress once I've tried all your suggestions.