Middle cylinder

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apwrigs
Posts: 205
Joined: Tue Apr 05, 2016 11:00 pm
Location: Oldham

Middle cylinder

Post by apwrigs »

Chaps, but of a carb head head scratcher, for me anyway...

77B model, fully rebuilt carbs. Bike starts and runs well but the middle exhaust is cool. Fuel is in the float bowls, strong spark all 3 plugs, but when I've removed the middle plug it looks dry and hardly used. Start the bike up, and when I turn on the choke the middle pot comes on hot, but if choke left off it remains cool. Any thoughts ..?....Paul
barney01
Posts: 3118
Joined: Wed Sep 26, 2012 7:33 pm
Location: Prestatyn.Wales

Re: Middle cylinder

Post by barney01 »

Almost certainly a blocked pilot jet or blocked pilot jet passage and bypass. I know you say the carbs are rebuilt but it impossible to see into the internal passages. You can test to see if they are clear by spraying carb cleaner into the passage and observing the amount of fluid exiting into the venturi. The pilot jet provides the fuel for tickover and up to 1/4 throttle.
NO SMOKE .......NO POKE
apwrigs
Posts: 205
Joined: Tue Apr 05, 2016 11:00 pm
Location: Oldham

Re: Middle cylinder

Post by apwrigs »

Thanks Barney, I take it you are reffering to main jet in the float bowl, remove it and spray down the passageway???
teazer
Posts: 1035
Joined: Mon Mar 07, 2011 8:10 pm
Location: Chicago

Re: Middle cylinder

Post by teazer »

Almost, but not quite. He is referring to the PILOT jet in the float bowl plus the pilot air jet and outlets in the carb body.
barney01
Posts: 3118
Joined: Wed Sep 26, 2012 7:33 pm
Location: Prestatyn.Wales

Re: Middle cylinder

Post by barney01 »

Not the main jet but the smaller pilot jet next to it.. the spray will exit into the venturi under the throttle valve and also in the bypass which is in the front of the venturi.
The only other possibility is an incorrect fuel level. Too high makes it run rich but too low means the jet doesn't have to power to suck up fuel. The reason it fires ok on choke is because the choke circuit draws fuel from the bottom of the bowl.
Have a look here
https://youtu.be/qz_ALOT24JA
This is for the early seperate carb but they are essentialy the same underneath
NO SMOKE .......NO POKE
apwrigs
Posts: 205
Joined: Tue Apr 05, 2016 11:00 pm
Location: Oldham

Re: Middle cylinder

Post by apwrigs »

Many thanks for that chaps.

First job tommorw.
apwrigs
Posts: 205
Joined: Tue Apr 05, 2016 11:00 pm
Location: Oldham

Re: Middle cylinder

Post by apwrigs »

Cheers fellers , that worked a treat.
It turned out to be some foreign matter in the passageway behind the iddle jet on the bowl.
barney01
Posts: 3118
Joined: Wed Sep 26, 2012 7:33 pm
Location: Prestatyn.Wales

Re: Middle cylinder

Post by barney01 »

Result! :D
NO SMOKE .......NO POKE
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