Author Topic: renewed carb jets has made it worse  (Read 567 times)

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maddogmargetts@yahoo.co.u

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renewed carb jets has made it worse
« on: 10 June, 2020, 18:33:18 »
My M21 was running ok but i would like to slowly go through the bike and sort out problems so i put new jets, needle, float and gaskets in the carb and now although it starts easy enough it runs for a few seconds and conks out. All the settings are as they were previously, the jet sizes are the same, so i just cannot understand it. The spark plug is sotty and yet weakening the mixture makes no difference. Maybe i should just put it back as it was before but now i am worried it still won't run properly. It ran fine before, just a few little ups and downs but i thought no harm can come of replacing old bits with new ones which are exactly the same. Would someone take pity on me please and give me a suggestion or two?
john

JulianS

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Re: renewed carb jets has made it worse
« Reply #1 on: 10 June, 2020, 18:42:23 »
Which type of carb?

If a monobloc, old type, which uses the thick O ring, make sure you  have not fitted a fibre washer between the float needle seat and carb body. This is only used on the new type which uses the thin concentric O ring, which has a recess to accept it. Used on the old type raises fuel level in float chamber and can cause flooding.

A few years back I replaced jet needle and needle jet in a monobloc but was supplied pattern not genuine Amal parts. It ran roughly until I replaced with genuine Amal parts.

maddogmargetts@yahoo.co.u

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Re: renewed carb jets has made it worse
« Reply #2 on: 10 June, 2020, 21:07:16 »
thanks. Yes monobloc 376 with thick o ring. I did put a fibre washer because petrol gushed out and i was wary of overtightening the bolt through the banjo. when you say needle jet, you mean the needle which is pushed in and out by the float, yes? Presumably then the seat is the bit that screws into the float chamber and holds the filter and banjo feed from the tank. So its just the banjo tightened onto the plastic lip of the filter that does the sealing?
i shall attend to it and see if that cures the problem
john

JulianS

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Re: renewed carb jets has made it worse
« Reply #3 on: 10 June, 2020, 21:26:59 »
Yes the seat is the bit which screws into the carb body.

In extreme cases the addition of the fibre washer on the early monoboc causes the float to touch the tickler and prevents the float needle seating and sealing of the petrol.

Petrol should not be so high as the leak from the float seat/carb body joint.

Yes the tightening of banjo bolt with fibre washer at the top seals that joint

Mike40M

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Re: renewed carb jets has made it worse
« Reply #4 on: 11 June, 2020, 06:14:06 »
I have the habit of reading different forums concerning the bikes I have. As always, you can't  believe everything on the net. But some people seems to have very long experience.  Some thoughts of them:
Measuring flow on new jets sometimes shows up to 10% difference.
The parts in a Monobloc that wears is the throttle slide and bore. Also needle and needle jet. On a worn carb, increased throttle air leak can be compensated by higher flow through needle jet. Replacing needle and needle jet can make it run weak.
Fuel level in float chamber can change replacing float, check.
Not certain if it applies to Monoblocs, but a new float don't always move free, check if it needs trimming.
Personally have bad experience with a new Matchbox float chamber from AMAL. Even after adjusting, it didn't supply enough for long straights, racing the Manx.

maddogmargetts@yahoo.co.u

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Re: renewed carb jets has made it worse
« Reply #5 on: 12 June, 2020, 08:45:12 »
took out the fibre washer and screwed the needle seat bit straight onto the carb and vrrmmmm. It worked! Thanks for the advise everyone. Who would have thought a humble fibre washer would make such a difference although it must have altered the distance where the needle goes into the seat. Next job : put new valves in.....
john