general (amal) carb question

Fast Eddie said:

Just for clarity, a faulty cold start plunger seat allows excess fuel to pass… not air.

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Fast Eddie said:

Just for clarity, a faulty cold start plunger seat allows excess fuel to pass… not air.

Click to expand...

Mart UK said:

The start point for the pilot jet on a single 34 mk2 Amal is 15. A 15 doesn't work on mine. I'm running a 20, just under one and a half turns out. Does that suggest more fuel is getting into the mix at idle, requiring more air? If so, I suppose it might be the choke (enrichener). No harm checking! I have some replacement fuel lines coming this week, so when I remove the carb again I'll take a look at the choke.

One notch leaner on the needle definitely introduced decel pop. So, I'm now doubting it was previously over-rich on this phase of the carb. I think this finding is pointing me to suspect the fuelling during the 1st 1/4 throttle as the source of the problems. The rich running and plug fouling might be connected to the intermittent missing at idle / low throttle in 1st gear. Possibly difficult to get to muck in the pilot bores and/or the enrichener being faulty.

So it's probably an ignition problem!

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Mart UK said:

Boyer ignition from 2013, with a Boyer Powerbox. I checked the 2 x 6V coils when I had a similar problem last time. They checked out OK. I've done runs well over 100 miles with lights on and using the electric starter. So, I think it's charging OK. Last time the plugs were again sooty and it was sorted with new plugs, but I still had the intermittent missing at tickover and slight unevenness at low speed / throttle. The battery is amply sized, fully charged and was new this spring. Plugs are new again. It runs well under acceleration and cruising on a steady throttle. I was thinking, this is when the ignition is under most demand, so I was ruling it out?

The soot on the old plugs was dry and powdery, not oily.

I took the choke out this morning. The brass plunger has a black seal captured at the base (viton?). It showed a light wear ring, suggesting seating all round it's circumference. It was clean. There is slight lateral wiggle movement on the plunger when tightened down, it does seem to seat fully down when I turn the enrichener off by lifting the lever.

I think it's a 3 1/2 slide, anodised, it moves smoothly, no sign of any particular areas of wear and I hear it click onto the throttle screw just before the twistgrip is fully closed. Is there something particular I should look for / check?

I'm trying to think what may cause richness at idle / pilot circuit only.
I checked the float level, just the bowl connected to the tank. It cut the fuel supply with the float parallel and level with the top of the float bowl. I then tested it with the bowl attached to the body and it is still cutting the fuel, no sign of fuel from the inlet side of the carb. The rubber air vent tubes had a film of fuel on the open end below the carb and there is a faint smell of fuel. Is this normal?

The other things I've been thinking about are:
1. The seal at the top of the slide chamber (although this would introduce air, not fuel). The plastic screw on cap looks ok. The cable exits through a rubber bell shaped sleeve. This is a bit ropey. Could air leak through here and disrupt the carb?
2. I recently adjusted (within 100 miles) the rocker clearances. Before adjustment the inlet clearances were tight. I adjusted, walked away for a cuppa and then came back and rechecked, then double checked! I think they're fine, but could clearances have an effect on this? That said, I had the problem before and after.

So, I'm still stumped! It seems to run rich at tickover and low throttle, intermittent missing. Rich enough to require new plugs at 2,000 miles. I think the pilot jet is correct. It ticks over most happily at 1200 rpm with the air screw out just under 1.5 turns. It doesn't like it if I open the air screw much more than half a turn in either direction. It will stall with the screw fully in, or out beyond 2.5 to 3 turns. It doesn't want to idle and roll on the throttle if it is set much below 1200rpm.

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