ET: Engine (Twin) Uneven Running in Lower Gears

[nigsey]

My C Rap doesn’t run smoothly at a steady 1/4 throttle and below, usually when I pull away or negotiate a slow bend or roundabout in 1st or 2nd gear up to a speed of 25mph or there abouts. It’s like it’s 8-stroking or even running on 1 or 1 & 1/2 cylinders. It accelerates well with no problem and runs well in 3rd & 4th gear and doesn’t miss a beat at higher speeds. It starts well from cold, ticks over fine and is responsive on the stand, it’s when I’m riding in the lower gears that the problem manifests itself. I have Amal 276 carbs which I have taken apart and cleaned recently, albeit they were very clean to begin with. I have Vokes air filters with K&N’s, At the moment I’m running NGK BP6ES bought from the Green Spark Plug Co, they’re gapped at 18 tho although the problem is the same on all makes of plug, I also have a BTH magneto. The fuel I use is Shell V Power. Any advice would be much appreciated. Thank you, Nigel.

 

[oexing]

That sounds like very rich mixture at quarter throttle. Try lowering the slide needle one groove. Also the slide cutout can be too low or the needle jet worn. These can be oval by wear from needle, better have new needle jets.

Vic

 

[Jim Bush]

Pilot jet blockage... dismantle and physically probe with 16thou wire

 

[nigsey]

That sounds like very rich mixture at quarter throttle. Try lowering the slide needle one groove. Also the slide cutout can be too low or the needle jet worn. These can be oval by wear from needle, better have new needle jets.

Vic

Thanks Vic, I’ll order some new jets & needles, although I didn’t detect any signs of wear on the needles when I had the carb stripped out, it’s worth a try though. Not sure about slide cut out because I don’t know enough about them, I’ll have to try and work out what I’ve got now and take it from there.

 

[nigsey]

Pilot jet blockage... dismantle and physically probe with 16thou wire

Pilot jet blockage... dismantle and physically probe with 16thou wire

That’s an interesting point Jim but I had the carb completely stripped out and cleaned it in an ultrasonic cleaner, I guess there’s a chance I could have missed something though.

 

[nigsey]

That sounds like very rich mixture at quarter throttle. Try lowering the slide needle one groove. Also the slide cutout can be too low or the needle jet worn. These can be oval by wear from needle, better have new needle jets.

Vic

Excuse my ignorance but how can I tell what size cutaway I have on my slide, is the number likely to be stamped on it or do I have to measure it? I see Amal do sizes numbered 2 - 6

 

[vibrac]

if should be stamped on the top of your slide
its a measurement of (I believe 1/16" increments) no metrics here

 

[Jim Bush]

That’s an interesting point Jim but I had the carb completely stripped out and cleaned it in an ultrasonic cleaner, I guess there’s a chance I could have missed something though.

Ultrasonic does not remove crusted blockages ...the jet does require physical probing. So many times I have checked jets and seamed to be clear, with gas coming through, but when probed there is still resistance which means there is build up on the walls, much like arterial plaque.

You can spend money on new needle jets, they will help the proper idle as long as the circuits are clear.. Needles seldomly wear.

 

[nigsey]

Ultrasonic does not remove crusted blockages ...the jet does require physical probing. So many times I have checked jets and seamed to be clear, with gas coming through, but when probed there is still resistance which means there is build up on the walls, much like arterial plaque.

You can spend money on new needle jets, they will help the proper idle as long as the circuits are clear.. Needles seldomly wear.

Many thanks Jim, that’s very helpful, I’ll strip the carbs out again and thoroughly check for a blockage.

 

[Jim Bush]

That sounds like very rich mixture at quarter throttle.

Vic

I beleive it to be the oposite - air will always find a way in and usually makes things lean. When the pilot jet is blocked, that metered amount of petrol can't get through, so the carb draws it from the main jet as the throttle is opened,, causing unstaisfactory low speed running. Unblock the pilot petrol passage, set the air screw to 1 1/2 turns and it will be fine. Needle jet really doesn't come into the mix until after 1/4 throttle.