Anyone converted a standard Amal 930 to "Premier" spec with:
- the fixed bush removed,
- variable jet inserted,
- the left hand tapered needle inserted. (What is the intention with this?)
Carb body is tapped for the jet and the tapered screw but this is obviously blanked.
Just looking to improve my slow running.
Other users have suggested a 30cc pilot which is far greater than the fixed bush.
Many thanks
Jon
Yes, I do it on a vertical…
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Thanks You John
Concise answer and mainly understood.
Bush is clear with number 78 drill, slide is good fit and no wear marks.
- variable jet inserted,
I have 20, 25 and 30cc jets to mount in the float chamber roof to control fuel flow. This I presume can be further attenuated by the new jet on the left side of the standard carb. As I understand concentric's the original idle adjust is air control, not fuel?
- the left hand tapered needle inserted. (What is the intention with this?)
From Jim's write up he describes accessing the .016" bush via the opposite side to clean and then blank off again. With the "Premier" they introduce a second needle which apparently you screw home after removing the blanking and bush; your metering is affected by variable size jets in the float bowl, as above.
But this new jet(brass/steel combination in various sizes)opposite the standard pilot air control also controls fuel flow. Have I got it wrong that the old "two stoke" jet locations are what we use to adjust? or is this new needle to fit opposite the air screw my adjust??
I may be getting confused (its allowed at 67 my daughter says)
I have written to Burlen for clarification....
Thanks again for the bush removal method which I am quite happy to undertake. I am just a bit in a 2 & 8 about what I play with next.
Best Regards
Jon
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Where are you getting this…
Where are you getting this variable jet idea from, you can change the jets to different sizes but once in place they are fixed to the size they are stamped. You vary the mixture by the air mixture screw which adds or reduces air to the fixed amount of fuel. The new premier pilot jet is the same, no air just fuel.
If you want to try the 2 stroke pilot position then first you drill the 4 stroke pilot bush oversize to remove it's metering, then you can add the 2 stroke pilot to the pre drilled hole under the carb.
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OK Good...
... that's what I'm getting at. There are, apparently two solutions in play:
1. - Remove bush and select two stroke jet of appropriate size (this is my interpretation of "varaible", [by selection]).
2. - The "Premier" route, by removing the fixed bush and inserting the new taper to the fuel port, to change as appropriate.
So my simple solution is to remove the bush, play with the two stroke jets to determine the best rate, then if its compatible with the fixed taper that "Premier" introduced, I may upgrade.
Thanks for your input John, I was getting confused by the two solutions.
Best regards,
Jon
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Removing the pilot bush can…
Removing the pilot bush can be a pain, drilling it with a 2mm drill will remove its influence and leave the 2 stroke pilot jets as the fuel restriction. Note that the first concentrics all had the pilot jet in the 2 stroke position but after a year or 2 the issues that position gave on 4 stokes were overcame by the pilot bush which then were made standard for all 4 strokes. You may be adding more problems not less. What are the issues you have with the idle on the bike.
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Problems, problems
Noted John, I have a response from Burlen, who recommend the same approach; (resize the existing bush).
Machine: 37 Norton Inter MS.
Starting is difficult, and even running falters after the engine warms. The carb is on a Mica insulator but I have a phenolic 3/16" insulator coming. It "pops" in the exhaust suggesting its weak, and hunts. Valve and ignition timing spot on to recommended settings, fresh 97 fuel an float adjusted to JB's set up plus compensation for 15 degree down draught.
I have a Model 50 on a Mono-block, its metro-nomic on tick over and clean though the phases.
Also 650ss on twin mono's; nice even tempo.
850 Mk 2.5 on twin concentric 932's no issues.
I'm suspecting its a bit hungrier for fuel at the low end as it breathes very well and runs well in the later phases, 310, 109, 3.5 slide..
All mods are reversible on the carb so will experiment further.
Many thanks for your knowledge and input
Best regards
Jon
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Yes, I do it on a vertical mill, I have the depth data somewhere.
To prepare the carb I drill and tap the opposite side to the mixture screw 2BA, the pilot bush is exposed and then I knock it out from the mixture screw side and out the freshly drilled and tapped hole. After a good clean and a bath in the ultrasonic cleaner the premier pilot jet can be screwed in, the chamfered tip fits into the hole left by the pilot bush. 30 old pilot jet is the same size as the new 19 premier pilot.
I do the change to improve access to the pilot jet, if the carb was giving idle issues before then I would only convert the carb after getting it to run right with the pilot bush.
Have you cleaned out the pilot bush with a 16 thou drill, prodding with wires and fluids will not clean it properly. Worn slides also give variable idle as they rattle about leaking varying amounts of air.
The pilot bush and jets are fixed sizes and meter the fuel to a fixed rate set by the size. If clogged that rate is reduced out of tolerance for the air mixture screw to cope.
This is the air mixture screw and meters the air, with the right sized and clean pilot jet the proper idle mixture will be from 1.25 to 1.75 turns out from closed, hence the initial starting point of 1.5 turns out before tuning.
Read this webpage for more detail.
http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html