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[adamscarborough]

Has anyone any experience of jetting an Amal Mk2 2932 and a trials exhaust?

I have a 1993(?) 500cc RE Bullet and have fitted our hosts:
Performance cams 90023
Alloy barrel 200071
Piston 90204
Trials exhaust 91012 and baffle
Performance valves 90052 and gas flowed head.

Starts fine, usually with a little throttle, tick over is fine, but not perfect as to sometimes splutters and stalls, running on a some throttle it’s very rough and worse on full, pottering about on low throttle and the plug is very sooty. I’ve taken great care that my exhaust isn’t leaking at the head and the carb rubber manifold isn’t leaking.

Currently the set up is:
Needle 2A1 (second groove up)
Valve 3
Pilot 25
Needle jet 106
Main jet 240
Air 1 1/8 out
Plug NGK BR8ES

For starters I'm thinking of ordering a smaller main jet to see if there is an improvement? should I order anything else while I'm at it, just in case?

[Marko]

Hi Adam, fitting a smaller main jet will not cure your problem unless you were doing your riding in a range from 3/4 to WOT. I'm only guessing & presuming that most of the time the riding is done at much lower throttle openings?
1st doublecheck that the "cold start" on the mk2 is definitely "OFF" & seating correctly when so.... the smallest leak there will cause very rich running.
2nd: mark the twistgrip temporarily so that you can check at which throttle range your bike is running rich.
I'm pretty sure that you'll find your bikes sooty condition is down to the throttle valve cutaway size (roughly up to 1/4 throttle) & you might need a valve with a larger cutaway. You can "tweak" the pilot air screw to compensate On the throttle valve cutaway but only so far: back the screw out & see if matters improve, if there is an improvement then ok, but if not then go up on the cutaway.
Anyway, by having your throttle/twistgrip marked you'll be able to determine exactly where/when the richness is occurring & concentrate on that area
Hope it all works out

[adamscarborough]

Thank you Marko, great advice, I'll try tweaking the air screw to let more air in to see if I get an improvement in that range. I've just checked the price of the valves, too expensive to just experiment with . I currently have a 3, from what I understand if I find I need a bigger cut out I need a bigger number 3.5 or 4?

[Marko]

Yes, if its still running rich (& its definitely not caused by the cold start assy) then one size larger on the valve. I know of people who have filed their throttle valves , even I've done that it the past, to achieve a larger cutaway - but you do need to be careful & confident in what you're doing. Oddly i'm currently working on a friends bike (a Triumph but with a Mk1 concentric) which is having similar problems to yours - everything was fine until new pipes & "pattern" silencers were fitted Hope it all works out for you

[Marko]

Temporary marks on the throttle

Temporary marks (I use artists acrylic paint, can be rubbed off when not needed) can help pinpoint at what throttle opening richness/leanness is occuring

[adamscarborough]

I’m sorry if this is a stupid question, but do you do the WOT testing and similar “on the road’?

[Marko]

WOT.... hahahaha yes... the trick is not to get caught speeding!

[ChrisD]

Hi Adamscarborough.
Marko’s suggestions are good and I concur. BUT remember the pilot jet is only about a tenth the size of the mainjet, so tweaking it a bit does nothing once you are riding, only for the starting and idling.<br>
I have fitted a 32mm Mk2 to my 535cc 1996 bullet, twin plugged. Tried two silencers, an EMGO reverse cone megaphone from our hosts (too loud even packed with real sound deadener, not the weak rubbish it came with) and one of our hosts “shorty” PN91007, cut open and stuffed with packing (see earlier post). <br>
I used a 2.5 slide (tried 3.0 but too weak).
Needle jet 105 (tried 106 and 107) but after latest mods of fitting a K&N filter and a large scrambles inlet valve (48mm) it now takes a 106. Uses an AceCafe large tubular air cleaner (sorry, Mr H). <br>
Main jet 220 (tried 230, 240, 250, 260. Now, after the latest mods above, takes a 230.<br>
Pilot jet 25 (tried 30, 35, 40, 45) <br>
Choke jet 45 (tried 25, 30, 35, 40, 45) <br>
Needle 2A1, clip at #1 but after the above mods, better at #2. <br>
Air jet 3.5 (tried 2.5 – don’t go there!). <br>
There are others out there with similar setups. <br>
WOT testing I do on a long upslope road where I can have WOT for a mile or more.<br>
Cheers, ChrisD.

[mauri]

there often a misconception about the workings of the pilot jet.

you'll only use the main jet at full throttle, and in no other situation.

as soon a you close the throttle you'll gradually and increasingly start using the pilot jet.

hence the pilot jet is of equal importance as the main jet as is often overseen and misunderstood.
and throw the position and shape of the needle in there, and it becomes les evident on how and where to adjust.

at start up and idle there is more than just the pilot jet working also.

on my own trials i have the same exhaust but with a amal MK1 premier dia 32.
i found that bending the filter away from the exhaust solved most of the problem.
with that high exhaust the airfilter is very close and takes in quit a lot of hot air.
wich advertadly efects your carburation also

[adamscarborough]

That's a very interesting point about the proximity of the exhaust to the carb, it never even crossed my mind and could have a big effect for some riders. I have an air quality meter in my house which gives a reading for CO2, I have noticed the CO2 in my house ROCKETS if I've started my bike in the garage or ridden it into there after a ride!