Jets again (I know, i'm sorry, but I got a 1200 Jota and need a start point)

Rufustbear said:
Well you could buy a Honda, >:D >:D >:D >:D >:D >:D >:D >:D >:D >:D >:D >:D >:D >:D >:D >:D
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Don't want a Honda......they don't make me smile, or scare me anything like as much as a Laverda triple  :-*


I am happy to fettle it, just know bugger all about jets & modern fuel. Don't want to learn by blowing the old girl up....
 
Rufus,

you cheeky mare.....you changed you post. ;)

80 or 90 is what I plan to ride at......next thing I know, I have 1** showing on the speedo  ::)

Guess I should get some 145s too then.

Pete
 
Modern fuel seems very light, presumably formulated for 'lean burn' engines and is full of additives and ethanol etc that weren't in 5 star.  If you are running the o/e airbox I'd be surprised if it needed much more than a 130 main, but if it's spluttering even coming off idle there are bigger issues. Hopefully not one of those over oiled Unifilters which IMHO should be bone dry to have even a chance of letting the motor breathe.

You need a base line- find the specs for a Mirage and set the carbs to those. Then book an hour on a dyno to find out what's really going on.

Andy Wagner produced a good guide to jetting- it's in one of his 'Rundbriefs' (sp?) Autumn 2002? on his website.
 
Unless UK fuel is very different to Australian, or you're running open bellmouths on 36mm carbs, I doubt you'll end up with jets as big as 145.

You can't really do jetting properly with plug chops these days. You either need to book a dyno session or fit a lambda sensor in your exhaust system.
 
Cam,

Suspect fuel is much the same. The Jota was jetted donkey's years ago, so desperately needs an update to accomodate modern fuel.  Just need a start point to set the bike up, before I go to the dyno with a bag full of jets.

This is all a bit new to me, but from posts I have read (and thanks for those), my instinct says 130-140.  Don't forget, it's a 1200 Jota, so probably a bit bigger than standard.

Pete
 
You must be a mindreader, I have just finished reading Phil's cam timing write up on Laverdamania!

Think it will take two or three re-reads, as it's a bit out of my area of expertise....  Having said that, since Phil did the 1200 conversion, there is every chance it has already had it's cam timing looked at (Phil is very thorough).

Pete

 
We have done this Dell carb setup thing many times. Search the archives.

Set valve clearances, cam timing, and ignition timing accurately.

Make sure choke plungers actually hit bottom and seal.

Pilot jets anything between 50 and 65, most likely 55, 57.5, 60. Mixture screw with proper o rings, out roughly one and a half turns. That will look after starting and idle

Needle jet AB 265, needle K 1, middle notch. Slide cutaway? Those are your major factors, 1/4 throttle to 3/4 throttle. If you only replace one thing, replace the needle jets and needles as a set, biggest bang for the buck.

Accelerator pump setup, and slide bump? 60/2, 60/3?

Main jet, last and least, only affects from maybe 3/4 throttle to wide open. "1200" high comp, anywhere from 120 to 145. my bet is you wind up around 128 or so. But, my high comp 1200 with 4 c, open exhaust, open air box with thin K & N flat filter, after a LOT of dyno time, unexpectedly wound up with 145's, and pulls like a train at wide open throttle

Make sure you don't have any air leaks, and get the three carbs mechanically synchronized equally on the bench before you start, then vacuum balance.

Paul LeClair
 
Paul,

That's great, thanks for that  ;). You have given me just what I need, with a start point for mains, pilots needles etc. AND from the experience of a 1200 h/c 4C motor!.  I will order up the parts later today, with a decent spread of mains and pilot sizes  (thumbup)

cheers

Pete

Paul LeClair said:
We have done this Dell carb setup thing many times. Search the archives.

Set valve clearances, cam timing, and ignition timing accurately.

Make sure choke plungers actually hit bottom and seal.

Pilot jets anything between 50 and 65, most likely 55, 57.5, 60. Mixture screw with proper o rings, out roughly one and a half turns. That will look after starting and idle

Needle jet AB 265, needle K 1, middle notch. Slide cutaway? Those are your major factors, 1/4 throttle to 3/4 throttle. If you only replace one thing, replace the needle jets and needles as a set, biggest bang for the buck.

Accelerator pump setup, and slide bump? 60/2, 60/3?

Main jet, last and least, only affects from maybe 3/4 throttle to wide open. "1200" high comp, anywhere from 120 to 145. my bet is you wind up around 128 or so. But, my high comp 1200 with 4 c, open exhaust, open air box with thin K & N flat filter, after a LOT of dyno time, unexpectedly wound up with 145's, and pulls like a train at wide open throttle

Make sure you don't have any air leaks, and get the three carbs mechanically synchronized equally on the bench before you start, then vacuum balance.

Paul LeClair
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chrisk said:
Pfft, 7C cams ..... take em out and weld'em together end to end.
Perfect as a letterbox post & finally they'd be useful!
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Thanks for your words of wisdom. I have decided to file it under ignore. :D

                                                                                              Dave
 
Cosi, probably safe to assume most racers will already have a fairly comprehensive choice of jets in their toolbox.
 
Someone mentioned main jets only affecting 3/4 to full throttle opening ? Sorry If I've misunderstood that but its just not true or at least is misleading to read..
You start with the mainjet and WOT 1st and work down. YOu cant set up any other aspect of jetting, needles, emulsion tubes, pilots until that is sorted.

Ray, do you really drill your mainjets, or is that bullshit...I may have missed your SOH  :D

Rob
 
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