OzVMX Forum

Marque Remarks => Spaniards (Bultaco, Ossa, Montesa etc) => Topic started by: southnztim on April 13, 2017, 02:48:06 pm

Title: Bultaco Mk11 370 jetting
Post by: southnztim on April 13, 2017, 02:48:06 pm

On the Mk11 (I think American model) a 40mm Amal Mk2 was used in conjunction with a reed valve and twin plug head. Out of interest, does anyone know the standard jetting? The bike had been set up to run on methanol ice racing. At present I have it running nicely except for coming off the pilot jet, where its a bit rich. Main 480, needle 2F1 position 4, needle jet 113, Slide 4 and Pilot 35. The plug colour is great from just below half throttle up.  It gets darker from there down. Idles nicely.

Title: Re: Bultaco Mk11 370 jetting
Post by: 80-85 husky on April 13, 2017, 03:34:10 pm

drop the pilot one size? or next size up cutaway??

Title: Re: Bultaco Mk11 370 jetting
Post by: southnztim on April 13, 2017, 03:57:00 pm

Tried a 30 but had to screw the air into only about a quarter of a turn left. 35 idles good at about a 1 and1/4. Responds cleanly on throttle

Title: Re: Bultaco Mk11 370 jetting
Post by: 80-85 husky on April 13, 2017, 07:02:58 pm

cutaway might be the next best ploy

Title: Re: Bultaco Mk11 370 jetting
Post by: southnztim on April 14, 2017, 05:41:23 am

Have tried 3, 3.5 and settled on 4. Am I right in thinking that 1/8 to a 1/3 throttle is governed by the needle jet size as the needle taper wouldn't have raised into the jet by that stage?

Title: Re: Bultaco Mk11 370 jetting
Post by: 80-85 husky on April 15, 2017, 09:11:20 am

yep, think your about right there

Title: Re: Bultaco Mk11 370 jetting
Post by: Slakewell on April 15, 2017, 05:39:36 pm

It is an old carby , how do you know it is not sucking air around the slide etc that make jetting it a complete waste of time. Please start with a good condition or new one.

Title: Re: Bultaco Mk11 370 jetting
Post by: southnztim on April 15, 2017, 06:26:20 pm

No. NOS carb body.  Slide and jets and needle from Burlen UK