Hitchcocks Forum

I've got a 1 1/8th 389 on the 350 Bitsa and whilst it goes like a good thing it does have a flat spot just off the idle when warm. It occurs to me that this means that it is likely to be too rich off the idle mixture but if I lean it up there it spits and stops ( clearly a sign of leanness ) it has a 3 1/2 cut away on the slide and my option is either to make that a number 4 or to alter the mid range fixed jet the the mono blocks have. I'm not entirely sure of the purpose of this additional pilot jet and how it interacts with the air screw. The question is for my 350 with 8.5:1 compression and improved cams do I go for a higher cut away or reduce the size of the pilot jet?

Hello SIMON----- usually a "flat spot" would indicate a slight LOSS of fueling ie a weak mixture ??? As it's just off the idle it could be the pilot jet needs increasing a tad OR --- as you say, the throttle slide could be lowered slightly to improve pick-up.."If I lean it up there it spits and stops"----- because it's already lean and by doing that you've made it worse !! ie it needs to be made slightly richer, NOT slightly leaner..The cheaper option ( and more likely IMHO,) is to increase the pilot jet first... The 1+1/8" Monoblock equates to 28mm...As you have the improved cams( Higher lift--greater overlap) then your engine is probably breathing equivalent to a std. 500 unit?? Just an idea but compare the settings (dot com) to see how they look...

I agree with Alan's reasoning. Pilot jet is easy to change and a small increase is a good idea. A couple of points, changing the slide is not supposed to alter fueling this low down the range but in my experience it can have exactly this effect and also needle position (with 5 grooves,) is quite finely adjustable on a Mono so try that too. REgards, Mark

The reason I think it's a richness issue is that the bike runs fine without a flat spot when cold where it would be worse if it were too lean. Also enriching the mixture makes the problem worse. Ill solder up the pilot jet and drill it a size down and see what effect that has I think. It is certainly a more reversible option than modifying the slide. Thanks for your thoughts. S

I had this with a Triumph Cub and traced it to the float level being too high.
The flat spot was actually a "cough" caused by richness. Try turning off the fuel while riding along and see if the problem dissappears as the fuel level drops in the float chamber. If it does then this could be the problem. Worth a try anyway.
Have a look here
http://www.whfolk.co.uk/REOCHUMBER/Arti ... nobloc.htm

Yes thought of float height as an option and tested it using the clear pipe over the pilot jet method but more through good luck than good management it lines up exactly with the pimple on the cover.