Hitchcocks Forum

Hi Guys.
My 2008 k/s 500 classic 5000kms spits back when opening up the throttle above halfway.
I have an unrestricted silencer and conical S&B air filter fitted.
Since fitting the air filter I have richened the Needle Jet P2 and Slide 2 1/2, also tried needle groove positions; lowest, 2nd to lowest and middle.
Present config. is Idle Jet--30, Needle Jet--P2, Slide-- 2 1/2,Main Jet 120. It starts well and ticks over fine. I have fitted a new inlet rubber and new coil (cos i had one on the shelf)Tappets checked ok,Boyer Bransen ignition installed.
It doesn't pull very well seems sluggish, up hill especially, since fitting the air filter. When I open it up it spits back through carb and hesitates, while this is happening, if I push enrichment lever down it revs freely stops spitting and storms up the hill. i have tried a plug chop but its abit inconclusive, cental electrode appears abit light in colour.
I know this problem is abit of old chestnut, any ideas why carburation appears so critical on the bullet,other old singles I have had would accept virtually any carb from the jumble and run reasonbly well.
Any help would be appreciated.

Hi Triton. Sounds like your main jet is far too small. I had to go right up to 150 on mine. Actually you can't buy a 150 so I used a 1.5mm drill on an old main jet and hey presto! she flies! Cheers Tony.

Hello TRITON--- I think you have appeared to answer your own question }-----"When I open it up it spits back through carb and hesitates, while this is happening, if I push enrichment lever down it revs freely stops spitting and storms up the hill." ----------which tends to make me think you have a mid-range/main jet weakness problem...Don't forget that although the carb has, in simple terms, 4 areas of function viz}---slide size,..piot,..mid-range,..main jet --- an alteration in one will also have an effect on its neighbour to a degree as well...From your present set-up then your easiest option would be just to increase the main jet to a 125(1mm) or even a 210(1.25mm)----- you can do this through the big nut on the base of the VM28 carb and also by slackening the air hose clips and rotating the carb. I have the same Classic 500/Vm 28 arrangement and can change a main jet in about 10 mins without removing the float bowl. I would leave the needle in it's No.3 position for now. If done correctly a plug chop is definitely NOT inconclusive..... Also give some consideration to the plug type/range being used because although the Compression Ratio(CR) has remained the same, you are now drawing in a greater amount of fuel/air mix and thus Mean Effective Pressures(MEP) and temperatures will be increased....Keep to the proven rule}------ ONE ADJUSTMENT AT A TIME, then road-test and evaluate. It also helps to have A.N.Other person on hand to play "Devil's Advocate" because we all do it---- ie}try "talking" a fault away instead of being hard with ourselves. I think you're not far-off so persevere and keep in touch.

Hi Guys -------- additional to my last}----try this}-----http://www.allensperformance.co.uk/mikjets.htm ------and then this }------http://www.allensperformance.co.uk/jetsizeconv.html --------and note that 110=500cc std.----130=0.986mm(ie almost 1mm),-----210=1.25mm,------and 310=1.522mm(ie a tad over 1.5mm)

----------bloomin computers and their ABSOLUTE ACCURACY !!! That first one has an "l" missing off the end.

Thanks for speedy replies,
Alan if you have the same set up what size MJ are you running. I have today ordered 125 and 130 Main Jet sizes. What is the 210 size you were refering to.
rgds Roy

----------Had to get my set-up notes for this answer }---Am currently running a Tea-Caddy air filter which--for the sake of tuning---I'm regarding as a free-flow type. I have a MIKCARB VM28 with}---- SLIDE=No3.... PILOT=25 (This is to be changed to a 30 as it's just on the weak side)....NEEDLE JET=08 (I now have a PO to fit)....NEEDLE POSITION=No4 (This will be returned to the No.3 position when the PO needle jet is fitted) .....MAIN JET=135.....My silencer is the standard "Long-Tom" shortened by 6", hence my slightly different settings to what you might expect...And finally an irridium plug is fitted......... The 210 size is on that Allens jet size chart and equates to 1.253mm......Don't forget that the Hitchcocks jetting chart is----as they themselves say-----only a starting point.

Thanks again for all your help. I have ordered 125 and 130 Main jets from our hosts, maybe 130 will still not be large enough. I can always drill it out as Tony suggested.
I have just realised the forum page doesn't do paragraphs, still it makes you concentrate I guess.
I will keep you informed of progress, I have been fiddling with the carburation on and off for 2 years. If this fails its going to be chopped in for a new Bonnie!

WOW brilliant service from Hitchcocks again, I ordered the Jets late yesterday afternoon, they have just arrived in Deepest Sussex at 0900 today. Just got get out of the shopping trip now!

------------ HARDER than any carb tweaking, that's for sure !! LoL