Anyone tried a cylinder spacer on their Bantam to improve pulling power?

the first couple of posts in this thread are on the money.. in the whole forum i can find no reference to to cylinder spacers..

Edited March 16, 201511 yr by totalshell

Why would you want to use a spacer in the first place? All using a spacer does is lift all the ports and decrease crankcase compression slightly. It's a 2 stroke not a 4 stroke if you havent noticed yes altering the port timing will have a marked difference on the torque and power of an engine but why you would think a spacer under the barrel would be a panacea is beyond me

Many thanks for replies. Saw a spacer for sale on the Progressive Classics web site and just wondered if anyone had first hand experience. The Bantam racing site has lots of advice and guidance on making a Bantam go faster - without spending pocket money on 'fancy' alloy bits, yet nowhere seems to have even basic guidance on tuning an engine output for trials purposes. Forks, frame, shocks, gearbox but little else.

Depending on what budget you are hoping to adhere to but I can recommend getting the barrell tuned / ported by Fahron tuning (Google it for number) Use Rex Caunts piston to bore to 186cc also. Ron at Fahron is always busy don't expect a fast turn round.

A spacer under the barrell works on some 2 strokes, at least for certain applications. It's not such a bad idea on such engines.

Raising the barrell 1mm typically increases total exhaust duration by about 4deg, and increases total transfer duration by 5 to 6deg but reduces inlet duration by about 4deg. (It affects transfer duration more because the crank is closer to BDC when they open/close compared to inlet & exhaust.) It will also reduce secondary compression noticeably but has little effect on primary (ie crankcase) compression. And it reduces squish clearance which affects combustion efficiency, alongside the change in C.R.

If you can adjust the secondary compression appropriately (eg thinner head gasket, higher Comp Ratio head) it works on engines that are a bit short on exhaust & transfer timing but a bit long on inlet timing, or if you adjust/extend inlet timing as well.

It's often done by just using an extra base gasket (rather than a spacer) together with a thinner head gasket, or at least trying that first.

Whether or not it works on a Bantam engine I do not know, but you can get some idea by looking at specs. I'm not 100% sure of the following specs but I'm lead to believe many Bantams have:

140deg Exh duration, 120deg Trans, and 120deg inlet (130deg for D14)

If you raise the barrell 1mm that will become approx:

144deg Exh, 125-126deg Tr, & 116deg In (126deg on D14)

Compare that for Villiers 9E engine:

152deg Ex, 126deg Tr, & 134deg In

Or Bul 250 (early SherpaT):

156deg Ex, 128deg Tr, & 122deg In

That looks to me like it may work for trials on a D14 cylinder. Otherwise it would be rather deficient on inlet timing.

If using a D14 head (C.R. 10:1) it would drop CR back to about 8.5:1 (similar to D10) which I would have tho't aint too bad for trials, depending on rider skill.

The beauty of it is that it's not hard or expensive to try it & see, & if it doesn't work to your liking it can all be put back how it was.

Hi andy m

Asked Ron some time ago for a guide - around £100. Pension sadly would not stretch far enough.

jc2

Many thanks for going to the trouble of explaining everything in an easy to understand manner. Much appreciated. Have an old George Todd head but for a 125. Stud spacing wrong for a 175 to try to increase comp ratio back to normal. I assume fiddling about with the ignition timing might help a little.

Hi jc2

Tried to send a pm but your box seems to be full.

Jim

i run an absolutely standard b175 engine straight from a mot'd road bike plus electrix ignition in my drayton, its flexible forgiving and more than enough to haul my 18.5 stone up anything , rides fantastic.. all weekend at the captains trial i ve been asked , who did your suzuki piston who built the engine, which barall did you use, whose gears have you got, what airbox is that ( its a standard b175 inlet rubber with a mini moto foam filter ...2.99) dont waste time and money..

Hi jc2

Tried to send a pm but your box seems to be full.

Jim

Try again Jim. I emptied it some