adam_winstone Posted January 13, 2010 Report Share Posted January 13, 2010 (edited) Hi All, After spending lots of time searching the web for info on the older Bultaco 2-strokes, I'm at a complete loss and need some help from anyone who knows about these bikes/motors. What I'm after is some basic cylinder porting information (e.g. inlet port duration, transfer duration, exhaust port duration) so that I can see how the 250 motors developed, specific to their applications (flat-track racing or trials). I managed to find Bell's tuning info regarding the Matador's 150 inlet, 126 transfer & 170 exhaust timing but that's all I've managed to find. Please note that I'm not looking to steal anyone's tuning methods/port timings, I just want to be able to see how port durations changed in comparison to carb size, compression ratio, gearing, etc. The BHP @ RPM figures for the various models suggest that cylinder porting changed between models, as do other factors (e.g. 27mm to 32mm carbs), and this is what I'm hoping to learn more about. If anyone out there happens to know the 'real' technical spec of these motors, I'd love to hear from you and talk technical Thanks in advance. Adam Edited January 16, 2010 by Adam_Winstone Quote Link to comment Share on other sites More sharing options...
adam_winstone Posted January 16, 2010 Author Report Share Posted January 16, 2010 No takers then If anyone knows the standard port timings of any of the models then I would love to hear from them. I'm not expecting any one response to cover all models. Any help appreciated. Thanks, Adam Quote Link to comment Share on other sites More sharing options...
feetupfun Posted January 16, 2010 Report Share Posted January 16, 2010 I can measure the exhaust and transfer port heights of a couple of Bultaco trials barrels, but am not offering to work out the timing from that nor am I offering to assemble anything to get the inlet timing The barrels I can measure at present are M198 (1977 238cc trials) and M27 (1967 250cc trials) Quote Link to comment Share on other sites More sharing options...
adam_winstone Posted January 17, 2010 Author Report Share Posted January 17, 2010 feetupfun, Thanks for the response. Yep, any info that you could offer regarding inlet, transfer or exhaust port measurements would be great If you could give me the measurements from top and bottom of port to the top lip of the bore (head gasket face) then I'll see if I can find a suitable port timing calculator on-line, of which there are many. A width of port measurement would also allow me to calculate the port's area but will not change the duration figure. Thanks for the offer.... YES PLEASE Adam Quote Link to comment Share on other sites More sharing options...
feetupfun Posted January 17, 2010 Report Share Posted January 17, 2010 Ok here are the measurements for some Bultaco barrels from my shed. There were a few more that I had forgotten about. All are 71/72mm bores. All are distances from the top end of the sleeve Stamping, Exhaust top/bottom, transfer top/bottom, inlet top/bottom 27, 37/60,46/60,64/85 107, 32/60,46/60,65/91 and had finger ports 157, 38/60,45/60,63/83 86, 34/61,47/61,67/91 and had finger ports no stamping but is from my M198 and appears unmolested, 40/61,47/61,65/85 Yes I know that a M157 is not supposed to be a 250 but I'm just reporting what I saw Regards David Quote Link to comment Share on other sites More sharing options...
adam_winstone Posted January 18, 2010 Author Report Share Posted January 18, 2010 Excellent stuff, thanks. Quote Link to comment Share on other sites More sharing options...
adam_winstone Posted January 18, 2010 Author Report Share Posted January 18, 2010 Okay, next (hopefully last!) question.... Do all of the 250s run the a similar length piston? The shorter the piston skirt on the inlet side, the longer the port open duration. I've managed to plug the measurements that you kindly gave into a spreadsheet but need to know what the measurement is from the edge of the piston crown (above the rings) to the bottom of the skirt length is? Even if the pistons did vary somewhat, I can plug a single piston measurement into the spreadsheet and it should allow me to compare port timings anyway. Thanks in advance, Adam PS - Regarding your M157 item... whilst most Bultaco model lists have this down as a Sherpa T 125, I came across one list that had it as a 'Lobito S 238 Prototype'... perhaps an error, perhaps a treasure!!! Quote Link to comment Share on other sites More sharing options...
adam_winstone Posted January 20, 2010 Author Report Share Posted January 20, 2010 Another step closer... I've come across a service manual that states the 250cc scrambler came in 2 options; the motocross version that had a 64mm long inlet side piston skirt and the TT version that had a 59mm skirt. I'll have a look through the porting measurements given again and work out the inlet timing for both piston options, although it is likely that the respective barrels also had different exhaust port timing to help produce power at the desireable rev range. Cool! Adam Quote Link to comment Share on other sites More sharing options...
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