Port timing for GT250A

[GraemeK]

Hi All
Have just added a 1976 GT 250 A to my collection and would like to build a set of chambers for it but at the moment I do not any cylinders. ( They were missing when I bought it ) So I am wondering is anyone can tell me the exhaust port opening and transfer port opening in degrees from TDC.

Thanks Graeme

[Vintageman]

I suspect the GT250A and B since they have the same Exhaust port height and stroke have similar timing to the Yam RD350. Search web for RD port timing USA model.

I am not sure about transfers would think similar too within a few degrees but again not sure. Might be like Suzuki X7?

Given how easy it is to check, pop off your head and take opportunity to decoke as well if no one has this data.

I will also attest that D&G for RD works real well on GT250A and great broad street pipe. Not adhoc You can do much worse I suspect. Jemco has a set but more race tuned I think

The GT250A with its port timing change versus earlier modes has nothing below 4500 rpm, but, that bike is a rocket. I could never ride it easy, never relaxing, always exciting. I agree it must have chambers

[tz375]

I'd recommend finding cylinders first and measuring port openings and widths. For timing, I recommend using a dial gauge AND a degree wheel to check one against the other. Then work out what port time are you want for your target revs and then port the cylinders and work up a pipe design that works best with your porting.

We have a bunch of different port time numbers on file from different GT250 motors but no idea which is which, let alone what timing yours will end up with. It looks like transfer is 120 degrees ATDC (43mm) and exhaust is 94degrees ATDC (32mm) for a pre A model . The onlky A model data I have is from a reed valved, ported high performance motor and if I told you those port timings, I'd have to kill you

What software are you using to design your pipes? Wizard is barely worth the little that it costs. In theory it uses the same formulae as everyone else's software but the pipes it generates don't seem to work very well from what I have seen and heard to date.

Jemco sells a pipe but it's the T20 race kit pipe and so far we haven't worked out a way to make them work on the street and they are not the best race pipes either. They are what was available at the time, but time kept marching on.

According to a tuning article from back in the day, a stock GT250A made 26hp at the rear wheel at 8,400. You could usefully raise that rev ceiling. The A is a better base to work from with a 4 bearing crank and two pairs of transfer ports - albeit in the wrong places. They mentioned raising the ports by 5mm but without any details and it's not clear if that was real or just a number to make a point.

That's why it's essential to start with facts rather than myths and measure your barrels when you get them.

[jabcb]

You should take a look at three 3 service bulletins that cover changes to the GT250A.

GT-30, timing mark changes during GT250A production run
GT-32, changes from 75 to 76 models
http://www.ozebook.com/compendium/techb ... 027-33.pdf

GT-35, cylinder head, spark plug & carb changes during GT250A production run
http://www.ozebook.com/compendium/techb ... 034-39.pdf

[GraemeK]

Hi Vintageman - I would love tone able to measure the barrels and would do just that but at the moment I am having no lick finding any barrels to buy - they seem to be very scarce in Australia.

Hi tz375 - I agree that there is a bunch of port timing info out there and none of it seems to correct for the 76 model where Suzuki introduced the 4 transfer ports with two sort of above the intake port at the rear of the cyl. I am an engineer and used to race 250 production in the seventies, mainly on early model 250 Hustlers. I ran a business building chambers for many years and have a set of calculations I use to design chambers that I developed and proved to work well which I will use to do the calculations. I have looked at a number of the software packages that are around to do chamber design and to be honest I am surprised that any of them work at all. Most don't seem to understand the underlying principals and have resorted to using a bunch of generalisations as the design criteria and people seem to believe that because there is an online calculator that it must be correct. I agree with you that having the cylinders to measure would be the best option but I have been looking for the correct cylinders for months now with no luck and I would at least like to do the calculations so I can see roughly how they will fit.

Hi jabcb - Thanks for the link to the info about the ignition timing marks - I had come across that problem while I was building race bikes in the seventies - I realised that the timing marks were different from one model to the next and that Suzuki had screwed up which ones went on which bike when I dyno tested a 76 and found that it made less power when the ignition was set to the ignition mark. I generally check all my bikes using a dial gauge to ensure the marks are correct these days.

Thanks everyone for your input - I would still like to hear from anyone who has measured the ports on a 76 model 250 A that has the 4 transfer ports because I have no idea when I will be able to find cylinders.

Regards Graeme