Sprockets

[Tripleking]

I want to lower the overall gearing on my 71 T250. After rebuilding the engine with some mild porting and a little boost to the compression I make good power from 5k to the 8K redline (it feels like it would pull through that). My 5th and 6th gears are basically unusable a my rpms are a bit low. The question is do I change the front or rear sprocket and to what? Both are in good shape so neither needs replacing.

[Coyote]

If you want more top end, you need to up the front by 1 OR reduce the rear by 2 or more..
Just the opposite for greater acceleration and don't care about any top end loss. i smaller front OR 2 or more larger on the rear. Be careful going in this direction as a little too much will render first gear useless.
I played that game with my GT550 and found the stock gearing worked the best all around.

[Cliff]

I went up 2 on the front, down 4 on the rear and paid for it. Over loaded the motor and burnt a piston!

[jabcb]

The Cycle World magazine review of the T250R list the top speed at 8k rpm in each gear.
Calculating the corresponding speed at 5k rpm results in:
gear / mph @ 5k rpm / mph @ 8k rpm
1 / 14.8 / 23.7
2 / 26.7 / 42.7
3 / 34.5 / 55.2
4 / 44.6 / 71.4
5 / 51.6 / 82.5
6 / 56.4 / 90.3

Are you unhappy with first gear performance?
Does your motor make minimal power below 5k rpm?
When you pull out from a stop in 1st gear, is your bike just slowly accelerating while you wait for 5k rpm?

If you answered YES, then you likely will benefit from a smaller front and/or larger rear sprocket.
If you answered NO, and you are happy with first gear performance, then you might be better off with the stock gearing you now have.

Its worth running the numbers in a spreadsheet before making any gearing changes.
Doing so is a lot cheaper than finding out you just wasted $$$ on new sprockets & chain.

[Tripleking]

My answer is is YES! I can do in excess of 25 (easily) in first gear without hitting 8,000 RPM. The engine in my 71 T250 is a completely rebuilt 73 GT250. However, I have the T250 sprockets. I am not sure if there was any difference in the GT250 gearbox ratios versus the early T250. Maybe there is a mismatch between my gears and sprockets which is has raised by overall gearing?

Also, I need to be in third gear to climb even modest grades at 40 MPH to 50 MPH, but when I am she pulls gloriously. As I said a few weeks ago in another post, my son rides a well-prepped CB450. It scoots for what it is and has been massaged a bit. He has all new suspension (as does my T250). We were blasting down a nearly abandoned paves road with twisties that leads to a state forest and I was on his A$$. Consider this:

He has 200 more CCs and is 70 lbs lighter than me and his bike is no slouch.

He says to me when we stopped down the road "Dad, you were in my mirrors the whole time." My little 250, which I had planned to sell, has now become my favorite bike out of my four.

[jabcb]

The book, "Cycle World on Suzuki Street Bikes 1971-1976", has their reviews for the 71 T250R & 73 GT250K.
Per those reviews they have the same overall gear ratios & same max speed @ 8k rpm (reviews have #s showing rounding differences).
But you might want to confirm that with the online parts diagrams.

3rd gear is now the sweet spot for 40 to 50 mph in that you have good power over that range.
I think I would leave it alone, but you might like a gearing change so that both 3rd & 4th cover the 40-50 mph range.
That 10% change in gearing would, however, also reduce you max speed in 1st gear by 10%.


You could also surprise you son by secretly upgrading the top end to a T350.
Then just tell him you are trying out different heads.
He might just end up saying, "Dad, I was in your mirrors the whole time."

[Tripleking]

I have knobby trials tires on. Still, if I wanted to be more aggressive I could have turned the tables. The shop did do some mild port work and we did take it out to 260cc. That alone with stock combustion chamber size would have taken the compression up a bit, but a little material was taken off the heads to true them up. Not much, just enough to get them true. The compression ratio should be a tad bit higher.

I think part of my "problem" is that my power ban was moved higher. I have to slip the clutch and launch at a higher rpm than before, but as I approach 8000 there are no signs of fading. The bike is also running a set of brand new properly jetted aftermarket VM26s with Uni filters. The bike really is a blast. I might sell my GT380. She is very nice. Just painted last year. I think I posted pics on a previous thread.

[titan performance]

If you can't pull 5th on a 6 speed bike, it's definitely over geared. It's common place for these old strokers not to have the power to pull top, and to achieve their highest speed in one from top, but having 2 over drive gears is wrong. Experimenting with sprockets could be costly, but from what you have said, I'd try for 2 teeth less on the front, which is probably equal to between 5 or 6 on the rear. It wont cost a lot for 1 front sprocket, but fitting a smaller front, you will have to watch out for the chain run on the swing arm....

[Tripleking]

I typically cruise 4th gear on local roads between 40 and 50. I'll figure it out.

[Tripleking]

Engine seems very "pipey," almost like a Kawasaki two-stroke. When it is on, it is on. When it is off, blah.

[jabcb]

Engine seems very "pipey," almost like a Kawasaki two-stroke. When it is on, it is on. When it is off, blah.

That's what I was asking about when I asked about the power below 5k rpm.
In that I case I think I probably would change the gearing.

Have something similar with my gray 75 GT750.
The vintage expansion chambers dropped the below 4.5k rpm power & made 4.5-7k power strong.
I'm either going to change the gearing or change the exhaust.
Just put the bike in storage for winter, so I have a few months to decide.

[Tripleking]

The best way I can describe it is:

3,000 to 4,000: a slug in first gear and impossible in any other.

4,000 to 5,000: good in first gear, ok in second, a slugh in anyother.

5,000 on up: the pleasure keeps building. She will pull through 8,000 if I let it. I have stock exhaust. I also have a nice set of mid pipes, but she will barely run at all with them and the carbs as currently jetted. My jetting is way to rich for the mid pipes.

[tz375]

You can change either the front or rear sprockets. You could start with a front sprocket with 1 tooth LESS. If that turns out not to be quite enough of a change try a rear sprocket with a few extra teeth. Typically 1 tooth at the front equals 2-3 teeth at the rear.

It sounds as if the sprockets on it now are not stock. Write down what you have at the moment. I did a quick search on line and found 13, 14 and 15t front sprockets listed as stock but I suspect that 13 is the right one. What is on now - front and rear? If it's 15 front, try a 14 and if that's not much better, try 13t and if that is too much try 1 tooth larger rear with a 13 or 1 tooth smaller with the 14.

On my 750 I fitted new front and rear sprockets that were perfect on the street for casual relaxed rising but not right for getting max performance. I went from 43 rear to 48 to 50 for the drag strip. If I had more power or put it back on the street I'd go back to 48.

The only way to know is to try. Fronts are cheap. Rears are slightly less cheap, so try changing fronts until you get an overall ratio that works for you in all gears for the way you ride and where you ride. But start by knowing what is stock and what you currently have and work from those two points.

[GT750Battleship]

Hi,I found a site on the Web search "Gearing Commmander" allows you to see how changing this & that can affect road speed in any given gear.lots of bikes listed in the program,very useful "tool" all free on line
Cheers,
Roger

[jabcb]

That web app looks useful: http://www.gearingcommander.com

Stock sprockets on the GT750J are 15 / 47
Stock sprockets on the GT750M are 16 / 43
Max optional rear sprocket per the parts manual is 51

Have lots of hills where I live.
My GT750M has the stock gearing.
The bike pulls out from a stop going up hill like its a slug slowly accumulating speed while you wait for 4.5k rpm, then its zoom ignore those square roads signs with numbers on them.
It runs fine below 4.5k rpm, starts easily hot or cold, & idles nicely around 1.3k rpm -- it just doesn't produce much power.

I was thinking about switching to 15 /51 sprockets.
That would give me a speed at 4.5k rpm in the 5 gears of: 21, 33, 44, 53, 66 mph.

Alternatively, I'm think about switching to Jemco 3-into-1s.