For those that advocate using 87 octane in these bikes

[Billy Ricks]

Just curious, what are the compression ratios, and what are the factory fuel recommendations? Of what vintage are they?

IIRC the cr on my old 305 was down around 7.3:1 or so. They are firing every stroke so combustion temps were probably higher than on a 4 stroke and detonation was a bigger problem on two strokes.

[Allie]

IIRC the cr on my old 305 was down around 7.3:1 or so. They are firing every stroke so combustion temps were probably higher than on a 4 stroke and detonation was a bigger problem on two strokes.

That's the kind of info I'm interested in. I guess a four stroke cylinder has a lot more time to cool between explosions. I wonder how number of cylinders affects cylinder temps? I'm far from an engineer, but it would seem that multis might have an advantage in that even more time elapses between a cylinder's firing? Are there any engines where more than one cylinder fires at once? I'm picturing a Ferrari v-12 here.... each little cylinder waiting it's turn to fire... Surely there there's some overlap? Or would that take away some of the smoothness? Maybe it's one at a time but one cylinder fires while the previous is not quite finished yet? Anyone ever driven a two stroke Saab? And, and, where do little mechanics come from? What is our purpose in life?

[Billy Ricks]

The last of the two-stroke GP bikes were running big bang engines to tame the wheelspin coming out of corners. Cylinders firing close together instead of at even intervals. More cylinders would just raise redline so temps would go up. The GT750 went liquid cooling mostly to keep that center cylinder cool. Before Suzuki built the Titan it was thought a two cylinder 500cc two stroke couldn't be built because of the heat problems. Trying to get three big cylinders to stay cool meant liquid cooling.

[7981GS]

Just curious, what are the compression ratios, and what are the factory fuel recommendations? Of what vintage are they?

Impossible to find any useful info on my '75 YZ 400 MX as so few were made.
I got it literally in boxes and could not find any manuals for it when I put it back together.

It looks a lot like this only my seat needs recovering now and one of my son's lost the right side cover out in the desert somewhere.
I have been looking for years for a replacement.



As for my (ugly) green '72 TS 250, I found this: http://www.bikez.com/motorcycles/suzuki_ts_250_1974.php





Eric

[Reaper0001]

na 93 less ethonal more power

[Billy Ricks]

The reason for the relatively low compression on two-strokes is everything has to happen in one revolution. Intake and exhaust are both happening at the same time at the bottom of the stroke. The fresh charge coming in pushes out spent gasses and part of the stroke is spent clearing the ports. Until the piston clears the port no compression is made.

[7981GS]

OS, the '75 YZ 400 MX was not only the fist mono-shock Yamaha MX bike (under the fuel tank), it was a one year only model.
Not easy to find parts for this dinosaur.

Eric

[koolaid_kid]

I remember that Yamaha as being a beast, but fun as long as you did not nail the throttle and stayed within the limits of the suspension.
Of all the TS models I preferred the 175, not as powerful but a bit lighter. Still would prefer the 250 over the 125 which was just too underpowered.

[7981GS]

I remember that Yamaha as being a beast, but fun as long as you did not nail the throttle and stayed within the limits of the suspension.
Of all the TS models I preferred the 175, not as powerful but a bit lighter. Still would prefer the 250 over the 125 which was just too underpowered.

It is actually much easier to ride in the tightest of rock-filled (boulders) turns on some of the most technical trails around here.
It is so low to the ground, easy to turn and has gobs of low-end torque.
Effortless compared to my big XR600R and XR650R Honda's and the Yamaha YZF400FK.

26 miles per 2.1 gallons of gas though.

I try to ride it at least once a year to remind me of what a beast it was when I was 16 YO when the dinosaurs still roamed the earth.

Eric

[Suzukasaurus]

Helium...

Less un-sprung weight.

Which is why if you're driving a tanker truck filled with helium cargo and you go across the scales overweight, you just have to add more.

Or maybe not....

[tkent02]

I need a few random TS 250 parts, wheel spacers and such. Anyone have some TS 250 or 400 stuff laying around?

[bellucci]

Speaking of gas mileage, I get 1 mile per gallon and I pay 8 bucks per... And am happy to pay it.
Curt

[boingk]

Good stuff everyone.

I fuel with and, more importantly, tune for premium fuels.

Down here in Aus we pay between a 5 and 10 cent premium for premium over normal, with 10% Ethanol 91 being cheaper than regular 91 by a cent or two a litre. Our Premium comes in two blends - 95 and 98 octane.

I couldnt give a stuff about cost, if you're that tight you can't afford to ride. One thing I do find is that most engines run better on premium if you tune for it. Some stock engine do not run better with it, of course.

An additional advantage with stock machinery is the richer mixture resulting from the denser high octane fuel provides an additional method of cooling for our already hot engines.

That's how I see it, anyway.

- boingk

[Nessism]

An additional advantage with stock machinery is the richer mixture resulting from the denser high octane fuel provides an additional method of cooling for our already hot engines.

That's how I see it, anyway.

- boingk

High octane fuel is denser? And if so, how does that result in a richer mixture?

What do you do to "tune" for high octane fuel? Advance the timing?

[boingk]

Not exactly. A higher octane fuel, should you decide to spend toe money on it, allows you to advance the ignition timing. Up to a point, this can increase power.
Having said that, I also suggest that it is not needed on a street bike.

That, in addition to fine tuning other aspects such as carberettor pilot, air and main jets.

Also if you use an AFR O2 sensor to tune bike, or dynotune it, you will almost definitely notice a difference between regular and premium. It may not be much, depending again on the type of engine and its state of tune, but it should be noticeable.

- boingk