Suzuki 2 Strokes

Hi... I'm new to this site but not new to motorcycles as I've own many over the years.

I'm currently preping a GT185 for a run in LEL (lake Erie Loop) and have read AG Bell's book on 2-stroke performance Tuning and recently purchased a software package to build a set of expansion chambers. While AG Bell's discussion doesn't include transfer port timing or intake port timing to build chambers, the software package wants to know this information. So I'm hoping someone on this forum has or knows where to find all GT185 port timing. FYI... exhaust starts at 90 degrees after TDC and lasts for 180 degrees (as measured by a degree wheel)

I measured the exhaust port using a degree wheel but one can't see the transfer ports w/o removing the heads and I'm hoping I don't have to do that. Certainly somewhere that info exists.

My GT is a 1975 w/2300 mi and I've completely restored the bike. The crank and cylinders look perfect and on several outings it seems strong, coming on stock pipes at about 5500 rpm and pulls well into low 80's and that's at 6000' elevation. A good set of chambers tuned for mid range would give me what I want; more power w/o extremely high rpm (8500 max) and improved fuel economy (now at 40 mpg running close to 3/4 throttle).

I've built 3 into 1, and 4 into 1 pipes for 4 stroke motors but my first with a 2 stroke but with good resources I should have success.

Thanks for any help,

Motodio

I don't have GT185 port timings in the database. The easiest and only way to get them is to pop the head off and measure the port timing and width. You can measure the depth down from TDC (most accurate approach) and convert that to degrees or measure the degrees (harder to be accurate).

That's what I wanted to avoid. I'm not sure how you convert measurements (ie. distance from tdc to transfer port opening) into degrees. If you have an equation for that please list it.

Why would distance measurement provide more accurate data than using a degree wheel. I think I know why but would like you're input.

If and when I get the data I'll pass it on so it can be included in your database for future reference, even though it may never be needed again.

dio

There is an inherent error in any measurement and deciding exactly when the port opens and closes is hard enough. Even with an accurate degree whel, it is lucky to be at +/- 1 degree and probably closer to +/- 4-5 degrees depending on the size of the degree wheel and how accurately TDC has been located. I use a dial gauge to determine TDC and distances down. That is accurate to .01mm or a tiny fraction of a degree.

The math is simple. We know the stroke, distance and if necessary we can guestimate conrod length.

Vintageman on this board has a tuned 185 with hot pipes and may already know the numbers.

I like the idea of using the dial indicator to set the degree wheel. Would be more accurate... thanks.

I pulled out the spare engine and on the bench it'll be easier to measure port timing. I'll start with the dial indicator to find TDC. Once past tdc the degree wheel is quit accurate since it's not playing with an almost dead movement at tdc and bdc as you've pointed out.

I never did degree timing and have forgotten port height.

I only removed 1/2 mm off top of Ex port. That is I wanted to take off 1/2mm to make them both the same

I did not change intake timing If I remember ( I usually do take 1 to <2 mm off skirt, but not this time for some reason....

I bored my carbs to near <22mm versus <20mm stock. Be careful to offset a little or you wont be able to reduce idle speed. I think I up main jet same proportion and richened needle one clip.

I opened the intake boot and port to match and followed that about 3/4" into intake where it opened up nicely anyway. I run the later (mine is a 1975) ignition timing (24 degree?). I like midrange torque. I run a 0.020" head gasket versus the 0.040". I run 2mm OS pistons (~200 cc)

The pipes are JEMCO and came as a kit with a few joints I had to weld up each.

Those pipes are radical. they hit real hard after 7000 rpm and my engine drops off fast around <9500 (not sure if pipe, exhaust port timing or those funny long OEM points). Tuning between 5k-6k RPM was tough and worse with pods. Not bad with stock air box. stock timing, and thinner head gasket. It is very street-able (all I do) and will run OK between 5000 - 6000. Under 5000 rpm it seams just like stock pipe. But stock pipe are very linear up to <8500. But they don't have anywhere near the boost Jemcos make Again when these pipes boost it like nitros for about 2krpm. Sounds wonderful too.

I changed my gearing to close ratio from TM100/125. This was best thing yet. I can drive just into where pipes kick in, shift and not drop too far away from power band and hold an mild incline for example. Can't change first gear for part of shaft and 1st to 2nd wider jump now (like GT550 ). But when riding street never drop to 1st. 2,3,4,5 closer ratio than GT185. Also less gear misses if lazy shift for the TM gears are beveled

The bike is fun, but its a little bugger for sure. No way I can run interstate +65 mph in VT with its hills... runs 55-60 mph all day long. Maybe >80mph max where I would be around +9000 RPM as geared

The transfer ports are very very tiny. The exhaust port is square. I should have widened or oval the Ex some. I fear if too far raising EX you will only have road race machine with no bottom or midrange. Need timing to know this better. I think a 1mm off skirt may allow a little more over rev.

I may get bored a try these changes.... If I could only bolt on a couple TM100 cyls

Check out a TM 100 Cylinder for size of transfers and other ports. The GT185 and TM100 use same piston and stroke if I recall (please verify). I would use the TM100 as a guide of what can be done for a hopped up street bike.


Fun Stuff

Running the GT185 seems fine right now. I live at 6k' elevation so there is some loss in power, like about 18% but all vehicles at this elevation suffer from the same loss of power. Also there are a lot of hills here, some fairly steep. Sometimes I can pull them in 5th and others I need 4th. Wish it had a 6 speed.

There is actually a noticeable kick at about 5500 rpm and it continues up to 8000. At 8000 in 5th it is running 80mph but for the LEL I prefer to keep stock gearing as the run is 650 miles non-stop and I want it to last.

I did find the extra GT185 motor and clean the pistons, honed the cylinders (good shape) and reassembled just one piston/barrel to meter the degrees of opening and closing. This is what I measured..

1. Exhaust port opens at 95 degrees and is fully opened at 180 degrees.
2. Transfer port opens at 120 degrees and fully opened at 180 degrees.

Of course the ports close at the same because they are fixed.

I'll use this info with the software to determine expansion chamber design and then run calcs against AG Bell to determine any difference.

I need to check the LEL rules to see if I can run modified chambers.

I'm impressed at the heart of the GT185. Too bad so many were treated like trash and left outside to rot.

Thanks for sharing your post. I'll look into the TM100 for fitment to the GT185 cases.

motodio wrote: Of course the ports close at the same because they are fixed.

Not necessarily. Only if the crankshaft and piston(s) are central to the bores.

On some 2 strokes (true Désaxé engine), the crank is offset slightly - some forward and some rearward depending on the effect required, and on others the piston pin isn't central to the piston (Désaxé engine style).
This offsets the port timing so that the same port opens and closes at different degrees of rotation relative to TDC or BDC as the piston rises and falls down the bore.
All 2 strokes are definitely NOT equal.
That is just one reason why on some 2 strokes, the piston has to be fitted the correct way round as some have an offset gudgeon (piston) pin, which then offsets the port timing on power and exhaust strokes as above.
The old Ariel Arrow/Leader bikes were like that and were designed in the late 50s, a good idea 'borrowed' by Suzuki in the 60s for the T20 among other engines.
Shame that Ariel/BSA didn't have the money - or the will - to develop the engine or we might have had a genuine alternative to the Japanese bikes.

Alan is correct that port timings are not as symmetrical as one might expect. That said, with say a 1mm piston pin offset, the difference is less than 1 degree.

Those timings are fairly tame compared to say a Yamaha DS7 and blowdown is really short. With more aggressive porting it should be possible to raise peak revs from 7500 to say 9,500 which is still very low for a short stroke motor. That assumes that there is enough metal in the ports to achieve the sort of port time areas one might want, it should in theory be able to get at least 25 and probably closer to 30HP with bigger carbs, chambers and big ports - and of course no bottom end at all.

Just wanted to say great read on this thread. Not too many conversations like this pop up on the GT185. Great stuff!

The Jemcos move my peak to between near 8500 on my gt185. It amazing what a pipe can do on its own. Actually they are bit too radical if there is such a thing. I do agree if the porting peak was tuned to match pipes even more power at that number or higher and better fit for pipe.

But no botton/midrange sucks on road unless you had a close ratio 6, 7 or 8 speed tranny

I may try a little more Ex timing. What is stopping me from getting to 10,000 or 11, 000 rpm?

But if you don't like it no way to put it Ex material back .. I need some spare cyls in hand first.

Ah its just a little bugger now who needs anything other than top end

Her's another little bugger for exmaple-> https://www.youtube.com/watch?v=CpciAQKL2lc" onclick="window.open(this.href);return false;

https://www.youtube.com/watch?v=pFIyyj8gHvQ" onclick="window.open(this.href);return false;

There are some nice scooter pipes out there for these little engine and may work great on the 92cc cyls of the gt185. A good starting point

http://www.ebay.com/itm/80cc-motorized- ... 339aa1a350" onclick="window.open(this.href);return false;

http://www.ebay.com/itm/Alsil-Quich-Exh ... ea&vxp=mtr" onclick="window.open(this.href);return false;

Thanks to all for your comments and the interest in the little Suz. By todays bike sizes, the GT185 is quite small but in stock trim and at sea level its advertised like 21 hp (at the crank I'm sure) but the whole package is so light and small, it actually performs surprisingly well. Those little wizzers weren't well maintained and there seems to be plenty of spare parts floating around... but for how long?

While extracting 28 - 30 ponies is probably possible, at what cost to the low end? I prefer tractability to high electric motor type power (very narrow power band) as I'll be riding it on the streets in Ohio and Canada. So lower end grunt is preferable to upper but still after reading AG Bell's book a chamber is perhaps the easiest and least costly way to extract a little more power and improved gpm. I'f I can get the patterns correct I should be fairly close to that goal.

I'll do periodic updates as I progress. I suppose I should find a dyno for a base line, as it is possible to go backwards.

Dio

I wouldn't recommend a full race tune approach for your application of course, but that doesn't mean that stock is necessarily best either.

With a small engine, light is obviously important, so there is less mass to drag around and that requires less power. There are two approaches to small engines. One is they are not powerful at any speed so let's at least give them some low end power to commute with. The other is to recognize that short stroke motors can rev much higher than long stoke motors with low stress levels. That is a more sporty approach and ultimately leads to all the power at the top end.

But there is a lot of ground between those two extremes and that's where the fun comes from - working out how to optimize what you have to best suit your needs. That may for example result in a powerband basically moved to the right ie more revs at all speeds and more top end. Or it might require less extreme porting changes to get more at all speeds.

With such a short stroke, it would be fine to increase peak power by 1000 revs or more without losing anything much at the bottom end. The way to do that is to work out the port time area you need in all ports to achieve a certain power at certain revs. Let's say the target was 25 crank HP at say 10,000. Work out a range of port widths and timings that could achieve that target and design a pipe. Then test for each of those combinations of width and height (say 5 pairs) and see what that looks like. That way you are not chasing port dimensions that are unnecessarily large and which would drop low end.

You are absolutely correct that it is possible to go backwards. A dyno test baseline would be a good place to start and you would need to accurately measure port widths as well as timing and compression and pop it all in the computer and see what comes out.

TZ375.... Thanks for the update info on 2 stroke tuning. You mentioned increasing compression and I've always been interested in this approach because higher compression generally results in better torque values, at least in 4 strokes. I can't imagine why this wouldn't also be true with 2 stroke, although a 2 stroke seems more finicky about some adjustments. Detonation is a consideration but mostly I think with racing 2 stroke motors and a bump in compression for street use shouldn't be a problem. But the question is how much. Do you have any recommendations?

I think the GT185 runs 7.0 cr adjusted (much higher unadjusted) and since I run premium anyway and I'm going to install head temp gauge for each cyl, I'm thinking I could manage any potential problems. Another thought I had was installing exhaust gas temp probes into the exhaust pipes just as they exit the exhaust ports to watch excessive heat issues, again being aware of potential problems and manage them before they create engine damage. Any experience with this also?

Dio

The trick there is to watch what the gauges do because what you see is not always what you think it should be doing. For example when it's detonating to death, CHT rises and EGT falls. If I got that the right way round.

It's a good idea to keep an eye on things or even better get a detonation light and back off the timing or throttle when it starts to flash too fast. There are several on the market and will probably be more useful to you because detonation is often bad at part throttle moderate revs because the mixture is on lean trim for economy.

How much CR is too much? the answer as always is "It depends". I cannot remember how bad the heads are on that motor but with squish heads you can get up to 165-175 psi cranking on pump gas. My 750 runs 165 plus on pump gas with no issues with a crappy quiescent chamber. With a decent tight squish band you can go higher before you run into trouble. Do you have a picture of the heads and do you have any idea how wide the squish band is?

To put that into perspective an old type TZ250 (54mm bore) runs about 15.5:1 or 7.9 after exhaust port closes on 100 (MON) octane gas and I just had a twin head machined to 17:1 to run with 105 MON fuel - that's about 110 (R+M)/2.