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Ftwelder
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Greetings, I altered a Suzuki 185 trail bike for trials service and it's coming along pretty well. The motor is stock and I made a pipe to get it going and now want to make improvements with pipe and port timing. Most of what I need can be found in factory data or engine measurements except engine RPM range typical in amateur trials.  I would guess 800~6000 with peak around 4K?  Does anyone know better?

Thanks!

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Does the motor have an expansion chamber creating a power band, (This is not desirable in trials, go with a long parallel pipe, and not too large, slightly smaller than enduro spec. I will increase the back pressure slightly and that will help the motor build torque). The big thing is smooth power and torque growth as revs climb, and as much low down torque as you can get and still have reasonable acceleration.

So, no expansion chamber, heavier flywheel maybe, (try the std one first for a while). I had good results with a smaller diameter and longer exhaust pipe and lighter flywheel. Motor was a KT250 so already a trials unit.

when riding you will (98% of the time), be trying to get or keep tires gripping the ground, so sudden uncontrolled bursts of acceleration from the rear wheel is not helpful. Typically you will need short bursts of acceleration to get over "stuff". at these times you will be "pushing down" with your weight through your legs to increase grip to handle the power increase. The timing of these two actions is half the battle, balance being the other. Oh and then there's stopping, reading the section, mental self control,.............But I'm sure you've got all this already,

Good luck with the build, bet your learning heaps, I did with mine.

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A correctly designed expansion chamber, with long front pipe and absorption type silencer can be very effective. Excellent low power and acceptable noise level.

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Posted (edited)

I have both a very long head pipe and a poorly designed expansion chamber!  The belly is probably too large and baffle cone too steep. I found a great series of Youtube videos by
"AU mechanic" explaining pipe design that I have begun watching.  The power down low is pretty good but I find myself slipping the clutch on steep grades in sections and carefully avoiding getting "on the pipe" which hits quite suddenly.

I am learning quite a bit and love the way the bike handles, Thanks much for your input!

I have lowered the cylinder .5mm by removing base gasket and it seemed to help. Compression is a low 6.5:1 in stock form. Ill make a new pipe before tampering withthe port timing any more.

Frank

 

 

Edited by Ftwelder
added something
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How is your jetting? If you are too rich on the first  1/4 of throttle then it will “boost up” once you open the throttle more and the mixture leans out. When the carb is properly jetted it fills in the lower power curve and then the transition to midrange is much more gentle. I just leaned the atomizer tube on my TRS XTrack  4 steps and it did wonders for my controllability. (it’s a Dellorto  phbl and went from a 270k atomizer to a 264k) Dellorto carbs are very easy to grasp the adjustment concepts, the numbers are in millimeters,2.7 mm to 2.64 mm change.

I recommend working on your carb before grinding on the ports,you should optimize what you have before doing something irreversible.

You can keep going leaner until it bogs, At trials engine speeds you aren’t in  very much danger of seizure 

 

I was working on a friends TRS Raga this past weekend and getting his jetting closer(Keihin PWK carb) and he commented on how it smoothed the abrupt hit compared to the way it was stock. And I really think we can smooth it more with a bit more tuning. 

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Posted (edited)
5 hours ago, htrdoug said:

How is your jetting? If you are too rich on the first  1/4 of throttle then it will “boost up” once you open the throttle more and the mixture leans out. When the carb is properly jetted it fills in the lower power curve and then the transition to midrange is much more gentle. I just leaned the atomizer tube on my TRS XTrack  4 steps and it did wonders for my controllability. (it’s a Dellorto  phbl and went from a 270k atomizer to a 264k) Dellorto carbs are very easy to grasp the adjustment concepts, the numbers are in millimeters,2.7 mm to 2.64 mm change.

I recommend working on your carb before grinding on the ports,you should optimize what you have before doing something irreversible.

You can keep going leaner until it bogs, At trials engine speeds you aren’t in  very much danger of seizure 

 

I was working on a friends TRS Raga this past weekend and getting his jetting closer(Keihin PWK carb) and he commented on how it smoothed the abrupt hit compared to the way it was stock. And I really think we can smooth it more with a bit more tuning. 

I am using a 24mm OKO (PWK) currently. I have been up and down the scale with jetting and carb size and the current combo runs well and idles down even when really hot. It's not building up any carbon and rarely if ever stalls. It does get a little fat at the bottom of a steep grade but runs very clean under any acceleration. I have a video to share that has good sound quality.

 

Edited by Ftwelder
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Damn, that sounds good. I think I would just turn that midbox you have into a packed silencer and see how it does. Just add a perforated tube through it and moderately stuff it,not too tightly. 
another experiment would be to cut up a stock 185 exhaust to fit the bike without changing header length and keeping midbox volume as close to stock as possible, just to see how it goes.

 

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Thanks! Toward the top of that climb, at that RPM the engine is ready to accelerate suddenly. Below that RPM it is weak, compared to my OSSA MAR.

I ended up cutting the cones out of my pipe and rolling new ones that were very gradual with a 65mm diameter mid section about 100mm long and hope to test it Saturday.

I have the stock pipe but I lowered the seat support tubes of the frame when I reworked it and and changed the exhaust port to use a normal MX type steel double wall flange. The original exhaust port was severely worn so it probably wouldn't fit. 

Ill keep you posted.

 

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I did a couple of trials back in the late 70's on my 185B but they were too hard (no dual routes in those days) but I did a lot of practicing with my brothers who had similar trail bikes. Yes the power comes in strong about 4500RPM but usually I kept it out of that. Dropping the barrel a little more might help and , like you say the compression is quite low. My B model had higher compression and the later models dropped it each time.

I've just bought a C model which has stood outside for the last couple of years and nearly have it back on the road.

Have you any parts left over would love an exhaust and tank like yours and I'm short of the air cleaner assembly

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Posted (edited)
13 hours ago, teamferret said:

I did a couple of trials back in the late 70's on my 185B but they were too hard (no dual routes in those days) but I did a lot of practicing with my brothers who had similar trail bikes. Yes the power comes in strong about 4500RPM but usually I kept it out of that. Dropping the barrel a little more might help and , like you say the compression is quite low. My B model had higher compression and the later models dropped it each time.

I've just bought a C model which has stood outside for the last couple of years and nearly have it back on the road.

Have you any parts left over would love an exhaust and tank like yours and I'm short of the air cleaner assembly

I removed the base gasket (.5mm) and it made a noticeable improvement. I am thinking about removing 1.5mm from the bottom of the barrel and using the gasket for a 1.0mm difference. My squish band is 1.75mm at the outer edge so I will likely cut 1mm from the head, I need some help with the calculations to make sure I don't increase the compression too much. The kicker has horrible geometry. 

The pipe is fairly easy to make but wouldn't fit the stock cylinder or frame. I am a rookie at sheet metal work and it took a very long time to make the tank and it's not even very good. The airbox is a fibreglass unit but also useless as the rear tubes are different. The bike came with a sock (like on your foot) for an air filter.

DS185

 

Edited by Ftwelder
typo
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5 hours ago, htrdoug said:

Here’s a thought for you,a modern 125,250,300 all take the same header pipe and silencer. 

That is very interesting. I am totally stumbling though the design process but this info cuts out a lot of the variables. I should make a shorter header see what the effect is.

 

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Maybe a straight kicker from the earlier TS 185 will fit the shaft? And give it the bend like this Cota. When it engages the gear it will be directly above the shaft and then rotate back and stop on the foot peg. I had a RL 250 that had a kicker like yours and it didn’t engage until it was even with the foot peg and it only gave a short effective stroke and the large offset made it wonky to kick. Fortunately it was very easy to start. 
montesa-cota.jpg

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