2 stroke tuning for beginners

Cheers for this bit eddie!

A true inspiration, for me anyway

Well the only 2 stroke i've got unfortunately is my mini moto. A little 3.5 blata supermoto. I've been half heartedly stripping and rebuiling it with intent on making it as much like my race supermoto as possible.

Tonight i removed the engine and have stripped it. Just wondering what tuning i can do with a steady hand and a dremmel if any?

I removed the rough edges that were on the intake but wonder if getting things really smooth would be of benefit at all? I'm sure i've read about creating a venturi effect by leaving things rough.

Any hints or tips are welcome. Parts for these things are peanuts on ebay so happy to try stuff out.

I'll post up some pics when its finally done. It already sounds like a race bike

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Take bits out of pistons rather than the barrels to adust intake / exhaust timing. If it work then make the change permanent on the barrel.

Keep using VERY good oil if its tuned !

Really smooth is bad btw... You want rough(ish) to create a good mix with the air and petrol... Turbulance almost... Struggling to get across what Im saying.. Someone will clarify

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Do you know anyone throwing out a plotter / projector.. any kit really that could be used in making large scale prints. I dont have money but would be willing to work / trade. Give us a PM if you know anything. Cheers. G

Only ever tuned my chinese 41cc engine & even though the stuff I did was just mainly to tidy everything up the engine responded well & does go very well now.
I did'nt alter the timing on the ports ie raising them, all I did was widen them a little about 1mm either side, on the intake on the bottom of the barrel and the crankcase (where the two bolt together) there was a big mismatch so I sorted that out.
Also the transfer (fuel/air) ports on the barrel itself I opened up the port channel & changed the angle a little to direct the mixture to the cylinder head better, make sure their the same height as well.

Doing these mods you can't go wrong, make sure everything in the barrel has a nice radius on it.

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KTM 660SX (ex-dynotorque)
SV650S

im_slo_on_my_sm wrote

I removed the rough edges that were on the intake but wonder if getting things really smooth would be of benefit at all? I'm sure i've read about creating a venturi effect by leaving things rough.

Venturi effect is what you get in carbs where you reduce the size of an orrifice to increase the airspeed through it and reduce the pressure. I think you are thinking of boundary layer effects. Current thinking is that a polished exhaust port is ok to reduce carbon buildup, but inlets and transfers should be left with a rough surface to promote turbulant mixing of the inlet charge at the boundary layer.

I wouldn't go changing any port timings unless someone can give you specific port dimensions to work to. Arbitarily hacking away at the ports can have a detrimental effect if you get it wrong. In general though, make sure opposing pairs of transfer ports open at the same time. If there's more than one pair of transfers, it's perfectly acceptable to have staggered opening of different pairs. Flatter port roofs tend work well, aiming flow arcoss the piston crown, rather than ones pointing flow upwards. Make sure there are no big steps or sharp junctions in any of the passages. Also any acute corners in ports will cause flow stagnation so either fill them with epoxy (Defcon F works well) or open them out a bit.

You can get appreciable performance increase just from making sure other parts of the engine system are matched to the barrel - a decent exhaust, making sure the carb is jetted correctly and checking the ignition is up to the job.

You could check your squish height, probably left gaping as most prodcution engines are, and get that right and also bump the compression up a smidge to increase power.

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Team RNR

Valves are things to hold air in tyres!

The inlet port itself was really smooth (its a little die cast tube connecting the carb and engine) and i'm considering making it a little rougher to try and improve things, also i've found several edges that are pretty sharp and poorly matched so will improve that.

I dont have much more machining tools than drill, files, dremmel and wet & dry.

I was thinking of squish but dont really know enough to start playing with that, though i'm interested in more info?

The exhaust, which is standard, as far as i'm aware is the only one made for the bike so i cant really make improvements there or can i? Its in perfect condition and last night was stripped, cleaned and will be re-coated tonight.

I'm interested in a book or a good site to really read up on this. I've always been keen on getting into tuning and as this bike is easy and cheap to replace bits on think it could be a good starting point.

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im_slo_on_my_sm wrote

I was thinking of squish but dont really know enough to start playing with that, though i'm interested in more info?

I'll see if I can knock up a sort of laymans guide to squish.

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Team RNR

Valves are things to hold air in tyres!

Ric wrote

I'll see if I can knock up a sort of laymans guide to squish.

That would be great Cheers!

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I've got a really good article by Rob Tulie which covers most of the stuff concerning squish and detonation. It's a "microsoft word" document at present, if someone could host it I'll gladly e-mail it, excellent article.

Is it this one? Like most or Rob's articles, it's pretty technical and mainly theory rather than application as such. A very good article, but maybe a bit heavy reading for a newcomer.

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Team RNR

Valves are things to hold air in tyres!

Ric wrote

Is it this one? Like most or Rob's articles, it's pretty technical and mainly theory rather than application as such. A very good article, but maybe a bit heavy reading for a newcomer.

That's it! Maybe a bit technical but very well explained and TBH anyone who can't understand it or get to grips with it probably shouldn't be messing with base gaskets and skimming heads anyway! The diagrams on it are worth quite a few hundred words alone.

Did a little spreadsheet for calculating head volume and squish velocitiy
Might be of some use.
http://myweb.tiscali.co.uk/teamsparrow/cylinderheadsv2_3.xls

MJ, nice spreadsheet. Either I'm using it wrong though, or it's giving out numbers about half of what I'm expecting to see. I compared it with 2 other packages which give similar results to each other, but double the numbers I get off the spreadsheet. Any ideas?

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Team RNR

Valves are things to hold air in tyres!

Don't know - which numbers do you think are out?

It's the MSV on the hemi-head sheet.

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Team RNR

Valves are things to hold air in tyres!

Dunno ? Equations are based on Blair's book. Only thing I can think of is I do the full stroke not the trapped volume. That might make a difference - will have a look at it tonight.

I thought I had checked it against another bit of code - might have been the Bimotion - it is a while back and I can't remember.

What do you use?

I'm using TSR and EngMod2T. Both give very similar results to each other. TSR is just a rip-off of Blair's programs anyhow.

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Team RNR

Valves are things to hold air in tyres!

tried an example of Macdizzy's where he was testing Bimotion v2.1 (Think I did my original checks against bimotion output though not the Dizzy example)

http://www.macdizzy.com/19892hopup.htm

the tweaked motor
On Engine data sheet
68 bore, 57 stroke 115 rod length
set deck height and gasket thickness to 0
Exhaust port duration to 189deg
RPM 8700

On Hemi Head sheet set
D1 = 68
Step 1 =0.9
D2 = 50
Squish Angle =10
Head radius =29.5
Piston Dome =150

Gives 14.97 Volume (Dizzy has 15.4 )

Trapped CR 7.99 Dizzy 8
Geometric CR 14.36 Dizzy 14.4

Peak Squish Velocity 27.76 Dizzy 27.6

I haven't got enough data from Dizzy to do it spot on but it is close enough. The slight difference in CR isn't going to make a big difference.

Will see if I can find some other examples to check against - but interested to see why TSR and other package are twice the squish velocity?

hello, you have to give me the bimotion
?