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2 stroke port timing and carb questions.

Posted: Thu Nov 09, 2017 3:01 pm
by gmc406
I’m currently working on a vintage snowmobile with a 2 stroke 436cc piston port air cooled engine. I acquired the machine from a fellow who couldn’t get it running properly. Upon inspection I could see a massive leak below the one cylinder. Once I removed the cylinder I could see that the lower gasket was installed upside down, causing the leak.
I have now torn the engine down and decided to replace crank seals and pistons. I plan on putting the engine back together without head gaskets and simply lap seal the heads. I did this to another identical engine, and it worked wonderfully. The engine had a lot more snap and upper rpm picked up. The operating range of this engine with be from 4500-7400ish. I’m wondering about shimming the cylinders with a thicker lower cylinder gasket. Does this generally move the power band up or down?
Right now the engine has a single 34mm Mikuni round slide carb. I have in my possession a 38mm flat slide Mikuni. I can’t seem to decide if it’s worth the trouble to mount the 38mm. I haven’t found any cfm ratings on these carbs. Arctic Cat and Polaris were both running dual carbs back in the day.

Thoughts? Thank you.

Re: 2 stroke port timing and carb questions.

Posted: Thu Nov 09, 2017 3:28 pm
by mitch
Raising the cylinder will raise the power band. As far as the carb I would probably stick with the stock one unless you are going to do more as far as porting an maybe exhaust.I would not raise the cylinder very much because a little can do a lot.You need to be sure an check the squish before an after an try an keep it around .030-.035 that also helps.
Mitch

Re: 2 stroke port timing and carb questions.

Posted: Thu Nov 09, 2017 5:14 pm
by jet1
Agree with Mitch also as you raise the cylinder the transfers raise also. something to watch for as depending on the height it can effect the mid range alot.

Re: 2 stroke port timing and carb questions.

Posted: Thu Nov 09, 2017 5:58 pm
by pamotorman
instead of cutting into the cylinder to raise the transfer ports I just notched the piston top .020 to .040 in the transfer port areas. if it is a piston port not reed valve engine notching the piston skirt in the intake port area will raise the power RPMs

Re: 2 stroke port timing and carb questions.

Posted: Thu Nov 09, 2017 7:22 pm
by Erland Cox
Highly recommended

http://www.macdizzy.com/

Erland

Re: 2 stroke port timing and carb questions.

Posted: Thu Nov 09, 2017 8:37 pm
by gmc406
Thanx for the link, I will definitely take the time to read.

As far as porting this engine, I don’t have the knowledge or confidence to do so. I’m too scared of wrecking the engine. Removing too much material or in the wrong place, would wreck it. All I would really do is polish the ports up and remove a few burrs.

I can’t notch the top of the piston as the upper piston ring is right at the very top. If I decide to take a little material off the piston skirt, how much would you recommend?

If I go with shimming the cylinder, any idea how much? The current gasket is 0.015. I have 0.025 gasket paper.

Kinda glad you recommended to keep the original carb. It would save a bunch of time retrofitting, not to mention all the tuning.

Re: 2 stroke port timing and carb questions.

Posted: Fri Nov 10, 2017 1:25 am
by jfn2
Also go to vintagesleds.com and get on the forum. These guys will help you a lot and they know what they are talking about. The most important part on a two stroke is the exhaust pipe. You don't say what year or make of sled your working on, so you won't get too many recommendation on what to change. You can usually raise the cyl's .040" or 1 mm and not get into trouble, but when you raise the cyl's you will also lose compression so you have to cut the head or the top of the cyl to get it back. Most old sleds had about a .040" or more head gasket. This was usually the squish clearance, so when you remove the head gasket you lose this clearance. You want to maintain about .035"-.040" squish on these old engines. It looks like your working on a mildly tuned sled with only one carb. With that said, the manufacture should have had a performance model in that same year and size. You should be able to use some of their parts to hop yours up. Also remember that when you change the rpm you have to adjust the clutch accordingly.
Those old sleds are neat. I have about 8 of them.

Re: 2 stroke port timing and carb questions.

Posted: Fri Nov 10, 2017 1:43 am
by kirkwoodken
Good start for Two Stroke engines:

http://www.vintagesleds.com/library/man ... ndbook.pdf

My experience suggests rings and pistons can get by with 100:1 oil mix; Bearings require more oil for long life.
I've always mixed 32;1 simply because it's easy: an ounce of oil to a quart of gas.

Re: 2 stroke port timing and carb questions.

Posted: Fri Nov 10, 2017 7:04 am
by Circlotron
kirkwoodken wrote:
Fri Nov 10, 2017 1:43 am
Good start for Two Stroke engines:
http://www.vintagesleds.com/library/man ... ndbook.pdf
Page 143 says 375 x 12 = 45,000

Re: 2 stroke port timing and carb questions.

Posted: Fri Nov 10, 2017 10:12 am
by old_chopper
I've had the privilege of working with a 2 stroke wizard but unfortunately he's gone. Calvin Pollett owned and operated Cast Precision Industries in Oklahoma and built some of the finest performance cylinders money could buy. Their Cheetah Cub cylinders for the Yamaha Banshee is a fine example. His wife Elaine is still running the foundry/machine shop and I do engineering work for her.
Sorry I can't help with any advice, I just loved reading a two stroke post on here. Plus every time you mention someone who's gone name they live on. God speed Calvin!

Re: 2 stroke port timing and carb questions.

Posted: Sat Nov 11, 2017 12:09 am
by kirkwoodken
Circlotron wrote:
Fri Nov 10, 2017 7:04 am
kirkwoodken wrote:
Fri Nov 10, 2017 1:43 am
Good start for Two Stroke engines:
http://www.vintagesleds.com/library/man ... ndbook.pdf
Page 143 says 375 x 12 = 45,000
Two things wrong with page 143: Looks like a simple typo, and/or Gordon didn't really understand how induction coils work. !2 volts are not stepped up. 12 volts only "excites" the primary coil with a magnetic field. When the points open, the capacitor makes a resonant circuit with the primary winding, and the sharp drop in the primary magnetic field/voltage induces a higher voltage into itself, usually about 350 volts. That 350 V. is what goes through the secondary, and that is where the 35,000 volts comes from in a 100:1 ratio induction coil. Without the capacitor in the circuit, only 12 volts will be sent to the secondary, the result of which is a very weak spark, if any.

Circlotron: I'm well aware that you know this and may wish to correct my fuzzy explanation.

Re: 2 stroke port timing and carb questions.

Posted: Sat Nov 11, 2017 3:51 am
by Juho_
Changing port timings longer in duration might not do anything for the peak hp rpm, or when the pipe starts working.
Highly depends if the cylinder itself is currently restricting compared to the pipe (if it was designed for a higher BMEP than what the cylinders could produce).

There are many things, that can have an effect when the pipe starts working ("powerband").

- compression ratio
- exhaust port opening, earlier the higher the exhaust gas temperature will be (within limits of course ! No more than 190* duration will be needed, keep the transfer ports at less than 128* duration to have more blowdown duration.. in 2-strokes the blowdown and transfer areas are what matter a lot, not the exhaust port area! And yes, it is not that simple at all, there are many things that'll need a lot of attention to reach a lot higher BMEP)
- exhaust wrap, this will keep the pipe hot longer -> revs higher and produces more power IF the cylinder won't run out of blowdown

Combustion chamber design is very bad at old 2-stroke engines.
Get the squish ratio to around 50-60% depending of your squish clearance (which should be kept as tight as possible).
But you do not want too high maximum squish velocity, if the ignition can't be made to work with it.. it'll just stop revving any further if the msv is too high.

And the best combustion chamber design will depend a lot, how much compression ratio your going to run.
Lower compression ratio (higher volume) will allow you to machine a "bathtub" shape, or toroidal.. and squish area should have a sharp edge, not rounded.

Mikuni TM or TMX 38mm carburetor is very good.
I have never found any losses at lower rpm power on larger carburetor on a 2-stroke engine.
I've mostly worked on 50-55cc 2-stroke race engines and 70-80cc, and used carburetors from 24 to 32mm. Funny thing is, a 55cc engine produces about as much peak power as 80cc because the cylinder bolt pattern is restricting the transfer ducts shape and volume.

So two 38mm carburetors should be just fine in a 436cc twin.

Re: 2 stroke port timing and carb questions.

Posted: Sat Nov 11, 2017 9:22 am
by pamotorman
this the tool I used to put piston fed boost ports in the old 5 port 2 strokes