Engine mods I may try.

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Aug 20, 2009
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Erie, PA.
Any opinions on the following will be read and considered with care. I've been a motor head for like forever, but my experience with micro engines is limited. Most of my experience comes from auto and motorcycle engines. So jump in with advice/opinions.

I have 2 robin engined MBs, the old Staton friction drive and the new GEBE bike. This gives me the oportunity of trying some things with the old MB to get a little more from a Robin. Higher compression? Hmmmmmm. I can shave the head but should I?

I intend to make a new lower back pressure exhaust. It should be a piece of cake since it is a straight shot out the back. I figured to cut the flange off the stock muffler, slightly hog out and polish the exhaust port a little, match the exhaust flange to the port, silver solder on a length of pipe gently bent 90 degrees towards the ground, and find a suitable muffler to attach.

The carb seems like an obvious way to punch things up. The stock carb is a Walbro diaphragm type with a 9.5mm hole. I could try a 10mm carb of the same type as stock. I would port the intake slightly, only enough to match the larger carb and also polish the intake port. The air cleaner can be replaced with a higher performance unit to allow freer breathing. One thing worries me; Higher top speed is only a small part of the goal, higher torque is the summit. Putting on a bigger carb may increase power at high rpms, but reduce low end punch, hurting overall performance more than it helps. By trying a little ram effect manifold length tuning, I might be able to bring the low end up. A longish intake tube could be added between the carb and the head. Likewise, the exhaust could be made to almost any length that would help with the bottom of the power curve. My problem is, I have no idea how long to make the intake and exhaust to bring the power on at the lower end. Are there formulas for 4 stroke micro engines regarding ram tuning, IE length to power/torque gains and at what rpms, or is each engine model unique due to port/valve/combustion chamber design.

A cam regrind is more than I want to try. I don't want to try to make a dragster from a 33.5cc engine, after all, just to maximize the scoot factor. LOL. :confused:
 
Be very careful what you do to the engine ports.
I wouldn't touch them till last and only with supporting data to show what i'm doing is correct.

Now i don't have alot of hands on with 4 Strokes so i may be a little off with the porting.

Definatly polish the exhaust as smoothe as possible and the intake should actually be roughened. Rougher intake causes better atomization of the fuel air mix. and polished helps the exhaust flow better.

But all transitions in the ports should be smoothe and step free including the gaskets.
Matching the gaskets to the ports will be a plus.

Bigger carb may not produce the results you are after.
With the 2 cycle i have found the carb size not to be a limiting factor to performance.
Only at super high rpm could my carb not provide enough.

High flow air cleaner would be a plus.

I am positive that higher compression will definatly give you more low end power.

There i would take a good thick pc of glass and wrap Emery Cloth around it.
Sand about .010 to .020 inch at a time off the head and see how it reacts.

Also you could measure the volume of the combustion chamber and figure out what your compression is.
Then search out what the optimal compression would be.

Remove enough material off the head to achieve it.

But be careful there too and make sure the piston wont hit anything.

I would start with compression and exhaust.

Other porting to make the engine breathe in my opinion allows it to rev higher but actually looses a little of the low end.

It depends on what your really after.
Good low end as in it will climb anything?
Good low end as in it gets up to speed faster?

Go small and slowly till you are content with where it is at.
But remember if you go a little too far you will get the oppisite of what you want.
 
The only other thing i tink about is the springs in the clutch.
Changing them to engage at a diff. speed "where your power band is at could also be a big plus"
 
Did I mention how impressed I was with your bike when we rode together? It was the first HT I've been around. Smoother and quieter than I expected but the exhaust smells as bad as my 2 stroke outboard. :) I don't want to gear the bike down any for better acceleration or to better climb hills. The friction drive bike I rode that evening tops out at about 26-27mph and I'm fine with that. If I need to pedal on hills I'm fine with that also, I do need a little exercise. I would like a little more brisk acceleration.

The small carb acts as a governor to limit rpm, it is a design feature, so putting on a slightly larger throat unit will automatically raise top speed even though it may also shorten the engine's life. The engine has a compression release and fairly high compression stock, so I'm not sure if I should take much, or even any, off the head. I will do it in small increments if I do raise compression. I use a bit of clay and squeeze it in the engine to check piston clearance. The exhaust is going to get some improvements and I'm looking for a low restriction intake filter, regardless.

You have my number so when you want to go riding again, let me know.
 
As far as i can figure there are two things to shoot for.

A. Higher Torque at lower rpm = more pulling power.

B. Higher Rpm to get higer speeds.

The only way to get higher torque at lower rpm would be higher compression or large combustion chamber.

To get higher speed means higher RPM.
Getting the engine to breathe easier will accomplish that.

A balance between the two will yeild the best experience.

I would look at thoose clutch springs.
Seems that springs that engage at a lower rpm are desirable.

As i said i have not beefed up any 4 strokes but it seems the same principals apply.

Only difference is your valves seal making one less thing to worry about.

Maybe you could slightly file the ends of the Valve Stems to Change the timing a little.

Really i don't know.

But if you can retard the timing a little that should give more lower end power.
 
To adjust spark timing I would have to move the magneto pickup. It could be done by filling the coil mount holes and then drilling new holes to remount the pickup coil. To retard or advance by only a few degrees is probably beyond my abilities, I'm afraid I would mess it up.

The valve lash is adjustabe at the rockers, just like a pushrod car engine, so I wouldn't need to grind any off the valve stems.

Clutch engagement speed is already only a little above idle. I was considering raising the engagement speed by installing stronger springs to bring the engine engagement speed on higher in the power band. Now I'm confused.
 
Well i'm sorry for confusing you.
As i said i am not real knowledgable with the 4 strokes.

I think you are corect about the clutch.
If it engages just above idle thats prolly what you want.
If you make it engage at higher speeds i don't know what the effect will be.
You would prolly have to peddle faster before hitting the gas.

Anyway sorry i cant help here.

Seems like the Exhaust is the best thing you could do.

I guess these 4 Strokes are alot harder to beef up.

I will have to lear though.

With the extinction of the HT i'll have to run a 4 stroke.

Have to try that Honda GX50 in the frame.

BTW
Your bike was much quiter than mine.
I bet the regular bicyclist were irritated with the sound of my engine.
 
I am stoopid. I forgot to RTFM first. When I bought my first Robin EHO35 I also sent for the repair manual. After looking it over (finally) I discovered the head is integral with the cylinder, doh! The advantage is I will NEVER have head gasket problems, but to raise the compression I would have to install a domed piston. I will leave the compression alone. It looks like carb, exhaust, manifold, and port mods are about it.

Read the _______ manual, Mike. Words to live by.

A few of the pedal bikers did complain about your exhaust noise or smell, but most were happy we were there, to add to the numbers and to block the intersections. One or two even complained about MBs in general, cheaters was the word I overheard. Heh heh. I had a ball with Critical Mass. I have an avid pedal biker friend in Chicago who loves their CM rides. They often draw well over a thousand bikes for their Critical Mass rides there.
 
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Well i would like to do some more critical mass rides.
It would be nice to get enough MB'ers together in ERIE.
Let me know... I'll ride anyday.
 
You can make some power gains with port mods and intake/exhaust tuning.
The gains are not huge but will give a noticeable improvement.

I have some links that offer good reading on the subject.


http://www.grapeaperacing.com/

Has several valid tech articles, You can start there.
 
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