Engine mods I may try.

Discussion in '4-Stroke Engines' started by motorbikemike45, Nov 14, 2009.

  1. 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:

  2. 210061741

    210061741 Guest

    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.
  3. 210061741

    210061741 Guest

    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"
  4. 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.
  5. 210061741

    210061741 Guest

    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.
  6. 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.
  7. 210061741

    210061741 Guest

    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.

    Your bike was much quiter than mine.
    I bet the regular bicyclist were irritated with the sound of my engine.
  8. 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.
    Last edited: Nov 17, 2009
  9. 210061741

    210061741 Guest

    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.
  10. Old Bob

    Old Bob Member

    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.


    Has several valid tech articles, You can start there.
  11. Thank you Bob. There is some great information in there.
  12. Mr.G

    Mr.G New Member

    Has anyone ever used the Ryobi 26cc four stroke engine off their string trimmer?
  13. Old Bob

    Old Bob Member

    I reviewed one once, its not really suitable for a MB. Very Low on power @ .9 hp and no hot rod potential.It is compact and light, but you wouldn't be happy with it on a MB.
  14. I have some experiance hot rodding small block Chevys and a 410 Edsel beastie I put in a '55 Ford tudor as well as several motorcycles, but there's not much in the way of hop up parts available for the Robin. I will have to wing it I guess.
  15. Old Bob

    Old Bob Member

    The problem is that you can only do some much before you need to work on cam timing and compression. Compression might be able to be increased using a techinque I developed for the Honda GX31.Although you can only increase compression on the stock engine a small amount because its too easy to push cylinder pressure to high and end up losing power.The rule of thumb is to have trapped CR not over 9:1.
    Then the next thing is the cam. The unilobe cam can be modified to a certain degree although to maintain valve train geometry and increase overlap, you have to add material at the top of the lobe, the trick of reducing base circle does not give enough of increase in lift or overlap to really make it effective.
    You could try it though, and just make new pushrods from drill blanks to bring geometry back.Unfortuntly the cam lobe is a sintered metal part molded in the cam gear, it would need to be ground, but this could be setup on rotary table on a bridgeport with the appropiate shank mounted grinding wheel.
  16. 210061741

    210061741 Guest

    Mr Mike

    Any Progress?
    I love your new bike it's great.
    When you get all the tuning done let me know.

    If you get bored over the winter i would love to build one for my wife.
    But thus far i have no experiences with the rack mounts.

    I'm sure you would be a great help.
  17. Helping to build a rack mount for your wife would be fun. Have you thought about what system you will go with? Friction drive is cheap, light, and simple, if hard on the tire. Staton's 18.5 to 1 gear box/chain drive is strong, bullet proof, and very high quality, but fairly heavy. The GEBE belt drive system is light and smooth with a nice choice of 4 final drive ratios, but the price seems a little high for what is included. I've built bikes using all 3 and the GEBE may be the best compromise between weight and ease of use.

    I haven't even started yet. Money will be very tight untill after the holidays. I've decided to go with a one size larger Walbro carb of the same series as stock, porting the intake/gasket to match the carb, and building a freeer flow exhaust. I want to keep the reliability, long life, and fiddle free running of the engine. The only difficulty these mods could cause is over revving the little mill. If I add a tach the revs will be my responsibility and I will have to learn to exercise some control over my exuberance.
  18. 210061741

    210061741 Guest

    I hear it my spending is cut back till next year.
    The bike you brought over was a Subaru?
    I think a bike just like that would be perfect for her.
    If something breaks down she would be stuck till i went to get her.

    Plus she wants the full suspension.

    And a Pink Motor with pretty flowers painted on it. LOL
  19. Ha ha ha ha ha! The engine is a Subaru Robin EHO35, 33.5cc, 1.6hp, and uses a Golden Eagle Bike Engines (GEBE) belt drive system. My bike is a Maelstrom. Full suspension = plush ride. Any full suspension bike will work, but mine has a rear disc brake and that made mounting the GEBE system a pain and voided the warranty. A rear disc is not recommended. You could certainly paint the engine cover pink with pretty flowers on it. It has a cover because it is fan cooled and the cover directs the air flow evenly over the engine. Let her paint it herself.
  20. 210061741

    210061741 Guest

    Will do.
    Now i know what to price anyway.
    Can't wait till christmas.