48cc Grubee GT-2B 1500 mile TEARDOWN

Discussion in '2-Stroke Engines' started by 210061741, Nov 7, 2009.

  1. 210061741

    210061741 Guest

    Well i have about 1500 miles on this puppy.
    Rode Hard and Fast Daily.
    SBP Chamber / Boost Bottle / High Compression No Head Gasket.
    Allways Cranking every last RPM I could Possably Get outta her.
    Only Limitation was the stock Carb but the Boost Bottle gives me extra volume to pump into the cylinder to get some more RPM.

    The Verdict is in.... I am Fairly Impressed.
    The things i found i didn't expect to see.

    Everything well oiled, Bearings in good shape.

    There were a few mistakes made on this motor at the factory.
    We will cover that later.

    I thought this motor was shot.
    It's not, I will be putting it back together today.
    With all the original parts except gaskets.

    Iv'e got alot to do so i will continue this later.

    And yes yes yes,,,, I have a Ton of Pics.

  2. DIYMark

    DIYMark Member

    As I found out the stock carbs are not as much a limitation as I thought they were - I got amazing revs and power with my stockie and the only thing I did was drill jet and adjust fuel air correctly.

    ...and the boost bottle is a nice addition - everyone says they are rubbish but If you make them correctly I experienced a 7.5Km/h increase in top speed! Not bad for welding up some scrap steel I had at home. Speaking of which, ill try and make one up for my current bike (I had one on my first HT and my second is the 70km/h one) now I can go 77.5km/h haha

    During typing this I had an idea - Ill machine up to halves of pipe (one capped off, one with a fitting for the transfer pipe to manifold) and each halve will be threaded (using a pitch that knowing the diameters of the pipes, one turn will equal 0.5cc) and then I can screw/unscrew the 2 halves to exactly tune the volume. In addition there will be a backing nut to lock it down once tuned! Might seem excessive but hey, if I got the time, skills and materials why not.
  3. 210061741

    210061741 Guest

    Yes you are correct about the stock carb.
    You will see my fuel air mix was way rich.
    And i still had alot of power.

    The problem which i'm sure existed when i got the motor is the bearind seal at the magneto side was stuck to the magnet and not sealing my engine.
    So i was allways sucking in air at the crank.
    Thats why i had to run so rich.

    Now i'm fixing that and doing some porting.
    Actually just takin a break.

    Right now i'm grinding out the transfers in the crankcase and matching the cylinder transfers.
    Should produce better flow and i should be safe there.
    The size of the transfer in the cylinder will actually be the limiting factor in the porting.

    Well back to work.
    And i will post pics later.
  4. 210061741

    210061741 Guest

    Well got her allmost all back together.
    Started to mount back to the bike.
    That won't take long but i want to add a spacer behind the rear mount.
    To take up a little slack in my chain.
    BMX chain stretching to ****.

    Other than that i will need to adjust the clutch correctly and i'm sure thats gonna be a PITA.

    Is there a wrench or tool for that darn star nut?

    Other than a hammer and screwdriver.

    I had enough fun for tonight.

    Finisher up tommorow and hope to have a Happy Time.

    Otherwise i'll be happy when i get a new motor.
  5. Al.Fisherman

    Al.Fisherman Active Member

    I remove the small screw, engage the clutch as much as possible and take a blunt instrument (Drift pin, flat tip punch) and care.
  6. DIYMark

    DIYMark Member

    I pull in the clutch and use my fingers to tighten it (once retainer screw is removed)

    *Yes I am Rambo*
  7. 210061741

    210061741 Guest

    Yes i figured it out.
    Actually i read it somewhere.
    Lock the clutch lever and screw in by hand.
    Think it will be ok.
    I can't wait to fire it up tommorow.
    Wondering what effect a good seeled crankase along with grinding out the transfers in the crancase.
    I should be able to adjust my fuel air mix leaner.
    Also now i have no head gasket or bottom jug gasket either.
    I got rid of em.
    This will give higher compression in the cylinder.
    It's been this way for at least 500 mi an no damage.

    The bottom jug gasket removal is new and will change the timing.
    I didn't mess with the cylinder ports yet.
    Wanna see how this goes first.

    Without the bottom jug Gasket I think a couple of things will happen.

    The length of the power stroke will be increased as there will be more stroke length past the exhaust.

    And a higher BMEP or pressure in the crankcase "which if sealed now" should kick things up a bit.

    Ultimatly The BMEP of the engine has a huge impact on the performance.

    What i would really like to know is why there is so much weight added to the crank with the two roughly machined disks.

    A lighter crank will allow the engine to rev higher and seems to be desirable.
    Did they do that to limit RPM.
    Also if they were machined out of aluminum would that be a bonus.
    They are needed to take up volume in the crankcase.

    There is no noticable offset in weight so i don't believe they balance the motor.

    A true running crank is what will produce better balance.

    Crank Balance in a Single Cylinder seeme to be a myth.
  8. Whizzerd

    Whizzerd Member

    Interesting read 2100..., have you considered a tuned pipe exhaust set up to increase top end performance? In modeling, amazing power increases are achieved. Gas or methanol, doesn't matter. Flexible coupler between pipe & expansion chamber allow one to fine tune the system. Also, would you tell what oil your using and mix ratio? So many people here say the HT's are junk. I'm satisfied with mine so far but still breaking in both. Thanks!
  9. Fabian

    Fabian Well-Known Member

    Last edited: Nov 10, 2009
  10. 210061741

    210061741 Guest

    Back on D road

    Ok i got it all back together and running very very strong now.
    I was worried for a bit when it wouldn't start but i had a couple things going on.

    #1 I put quite a bit of 2 stroke oil in the crankcase before i put the head back on.

    #2 messed up my magneto coil pullin on the wires.

    So when i tried to start it up it blew all that oil out the expansion chamber.
    And fouled the allready not sparking plug.

    Peddeled it around a bit to get the rest of the oil through the engine.
    At least she is lubed up as good as possable.


    I took the motor apart cause i thought i was about to loose my needle bearings. What i found "after dissassembly" is that the oil seal behind the magneto popped out of the crankcase. Since my chain broke a hole in the magneto housing when it came off i was sucking a tremendous amout of air into the motor.
    See the picture of the seal stuck to the back of the magnet.
    That is how it came out of the motor.
    That seal was not installed properly at the factory.

    Fortunatly i was running a very rich Fuel / Air mix with ample oil so no major damage was done due to running lean.

    Actually look at the picture of the cylinder head it is black with buildup from running too rich.

    I have an SBP Expansion Chamber and a boost bottle with the lame stock carb. I was never able to tune the carb 100%
    Either i run too lean and have not enough fuel at high rpm
    Or I run rich and get a little more 4 stroking at low rpm.
    There is a picture of my DIY boost bottle welded to the intake.

    So i opted to run it rich. Allthough a new carb that has an adjustable air fuel mix not using the E-Clip would be a big help. Now that i fixed my motor instead of buying a new one i can buy some parts this week.

    Including a carb.


    The needle bearings looked good.
    But there are no thrust washers at the crank pin or Piston pin.
    Once the cylinder is off i can move the piston alot.
    Side to side and back and fourth.
    Tons of play in this thing.
    The Connecting rod has 3 oil holes drilled in each end.
    And was very oily especially at the bearings.


    The 6202 Bearings at the crank are not sealed on either side.
    Except for the rubber oil seals on the case.
    They were both pretty dry.
    I packed these with grease on both sides before reassembling.

    It seems 6202 bearings that are sealed on both sides would be the best here.
    I would like to replace them with fully sealed bearings.

    Dose anyone know a reason why that wouldn't be good?
    Has it been done before?

    The 6202 bearings at the Clutch / Drive shaft are sealed on a single side.
    The outside.
    That works good here cause the area in the Crankcase where this shaft goes thru is sealed off from the Crank part of the case.
    When you pump grease into the clutch post hole on the top of the motor it lubricates them quite well. Along with the clutch spring that is on that shaft.
    I had lots of grease here.

    I did notice several dents in the bearing faces on the sealed sides of theese bearings. They were definatly hammerd in at the factory.
    I didn't replace them yet but the next time i tear this motor down i will.

    To answer some peoples question about removing that shaft.
    It may be possable to remove it without splitting the cases but i wouldnt chance it.

    Splitting the cases is fairly easy. I tore allmost the whole motor down wile it was on the bike. I loosened all the case screws and removed everything i could befor pulling the motor.
    That way i didn't have to fight the screws and try to hold the motor at the same time.

    It took maybe 1 1/2 hours for me to completly tear the motor down.
    And it was worth every minet.
    I did not split the Crankshaft. It looks good so i left it alone.

    The worst damage to the engine.
    The piston was pretty well scuffed up on 2 sides of it.
    The other 2 sides of it had buildup.

    The cylinder was the crome plated type.
    Alot of the crome was wore off down to the aluminum.
    Seems to match the piston wear.

    The top of the piston had no holes or pitting.
    So running with no head gasket and higher compression did not hurt it.

    All in all i should replace the cylinder and the piston and the rings.

    Since i didn't have to buy a motor i can buy thoose too.


    I ran the **** outta this thing allways as fast as i could go.
    36T on the rear wheel.
    4 to 6 oz NOS Octane Booster Per Tankful of gas.
    And most of the time 1 1/2 oz oil per tank of gas.
    I allways filled it at the station and mixed it in the tank.
    Never have a problem.
    Shut off fuel valve / Add Oil / Add Booster / Add Gas / Shake Around.

    Anyway thats all i got time for right now.
    I'll be back to tell you about the porting work i did to the crankase.

    Attached Files:

  11. happycheapskate

    happycheapskate Active Member

    What is a boost bottle?

    What is a boost bottle?

  12. happycheapskate

    happycheapskate Active Member

    What is a "transfer"?

    I understand what a cylinder is, and some about 2st. engines. What is a cylinder transfer?

  13. happycheapskate

    happycheapskate Active Member

    From what I've seen of bicycle motors, they are all junk save for a few one-offs or actual moped and track bike engines people have carved out their pedal bikes to install.

    Maybe it will be like cell phones: 20 years ago, they weighed a ton, were thousand dollars, considered silly or pretentious, and irritating to use. But USEFUL! So eventually now they are cheaper and majorly mass produced, with lots of design competition and fan/brand marketing, trickle down technology, and better reliability (for the most part).
    maybe in 20 years , motor bicycles will be awesome. For now they are pretty much either just useful, or pretty but painstaking.

    good luck!

  14. happycheapskate

    happycheapskate Active Member

    re: no head gasket

    Did you use some type of gasket maker on the engine? Did you mill the surfaces any?
  15. happycheapskate

    happycheapskate Active Member

    I have a Grubee Skyhawk 48cc I am still putting together (muffler doesn't fit. Have to bend it or something, not while attached to motor! I know)

    The instructions, kind of lame as they are, say to use the clutch lever lockout when adjusting the clutch. I think that star nut is for fingers. It came with some kind of puller tool, probably for the 10 or 11 tooth cog. It didnt' come with any star nut tool.

  16. 210061741

    210061741 Guest


    I use aluminum gaskets cut from very thin disposable cookie sheets.
    And yes i mill all the gasket surfaces on the engine with the sandpaper and glass.

    If the muffeler dosen't fit check how much the motor is tipped to the right.
    Is it getting blocked by the bicycle downtube?.

    The cylinder is the part with all the fins.
    The piston is inside the middle of it. "cylinder bore"

    The Transfers " Transfer Ports " is the passage where the fuel air gets sucked into the crankcase.
    We want good flow through there.
    The transfers are located on each side of the cylinder.
    The front of the cylinder has the exhaust port.
    Where you connect the exhaust.
    The rear of the cylinder has the intake port.
    Where you connect the carb.

    The HT is piston timed which means there are no valves.
    Only 2 Transfer Ports the Intake Port and the Exhaust Port.

    I know i have pictures of this stuff around here.
  17. happycheapskate

    happycheapskate Active Member

    Thanks for the info

    Ok. I understand. So it doesn't have a reed valve? Very interesting. I understand the transfer port then, I just didn't have a word for it.

    I like 2 stroke motorcycles for some reason. Maybe because I like the sound and because they are always changing their own oil if you do the mixing right. :)

    Can anybody link me to a thread on various oil for 2cy bike engines?

    I want to read reviews of the different oils, like Echo 50:1 premo oil (can use more if you want), Walmart Ashless 2cyc ski-doo/atv oil, Power Blend Home Depot, etc.
  18. 210061741

    210061741 Guest

    I have used MC-1 Synthetic Racing oil and Lucas Synthetic.
    I like the Lucas.
    I think any synthetic oil is good.
    You use less and it leaves less garbage in your cylinder.
    If i use normal oil i have to run the carb richer.
    With the synthetic i can lean it out a bit.
  19. happycheapskate

    happycheapskate Active Member

    You can adjust the star nut with a $1 pair of pliers. Be careful.