tb88 stock head vs race head

M

Merz

Member
Ive had the tb88 stock head, 20mm carb for a while now. I was happy with the power gain over stock, but wanted to go a little faster, so i purchased the new tb race head. It came with a different piston to fit the newer style head. The bike is now a little slower than it was with the stock head. It feels like there is less compression.

Should the new style race head be much faster than a stock older bigger dome head?

I dont think jetting is the problem. I have a 92 main installed now, and have tried an 89 and 95. 89 was way lean. 95 bogged at wot.

I have also set the valves twice.

It will get up to about 40mph now. Would it go this fast if the timing was off a tooth? the marks seem to line up perfectly.

What could the problem be?



thanks
 
P.C.

P.C.

Active Member
how fast was it before?call or email and ask who you bought it from if you should use your original piston or theirs?
 
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M

Merz

Member
I had it up to 50 with the stock head. I dont think the old piston would work because it has the bigger dome. The newer style heads have a smaller dome.
 
P.C.

P.C.

Active Member
10 mph is a big difference,i checked tb website and they list a race head for a 82- 91-94 year.which originally i think had smaller valves and less compression then the 71 style head ''like my bike''.i would take it off and sell it,it seems wrong for the bike unless its 82 91 94 model year.lower compression with big valves isnt a good formula for more power.
 
MSZ

MSZ

Moderator
12V CDI 3 speed
Trailbikes manual clutch
16/35 gearing
TB88cc Kit with Race Head, cam, etc.
20mm Keihin Carb
Good aftermarket Exhaust

= 55mph top speed and will cruise at 45 without much effort other then very large hills.

I ran the above combo on my 1993 CT70 with 3.5X10" wheels before I upgraded to a 117.
 
6

69ST

Well-Known Member
P.C. said:
10 mph is a big difference,i checked tb website and they list a race head for a 82- 91-94 year.which originally i think had smaller valves and less compression then the 71 style head ''like my bike''.i would take it off and sell it,it seems wrong for the bike unless its 82 91 94 model year.lower compression with big valves isnt a good formula for more power.
Click to expand...

12v motors have longer conrods, ball bearing cams and CDI. Installing a 12v piston on a 6v crank would drop CR dramatically and power along with it. Judging by the generally stronger performance of the few 12v CT70s I've ridden, even if the ports & valves were smaller than earlier 6v equivalents, the heads flow at least as well. Doesn't really matter if you're replacing the head anyway. The lion's share of aftermarket performance parts are for 12v motors, which are still in production. It's possible to retrofit them on a 6v lower end, but, at that point may as well go for a 110 or 117. There's a lot of dominos to stack before completing that chain.

FYI...compression ratio is matched to cam profile, not valve size. All things being equal, a big-valve/low compression motor should make more power than the same lump with a small valve head. Optimal valve size is a function of port size/flow and limited by bore diameter.
 
M

Merz

Member
Crap.

I think its about time i gave up on this motor and bought a big chinese one. I almost bought a yx 150 instead of the race head.

Here was my setup before:
1977 3spd auto
TB88 stock head, stock cam
Cheap aftermarket exhaust
cheap 20mm carb
16t front sprocket

50.3 mph on a gps on a slight downhill. Maybe a couple less mph on the flats.


I think i will put the old head back on with some new rings, get it running good, and sell the motor and the new head and buy the yx150. What do you think? Then i would have a manual clutch also, and cdi.
 
M

Merz

Member
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P.C.

P.C.

Active Member
going into this thread not knowing the year of the bike and doing some mild poking around,i managed to answer his question.thanks for the FYI Racer,but thats not the case with Merz.he lost 10 mph.why?with his particular amount of compression drop, all is not equal. i suggest talking with faatcat about the tb108 he has for sale.if your hell bent on a china motor,i strongly suggest buying from pitster pro they actually stand behind their products!very rare with china stuff,if you have any problems in the first couple months they honor a warranty!
 
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M

Merz

Member
MSZ said:
12V CDI 3 speed
Trailbikes manual clutch
16/35 gearing
TB88cc Kit with Race Head, cam, etc.
20mm Keihin Carb
Good aftermarket Exhaust

= 55mph top speed and will cruise at 45 without much effort other then very large hills.

I ran the above combo on my 1993 CT70 with 3.5X10" wheels before I upgraded to a 117.
Click to expand...


What speeds do you get with the 117?
 
MSZ

MSZ

Moderator
Merz said:
What speeds do you get with the 117?
Click to expand...

Ask me in the spring. I have 4 other projects ahead of it. Here's the last pic I have before I installed the front wheel.

The 117 should go around 70mph when tuned and sprocketed correctly with the FCR28.
 

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M

Merz

Member
To build a 117 or a 108, you need a 82' or newer motor right?

I think i read that you cant get stroker cranks for 6v motors anymore, except for the 4spd manual motors. but you can make those work on a 3spd if you buy a $200 flywheel or something. I wonder if you could get a used flywheel off a 4spd ct70 instead of buying the dratv flywheel?
 
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HymieP

HymieP

Member
Merz said:
To build a 117 or a 108, you need a 82' or newer motor right?

I think i read that you cant get stroker cranks for 6v motors anymore, except for the 4spd manual motors. but you can make those work on a 3spd if you buy a $200 flywheel or something. I wonder if you could get a used flywheel off a 4spd ct70 instead of buying the dratv flywheel?
Click to expand...

Not sure about the "82 or later, I know mine is a '93 12 volt and was converted to a TB117. Fast little bike.
 
M

Merz

Member
Maybe i should buy a newer style engine case and build a 117 instead of getting a yx motor. It would probably be more reliable.
 
M

Merz

Member
racerx said:
12v motors have longer conrods, ball bearing cams and CDI. Installing a 12v piston on a 6v crank would drop CR dramatically and power along with it. Judging by the generally stronger performance of the few 12v CT70s I've ridden, even if the ports & valves were smaller than earlier 6v equivalents, the heads flow at least as well. Doesn't really matter if you're replacing the head anyway. The lion's share of aftermarket performance parts are for 12v motors, which are still in production. It's possible to retrofit them on a 6v lower end, but, at that point may as well go for a 110 or 117. There's a lot of dominos to stack before completing that chain.
Click to expand...


Can you explain why the 12v piston drops compression when installed on a 6v crank? If i were using the stock head, that would make sense because the newer piston has a smaller dome than the stock head. But, since i have the 12v piston with the newer style head with the matching smaller dome, shouldnt it run the same compression as if it were a newer style engine? Or does it have something to do with the longer connecting rod? thanks.
 
Grant's trail

Grant's trail

Member
6V. eng. have a shorter conecting rod and the piston's wrist pin hole is lower on the piston. 12v. eng have a longer conecting rod and the hole is located higher up on the piston. so even if the 2 pistons have the same dome on top when you mix the parts the 12v. piston on a 6v. rod comes up short in the cyl. I hope this helps.

Grant:4:
 
6

69ST

Well-Known Member
Grant's trail said:
6V. eng. have a shorter conecting rod and the piston's wrist pin hole is lower on the piston. 12v. eng have a longer conecting rod and the hole is located higher up on the piston. so even if the 2 pistons have the same dome on top when you mix the parts the 12v. piston on a 6v. rod comes up short in the cyl. I hope this helps.

Grant:4:
Click to expand...

Exactly.

The location of the wrist pin hole, relative to dome height is known as compression height. Make a note, there will be a surpsrise quiz later...:3:
 
M

Merz

Member
Interesting. Seems like the guy at tbparts would have informed me of this before selling me $250 worth of parts to make my bike slower. Maybe i should get my stock head ported and put a bigger cam in it?
 
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