carb issue

E

emso

Member
Finished rebuilding a 1977 ct70. New top end, new valves, new cam, cam chain etc... Valves are adjusted at .002, timing is correct, new points set at correct gap has per manual. Carb is rebuilt, new jets. Needle is set has per manual but I'm still getting a bog when I it the throttle!? Going from 1st gear to 2nd it it will bog a bit. I've adjusted the air mixture at 1 1/4 turn and still no better. Anyone have an idea at what else I should look at?

thanks
 
6

69ST

Well-Known Member
Try dropping the jet needle C-clip to the lowest groove. That should cure the stumbles...and hopefully leave you with an overrich mixture, i.e. room to tune it back toward the lean direction.

This assumes that you've rodded out every orifice in the emulsion tube, have allowed the engine to come up to full operating temps and have no vacuum leak(s).
 
E

emso

Member
racerx said:
Try dropping the jet needle C-clip to the lowest groove. That should cure the stumbles...and hopefully leave you with an overrich mixture, i.e. room to tune it back toward the lean direction.

This assumes that you've rodded out every orifice in the emulsion tube, have allowed the engine to come up to full operating temps and have no vacuum leak(s).
Click to expand...
All the parts are new in the carb... I'll try the needle idea
 
6

69ST

Well-Known Member
emso said:
All the parts are new in the carb... I'll try the needle idea
Click to expand...

I've never seen a carb rebuild kit that included a new emulsion tube (which doubles as the main jet holder) and, if that's not 100% clean you'll never get normal throttle response.
 
E

emso

Member
had both ct70 out today, my daughter drove the 1973 and I drove the 1977. The 77 has no power, bogs down. I played with air mixture, I changed the clip on the needle jet and nothing made a difference. I'll recheck emulsion tube and I think I'll also check the timing. No backfires but it just doesn't have the power like the 73.
 
6

69ST

Well-Known Member
The "air mixture" only controls the amount of air entering the pilot (idle) circuit and won't make any difference beyond ~1/8 throttle.

On the other hand...emulsion tube blockage is equal parts art & science. It's not only easily overlooked, it can be a real mofo to correct. I've had emulsion tubes that looked absolutely new and had been rodded-out...that ended-up needed 2,3...up to 6 additional passes to get every last orifice truly clean and without removing any metal. Those minuscule orifices are high-precision.
 
E

emso

Member
I just had a look at the tube under a magnifying lense and they look clean. Checked my point gap and I'm between .012 and .016. Removed my starter plate to check the timing with the key on the shaft and the "0" that's stamped on the cam gear. The key is up, toward the head, the "0" is also at the top. But at what stroke are they both supposed to be lined up ? I'm thinking I may have messed up. Are they lined up at top dead center ? Hope my ranting makes sense
 
cjpayne

cjpayne

Well-Known Member
The "T" on the flywheel should be on the mark at the top of the motor when the "O" is dead on the mark at the front of the head.
 
6

69ST

Well-Known Member
emso said:
I just had a look at the tube under a magnifying lense and they look clean.
Click to expand...
They may be clean...or not. Ask me how I know this.

I suspoze the acid test would be testing the carb from the other bike. If that eliminates the stumbling, the problem is your carb and the emulsion tube the most likely source.
 
E

emso

Member
Ok, I've cleaned the emulsion tube has recommended. I'm checking the timing and noticed that when I'm on "t" with tension on the flywheel, when you slowly turn it left or right you feel some tension, I think it's called valve overlap, the the key on the shaft is facing toward the head but the "o" on the cam gear is facing down. Counterclockwise till it's back with "t" line up on the notch, no tension left or right on the flywheel, the key on the shaft is up toward the head and the "o"" is facing up. Is this correct or did I time it incorrectly .
Thanks for all the advice
 
E

emso

Member
ok looking into it, looks like it doesn't matter if it's on valve overlap, just need to have the "t" on the notch and the "o" facing up. So I checked something else, did a compression test and it's only reading 120 psi!! It's a bran new cylinder with new piston and rings, the valves are also new and seated properly...or I thought! What should the compression read, 140-150? But would that make it bog down and have no power?
 
cjpayne

cjpayne

Well-Known Member
The T needs to be on the notch at the top of the motor(12 oclock). The O needs to be at the notch towards the front of the head(9 oclock).
 
6

69ST

Well-Known Member
If you're unsure of the cam timing, the crankshaft key, cam sprocket "o" and notch in the head should all align, when the piston is at TDC.

120psi is weak, especially for a fresh rebuild. How were the valve seats prepped?
 
E

emso

Member
I used valve lapping compound and then used a very bright light, shunned it in the ports turn the shop lights out and if no light could be seen around the valve then called it good....that's probably the issue, but when I did the compression test I didn't crack the throttle wide open...would that throw off my readings by a lot? Hopefully I'll have some time today or tomorrow to have another look, I'll do another compression test with the throttle open if that doesn't work then I'll lift the head and check the valves. Funny how I rebuilt the 1973 the same way and that bike runs great.
 
6

69ST

Well-Known Member
emso said:
I used valve lapping compound and then used a very bright light, shunned it in the ports turn the shop lights out and if no light could be seen around the valve then called it good....that's probably the issue, but when I did the compression test I didn't crack the throttle wide open...would that throw off my readings by a lot? Hopefully I'll have some time today or tomorrow to have another look, I'll do another compression test with the throttle open if that doesn't work then I'll lift the head and check the valves. Funny how I rebuilt the 1973 the same way and that bike runs great.
Click to expand...

Sometimes, light lapping is all that's needed...other times, you can "get by", for a while. One should expect to end up getting bitten. It is, literally, the luck of the draw.

This is what the valves & seats should look like, after a rebuild...a.k.a. cleaning, cut & lapping. They should be tested using brakleen; it'll find any leak, not matter how tiny.

DSC08352.JPG
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DSC04116.JPG
 
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