Carb tuning

[Addison]

hi all!!

I am having the hardest time tuning my carb on a 1974 ct70. I cannot get it to idle right, and it back fires every so often.

Are there any tricks to tuning the idle screw and air mix screw? It seems when I turn the idle screw, the idle RPM is not changing and it keeps bogging out.

 

[69ST]

First off, check your point gap. Occasional backfiring seems more consistent with too small of a point gap...which causes ignition malfunctioning. Next up, gotta be sure that there are no vacuum leaks (intake gaskets at both ends should be checked, along with the carb-to-spacer/insulator O-ring), the air filter element is clean & in good shape and the inlet side airbox boot is not crushed. After that, it's onto the pilot jet, main jet, emulsion tube orifices all of which can only be properly cleaned by using a thin wire. And, lastly, verify float level.

 

[Addison]

Ill have to double check all of those things again. It ran great about a month ago (last time i ran it) after i rebuilt the carb without needed to tune the carb at all. not sure what changed.

Would the points and backfire issue have anything to do with idle?

 

[69ST]

Oh yeah...
When the points close-up, ignition parameters (timing, coil saturation time/dwell, etc) go all to hell and the engine becomes temperamental for no apparent reason. Fouled points, a dying condenser or carb/induction issues can cause this, as well. Checking point gap and even cleaning the points, is quick & easy. If that's the issue, you'll have saved yourself a lot of frustration. If it's not, you won't have "wasted" more than a few minutes.

FWIW, point cams can flash rust and wear down the breaker point rubbing block. I've seen points close up after the lock screw (the one that gets loosened to tweak the gap) got loose, for no apparent reason...while riding. When in doubt, it usually pays to start with the easy stuff.

 

[Addison]

OK sounds great! Good thing i purchased the flywheel puller lol. Ill give it a shot and see what happens.

Just seems weird that a month ago the bike ran great... and hasnt moved since.

 

[69ST]

A loose adjusting screw, a minuscule amount of rust, or oil, on the point contact...or something else really minor...is all it takes, sometimes. Can all but guarantee a simple problem with a simple fix. It's always the diagnosis that raises blood pressure and hairlines...

 

[Addison]

i realized that part of the struggle to stay idle was due to someone repairing the flywheel cover; in turn the repair caused an obstruction on the flywheel which was grinding each time the flywheel spun. see the photos for a more clear understanding

well it appears the problem is worse than just the cover obstruction. It back fires almost constantly and struggles to stay running.

Im going to try and sand the points a bit, and than replace the points and condenser if that doesnt help. I just rebuilt the carb and it ran great right after so i am waiting until the end to break down the carb again and check the jets and emulsion tube.

I realized last night that the intake tube from the cylinder head, one of the bolts doesnt want to tighten... almost feels like the head portion may be stripped a bit. However, when i remove the intake tube and tighten just the bolt itself into the cylinder head, it tightens fine; with the intake tube in place, when it hits the point of tightening on the cylinder head, i can feel it loosen and not tighten anymore (like a stripped feeling). Im wondering if a slightly longer bolt there would fix this issue or if a helicoil is required...

ugh im starting to lose steam on this project.

I so baffled over it running great a month ago, not moving, now its mysteriously runs like garbage.

I am also considering replacing the brand new spark plug again to see if that makes any difference.

Any suggestions would be greatly appreciated!

 

[69ST]

Take care of that loose bolt, before changing anything else. With a huge vacuum leak, right at the head, this thing's never going to run right; this may turn out to be THE problem. Should be enough healthy threads, deeper inside the hole, to allow use of a longer bolt. Intake bolt holes can be helicoiled without disassembling the engine...if necessary. It's one of the easier repairs.

 

[Addison]

ok. ordered the OEM gasket, i was using gasket maker to make my own. Ill also try a larger bolt; 25mm/30mm when the gasket comes in. I could not feel a leak by hand when the bike was running, so hopefully the bolt not tightening is causing a vacuum effect and that is the root to all this lol

 

[69ST]

I've seen that happen, firsthand.

 

[Addison]

...there is hope lol