Cracking up and Popping

[9842]

Hi
Got a nice 73 LWB Green R75/5 toaster. Has 36k on it and was tuned up last spring and ran well when put away last fall. I completely drained gas and removed float bowls for the winter snooze. I recently filled it with 98 oct race fuel and went for my first ride since the winter snooze. Started fine, would pull strong, but was cracking up and intermittent when cruising along at 3500 rpm, and will backfire and pop a little when rolling off throttle. At one point it was stumbling and stalled out. As I was still rolling along I pulled out key and reinserted it and popped the clutch and bump started ok but had to limp home.
I have had a little trouble with the switch before so Im thinking its the key switch. Is there a way to clean the contacts on the switch. I repeat I want to clean the switch for now not replace or go deeper for now. Any info on cleaning for better contact is appreciated.
Thanks
Brian

 

[collingsbob]

I think you have an air leak, possibly at the exhaust pipe/head joint..or somewhere in one of the intake manifolds. Intake manifold leaks can be detected by spraying wd-40 and listening for a decrease in idle speed. Hardened rubber manifolds can be softened by soaking them in oil of wintergreen.

 

[kbasa]

I think you have an air leak, possibly at the exhaust pipe/head joint..or somewhere in one of the intake manifolds. Intake manifold leaks can be detected by spraying wd-40 and listening for a decrease in idle speed. Hardened rubber manifolds can be softened by soaking them in oil of wintergreen.

Yep. When you roll off, any opening in the exhaust side will start pulling air in and giving you that crackling/popping sound. I'd check the exhaust nuts and if I was feeling ambitious, I'd just order the little sealing bits and replace them.

But that shouldn't make it stall or run poorly (presumably breaking up = misfire). I think Bob's on point that you need to take a look at the intake and see if you have a leak there. The WD40 test has always served me well.

Good luck @9842!

 

[69zeff65]

It may not like the 98 octane race fuel! You might have a detonation problem that can break your motor so I would suggest cleaning out your tank and carbs and run some 89-91 octane NON ETHANOL gas for a while and see if that cleans things up. (8 stuff wasn't common in the early '70's so you shouldn't try it out in a nice '73 toaster. Just my thoughts!

 

[kbasa]

The higher the octane rating, the less likely gasoline is to detonate, so I think it's unlikely that 98 octane is going to lead to a detonation problem.

If your bike isn't pinging or detonating on cheap fuel, you get no advantage with higher octane.

 

[stevenrankin]

A lean condition will cause popping and sputtering at throttle off or on down hill runs with the throttle closed. As has been suggested, check the carb to head interface for a vacuum leak, then the rest of the intake system.

What color are your plugs after a good ride? Very light or white would be a lean condition. And again, it may be caused by a leak. St.