Timing Q of the day

[plateaucal]

That dashpot, if I recall correctly, would be used to dampen the throttle upon closing, to prevent backfire.

Also, maybe it missed it but did you adjust your mixture and how so?

Also, did you try plugging the vacuum port on the carb to see how it runs.

I assume the accelerator pump is adjustable but can't remember. That would be the last thing to check after mixture is verified.

[Slackd]

Thanks to everyone on this post, backfire issue resolved. Not one, but several intake/exhaust manifold bolts were loose. Sometimes it’s the simple things that make the a world of difference.

Dave

[desertrat]

...Patrol runs ok at speed but lots of backfire when I let off throttle, and will sometimes stumble and cough when accelerating from low idle.

Any ideas? I’ve played with timing but cannot seem to eliminate the backfire.

Thanks, as always.
Dave

Backfire when you let off the throttle can be a sign of leaks on the exhaust pipe, and nothing related to timing. With the engine running put you hand, carefully, around the exhaust pipe at the flange with the manifold to feel if there's some exhaust leak. Also down the pipe at the joints.

Stumbling when accelerating from low idle could be cause by some slack in the linkage of the accelerator pump for the carb. I'm not familiar with the Rochester at all, but on the Hitachi carbs that was the case. The throttle opens a bit, but no injection of fluid is coming from the accelerator pump or piston, and you have that stumbling.

Correct and yes. Sometimes on the Rochester it's the accelerator pump cup that drags in the bore.

Thanks to everyone on this post, backfire issue resolved. Not one, but several intake/exhaust manifold bolts were loose. Sometimes it’s the simple things that make the a world of difference.
/quote]

For future reference there are two kinds of backfire. Exhaust backfires and carburetor backfire. The exhaust air leak causes popping in the pipe. Carburetor backfires are a bit more complex and can be caused by a range of issues and can be very destructive to the carburetor and air cleaner and can result in a carb fire, collapsed floats, etc.