Slight mis-firing noted at cruising speed left bank steady throttle?

imported post

Okay guys, this is Kinda long but heres the run-down on what Ive completed in the last three months:

Overhauled carbs / complete tune-up / new timing belts / new left head gasket

When I first got her she had sat for a long time and didnt run. did the carbs/tune-up got her running very good. Took one trip and she started fouling the #4 plug then blew the left head gasket. So I replaced the head gasket and timing belts while I was in there (timing has been triple checked).

So after all that she runs good but has a slight mis/backfire coming from the left pipes at steady throttle speeds above 2000 rpms. So I reset the carbs again today and noticed that if I dialed them in at idle, they were off buy 3-5" Merc on the gage set at 2000 rpms . If I dial them in at 2000 (vice at idle)rpms then check idle there off that way also (but off from 2-3INHG).

While checking the exhuast for rich/lean smells I noticed a big differance in exit temps on the exhuast eflt to right.

Checked it with a thermometer and found right bank running around 480degf and left bank running around 280-300 degf.

Could this be the in-famous air cut-off valve giving me grief? Oh yeah its a 75 1000..


It runs very nice, crisp throttle response (slight smoke from left bank on hard throttle) and idles smooth enough to pass the old dealership nickel test (stand a US nickel .05 coin on its side on the fuel pump and it wont fall over) and is getting around 35 MPG.

Any idea's would be helpful, I'll be running a full compression test tomorrow night.
Thanks for reading:baffled: