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

1147 Views 9 Replies 4 Participants Last post by  AZWinger
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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:
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fatalattraction wrote:
Harry, isn't 480 degrees damned hot? I don't know what they are supposed to run but that sure seems hot to me. Oh well, one of the smart guys will be along to help. Wish I knew more and someday I will need to. Thankfully I don't have any problems. Good luck.
Mike, remember this is exhuast gas temp (readings taken from the exhuast pipe via laser temp gage) this readings are always high from the combustion process (remember race car headers glow when ridden hard 1000 plus deg's).

Im just worried over the extreme differance in temps from the side that doesnt run as good as the other.....................

I know that differance's in timing (changing the combustion) can cause temp variations in cylinders but for both cylinders on the left bank alone to be that far off it almost has to be a common fault cause.

Ive ran this over in my head several times, it cannot be ignition related per-say because each coil feeds one left/right cylinder as does each set of points.

The belts have been triple checked by myself and another person.


So I left with a fuel related problem, however everything ive read so far on air cut-off valves seems as the odds are low but plausible. However, I just read "Randakks, aka Randall Washington'snotes on the air cut-off valve issue's" and do have the problem of having to use lots of choke with poor throttle response until warm. Guess maybe that may just be the problem.....................

I guess sometimes you just have to write poop down in order to figure it all out!:jumper:

Then again maybe you guys know something different?:baffled:
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Thats what I thought too Paul, but both carbs?, Unless its the porting from the cut-off valve because it does port left and right before branching out to each carb??

Maybe just one of the 2 o-rings under the valve are leaking causing this problem?

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Well, I did alot of thinking today. Discussed with several motor head friends of mine as well.

Came home tonight and checked my compression on all cylinders, everything looked good with 150PSI cold +/-5 then checked it hot later found 165-170 on all cylinders.

Looked really good, so I ruled that one out.

Now keep in mind Ive had the carbs off and on 5 times and each and every time I always re-assembled everything as found. SO TODAY Im looking at the air cut-off valve, took it apart again and Im trying to figure out how it really works inside (cause the tech manual pictures can be confusing at times. Im blowing thru all the ports and looking at the diaphragm and thinking how this thing cant possible work and them

BLAM it hits me, the last owner (or who ever had this thing apart) had put the diaphragm in upside freaking down......................................

Go figure, re-installed correctly and had to completely re-tune the carbs again cause now there way the heck out of wack.

Now she's running like a champ, doesnt require choke anymore to start cool/cold and idles great with no crazy changes in the inches Merc gage!

Ive told myself , lord knows how many times "never trust somebody else's work" and still forget it once and a while..............

Thanks for reading and letting me vent till I figure it out! :grinner:
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