One carb sooty and one not

carl

carl

True Classic
I run dual 40 IDFs on my Fatrat. Very trouble free but idle was dipping and I thought I'd get the vacuum gauge out and check the mixtures to see if anything was off. Imagine my surprise when the top of one filter was black with soot and the other was the normal squeaky clean. Also the velocity stacks were sooty on the one carb and clean on the other.

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I have played with dual IDFs on my spiders, Xs, 128s for decades and never seen this. Vacuum gauge says they are all balanced and so does my ear. What could be causing this? I'll check the plugs tomorrow when the engine cools down.
 
Perhaps a backfire or other strange condition in one of those cylinders left some deposits. It will be interesting to see what the plugs look like.
 
IDF's use a fuel enrichment plunger for cold starts, it's not unusual for the plungers to leak / get sticky and dribble a little raw fuel in ... that would certainly cause some soot.

SteveC
 
Oddly enough, the four plugs look identical and do not indicate rich jetting. Steve, I'm aware of the leaking cold starts on IDFs and routinely block off the port at the base of the carb.

I assume all the carbon is caused by some kind of combustion at the top of the carb? I routinely get popping and mild backfiring on decel which I had thought was in the exhaust due to a disintegrated gasket at the header flange.

At this point, I'll clean things up put it back together and check the air filter top on a routine basis to see if it's gone away.
 
The only time I've ever had popping on deceleration was when the IDFs were not as balanced as I thought they were. That was my first thought when I saw the original post. But, you seemed to confirm yours are balanced.

I suggest that if the bypass screws are not used, the carbs are not balanced. I've never balanced dual carbs without using them, but I have seen advice on the internet to keep them closed.
 
Ernie, the advice on those air bleed screws are all over the place. Some folks say to keep them closed, some say a set amount open. I usually keep them closed but did notice #1 cylinder was lower than the other three so I cranked out the bleed screw on that one.

With all my decades of playing with IDFs I have never seen this sooting up.
 
A google suggests it could be exhaust gasses getting back into the inlet, popping and back fires through the carb would be an indication.
 
I guess that would be caused by improper valve shimming? Easy enough to check. I cleaned up the filter and velocity stacks and took the car for a ten mile drive and there was no soot. Maybe it was a one time event? Don't forget, an IDF is like two one barrel carbs and other than a shared float and cold start (blocked off) what happens in cylinder 3 should not impact cylinder 4. And yet both exhibited the same soot.

We love mysteries, just not in our own car.
 
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