Quote:
Originally Posted by
KEITH KRAUSE 
I'm not sure if you're testing with the vacuum port at #3 capped already or if you're running with the vacuum line attached...
An orange spark at just the #3 carb could suggest a rich mixture or weak spark or a combo of both.
Be sure the plug lead is in good condition and no arcing from any cracks. Also, remove the plug cap and unscrew the connection and inspect the cap to lead connection for corrosion. Be sure the plug itself is good.
If all OK...
Test for a leaking petcock leaking fuel into the #3 carb via the vacuum line.
Remove the vacuum line and cap off the port at the carb tightly. Cap or safety the open vacuum port at the petcock, in case fuel DOES come out at all. Turn the petcock to PRIME. Be sure the plug is reasonably clean too.
Fuel should flow. After warm up, see if the #3 tunes like the others now.
If not, sounds like something's been missed inside the carb.
I can see a good spark from the colortune but I can't tell the exact strength of the charge. I will ohm out the coils and wires to see if I can find anything wrong there. When adjusting the fuel or pilot air mixture, I get no explosion (just spark) or I get an orange explosion (somewhat weak). Adjusting the slide screw when syncing the carbs, I was able to get a slight blue spark when the vacuum was reduced to minimum.
So right now I have a faint blue combustion with periodic orange (very weak compared to other cylinders. Any air or fuel adjustment I make does not improve the situation. It acts as if neither adjustment has an effect. Really perplexing.
OK. I have zero experience with the colortune tool but I gather a blue flame is good, and an orange flame is...rich?...or lean? I'm guessing rich.
My first post suggested checking the plug, plug cap, and lead for good/clean connections. Quick and easy to do. I'll assume that's been done. I'm also assuming compression is good. Without reading back, I think you said the valve clearances were good and no chance of intake leaks?
That leaves us with the carb.
Since you apparently serviced and adjusted the other 3 with good results, it seems you know what you're doing. Carb is clean and synched. Float level is good.
If turning either the side air screw or pilot fuel screw does little or nothing to change the colortunes color or the idle rpm, then something's wrong with the carb. Specifically, the pilot circuit, since you also say the tool burns blue when you open the throttle (running on the jet needle and/or main circuit.)
If you DECREASE the vacuum level to the carb, the tool begins to burn blue. Less vacuum means you're drawing less fuel through the pilot circuit, improving the situation.
So it appears the problem is the pilot circuit running rich. So rich, that basic screw adjustments have no effect, which makes sense if it's excessively rich and beyond what the screws can effect.
The VM pilot circuit is simple. If it's clean, very little can go wrong, beyond abuse. The first 2 things I think it could be that you may not have paid attention to is a partially clogged air jet and/or the wrong pilot jet.
The air jet (at the filter side/near bottom of the carb throat) is fairly small and if it's partially clogged, would cause your problem. Verify it's 100% clear and compare exiting carb cleaner/high pressure air to another carb. Many times you can hear a clog effecting the air escaping. Look and inspect for a damaged opening to the jet from a previous owner. If the opening is buggered up, you'll be rich.
The wrong size pilot jet would cause the same problem. I'm thinking you have a #15 jet there (does it say 15 ?) but is it possible it's been modified/drilled out by a PO? When compared to another jet, are you positive the holes are identical in size? I'm mentioning it because it does happen.
Is this pilot jet identical in all other ways? Same overall length? Same tiny holes/number of holes?
If all above is good, next thing I'd look for would be a poorly operating float needle valve. Usually, you'll see some fuel spillage out the bowl dump line but not always if it's small. A high fuel level will cause your problem. The float valve can be a pain in the ars to inspect sometimes, showing no real signs of wear. Any chance you have or can make a clear tube tool to verify correct float level? You could swap parts too and see if the problem follows.
Less likely, but still possible, a large piece of the air screw o-ring could have broken off and lodged in the passage? Also, a member here once said he saw a crack in the carb body.
Hope something here helps. Just so we know, did you inherit this problem? Did these carbs work well before or do you have no idea of their history?
I have sychronized carbs using analog sync gauges. Now I'm trying to fine tune carbs. Carb #3 doesn't respond to any adjustments or just slightly. I am using colortune to view the spark. Most of the time all you can see is the spark with no real "explosion". Occasionally I can get it to run orange but weak. I took the carbs apart and recleaned them. Didn't find any clogs but used carb cleaner in all orifixes. Put it all back together but #3 still not changing no matter what I do with air or fuel mixture screw. The other carbs are just fine. There is "new" intake boots on all ports, so I don't believe there is any air leak.
