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Pulled carbs again, blew them out with compressed air, pilot jets clear #4 head pipe not getting hot, #3 better but not close to cylinder  1 and 2. temp at spark plugs are pretty close on all 4. Switched plug wire from 1 and 4 no change, switched out spark plugs no change, hot compression on #4 150, other 3 between 135-150. I guess this narrows it down to carburetor ? Float height same on all carbs and all getting fuel.  Ultra sonic next. Any ideas ?

You make a very confusing comment if I read correctly. You say the IR temps are very similar at the spark plugs, but you have extreme temp difference at the down (head) tube. This doesn’t seem possible. Is it possible you have a blockage in the down tube. Critter nest? I bought an once engine that had short 6-8 inch sections of down tubes affixed and mice had called up into the open tube and built nest in one of them.

Mikaw funny you said that it did cross my mind but I thought between the exhaust heat and pressure it would have blown it out. 

Not sure what else could cause such a temp differential between the plug and the down tube. 

Remove the bleeder tubes under the main jets and give them a clean. Compressed air is not a cleaning medium!Did you inspect and set the pilot air mixture screws to 1 1/2 - 1 3/4 turns out? A broken air mixture screws tip can made your day!Are the carbs mechanical/vaccum synced? 

 
 
 
 

Scirocco, all above mentioned has been done air screws are 1 1/2 turns out. Mechanically synched for now. Thanks for the suggestions. These are Z1b carbs, are pilot jets supposed to have o rings? none of these do.

These are air/fuel mixer screws were the tip was broken and stuck in the carb body.

 
 

Here this will help with item terms. There are no ORings on the pilot jet #21, non on the main jet emulsion tube/ jet holder #20, nor on the pilot air adjustment screw #23.

 

This is a picture into exhaust port of #4 cylinder 

Looks like it’s been running rich or burning some oil, but I don’t see anything “Abbynormal”. 

mvent wrote:

Pulled carbs again, blew them out with compressed air, pilot jets clear #4 head pipe not getting hot, #3 better but not close to cylinder  1 and 2. temp at spark plugs are pretty close on all 4. Switched plug wire from 1 and 4 no change, switched out spark plugs no change, hot compression on #4 150, other 3 between 135-150. I guess this narrows it down to carburetor ? Float height same on all carbs and all getting fuel.  Ultra sonic next. Any ideas ?

Have you tried switching the actual coils ?

Wookie I have not, switching wires between 1 and 4 then spark plugs without change my assumption was that would eliminate any electrical problems with coils, wires and spark plugs.