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Have 240 ,crane ign. k+n,supertrapp,and 3 older
40 dcoe 2's (3progression holes not 4) with short stacks.
No matter what i do i can't seem to get rid of the lean spot! I've tried emulsion tubes, venturi's, jetted up and down and round and round!
The engine is basic but fresh. the lean spot that sticks between 1500-3000 RPM depending on load, jetting, ... causing backfire through one or all carbs. PLEASE ADVISE
 

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Re: a very long reply...

> Have 240 ,crane ign. k+n,supertrapp,and 3
> older
> 40 dcoe 2's (3progression holes not 4) with
> short stacks.
> No matter what i do i can't seem to get rid
> of the lean spot! I've tried emulsion tubes,
> venturi's, jetted up and down and round and
> round!
> The engine is basic but fresh. the lean spot
> that sticks between 1500-3000 RPM depending
> on load, jetting, ... causing backfire
> through one or all carbs. PLEASE ADVISE

Here's a list of things to check.
Check the fuel pressure. (use good elect. fuel pump and fuel pressure regulator.)
Check the float level.
Check the carb. sync. (Get a manometer to do this if you want a good sync job)> Can be bought at any motorcycle shop. They use them for multi carb cycles and Mikuni recomends this tool only for syncing their carbs.
Check the Accel. pump operation to make sure it's getting the proper amount. These carbs like to backfire when they don't get enough gas.
Check the pump jets for dirt/blockage Only use air pressure to clear them, don't run a wire through them or they will be damaged.
Check the Starter jet for same.

Also you may want to take the carbs off the manifold and check to see if the throttle shafts have been bent, it's not fun but they do bend easy, I had this on a set of Mikuni's that gave quite a headach, you can also find this with a unisync tool (this is all this tool is good for in my opnion), check the flow from each stack and compare the flow, if you get more from say the left side (say cyl #1) of one of the DCOE's than the right (cyl #2) it may be bent and you'll never get them synced. Check all 3 carbs this way (compare cyl 1,2 then cyl 3,4 then cyl 5,6)
This is caused by bad carb throttle linkage adjustment, you only want to go full throttle without putting anymore stress on the carb and too much can bend the shaft like slamming the gas pedal to the floor when they would be at full throttle 3/4 of the way down and putting more stress on the carbs then they should see.
I've had tripples stumble a bit on the lower end because the pilot jet. You should also take a look at the idle jets. It might be caused by a few things instead of just one so you may have to make sure everything is well cleaned, I don't know how old this carbs are or their history so I'd start with the float bowl area and make sure all jets are cleaned.
One last thing is to check the throttle shaft that actuates all the carbs, it may be worn and as you give it throttle the sync goes out. I hope you have a book on weber carbs, if not get one.
But if you can start with the easy things first like fuel pressure and float level and go from there. I may have missed something but I stick to Mikuni's so good luck and hope you find the problem.
Also make sure you have a top quality fuel filter, not one of those cheap ones, one that can go into the low micron level, they cost but these carbs will give a lot of trouble free life if you invest in this area. These carbs are great but have to be treated better than most standard carbs.
If the problem you described happened with my Mikuni's I would be looking at the pilot jets, they say from 0-20% throttle if the car stumbles/backfires check/clean pilot jets, if clean then go to the next larger size and so on. But also in their list is other stuff I mentioned above, I hope this ton of typing helps.... sorry its soooo long but I wanted to make sure I covered everything I could think of to help....
 
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