Wot sputtering

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I have the jrc carbs on my '72 pretty well dialed in except for 3/4 to wot sputter. Main jet right? Plug chop shows lean if anything and when I go to progressively larger main jets it gets progressively worse. I'm down to my smallest main jet, a 35. And before I send off for smaller ones I figured I'd plumb the depths of our knowledge pool. The only thing I can think of is fuel starvation. Is this a common tuning problem? I'm running 1/4" line no fuel filters. Thanx in advance Rick.
 
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Go back to middle of the setting on the mains and DROP the needle. (Raise the clip).
I know this sounds backward and I am sure others may explain but, I run a TM flatside and had the same behaviors. Thinking lean i raised the needle and it got worse. Then lowered to improve. Put it to the top groove lowering all the way did the trick.
This may be a good place to check.
While your at it, it sure will not hurt to go to 5/16 fuel lines. Also run it with both petcocks open, if you haven't already, to check your suspect starvation.
 
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I had a similar problem with my 850 when I was running Amals. This is probably not the same thing but I've heard of a couple of people that did find the same thing going on their bikes, so it's worth mentioning. I had a skimmed head with around 10:1 CR and it was behaving just as you describe. It crapped out at WOT under max load. Somebody told me to try a new coil. It turned out that the coil I was running was being overcome by the high pressure in the cylinder. I'd never heard of this before but apparently if the coil is weak the spark can be quenched or shut down. I chased the jetting with the mains all up and down and nothing worked. Swapped out the coil, went back to stock jets and it ran great. The coil must have been on it's last legs.
 
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Thanks for the replies. I'm going to put on an old set of coils one at a time and see what happens. I've moved the needle around a bunch w/ different main jets and it improves but doesn't Eliminate the sputter. I'll follow up on the coil substitution.
 
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rgrigutis said:
I have the jrc carbs on my '72 pretty well dialed in except for 3/4 to wot sputter. Main jet right? Plug chop shows lean if anything and when I go to progressively larger main jets it gets progressively worse. I'm down to my smallest main jet, a 35. And before I send off for smaller ones I figured I'd plumb the depths of our knowledge pool. The only thing I can think of is fuel starvation. Is this a common tuning problem? I'm running 1/4" line no fuel filters. Thanx in advance Rick.

Not sure how you're approaching getting your carbs dialled in but the main jet is step 1, I think you might be a long way off "dialled".
What does that plug look like after the plug chop, what are you looking for?
 
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Well like I said the plug looks ok, lean if anything. Larger mains increase the sputter. Bike runs and idles fine. If I roll the throttle on gradually the sputter is delayed but still present. I'm going to check my valve clearances inthe morning, substitute the coils and go bacK to square one.

This is a '72. The seller told me it was a non combat. It had 30mm amals and no c stamp but the head is sitting really tight to the barrels, smaller gap between fin interface. I've adjusted the valve clearance like a noncombat but after I recheck I'll open the clearance to combat specs and see what's up.
 
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rgrigutis said:
Well like I said the plug looks ok, lean if anything. Larger mains increase the sputter. Bike runs and idles fine. If I roll the throttle on gradually the sputter is delayed but still present. I'm going to check my valve clearances inthe morning, substitute the coils and go bacK to square one.

This is a '72. The seller told me it was a non combat. It had 30mm amals and no c stamp but the head is sitting really tight to the barrels, smaller gap between fin interface. I've adjusted the valve clearance like a noncombat but after I recheck I'll open the clearance to combat specs and see what's up.

Where are you looking to tell the that plug is lean, what are you actually looking for?
 

L.A.B.

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rgrigutis said:
The seller told me it was a non combat. It had 30mm amals and no c stamp but the head is sitting really tight to the barrels, smaller gap between fin interface.

All Commandos tend to have a noticeably narrower gap between the fins at the head to barrel joint, so that's not always an entirely accurate means of identifying if a Commando is fitted with a Combat head.
 
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