Mikuni SBN fuel dribble

302much

302much

Location
Highland Village, TX
I have a 839cc big pin, ported, dry pipe, vforce Reeds, with dual sbn 46 carbs. I've been having a tough time with tunning the mid-range/transition. I noticed that it was loading up pretty bad so I rebuilt carbs with new needle and seats. On the trailer trying to tune low end I noticed that at idle I have fuel dribbling out of the high speed jet (from the Venturi above the throttle blade)
I have tried upping pop off to 30 psi (verified with gauge) and it made it a bit better (after trying a few low speed jets) but still there... Then up to around 50psi (heavy spring, 1.5 NS) still dribbles.. I was able to get it running better, but it's really hard to get it right with the uncontrolled fuel delivery.

I use two of the check valves that cover the high speed jet (not sure if that matters)

Right now I'm at 30psi pop off. 125 LSJ. 145 HSJ. And it runs best at about 1 turn out on LS screw. (But I doubt the screw setting matters much)

Any ideas on how to stop the dribbles (happens in both carbs, front may be marginally worse)

I was thinking about drilling the returns out and adjusting the "fuel pressure" down a bit to see if that helped. But I wanted to check and see if anyone had any ideas before I go HAM on it with a drill and ruin something...

Edit: compression was set up for 92 octane by engine builder (Dale's jet sport) and I'm running msd enhancer with advance plate and retard module..
The reason I'm thinking "fuel pressure" is because I think the additional CCs would have increased the pulse signal quite a bit.
 
302much

302much

Location
Highland Village, TX
Here is my kindney deal..
Also, from this cut away it seems that the low speed jet flows fuel as a result of the manifold pressure (low pressure beneath throttle blade when closed).. to me this means that the pressure above the throttle blade, acting on the high speed jet will be basically atmospheric pressure (when throttle is closed). This would be the pressure as what's acting on the diaphragm. So it follows that inorder to get fuel through the high speed jet at idle we have a couple options.
1) faulty check valve seal allows fuel in.
2) fuel pressure inside fuel chamber ( leaky needle and seat. Or maybe too high fuel pressure (pump side) at idle.
3) high vibration cause needle and seat or check valve to leak.

What do you think.. I could try three check valves? (Allowed per mikiuni in sbn tunning manul)
 

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Harbor

Harbor

Location
Stockton
CARBONX2 said:
You need to change them to the ones with offset hole, it must not be directly above check valve. Yamaha XL1200 uses them.
Click to expand...

Thanks, question though, the micro fish shows the 98 XL uses the same part number as the 38's in a super jet. The 99 and 2000 XL show a different part number that is more expensive, can you tell if this the one, part 41? The 2001 is a XLT and has a different number again.
 

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Harbor

Harbor

Location
Stockton
Harbor said:
Thanks, question though, the micro fish shows the 98 XL uses the same part number as the 38's in a super jet. The 99 and 2000 XL show a different part number that is more expensive, can you tell if this the one, part 41? The 2001 is a XLT and has a different number again.
Click to expand...

A pick i found of 66E-24412-00-00 matched your pick, ordered will have in a week.... thanks @CARBONX2

CARBONX2 said:
You need to change them to the ones with offset hole, it must not be directly above check valve. Yamaha XL1200 uses them.
Click to expand...
 
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