Split choke pickups and erratic fueling with VM 26 carbs

[MadCapsule]

First, here's a pic of the suspected problem:

All four pickup tubes are like this

I've been beating my head against the wall trying to get the fueling right after installing a set of pistons and barrels from an 850 and a Vance & Hines 4-1 exhaust.

All of the adjustments I've made have produced results that are inconsistent and I suspect the cracked choke pickup tubes may be letting unmetered fuel into the air/fuel mixture. 

Plug chops show that the 1 and 4 cylinders are fouling the plugs and they are often wet, while 2 and 3 are lean. Adjustment of the air and fuel screws has little to no effect on this.

The carbs have been fully disassembled and soaked in carb dip, had all passages sprayed out with brake cleaner and reassembled with fresh o-rings.

I'm currently running the stock air box with K&N air filter, fresh air box boots and fresh intake o-rings.

The carburetors have 112.5 mains, pilot jets are stock and the needles are set with the clips in the center position. All parts are OEM, not from aftermarket rebuild kits. Float height is set at 25mm and the float needles all move freely.

The bike will usually run fine on initial start up, but the 1 & 4 cylinders stop firing as the plugs become fouled, with occasional surging when they happen to fire. I have confirmed that 1 and 4 are receiving good spark by placing a clean spark plug in the boot for 1 or 4 and holding it against the motor while pressing the start button.

So, could the split choke pickups be the cause of all this? If so, can they be removed and replaced? I can't seem to find them for sale, unless I'm using the wrong search terms.

Any thoughts or advice would be greatly appreciated.

[Gixer1460]

Thats a wierd one!! Can't even imagine how they got split? Don't think they are removable as not a service item! As a temporary check / fix how about sealing the cracks with epoxy then see if adjustment is easier / has more effect?

[jameskat]

Have you checked your ignition timing and wiring, just seems strange it is on 1 and 4 which is on the same coil and pickup.

[markfoggy]

I'd switch the coils, see if it moves to 2/3. Coil breaking down is easy to check. and I would be suspicious.

[MadCapsule]

9 hours ago, Gixer1460 said:

Thats a wierd one!! Can't even imagine how they got split? Don't think they are removable as not a service item! As a temporary check / fix how about sealing the cracks with epoxy then see if adjustment is easier / has more effect?

Yeah, it's quite the mystery. I'll look into some kind of fuel resistant sealant, I think JB Weld makes one. Brazing or soldering may be an option, but I'm not sure how well they would withstand the heat.

3 hours ago, jameskat said:

Have you checked your ignition timing and wiring, just seems strange it is on 1 and 4 which is on the same coil and pickup.

Ignition timing checked out fine with an inductive timing light. Inspection of the ignition wiring didn't reveal anything that was obviously damaged or disconnected, but a second look may be in order.

I suppose it could be an intermittent problem with the 1 & 4 coil or the electronic ignition (both are Dynatek pieces). If that's the case, I may just have to swear off Dynatek products and go back to stock coils and and points ignition; I've already had a Dyna coil and a Dyna ignition take a shit on me just out of warranty.

[MadCapsule]

20 minutes ago, markfoggy said:

I'd switch the coils, see if it moves to 2/3. Coil breaking down is easy to check. and I would be suspicious.

I'll try that as well.

[MadCapsule]

I switched the 2/3 coil to 1/4 and the problem remained on the 1&4 cylinders.

[arnout]

On 7-5-2017 at 0:45 PM, Gixer1460 said:

Thats a wierd one!! Can't even imagine how they got split? Don't think they are removable as not a service item! As a temporary check / fix how about sealing the cracks with epoxy then see if adjustment is easier / has more effect?

Yeah.. I'd start there.. So long the cracks are there you cannot rule out these being the culprit.. So find a way to seal the tubes or if that fails, a way to block or disable the whole choke circuit for now..

[johnr]

i wonder if theyve been stood outside with water in thm and its frozen?

[Gixer1460]

8 hours ago, johnr said:

i wonder if theyve been stood outside with water in thm and its frozen?

They'd be in the ice so no effect - if there was ice, the bowls would have split first.

[johnr]

if water got into them perhaps even by running down from above, it might explain it. its as good an explanation as any other complete guess based on speculation!! ice would only split the bowls if they were completely full of water, but weve all seen small ammounts of water in the bottom of carbs, and these do stick into thee bottom of the carb, so its not implausible to imagine water being pulled up from thee bottom of the carb via this jet.

 

[KATANAMANGLER]

Before you go too far down the carb rabbit hole. I would fo a compression test. I spend a few months 10 years ago chasing a fueling issue on a GSX 750 and eventually a compression test revealed low compression on the affected cylinders. If you rule that out, i would be tempted to buy another complete set of second hand carbs. At lesst you have the confidence at that point that you have ruled out spark and compresion.

Did you swap the feeds to the coils when you swaped them to 2 and 3 or did you just swap the coils?

[MadCapsule]

I have two full sets of used carbs from Eblag on the way. I got the pair of them for 80 USD, plus shipping.

Between those and some extra carb bodies my friend has, I should be able to put together a complete rack of good carburetors.

Edit: Compression test shows all cylinders are over 100 psi.

Edited May 13, 2017 by MadCapsule
Additional info.

[wraith]

The cracking will not help on the set up or running of the bike, is this on all 4 carbs or just some?

If all cylinders are above 100 psi it should run (not the best would be better 120/140 psi or more) anything below 100 is very low and the bike would not run very well. Are the values clearances OK?

Are the carb rubber inlets OK no cracking and sealed to the head ?

Did you use the gs850 head as well as the barrels or just the gs850 barrels as the 850 had CV carbs and a much bigger carb rubber and the slide carbs won't seal on the 850 rubbers.

[BigT]

I've been contemplating your situation and I have a few insights:

1. The cracked pick up tubes are odd. Ed or one of the guys on the GSR wrote a tutorial on replacing them a while back. Sorry, no link

2. The choke system should not be supplying any extra fuel into the carb once the choke plungers are pushed back down. The plungers block the holes so no fuel can come through. Therefore, perhaps your issue is that the #1 and #4 choke lifter arms are installed upside down? They are asymmetrical and would therefore not close off the hole in the choke system and extra fuel would be coming in, causing the plugs to foul?

3. If moving the air screw doesn't create any changes, there's probably an air leak on top of the other issues. How are the head to boot O rings and the carb boots themselves? Also, did you check the pilot fuel screw holes to see if they are reamed wide from someone tightening the screws too tight/snapping off the tip? Are the fuel screws set around 7/8 turn out?

4. I didn't see your build specs, but if you used the later big port head, what carb boots are you using? CV carbs or VM carbs