VM carb circuits in relation to insulator depth

[Guest]

I've read the VM carb plug chop by TKent02 at Plug Chop Primer for VM carbs.

Does the depth of the insulator roughly correspond to the various carb circuits? (pilot, needle clip, main)

For example, the color on the deepest part of the insulator would tell you about the main jet while the tip of the insulator would tell you about the pilot circuit.

Please see the pic below, would this indicate the pilot and needle clip circuits are pretty good but the main is lean?

No real problems on the bike, just doing some fine tuning.

[TheCafeKid]

No. Not really. To perform proper plug chops, you'd want new plugs to read each circuit with. Mark your throttle with tape in roughly 1/4 turn increments, ride the bike with the throttle steady at each increment, hit the kill switch without changing throttle position, remove plug and read.

Pilot- 0-1/4 throttle
Needle 1/4 to 3/4 throttle
Main 3/4- WOT.
The insulator depth means nothing.

[KEITH KRAUSE]

I've never heard of using insulator depth as a method of determining what each jetting circuit is doing though I don't read as much as I used to.
Photos of plugs do help but I've found it's innaccurate to judge the color of a plug from a photo. You have a nice tan going on there. Tan is always good. How many miles on the plug matters too. Plug probably isn't done coloring yet and it will typically color up further down into the plug. I assume the other 3 plugs are similar.
You say the bike runs OK. Just curious. What model and mods do you have? What main jet, jet needle e-clip position and pilot jet have you selected? How far out are the pilot fuel screws (underneath) and have you tuned the side air screws for highest rpm? Are you using stock jet needles or a jet kit? Have you correctly adjusted the float levels and have you done a bench synch followed by a vacuum tool synch?

[btbarb]

Thanks guys, I just want to make sure it’s white down in the insulator because the ‘color doesn’t belong there’ vs. it’s white because it’s too lean.

• '78 750E – B8ES plugs with about 1000 miles on ‘em, all look the same.
• Valve clearances – checked about 400 miles ago, 2 of 8 were still in good range.
• Air box - stock, rubber is good, weather stripped and sealed well. Filter recently cleaned and oiled.
• Intake boots – new, used the Kaw boots with built in oring ridge and vacuum fitting, no leaks
• Exhaust – stock, replaced exhaust bolts at head, no leaks at collars
• Ignition – stock points, did coil relay mod, good voltage to coils, good blue spark at plug
• Carbs - VM26, carb id 45030
• Pilot – 15
• Mains – 102
• Jet Needle – 5DL36, set at second position down from the top
• Needle Jet – unknown, could not find marking
• Pilot fuel screws are about 1-1.25 out from lightly seated, all tips are intact
• Air screws – set with highest RPM method, about 1.5-1.75 turns out.

I just finished the full carb maintenance – dip, new o-rings, poked passages, etc.. Float levels set with clear tube method, fuel level is either spot-on or maybe 1mm low. Bench sync, no vacuum sync yet. It idles and starts fine. 0-1/3 throttle is pretty good, 1/3-2/3 is really good, near WOT doesn’t stumble, it just sounds and feels a little lean. Ater I get the tool and do the carb sync, I”ll get the real picture with some legit plug chops.

[KEITH KRAUSE]

OK Bryan. Sounds good. For some reason I thought you made intake/exhaust mods but since it's stock those jetting stat's you gave should work. I believe your model came with 102.5 mains?? And your jet needle e-clip position I assume is factory?? If you're SURE then the bike should run fine. And I know the factory pilot jets are #15. So if the carbs are operating as they should in all areas then you should be good. The vacuum tool synch will improve things because most likely the levels aren't as close as they should be. Do that first and re-test. If you still think it "feels and sounds lean", then maybe double check things. Fuel level in the bowls needs to be within a particular range. For example my '79 1000 levels must be between .90 and .98". If I were to adjust them I would set them at .95". Since there's always a chance for human error, setting them right in the middle of the range allows a margin of error. You say your levels COULD be off by 1mm. That's .04". So if you did set them near the "lean side" of their range that possible extra .04" could put you beyond the acceptable range.
Of course, if you are testing in colder conditions, that could contribute to a lean feel too. Put some more miles on the bike after the synch and we'll see what you've got. If you actually can feel surging then you know somethings not right.
By the way, be sure yiou have the correct size fuel line and be sure those two float bowl vent lines are clear/not kinked and routed as per factory (under the seat I believe). Also, if keeping the bike long term, do yourself a favor and try to pick up a Dyna S or other quality pointless ignition. Overall performance will be better, especially at high speeds/rpms and future tuning/maint' is a snap. Points lose their adjustment, get dirty and condensors get weak. One of the best bang for your buck mods you'll ever make.

[btbarb]

According to the carb database here and the pdf service manual from bassCliff's site, the stock 750 main should be 100 but the typical replacement seems to be 102. My jet needle matches stock GS1000 not 750, I'm guessing the PO got hold of a GS1000 carb set and left the jet needle and needle jet alone and just changed the mains to 102 because the stock GS1000 main was 95 (yes, smaller). The PO then leaned it out a bit by setting the needle clip at position 2 instead of the stock 3 position. I know one of the POs worked their magic on the carbs, orings were improvised/hacked and 2 of the 4 fuel pilot screws had to be replaced, I have a set of carbs that are mostly broke but they did have good fuel screws I was able to salvage.

I scrounged one vacuum gauge and measured each carb, 2 & 3 were low-30s Hg and 1 &4 were low-20s Hg - I adjusted 1&4 to 25 Hg and that evened all four out to within 1-3 Hg of each other. This change made enough of a difference in how the high-mid to top end performed to tell me when I get a real sync tool and check (again!) the float levels, it will be worth it. I didn't get to chop or ride long enough yet to check the plugs.

My fuel line isn't perfect, it's real fuel line but I'm using 5/16 ID, I think OEM should be 7mm ID. I am taking a long term view with the bike and will be reading/taking advice on ignition and suspension, probably ignition first. Thanks for the input.

[Steve]

Valve clearances – checked about 400 miles ago, 2 of 8 were still in good range.

Which way were they off, by how much, and why didn't you adjust them?

I scrounged one vacuum gauge and measured each carb, 2 & 3 were low-30s Hg and 1 &4 were low-20s Hg - I adjusted 1&4 to 25 Hg and that evened all four out to within 1-3 Hg of each other. This change made enough of a difference in how the high-mid to top end performed to tell me when I get a real sync tool and check (again!) the float levels, it will be worth it. .

Hopefully you realize that this is not a very good way to do this.

.

[btbarb]

Which way were they off, by how much, and why didn't you adjust them?



Hopefully you realize that this is not a very good way to do this.

.

#3 intake was #4 exhaust were both somewhere between .06-.07, the next thinnest shim took me out of range, so I left those two as is.

Yep, I know the one gauge method isn't the way to go, I just wanted to probe a bit and confirm, I gotta wait a month or so before I can get the real tool and wanted to do something in the meantime. The set isn't properly sync'd yet, but it's better than it was.