3 out of 4 sooty plugs

Looking for ideas here...

If you have a 1000 with good spark, clean and rebuilt VM28 pumper carbs that are all setup exactly the same with working pumps, your valves are in spec and plugs 1,2 & 4 are sooty but #3 is just perfect... what would you think is causing this to happen, where would you look first???

First thing I would do is make sure the fuel levels in the bowls are set right (adjust the float tangs).

Do you have good, smooth power from low speed to full throttle? Good throttle response?

On mine, the pilots run the show from idle up to maybe 2500. Needle position strongly affects throttle response in the 2000 - 3500 range. mains run the engine from about 4k up at large throttle openings.

Just for info, I might try this:

1) Clean the plugs

2) take your spark plug wrench and do about 10 miles down the freeway as fast as possible and check the plug color. Is it sooty, white, tan?

3) Clean the plugs and try low speed city driving and keep the revs down and the throttle closed as much as possible. Check plug color.

Do you get sooty plugs both times? The pilots will be doing most of the work in low speed, more the mains and needles on the freeway.

The "one carb good" is really puzzling. I wonder if it's possible the jets are set up rich and one carb has a vacuum leak leaning it out a little?

The only other guess might be the fuel level in the bowl is low and that would make it run a little leaner?

bountyhunter wrote:

First thing I would do is make sure the fuel levels in the bowls are set right (adjust the float tangs).

I have as best you can with the clear tube method.

Do you have good, smooth power from low speed to full throttle? Good throttle response?

I did with the 20 pilots but things are not going well with the 17.5's. Both sets though resulted in sooty plugs.

On mine, the pilots run the show from idle up to maybe 2500. Needle position strongly affects throttle response in the 2000 - 3500 range. mains run the engine from about 4k up at large throttle openings.

Should be the same for just about all VM's I'd guess.

The "one carb good" is really puzzling. I wonder if it's possible the jets are set up rich and one carb has a vacuum leak leaning it out a little?

I guess that is possible. I do find it odd that hte reference carb is the one carb that is running right.

The only other guess might be the fuel level in the bowl is low and that would make it run a little leaner?

They are all supposed to be the same height but I guess it could be splitting hairs as to whether they are all the exact same height or just a tiny bit higher than this one. I would have though it would take more than what I saw as differences based on the FSM height range shown.

Before you make yourself crazy doing all kinds of carb adjustments, take about 5 minutes and check your voltage at the coils. If they read low do the Wired George coil mod and get them back up to 12V where they should be.

Gringo - I made a relay coil mod already and even had a question as to the running voltage at the coils with an igniter answered that pretty well confirmed that th electricals are fine.

Here is the link to the electrical post and it will give you more info on the rest of the engine as well:
kzrider.com/index.php?option=com_kunena&...d=4&id=203799#203799

I'd still dot he "freeway test" and cruise it for a few miles at high RPM wide throttle and check the plug color. If they look good, the mains are probably OK.

I can say this: when I was dialing in mine to the new headers, I was sooting plugs because my pilots were too rich. I was trying to get better low speed throttle, but it turns out that riching it up at the needles was the best thing and then I went to leaner pilots and now it runs perfect and the plugs are clean.

Don't know if your pilots are too rich. Do the low speed run and see if the plugs blacken up on you. If they soot up at low speed and run right at high speed, the pilots probably are too rich.

The tuning procedure I have seen is always to get the right main jet first (by running high RPM, wide throttle opening).

Then do the needles, pilot jets last.

YMMV

Right now it won't run well enough to even idle for long and won't go above anything around 4500 and starts to bog down and die unless you let off of the throttle. Acts like it did when I had a vacuum leak but can't find one now.

I'll pull them back off again and crack the rack this time.
I'll drop in 15 pilots and drop the 117.5's mains down to 115's plus lower the floats a little again. I will go over every orifice, o-ring and gasket.

When done if it does this crap again, these 28's are going to the shelf and I'll throw on a set of 26's I have until I can find some newer flatslides.

My bad I hadn't read the other post. From reading this thread only, the coil mod was the first thing that popped into my mind. Carry on :blush: .