Tiny Stumble and Ignition Timing

[MisterCinders]

I am rooting out a small miss, stumble in my engine. All the maintenance is up to date (valves, clean carbs, synched, etc.). Battery is good, and coils are powered through a relay.

The stumble is very slight and only happens under load. The bike idles very nicely, no hanging or sagging. Under load and at low speed, the bike feels like it is barely missing stroke. It slips just a hair and feels similar to the kind of stumble one hits when it's time to kick over to reserve, but not nearly as severe.

I cleaned out the pilot jets again and adjusted the fuel and air screws, but there wasn't time to take the bike around the block to test those changes. While I am ferreting out this issue, I want to double check my ignition timing, as the hiccup might just be a slightly off-timed spark.

When I installed my Dyna, I followed the instructions and set the advance timing statically. I have an induction gun, but when I first tried it out (brand new), it sparked quite a bit at the battery connection. So I am focusing on the less explosive static light. If anyone has thoughts on the sparking gun issue, have at . . .

For whatever reason, Dyna says to use the advance marks for static timing instead of the forward marks. So I wire up the light to the 1/4 ignition wire, rotate and hold the advancer forward, and then turn the engine until the light comes on. The F | (full advance) should line up with the mark. I did all this, but here's the thing. Finding the edge of the lighting up and aligning the F | mark to hit just as the light flickers on is tough to do while holding the advancer against the spring tension and turning the engine. Has anyone tried timing the Dyna against the idle mark (| F) and adjusting until the light flickers out? I understand that is how you time the points ignition, and it would free up the hand used to hold the advancer in place.

Also, the advance/retard details bewilder me. Moving my Dyna modules counterclockwise advances the timing, and moving them clockwise retards the timing. Translating that into the F mark position makes my head hurt.

Say I am using the Dyna method and find the light coming on with the mark and F | misaligned. In terms of the F | mark, which direction is advancing versus retarding? In other words, if F | is to the left of the mark when the light hits, do I need to advance the timing or retard it? I think that means the ignition is too early, so I'd need to retard the timing so that F | meets the mark a bit later in the rotation. Is that correct?

TIA

[OldVet66]

At what RPM does this happen?

[MisterCinders]

At what RPM does this happen?

Hard to say. With all the carb work, I've been more focused on throttle position than RPMs. It's definitely at the lower end of the throttle range, might even be at the transition from pilot to needle (if it's a mixture issue).

I live in the city, so all my short test runs are very low speed and RPM. Probably nothing above 3,500 or so.

Whether its throttle (carbs) or RPM (timing), it's not a really narrow hiccup. In other words, the subtle miss appears sporadically as I pull away from a stop or accelerate. It doesn't seem to be a discrete "patch" of stumbling. That's what got me thinking about timing and ignition, since the symptom doesn't seem to correlate directly to throttle position.

Further description of the hiccup itself. When rolling out, it will sometimes lurch the tiniest bit, like a missed cylinder fire but more subtle. It doesn't really pop, cough or make noise when this happens, but you can definitely feel it as the rider.

Electrical came to mind because I had similar lurching once before when my ignition power connection frayed. The loose wire jiggled and caused the power to the Dyna to fluctuate. When that happened the bike would lurch and miss and sometimes die, but that was far more pronounced than what I find now. I did check those connections again to see if anything was loose or cracked. Nothing there seems out of place.

[OldVet66]

I had a mid range stumble with my vm's that was caused by being too rich. Raised the clips one notch to lean it out. That was trying to run to red line though. It didn't show up at moderate acceleration as long as I was under 6200RPM. Where are your fuel needles set?

[MisterCinders]

Jetting for pipes and pods is currently:

Fuel screws 1 turn out (was 3/4 turn when stumbling)
Air screws 2 turns
Pilot 17.5
Needle in 4th slot down
Mains 122.5

[OldVet66]

Are you using stock parts or a stage three jet kit?

[MisterCinders]

There is no Stage 3 kit for the 78 GS 750. I am using Mikuni parts, but have changed sizes from the stock settings. Specifically:

Pilot 17.5 (Stock is 15)
Needle in 4th slot down (Stock is either 2d or 3d slot)
Mains 122.5 (Stock is 100)

Before my latest carb tear down, the bike had no stumble with pipes and pods and roughly the same jetting (pilot 15 and 125 mains), but ran too rich, had poor mileage and dripped fuel from the bowl drains. The carbs were slightly dirty (I had done a full strip a year ago), and I think the floats were set a bit too low (making fuel level too high). Old float setting was around 22mm, I have them now at about 23.5mm to 24.5mm. I also did a complete strip and dip this time.

After a long bout of chasing my tail to address sticking needle valves and floats, I finally rid the carbs of overflow problems and no longer have any drips from the bowls or overflow tubes.

My petcock is working properly.

[OldVet66]

From what I am hearing, the stock needles don't have the right taper to transition smoothly and are a lot harder to dial in. Are you using a stock or electronic ignition? I ask because I replaced all the stock ignition components and it gave me fits until I scrapped it and went with a Dyna-S, Dyna green coils and Dyna 8mm suppression wires. I'm using the stock 15 pilots with my stage three jet kit. Me thinks you are a bit like me, the princess and the pea syndrome, I fiddled with mine for a couple of years before I decided I had them just right.

[MisterCinders]

Have Dyna. Tinkered with timing. Swapped back to stock pilots at 15. Fuel at one turn and air at two with some tuning for idle. Pilot is running nice. Small flat spot at about 1/4 throttle. Will see if I can smooth it out at pilot screws, but may need to shim needles.

[OldVet66]

I don't think timing at anything but stock settings will help. Set it and forget it with the Dyna-S per instructions for both pickups. The half space shimming needles might be the ticket. After you get it sorted, try the 1-4, 2-3 vacuum crossover mod. As "willie" says you'll be glad you did. It won't fix your flat spot, but it will add some low end grunt and smoothness.