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Discussion Starter · #1 · (Edited)
I have iridium spark plugs and my aftermarket ignition controller in. On a whim, I decided to try a larger plug gap. Based on the graph found at the link below, a 1.4mm gap can fire a 17:1 AFR. I didn't use a standard gapping tool, as those damage the iridium tip.

I bumped the rear plugs up to that gap (they were at 0.9mm) to see what happens. I only did one cylinder because I want to compare results. With true dual pipes, I should be able to hear misfire when one side gets quieter.

 

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Discussion Starter · #2 ·
I set the rear plugs to 0.045" (1.15mm) and kept the front at 0.035" (0.9mm). I found during decel, the front cylinder pops more than the rear. It took me awhile to get my head around this, but the pops during decel are because the mixture is too lean to burn in the cylinder, causing unburned fuel to collect and burn in the exhaust. Since the rear cylinder is still managing to ignite in the cylinder some of the time during decel, there's less fuel to ignite in the exhaust.

I'm also having a lean misfire coming off idle. I actually had it die while pulling out into traffic! I can reliably recreate it by trying to accelerate in 4th gear with RPM at 1500. (Not recommended for lugging reasons!). I noticed each time that it was the front cylinder that was misfiring.

In both cases, it seems like the wider gap is helping with lean burns. I don't know if copper plugs and/or the stock ignition system can handle gaps wider than the recommended 0.035" (0.9mm), but it seems to help with Iridium plugs and my aftermarket ignition controller on stock coils. It is band-aiding a lean condition that I need to solve, so I'm going to leave the front gap alone until I'm certain the lean condition is gone.
 

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Discussion Starter · #3 ·
The misfire was due to a configuration error in my ignition controller. I am now running 0.045" gap on all 4 spark plugs.
 

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our Vulcan 750 seems like its running on 2 cylinders. Does anyone know the correct spark plug gap? Or is the front and back different gaps as someone previously stated
 

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We have a Vulcan 750, I’m most likely positive it has 4 cylinders, it has 4 spark plugs so?
Four valves per cylinder, two spark plugs per cylinder, it's a V-twin. Two cylinder. Oh, four camshafts too.
 

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If you're degoated without an H pipe, then the rear pipe is probably shorter than the front. I run 145 main in the rear carb and 142 in the front to balance the difference in pipes - HD sporty drilled mufflers with filter pods. She's a mover.

Another trick is to trim the spark plug ground strap so you can 'side gap' the plugs. Boom. Difference you can feel.
 

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If you're degoated without an H pipe, then the rear pipe is probably shorter than the front. I run 145 main in the rear carb and 142 in the front to balance the difference in pipes - HD sporty drilled mufflers with filter pods. She's a mover.

Another trick is to trim the spark plug ground strap so you can 'side gap' the plugs. Boom. Difference you can feel.
What do you mean by “ground strap”?
 

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Hey, When i started our 750 i felt gas spray on my leg, i was wondering where it was coming from, turns out it was coming through where the air gets sucked in from. How do I fix this? Anyone have a fix?
 

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What do you mean by “ground strap”?
Hey, When i started our 750 i felt gas spray on my leg, i was wondering where it was coming from, turns out it was coming through where the air gets sucked in from. How do I fix this? Anyone have a fix?
Don't run it without air filters, it might be normal "reversion" on the carbs. If it's not reversion, it could have leaky intake valves, depends how much gas is actually there. Some reversion is normal without an air filter on motorcycle carbs.
 

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Alright but the bike is really hard to start now i do not know why, some times the cylinders are flooded with gas, sometimes it does not crank because its flodded, and if it cranks it will just crank until you pull back on the accel a little. Before it never did any of this. I would open the choke turn the bike on and it would run until i turned it off. Now i basically have to force it to run and if i let go off the accel it will die as soon as the rpms go down. I have new spark plugs on order its NGK DPR7EA-99, i cleaned up the spark plug but this bike is funny, it wants to run when its in the mood. I need it to run all the time. There is more than enough gas in the tank. I searched it up on youtube saying its the float in the carb which is messed up? Is this correct, anyways what do i have to do to fix this issue? Thank you guy!
 

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Alright but the bike is really hard to start now i do not know why, some times the cylinders are flooded with gas, sometimes it does not crank because its flodded, and if it cranks it will just crank until you pull back on the accel a little. Before it never did any of this. I would open the choke turn the bike on and it would run until i turned it off. Now i basically have to force it to run and if i let go off the accel it will die as soon as the rpms go down. I have new spark plugs on order its NGK DPR7EA-99, i cleaned up the spark plug but this bike is funny, it wants to run when its in the mood. I need it to run all the time. There is more than enough gas in the tank. I searched it up on youtube saying its the float in the carb which is messed up? Is this correct, anyways what do i have to do to fix this issue? Thank you guy!
It sounds like a carb is flooding, sticking float or leaking float valve.

Should probably start a new thread on the subject before we get too deep off topic in this thread.

Maybe a rebuild kit for the carbs.
 

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What do you mean by “ground strap”?
I was waiting for dirtrack650 to answer, but I've got nothing to do.... And he's probably doing a weekend ride.

It's the ground electrode that arcs from the threads over to above the center electrode. The part you tweak to get the plug gap.

Side gapping places the end of the ground electrode beside the center electrode instead of directly over the tip. The idea is better spark and flame propagation due to the original setup shrouding some of the center electrode.


Some racing plugs have no ground electrode at all, and some have side gapping already done, along with multiple ground electrodes.

Google won't me have a link... Just search.. side gap spark plugs .. on Google Image and you'll see many examples.

Now if you want to get really crazy, go through and index each spark plug. That's where you get the ground electrode to face a certain direction, the tip pointing toward the center of the cylinder, or pointing toward the intake valve.

You draw a line up the side of the plug, in line with the ground electrode. Then you thread the plug into the head and see where the line ends up. If it's no bueno, you try another plug until you get what you want, or you can buy indexing washers so the line lands where you want it.

I can't wait to see if transient plasma ignition is as good as they say. Could be a game changer for gasoline engines, huge advantage for power and fuel efficiency, so they are claiming. No more spark plugs with plasma. 20% increase in fuel efficiency?
 

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Excellent reply, thanks! I've indexed in a past life 😆, don't know if it's worth the effort on a head with two plugs and four valves. I knew a guy that swore by indexing on a 2-stroke 🤣.

Riding? You bet! Did 250 yesterday out to Westport for Rusty Scupper Pirate Dayz on my Wing. Bike show today on the VN, Morton Sunday on my ADV GL500. The Chevron up there (on the way to White Pass) is a favorite stop for all types of bikers: baggers, ADV, Rocket III's and there's a guy that shows up on a massive R18.

Then there's The Skate...

Ride safe boys
 

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Excellent reply, thanks! I've indexed in a past life 😆, don't know if it's worth the effort on a head with two plugs and four valves. I knew a guy that swore by indexing on a 2-stroke 🤣.

Riding? You bet! Did 250 yesterday out to Westport for Rusty Scupper Pirate Dayz on my Wing. Bike show today on the VN, Morton Sunday on my ADV GL500. The Chevron up there (on the way to White Pass) is a favorite stop for all types of bikers: baggers, ADV, Rocket III's and there's a guy that shows up on a massive R18.

Then there's The Skate...

Ride safe boys
Thanks. I never got to that point either, I was happy keeping all the wheels on and the engine cool.

Amazing how many good plugs are scattered on the ground in the pits after the races. Three laps and thrown away.
 
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