Kawasaki VN750 Forum banner

1 - 16 of 16 Posts

·
Registered
Joined
·
105 Posts
Discussion Starter #1
So I plan to try my hand at the pickup coil mod, but I'm not exactly understanding where the 0.020" Gap is supposed to be. I know it's supposed to be against some kind of plate on the stator? On a photograph I saw a "trigger magnet", is this the piece that's supposed to have the 0.020" clearance from the coil, or another?
Also, should the engine be cold, or can I do this after a ride? And which way should I rotate the motor if I have to? Clockwise or counter?
 

·
Super Moderator
Joined
·
2,569 Posts
the trigger magnets on the flywheel are on the outer diameter. the stick up slightly from the surface (very slightly). you set the gap between the coils and the magnets to .020". if I recall, I turned mine counterclockwise when viewed from the left side of engine, where the flywheel and coils live. I did mine when cold.

you will find that you need to elongate at least one mounting hole on each coil, as well as remove the locating tabs that are cast into the cover in order to get the adjustment made. I used a rat-tail file for the holes (be careful, the wires are likely to be fragile), and a small pair of diagonal cutters to snap the tabs off (its just aluminum, and fairly soft there)
 

·
Registered
Joined
·
7,687 Posts
I think counterclockwise rotation is correct, but it wouldn't hurt to check by bumping the starter with the stator cover removed, just to check.

Overhead cam setups are sensitive to rotating backward, it puts the chain/belt slack on the wrong side of the loop (opposite the tensioner), and could cause problems.
 

·
Registered
Joined
·
105 Posts
Discussion Starter #4
Did it counterclockwise on a cold motor, and put the coils at 0.020". Started right up for a few seconds, but I'm gonna let the loctite dry, so I'll test it out tomorrow to know for sure how it went. Next in line: 2 wire mod & fixing a broken ground wire.
 

·
Registered
Joined
·
472 Posts
Any chance you can measure the ignition timing after the mod?
From what I read, this mod advances the timing, but I didn't see any measurements after the mod.
 

·
Registered
Joined
·
105 Posts
Discussion Starter #7
I will be 100% honest, I'm new to carburetors & distributor systems, so I'm not exactly sure how timing is measured. All I know is, changes in acceleration seem minimal right now, but starting up seems a lot easier, which was always my main problem.
 

·
Registered
Joined
·
472 Posts
the amount the timing changes is minimal. many (me included) feel like performance is slightly better, but thats just the butt dyno measurement
I have considered messing with the stock timing (5-25 BTDC) to try to get a bit more out of it. Maybe bump it 2 degrees for 7-27 BTDC? Some guy asked the question on another forum 6 years ago, but the thread died.

My truck (I know, I know, vastly different engine...5000CC...V8...Single plug per cylinder...etc) has ignition advance starting at 14 BTDC base timing, with both vacuum and RPM advance on top of that. For the Vulcan to have a base timing of 5 seems like an opportunity for a bump in HP, throttle response and MPG, but it all depends on how close to the edge Kawasaki set it from factory. The dual plugs reduces the need for spark advance.

I will be 100% honest, I'm new to carburetors & distributor systems, so I'm not exactly sure how timing is measured. All I know is, changes in acceleration seem minimal right now, but starting up seems a lot easier, which was always my main problem.
I grew up on Fuel Injection, so I'm learning all this stuff too. Head over to the link I posted above from the other forum, as the 2nd to last post talks about ignition advance theory better than I could write up. The timing measurement is outlined in the Kawasaki service manual on page 272 (if you download the service manual from this thread). It requires a timing light to measure.
 

·
Super Moderator
Joined
·
2,569 Posts
the problem with changing the timing on the vn750 includes, but is not limited to:

1. designed as non-adjustable.. there is no way to change it without modifying the pickup coil mounting (left cover)
2. all the advance is handled in the igniter module, probably based solely on rpm.
3. with 10 to 1 compression, in order to live happy on 87 octane, you cant have much timing. also one of the reasons we have 2 plugs per cylinder, to light the fire from both sides of the chamber
 

·
Registered
Joined
·
472 Posts
I did the .020" mod yesterday and I honestly didn't notice anything different. Factory gap was .033 on both coils.

I did the math though and moving the coils clockwise by .041-.045" will advance the timing by one degree. The timing module is under the assumption that the pick up will be triggered at a particular position of the crankshaft. My guess is the coils are triggered at or before the maximum advance (25 degree). The module then retards the timing based on rpm (5 degrees BTDC at 1k rpm). The module would be unaware of any manual advance, and would assume it's in the factory position. Since a module handles the timing advance, a manual advance of 1 degree would shift the entire range (5-25 becomes 6-26). Now whether this would result in detonation or not is the real question.

The goal is peak cylinder pressure right after Top Dead Center. It takes a finite amount of time for the pressure to build up. Theoretically, twin spark plugs means it takes half as much time. Too much advance results in either: peak pressure before Top Dead Center, or the pressure builds up to the point that a portion of the yet unburned fuel spontaneously explodes. The first is the hard limit of timing. The second can be resolved through higher Octane fuel.

Having grown up on a low carb diet (all fuel injection cars), I am not well versed at identifying the symptoms of too much ignition advance. I suspect that Kawasaki was conservative from the factory. Is there a possible improvement here, or am I chasing a fools dream?

If I attempt this change, what should I be listening for, and under what conditions are considered "worst case" ?

All the Google results say that too much timing advance can damage an engine. Is this one single rev with 1 degree too much timing and BOOM, or is this sustained operation for 3 hours with detonation? Is it possible to find the limit without causing permanent damage?

If this works, I expect to see an increase in both power AND mpg.
 

·
Registered
Joined
·
7,687 Posts
I've made ashtrays (burnt pistons) in as little as 45 seconds, others have taken 6 miles at full throttle to get results.

The VN750 has a moderately high compression ratio and is designed to run on low octane gas, so it's right at the edge.

What to look for:

Ping - the sound of detonation/pre-ignition, described as marbles dropped on a glass plate. The 45 second ashtray gave no warning signs.
Overheating - or just running a touch warm
Spark plugs burned white, melted or missing electrodes.
Driveability issues - bucking, surging
Starting issues - hard cranking when warmed up

Warning - Terminal Engine Damage Ahead

One major problem is the lack of precise marks to gauge the adjustments, and precise means of making adjustments. Also, it will be very easy to have one cylinder advanced more than the other.
 

·
Registered
Joined
·
472 Posts
I am seeing several polite responses suggesting that this is not a good idea. :smile2:

It's so tempting to try though >:) ...but at what cost :doh:?

Combining what I've read hear and some of my own research on this, I see that worst case (most prone to detonation) is full throttle at low RPM on a hot engine, with a rich mixture. Part throttle means a less dense charge in the cylinder and a slower burn. Heat makes it more prone to auto-ignite. A lean mixture (I read) burns slower. I have the stock jets in my carbs. With open pipes and K&N filters, I may be a bit on the lean side. Plug color says I'm normal.

If I use a dremel to grind off the upper ear of each coil, I can precisely measure my advance by measuring the gap between the lower ear of each coil and its locating pin. Alternatively, measuring the total length of each coil with calipers would make it possible to match them. Between 0.041" and 0.045" is 1 degree of advance. It should be non-trivial to match the two cylinders to well within 1/2 a degree.

My motivation for this mod is as follows:
Most car engines use vacuum advance, and this bike doesn't have one. I can't conceive of a way to add vacuum advance, so I thought advancing static timing would help. If there is room to safely adjust, I should gain both power (at WOT) and mpg at part throttle. But I also may gain a 500lb heap of scrap metal if I go to far.

Oh...what to do?!?
 

·
Registered
Joined
·
7,687 Posts
I've only run into burned pistons at full throttle and high rpm. But an engine can ping easier under load at lower rpm with advanced timing and/or poor tuning.

Many unmodified carbs will go lean at sustained full throttle, which contributes to burning a piston. Two-strokes are highly sensitive with respect to timing and fuel mixture.

Seems like a lot to go through for an extra few tenths of a second on the street. You might gain a little mpg, but it's easy to jump the shark too.
 

·
Registered
Joined
·
472 Posts
I can't let this go...

I keep coming back to this, and I think I'm just going to try it.
Near as I can tell, lean burns faster than rich (I've heard some conflicting info).
I know that part throttle burns slower than WOT. This is what vacuum advance is used for in cars. Based on that, I know I can get away with more timing advance at partial throttle. I would gain some MPGs. The danger is at WOT.

I know people have stated the bike runs lean from the factory. It probably also uses conservative timing to account for bad gas. I plan to first richen up the AFR through a larger main jet. The chart shown here indicates the main jet covers 3/4 to WOT. If I can get it rich enough, it should tolerate more advanced timing at WOT as well. All in all, more MPG and more acceleration. I'm running no mufflers, and I'm doing the ear shave. I'm planning on a 138 main jet, based on that post. I'll probably buy a set of 140s too, just to try.

The chart in the service manual (page 15-7) shows a range of timings. I'm assuming that's the tolerance band. At 1100 RPM, it shows ~3.5-7 and at 3500 RPM it shows ~24-27. That tells me that without any other modifications, I should be safe to push the 1100 advance to 7 degrees. I'd use a timing light to verify the timing of each coil. With a rich WOT mixture, I MIGHT be able to push 8 or 9 degrees. >:)
 

·
NewB to Vulcans
Joined
·
220 Posts
There is a wire that runs from the starter switch to the ignition system. I strongly suspect it is there to change the ignition timing during starting to make it easier.

I think it retards the timing so you get a soft start, similar to the manual retard you find on some historic bikes. Others think it advances the timing.

Might be an idea to test the theory with a timing light while connecting the ignition input to 12v and leaving it open.

If it advances the timing you have a simple way to test your theory about performance without having to dremel parts.
 

·
Registered
Joined
·
7,687 Posts
There is a wire that runs from the starter switch to the ignition system. I strongly suspect it is there to change the ignition timing during starting to make it easier.

I think it retards the timing so you get a soft start, similar to the manual retard you find on some historic bikes. Others think it advances the timing.

Might be an idea to test the theory with a timing light while connecting the ignition input to 12v and leaving it open.

If it advances the timing you have a simple way to test your theory about performance without having to dremel parts.
Planned to do that this past summer with the wire ... never got a round tuit.

I sense a new mod, maybe, a "Push-to-Pass" mod, like F1, or is it Indycar. (only if the wire makes it advance)

Pretty sure it's the red/white on the CDi, but I didn't check the diagram.
 
1 - 16 of 16 Posts
Top