|Topic Review (Newest First)|
|11-30-2018 05:40 PM|
Originally Posted by TonyO View Post
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.
|11-30-2018 04:56 PM|
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.
|11-30-2018 04:21 PM|
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.
|05-29-2018 02:14 PM|
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.
|05-29-2018 12:22 PM|
I am seeing several polite responses suggesting that this is not a good idea.
It's so tempting to try though ...but at what cost ?
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?!?
|05-28-2018 09:58 AM|
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.
|05-28-2018 12:48 AM|
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.
|05-09-2018 04:00 PM|
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
|05-09-2018 11:51 AM|
Originally Posted by michiganteddybear View Post
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.
Originally Posted by SergheiS View Post
|05-09-2018 10:36 AM|
|SergheiS||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.|
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