Ignition Timing Question (long)

G. Gearloose

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#1
I need some advice, please bear with me on this long story.
Over time, my ’91 200 had developed a flat spot from off idle to perhaps 2000 rpm. Above that it would be strong. The transition always occurs at the same rpm, like someone hit a switch. (not fun in the rocks!). Not sure what I dinked with to perpetuate this, but it was not as detectable until I tried improving the rich stock jetting last year after consulting JustKDX. I had tried the pilot from 48 to 45, 1173N vs 1172N needle, airbox mod, Main 150 summer, 152 spring and fall. KIPS was operating and assembled correctly. AS (.5-.75 typ) had some effect of course, but no setting resolved the symptom. Overall the bike was running much better with jetting, but off-idle didn’t keep pace.

Over the last few years I added a DG pipe and Delta-V reed block, I think the low and midrange came up significantly, but the Off–idle stayed flat, exaggerating the rough transition. All attempts at jetting improved low, mid and top, improved plug color, but this only accentuated the lackluster off-idle.

This symptom is also independent of throttle position (cracked to WOT), off-idle remains blah; no bog or gurgle, just bland. I was beginning to doubt jetting was the cure nor cause.

I discovered a blown out crank seal on the mag side (Ah-HA! I thought), so I recently had it torn down at the local dealer (they did a nice job) new top end, crank bearings and seals. But the same symptom persisted, despite more attempts at jetting and AS adjustments.

I remembered that while installing the flywheel, although there was no visible damage to the keyway nor slot, there was about 1 to 2 degrees slop where the flywheel could settle on the crank and key but still be seated.

I reinstalled the flywheel but let it settle where it could be more advanced, then noticed marked improvement off-idle. It even sounded different the first time I started it. Encouraged, I gave the stator a nudge more advanced; only halfway between the middle line and the adjacent ‘advanced’ mark.

It runs really great now, really strong off idle, and then gets stronger and stays crisp with more RPM and throttle. Front tire comes up in first 3 gears, w/ 13:48 ratio. In two words, just right. Rockcrawling control is great.

I suspect the CDI advance is minimal at idle and I felt it kick in at a certain low RPM, resulting in the ‘switch-like’ change in performance I observed at 2000 rpm or so? But I’m just guessing.

So, do you think this a sign the CDI or coil is suspect, or just manufacturing tolerance? Know of any timing light procedure? I am hoping the factory may have been as conservative with the timing as they are with the jetting.

Before I put more time on the engine, I wanted to hear what you think or any related experience, as to whether this solution is viable or risky before I damage something.

Thanks!
G. Gearloose
 
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#2
Sorry to not see some input on this timing question.

I have read (in the not too distant past) from either eric gorr or rich r. that they'd seen far more engines destroyed from advancing the timing than improved by advancing the timing. Generally, delaying the spark is an improvement for most 2-cycles.

You might try posting this in the performance forum.

I'm curious about the 2º slop. Don't understand how that could be. Something's got to be off..the key, the crank or rotor keyway.

How do you determine the 1-2º part? That's a pretty small difference. Using a light? Once you determine TDC, any mark anywhere can be relatively used to set, check or change the timing. Measure the circumference of the rotor, divide by 360º and make your own scribe marks based on your previously found TDC.

Altogether not too handy cuz you can't change the timing without removing the rotor..and if there's slop putting it back together, it would be tough to know what you were ending up with.

If you get some definite input as to advancing/retarding KDXs and its effect either way, hope you post it back here.
 

Canadian Dave

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#3
Here's a blurb from Eric Gorr's book: Motocross and Off-Road Performance Handbook. I cut this from his web page @ www.ericgorr.com

David


AFFECTS OF THE IGNITION TIMING

Here is how changes in the static ignition timing affects the power band of a Japanese dirt bike. Advancing the timing will make the power band hit harder in the mid range but fall flat on top end. Advancing the timing gives the flame front in the combustion chamber, adequate time to travel across the chamber to form a great pressure rise. The rapid pressure rise contributes to a power band's "Hit". In some cases the pressure rise can be so great that it causes an audible pinging noise from the engine. As the engine rpm increases, the pressure in the cylinder becomes so great that pumping losses occur to the piston. That is why engines with too much spark advance or too high of a compression ratio, run flat at high rpm.

Retarding the timing will make the power band smoother in the mid-range and give more top end over rev. When the spark fires closer to TDC, the pressure rise in the cylinder isn't as great. The emphasis is on gaining more degrees of retard at high rpm. This causes a shift of the heat from the cylinder to the pipe. This can prevent the piston from melting at high rpm, but the biggest benefit is how the heat affects the tuning in the pipe. When the temperature rises, the velocity of the waves in the pipe increases. At high rpm this can cause a closer synchronization between the returning compression wave and the piston speed. This effectively extends the rpm peak of the pipe.



HOW TO ADJUST THE TIMING

Rotating the stator plate relative to the crankcases changes the timing. Most manufacturers stamp the stator plate with three marks, near the plate's mounting holes. The center mark is the standard timing. If you loosen the plate mounting bolts and rotate the stator plate clockwise to the flywheel's rotation, that will advance the ignition timing. If you rotate the stator plate counterclockwise to the flywheel's rotation, that will retard the ignition timing. Never rotate the stator plate more than .028in/.7mm past the original standard timing mark. Kawasaki and Yamaha stator plates are marked.
 

G. Gearloose

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#4
Thank you both, CC and CD!
Originally posted by canyncarvr
How do you determine the 1-2º part? That's a pretty small difference. Using a light?
I'll try to elaborate; It was a 'feel' thing..
After seating the flywheel on the shaft, being careful it doesn't bind on the key and is seated sqare to the tapered shaft, then add the nut finger tight. The engine sticktion holding the shaft stationary, I can rotate the flywheel a little each way, slipping on the shaft before the key binds, all before torquing the nut. This is the 1-2 degrees I feel, just a guestimate. This slop could reduce repeatability every time the flywheel is attached.

I'll reinspect the parts, take some measurements.

Heeding your advices, I'll treat as a clue intead of a fix, check I didn't rotate the stator too far, and see if I can play with a timing light for more clues as to whats occuring. I'll certainly share any discoveries.
 

Rich Rohrich

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#5
If the timing was substantially retarded initially there is no doubt you could effect a fairly dramatic change by advancing it. Overly retarded timing will tend to raise the exhaust gas temperature and change the pipe tuning to a higher rpm causing an even more pronounced "hit" during transition and a general loss of bottom end response. Given the rather poor combustion quality of two-stroke engines at low rpm and part throttle, running too little timing can be pretty obvious. As everyone has pointed out, it's clearly a double-edged sword when you advance the timing. How far you can go advanced has a lot to do with the loads, heat, and rpm range the engine will see. An engine that sees mostly part throttle , moderate load and rpm duty can run a lot more initial timing advance than a engine that sees extended high load part throttle or WOT operation.

The basic idea behind ignition advance on any conventional piston engine is to try and time the peak cylinder pressure to occur so as to apply the pressure on the descending piston in the most mechanically efficient way. This tends to be in the range of 15-20 degrees ATDC. As rpm increases and less time is available for charge burning to take place the engine will
need to fire earlier (more advance) to account for the diminished time. The overall idea is to use the minimum timing necessary to obtain the best possible torque.

Higher engines speeds reduce the overall burn time by virtue of additional turbulence in the combustion chamber, less time available to lose heat to the surrounding chamber surfaces, higher flame speeds, and increased mixture density. In four-stroke engines with narrow valve included angles, and efficient combustion chambers these factors can more than offset the reduced time available for combustion to take place, and retarding the timing will actually produce more torque. We are starting to see newer
four-strokes take advantage of this fact and using more advanced digital ignitions that have the ability to retard the timing at the appropriate time. This is a VERY general explanation of the four-stroke side of things.

Two-strokes tend to have fairly specific reasons to retard the timing at high speeds. Including the above points, we can add, more efficient scavenging at high speeds which improves mixture density and speeds combustion, plus the mixture density as a whole is much more dense than on a four-stroke because of crankcase compression. Dense mixtures burn considerably faster, so less timing is needed. Once the engine is operating in the range of the pipe tuning, the volumetric efficiency will be at it's highest which will obviously be the point where maximum cylinder filling occurs. Again this will improve the mixture density and speed things up, so less timing is needed. In a two-stroke race engine this tends occur at high rpm Two-stroke exhaust valve equipped engines will have higher dynamic compression ratios at high rpm which will also help reduce the burn time. But probably the most important reason for retarding the high speed timing on a two-stroke is to use some of the available combustion heat to manipulate the pipe wave timing and improve the scavenging/ram tuning. Less efficient combustion tends to raise exhaust temperatures, so they are effictively trading off some thermal/combustion efficiency to improve the scavenging and increase the delivery ratio of fresh charge. Like most things in engine design, it's a tradeoff based primarliy on the intended use of the engine. More radical two-stroke road race designs like the TZ and RS motors have fairly narrow power bands, so they use these effects to maximum advantage. Add to all this the basic simplicity of a two-stroke combustion chamber, which by it's very nature will provide a fairly fast burn rate and it's pretty apparent that there are probably more similarities than differences between the 2 engine types
, and they have nothing to do with the physics involved. It's more a case of the differences in the basic environments.

This probably seems more complicated than it really is. Ultimately if you decide to vary (advanced or retarded) from the stock spec on initial timing you'll need to keep an eye out for excessive heat as well as signs of spark knock. The sprakplug has a remarkable ability to clue you in on these things. Anything stronger than trace knock will tend to show up on the surface of the plug's nose, and excessive timing has a nice consistent habit of overheating the center electrode and ground strap. Blueing and scaling are sure signs that there is more timing than the engine really needs. IT's worth experimenting, but take small steps and it's advisable to check the stock timing marks against the manufacturer's spec for piston positon. The procedure is listed in most service manuals.

Good luck.
 

G. Gearloose

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#6
Thanks Rich for your detailed response!
Looks like we'll all learn a lot on this one.
I can't retieve my timing light until midweek, I'll try to degree the flywheel and post back numbers.
my manual says 19 BTDC @ 6000 rom for a '91..It mill be a trick to keep it steady at 6000 I suspect.
 
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#7
Maybe it's not complicated for YOU..but...

CDave quotes from EG's writing to say:
Advancing the timing will make the power band hit harder in the mid range... Retarding the timing will make the power band smoother in the mid-range
RR sez:
Overly retarded timing will tend to raise the exhaust gas temperature and change the pipe tuning to a higher rpm causing an even more pronounced "hit"
I'm looking at two different situations making a harder 'hit'? Is it the 'overly' part? Is the harder 'hit' in different places?

In the first part, I read it as, 'Advancing timing makes the power band hit harder.'
In the second part, I read it as, 'Retarding timing makes the power band hit harder.'

Where's my advil..........
 

G. Gearloose

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#8
Mo data…

Short story:
I set the timing back to ‘straight up’. I thought I heard a knock at about 2000rpm on my first real ride with a decent load on it, and at temp.

Longer story:
I tried to get some timing numbers but this is where its gets ambiguous.

To get an idea of rpm, my DVM has a ‘frequency’ function, so I wrapped the leads around the plug wire 4 times to make an inductive pickup. :silly:

One would think that if I aligned the factory score on the flywheel with the pointer on the case that the piston would be at tdc? Apparently not...

Finding tdc really needs a dial indicator, but after some probing in the spark plug hole with a Teflon dowel and midpointing similar deflection in each direction, I concluded that when the factory marks on the case and flywheel align the piston is really at about 20 degrees btdc.

Idling, the timing light indicated the factory marks aligned right on at 1200rpm, (about 20 degrees btdc) and gradually retarded with rpm to an observed 8 btdc at 6000rpm using a light with dial-in retard. Keeping dial-in to 0, got same numbers by indexing some marks in 5 degree increments on the case after calculating flywheel diameter with a caliper.

But this data conflicts with my manual, that states the spec is 19 BTDC at 6000rpm.Thats my @ Idle advance. :confused:

I know my method to find TDC method was kludgey, but I suspect I was not off by 11 degrees.

All I could conclude from the exercise was the CDI did not have apparent abnormalities at the transition where the flat spot subsides, and that to measure or set the timing with real accuracy, a dial indicator and a timing light with built-in retard is necessary. (and better specs from Kawasaki.)

Guess I can swap the V-force or pipe with stock to see if they are responsible; good thing its ridable as it sits.

Thanks to you all for getting me this far, I just wanted to share the info. Knowing what the problem is ‘not’ is almost as good as knowing what it is.
Maybe is time for carb mod ;)
 
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#9
One would think that if I aligned the factory score on the flywheel with the pointer on the case that the piston would be at tdc? Apparently not...
You are correct in the second instance (you know that..just agreeing with you). They are TIMING marks..not TDC marks. If they WERE TDC marks, they wouldn't do much good for timing..they are too close to what would be TDC to accomplish anything.

re: finding TDC. Dial indicators don't do too well. There is too much of a flat spot at the top (and bottom) of the reciprocation motion of the piston to determine exactly where TDC really is. Best way to find it is a positive stop method.

Which sounds like what you used with the possible exception of the stop maybe not being real positive.

Why would timing at 6k be the same as at idle? It SHOULD retard considerably at higher rpm. The retarding of spark coincides with power valve actuation..if I recall correctly..which doesn't apply to a '91. Point being that spark is retarded in the upper ranges..as rich said:
Two-strokes tend to have fairly specific reasons to retard the timing at high speeds
I'd bet you are correct in all your observations..and that the spec is not. Your method AND the documentation of it seem right on the money.

And, yes...it IS time to send your carb off to RB Designs!
 
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#11
I remember seeing this post a while back. One thing that struck me is you are focused on the ignition timing being the problem & seem obsessed with finding the correct timing mark but you don’t mention (well at least that I can see) if you tried retarding it (which is always safe) to see if it made a difference.

Other thing I would try is a leak down test. The bottom end rebuild should eliminate this but you may still have a leak between the cases or at the reed valve mouth.

You could try checking out your sparkplug cap/coil to see that they haven’t degraded which may affect low speed (hence spark energy) high load conditions. Usually this will manifest itself at high rpm but stranger things have happened… apparently at sea, but I’m not keen to find out.