Timing Question

[scottessex]

I am reassembling the Arctic cat for the 4th time, Since I burned holes in the pistons last time, it looks to be a timing issue.
The AC engine is timed at 25 Deg BTDC (.151")
I have been looking at the rotax info:
The air cooled 503 is timed @ 15 deg BTDC
The 532 and 582 are timed @ 18 deg BTDC (.077")
and the 618 is timed @ 16 deg BTDC

My Arctic Cat has a stroke of 65mm the rotax 582 has a stroke of 64mm
How much difference is the 1mm in stroke going to make when I measure
with the dial indicator on top of the piston?

I want to split the difference and try to figure out how many thousandths
BTDC 20 degrees is.:noidea:

Also, Is there any way to check and see when a CDI ignition actually fires, when checking statically?

The rotax points engines are easy, I use a buzz box.
The rotax CDI uses alignment marks.
The AC uses an adjustable stator plate behind the flywheel. with hash mark that may or may not be real accurate.
I guess I just measure my distance and the mark the flywheel and check with a timing light once it is all together. ? :wacko:

Any suggestions?

[Passin' Thru]

Scott, it will be .098 in. from TDC

[GrantR]

Scott,

Why not start out at 15 to 18 to be safe. 20 could burn your pistons. Just saying better safe than sorry.

[Passin' Thru]

15° would be .057 inch BTC

18° would be .081 inch BTC

[Steboe]

Scott, 5deg is a large amount to retard an unmodified 2stroke, 2deg should make a noticeable difference
The ignition timing varies from one type of engine to another, I don't think that the timing rotax is using is very relevant

I think fuel is the real issue,are you running the main jets and needles as rich as you can?
Ken

[Lee Scatt]

Scott, If you start well retarded, you won't hurt a thing. The engine will not get up on the pipe is the only result. I would use a degree wheel, instead of a dial indicator myself, and check the factory marks. Slowly advance the timing till it will come up to speed. Start at about 12 BTDC.

[scottessex]

Thanks guys for the advice.
Ken,
yes I am jetted fat, and will be jetting fatter, and even fatter on the MAG cylinder.
The AC ignition/CDI has a built in advance curve, it goes to about 25 deg,@6000 rpm, and then it backs off the timing as the RPM increases.
The Rotax uses a fixed timing.
Just using the Rotax as an example here since it is a proven engine, and all the specs are documented and published for the type of load the engine will see.

I'll take some pictures when I get to that point, hopefully I can get the engine all buttoned up after work today.

[dinoa]

Not that it makes much differance but my manuals show the Rotax 503 dual cdi ignition model at 16 deg advance


Dino

[Lee Scatt]

Scott, IMHO , Unless you are running at about 10,000 rpm, 25 degrees BTDC is scary as an advance. Were you running in that rpm range (6000)at the time? Maybe the way the torque converter is set up on the sled is determining the advance at that rpm. I think a linear advance is better suited for your purposes.

[RotoPlane]

I know I don't no nothing about this....but from what I've read so far, I will not go more than 18* BTDC on my Arctic Cat 600. Even that advance sounds like bordering on the extreme to me.

Okay, I just checked my 2002 manual….it says my 600cc is 24* or .133" BTDC. I'm still going with 18* to start with, because I am limiting it to 6800 rpm.

Scott - this 2002 manual says the 570cc is 18* or .081" BTDC. (Stroke of 2.598", which is longer than the 600cc)

[Passin' Thru]

nevermind ...

[scottessex]

Lee, the max advance is @ 6000rpm 25deg, then it starts to back off the timing as the rpm increases, How much? I do not know.
I guess I will shoot for the 15-18 degree range re mark the flywheel,
and then check it @ 6000 rpm with a timing light,
Hopefully here in the next week or so, depending on how busy I am with
all that other stuff that gets in the way. :)

[Friendly]

nevermind ...

Pete,
I am amazed at how quick you do the math on those things!

[scottessex]

Yes, Thanks Pete, I was always slow at "ciphering" :)

[billygyro]

Scott, what prop are you running? is it Ground adj.?

Could it be that at higher RPMs the prop is unloading enough to over speed the crank and at that point lean out enough to cause the damage?

Every time it happens you might be cranking and banking and not notice it and running on the high side of rpms. And mix that with maybe a small intake leak and wala,, cooked piston? just wondering.

If timing was that far off it could do it, but just for thought- I had a 503 points engine and made a mistake of setting the timing at .086 and that was the strongest 503 I had ever seen for about 10 hr before it shut down. I didn't find that until t/down inspection.

I hate to see you cook another one, look at all possibilities before moving on. I hope you get it fig out soon. Take care billy

[scottessex]

Thanks Billy, I already changed out the IVO for a warp drive for that reason.

[scottessex]

Excellent article: about detonation and pre-ignition
http://www.contactmagazine.com/Issue54/EngineBasics.html

[edypaul]

Nice article. Thanks

[Arnie Madsen]

Excellent article: about detonation and pre-ignition
http://www.contactmagazine.com/Issue54/EngineBasics.html

This is one of the best articles I have ever seen on the subject Scott . Thanks

This is a "must read " for anyone running any engine in anything anywhere.

[RayPierce]

Scott
Take the guess work out of it and go down to your local McMaster Carr store and get one of these:
http://www.mcmaster.com/#protractor-dials/=700nyr

We use them at work for timing things.
All you need to do is attach it somehow to the crankshaft. Then make a pointer attached to the case.

[RotoPlane]

Thanks Scott....I agree, that is a article worth saving.....need to do some rethinking on my engine timing....

[scottessex]

Thanks Ray, But I can only see the side of the flywheel. There is a pointer on the case and I have it marked at 16 18 and 20, the factory marks start at 21 and go to 29.
The problem is major dissassembly required to adjust the stator plate, then re assemble and check again. The engine is back together hopefully going back on an get started in the following week.

Ed I'll keep you posted on the changes. :)

[All_In]

Good luck Scott!!!

[NoWingsAttached]

1. Verify your true TDC with a plunger micrometer, and mark the spot on your timing wheel. You can make one easily with any dial micrometer. It sounds like you have one, cuz you are asking for distance measurements early in the thread. It's next to impossible to verify and mark true TDC without this if the factory mark is missing or wrong. I've never found the factory marks to be wrong, just missing from a place where I can see them easily when playing with timing and strobes.

2. If your factory marks are 21-29, then the sweet spot is supposed to be somewhere in the middle, like the 25d you say. Stick with that, not a Rotax firing pattern.

3. Correct firing position is determined by compression ratio and RPM, and I dont' see it mentioned here.

4. Which brings us to compression ratio. What is it supposed to be? What is the Rotax's? What do you find with a compression guage? Fuel burns at a fixed rate for a given mixture. The higher the compression, the less advanced the timing must be, because at higher compression the fire expands that much more rapidly because the molecules are that much closer together, and MUCH hotter. You can change compression easily by changing the thickness of the head and jug gaskets.

6. CHeck the CDI map with a timing light at all RPM ranges and be SURE it is working properly. If it does not retard properly (should be 5-10 degrees retard at redline from peak advance) then you can set the static advance back that same amount. THe retard drop off curve is typically very steep, the advance falls of very rapidly. So, say you find the thing does not drop off the expected ~8 degrees, say it drops off only 2 degrees. If your static timing is normaly 25, then you can set it for about 18-20 BTC.

7. When in doubt, here is what I have always done: Set the advance to what you think it should be, so the engine starts easily. Retard the spark until you can't start the motor easily, on the first crank or two for a warm motor. Return the setting to the next closest mark where the motor again starts easily, and mark it (liquid white out works well, and you can see it with a strobe easily while running the motor). Run the motor up to midway in the power band. Play with the timing as you monitor the RPMs, pick a sweet spot and set it. Stop the motor, mark this spot, and measure the difference in degrees from the first mark where your engine started easily. Compare that to your factory specs, and continue to tweek until it feels right, where the motor starts easily and you also get the best power and temps. Put the nose wheels against the side of the building. Run the thing at each regular running RPM for long enough to get the head temps for that RPM, say 4500, then 5000, 5500, etc.

8. Have the gyro blessed to run off any gremlins by a priest or a rabbi.

9. OK, after I wrote this I went back and read the article posted by Arnie. THAT's a great article, eh?

[scottessex]

Gregg, I wish it were that easy, To adjust the timing I have to pull the starter, case cover, water pump pulley and belt, then pull the flywheel, and make my 15 second adjustment, then assemble in reverse order.
It could take the better part of a day just to try two or three different settings.

[scottessex]

OK I ran the engine, I had my son home to help me out.
I have the timing set to where I believe it will work out.
The funny thing is that while checking it with the timing light,
the timing is about 29 degrees at 2000 rpm, and progressively retards
as the RPM increases.
I am right about 18 degrees at 6000 rpm.
There was a small reduction in power, My ground rpm before was 6500,
now it is about 6100 with the same pitch setting on the prop.
I also went from 330 jets to 350 jets, depending on how it works out I may have to jet
one size down on the PTO cyl, or still go up one more size on the Mag cyl.
Water temps seemed to be lower also.
Keep your fingers crossed.
Now to safety wire everything back up, and double check everything, before final engine break in and flight testing.

[RotoPlane]

Man I hope this time it works out for you. You've had to break-in way too many engines!

[Friendly]

Thanks for the details Scott. I pray you found the solution
Why did you elect to start with 29 degrees?

[Passin' Thru]

Thanks for the details Scott. I pray you found the solution
Why did you elect to start with 29 degrees?

Yeah, me too! I'm really puzzled by this:
The funny thing is that while checking it with the timing light,
the timing is about 29 degrees at 2000 rpm, and progressively retards
as the RPM increases.

That's just plain backwards to what I THOUGHT I knew about ignition advance. :confused:

Scott, how does this thing advance, or retard in this case? Is it centrifugal? or some of that new fangled programed micro chip Voo Doo? :twitch:

[Lee Scatt]

I am sure you are right Pete, I have never heard of an ignition system designed to retard the spark as the rpms increase. I am surprised that it even cranks over with that much starting advance. Scott I advise against even starting your engine untill you get this figured out.
I think your cdi box is whacked. It should be fairly easy to adapt another type to your needs.
You should hit GyroJake up on this. He'll build you one up. An outboard motor cdi box will work fine.

[scottessex]

I did not elect to start at 29 degrees.
The factory setting is to set the stator at 25 degrees.
I measured and set the stator plate at 18 deg.
So The factory spec timing is checked at 6000 rpm.
At 6000 rpm I now have 18 deg instead of the 25 deg.
But to watch the timing mark move backwards and retard as the rpm increases
was really weird compared to what I am used to.

The timing is controlled by the CDI box.