Timing Question

scottessex

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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?
 

GrantR

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Scott,

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

Steboe

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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

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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

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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

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


Dino
 

Lee Scatt

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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.
 
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RotoPlane

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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)
 
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scottessex

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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. :)
 

billygyro

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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

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

RayPierce

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360 degree dial

360 degree dial

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.
 
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