2pg timing

TheGDFP · 11-29-2014, 04:12 PM

Yeah, I looked...

On an 88, there are:
2 trigger bumps on a plate that bolts to the flywheel.
1 pickup

Does it fire the plug at roughly 20 degrees BTDC and 20 degrees ATDC?
If not, how does the single pickup "know" which bump to be aroused by?

TheGDFP · 11-29-2014, 04:52 PM

And then, since fixed timing is such a compromise, and sustained higher-rev running at 20 degrees may be deadly in a tuned-up engine, a retard function 5-pin maybe?

Instead of a boost switch, a full-throttle micro would do it, or tied in to the stock governor, etc
http://www.instructables.com/id/Dirt...harged-engine/

1989Marathon · 11-29-2014, 06:45 PM

If you retard that motor 10° at the higher rpms, it will fall on its face, and run much hotter.

TheGDFP · 11-30-2014, 09:21 AM

Quote:

Originally Posted by 1989Marathon

If you retard that motor 10° at the higher rpms, it will fall on its face, and run much hotter.

Thank you. While that may be true with a stocker, I'm running a substantially different setup, with snowmobile bits. I'll be making peak power at twice the revs of the iron jug they come with. Fixed timing is fine with low revs, low compression, very mild port timings, but more aggressive setups will melt pistons at 20 degrees and high revs. My thoughts were, to advance to 22-25-ish, retard for starting and high revs.

TheGDFP · 11-30-2014, 09:24 AM

Post #1 is the one I could use an answer to :)

1989Marathon · 11-30-2014, 10:36 AM

Got it. If you are going over 6000 rpm, it's a good idea to have that curve.
I was under the impression that you were looking to retard the timing at higher rpms. I guess a better description would be that you are looking to advance the timing only in the low-mid range. You won't be retarding below base timing.

There are two bumps on the flywheel for forward and reverse timing. One bump is for fwd, the other for rev. I believe they are at 18° for fwd, and 21° for rev. +/- 3°.
The second hit is just a wasted spark.
What software is that in the pic?

1989Marathon · 11-30-2014, 10:48 AM

What is your avatar showing? It looks like a hypoid pinion welded to a ring gear?

TheGDFP · 11-30-2014, 11:23 AM

Sweet, I was hoping it triggered on both, and there was no real voodoo inside the box.
I'd seen the 18/21 numbers, and am basing my assumptions on that.

Pic is a generic interweb grab, just a visual of my intent, showing the low-high-low "curve", even if mine would be shaped more like an outhouse silhouette.

I'm looking to advance the timing manually, via rotating the pickup, retard electronically for start/idle, and sustained higher revs. +7, for 25 degrees forward, 14 degrees reverse, should be fine? Dropping 10 degrees when it's up "on the pipe", should save the piston and make more power

Sure, I'm complicating a simple setup, but it seems a lot easier than adapting a bike/sled/quad system, and dealing with no reverse capabilities. I'd rather have a nice digital curve, and looked at using a pair of pickups, 40 degrees apart, on the outside of the flywheel, single bump on the OD, and switching them along with the F/R lever. Then a second bump at BDC, only to trick the CDI in to thinking it's on the higher-revving scooter, halving the RPMs of the curve points.

TheGDFP · 11-30-2014, 11:25 AM

Quote:

Originally Posted by 1989Marathon

What is your avatar showing? It looks like a hypoid pinion welded to a ring gear?

Yes, I heard that all the cool kids had welded diff gears

1989Marathon · 11-30-2014, 11:53 AM

Quote:

Originally Posted by TheGDFP

Yes, I heard that all the cool kids had welded diff gears

Not really sure how that would work... but, OK!

11-29-2014, 04:52 PM	#2
TheGDFP Gone Wild Join Date: Apr 2014 Location: Flint, MI, USA Posts: 656	Re: 2pg timing And then, since fixed timing is such a compromise, and sustained higher-rev running at 20 degrees may be deadly in a tuned-up engine, a retard function 5-pin maybe? Instead of a boost switch, a full-throttle micro would do it, or tied in to the stock governor, etc http://www.instructables.com/id/Dirt...harged-engine/

11-29-2014, 06:45 PM	#3
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730	Re: 2pg timing If you retard that motor 10° at the higher rpms, it will fall on its face, and run much hotter.
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730

11-30-2014, 09:24 AM	#5
TheGDFP Gone Wild Join Date: Apr 2014 Location: Flint, MI, USA Posts: 656	Re: 2pg timing Post #1 is the one I could use an answer to :)

11-30-2014, 10:36 AM	#6
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730	Re: 2pg timing Got it. If you are going over 6000 rpm, it's a good idea to have that curve. I was under the impression that you were looking to retard the timing at higher rpms. I guess a better description would be that you are looking to advance the timing only in the low-mid range. You won't be retarding below base timing. There are two bumps on the flywheel for forward and reverse timing. One bump is for fwd, the other for rev. I believe they are at 18° for fwd, and 21° for rev. +/- 3°. The second hit is just a wasted spark. What software is that in the pic?
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730

11-30-2014, 10:48 AM	#7
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730	Re: 2pg timing What is your avatar showing? It looks like a hypoid pinion welded to a ring gear?
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730

11-29-2014, 04:12 PM	#1
TheGDFP Gone Wild Join Date: Apr 2014 Location: Flint, MI, USA Posts: 656	2pg timing Yeah, I looked... On an 88, there are: 2 trigger bumps on a plate that bolts to the flywheel. 1 pickup Does it fire the plug at roughly 20 degrees BTDC and 20 degrees ATDC? If not, how does the single pickup "know" which bump to be aroused by?

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11-30-2014, 11:23 AM	#8
TheGDFP Gone Wild Join Date: Apr 2014 Location: Flint, MI, USA Posts: 656	Re: 2pg timing Sweet, I was hoping it triggered on both, and there was no real voodoo inside the box. I'd seen the 18/21 numbers, and am basing my assumptions on that. Pic is a generic interweb grab, just a visual of my intent, showing the low-high-low "curve", even if mine would be shaped more like an outhouse silhouette. I'm looking to advance the timing manually, via rotating the pickup, retard electronically for start/idle, and sustained higher revs. +7, for 25 degrees forward, 14 degrees reverse, should be fine? Dropping 10 degrees when it's up "on the pipe", should save the piston and make more power Sure, I'm complicating a simple setup, but it seems a lot easier than adapting a bike/sled/quad system, and dealing with no reverse capabilities. I'd rather have a nice digital curve, and looked at using a pair of pickups, 40 degrees apart, on the outside of the flywheel, single bump on the OD, and switching them along with the F/R lever. Then a second bump at BDC, only to trick the CDI in to thinking it's on the higher-revving scooter, halving the RPMs of the curve points.

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