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Unread 02-23-2012, 05:37 PM   #1
hunts2much
Registered User
1978 CJ7 
 
Join Date: Jan 2010
Location: Michigan
Posts: 13
Timing curve for propane?

Hello all, I could use a little help dialing in the timing on my '78 CJ7 that I have recently converted to propane with a Gotpropane kit.

It seams I have too much total advance (and not all in fast enough) even after changing out the springs in the distributor for the 2 lightest ones. I'm still getting a back fire/popping on deceleration even when the rig is just parked in the drive with no load.

Specs:

'78 CJ7, AMC360, Impco model E regulator, Impco 425 mixer, Performer intake, unknown (lumpy) cam, MSD 6al, Ford E-coil, big cap conversion, MSD wires, plugs gapped to .035, stock duracraft dizzy with the 2 lightest springs installed and a R15 advance plate(I think that is what it's called), Vacuum advance is hooked to manifold vacuum. MSD timing tape installed.

Initial (vacuum advance pulled and plugged) is currently set at 13 degrees. I'm not sure how to post a graph but I have graphed out the timing curve. I'll type it out...

800rpm - 13deg
1000rpm - 13deg
1200rpm - 14deg
1500rpm - 17deg
1700rpm - 22deg
2000rpm - 28deg
2200rpm - 31deg
2500rpm - 36deg
2700rpm - 41deg
3000rpm - 45deg
3200rpm - 48deg
3500rpm - 50deg

I've done a ton of research on timing for propane and from what I gather I need to be around 30-36 degrees total all in around 2500-2800 rpm. I also read 14-14-14 for a good starting point...I'm open to suggestions here if any one cares to comment.

So I guess my question boils down to how do I limit all that total advance? Different advance plate? Limit my current advance plate?

I'm open to any and all advice. I will provide any more info I can if needed.

Thanks, all.

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Unread 02-24-2012, 12:46 PM   #2
hunts2much
Registered User
1978 CJ7 
 
Join Date: Jan 2010
Location: Michigan
Posts: 13
Well, now I'm even more confused. If you look at the pic I've posted you can see I have a 15R and a 21R advance plate. What I don't understand is how the 15R slot is wider than the 21R slot.

That seams to be backwards according to everything I've read. Shouldn't the 21R slot be wider than the 15R slot??

I'd like to take the whole plate assembly off so I can weld/fill in the slot that is currently being used in order to limit the centrifugal advance. Problem is I can't figure out how to get the advance plate to lift up and off the distributor?? I pulled that foam piece and the clip right underneath it off the shaft but I still can't get it off. What am I missing?

If I do get the advance plate off and fill in the gap, will I have to pull the distributor and rotate 180 degrees if I don't switch to the 15R slot?

Lastly, the limit "pin" seams to have a tiny bit of slop. The pin itself moves just a few degrees. Is that normal or should it be rock solid and not move at all?

Hopefully the pic will show...


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Unread 02-24-2012, 12:58 PM   #3
CJ7dreamer
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1997 TJ Wrangler 
 
Join Date: Oct 2010
Location: Trabuco Canyon, CA
Posts: 141
I would take it to a good distributor guy and get it curved to what you want. They can work miracles. It's popping because you have WAY too much total advance.
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Unread 02-24-2012, 01:03 PM   #4
CJ7dreamer
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1997 TJ Wrangler 
 
Join Date: Oct 2010
Location: Trabuco Canyon, CA
Posts: 141
I would take the diatributor to a good distributor guy and get it curved to what you want. They can work miracles. It's popping because you have WAY too much total advance.
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Unread 02-24-2012, 01:11 PM   #5
hunts2much
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1978 CJ7 
 
Join Date: Jan 2010
Location: Michigan
Posts: 13
Yep, too much total. It really pops at ~4000 rpm. I bent the tabs in today that hold the springs so I should get the advance to come all in sooner but I still need to limit the total somehow. I just need to figure out how to get the advance head off to weld it/fill it in...
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Unread 02-24-2012, 01:56 PM   #6
JeepHammer
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1973 CJ5 
 
Join Date: Aug 2007
Location: South West Indiana
Posts: 9,950
Someone has opened up that '15' slot, it should be MUCH smaller than the 21 slot...


*IF*
You are 'Experamenting' and just want to try this and that.
Try slipping a piece of fat vacuum line over the limiter peg in that distributor...
That will take a little timing out for testing.

So will fat zip ties around the peg.

Heavier springs will bring in the centrifugal SLOWER, and start higher up in the RPM range.
You have some pretty small springs on that thing, so it's coming in REALLY fast.
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Unread 02-24-2012, 03:18 PM   #7
hunts2much
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1978 CJ7 
 
Join Date: Jan 2010
Location: Michigan
Posts: 13
I was thinking the same thing about someone opening the slot up but if someone did it they did a near perfect job of machining it. I finally got the advance head off and after closer inspection i see no evidence of tooling marks. So not sure what to think here...

Zip ties or vacuum hose on the pin would have been a great idea however I just took the head in to have it welded and machined to the equivalent of a 9r head before I read your post... .385" slot width i believe.?. that's what I requested anyway. I'll just dremel it open further if need be.

As far as the 2 springs I was expecting the advance to be all in a lot quicker than it actually proved to be according to the graph I posted. The tabs were bent out quite a bit so I'm guessing thats the reason I couldn't get all in under 3000 with the 2 lightest springs in the recurve kit.?.

Jeephammer- I think I've read you have a propane powered rig? I'm wondering if you agree with what seams to be the consensus amongst the propane community that total (initial+cent) advance should be all in under 3000 rpm?
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