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GMC 1928 T20 Truck Restoration

Oldtech

Subscriber
Last Subscription Date
12/14/2019
Re: GMC 1928 T20 TRUCK restoration.

Hi GMC. Yes, I have seen the ones without the cap on the bottom, and yes, I think you should get it to the closest tooth. The point cam can then be loosened for fine tuning I think. Timing mark is a bit of a puzzle, Yes, there's a 17 degree mark but there should also be a 0 or a TDC. Maybe truck flywheel is different, I have one in shop, maybe I can check this weekend. # 1 cylinder is the front one.
Firing order... I will double check. There was a change somewhere in there. One instruction I have says use the 7 degree mark with the lever in full retard, which makes sense.
 
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GMCtrucker

Registered
Re: GMC 1928 T20 TRUCK restoration.

Hi there Oltech.
I will closely check the flywheel this weekend too and try to make photos of the marks on it.
I really appreciate your effort in helping us.
 

Oldtech

Subscriber
Last Subscription Date
12/14/2019
Re: GMC 1928 T20 TRUCK restoration.

Back at it. I looked at 2 flywheels and the mark reads either UDC or DC. over 1-6
Translation . Upper dead center cylinders 1 and 6. I had mistaken the C for an O. in the rust. The engine I looked at is a 1928 and it appears the firing order is 142635, so yours is likely the same. I have a 27 with wires still on it but it is out in the snow.
Your distributor is likely the modern type with the mount clamped to the dist body so it can be loosened if you need to adjust the position. So... To time it, from the start.
get #1 on firing position - either by watching valve action or holding thumb over the spark plug hole while cranking, and set to the UDC 1-6 mark. Move the lever on steering column to full retard position. (distributor full clockwise). Decide which rotor position will be number 1 and set the distributor so the points are JUST GOING TO OPEN. Clamp it there, then install the wires clockwise 142635. Should work. My apologies for the confusion, sometimes my memory isn't what I think it is. I will watch this thread if you have any more questions.
 

GMCtrucker

Registered
Thanks Oldtech, your help is very appreciated.
Will try the procedure you just sujested and will tell. We have the same flywell marks you have.
Do you have a photo of your car to share? Hope I can see yours :brows:
 

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Oldtech

Subscriber
Last Subscription Date
12/14/2019
I'm not sure what the 17 degree mark is about. Mine doesn't seem to have it. Ill try to put a couple of project pics.
 

Oldtech

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Last Subscription Date
12/14/2019
I found my 1926 Reference book. They have a convoluted method of timing the distributor using the 17 degree mark. I like my method better. It confirms firing order 142635
 

GMCtrucker

Registered
I found my 1926 Reference book. They have a convoluted method of timing the distributor using the 17 degree mark. I like my method better. It confirms firing order 142635
Thats cool. Ours is running better. Just dealing with the idling which I believe is the Marvel carb. Adjustment.
Thanks for all your help Oldtech.
 

Oldtech

Subscriber
Last Subscription Date
12/14/2019
Yes, adjusting marvel carbs is a bit of black art Basically... set the big knob flush with the end of the lock spring, then adjust the mainjet on the bottom for best high speed operation, then fine tune the big knob for idle. I think the settings change whenever the temp or barometer changes. Are you running a vacuum tank?
 

GMCtrucker

Registered
Yes, adjusting marvel carbs is a bit of black art Basically... set the big knob flush with the end of the lock spring, then adjust the mainjet on the bottom for best high speed operation, then fine tune the big knob for idle. I think the settings change whenever the temp or barometer changes. Are you running a vacuum tank?
Hello Oldtech. Sorry for the late answer. It runs better now thanks to your advise in the carb. Yes we are using the vacuum tank.
 

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GMCtrucker

Registered
Re: GMC 1928 T20 TRUCK restoration.

heres the pics of my truck..Canadian made.. has been in the family since new..
ive had a good start on it...doors are off, all new glass, just about time to do the body work(just the fenders) im leaving the green the way it is

cheers
Nice photos. The dataplate one have the info I was looking for. Thanks a lot.
 

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