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Jul 21st, 2025 | | | | Joined: Nov 2009 Posts: 639 'Bolter | | 'Bolter Joined: Nov 2009 Posts: 639 | While looking for a 270 for my truck, I ended up with two of them.
The first is out of a '58 two ton with a sealed valve cover and what looks like a rudimenery PCV system. It also has a sticker of some sort on the side that says its a remanufactured engine.
The second is from a '59 1/2 ton with a vented valve cover.
I noticed other subtle differences between the two, but the thing that puzzles me the most is the engine from the '58 2 ton does not seem to have vacuum advance. The distributer is fixed in place and has no vacuum line to it.
Is this normal on a two ton truck?
The engine from the '59 1/2 ton has vacuum advance.
| | | | | Joined: Mar 2005 Posts: 1,629 'Bolter | | 'Bolter Joined: Mar 2005 Posts: 1,629 | Could be. The vacuum advance works when there is some vacuum to operate it. In a larger truck under a load there may not be enough vacuum to operate it. The mechanical advance in that distributor will do the timing advance instead of depending on a vaccuum advance.
Even in the half ton trucks if the throttle is wide open under a load (pulling a long steep hill) the vacuum advance is all but useless. The mechanical advance takes care of the timing.
| | | | | Joined: Sep 2001 Posts: 29,262 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,262 | Have you checked a GMC Maintenance Manual or Master Parts Book?
Recently, I have been working on a GMC 350 (2-ton) 270 engine. It does not have a vacuum advance unit, and I was told this is normal in GMCs larger than 1 ton. I am not near my manuals, so I cannot check this out.
| | | | | Joined: Apr 2011 Posts: 181 Wrench Fetcher | | Wrench Fetcher Joined: Apr 2011 Posts: 181 | I have a 248 with an HEI on it and multiple carburetors and have no Vacuum Advance, It has run this way for over 20 years with no problems. Not too god for fuel economy but great on power.
Normbc9 | | | | | Joined: Apr 2005 Posts: 1,107 'Bolter | | 'Bolter Joined: Apr 2005 Posts: 1,107 | since they are work trucks they dont require one. the ashton does'nt have one and every gmc 350 and up ive seen didnt have one. as roy stated not enough vacuum to operate it. since i have a vacuum gauge mounted on the dash for the fire side of the truck. the only time the vacuum is above 15" is when im at idle 20-22" or going down a hill with 25" plus. as for the pcv set up if you have the curved pipe from the valve cover to the intake then its original and works well over the vented system which inturn keeps the top cleaner.
my 6100 235 does have a vacuum advance with a vented valve cover. | | |
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