Engine Pinging/motorvac

[dave1]

[Admin note - "Bounced", text-enriched. Dave, set Eudora Pro for
"plain text" only when posting to GMCnet. Patrick]

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>

>
>
>If you have any deposits on the pistons or in the head they can also
>cause preignition and may require timing adjustment below 8 deg. tdc.
>Use of a good grade gasoline might eliminate these deposits or, if
>really bad, the heads may have to be removed and parts scraped and
>cleaned out. Which, of course, means that you should do a valve job,
>etc. at the same time.

Here is something I came across that is supposed to remove deposits from
the combustion chamber, intake, exhaust, rings etc. To the best of my
knowledge Sun/Snap-On is a reputable company but the skeptic in me says
that its too good to be true.

http://www.motorvac.com/

http://www.motorvac.com/Professional%20Section/tradeindex.htm

Dave
Ann Arbor, MI.
73 Sequoia (26'/455/EFI/HEI/160=B0)

 

[dave1]

>Hi Dave
>
>I really don't think I have deposits on pistons, Atleast I hope not, have
>been running hi grade gas(CA/AZ) till second to last trip, and then only
>went to mid grade gas. for about three/four fill-ups. Also a sucker for
>those hi priced additives, hope that didn't do it.

I agree, you probably are not having the pinging problem because of
deposits. I just posted the Motorvac thing because it was interesting. I
suppose I should have changed the thread.=20

But... I do have a 73 Sequoia with a 455 and I too had the pinging problem.
I always had to run 93 or 94 octane gas to get it to stop pinging. I tried
retarding the timing and that helped but then it didn't start as well. So I
fixed mine by upgrading the ignition to the Delco HEI ignition system and
upgrading the fuel system to the Howell closed loop EFI system.=20

You could probably solve the mixture problem for less money (than the EFI
system) by re-jetting the carb to run a bit richer. It seems that the gas
avalable today makes these older engines run leaner. That means hotter and
more likely to ping. A number of people here have posted that they have
gotten good results by going one or two sizes larger on the main (primary)
jets.=20

It is also possible that your centrifugal advance and/or vacuum advance
springs have become weaker over the years. And, if I recall correctly, the
conventional ignition system has a more aggressive vacuum advance than the
HEI system. Maybe a good (ie cheap and easy) starting point would be to
change your vacuum advance to 10 degrees (make it the same as the HEI
system). Unfortunately I don't have a part number for the 10 degree vacuum
advance motor used on conventional Delco ignition systems.

If you decide to change over to the HEI system, I found that the curves on
a 75 Toronado distributor (Cardone 1893) are very close to those needed for
the GMC when you change the vacuum advance to a Delco 1973577. You will
also have to run a wire directly from the ignition switch to the HEI
distributor. The B+ wire that supplies the conventional ignition is a
resistor wire and can not be used with the GM HEI system. I bypassed mine
with 12 gauge stranded copper wire.=20

Given the cost of gas these days, a new vac advance, new centrifugal
advance springs and new jets in the carb would have a quick payoff if those
changes will allow you to go back to using regular gas.

Dave
Ann Arbor, MI.
73 Sequoia (26'/455/EFI/HEI/160=B0)