I will do a compression check in the morning as well as try to get a vacuum reading. This carb is brand new however, I have also put a slightly used carb(750) that was too much for another truck about four months ago onto this truck, as well as a 670 Holley that I know worked great on a Ford 351 earlier this year. As far as the timing goes the only thing I’ve been able to do, I advance my timing light 32° because I can only time it while it’s revved up so while doing that at about 3000 RPM my harmonic balancer reads zero. I haven’t been able to check what my idle timing is because I can’t get it to get below 1800 and you constantly have to be pumping the gas. I’ll post compression readings as well as a vacuum reading and hopefully everything will start to fall into place. Thanks again for all the advice.
Greetings Cookscustoms,
I've got an idea, but first I wanted to recap/comment on the good troubleshooting steps you've taken to date.
First, setting your timing light to 32° advance & then setting the timing mark to 0° at 3000 rpm was a good move
to combat not being able to set the timing at idle speed. (As far as I'm concerned, the base timing should be close
enough at least to get the engine to idle. (Depending upon how much total timing curve is in the distributor it could be
close, or still need a small handful of advance.)
Also, substituting a 'known-good' carb just to cross-check the idle fuel delivery was very good -- that takes a *lot*
off of the troubleshooting table.
****
OK, punchline first, followed by the theory after. Here's a passage from page 5 of the Comp Cams installation guide.
Check out what I underlined:
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(credit: Comp Cams installation guide .pdf (
LINK)
The older 454 big blocks used an adjustable valve lash setup, via studs & self-locking nuts. On the other hand, the
'99 L29 454 did come with factory roller lifters, but at the same time a
non-adjustable valve train setup is
employed, replacing the studs with
bolts torqued to 40 ft/lbs to hold the pivot balls/rocker arms in a set position.
We haven't discussed pushrod length yet, but IF you used the ones that came with the engine then the following
might be the key to the no-idle mystery:
I found a donor 454 out of a 1999 three-quarter ton suburban so That way it was a roller motor. I then took it to my machine shop where they bored it .30 over,
decked the heads, did a three angle valve job, and put on beehive springs to go along with the cam That I had chose for this truck.
^^^^^^^^^^^^^
IF you are still running the non-adjustable valvetrain AND the stock length pushrods,
then if the heads and/or block were decked a sufficient amount, *then* you now have
effectively even
longer than stock pushrods, which would lead to excessive hydraulic
lifter pre-load. Which, according to Comp Cams, will give you "low manifold vacuum."
Again, your willingness to temporarily swap in a known-good carb, install a new
fuel pressure regulator and measure the output PSI, and get the timing set at
32° at 3000 rpm are solid troubleshooting steps. In addition, the compression
test you made at
*cranking speeds* tell us that the basic valve seal & piston
ring seals are good enough to support a running engine.
So, it really looks like the only variable is that when the engine is no longer
cranking over at 150+ rpm, but is now (trying to) run at 1000-3000 rpm,
now the oil pressure is way up, and with too much lifter preload the valves
are being held just far enough off the seat to cause the low vacuum that
Comp Cams warns about? (Either loss of dynamic compression from
the intake valves not completely closed, or dilution of the intake charge from
uncontrolled EGR from non-seating exhaust valves...or some combo of both?)
****
This is a tough problem. And for all I know the machine shop already measured
everything during assembly and had to order a set of custom-length (shorter)
pushrods in order to make the non-adjustable valvetrain work with the 'decked
heads' and/or block. OR they upgraded to the adjustable old style screw in
studs...yet still managed to set the preload too tight? It happens.
Q: Does all this make sense? I'm hoping that I am describing all this clearly.
The bottom line is that the factory length pushrods are correct for the L29
motors being built to stock specification on the original assembly line. But as
soon as any changes are made to any of the parts involved, then pushrod length
checks are a must.
You know the old truism -- "Spark + Fuel + Compression = it has to run."
The fuel delivery looks good enough to idle. Assuming the spark system
is good & a set of fresh plugs still don't cure the no-idle issue, then all
we have left is compression. And the basic mechanical setup is healthy...
we just gotta figure out the lifter preload per the Comp Cams installation
manual.
Best of luck sorting this out...we're rooting for ya!
