Well, I finished up my project truck last week and I havent got out all
the bugs yet. Its a 1989 GMC Ext cab 4x4.I dropped in a new GM crate
engine(350/290hp)and converted it from TBI to carby.Heres a list of parts
I used,Edelbrock intake(2701),Edelbrock 650 AVS off-road carb, Mallory HEI
distributor, Taylor wires, that might be all you need to know. Actually, I
used the stock electric fuel pump in the tank and used a holley by-pass
style regulator.The instructions suggested making the return line the same
size as the feed line(its 1 size smaller)I left it,maybe its causing my
problems, I dont think so.Anyway,I have a fuel pressure gauge,and I tried
different fuel pressures(4,5,6,and now 7 psi)The truck seems to bog or
starve for fuel after it gets warmed up.Putting around town seems OK. On
the highway,it tends to bog.It really dosent have a whole lot of
power.Having trouble breaking the 120km/hr mark. It seemed to work better
with the higher fuel pressure,but at 7 psi,its still not working all that
great.The instructions with the carb suggested no more than 6.5 psi.Starts
up great in the mornings, burning rich(darn mixture screws)in the
mornigs,black crap all over the side of the garage.Its a 3 speed
automatic, in second it climbs the RPM's pretty fast.In third not much
power.To much fuel,not enough Idont know. I set the timing at 12 BTDC at
1000 rpm. Please help,sorry it was so long.
Set it at 12 BTDC but at a lower RPM (about 600) to make sure no
mechanicale advance is starting yet and with advance disconnected as
well. YOu want about 38 degrees of total advance by 3800 RPM or so
with most of it by 3000 and atleast 20 to 22 at 2000 RPM. YOu want to
use 89 octane or better to as do not waste your time with 87 if you
want it to run. Yu can try 14 BTDC if it is not ping but keep total
WOT advance at around 38 max. Check you plug color too. Also I am not
familar with that intake but if it has a big phlenum and big runner it
will be soogy at lower RPMs and hurt responce. Carb size is okay but
need plug data to say more about mixture. One more thing, dual exhaust
can also hurt lower RPM responce as well so you may have several
things that are not quite right yet. I need more info to suggest any
Oooh... the 290hp... My nemesis. I got one too, it's in my 1986
Blazer. Put on a fresh pot of coffee then Google my name in this
newsgroup. Have fun. For kicks, Google the GM part number as well. I
found more than a few forums of people discussing similar problems with
Seriously, I've had one hell of a time getting that thing to run right.
I've got the Quadrajet manifold off the old 305 motor on there, and
it's topped of with an Edelbrock 600cfm carb. Very similar to your
setup, as I believe the AVS is based on the old Carter AFB carbs, same
as my Performer series. Only difference is that you're probably not
using a carb adapter.
I started checking everything I could find and then did more learning
and more checking. I don't know if you've checked, but I'll be willing
to bet that you, like *everyone* else who has that engine will measure
lower than normal manifold vacuum readings at idle, and notice a very
rich-running engine. If it's like mine, you can probably smell the gas
fumes from 3 houses away while it's idling.
I don't think it's fuel pressure related. You seem to have addressed
that before you got started, so I think you're in the right range. All
my knowledge on this subject pertains to my own truck. You can try
playing with the metering rods and jets to try to get it running
alright, but I wasn't successful in doing much with it.
When it was all said and done, I realized that the cam profile was from
an old 60's-era Corvette, and was intended for use in a
higher-compression engine (10:1 as I recall). The 290hp motor has
around 8:1 or so. The power doesn't peak in that engine until you hit
around 3700rpm. I don't know about you, but I don't really break
2000rpm that often in mine, and that's only under heavy acceleration or
while doing 80mph down the interstate.
I am going to try advancing my cam a few degrees before I replace it.
exhaust lobes overlap more than what would be ideal for an engine being
used to move a heavy truck. I wasn't able to prove my theory, but I
think it's logical to assume that a lower than normal vacuum level may
throw off the carb operation. The engine just seems like a mis-matched
collection of parts/specs.
BTW, you didn't mention it, but I'm assuming you're getting around 6mpg
Well, the electrode and ground on the spark plugs seem fairly
clean(blackish residue).The bottom cicumference of the plug however(ring
around the bottom under the threads)is quite black and sooty.I advanced
the timing 3 or 4 degrees,than backed off a degree or two(until I couldnt
hear spark knock under load.My fuel pressure is up to 7.5 psi. Anything
less seemed to create a fuel starvation problem. Maybe the extra fuel
pressure is compencating for the lack of vaccuum? But still creating rich
burn at low RPM.Anyway, the truck seems to pull all the way through the
rev range now.Still seems like a 200hp engine instead of a 300hp
engine.Maybe your're right, maybe the cam wasnt made for a 5400 lbs
truck.Maybe 35 inch tires and 3.42 gears arent really the answer. If I
geared down,maybe 4.10's,swap in a good set of edelbrock heads,change the
cam shaft, maybe then I'd have the power I was looking for.I'm not sure
about gas miledge,(havent been running it enough),I know when I was
stepping on her, opening the four barrels for tuning purposes, comin right
on to her, Uh,ok,you get it ,she was drinking back alot of fuel.The truck
is working OK,but there somthing not right I beleive.I may have to get
some old hot rodder to tune this damn thing. The carb probably needs to be
tuned for the heavy truck,althought it is an off-road carb,maybe its set up
for a heavy truck.(Remanufactured Carb,maybe problems from the factory.I
dont know.Maybe timing.Either I'm loosing 50hp in the tuning, or maybe my
expectations for the engine were to high.
Thanks for help Guys, Keep it up.
Ick... 35's and 3.42's won't pull well. I had 3.08's with 31's and
according to the calculator at
http://ringpinion.com/content/calculators/RPM.asp , my RPM's @ 60mph
would be 1402rpm. Yours at the same speed would be 1379rpm. Slightly
less, but I know how mine felt, and couldn't imagine yours being any
better. I got better performance after switching to a 3.73 gearset. I
also don't know what my Blazer weighs, but I'm assuming no more than
your truck, if not less, so that's probably not helping much
either...not to mention the added rolling resistance of your bigger
I ended up semi-permanently mounting my vacuum gauge on the dash to
watch things happen real-time as I drove down the road. It was
connected to the front manifold vacuum port on the carb. Basically, I
was watching to see when "power mode" would kick in on the carb, which
is when the metering rods hit the full "UP" position to richen the
mixture. I could definitely feel it as I saw it on the gauge, and it
may be worth your time to experiment with different metering rods/jets
to get it to run "decent". I say "decent", because I've not yet been
able to achieve anything close to what I'd consider "great" with my
Concerning getting an "old timer" to help...well, an Old Timer put the
motor in mine, and deemed the stock Quadrajet to be in need of
replacement based on the fact that he couldn't tune it properly.
That's where it started. I immediately got the 1406 Edelbrock (elec.
choke, vacuum secondaries, 600cfm) based on someone else's
recommendation and went from there. I've had an experienced mechanic
friend assist as well, and we've never been able to get it to where
either of us thought it ran as it should.
As it stands now, I'm still saying it's the cam. It's simply meant for
a smaller, lighter car, in a higher-compression engine, for higher
RPMs, since like I said, it *is* from the old 1960's 327ci/350hp
Corvette motor. The overlap between the intake and exhaust valves
isn't good for idling or low-end response. I'd investigate that before
considering different heads. From what I hear, the standard Chevy cam
(the cam that would've come in the 260hp motor) is a better choice, and
is cheaper from GM than an aftermarket equivalent.
At 7.5psi of fuel pressure, you should be having ZERO fuel starvation
problems. My mechanical fuel pump (stock) puts out less. I'm still
leaning toward low vacuum causing problems atomizing the fuel properly.
It's getting gas--and PLENTY of it obviously, same as yours. I just
wonder what it's doing with it once it hits the carb. I did just
recently (in the last couple of weeks) get interested in booster
venturis and how one size or design may work better in this engine.
But in the end, I think I'd simply be trying to compensate for
something that's very wrong to begin with.
You said there's something just "not right" about it. That pretty much
sums mine up too.
Regardless, I am *very* interested in how this all plays out for you,
as it seems you're in exactly the same situation as me. I ended up
buying a smaller car for my 30 mile commute to work, and the truck is
parked and has been declared as being in "permanent project status".
I'm currently regrouping and trying to decide what my next move will
be. Luckily now, I can take my time because it's no longer the daily
driver, and I don't have to worry about whether any changes made will
result in me not having a ride to work the next day ;-)
I'd appreciate you keeping us posted on your progress, as your success
in fixing the truck will most likely influence the direction I go with
Thanks for the input. I will let you know if I am able to "wake her
up". If I cant dial her in than I guess I'll be doing some research on
cams. We need one that brings on the torque. A cam designed for getting
heavy machines moving.Maybe all I need is lower gears and 100 more ponies.
Anyway, I let you know if I make any progress.
35s with 3.42 ratio!! That engine probably doesn't start making power
until it hits 2,000 rpm at least. With those tires and gears your
probably not turning 1900 at 60 mph. Switch the gears to about 4.88:1
and you will be in the power band of the engine.
That engine was designed for something like a lighter car NOT a truck.
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