Hey guys, 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
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 more.
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 it.
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 in town...no?
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.
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
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my RPM's @ 60mphwould be 1402rpm. Yours at the same speed would be 1379rpm. Slightlyless, but I know how mine felt, and couldn't imagine yours being anybetter. I got better performance after switching to a 3.73 gearset. Ialso don't know what my Blazer weighs, but I'm assuming no more thanyour truck, if not less, so that's probably not helping mucheither...not to mention the added rolling resistance of your biggertires...
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 truck.
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 mine.
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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