Worthless Midrange

[rockman20]

Okay, I have my 82 Chevy Scottsdale that use to be diesel but now has a 350 gas in it. (NOT MY CHOICE!) This engine has around 7 or 8,000 miles on it now since I got it from the company that built it. Everything should be pretty much stock inside, but I did ask them to cam it for a 4WD tow vehicle. (What cam they used, I am unsure of. I am still trying to find my papers to see if I can find what lift and duration they used)

All I can say is that I am not pleased. The pickup has tremendous power if your foot is in the floor. It gets up and moves pretty dang well. But when my foot isn't in the floor (hard on gas that way!) I have to say that I am not pleased at all with the power.

On its own, it doesn't do too bad. But if I am towing something, I loose all power. If I push on the throttle a little harder, I can feel the pickup kind of jerk very slightly almost like it is low on fuel or something. But if I push slightly harder and it kicks down a gear, it moves just fine.

I haven't done much diagnosing here yet, but does anyone have any suggestions? I want to pull the plugs soon to see what they look like.

Everything else seems fine. It has new plugs, used wires, new cap and rotor, old coil (HEI), it has a new module in the distributor, stock everything else less the cam and intake.

Any ideas to throw my way on what to look at? (TH700R4 transmission with stock converter and 3.40 gears also)

[TIMSPEED]

If I was towing something, I'd move up to a 3.73 gear. But what it sounds, is since it's a 700R, it's going into 4th (.70 gear) and it's not spinning the tires with enough power, then once you get it to downshift to the 1:1 gear, it does ok.

[rockman20]

It has this lack of power around town to. It seems to be right in the mid range. I am kind of wondering if they didn't cam this engine wrong. WOT it's like a power demon, but in the mid it just isn't there.

If I get into 2nd and keep it there, get it under some power just to where it is ready to downshift, I get the same results.

I am thinking of going to 3.73 but I do not have the cash to replace front and rear. I do need to do something soon though. My carrier in the rear is shot. The spider gears are pretty loose in there. The pin is actually worn and the teeth on the pinion has seen better days.

The guy before me must've really beat on this thing hard.

[plumbcut]

sounds like its a carb/ignition thing.. you seem to know a bit about trucks, so I think you'd know if it was just not very powerful.. you say it jerks at a certain point then picks up power..

4th gear is pretty high on the 700.. what size tire do you have? if you manually put the truck in 3rd gear does it act the same? Im ruling out trans/gearing. Sounds like your carb might be running a little lean on the the first half of the throttle.. that, or your timing advance doesnt kick in soon enough..

highly unlikely that its cammed "wrong" .. a stock to RV grind cam is going to be good no matter what.. .. the wrong cam would be something really radical, and youd know by the idle, etc... I would look into getting the carb tuned and ignition advance checked.. look around for a shop that will curve old distributors, it would be money well spent. keep us posted

pc

[rockman20]

I am going to rule out timing right now. I have gone through and put some lighter springs in the mechanical end of things and although I do not have an advanced timing light, I can see that my timing is right there and is all in right around 2500 - 3000 RPM. The only thing that I dislike is that my stock distributer really does not have much advance in it. I have a MSD on my Cutlass and love the ability to control my total advance. I may think about upgrading in the future on the pickup.

I thought about a carb issue, but it is a Quad that I am running on the pickup right now. I just went through the carb and put a kit in it and got it all cleaned up. I brought the base plate in and had new bushings put in the throttle shafts because they were pretty loose.

Since this is a Quad and I am not running anything really radical for engine, Headers and intake I can not see making a stock Quad lean on gas, I kind of ruled the carb out right away. But I will see what the plugs look like this weekend. I am going to pull all of them and see what they look like and see if I can find anything that may be wrong.

I will keep you guys posted on what I find.

The only thing that boggles me is that very faint jerky feel when under power. The only other thing that I can think is that perhaps I have a small vacuum leak somewhere. I was going to plug off the heater stuff for now to see if maybe it is something in there. We'll see. Just looking for guesses on what I can look at.

[RockHQ]

You need vacuum advance under part throttle, midrange rpms with a steady load to have any power. So, check your vacuum advance and fer ****s sake get a dial back timing light so you don't have to guesstimate?

[rockman20]

But it is so much cheaper to guestimate! I have always been wanting to get a new timing light although I love my Sun. I see that they make a dial back exactly like mine.....in fact......there is a sticker on mine where the dial back would be at.......but that dial looks so cheap. I would like to get a hold of a digital with RPM's on it. We'll see.

Off Topic:

I see you have a 69 455 W30 you are working on.........SWEET!

I have a 69 Cutlass S with the rocket 350 under the hood. Fun cars.

Also, vacuum advance is brand new. Just put it on the pickup about a month ago thinking that may be the issue. And I did check and the advance is hooked to the ported of the Quad. No vacuum until the throttle is touched.

[RockHQ]

Most people hook vacuum advance to manifold vacuum, like was done during the muscle car era. Ported vacuum advance is all emissions stuff.

I'm anxiously going over different parts catalogues for the 455. Most of my friends tell me i'm a moron to be building up an Olds engine for power. It's true i could build 4 stout SBCs for the same money, or buy a crate BBC 454 with more power&torque for the same money, but there's just something intriquing in making an underdog perform. And the fact that unlike SBC's i can't buy every single part i need through summit or jegs.