TOPIC: 1987 Chevy C70 Wrecker 427 cid The Gas Hog

87' Chevy C70 20 ton wrecker
76,000 miles
427 High Deck motor
Holley 4 barrel carb
HEI ignition

This vehicle is owned by a friend of mine, he agreed to let me play with it.

Grooved only, he will play with the plug gap at some point

FUEL MILEAGE before 4 mpg after 6 mpg

ADDITIONAL starts better, holds a gear longer on hills, runs smoother

Woo-Hoo Prescott! 50% ain't nuttin' to sneeze at!

A nice 'lil 20-ton capacity grocery getter, eh?

Have ya thought about a PCV re-route (does it have one?)

So does Big Mo even have a computer is it a feedback carb, or just straight up old
school GM w/ an HEI distributor? Tell about the carb, what style type model#
Holley is it. Is it the factory carb. or is it a replacement one? Pics tell a Lot!

If it's old-school--a Colortune clear test spark plug could be a great tuning aid
for setting fuel ratio (jetting & idle mix.) Google Colortune to see what I
mean... BTW, a Colortune is used strictly in driveway (no load) Do not Drive on
the street w/ a Colortune installed it will blow out!

(If Big Mo was a Real Battleship we would ask for a full 9-gun Broadside on Video
to celebrate your sucess!)

Good for You, Prescott!

TracyG Gadgetman Reno

Hey Prescott,

Nice

Something to look at, along with the PCV, tuning the idle screws for max vacuum.

Also, with the carb, need to look at fuel pressure.

My 454 with the Edelbrock 750 cleaned up real nice after the groove, moving the PCV and tuning the idle for optimum vacuum.

Typical fuel pressure is approx. 6-7psi, if you can get it down to 4-5psi at idle
you would probably see more improvement on the mpg. Can't trim to much though, can't starve a work horse.

Karl Fortner
Tacoma, Washington

I can assume this vehicle is pre-1986, so put that in the original post as well if you don't mind. I would also like to see this reported under the Success Stories section!

As it is carbureted that is the main point and demonstrates the effects without re-jetting. 50% is EXEMPLARY mileage gain!

Before going further with that aspect, I would recommend getting some carb spray and check for vac leaks on the manifold. Many of these GM engines are very weak at the intake-EGR ports on the intake gaskets. A check for torque would also be advised.

Also, remember the fuel burns faster, so you may recommend to the owner to retard the timing by 2-4 degrees to see its response. Now, you guys know these must be in order, right?

1) Eliminate vacuum leaks.
2) Adjust timing for performance.
3) Ensure good plug wires.
4) Regap plugs.
5) Adjust timing.
6) Rejet carburetor.
7) Rejet again.

With the Holleys, most of modern design have metering rods that can be changed, but Tracy is the resident expert on those. This makes it a WHOLE lot easier to rejet.

SOME things the manufacturers design actually ARE beneficial!

Let's all work together to make this Wrecker a Poster Child for The Gadgetman Groove!

How about we shoot for double digits?

Another Thank you to all on this Thread!

Prescott, could you tell us what year Big Mo is? Come to think, it likely has little in way of Emission controls, being that big of a truck. (Something that big was likely Launched instead of rolled off an assembly line!)

Is the Holley carb original to the engine or a replacement? Is is a square or spread bore. If original equip. it might be an unusual one as this is a Heavy duty application. Several close up shots without the air cleaner would help a lot.

In any case Holley's use open jets for main circuit metering, no Metering rods. There should also be a vacuum controlled Power Valve.

It's the Carter AFB/ Edelbrock Performer carbs that use Merering rods w/jets in the primary barrels.

Anyhow Ron is correct in his Order of Battle for tuning.

And Karl is on about fuel pressure issues.

Prescott you have started a Landslide w/ this one!!

I hope everyone likes my Big Mo nickname for this one (I'm a Battleship nut!)

So Prescott when you are able to give us more data and hopefully some pics then we can all dig in w/ specific advice for this truck:)

TracyG Gadgetman Reno

This is a prime example of the need to document your modifications. Not only with the Mod Report Form, but with Video and/or stills.

If it's the first time you've performed the modification on a particular configuration, then document it thoroughly and present it here and on your own YouTube channel!

If you don't already HAVE one, GET ONE!!! At first, you are limited to 10 minutes, but that is more than enough for anyone that I have ever done.

This mod was performed in August, I received feedback on 01/08/13. Not sure if the Carb is a spread bore or not, he did not know.
Rerouting or capping the PCV is part of The Groove procedure, so is a given. Checking for vacuum leaks is also part of The Groove procedure, so another given.

My friend is a mechanic with his own shop. He understands the capabilities of the HEI ignition system and the potential to gap the plugs to .050 and timing. This truck would make a great poster child, but given the frequency of use, I will not be spending anymore time on it. If he gets an itch to play with this truck some more, I'm sure he will call. I could do over $1000 worth of mods and get the mileage into double digits. Hay, the gas mileage is great when the vehicle is sitting in his garage. You agree?

True, Prescott!

(Battleship stuff I can't help it w/ this one!)

And Big Mo isnt using any Navy Distillate fuel or using any 16 inch shells now that she is a Memorial at Pearl Harbor either...USS Missouri BB63 where Japan signed the WW2 surrender documents on the quarterdeck w/ Gen Douglas MacAurthur and others Sept.2 1945 in Tokyo Bay...(I've been aboard briefly and stood there there's a Plaque in the deck where the tables were placed!)

But the Gas Hog will sail forth to rescue a Semi truck again one day...and earn her Owner some $$!

So if there is ever a need, post info about the carb and I will pitch in if needed!

TracyG Gadgetman Reno