GMC Western States

 Tech Center Number 19 - October 1997


Edelbrock is now offering an air / fuel (A/F)ratio monitor that works great to monitor the effects of changes in altitude, fuel quality, engine load or throttle position. The monitor can also be used as a tuning tool to help calibrate your carburetor for reformulated gas. The unit consists of a LED display that covers ratios from 12:1 to i5:1 (14.7:1 is perfect combustion). A standard Bosch oxygen sensor, wiring harness, exhaust pipe bung and installation and carburetor tuning instructions are also included.
Installation is simple using hook and eye tape to mount the display to the dash. A hole must be drilled in the left side exhaust pipe as close to the manifold as possible and the bung welded in place. This is an easy job for a muffler shop. If you are running headers, the sensor installation should be either in the collector or just aft of the exhaust pipe flange.
I installed the unit before the trip to Bryce Canyon. Near sea level and at part throttle, my coach was running at an A/F ratio greater than 15:1 even though I had increased the primary jet size from 69 to 71. While climbing or under load, the meter indicated that the power piston was opening at about 5 in. of vacuum and that the coach was running between 13:1 and 14:1 which is about right for maximum power but the vacuum point was slightly low. The power piston should begin to lift at 8 in. The low lift point was the result of my cutting off about 11/2 coils from the spring in an attempt to increase mileage. As I climbed to the higher elevations around Bryce, the air fuel ratio fell into the 14.7 range at cruise, but was a little too rich under full throttle. I later replaced the power piston spring with a correct one. This solved the low vacuum cut-in problem. My next change will be a 72 primary jet to enrich my cruise mixture.
While the A/F monitor does not provide the benefits of a modem, computer controlled fuel injection system, it does allow you to spot carburetor problems under driving conditions. Of equal importance, the monitor allows one to achieve an enriched mixture ratio by throttle position during long hard pulls. The enriched mixture should minimize the potential for burned valves and cracked manifolds. The unit sells for about $140 and can be obtained through Pep Boys or any other Edelbrock dealer.


Cracking of the lower control arm on the front suspension has been evident on several high mileage coaches that have had ball joints replaced. The cracks develop around the lower ball joint attachment point and have caused complete failure and dropped suspension. Reported failures to date have been under low speed turning conditions. The problem is thought to be related to loosening of the side bolts and enlarged holes resulting from drilling out of the old rivets when the ball joints are replaced. Preventative maintenance includes inspection and re-torquing of the bolts. If your coach has about 100,000 miles on it, have it inspected and repeat the inspection at intervals, such as every time the wheel bearings are packed. Replacement or remanufactured heavy duty arms are available from several sources including Cinnabar, Buskirk Enterprises and Eric's RV of Fresno, CA. In an emergency, a Toronado or Eldorado arm can be used but it should be beefed up by a competent welder.

California Smog Update

For Californians: The State Assembly and Senate have passed, by wide margins, two bills that should be of interest to most of us. SB 42 allows any vehicle built before the model year 1974 to be exempt from the annual smog test. After the year 2003, a rolling thirty year exemption is provided. The other, SB 772, eliminates the visual inspection and allows the use of aftermarket equipment if the vehicle passes the tailpipe test. SB 42 has been signed by the Governor, but SB 722 that would have allowed aftermarket equipment has been vetoed.
Visit our web page on the World Wide Web (http// if you have access to the Internet. Parts and service suppliers are listed along with this news letter. If you have any service tips, questions or other useful information this is the place to post them. Postings will also be printed in a subsequent news letter.


Since Darrel has rebuilt and replaced dozens if not hundreds of engines, some of his thoughts seem to bear repeating. Some are beyond our usual capability, but they all provide an idea of the care that Darrel takes:

* When checking the torque converter, the height {thickness) should be 6 9/16" plus or minus 1/16". Any more indicates a bulged converter.

* After an engine failure, both the oil cooler and lines should be flushed after replacing the engine to remove any debris from the failure.

* Check oil lines for evidence of seepage.

* Remember, when replacing oil lines that the fittings have left hand threads.

* Replace with "high temperature" hose (blue line) using the old fittings, which can be swaged on by a supplier or implement shop.

* Up to a 1/4 turn on the crankshaft pulley nut without any resultant movement of the distributor rotor is permissible. Any more is too much.

* Robert-Shaw thermostats provide optimum flow. 195 F is recommended.

* Use soft radiator hose, not corrugated which may strain fittings on the radiator.

* Replace valves with stainless steel.

* Use 91 octane fuel at all elevations! [No wonder Darrel gets so much for his engines if he is using 91 octane!]

* When replacing the radiator, use a minimum of 14 fins/inch on the core. (Darrel uses four core, 20 fins/inch with his premium engines.)

* Never grind valve seats more than once. The induction hardening will be ground off with the second grinding.

* Maximum rebore on an engine is 0.040". Use a torque plate when reboring.

* Index the crank so that the throw is the same on all pistons.

* Finally, be sure to prime the rebuilt engine with oil before starting.


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