Home » Public Forums » GMCnet » Tune and timing B4 the Big Trip
| Tune and timing B4 the Big Trip [message #214424] |
Mon, 15 July 2013 08:55  |
xplorid
Messages: 213
Registered: February 2012
Location: Boise
Karma:
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Senior Member |
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About set to leave on our 1st RV trip ID-NY and back, I want to tune the 455. I have a timing light, tach meter, a new vacuum gauge, and very little experience on this vehicle. I have read all the past posts I could. They seem to say (with a Patterson distributor on a '76 engine) -
plug the vacuum line to the distributor
set idle at 1100 or set idle at 550-600 (different posts)
aim for 12 deg initial timing with this distributor
reconnect vacuum, start engine, and set idle
adjust the 2 mixture screws on the distributor 1 at a time to max vacuum
reset idle if necessary
drive it and adjust from there, with pinging under load as the guideline for further timing adjustments.
Something like that. So here's my thing: I drove from Florida to here last year after purchase, and other than some vapor lock it ran well enough, 9+mpg, and about a quart of oil every 800 miles.
Last fall the distributor failed because of an improperly mounted cap and rotor so I bought the Springfield Ignition thru Applied. A GMC buddy here (another Jim) plopped the new distributor in and dialed it quickly saying "that sounds about right". Ran great and I did not think about again until this weekend, prepping for the big trip.
So, I hooked up the light and meter (no vacuum gauge yet) and found the timing was at around 17 at 1100 rpm. Remember, it was running OK, no pinging even pulling up the hills around Boise on a hot day. I backed it down to 12, reset the idle and never touched the mixture. Started fine, maybe a little surging on idle, not sure yet. On the test drive it ran OK, but maybe did not seem quite as anxious to get up and go.
The other strange thing is that the HVAC system misbehaved when the engine was under load, like it losing the vacuum needed to direct air flow - it moves to the floor vents until the engine load lightens.
So, big question is should I advance the timing back up to the 17 range since it was not pinging before? Little question is does the HVAC behavior indicate a vacuum leak somewhere?
Sorry for the longer post, but the anxiety before the big trip is getting to me. I want to make this as worry free for my wife as possible, as this experience will color everything that comes after.
Thanks,
Jeff
1974/94 GMCII by Explorer
Manny 6.5 TD Al radiator
1 ton front 4 bags back
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