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Guys....I got a 67 GTO stock 400 with 1973 heads. I got pinging problems. My GTO had a HEI distributor. I need to know if the vacuum line should be on port or manifold vacuum first thing before I try to reset the timing. Any info or advice would be greatly appreciated.......Joe
 

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Port is for yuppies and cheese.

Ya. The port timing is for pollution controlled devices, mostly. I experimented with that on my 455 w/ '73 heads, and thought it ran fine at one point w/ ported, (which it did), but that was part of my hot running at idle issue that had me ripping my hair out. You want FULL ADVANCE at idle. (Lest ye be running smog crap). Most anyone on this forum can attest to this. (That's where I got my info! ;)) There may be special circumstance where you'd want ported, but you'd have to do a little research on that. I can run mid grade gas w/no pinging or clatter @175 psi compression, and cause an uproar with the neighbors. Make sure you follow these folks advise on how to set your timing advance curve, valve lift settings, carb and all that, and you should be a happy camper! For best results, DO NOT cut corners. Do it right the first time and save yourself $$$ and heartache!!! Take it from a hardhead who "thought" he knew GM! This he'ar tiger is a totally different animal! :cool
 

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Manifold. But disconnect and plug the vacuum line when you set the base timing. THEN reconnect the vacuum line and you can set the idle speed and carb mixture.
 

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Excuse my ignorance, but when you say port, are you talking about carb?

I have a regular distributor and my vacuum advance is connected to my carb at the base.
67 GTO 400 block, Dominator carb.

Should I have the vacuum advance connected at the manifold then?
 

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Excuse my ignorance, but when you say port, are you talking about carb?

I have a regular distributor and my vacuum advance is connected to my carb at the base.
67 GTO 400 block, Dominator carb.

Should I have the vacuum advance connected at the manifold then?
Port would be on the carb, which only provides a vacuum signal at part throttle. Manifold vacuum is found at of course the manifold, and/or at the base of the carb. Simply check the port with your finger when the engine is at a closed throttle idle. Some of the carb ports have vacuum at various throttle positions. Best way to check is buy an inexpensive vacuum gauge, which is a high value tool when tuning these cars!

There are different schools of thought on port vs. manifold vacuum for the dist advance. Most would say manifold, however in the Cliff Rugles qjet book, he leans towards ported vacuum, and performance tests did not show an advantage with either source. I have tried both on my "mid-performance" 455, and really cannot tell much difference, although it will idle better with manifold vacuum.

Do a search for the tech article "tuning your gto" or something like that, which was posted by LARS, it has some good information.

STEP #1 -- Buy yourself an adjustable timing light, and FORGET about initial timing settings at idle. Only concern yourself with total mechanical advance with the vacuum canister disconnected, should be 34-36 degrees. AND, total advance with FULL vacuum advance connected, (i.e. hold the throttle at steady 3,000 rpm while checking total advance) which will be in the high 40's or low 50's. As long as the engine turns over and starts ok, your initial timing is fine. If you are getting pinging or knock at any RPM, you either need to run higher octane fuel, or back your timing off a little.

It makes sense that car-guys with limited experience are concerned about initial timing, as that is how the factory sets it up, but how many of these cars are actually "factory" anymore? Almost none. The above process is really easy to get the hang of with just a little reading & practice.
 
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