Author Topic: Two part question  (Read 2411 times)

Offline runnergeorge

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Two part question
« on: March 19, 2021, 12:36:51 AM »
Greetings! Iím a relatively new DIYer on a 75 c10 Cheyenne with a 350. This is a two part question related to vacuum advance and warping, I will try to keep them separate as I donít wish to cause any confusion.

Vacuum advance: I installed a brand new distributor and set the timing (with vac line removed and capped) to 8* BTDC and reconnected the vac line. Iíve always been told that the vac line goes to ported vacuum on the distributor and not manifold vacuum. However I just read on another thread that it should always be connected to manifold vacuum to avoid overheating probs and idling issues. Which is correct??

Warping: I recently had an issue where I had to remove and reinstall my aluminum intake manifold and I noticed that when I went to put it back on it rocked forward and back. Iíve heard of intake manifolds warping, however my actual question is if the heads are warped would that mean the block is warped as well? I donít have a head gasket issue as of yet.

Thanks for the replies!

Offline VileZambonie

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Re: Two part question
« Reply #1 on: March 19, 2021, 04:04:30 AM »
You can easily check for warpage using a precision straight edge and feeler gauge. Make sure nothing is under the mating surface, wires, gasket material, foreign object, etc. but yes they can be warped. Using the tightening and loosening sequence is the way to avoid warpage.

You will find increased performance with more advance and better throttle response with full vacuum. Using the factory method for setting timing is fine, but will not optimize performance. Back in the day, we used to pop the vacuum advance hose off and set the timing to factory specs, then go through the emissions test lane. Before leaving the parking lot, hood goes up, vacuum hose back on, distributor cranked and rubber laid.
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Offline dusterdude

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Re: Two part question
« Reply #2 on: March 19, 2021, 04:05:21 AM »
Ported vacuum and ive had aluminum intakes tweak a bit before and welcome

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Offline TexasRed

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Re: Two part question
« Reply #3 on: March 19, 2021, 09:32:07 AM »
Ported vacuum was a crutch for emissions testing, hotter engine temps at idle to get the catalytic converter to be more efficient.

It only affects the idle, off idle manifold and ported are the same.

MOST engines prefer full manifold. Try both and see.

You may want to restrict the amount of degrees on the can (today's fuel isn't what it used to be). You may also want to speed up the mechanical advance (by a slightly weaker spring) and advance the initial to maybe 10 or 12* with the can disconnected.

Offline runnergeorge

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Re: Two part question
« Reply #4 on: March 19, 2021, 01:41:13 PM »
Thanks for the replies everyone. I will try both vacuum ports and see which one I like better and probably stick with that, for now. For the warping; if the heads are warped then the only way to verify if the block is warped is to take the heads off and check the block. I am looking to install a Proflo4 system and I just want to prepare myself, and my wallet, in case the heads/block are warped.

Offline runnergeorge

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Re: Two part question
« Reply #5 on: March 20, 2021, 05:19:05 PM »
So update on the vacuum advance. When I switched over to manifold vacuum this morning I noticed the RPMs even out and rise a little compared to the ported vacuum. The truck ran great while I was driving, even kicked the AC on and didn't notice as much of an RPM drop during stops. However, when I turned the truck off the engine dieseled for about 5 seconds after the key was taken out. This didn't happen with ported vacuum. Is this because of the greater advance from the manifold vacuum? I liked the performance of the manifold vacuum, but I don't like the run on. Thoughts?

Offline ehjorten

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Re: Two part question
« Reply #6 on: March 20, 2021, 06:14:23 PM »
You should check your idle speed...hooking up manifold vacuum will increase your idle speed and you might need to pull a few degrees of timing out of it.
-Erik-
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Offline JohnnyPopper

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Re: Two part question
« Reply #7 on: March 21, 2021, 08:13:03 PM »
Some carbs came with a solenoid that steps up the idle lever, never knew what it's function actually was. Might be stuck and not retracting when 12v is killed.

In the world of VW's, the run on was such a problem that they used a idle circuit solenoid/needle to kill the idle circuit completely.

Regarding Timing: VZ did I read in another thread that you should set your timing to 32 degrees advance at 3600 RPM?
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1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
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Offline runnergeorge

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Re: Two part question
« Reply #8 on: March 23, 2021, 04:35:16 PM »
I have the standard 1406 Edelbrock carb installed right now and I don't think it has the fast idle solenoid; if it does I have no idea how to find it lol. But the idle is set to about 850 in park and about 600 in gear. When I get a chance I will be changing the timing and see if that helps the run on; I've always been told that SBC that are closer to stock like to be around 8 degrees BTDC give or take. I'll change it and update when I get a chance!

Offline TexasRed

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Re: Two part question
« Reply #9 on: March 23, 2021, 06:25:10 PM »
8 is probably a little low and not really causing your issue.

Is 850 rpm in park really only giving you 600 in park? I set it at about 750 and it kicks down to 600 rpm in gear.

The solenoid you'll notice. It's at the front of the carb and just pushes the linkage back when the A/C is activated. You probably don't have one because they aren't cheap.

Offline JohnnyPopper

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Re: Two part question
« Reply #10 on: March 23, 2021, 10:47:58 PM »
Best clue; it has a wire attached to it... 8)
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline runnergeorge

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Re: Two part question
« Reply #11 on: March 25, 2021, 03:33:05 PM »
I definitely don't have that solenoid then, the only wires going to the carb are for the black circle thermostat housing on the side. And yea, 800-850 gives me about 600-700 in gear. Since I got the truck I rebuilt the carb using one of Edelbrock's kits, and even before that it wouldn't have the best idle by not being consistent. I never really paid a huge attention to it since my goal this summer is to swap it for the pro Flo 4 EFI System. How much should the RPMs kick down once in gear?