Idle Surge/Starvation/Miss/Vacuum Leak

[dz1087]

Truck has been surging or missing or starving at idle since I rebuilt it. Wanted to get some opinions on what it might be or if it’s just normal. The main reason I notice is because when I’m idling in to someplace, say the garage, or at the drive-thru, the whole vehicle surges forward intermittently. Or maybe it drops off instead of surging. Either way, it won’t idle smooth. It kind of bucks slightly if you just have it in gear and let off the brake.

At speed, there is no discernible miss or anything. Comes off idle quick and is very responsive. Just looking for some info from you all to see if you think this is normal or how I might be able to correct this.

‘71, stock rebuilt 350, rebuilt Q-Jet, stock manifold, breather, etc.

Link to video of engine: http://imgur.com/gallery/6d3nYti

[geezer#99]

Looks like you have the distributor vac advance hooked up to manifold vacuum.
Plug off the vacuum advance to the distributor as a first easy test.

[garyd1961]

Check for vacuum leaks and try advancing the timing a little

[dz1087]

Originally Posted by geezer#99

Looks like you have the distributor vac advance hooked up to manifold vacuum.
Plug off the vacuum advance to the distributor as a first easy test.

That practically cleared all of it up. Video below. I put it in gear after adjusting the idle a bit, and it was smoother than it has ever been just idling back and forth on the driveway. Engine sounded a lot better as well - I could actually hear a bit of the cam lope from the mild cam that is in it.

What does this point to? Bad vacuum advance on the distro? Bad distro?

Video: http://imgur.com/gallery/A66rjB0

[Rich72C10]

I would get two things:

A vacuum gauge and a pump.

The gauge is a must, IMO, you need that for tuning the carb's idle screws (assuming you have a Qjet). Well, unless you are like some guys that can just "listen". Anyways, you can get what the vacuum is doing off your carb and/or intake. Is it strong, weak, bouncing?

The pump has come in handy for all kinds of things but you can use it to test your distributor vacuum thingy to see if it hold vacuum or leaks horribly. I have used it to test anything that takes vacuum to actuate (testing the heater control valve just this weekend).

With that clear/solid information you'll be able to narrow down the issue (or at least have a better time of it).

[geezer#99]

Originally Posted by dz1087

That practically cleared all of it up. Video below. I put it in gear after adjusting the idle a bit, and it was smoother than it has ever been just idling back and forth on the driveway. Engine sounded a lot better as well - I could actually hear a bit of the cam lope from the mild cam that is in it.

What does this point to? Bad vacuum advance on the distro? Bad distro?

Video: http://imgur.com/gallery/A66rjB0

That points to too much advance at idle.
There’s a couple cures.
Limit the amount of vac advance. If you have an hei there are kits to do that. Points distributor is harder to do.
Or the easy solution is hook up your vac advance to a ported vacuum port.
What distributor do you have?

[dz1087]

Originally Posted by geezer#99

That points to too much advance at idle.
There’s a couple cures.
Limit the amount of vac advance. If you have an hei there are kits to do that. Points distributor is harder to do.
Or the easy solution is hook up your vac advance to a ported vacuum port.
What distributor do you have?

HEI distro. I believe it's Summit Racing branded. I don't think it has an adjustable diaphragm.

Should I look at total advance at idle with a vacuum pump hooked to the vac advance prior to moving it to ported? If it idles rough on full vac, is there really any benefit I might see with getting an adjustable advance, like cooler temps while idling?

[dz1087]

Originally Posted by Rich72C10

I would get two things:

A vacuum gauge and a pump.

The gauge is a must, IMO, you need that for tuning the carb's idle screws (assuming you have a Qjet). Well, unless you are like some guys that can just "listen". Anyways, you can get what the vacuum is doing off your carb and/or intake. Is it strong, weak, bouncing?

The pump has come in handy for all kinds of things but you can use it to test your distributor vacuum thingy to see if it hold vacuum or leaks horribly. I have used it to test anything that takes vacuum to actuate (testing the heater control valve just this weekend).

With that clear/solid information you'll be able to narrow down the issue (or at least have a better time of it).

I've got both of those, thanks for the suggestion. I used the vac gauge when setting my idle screws. Probably going to use the pump to check on the amount of advance I get out of this canister.

[geezer#99]

Originally Posted by dz1087

HEI distro. I believe it's Summit Racing branded. I don't think it has an adjustable diaphragm.

Should I look at total advance at idle with a vacuum pump hooked to the vac advance prior to moving it to ported? If it idles rough on full vac, is there really any benefit I might see with getting an adjustable advance, like cooler temps while idling?

If you have a timing light just see what the timing is with or without the vac pot hooked up.
Like I said, simple thing is hook to ported.
Cures your rough idle, doesn’t effect anything else on your timing set up.
The limiter I suggested is one that limits the amount like this one. Not an adjustable pot. The instructions pdf explains it.




https://www.holley.com/products/igni...ts/parts/84281

[dz1087]

Originally Posted by geezer#99

If you have a timing light just see what the timing is with or without the vac pot hooked up.
Like I said, simple thing is hook to ported.
Cures your rough idle, doesn’t effect anything else on your timing set up.
The limiter I suggested is one that limits the amount like this one. Not an adjustable pot. The instructions pdf explains it.




https://www.holley.com/products/igni...ts/parts/84281

Just looked up the distro I bought from Summit and looked at the directions. The vac adv is adjustable on this unit, and from reading the directions, they expect you to hook it up to manifold vac. I'm going to pull the timing light and vac pump and gauge out and do some diagnostics on this today and see what I'm getting from the system.

The instructions state to expect 8 to 10 degrees more timing at idle, so they are assuming manifold here. I'm going to start by checking my timing with the vac adv unhooked and the vac line plugged, then with it hooked to manifold to see what it's giving me. Then I'll play around with adjusting the unit and see what I get. Worst case scenario, I'll just go to ported if I can't get it to stop with the erratic idle.

[geezer#99]

Good to experiment!
Every motor is different in how much and how soon timing needs to be set at.
Have fun!

[dz1087]

Have some more questions after looking in to some things today. I hooked the distro up to ported and set idle rpm and then hooked up the vac gauge and messed with the idle mixture screws to get the highest possible vacuum. Felt like I had to screw them out quite a ways.

Anyway, I’ve included video of what I saw on the vac gauge and the timing light rpm. It seems to bounce around a bit. Is this normal?

http://Imgur.com/gallery/FA6o0rG

I also included video of the distro hooked to manifold vac. Again, I adjusted the rpm’s down. Of note in this setup, this was as low as the RPMs would go. I believe because I had the idle mixture screws out so far.


In adjusting the idle mixture screws, I screwed each mixture screw out until the vac no longer increased. Adjusted in until it went down a little. Then backed out slightly again. Repeated the operation on the other side, then finished with the first side again. The bouncy-ness of the vacuum got better, but the videos show as good as I could get it.

If this erratic vacuum is not normal, what does it look like is going on here? Vacuum leak? Bad distro? I sprayed around the base of the carb with starter fluid, but didn’t find any obvious leaks.

[geezer#99]

That’s not much movement.
One concern though.
You shouldn’t see any vacuum on a ported vac source.

Best way to adjust mix screws is turn them in slowly until the idle gets rougher. Then turn out an 1/8 turn. Do other one the same way.

What’s your initial timing set at?
Post a pic of your ported vac source.

[dz1087]

Originally Posted by geezer#99

That’s not much movement.
One concern though.
You shouldn’t see any vacuum on a ported vac source.

Best way to adjust mix screws is turn them in slowly until the idle gets rougher. Then turn out an 1/8 turn. Do other one the same way.

What’s your initial timing set at?
Post a pic of your ported vac source.

Oh, that vac gauge was hooked to manifold vac for both videos. I was showing the difference in the two levels of vacuum when the distro was hooked to ported vs. manifold. There was zero vac on the ported outlet and it showed high vacuum when the throttle was cracked slightly, as it should.

After I got either screw out to where the vacuum was no longer increasing, I screwed them back in a half turn at a time until the vacuum decreased. Then I backed it out slightly.

Initial timing set to 12*.


Good to know that is not much movement. I believe one of my culprits was the idle screws. I initially hooked back up to manifold vac to see what the vacuum was doing with the erratic idle. The vacuum was flopping all over much much worse that in those vids. I decided to go ahead and play with the mixture screws a bit. I had to back them out probably another 2-3 turns to get the vacuum highest and the idle calmed down.

I’m going to keep the truck on ported for a few weeks and see how it behaves on the road. Then I may go back to manifold and compare.


One concern I have is that the idle would not go below 780 when hooked to manifold. Do I need to screw my mixture screws in more, even if it would decrease vacuum at idle?

[geezer#99]

What do you mean by ‘it wouldn’t idle below 780 with manifold vac hooked up’?
Was the main idle screw backed off all the way?
Was the throttle closed completely?
Or do you mean it won’t idle well enough lower than 780.

[dz1087]

With the throttle plate closed, and the idle screw backed off completely (not touching the throttle arm at all) it would not get below about 780. I'm assuming that my primaries are closed all the way since there was zero vacuum on the ported vac port.

[geezer#99]

Should go lower.
Check the high idle screw isn’t holding it open.
Try that first.

[Steeveedee]

OP, you checked for vacuum leaks. Did you squirt the primary throttle shaft on both sides of the carburetor? If the throttle body is worn enough, the primary blades can get enough off-alignment to cause high idle.

[garyd1961]

Edited.

[dz1087]

Originally Posted by Steeveedee

OP, you checked for vacuum leaks. Did you squirt the primary throttle shaft on both sides of the carburetor? If the throttle body is worn enough, the primary blades can get enough off-alignment to cause high idle.

I'll giver her a squirt later today and check on that. The carb was rebuilt 8k ago by Quadrajet Power. They didn't say anything was worn out beyond spec.

Originally Posted by garyd1961

I didn't see where you said what the compression ratio of your motor was but one thing I have learned over the years is you don't want to run ported vacuum on a high compression motor. With ported vacuum you increase the timing while accelerating which can cause spark knock. With manifold vacuum you increase timing at idle and cruise which is best for a high compression motor. With manifold vacuum when you accelerate the vacuum advance drops out the equation and you run on initial advance and mechanical advance.

The only difference in ported vs. manifold is that the vacuum signal at idle with the primaries closed is zero. Once the primary blades are cracked slightly, the distro sees the exact same vacuum signal that it would on ported. Ported absolutely does not increase advance, or vacuum as you open the primaries or secondaries. It goes to zero, just like manifold.

On my engine, the difference between the two schemes are as follows:

Manifold: 20 in/hg, then decreasing to zero as the blades are opened fully.

Ported: 0 in/hg, then ~20 once the blades are cracked slightly open, then decreasing to zero at the same rate as manifold as the blades are opened fully.

I've verified this with both my vac gauge and my timing light.

Otherwise, I'm running a stock rebuild with dished pistons and 883 heads. I'm definitely not high compression. I'm going to continue to check for leaks and adjust the choke mechanism on the carb to ensure it is working properly. Once I messed with the idle mixture screws, my idle settled really settled out.

[dz1087]

I appreciate the responses, fellers. I believe my primary issue was bad adjustment of the idle mixture screws. after playing around with both ported and manifold vacuums, the idle was settled on either vacuum source. Since the idle was no longer bouncing around on manifold, it was also no longer changing the vacuum around enough to mess with the timing.

I will continue to mess with the choke mechanism and search for any small vacuum leaks in the coming weeks. The reason I have the truck set to ported right now is that it won't idle any lower than about 780 like in that video above. Not that it stalls, it simply can't be adjusted any lower right now. With ported, I can set it at 700 or lower. Aside from my higher idle, I noted no difference in manifold or ported sources as far as driveability is concerned.

[Rich72C10]

I also had my 72 Q-Jet rebuilt by Quadrajet Power - I also could not get it to idle much lower than yours and then I had all kinds of other issues where it wasn't actually useable on my truck (I have a '78 Q-Jet for comparison and it was fine). Also, my idle screws hardly did anything until screwed out a lot.

So I ended up finding out about and sending my Quadrajet Power rebuilt 72 Qjet to Cliffs High Performance Quadrajets. Long story short, the rebuild was a mess and he said it wouldn't be able to run on any motor. I have no good words for Quadrajet Power, where as Cliff is one awesome guy. Detailed emails and even called me to speak to me to get a clear picture of WTF happen to my Q-Jet. I suppose you can image the $$$ I ended throwing at my Q-Jet for all this work.

My truck has been a beast since Cliff's work (or as well as a beast it can be with my mild 350 rebuild).

[Steeveedee]

Originally Posted by dz1087

I'll giver her a squirt later today and check on that. The carb was rebuilt 8k ago by Quadrajet Power. They didn't say anything was worn out beyond spec.

Mark would have put bushings in if the throttle was excessively worn. So probably OK, but good to check. Looks like you got 'er licked, anyway.

[garyd1961]

Originally Posted by dz1087

I'll giver her a squirt later today and check on that. The carb was rebuilt 8k ago by Quadrajet Power. They didn't say anything was worn out beyond spec.



The only difference in ported vs. manifold is that the vacuum signal at idle with the primaries closed is zero. Once the primary blades are cracked slightly, the distro sees the exact same vacuum signal that it would on ported. Ported absolutely does not increase advance, or vacuum as you open the primaries or secondaries. It goes to zero, just like manifold.

On my engine, the difference between the two schemes are as follows:

Manifold: 20 in/hg, then decreasing to zero as the blades are opened fully.

Ported: 0 in/hg, then ~20 once the blades are cracked slightly open, then decreasing to zero at the same rate as manifold as the blades are opened fully.

I've verified this with both my vac gauge and my timing light.

.

Never too old to learn, I believe you are correct. I got my erroneous information from a supposedly top mechanic of a hotrodder forum that I was once a member of. The way you explained it makes more sense than his explanation.
ETA, I concede to what you say about port vacuum not raising timing advance under acceleration. Although when running vacuum advance I always run manifold vacuum and it seems to be the best from my experience. From what I have read port vacuum was an early attempt at pollution control by delaying the burn at idle so the exhaust gas would still be burning when it entered the exhaust manifold. This and very little initial timing helped reduce pollution but also killed performance. Manifold vacuum gives a better idle.
Saying this I don't have vacuum advance on my vortec 350, not that I think it is a bad thing, I just had a distributor without it so that's what I used. I get away with no vacuum advance by upping my initial advance and limiting my mechanical advance. Not the best for a street motor but it works for me.