Using vacuum gauge to tune engine?

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LovetheA's

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“I moved this update to a this new thread because I wasn’t getting feedback on what to do next in this situation.”

I recently took my edelbrock performer rpm intake to a machinist I trust explained the situation to him and had him mill my intake .040 on each side. That is the same thickness as the two felpro gaskets under and above the valley pan. The intake lined up perfectly now with the gaskets and I’m glad to report no intake or carb leaks. Checked for leaks with starter fluid previously based on the engine idling much higher than before it was taken apart. It used to idle at @1000rpm and with the leaks I had it idled noticeably higher at 1500. Now it is back idling at 1000 again. I sprayed the starter fluid at various points where intake meets heads and carburetor gasket with no sucking in of fluid or change in rpm as it did before. I hooked up a vacuum gauge to the constant vacuum draw on the side of my speed demon. I hope this is the correct port to use for vacuum testing? At idle it currently has 10-12 in vacuum that is after I moved the distributor around to find the steadiest and highest vacuum reading at idle.

However, it seems like I have another issue. The engine doesn’t idle as smooth as it used to and the gauge bounces around by about 2 inHg. I should mention that it is a 383 4 speed 67 Dart with the older Mopar performance camshaft 484 lift 284 duration. What would be my next logical step check plugs, wires and distributor cap or is idle mixture screw on speed demon carb off? Should I pull each plug wire off of distributor cap one by one and listen to see if I can find if one is misfiring or dead? Some are a bear to get to with TTI headers. I didn't touch the carb just put it back on after changing old cast iron to edelbrock performer heads and updating Harland sharps with Mikes geometry kit from B3 racing. I’m trying to get it running as smooth as it did before by process of elimination and learning as I go. I have included a video of the engine idling and the gauge. When I rev it up it also feels like it is running rough.
 
Here is the engine running warm at idle and what the gauge says. Is this vacuum too low for the combination I’m running?
 
You’ve made a bunch of changes to the engine. That cam if I remember is around 241degrees at .050” it will be some what choppy especially in a 383, the vacuum will fluctuate some. How did you set the preload on the lifters? Hot cold. The aluminum heads have a different growth rate than iron. There shouldn’t be lash with hydraulic lifters, they should be preloaded and I believe I hear lifter clatter in the video.
 
"The engine doesn’t idle as smooth as it used to and the gauge bounces around by about 2 inHg"

If the only thing you did is fix all the air leaks then I'm tempted to say that it needs more air????????? When it was leaking the engine ran more smoothly? Of course this was at a higher RPM which might have been hiding the lack of smoothness at a lower RPM??
A healthy cam usually equals a rough idle, especially at the lower RPMs.
 
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10 in vacuum is about right with the 284 cam.
you didn't say where the initial timing ended up.
It should be in the 20 -22 deg range in most cases
 
I'm betting needle bounce is the result of idle mixture
 
Start by warming up the engine fully. Get it to idle best you can and closest to target idle speed, by adjusting mixture and idle speed screws. Then carefully adjust timing for maximum smooth idle vacuum. "Temper" this with timing light readings so you don't get down the wrong road. This setting may be temporary, depending on your advance curve, IE you may need to re-work that. Temporarily disconnect and plug vacuum advance if used. On a side note I dislike full manifold vacuum for advance

Now readjust idle speed and mixture. Set mixture alternately for maximum vacuum, readjusting idle speed if needed. "Finish' by touching up idle mixture towards lean just a touch. Vacuum should not drop or just barely start to drop at that point.

Now see what you have.

If you suspect port to manifold leaks don't forget to check around UNDER the manifold as well!!! And ports can leak from within the valley if mismatched enough. At least on a B/RB you can examine the valley gasket to port match on the heads.
 
Initial timing needs to be sorted out 1st, 383's like a fair amount of initial timing .
 
Thank you everyone for all the advise. I’m testing the vacuum off of the vacuum port on the carb not the manifold and the distributor is an electronic one without vacuum advance on it. Should the timing be about 10-15 BTDC on initial and about 30-34deg BTDC in when fully advanced?
I set the pre-load on the Harland sharps when I was working on that particular cylinder intake and exhaust by feel. I set the rocker so that it just starts to compress down on the pushrod and hydraulic lifter then went another half turn about .030-.050 thousands to lock it in. Then tightened down on the lock nut above to lock it in. It does sound like there is a lot of clatter with the rockers. Do you think I should recheck the settings?
 
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"Depends". Different engine builds "want" different timing, a hot cam, etc, but you can not just "pick" numbers because of the curve in your dist. The engine may "want" more at idle (initial) but if you are driving it you don't want it to over--advance, so you may have to adjust it for max performance at high RPM and "put up with" less than optimum at idle.

The answer..........recurve the dist.

But since you seem to be trying to get it to idle, I didn't bother you with all that.

So what you need to do is find out where it needs initial / low RPM, and find out what it wants under full power/ mid-high RPM, then take those figures and recurve the dist. to suit.
 
I finally adjusted the carburetor air fuel mixture screws and set the idle screws after dialing in the timing the best I could. It idles at 1000 rpm, has 12-13 inHG and runs much smoother but what’s great is the choppy cam at idle. I’m over the moon for now.
https://youtu.be/L1f06aij-pY
 
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I had 14" vacuum on a '68 spec 340 with that cam and a Strip Dominator with a 625 Demon. I got my idle down to 800/750 in gear with a MP 175K convertor. It idled better with a Holley spread bore though. Hated that Demon, it was posessed with an ill machined needle/seat boss that would shear the O-ring every time you installed one and would always end up fouling the needle seat and flooding it. I would think a 383 would be more mild than a 340 with the same cam profile as that cam (031?) was called the "Road Runner cam" thinking it was probably mild enough for a stock 383 RR?
 
Than you everyone for all the advice and mopardude318 for the video. I do have another question for everyone. Everyone talks about ported and manifold vacuum. If my 383 has an electronic distributor with no vacuum advance how do you set the advanced timing? Also if my manifold doesn’t have a manifold port how can I take vacuum off of the manifold? I adjusted the timing and fuel/air mixture by using the rear port off of the demon carburetor on the passengers side. I read that is what you use. See photo.

4F00923E-04A9-4883-92CF-2789EE265D9C.jpeg
 
LovetheA's said:
Than you everyone for all the advice and mopardude318 for the video. I do have another question for everyone. Everyone talks about ported and manifold vacuum. If my 383 has an electronic distributor with no vacuum advance how do you set the advanced timing? Also if my manifold doesn’t have a manifold port how can I take vacuum off of the manifold? I adjusted the timing and fuel/air mixture by using the rear port off of the demon carburetor on the passengers side. I read that is what you use. See photo.

View attachment 1715616671
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You did it right; "manifold vacuum" is present at that rearward nipple and is what you want the vacuum gauge hooked to for tuning. The front or "ported" nipple flows only as the throttle opens whereas the rear nipple flows all the time. It's all in how their internal passages are routed. As far as a port in the intake manifold itself, the Edelbrock has a big plugged hole in the rear driver side intake runners. That is used as a vacuum source for power brakes, if yours are manual that better be plugged! It's not used for any carb or distributor functions.
If your distributor doesn't have a vacuum advance unit, well, no vacuum advance for you obviously--all you get is the mechanical curve. It can be adjusted in a couple of ways but it depends on your distributor....open it up, see what style it is and start researching. There's plenty of good info here and out on the web. If you get stuck of course post up lots of pics here and ask questions..
 
I'll tell what OP,
that rocker clatter should not be there; you need to get that sorted, because, your cam is only a 284/484 when the lifters are working right.
Also
I had a 292/292/108 cam in my 360 that idled down to 600 and less in first gear with a 4-speed and 3.55s, pulling itself along, with the timing set to 5*.
The point is; give the engine what it wants. Not what you think it needs.

Your idle quality, at 1000rpm, is telling you the tune is sick.
It should be idling;
ba-bump-bump-bump,bump-bump-bump- ba-bump,
or even better is something like
ba-bump-up, ba-bump-up, bump,bump- ba-bump-up.....
ticking over at 700/750;
I don't have that cam anymore, and the replacement, I like it ticking over at 650
This is not rocket science.You just need to get the transfer slots and mixture screws synchronized, no big deal.
But that cam (in a SBM) is gonna want some bypass air, more than what the PCV can provide,and it will NEVER be happy until you put a big V-can on it.Here's the deal; your mechanical advance system can only provide the correct advance at two times; the first is at WOT, after it is all in, usually after 3600rpm; and the second is never at no other time. At ALL OTHER TIMES THE TIMING WILL BE WRONG.
Now, you can try and get the WOT timing close, at WOT, by dinking the timing curve and initial timing around, but you cannot ever hope to make it perfect below about IDK 2800 rpm. And that mechanical curve is DEAD WRONG AT ANY OTHER THROTTLE SETTING BESIDES WOT. That is where your V-can ATTEMPTS to provide the missing Part Throttle timing. Namely from idle to about 3600.

If you don't believe me, do this.
Start your car and warm it up. Loosen the D, and start cranking some timing into it keep on cranking until the rpm stops rising. Now read the timing. This is the correct amount of timing at that rpm and load setting.
What did you get?
I tell you what you didn't get; not5, not10, not15 and not even 20 degrees.IDK about BBs but my 367 likes in the 30s/deep 30s . By 2400 it is liking 40s /deep 40s, and by cruising speed it is liking 60s!
Meanwhile;
My 360 with the 292 cam, and the T-slot synced;
liked one bypass hole drilled in each primary throttle blade. I put it on the front part, between the transfer slot and the mixture screw, about 1/4 inch back from the edge, and chamferred both sides. This was my first time so, I started with a 1/16 hole and ended with 1/8 (in each). I discovered that 1/8th was too big, and I couldn't idle the engine slow enough and still maintain the T-slot sync. So I had to solder up the hole and move over. I ended up at, IIRC, at 6 or 7, 64ths., which being .094/.109 inch.. I can't tell you how much your 383 might need, or even if it will need any at all; you won't know until you get the lifters quiet, and Transfers synced up, and the timing backed up to set the idle speed where you want it.
I highly recommend to leave the vacuum gauge in the tool box; it will just drive you crazy..
 
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AJ/Forms
Thank you for all the advice and suggestions. If I’m understanding most of what you said. You’re saying first get rid of the rocker clatter? Is that usually accomplished by removing all the lash or slop between the rocker & pushrod when dealing with hydraulic lifters?
Then when I can see the rpm of the engine at idle and listen more carefully. Then judge the proper adjustment by slowly turning the distributor to monitor the change in rpm. When you are saying set the degrees BTDC at idle is that when 30 degrees is not uncommon? Is that because the more high performance the engine the higher BTDC it likes at idle? There are some things that are beyond what I understand so I apologize if I don’t entirely understand. Then proceed to adjusting the idle screws and then setting the air/fuel mixture screws. The mixture screws synchronized I get but the work on the transfer slots I don’t understand demon carbs well enough to know how that ties in?
Carl
P.S. What is a v-can?
 
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LovetheA's said:
Here is the engine running warm at idle and what the gauge says. Is this vacuum too low for the combination I’m running?
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That's about right imo,close. I know it's the hyd cousin of the .528, so they say... and it's right about there installed with 2 adv@10inches.
Install it @5 degrees adv and itll pull 12 inches with a dual plane.
 
Moparofficial
I recently after fussing with it a little more got it to about 12-13deg in Hg. But I haven’t right now been able to get it below 1000rpm without it running rough. When I adjust the rpm’s up to @1000m it smooths out. But now that AJ/Forms has chimed in I’m rethinking what I have done. I think I may have more to do?
 
If your distributor doesn't have a vacuum advance, put a rubber cap on both, the ported and full manifold vacuum ports.
 
LovetheA's said:
Moparofficial
I recently after fussing with it a little more got it to about 12-13deg in Hg. But I haven’t right now been able to get it below 1000rpm without it running rough. When I adjust the rpm’s up to @1000m it smooths out. But now that AJ/Forms has chimed in I’m rethinking what I have done. I think I may have more to do?
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That reading and 1000rpm makes sense.
Tell me what carb again? It sounds like you need to do what someone may already have said.. and increase the initial timing so you can get the blades in the transfer slot set the correct mixture for it too idle. Another trick is If you take the heavy spring out of a stock distributor and run just the stock light spring it will with 35 degrees give 18 initial. It will be half way into the centrifical advance at a 800 idle rpm.
 
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