[dv657172]

72 GTO basically all factory specs
455 30 over
068 GM cam in place of original 067
TH400 + stock Power Brakes
Points system

Recently rebuilt Quadrajet after car seemed to be having fuel delivery issues which are now solved but....

Shop that rebuilt engines years ago (although very limited miles since) did not connect vacuum advance so I decided to reconnect it to a manifold vacuum source.

Initial timing is 10 degrees and with vacuum advance connected it increases to 30 degrees (which I know is too high). Dwell is 30 degrees. I have not try to determine a timing curve yet.

The issue is my manifold vacuum varies between 15-16 inches in park at about 800 rpm but in gear it drops to only about 12-13 inches and the idle drops to only about 600 rpm which the car doesnt like

If I put my hand just above the choke plate the vacuum increases to a very steady 18 inches and the rpms go up by about 100 rpm. I dont think I should have to have a curb idle of roughly 900 to get the car to run decent in gear with just an 068 cam.

Does this indicate a vacuum leak, timing issue or poor tuner (myself) issue?

Thanks for your thoughts
DV

[steve25]

The fact that choking off the primary’s at idle will increase rpm is a concrete sign that 1) you have a vacuum leak, or 2) your carbs idle circuit is too lean for the 068 cam now in use.
The 068 cam in a 455 should not need to idle anywhere near 900 rpm in gear and at hot idle.

Does the adjustment of you idle mixture screws in make the motor stall and when backed out make the motor run ruff?

[dv657172]

Thanks for the reply Steve, always appreciate hearing your thoughts.

Turning the idle mixture screws in does make the car nearly stall out while backing them out kept increasing the rpm but did begin get to get a little bit rougher sounding.

Am I correct that backing them out increased the richness of the idle mixture which it likes because it has a vacuum leak?

The car has had the 068 cam in it for a long time without any issue prior to now and the carb rebuild involved basically just a basic rebuild (one of Cliff''s Quadrajet SR Rebuild kits), just fyi

DV

[gtospieg]

If the problem did not exist before the carb. rebuild then that is where you should start looking. Don't start looking for a different problem. Go back to where it all started...the carb. rebuild.

[dv657172]

I hear you gtospieg but as mentioned I also went from no vacuum advance to vacuum advance.

I have also heard many times that many carburetor problems are actually ignition/timing issues. Thats why I posted the question as the carburetor appears to be working fine.

[Cliff R]

I would not use MVA on a 455 engine with an 068 cam in it, even a low compression one.

Switch back to ported vacuum to the advance if it's available. If not reduce how much timing the VA adds if you only have a manifold source available for it.

If the carburetor is lean at idle speed with or without the added timing at idle the cure is to make more fuel available to the mixture screws......

[dv657172]

Thanks Cliff for the advice!

Curious why the engine doesn't like the MVA? My 71 400 auto with a cam much like the Summit 2801 seems too

Also would you try to hook up the TCS system, assuming it works, which as I think I understand results in a ported vacuum advance until a certain temp or speed is reached?

DV

[Cliff R]

Some engines, especially well thought out ones will not like, want, need or respond well to running a butt-load of timing at idle speed.

Good rule of thumb for tuning is to set the initial timing at 10-12 degrees, set idle speed, fine tune the mixture screws and see how much vacuum the engine makes. If it's making good vacuum, little change in RPM in and out of gear and full control with the idle mixture screws there is no need to add more timing.

I ALWAYS get the carbs idle system up to par before running a lot of timing at idle or trying to tune with MVA. 9 out of 10 engine combos are fine without any additional timing at idle speed. Over the years I've had literally hundreds of troubled vehicles brought here for custom tuning. The biggest problem with most of them were distributor and timing issues not carb problems.

The FIRST thing I do when one shows up is to check timing. 9 out of 10 of them have some POS spring/weight kit in the distributor which allows some of the mechanical curve in at idle speed, plus folks follow POOR advice on the Forums and are adding another 20 or even 30 degrees at idle speed with manifold vacuum to the advance. Talk about IMPOSSIBLE to tune!

I yank out all that cheap Chinese advance curve crap and put the stock parts back in them. Once I get control of the timing curve I'll find the optimum initial timing, then go into the carb and get it up to par. You'd be surprised at how more times than not these engines love that scenario vs trying to run all that timing at idle. Most like the smooth/steady advance curve better as well vs dumping ALL the timing on the engine right off idle.

In any case the big 455 with the 068 cam should be perfectly fine with about 10-14 initial timing, another 20-22 from the mechanical advance, and 10-14 from the VA. I'd have the advance curve start right off idle and all-in around 2800-3200rpm'.

I don't like the TSC systems and don't use them when tuning any of these engines. If the carb doesn't have a port for ported VA I may add one if it doesn't like adding another 10-12 degrees timing at idle with manifold vacuum.........

[Cliff R]

Some engines, especially well thought out ones will not like, want, need or respond well to running a butt-load of timing at idle speed.

Good rule of thumb for tuning is to set the initial timing at 10-12 degrees, set idle speed, fine tune the mixture screws and see how much vacuum the engine makes. If it's making good vacuum, little change in RPM in and out of gear and full control with the idle mixture screws there is no need to add more timing.

I ALWAYS get the carbs idle system up to par before running a lot of timing at idle or trying to tune with MVA. 9 out of 10 engine combos are fine without any additional timing at idle speed. Over the years I've had literally hundreds of troubled vehicles brought here for custom tuning. The biggest problem with most of them were distributor and timing issues not carb problems.

The FIRST thing I do when one shows up is to check timing. 9 out of 10 of them have some POS spring/weight kit in the distributor which allows some of the mechanical curve in at idle speed, plus folks follow POOR advice on the Forums and are adding another 20 or even 30 degrees at idle speed with manifold vacuum to the advance. Talk about IMPOSSIBLE to tune!

I yank out all that cheap Chinese advance curve crap and put the stock parts back in them. Once I get control of the timing curve I'll find the optimum initial timing, then go into the carb and get it up to par. You'd be surprised at how more times than not these engines love that scenario vs trying to run all that timing at idle. Most like the smooth/steady advance curve better as well vs dumping ALL the timing on the engine right off idle.

In any case the big 455 with the 068 cam should be perfectly fine with about 10-14 initial timing, another 20-22 from the mechanical advance, and 10-14 from the VA. I'd have the advance curve start right off idle and all-in around 2800-3200rpm'.

I don't like the TSC systems and don't use them when tuning any of these engines. If the carb doesn't have a port for ported VA I may add one if it doesn't like adding another 10-12 degrees timing at idle with manifold vacuum.........

[steve25]

The 455 with its far greater piston speed over a 400 does not need a Butt load of low rpm timing.

The piston speed due to stroke produces a lot faster cylinder pressure rise.

[dv657172]

Thanks Cliff and Steve for your thoughts, I really appreciate the education and your willingness to share your experience.

I have switched the car back to no VA for the moment and it runs good at 750 rpm at idle (could maybe even go a bit lower) dropping to 625 in gear at 11 degrees initial timing. Vacuum is 16 inches in Park and 13.5 in gear so the car does seem to like no VA at idle.

Will work on going to a ported vacuum source or trying to limit VA to 10-12 degrees and try running it off of manifold vacuum again.

Hopefully thats a good game plan

Thanks again
DV

[Cliff R]

Ported vacuum can be added if not present. If ported vacuum is used for the VA it MUST have a correctly located source so it applies FULL manifold vacuum with the slightest movement of the throttle. The factory actually used a "slot" vs a round hole on Q-jets (except very early models) when they used ported vacuum to the distributor. EGR sourced ported vacuum will not work, the source is too high and/or they used a slot to bleed off vacuum for the EGR.

Below is a baseplate designed for ported vacuum to the advance showing how and where the factory put the vacuum source.

[srmmmm]

Originally Posted by steve25

The 455 with its far greater piston speed over a 400 does not need a Butt load of low rpm timing.

The piston speed due to stroke produces a lot faster cylinder pressure rise.

On my 350 with a 4.25" crank we wound up with 12 degrees initial and 23 mechanical in full at 3200rpm. Ported vacuum advance for another 10 degrees. Engine was happy on the dyno with that. Haven't had a chance to road test myself yet, but restoration shop said it's very "responsive".

I'm running the Comp XE276HR cam.

[Cliff R]

Since you are making 16" vacuum at 750rpms' with 11 degrees initial timing there would be no benefit whatsoever to add more timing at idle speed. Matter of fact it may not work nearly as well as you end up lowering the throttle angle too much which reduces how much transfer slot is showing under the throttle plates at idle speed. This cuts off some idle fuel and requires more fuel from the mixture screws to compensate.

There is another whole crowd who advocates manifold vacuum, advance for ALL of these engines claiming things like improved fuel economy, lower engine running temps, and most of those folks regurgitate another half dozen claims/benefits to going that route.

I'm not here to argue but most if not all of it is inaccurate and more times than not running a BUTT-LOAD of timing at idle to make an engine happy is more of a "crutch" fix to compensate for other issues with the engine build.

There is another entire crowd out there forming behind a self proclaimed tuning "expert" that uses and recomends "locked out" timing for these engines and adding even more timing with MVA. It appears that this controversial individule is making quite a debute these days. He even called here once tring to "sell" me all that crap which I know for sure WILL NOT work as I've been tuning these engines coming up on half a Century now I've knocked the starters off a few blocks and/or chunked some ring gear teeth off flywheels messing around with that deal early in my learning curve.

Rather than argue with resident "experts" and "guru's" I simply tell folks for any and ALL applications to give your engine the timing and fuel it wants at idle speed for the best end results. I also tell them that the better job you do with your engine build, in terms of compression, quench distance, combustion chamber shapes and cam events that the LESS timing and fuel it will want at every RPM.........FWIW......

[dv657172]

Cliff thanks for the advice, I have followed it as best I can and it definitely seems to have fixed the issue by not using MVA.

I will ask what is the best way or product to use to be able to limit my vacuum advance? I have seen some big name companies offering adjustable ones with reviews that are either great or terrible all for the same product. Any help here would be great as I have a handful of cars all with vacuum canisters with one that provide 2 degrees and in other cases near 20 degrees. Thanks again

DV

[Cliff R]

I have limiters for stock points vacuum advance units that are basically just "drop-in". I can also modify the factory VA units to add exactly how much I want them to.

I would NOT recommened an "adjustable" vacuum advance, pretty much a waste of time/funds. Therre are also several varities. Some only change the spring tension inside the until so only change stop/start points. Others adjust how much is added.

It's really easy to modify a factory VA unit so that's what I do here when I build distributors.

What you are basically doing is to limit the travel of or length of the slot in the VA unit so that when vacuum is applied it travels less distance therefore adding less timing.

Here I use a MIG welder and round file to "set" the amount of timing I'm looking for....pic attached......

[dv657172]

Thanks Cliff

Very helpful, definitely like that approach better. Will be in contact in the future to see about the "drop in" limiters if you have them availabe or getting some distributor work done. Thanks!

[Cliff R]

Me too. Not fond of using a "cam" behind the pull-off to limit travel. They did make some nice VA units at one point that you could adjust with an allen wrench but I still prefer to modify stock units when building distributors here....

[MPKind]

Cliff has great info on this stuff. He's right about the different camps that advocate MVA, or something else. Just trying to understand how all the variables of interact is difficult, but one thing these MVA crowd don't say is that if your idle is on the rich side, there's no reason to have any advance. I tried it recently and the engine puked once I tried light acceleration. The timing drops substantially at that moment when the mix becomes more dense.

I'm using ported now and the engine likes it, but in this thread I learned that it should go from 0 to MV once the throttle blades are moved the slightest bit from fully closed. The base plate for a 7042262 has the slot where the ported vacuum increase linearly as the plates are opened further. It's not affecting the 0 to full throttle situation, and with 3.42 gears and 27" tires to create some ported vac, I just don't know how much...guess I should look into that. BTW, Cliff, I got my carb dialed in to my liking...as far as I know. ended up with a 75/43 jet rod combo, no bog, idles as low as I want, starts cold or hot instantly with no starter groan. It was a lot of trial and error, and almost got into a fight with my neighbor for being too noisy....