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Old 11-04-2005, 02:45 PM
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Ported Vacuum Reading at Idle?

I checked the ported vacuum on my QJet and got the following readings:
850 rpm, 7 in. Hg
1000 rpm 9 in. Hg
1500 rpm 13 in. Hg

I think this is bringing in my vacuum advance early and possibly contributing to some pinging under hard acceleration.

This is an Olds 350 with HEI and initial advance at 19*. At 1000 rpm I have 19* initial, 2* mechanical, and 6* vacuum for a total of 29*. QJet carb with idle mixture screws out 3.5 turns for best idle. (But manifold vacuum is only 14 in. Hg at 1000 rpm, and this is another issue.)

Shouldn't ported vacuum be about zero at 850 idle in park?

John

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Old 11-04-2005, 05:53 PM
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What do you get at 600 or 700? I think you are a little high in your idle speed. I think w/ auto you should be at 600 and with a manual trans you should be at 700 rpm. Vacuum should be at zero at idle on the ported side. Depending on your elevation and condition of your motor you should be reading about 17 to 20 on the manifold side. Needle should be steady. If you are getting that much you need to check the vacuum advance unit. It might have a leak.

You are also not getting enough mechanical or vacuum advance. I am running at 14 initial and 36 mechanical @ 2500 and almost 50 with vacuum @2500. I think that is pretty standard. You should check your springs on the advance weights to make sure it is working correctly. There are different vacuum advance units that work on different amount of vacuum depending on the cam. If your motor is stock and has good vacuum it should pull in about 10 to 12 on the advance.


I am also running my vacuum advance off the manifold side and am not having any problems. This is a great debate among tuners. Try which works best for you. I am trying to get better mileage so I want as much advance as possible without ping.

Sound like your motor will run much better if you can get the advance straightened out.
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Old 11-04-2005, 07:55 PM
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You need to look at your initial advance or centrifugal advance because you should have very little ported vacuum under hard acceleration.
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Old 11-05-2005, 07:20 AM
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IMHO the problem lies in the carb. At idle, the ported advance outlet off the carb should be zero. If you are getting a 7 in. reading, I think your throttle plate is open too far and is allowing vac (on most carbs the throttle plate position determines ported vac. On some older carbs the open throttle plate position can be do to other malfunctions (leaks in the manifold or carb vac system) or a corroded throttle place etc. Unless you have a very lumpy cam, you should be able to get your idle down to 600-700 as Mertz suggests. If it won't idle down there then my guess is a vac. leak or a fuel delivery problem inside the carb. I had a similar problem with a stock Ford carb and it turned out to be a leaky ERG valve which was forcing a higher idle speed adjustment which was then opening the throttle plate and allowing a reading on the ported vac. outlet.
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Old 11-05-2005, 08:20 AM
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I have checked for vacuum leaks using carb cleaner around the carb base and manifold; also did the smother the carb with a rag test and the engine dies instantly without any momentary speed up before dieing. All hoses are new. So, I don't think I have vacuum leak.

The engine has a new Performer cam which was not degreed in but installed straight up along with a new Edelbrock double roller timing chain.

With vacuum advance disconnected and plugged, I have total advance of 35* by 3000 rpm using Moroso med springs with the original mechanical advance weights.

I had never put a vacuum gauge on the ported fitting before, but it didn't feel like it was pulling any vacuum by putting my finger over it. The vacuum advance is a new adjustable one from Mr. Gasket supposedly adjusted to give 14* max advance by 8 in. Hg.

So, I thought I had it set for 19* initial, 16* mechanical, and 14* vacuum for a "total total" of 49*.

But the discovery of this high ported vacuum has me needing to check the carb tuning some more I guess. I am also afraid the low manifold vacuum of 14 in. Hg isn't going away with carb tuning and I may have a cam timing problem. Cylinder pressures are all between 160 and 170 psi. Pinging occurs with 89 octane gas. Don't think I had pinging with 92 octane, but I expected this mild 350 to run on 87 octane (stock low compression pistons, no head work).

Thanks for feedback,

John
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Old 11-05-2005, 08:22 AM
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cboy is correct....ported vacuum should be "0" to about 1200rpm with no load on the motor, carb is messed up

manifold vacuum with a performer cam at 700rpm should be 15Hg+ (18 is great)

the 87 octane ping is very probably due to the 19 deg base

with stock compression, 12deg base/vacuum disconnected, 24deg mech is more the norm to use 87 octane as a start point then "creep" it up on the base (13...14) if it helps with throttle response

how much vacuum advance is subjective and depends on vehicle weight/gears/altitude/, it's a "test and tune" but whatever the factory setting/curve was it will put you in ballpark to start

Last edited by red65mustang; 11-05-2005 at 08:53 AM.
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Old 11-06-2005, 01:52 AM
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John, I think you missed something.

That advance can does NOT give you 14* at 8" vacuum.... I think that the can BEGINS to move (advance) at 6-8" vac and reaches 14* at probably 14-18" vac... for backing off the throttle and coasting= advance. Of course, what it is and what it is suppose to be might be two differnce things.

also most aftermarket cans are adjustable through the hose fitting to limit the total amount of degrees... rather than the rate of advance increase. That makes them tuneable for your mechanical curve and driveability.

You might also look into the old trick of drilling tiny holes into the throttle plates to allow the plate to close farther to control the idle circuit better and regulate the ported vacuum better.

Also consider that you have stock "low compression" piston, stock heads, and 170 pounds of compression...... DUH... what.......????...... That must be a mild cam to build that much pressure... and you have above 9/1 static compression. 160-170 pounds dynamic compression will need more than today's 87 octane.

Another consideration is that dished pistons ruin the quench effect and function a lot like open chamber heads, which allows pinging at lower than anticipated compression ratios.

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Last edited by xntrik; 11-06-2005 at 02:17 AM.
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Old 11-06-2005, 05:23 PM
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xntrik,

Made a mistake on the vac advance. It is adjustable made by Accel and the chart that came with it shows that it starts coming in at 5 in. Hg and is fully in at 14* by 8 in. Hg -- that is with 4 complete turns of the adjusting screw.

But, from the readings I took at the ported fitting on the carb, vacuum advance isn't coming in quite that fast.

I have read about drilling small holes in the primary throttle blades. I also read that later QJets had smaller idle mixture screw holes (.080) than earlier models (.095). So, that is another adjustment that can be made to provide more fuel at idle without setting the throttle valves open with the idle speed adjusting screw. I don't know what size holes mine has (remanufactured QJet).

So, current situation is:
manifold vacuum is low (14.5 in. Hg)
idle mixture screws have to be out 3.5 turns for smooth idle
idle mixture screws don't seem to have much effect on idle after 3.5 turns
ported fitting on carb is pulling 7 in. Hg vac at 850 rpm
warm idle speed is 800-850 in park, 650 in drive.

My next step is to try and reduce idle speed, see if ported vacuum is reduced and idle mixture screws have more effect, and if carb dies when put in gear at lower idle.

Thanks for input.

John
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Old 11-06-2005, 05:42 PM
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You have a ported vacuum problem.
And that Accel can is very agressive. Much more so than you probably need. It sounds like it is made for a very big cam to help smooth out the low rpm fuel delivery problems.
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Old 11-07-2005, 06:21 AM
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clarification/correction

please test and post the manifold Hg at 700rpm, 12deg base timing......(should be 15Hg...18 is great with the performer cam)

repeat: I don't like your 19base/16mech settings!

9.0 or less C/R small block Chevies WITH 92 octane like 16 base and 20 to 22* mech on the crank, starting about 1200rpm, all in about 3500rpm, 38 deg max! (from a Marlan Davis article)
*I wrote, before, start with 12/24, (blue oval numbers), start with 12/22

plug the ported vacuum, test drive it for knock and medium and heavy quick throttle response

(xntrik, if the motor is high C/R, above 9.0, it would be a real beeeetch to start hot with his 19deg base timing)
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Old 11-07-2005, 06:40 AM
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John,

Is that Qjet brand new or has it been around for a while? If it's got a lot of miles on it you might want to do a quick rebuild and also make sure your throttle plates are set properly. It is rare for them to get screwed up but until you can draw zero vac at idle, it's going to mess up your timing and advance. There isn't a second "ported" outlet anywhere on that carb is there?
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Old 11-07-2005, 07:58 AM
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cboy, nxtrik, red65mustang,

The QJet was bought online from a company that sells remanufactured units. It appears to be intact and fully functional. I did change out the primary jets and rods to get close to the original setup on early 1970 Olds engines. But I do not know if it has been hacked, drilled, plugged, etc. as some people have warned me about remanufactured carbs. The model number corresponds to a 1981 carb from a non-GM application.

The engine, cam, intake, carb were put together about a year ago, but we have only put 600 miles on it since getting it on the road this spring. I have been concerned about the low manifold vacuum, but am trying to eleminate all other issues before giving up and checking the cam timing.

Again, this is an Olds engine, and they used 20* initial advance with HEI distributors, which I have. I tried 12* initial once and it was really a slug.

I will try reducing the hot idle to 700 rpm and get a manifold and ported vacuum reading. I expect the manifold vacuum to drop even lower, but maybe followup adjustment of the idle mixture screws will bring it back up.

I will also try driving it with the vac advance disconnected to see if the pinging goes away.

This cam is the Edelbrock Performer (204/214), .448/.472 lift. Still stock compression of around 8.5:1.

Again, thanks for the inpput.

John
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Old 11-07-2005, 09:54 AM
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John, check to be sure the (no vacuum) timing is not all in 35deg till 3500rpm+ (factory was around 4,000)=bad advance springs

yup, 12base+ your 16mech=28=sucks,forget that....
try 16+16=32 and bump it up if no ping...depending on the car weight and gears with 87 octane/8.5C/R=??? 34deg practical max probably

from your "early 70's" (pre-unleaded?) build spec....what was the Old's "recommended" octane for 20deg stock base....

heck, Old's were 10+C/R till 72?, are you sure it is 8.5C/R
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Old 11-07-2005, 10:55 AM
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red65mustang,

From many sources, 1970 was the last year for high compression. 1971 the compression dropped to 8.5:1 by dishing the piston tops (still used high compression heads though) -- this is the engine I have. It originally had a two barrel and points distributor. I changed out the cam, intake, carb, and went to HEI distributor. In 1972 the compression dropped further to 8.0:1 by going to higher volume heads.

The reproduction Emissions Control Information sticker for that year says to use 91 octane, but were the octane ratings calculated the same way back then as they are today? I currently have 89 octane mixed with a little 92 octane in the tank. When I was running 92 octane I wasn't getting any pinging.

Back to the carb tuning:
This morning I adjusted idle speed screw for lower idle with vacuum advance plugged and vacuum dropped at both the ported fitting and the manifold fitting: rpm/ported vac/timing
800/5.5/23 (manifold vacuum 13 in. Hg in park)
700/2.5/21
650/1.5/20
600/0.5/20 (manifold vacuum 11 in. Hg in park)

With the idle speed screw at 625 rpm I then adjusted idle mixture screws. It seemed to like the range 3.5 to 4.0 turns out, but the exhaust still smelled a little rich.

I then hooked vacuum advance back up and idle speed increased to 675 rpm, manifold vac 12.5 in. Hg, and timing 21*. So, still getting a little vac advance apparently at idle.

I didn't check total advance (initial plus mechanical) today, but it has consistently been about 32* at 2500 rpm and 35-36* at 3500 rpm. I am using the stock mechanical advance weights but Moroso medium springs. These springs let the mechanical come in a little faster than stock.

The mechanical weights and pins show some obvious wear (holes in weights oval rather than round, pins grooved). This might cause some slop in the mechanical advance -- I do see the timing jumping around about 1 degree at any fixed rpm.

I can probably get rid of the pinging by either turning back initial timing a couple of degrees or running 92 octane. A test drive with vac advance plugged still gives me some pinging in 2nd and 3rd gear under hard acceleration (my hard acceleration equates to only mild acceleration for most cars with 350 engines.)

As for the low manifold vacuum, I feel confident the mechanic installed the Edelbrock cam and timing chain correctly. So the cam timing should only be off if there is a grinding error, or the lifters/lobes didn't break in correctly. I haven't found any metal in the oil.

John
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Old 11-07-2005, 12:29 PM
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yup, 91 octane is still 91 octane today....

that was called "regular" , "premium" was 100+octane

till the ported vacuum is fixed, I'd disconnect and block it, use no vacuum advance, won't hurt the motor, will cost you 1-2mpg till it's fixed,

I flipped thru a bunch of 70-72 Old's motors timing specs (Autozone.com/shopping/specifications/), all were the typical stock 8-12 base.....(so 12-16 base makes sense)
I'd phone Edelbrock tech service, ask for a suggested timing combo start point for the performer cam....
Iv'e never seen a factory 20base

Last edited by red65mustang; 11-07-2005 at 02:31 PM.
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