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Old 09-14-2006, 10:18 PM
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Bizarre High RPM Lean... Why?

Okay guys, got big problems and the car is supposed to see track time tomorrow and I'm in points contention.

Car (truck?): 1982 El Camino

Engine: 383 9.8:1 compression, fed by an Edelbrock 1910 (Q-jet for all intents and purposes) and the fire is made by a MSD Pro Billet HEI with total timing at 38 (seems high, but it likes it).

The fuel pump has the ability to push way more fuel than it will need. I have an Aeromotive that can push up to 20 psi (18 on the average ). I have it regulated down to 7 psi in a return type system. The fuel is the factory hardline from the tank to the pump and up to the front where I have used 6 AN line to the the Aeromotive return type regulator, 6 AN to the carb and 6 AN all the way back to the tank.

I have no doubt the pump can keep up with it. It can suck my tank dry in about three minutes (17 gallons later...)

Now the problem, The car seems to go lean and surge above 4500 rpm at WOT. Keep in mind the shift point is 5500 rpm. I have tried several things including using the vacuum/boost port to give me a little more at WOT. I had it set to 12 psi at WOT and came to about 6 psi at idle. Ran it out the same way, started to surge like it had before around 4500 and up. I tried bringing the pressure down the other way and came down to 5 psi, just in the odd chance it worked. To say the least, it didn't help. I even replaced the distributor cap and rotor in case there was a chance the distributor had something to say about it. The parts that came off looked fine, a little worn, but in good condition. I pulled the carb apart to and checked all the float levels and little channels that Q-jets have, and it was as clean as it was when I put it on the first time. I have change the coarse filter before the pump and the fine filter at the carb.

Now, I can run it up to 4500 and nail it to WOT and it won't surge till 5000. It seems to me that I have a problem with getting gas into that tea cup of a fuel bowl. I moved the float level a little higher to see if it make a difference. It had no effect on the performance on the car.

I am out of things to try in an effort to make this run right.

Recap, new filters some improvement, pressure up no improvement, pressure down no improvment, new cap and rotor no improvement, tear carb apart everything checked out, no improvment.

Can you guys help? This really has me frustrated and the car has to be somewhat race ready for tomorrow.

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Old 09-14-2006, 10:34 PM
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What is your total timing @ 3000 rpm?
Are you running a vacuum advance?
If so, is it from a ported source or from a full source?

7lbs fuel pressure is more than enough for a streetable car runing a 750 QJET
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Old 09-14-2006, 10:39 PM
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Total is all in by 3000, it likes a lot of advance hence the 38.

Has Vacuum advance to a ported source, but it doesn't matter at WOT. No vacuum.

It's actually and 850 cfm Q-jet.
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Old 09-14-2006, 10:51 PM
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Quote:
Originally Posted by 1982 SS
Total is all in by 3000, it likes a lot of advance hence the 38.

Has Vacuum advance to a ported source, but it doesn't matter at WOT. No vacuum.

It's actually and 850 cfm Q-jet.
The loss of vacuum causes the advance... so, you are shy
You should be seeing atound 48* @2,800-3,000 rpm

I would really like to see pics of the QJ with the choke wide open and venturies showing completely, this will require 2 shots, front and back of the choke plate
Thanks
Bob
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Old 09-14-2006, 11:02 PM
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Did you want the advance with the vac advance working or not? Cuz it still wouldn't matter at WOT. But it is something like 48 with the vacuum advance hooked up and held at 3000.

I'll have to get the pictures tomorrow when it's light out. It's really too bad Edelbrock dropped the Q-jet, I think they are a great carb when they aren't making a mess of things like mine seems to be doing.

It really is 850 cfm, I have the tuning manual that came with the carb. It's not so much that the primary side is larger but a little reworked. Most of the extra flow is in the secondary side . I think the secondary butterflies are a shade larger than a stocker.

I just had a thought, what if the regulator has some piece of junk internally that is stopping it from closing the return side properly to supply the carb? I'll take the regulator apart tomorrow.
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Old 09-14-2006, 11:21 PM
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Quote:
Originally Posted by 1982 SS
Did you want the advance with the vac advance working or not? Cuz it still wouldn't matter at WOT. But it is something like 48 with the vacuum advance hooked up and held at 3000.
Which is it?
The advance comes in as the vacuum drops
Quote:
Originally Posted by 1982 SS
Total is all in by 3000, it likes a lot of advance hence the 38.

Has Vacuum advance to a ported source, but it doesn't matter at WOT. No vacuum.
It seems like you need to try different springs/weights to get the full 48-53* with zero vacuum

There is no replacement for a properly tuned Rochester Quadrajet as long as you dont require more than 850 CFM

I am still leaning towards timing rather than Carburetion
What are your engine specs HP and torque and carb specs....primary jets/rods Secondary rods and rod hanger?
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Old 09-15-2006, 07:25 AM
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Quote:
Originally Posted by BC 73Vette

It seems like you need to try different springs/weights to get the full 48-53* with zero vacuum
WOW!!! This would be way too much advance!!!

Chevy engines do like a lot of advance to run best, but do NOT exceed 40 degrees with the vacuum advance dis-connected and plugged off. This is the same as zero vacuum. At cruise RPM with vacuum advance hooked up (does not matter whether it is hooked to full or ported source) the Total advance will be greater than 40 degrees and this is normal and correct. However, when going to WOT the vacuum will drop to zero (or very close to zero) and the timing will drop to whatever the Total Mechanical setting is. In 1982SS original post he said this was 38 degrees, which should be OK.

As to solving the problem, check that the spark plug wires are routed correctly. Easy to swap out 5 and 7 in error.
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Old 09-15-2006, 07:33 AM
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Are you sure your secondaries are opening? Sometimes the choke lockout on the secondaries remains engaged. This would cause problems above 4500 rpm or so as the manifold vacuum begins to rise to 5 or 6 inches. Q-jets work well at low fuel pressures, I run mine at 3.5#.
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Old 09-15-2006, 07:43 AM
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Quote:
Originally Posted by Frisco
WOW!!! This would be way too much advance!!!
Sorry you disagree with me but this is where they run best
Both my ElCamino and Corvette are set up this way (along with thousands of others SBC's from a different forum) and there is no detonation unless you have a very lopey cam

Last edited by BC 73Vette; 09-15-2006 at 07:49 AM.
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Old 09-15-2006, 07:56 AM
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Vacuum advance does not work without vacuum. BC, I think you have something backwards in how your explaining things. At wide open throttle there will be no advance via vacuum advance. His 38 total would be correct.
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Old 09-15-2006, 08:11 AM
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Quote:
Originally Posted by Rick WI
Vacuum advance does not work without vacuum. BC, I think you have something backwards in how your explaining things. At wide open throttle there will be no advance via vacuum advance. His 38 total would be correct.
Quote:
The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the
limits of the mechanical advance (mechanical advance consists of the initial timing plus the centrifugal advance that
the distributor adds as rpm comes up) under light to medium throttle settings. When the load on the engine is light
or moderate, the timing can be advanced to improve fuel economy and throttle response. Once the engine load
increases, this "over-advance" condition must be eliminated to produce peak power and to eliminate the possibility
of detonation ("engine knock"). A control unit that responds to engine vacuum performs this job remarkably well.
Full text: http://66.102.7.104/search?q=cache:k...&client=safari

Last edited by Jon; 09-15-2006 at 10:58 AM. Reason: Copy-pasting. Please see copy-pasting guidelines at: http://www.hotrodders.com/help/board/copy-pasting.html
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Old 09-15-2006, 08:27 AM
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Quote:
Originally Posted by BC 73Vette
Both my ElCamino and Corvette are set up this way (along with thousands of others SBC's from a different forum) and there is no detonation unless you have a very lopey cam
I don't know how you are checking/setting your timing but here is how you should be doing it for a non-computer controlled engine.

After the engine has reached full operating temperatures, remove the vacuum advance hose from the vacuum advance cannister and plug the end of the hose. (A golf 'T' works well for this.)

With a good timing light connected and the engine idle set at the RPM you desire, check the initial mechanical timing in degrees. Note this figure. Note: If the idle RPM is high (900 or above) the advance springs may need to be stronger or the mechanical advance will begin to work too soon.

Slowly increase the engine RPM until the timing light shows no additional increase in the timing. Note the RPM and the amount of degrees of timing.

The RPM range where you no longer see a timing change should be in the 2500-3000 area. If not the advance springs and/or weights need to be changed to get the Total Mechanical timing to show up in this RPM range.

The Total Mechanical timing should be set (for small block Chevy's) somewhere between 32 and 38 degrees BTDC. The number of degrees will vary on different engine combos. You will have to decide what works best for your combo. The initial timing will fall somewhere around 12 and 18 degrees BTDC for most GM HEI distributors.

After the timing has been set as above, then re-connect the vacuum advance hose to the cannister.

If you are using the full manifold source the engine idle should increase at this time and should be adjusted accordingly. Look for around 650-750 RPM with the trans in DRIVE if automatic or neutral if standard shift.

Now to actually check the Total Timing (not to be confused with Total Mechanical timing), increase the RPM back up to the figure where all timing stopped. Again this should be in the 2500-3000 RPM range. Since there is no load on the engine the timing you see will include an increase due to the vacuum advance being connected. It will be in the 40+ degree range. This is because there is no load on the engine. At cruising speeds (light load on engine) this will also be true. At WOT, there is relatively no (0) vacuum and the timing drops (retards) to the mechanical timing.

Timing is NOT set with the vacuum advance hooked up. This is because the vacuum will vary continuously during engine operation. Vacuum advance is used for an increase in drive-ability and for an aid in fuel consumption. It is not required to run the engine.
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Old 09-15-2006, 08:31 AM
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Quote:
Originally Posted by BC 73Vette
The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the
limits of the mechanical advance (mechanical advance consists of the initial timing plus the centrifugal advance that
the distributor adds as rpm comes up) under light to medium throttle settings. When the load on the engine is light
or moderate, the timing can be advanced to improve fuel economy and throttle response.
Excellent "copy & paste".

However, it is not saying what you claimed in your post above. It does not say that the timing is set with the vacuum advance connected.
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Old 09-15-2006, 08:40 AM
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Quote:
Originally Posted by Frisco
Excellent "copy & paste".

.
Obviously... do you think I typed that article in 30 seconds
It comes from one of the best ignition tuners in the country

If you dont agree with it thats fine
But it is a standard for gm engine performance.

Again, read your own words
32-38* plus the vacuum (or loss of) = 42-50 degrees total timing

Last edited by BC 73Vette; 09-15-2006 at 08:54 AM.
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Old 09-15-2006, 08:43 AM
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Quote:
Originally Posted by BC 73Vette
Obviously... do you think I typed that article in 30 seconds
It comes from one of the best ignition tuners in the country

If you dont agree with it thats fine
But it is a standard for gm engine performance.
I am in total agreement with what you "copy & pasted".

However, this does not preclude the fact that you don't seem to understand that there is no vacuum advance occuring when going to WOT and therefore the Total timing is limited to the mechanical timing only at that time.
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