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Bizarre High RPM Lean... Why?

6K views 52 replies 10 participants last post by  machine shop tom 
#1 ·
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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#4 ·
1982 SS said:
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
 
#5 ·
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.
 
#6 ·
1982 SS said:
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
1982 SS said:
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?
 
#7 ·
BC 73Vette said:
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!!! :nono: :nono: :nono:

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.
 
#10 ·
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.
 
#11 · (Edited by Moderator)
Rick WI said:
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.
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:...butor"&hl=en&gl=us&ct=clnk&cd=1&client=safari
 
#12 ·
BC 73Vette said:
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.
 
#13 ·
BC 73Vette said:
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.
 
#15 ·
BC 73Vette said:
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. :D
 
#16 ·
Frisco said:
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.
I know how to set the timing, ive been doing this for almost 30 years
Read the article I posted and learn something.

ps: the 1/4" stud which holds the air cleaner in place works just as well as a golf Tee and its always avail.
 
#17 ·
BC 73Vette said:
I know how to set the timing, ive been doing this for almost 30 years
Read the article I posted and learn something.
Well, yes I have learned something from you. You have been setting the timing for almost 30 years and still don't know or understand that the vacuum advance is NOT hooked up when checking/setting the timing. :nono: :nono: :nono:
 
#18 ·
Frisco said:
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. :D
Did you miss this?
BC 73Vette said:
The loss of vacuum causes the advance... so, you are shy
The vacuum at idle or low revs keeps the plate from advancing..correct?
 
#19 ·
Frisco said:
Well, yes I have learned something from you. You have been setting the timing for almost 30 years and still don't know or understand that the vacuum advance is NOT hooked up when checking/setting the timing. :nono: :nono: :nono:
Sorry if the arctile I posted is a bit confusing (we had a discussion about the terms used as far as total timing goes)...so call it "Maximum" timing...then it will make sense
My apoligies for not editing the post.
 
#20 ·
BC 73Vette said:
The vacuum at idle or low revs keeps the plate from advancing..correct?
If the vacuum advance is connected to a full vacuum source the vacuum advance will pull in additional degrees of timing at idle.

If the vacuum advance is connected to a ported source the vacuum advance will not pull in additional degrees of timing at idle.

The low revs is to prevent the mechanical advance from pulling in additional timing as well as to keep the carb in the "idle" circuit.
 
#21 ·
I'm glad I missed that string of posts cuz it's a Drag car that happens to have license plates! It sees WOT where there in no vacuum and no vacuum advance. It has 38 total mechanical.

For those who think it's the carb, it has a K rod hanger CC rods (about .030 at the tip) 69 main jets and not sure right off on metering rod. I have richened the secondaries and it does not improve. I am still thinking it has to do with fuel getting to the carb rather than the metering. Yes I'm going lean but that doesn't mean the metering is wrong, I'm just out of fuel in the fuel bowl. I'm going to go out and take the regulator apart and see if there is somehting stopping it from regulating properly.

FWIW, before this problem, that car was very close to perfection mixture wise at WOT. This is a recent occurence. I richened it up to that combo listed above just to see if it would help, it didn't. Before it had a P hanger and CE rods (about .041).
 
#22 ·
BC 73Vette said:
The vacuum at idle or low revs keeps the plate from advancing..correct?
Not correct. The vacuum advance mechanism rotates the baseplate counterclockwise with increasing vacuum which increases the advance under high vacuum conditions. When the vacuum goes lower, the mechanism rotates the baseplate clockwise to retard the timing. The vacuum advance and the mechanical advance are two entirely separate mechanisms.
 
#23 ·
onovakind67 said:
Not correct. The vacuum advance mechanism rotates the baseplate counterclockwise with increasing vacuum which increases the advance under high vacuum conditions. When the vacuum goes lower, the mechanism rotates the baseplate clockwise to retard the timing. The vacuum advance and the mechanical advance are two entirely separate mechanisms.
I do not run full vacuum for the advance, I use a ported source... so I am correct

Honestly, this forum looked loke a pretty cool place but now I'm not so sure....I posted an excerpt from paper written by one of the top GM tuners in the country and have been bashed because of a difference of a term "total timing and Max timing" and also criticized for a "copy and paste"
Please excuse me for my ignorance................
 
#24 ·
1982 SS said:
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...)
Just a thought. Is the factory hard line from the tank to the pump large enough to supply fuel at the rate your engine needs? Is the tank pickup staying in the fuel under hard acceleration?

If the 6 AN return line offers the path of least resistance, is it feasible that the fuel is returning to the tank rather than supplying the carb at the higher RPM?

Just some thoughts. Not sure of any correct answers.
 
#25 ·
BC 73Vette said:
I do not run full vacuum for the advance, I use a ported source... so I am correct

Honestly, this forum looked loke a pretty cool place but now I'm not so sure....I posted an excerpt from paper written by one of the top GM tuners in the country and have been bashed because of a difference of a term "total timing and Max timing" and also criticized for a "copy and paste"
Please excuse me for my ignorance................
A couple of things. If you are going to quote someone, give them credit with a link to the full text. Lars Grimsrud has a lot of stuff in print about engine tuning and is one of the most respected tuners around and wrote this:

http://www.teufert.net/distrib/vacadvspecs.doc

It makes no difference to the VA mechanism whether you use ported or manifold vacuum, it advances the timing when high vacuum is put to it and releases the advance when the vacuum goes lower. It does not do any of the following:
BC 73Vette said:
The loss of vacuum causes the advance... so, you are shy
BC 73Vette said:
Which is it?
The advance comes in as the vacuum drops
BC 73Vette said:
The vacuum at idle or low revs keeps the plate from advancing..correct?
Here's what you pasted from Lars:

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.

Light or moderate loads are synonymous with high vacuum and increased timing. When you increase the load on the engine, the vacuum is reduced and the vacuum advanced timing goes away.
 
#26 · (Edited by Moderator)
onovakind67 said:
A couple of things. If you are going to quote someone, give them credit with a link to the full text. Lars Grimsrud has a lot of stuff in print about engine tuning and is one of the most respected tuners around and wrote this:
Did you ever think that Lars does not want his papers all over the net?

Thats why I only posted an excerpt

Now I have some words for this board
It seemed like a nice place but was wrong as frisco has been breaking my balls for no rason at all

GO ***** FRISCO!
Guess this will get me banned, easier than asking to have my account removed
Too bad too, seemed like a nuce bunch of guys
 
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