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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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#27 ·
The factory hardline has been enough for the first part of this season and all of last season. The new pump and regulator have been more than enough for at least half this season. The only other thought I have is that maybe the canister filter I have before the pump has a bubble in it and it shows up high RPM the most. I am going to bypass the filter and see what happens. While I have the line undone, I will blow some compressed air backwards into the tank in case there is some blockage there.

The regulator is fine I just had it apart and looks good. No junk anywhere in it.

Also, BC that was uncalled for. you know where the door is if you are going to be like that. Too bad it happened in this thread though.
 
#28 ·
BC 73Vette said:
Did you ever think that Lars does not want his papers all over the net?
Lars Grimsrud's papers are all over the internet whether we like it or not. His generous work is linked through several Corvette sites, like this one:

http://www.corvetteclub.org.uk/kb.php?mode=cat&cat=3

His email address is linked with each article so you can contact him directly.
 
#29 ·
Latest development. I bypassed the filter I have before the pump. I used a piece of hardline and bent it to shape and connected my line from the tank right to the pump (before it went tank filter pump). It is no better than it was with the filter plumbed in. I even un-screwed the gas cap in the event that my tank was being sucked flat. No change. It starts to surge at 4600 and gets worse the higher it goes.

I am at a loss as to how to fix this.
 
#31 ·
BC 73Vette said:
It seemed like a nice place but was wrong as frisco has been breaking my balls for no rason at all
I am NOT trying to break anyones balls. If you are going to post info in an attempt to help another person on here then the info should be correct or at a minimum stated that it is your opinion. What you originally suggested ,"It seems like you need to try different springs/weights to get the full 48-53* with zero vacuum" and I originally responded to was outrageous.

The remaining posts by me to you were an attempt for you to get a better understanding of timing and what the vacuum advance unit does and when. I obviously failed in my attempt. So be it.

If you are so thin skinned that you are unable to take constructive criticism that is your problem. I guess if I wasn't able to come to a reasonable end and had nothing else to offer then I'd 'cuss too. When all else fails 'cuss 'em out! That will really help! LOL.
 
#32 ·
onovakind67 said:
You're investing a lot of energy and time in your fuel system with few results. Are you sure your secondaries are opening? Have you thought of checking the ignition system? Got good valve springs?
These are some good areas to check. I tend to agree with onovakind67 at this point. Especially the valve springs if they have a lot of runs on them.
 
#34 ·
That does seem like a good possibility. I will admit the valvesprings on there may not be the best ones in the world, but they do hold the valve to the seat. I looked at the ignition when I had the cap and rotor off and made sure the magnetic pickups were clean and dust was clear of everything. Also, really noticed how nice the Pro Billet looked on the inside, especially since I am going to school to be a machinist, a gorgeous bit of work, but I digress...

Anyways, I went to see a friend of mine who is the auto shop teacher at the high school I went to. I had the car on a lift looking to see if I had any crushed or mangles fuel lines. I asked him what he would check and he asked if I had tried another carb. I told him I had thought of it, but didn't have another carb to try. He had another Q-jet that he had rebuilt for his Camaro, but it wound up with a holley leaving this perfectly fine Q-jet. Anyways, rapidly approaching race time, I threw it on thinking it would be better than the existing carb if it worked. It worked. No lean spot in high rpm, it is a bit rich and the idle circuits are not set up for my car, but it ran better than the one that was on there. The event got rained out (thankfully) so I have another week to tear my carb apart and go through it very thoroughly. When I took it apart the last time, I was looking for something obvious, and apparently something not so obvious is the culprit. Somehting must be hiding in the corners and making it not run right.

Time for a few hours with some compressed air, carb cleaner and a small brush. Here's hoping it runs like it did before this little gremlin popped up.

Also, about how many runs before a modest hydraulic cam with .500 lift and about 230 @ .050 needs new valvesprings?
 
#36 ·
1982 SS said:
That does seem like a good possibility. I will admit the valvesprings on there may not be the best ones in the world, but they do hold the valve to the seat. I looked at the ignition when I had the cap and rotor off and made sure the magnetic pickups were clean and dust was clear of everything. Also, really noticed how nice the Pro Billet looked on the inside, especially since I am going to school to be a machinist, a gorgeous bit of work, but I digress...

Anyways, I went to see a friend of mine who is the auto shop teacher at the high school I went to. I had the car on a lift looking to see if I had any crushed or mangles fuel lines. I asked him what he would check and he asked if I had tried another carb. I told him I had thought of it, but didn't have another carb to try. He had another Q-jet that he had rebuilt for his Camaro, but it wound up with a holley leaving this perfectly fine Q-jet. Anyways, rapidly approaching race time, I threw it on thinking it would be better than the existing carb if it worked. It worked. No lean spot in high rpm, it is a bit rich and the idle circuits are not set up for my car, but it ran better than the one that was on there. The event got rained out (thankfully) so I have another week to tear my carb apart and go through it very thoroughly. When I took it apart the last time, I was looking for something obvious, and apparently something not so obvious is the culprit. Somehting must be hiding in the corners and making it not run right.

Time for a few hours with some compressed air, carb cleaner and a small brush. Here's hoping it runs like it did before this little gremlin popped up.

Also, about how many runs before a modest hydraulic cam with .500 lift and about 230 @ .050 needs new valvesprings?
Tell us a little about your carburetor. Does it use a divorced choke?
 
#37 ·
The Edelbrock 1910, now out of production, is a 850 cfm Q-jet. It uses an electric choke. I bought the carb used for $25, new was somewhere around $500. It had been used for mainly propane before (figure that one), but had seen some gas through it. I pulled it apart when I got it to make sure it was clean. It was as if it had come right from the factory. I bolted it on the 350 that was in the car at the time, it was too much but it got me away from the electric q-jet that had defaulted to full rich mode. When I did the engine swap, the carb went on the 383 and ran like crazy with minimal adjustment (guess I got lucky!). It has run on the 383 since May of '05 and went that season and this season up until a week ago when it started to surge, you probably have read what I have done in a quest to make it right. I have not had time to dig into the carb today, yard sales, wedding receptions, blah blah blah.

I was talking to a friend of mine who works auto parts. He says he has seen a handfull of the edelbrock q-jets (including the 1910) come back with casting flaws that lead to a surging condition. I have no data to back up a complaint like that but I am searching to see if such a complaint has been documented or is known to exist. Just one more thing to look for when I pull the thing apart. I am guessing that a casting flaw such as that would be on the secondary side because I can run it up to 4500-5000 with no problemon the primaries, and nail the gas and have it surge with the secondaries opened up.

For now I have a borrowed Q-jet that mixes fuel (albeit not set up for my car), but it does not surge. I will let you know of my findings as soon as I can get the thing opened up.
 
#39 ·
I'm pretty sure they are opening up. The car runs 16.9's without the secondaries. It went 15.4 surging. The first thing I looked at was the lockout pin, it was fully away from the throttle shaft (you know what I mean, it was disengaged). I made sure it was disengaged everytime I made a test run of it.

Anyways, I have the carb disassembled and I am waiting for the epoxy to dry on the caps over the primary and secondary wells and one other cap on the underside of the main body. I did this in case it was sucking a little air past the outside of the caps.
 
#42 ·
Small enough to make the motor go rich rather than lean. It's a 10 or 11 inch round by 2 inches tall k&N with an Xtreme lid. As far as I can tell, it's just big enough for the motor as long as I keep it clean. I clean it about once a month and it had been cleaned about two weeks ago, so it has seen a good night of racing before this surging issue came up. It has been dirtyier than it is and still run like crazy.

Nothing interferes with any of the linkage. I had to get the velocity stack style base so it would clear the accelerator pump. And it seems small because it is all I have room for under the hood with a 1" spacer under the carb.
 
#43 ·
high rpm lean

Needles and seats for Q-jets come in different diameters. In the Doug Roe Q-Jet book there is a section on drag racing where the seats are progressively changed to smaller ones until a lean surge is achieved.

Not sure what Edelbrock is using but at one time four or five different diameter seats were available from GM.

Paul S
 
#45 ·
For those of you still following this stroy and those who want to get to the end of the post to see the answer...

A combination of a thorough rebuild and putting epoxy all over and around the plugs, it runs like crazy now. It works real good now, pulls clean to 5500 then next gear and 4700 to the end of the 1320. I may need to adjust the secondary air door tension a little bit, but it is very close to the tension it was when I pulled it apart.
 
#46 ·
Glad to here the problem was solved, but I am guessing that the epoxying the plugs didn't solve it. The plugs should basically only give you a massively rich condition at idle. Quadrajets can be so wierd. I was chasing a bog off idle and went through it without really finding anything strange. Back on the car for a test drive and I almost ate the steering wheel at 4500 RPM, with it catching and pulling me back in the seat at 4300 and off again at 4500. That high RPM drop off wasn't there before. I haven't had the chance to go back through it, but I'm betting more on the top bowl gasket not positioned exactly, allowing a large enough air leak to prevent the secondaries from sucking fuel properly. You might have just positioned the gasket better and solved the problem on the last assembly.
 
#47 ·
One of the plugs could have been leaking but instead of dripping, sucking air. Who knows. I spent the better part of an hour just figuring which part of the secondary went where. Same with the idle circuit, but I didn't have a problem with that, I just figured if I was going to do it, I had better do it right. I'm back to where it was before now. Also, I had noticed my consistency was slowly disapperaing at the track, I wonder if this was something that had been there a while but had just gotten bad enough to finally seriously hinder performance. I still think I need to loosen the secondary air doors just a little though... Things to do, but not enough track time.
 
#50 ·
I'm not talking about the choke, just the choke lockout mechanism. It prevents the secondaries from opening if the choke is applied, and can hold the secondaries out even when the choke is open if it is misadjusted. You wouildn't want this to come into effect at the strip as it will make a huge difference in the carb. Are you familiar with the choke lockout mechanism on your carb?
 
#51 ·
The lockout is actuated by more than just it's weight alone. When the choke is off and the lockout is disengaged There is a pin on one of the other choke arms that pushes the top of the weight back swinging the bottom foreward. There is no way that that lockout can be engaged if the choke is off. Plus it does not have enough ckoke to be on more than about 2 1/2 minutes before the lockout comes off. The lockout arm also acts as a spacer and I'm not about to hack my linkages up.
 
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