Perhaps you guys can help me sleuth another elusive problem. Seem to have one every time I drive the car these days, but intent on working out all the bugs over the winter.

A few Sundays ago, I took her out for one last drive before the winter snow set in. Drove around for about 1 hour and it ran absolutely perfect..just a blast to drive, real smooth and gets way better gas mileage than my 2003 Dodge Durango.

Before bringing her back to the garage, I thought I would give her one last really hard run..if anything is going to break, best if it happened then so I could fix over the winter. I also wanted to test the 6600rpm pill I put into the rev limiter. I learned the hard way that the 6200rpm MSD pill I had in the 6AL could only rev cleanly to 5800. I wanted 6200rpm out of it so stuck in the 6600.

Found a nice big empty parking lot and dumped the clutch at 2,500 rpm. Being traction limited, she immediately jumped to 6200-6400 rpm or so. In the past when I did this sometimes it would bounce off the rev limiter and run crappy for a minute or so afterwards, then clear up and run fine. I figured the cylinders where loading up because the spark cuts out floored at peak rpm from the rev limiter, and that it was normal.

I then did a second high RPM blast and she bounced off the rev limiter, and the engine started running crappy when it idled down, but this time it just wouldn’t clear up. It won’t idle below 2,000 rpm and there is a slight constant popping in the exhaust. Played with it for a few hours in the driveway, noticing that it would start right up and run for a few seconds, then try to stall like it was running out of gas, if below 2000rpm. At 2000rpm+ it seems to run just fine, but below that she tries to die. I also have an air/fuel guage in the passenger side header and noticed the indicator lights would go out when I was below 2000rpm like it wasn’t working, but came to life and read normally above 2000rpm.

The following are the things I looked at or tried thus far, but have been unsuccessful in solving the problem?

-Fuel?: I have 2 fuel pressure gauges..one at the carbs and one in the cockpit. Both are reading a healthy regulated 6 pounds, so plently of fuel pressure. I have heard that anything above 6 pounds, will push the needle off its seat in the Edelbrock carbs and could cause too much fuel at idle. Therefore I lowered the fuel pressure via the regulator to 3.5-4 pounds and it did seem to run better below 2000rpm, but the general problem described above remained.

By the way, the plugs where a pale tan as I had been pushing the motor hard the last 2 times out…slightly lean at full throttle. She is slightly rich at low to mid-rpm. Needs a little more fine tuning, but close. Air fuel guage shows 13.5-14:1 at idle and about 12.5:1 at full throttle. I need to richen it to no more than 12:1 at full throttle, I think.

-Stuck float?: I tapped both carbs with a wrench in case a float might be stuck in the twin Edelbrocks.

-Spark?: I have had cases where plugs would become carbon fouled and NG after loading up on a motorcycles, so I put in all new spark plugs, freshly gapped. I also put a different pill in the rev limiter. All in vain.

-Valve train problems holding a valve open?: I looked under both valve covers..I thought I might find a broken valve spring, even though no strange noises where coming from engine. Everything looked fine, springs where all in one piece and I wiggled all the push rods and everything seemed to have the right amount of tension.

-Vacuum leak?: I have only one vacuum line for the power brakes and that looked fine.

Hhmmmm.. I have run out of ideas, but still thinking about the carb and vacuum. Everything is brand new with only about 600-700 miles on engine.

Thanks for any help/ suggestions you can provide.


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1973 RS Z28: Littlefield race series 6-71 supercharger on a 355 w/ AFR 195 heads, 10-12 lbs boost, 7.6:1 static compression, CompCams hydraulic roller (276/288 advertised, 224/236 @.050 duration, .510/.527 lift w/ 1.52 rockers on a 113 lobe separation), AFR Hydra-Rev kit, Eagle 4340 Forged Steel crankshaft, ARP studded bottom end, Eagle ESP H-beam rods, TRW forged reverse dome pistons, Comp Pro-Magnum 1.52 roller rockers with 7/16 studs, twin 750 Edelbrock carbs, 1&3/4 inch Hooker Super Comps, 3" dual exhaust with cross over pipe. MSD 6AL ignition & Pro-billet distributor, Tremec TKO-500 5-speed, Moser 12 bolt with Eaton posi, 33 spline axles & 3.42 gears.

Start with the simplest most failure prone stuff first.

spark plugs: Not fouled, bent ground strap, broken, etc. I usually just pull them, inspect, and replace.

Wires: Arcing, good connection, not sitting on header...

Cap and rotor: Visually inspect, watch for arcing.

Next I would do a misfire test by pulling wires.

Then I would check compression, if I found one low, I would leak test...
Coulda bent a valve.

Spark, fuel, air in and out..., compression, and timing...

Tokyo,
I have this exact same problem with my 600 cfm eddy, my motor will run great above 2000 rpms. I am totally stumped also. Ive got a new cap, rotor, wires, plugs, re-did intake gasket, new carb gasket, re-adjusted valves, re-did the timing and have done just about every other imaginable thing but pull the carb apart. I personally suck with carbs and dont want to mess it up worse than it is now. I think it has to do with the idle jet since that is the only thing that gives the motor gas when you are not in the throttle. Let me know if you figure it out, I am probably going to get a new idle jet and see if that cures the problem!
Thanks Travis

Could you have run lean and burnt a valve?
I had a motor with 3 burnt valves in it run like that once (admit without all the hightech goodies that your engine has)
But thats the way it ran

Missing and popping out of the exhaust is a burnt valve sympton, I would do a compression check first thing.

Could a burnt valve cause the car not to want to idle properly? Sorry Tokyo, I am kind of hijacking the thread but im in the same perdicament. My heads are new and only have maybe 200 hours on them. What causes a burnt valve?
Thanks Travis


[This message has been edited by 78RSCAMARO (edited December 25, 2004).]

<font face="Arial,Verdana" size="2">Originally posted by 78RSCAMARO:
Could a burnt valve cause the car not to want to idle properly? Sorry Tokyo, I am kind of hijacking the thread but im in the same perdicament. My heads are new and only have maybe 200 hours on them. What causes a burnt valve?
Thanks Travis

</font>

Anytime you have compression leaking that causes a missfire (valves, rings, head gasket etc) it will be more noticable at idle because of the slower engine rpm.

Valves can become burnt by many things, dirt, carbon or metal shavings that get caught between the valve seat and the valve face is the most common way. The dirt, carbon or metal lets a small amount of combustion flame leaks out, this acts like a torch and will very quickly burn a groove into a valve and seat. Also running too lean can cause a valve to over heat (mostly happens to exhaust valves) and as the valve heats up it start to melt, sometimes the tulip can fall off in very extreme instances.

[This message has been edited by Joekool (edited December 25, 2004).]

<font face="Arial,Verdana" size="2">Originally posted by Joekool:
Missing and popping out of the exhaust is a burnt valve sympton, I would do a compression check first thing. </font>

Yep, I second the motion.


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'Silver Bullet' 1977 Z-28 413" SB
11.73 @ 115.10 and dropping...
Old Vortec combo:
12.15 @ 110.52
1989 GTA L98
Daily driver

<font face="Arial,Verdana" size="2">Originally posted by 78RSCAMARO:
Could a burnt valve cause the car not to want to idle properly? Sorry Tokyo, I am kind of hijacking the thread but im in the same perdicament. My heads are new and only have maybe 200 hours on them. What causes a burnt valve?
Thanks Travis


[This message has been edited by 78RSCAMARO (edited December 25, 2004).]</font>

Why sure they could. http://www.nastyz28.com/ubb/wink.gif It's bleeding compression and making things uneven.

Any chance you "floated" a valve on your over-rev. Perhaps it smacked the piston slightly and just won't close all the way.

Sounds like bent valve worse, bent spark plug stap best if it happened durring a high rpm blast!

Thanks guys..next thing I do will be a compression test.

I would be surprised that it ran lean enough to do damage. Spark plugs have been all pulled & examined and they look fine. I also don'y think the valve could have contacted the piston..I have a low 7:6:1 compression w/ 74cc heads and my valve lift is conservative.

JoeKool's comment has got me thinking about a blown head gasket. I am not leaking coolant into the cylinder, but I wouldn't be surprised if I blew a head gasket as I am running on the high side of boost.

If that is the case, I will switch to the cometic MLS gasket which I hear can handle much higher cylinder pressures.

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1973 RS Z28: Littlefield race series 6-71 supercharger on a 355 w/ AFR 195 heads, 10-12 lbs boost, 7.6:1 static compression, CompCams hydraulic roller (276/288 advertised, 224/236 @.050 duration, .510/.527 lift w/ 1.52 rockers on a 113 lobe separation), AFR Hydra-Rev kit, Eagle 4340 Forged Steel crankshaft, ARP studded bottom end, Eagle ESP H-beam rods, TRW forged reverse dome pistons, Comp Pro-Magnum 1.52 roller rockers with 7/16 studs, twin 750 Edelbrock carbs, 1&3/4 inch Hooker Super Comps, 3" dual exhaust with cross over pipe. MSD 6AL ignition & Pro-billet distributor, Tremec TKO-500 5-speed, Moser 12 bolt with Eaton posi, 33 spline axles & 3.42 gears.

Without knowing the results of the compression test I'm leaning towards something valvetrain related.

If you revved it way up and it ran cruddy for a little while when it went back down to idle that sounds like floating the valves, causing the lifters to pump up and hang off their seats for a little while before bleeding back down again and running normally.

Then you went a little further with it and now it WON'T return to a normal idle. I'm thinking you bent a pushrod, killed the guts of one or more lifters or smacked a valve a little crooked due to some valve float-related issue.

If you have valvetrain issues you'll see it when you do the compression test. Should be very obvious.

I did a compression test today.

First test with engine warm (but forgot to open the carb blades). Did the test a second time with carb blades open, engine a little cooler, but still warm. Not much difference anyway. My average cylinder compression is 136 PSI. This is a 7.6:1 static compression engine, thus the lower PSI values:

Cylinder PSI#1 PSI#2
1 130 132
3 135 140
5 131 130
7 142 132
2 132 152
4 137 140
6 148 130
8 132 135

Since the lowest value is within about 12% of the highest value in either run, it looks like I do not have a head gasket or valve train-related problem.

However, I did notice something when I removed the new spark plugs I had just put in while trying to figure out this problem. I had only run the engine in the driveway between 1000-2000 rpm, but the front 2 plugs on both cylinder banks where white.. indicating a lean condition. The back 2 on either side looked ok. This may have to do with why my air-fuel guage reads fine when I rev it to about 2000+, but the air-fuel guage lights go completely out at any rpm below that.

With this in mind, I would guess that the primaries on the front edelbrock carb are not flowing fuel. The exhaust popping I hear at only low rpm is the lean condition, I think. I have two 4-barrel 750 edelbrocks on the blower, so at less than 2000rpm, it seems that I have no flow from the primaries to the front 2 cylinders, but only the back 2 cylinders since on the primaries in the back carb are working. Above 2000rpm, the secondaries in the front carb kick in and that probably explains why the lean condition seems to go away at higher RPM.

Does this seem to make sense? I have a big FRAM fuel filter on her, so I wonder what would happen in the carb to prevent fuel from flowing from the primaries in one of the carbs?

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1973 RS Z28: Littlefield race series 6-71 supercharger on a 355 w/ AFR 195 heads, 10-12 lbs boost, 7.6:1 static compression, CompCams hydraulic roller (276/288 advertised, 224/236 @.050 duration, .510/.527 lift w/ 1.52 rockers on a 113 lobe separation), AFR Hydra-Rev kit, Eagle 4340 Forged Steel crankshaft, ARP studded bottom end, Eagle ESP H-beam rods, TRW forged reverse dome pistons, Comp Pro-Magnum 1.52 roller rockers with 7/16 studs, twin 750 Edelbrock carbs, 1&3/4 inch Hooker Super Comps, 3" dual exhaust with cross over pipe. MSD 6AL ignition & Pro-billet distributor, Tremec TKO-500 5-speed, Moser 12 bolt with Eaton posi, 33 spline axles & 3.42 gears.

I don't know? Sounds strange to me but I would think all the plugs would look lean if there were no fuel flowing in the front carb just because after it squeezes through that big box then into the open intake it would all pretty much end up coming out completely mixed.
This is not the same but if you remember the big block Malibu I pointed at before, that guy had an 800 and 850 Holley on the shelf so he called Dyers and the guy told him mis matched carbs don't matter ,they've done all that and as long as the jetting is right it's fine because it all comes out the bottom the same.Look down the carbs and check it out, if you see fuel drawing pull the intake and see what you find.

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1978 Z-28 purchased new by me -$6750
Click here (http://f1.pg.photos.yahoo.com/ph/someonehasthisid/detail?.dir=/d752&.dnm=aefd.jpg) to view present state photo
Look above for glory days.
1994 Formula
1982 350 S-10

[This message has been edited by Mike-78 Z-28 (edited January 02, 2005).]

TT3 what jets and rods are you running? I'm pretty sure with the Edy carbs that they will always flow fuel as long as you are drawing enough air through them. Your blower is more efficient than mine was since you're running with the teflon strips(more air means you need more fuel), maybe you're on the lean side with your jetting. Also what are you running for a fuel pump?

The Edelbrock carbs share the same fuel bowl for the primaries and the secondaries.

Mike78Z28: I checked the carbs and both primaries are squirting fuel. I am really stumped at what would make just the front 2 cylinders on **both** cylinder banks run lean.

Flash: I forget the jets sizes that Al put in, but I have the specs somewhere. We jetted it as fat as possible. It was jetted/ carburated perfect all season until this problem suddenly happened. I have an air fuel guage and always pull the plugs, so I know it is not a jetting issue. The problem suddenly occured. I use a Holley mechanical high volume pump regulated down to just under 6 pounds. I have plenty of fuel pressure..the other day at the track she was still at a full 6 pounds going through the traps and the air:fuel mixture was spot on then too. I made a point to check my boost/ fuel/ air:fuel guages at the end of the track, but everything was great until this happened.

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1973 RS Z28: Littlefield race series 6-71 supercharger on a 355 w/ AFR 195 heads, 10-12 lbs boost, 7.6:1 static compression, CompCams hydraulic roller (276/288 advertised, 224/236 @.050 duration, .510/.527 lift w/ 1.52 rockers on a 113 lobe separation), AFR Hydra-Rev kit, Eagle 4340 Forged Steel crankshaft, ARP studded bottom end, Eagle ESP H-beam rods, TRW forged reverse dome pistons, Comp Pro-Magnum 1.52 roller rockers with 7/16 studs, twin 750 Edelbrock carbs, 1&3/4 inch Hooker Super Comps, 3" dual exhaust with cross over pipe. MSD 6AL ignition & Pro-billet distributor, Tremec TKO-500 5-speed, Moser 12 bolt with Eaton posi, 33 spline axles & 3.42 gears.

With dual carbs and a lean issue with cyls 1 to 4 it sounds like the front carb isn't doing equal work with the rear carb. Check throttle linkage for even actuation, richen up the front mixture screws, and turn up the idle speed on the front carb. Naturally, you will have to lower the idle speed on the rear carb a little to get your base idle where you want it. I prefer to back both idle screws off until the throttle blades seat then tighten the screws equal numbers of turns until base idle is achieved. I do the same with the 4 mixture screws. You may also have your front carb's floats lower than the rear, causing your lean condition.

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<font face="Arial,Verdana" size="2">Originally posted by Tokyo Torquer3:

Flash: I forget the jets sizes that Al put in, but I have the specs somewhere. We jetted it as fat as possible. It was jetted/ carburated perfect all season until this problem suddenly happened. </font>

OK, if it was good most of the season, it's probably not internal to the carbs. Rust has some good tips, I would check the dual carb linkage to make sure that they are not just both working together, but that both carbs are closed all the way when you're not on the gas. Also recheck the idle screws, I think Al suggested 1.25 to 1.5 turns out from being bottomed out.

USUALLY...
You set the rear carb to be the idling carb on a dual carb setup.

The front carb and primaries are not used until tip-in....

The rear carb's primaries are centered over the motor, the front carbs are up at the front....

On a Roots type blower, it may very well need the front primaries feeding at idle..