Breaking Up Above 3400 RPM - Team Camaro Tech
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post #1 of 38 (permalink) Old Nov 14th, 05, 08:19 PM Thread Starter
 
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Breaking Up Above 3400 RPM

I get a crackling noise out of the exhaust and a rev limiter effect only in gear with a load acclerating when the engine gets up to 3K plus rpms. I get a missing or a roughness when holding rpms above idle in neutral, and it gets progressively worse as the rpms increase. Idles fine (except for inconsistant idle and vacuum), flat revs fine to wherever, and generally accelerates fine until I get up around 3K rpms. I can pinpoint the rpm at 3400 rpm where the exhaust starts making a different sound. The engine feels rough like it's missing and vibrating up until exactly 3400 (all the time) on the tach, where the exhaust makes a rougher tone and the engine starts getting that rev limiter effect. I did this in neutral with no load as well and it also feels rough up until 3400 where the exhaust starts to make a different tone and it gets that rev limiter effect. I can free rev it until wherever, but I first noticed this driving and accelerating.

I shorted out the plugs with the engine idling and couldn't find any missing cylinders at idle. Can this rule out ignition problems?

Then I reset the preload on the valves to roughly 1/4 past zero. Went for a test drove and the valve adjust didn't help any. One thing that I did notice while adjusting the valves is that a couple of the pushrods didn't turn as fast as the rest. But, all the valves seemed to open the same. The springs all looked to be in good shape. Can a bad valvetrain be ruled out?

I think I have a problem with my valve seals or guides on the one head, as previously determined here. Would the increase in rpms have enough effect to cause the engine to miss when the rpms are up?

Another thing that I thought of is a vacuum leak causing a lean condition which causes missing or popping. Possibility?

What kind of condition am I actually seeing with this driveability problem?

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post #2 of 38 (permalink) Old Nov 14th, 05, 08:22 PM Thread Starter
 
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Re: Breaking Up Above 3400 RPM

Plugs were gray on the porcelain, electrode, and ground strap. Not full of metal flakes, but just a grayish tint instead of tan or black. Never seen gray, what does that mean? I cleaned them all up and stuck them back in. I checked the plug wires for an ohms reading. I'm using parts store brand (carbon core?) wires, currently, that haven't been on long enough to go bad. Here are the results (in K Ohms:

1)7.3 - 32"
2)7.2 - 32"
3)9.0 - 40"
4)6.1 - 26"
5)4.7 - 22"
6)5.8 - 26"
7)4.7 - 22"
8)4.6 - 22"

Just for comparison I did a test on an old pair of Moroso Blue Max 8mm pre-crimped wires that I thought were shot.

1)1.8 - 32"
2)1.8 - 32"
3)1.8 - 32"
4)1.5 - 26"
5)1.3 - 22"
6)1.8 - 32"
7)1.3 - 22"
8)1.3 - 22"

Is this a good test? Does it distinguish that one is shot and the other is ok?

I put the parts store wires back on so I could get home. Fired it up, ran it hard a bit, and shut it off right away. Pulled a plug and it was oily right away. I believe I have a valve seal problem, so that can't be helping much.
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post #3 of 38 (permalink) Old Nov 15th, 05, 11:01 AM
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Dave
 
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Re: Breaking Up Above 3400 RPM

Man, could be a few things here;

clogged fuel filter or line (check this FIRST!!)
break in fuel line causing it to suck air (rubber hoses notorious for this)
fuel pump
cap
rotor
coil
vacuum leak
loose/clogged carb

Of course, there are many more possibilities, but I bet $100 that it's one of those.......

Please post results

Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
Bought my first big-block Chevy in 1970
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post #4 of 38 (permalink) Old Nov 15th, 05, 11:43 AM
Derrick Schmidt
 
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Re: Breaking Up Above 3400 RPM

I had a simular problem years ago and it ended up being the pickup coil on the HEI. If you have another distributor to try it would probably be worth a checking out.
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post #5 of 38 (permalink) Old Nov 15th, 05, 11:46 AM
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Greg
 
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Re: Breaking Up Above 3400 RPM

PLUG WIRES! If you are using suppression core wires, that is how they do when heat breaks them down. Put an ohm meter on 'em to check.

I used to eat wires like candy with my headers. Finally went to solid copper core wires and problem solved.

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post #6 of 38 (permalink) Old Nov 15th, 05, 12:31 PM Thread Starter
 
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Re: Breaking Up Above 3400 RPM

About the only way to check for fuel when running is a inline pressure gauge, correct? As far as I know, i've got plenty of fuel to the carb. It will rev up fine if I gun it, just is rough and misses when rpms are slowly increased and are held.

I've been into the distributor and the cap looks good. The posts on the top outside look like they've been scraped, but the distributor rebuild is fairly new. The coil was checked, the module was checked, the cap/rotor was put on new, etc. Any advice as to how to check those out now? I don't have another HEI on hand. Well, I do, but it's in the grain truck (and that runs great ). I know the plugs are getting juice and the cylinders are firing, but that's at idle. Should I try to short them out at 3400 rpm, or isn't that safe? I believe these are suppression carbon core wires as they are OEM replacements, the quickest wires to get and try at the time. I was running on 4 cylinders at the time before I reset the valves, put in new R44T's, and put in these wires.

I've sprayed aerosol cleaner and also tried using an unignited propane torch in around the intake sealing areas in the front-back-and on the sides, and on the carb base area. I've got a fluctuating vacuum at idle, but the idle is also fluctuating at idle.

I've gone through and reset the lifter preload at 1/4 past zero, or actually 1/4 past quiet while it was running.

I tested both the resistance of the wires that are on there now, and the old Moroso Blue Max 8mm spiral cores, and listed that in my second reply.

One other thing to note is that the plugs on the driver side (side with a possible valve seal leak after revving it hard) are light gray, but show a heat range. And the plugs on the right side are the normal tan with a heat range.

Thanks much!
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post #7 of 38 (permalink) Old Nov 15th, 05, 02:00 PM
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Eric
 
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Re: Breaking Up Above 3400 RPM

Sounds lean to me, if everything else checks out. I would check the fuel filter and if your carb has a screen on the inlet check that too...then if that checks out I would guess a vacume leak.

That is my 2 cents that are worth half that.....

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post #8 of 38 (permalink) Old Nov 15th, 05, 02:21 PM
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Dave
 
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Re: Breaking Up Above 3400 RPM

Clogged fuel filter or bad fuel line or weak fuel supply would all let engine rev just fine. Float bowl fills up and a rev or two won't empty it. However, it (float bowl) will empty quickly under load, then produce conditions exactly as you describe.

Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
Bought my first big-block Chevy in 1970
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post #9 of 38 (permalink) Old Nov 15th, 05, 02:58 PM Thread Starter
 
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Re: Breaking Up Above 3400 RPM

Can I simply just disconnect the fuel line and crank the engine over to check?
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post #10 of 38 (permalink) Old Nov 15th, 05, 02:59 PM Thread Starter
 
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Re: Breaking Up Above 3400 RPM

Oh, and the carb has a new filter in the inlet, and no screen. Don't know if the Quad's have the screens.
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post #11 of 38 (permalink) Old Nov 15th, 05, 03:41 PM
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Re: Breaking Up Above 3400 RPM

Quote:
Originally Posted by Malibu Man
Can I simply just disconnect the fuel line and crank the engine over to check?
Yes, I think if you idle the engine and let the fuel run into a can, you should get about a pint of fuel in 10 seconds, depending on fuel pump and lines. My math could be off here, so someone correct me if I'm wrong. The engine will idle for a bit from the fuel in the float bowls.

I never liked to test this way as it's VERY dangerous (as in "life-threatening") to have the engine running and gas coming out of the disconected line. One spark from a bad plug wire or anything else could have really bad results. I always use a fuel pump pressure gauge and that's what I would strongly recommend. Go borrow one from someone.

Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
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post #12 of 38 (permalink) Old Nov 15th, 05, 05:51 PM Thread Starter
 
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Re: Breaking Up Above 3400 RPM

I've got a hand-held fuel pressure gauge that I used on my S10 (w/ the schrader valve tap), and a Mac hand-held fuel pressure.vacuum gauge. It's just a matter of adapting it the the system, which shouldn't be a big problem. I don't think the Mac gauge would work the best with this system, although I did used it on a '61 Chevy with a 261 inline. I've got the Mac gauge over by the car and I have to search for the other gauge. Now I just remember that my Quad has the smaller float bowl of the different sizes, but it does have the smaller float as well.

I pulled the plugs again to look them over, and to replace them when I get a hold of a set. They all have oil on them, on the threads and on the straps. It's hard to tell a pattern now. The driver side plugs are slightly grayish-black carboned on the flat part of the strap and on the porcelain/electrode. I believe the inside plugs were oily where the ground strap connects to the base, and the outside plugs looked like new where the ground strap connects to the base. The passenger side plugs had slightly grayish straps and tan/gray porcelain/electrodes.
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post #13 of 38 (permalink) Old Nov 16th, 05, 04:33 AM
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Re: Breaking Up Above 3400 RPM

Quote:
Originally Posted by Malibu Man
I get a crackling noise out of the exhaust and a rev limiter effect only in gear with a load acclerating when the engine gets up to 3K plus rpms. I get a missing or a roughness when holding rpms above idle in neutral, and it gets progressively worse as the rpms increase. Idles fine (except for inconsistant idle and vacuum), flat revs fine to wherever, and generally accelerates fine until I get up around 3K rpms.
First, you need to isolate the inconsistent idle. Vacuum gauge on full time vacuum reads? Should be steady, if a stock camshaft, usually around 18-21 in/Hg. if mixture screws are adjusted evenly, ignition timing is correct, and engine is mechanically sound and intake gaskets in good shape.

If vacuum varies quickly, then its either no fuel or no spark to a cylinder. Or a flat cam lobe, usually accompanied with an upper engine knock.

If tis lack of fuel at higher rpms, spray into the carb WD-40 and listen for the engine to smooth out. Usually shows a leaky intake manifold gasket. Temp gun on the exhaust manifolds might show a temp difference between live & dead cylinders, but hard with a cast iron manifold.

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post #14 of 38 (permalink) Old Nov 16th, 05, 07:55 AM Thread Starter
 
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Re: Breaking Up Above 3400 RPM

I shouldn't really say inconsistant idle, it occasionally drops or goes up slightly on the tach, but it's a big cam. It's a 292H with 72deg overlap. So vacuum is lower as well. I believe manifold vacuum was like 11" out of gear with the Holley, and is around 9" now with the QJet. I believe the idle screws are 2.5 turns out, and I tried to set them with the vacuum gauge. Running 14deg initial timing. Can't be totally sure if the intake gaskets are sealing good. I can't find a vacuum leak with spray or a propane torch. I do have oil leaks on the front and rear intake valley's.

I checked all the valves to make sure they moved evenly, and they looked to be. All the springs look to be good. I reset the valves, and that didn't help.

I've got a set of headers on the car so the temp gun is a possibility, if I can find one at a fair price.
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post #15 of 38 (permalink) Old Nov 16th, 05, 09:11 AM
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Re: Breaking Up Above 3400 RPM

Big cam = crappy and inconsistent idle. But who cares, it sounds a lot better loping along that way!

Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
Bought my first big-block Chevy in 1970
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