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Old 11-06-2009, 06:44 PM
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350 light valve rattle under no load

I've got a little '23 T with a fresh 350 that's beautifully set up, strong, fast, smooth and all, but there's one little thing. As it hits 55-60 mph, the load drops off, the vacuum comes up, and it gets a very light valve rattle until over 60 when the aero load increases and it goes away. I have been from one end to the other with initial advance, cent. adv. and vac adv., lifters from loose to tight, higher octane gas, and nothing makes it go away. It's clearly not hurting the engine or affecting performance, just frustrating. I'm sure the big engine/little car thing has a lot to do with it. Any ideas? Specs follow to keep post short.
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Old 11-06-2009, 07:23 PM
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Engine specs

350, +.030, 327 Power Pack heads, Comp 252H kit (cam, lifters, timing set) Delco HEI, Weiand dual plane intake, Holley 600, 3.00 rear end.

18' initial adv, yellow springs (all in by 1600), short vac can with 10' adv.
Total adv is 48' but I have run with much less and more and rattle is always there. I have locked out cent adv to 10' but it runs better with 20'. Even stock vac can runs fine except for brief rattle going thru 55-60 mph.

It's not like the car runs poorly, quite the opposite, great idle, jackrabbit launch, tire-chirping shifts, beautiful shifts and downshifts. But why does it rattle when the load drops off?

Last edited by Aerotech; 11-06-2009 at 09:31 PM.
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Old 11-07-2009, 05:57 AM
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I would try stronger advance weight springs. If it is a crane kit...try the blue ones or maybe a blue/silver combo.

That's a great little engine! I can see why why it is so snappy...outa be a tire fryer.
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Old 11-07-2009, 06:03 AM
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Do you have your vac advance hooked to ported or full vacuum?
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Old 11-07-2009, 06:40 AM
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Full vac. Does the same on ported vac too. As for springs, I've been all the way up to 2 blues so the cent comes in at 3500, which this engine rarely reaches. Even locked it down to 10' with a screw through the rotor base.

Last edited by Aerotech; 11-07-2009 at 06:45 AM.
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Old 11-07-2009, 08:40 AM
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Power pack heads with that small cam probably has lots of cylinder pressure. Do a compression test give us the readings.Are you running 91 or better fuel.
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Old 11-07-2009, 08:54 AM
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You may be on to something there. The compression is about 10.25 or so, didn't check pressure but miked the piston at TDC to calculate the CR. I think the cam would prefer lower compression. Oddly, the octane seems to have little to do with it. 93 works fine, octane boost does not help. Accidentally pumped some 87 yesterday (selected wrong grade) and it ran just fine. It never, ever, ever knocks under load. Cam is supposed to be ok up to 11:1 cr, but I suspect it is much happier at the lower end of its range.
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Old 11-07-2009, 09:04 AM
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Check #8 cylinder for signs of detonation. (spark plug appearance, cylinder scope) What do you have connected to the vacuum port above # 8 cylinder's intake runner on the top of the intake manifold?
What ever is connected at that point (P/B's A/C, auto trans modulator, cannot be leaking air (vacuum) or that cylinder will run leaner and ping at PT. (yes it can damage the motor). If you have the PCV connected there, move it to the carb base or manifold base plenum. If the PCV is drawing in oil from the valve cover in the intake it will knock also. Fix the valve cover grommet baffle. (typical aftermarket valve cover design issue.)

Your advance curve is too aggressive also. take a few deg of timing out of it to start. recurve it for a little less, a little later.
Possibley a hot spot (sharp metal edge) in one of the combustion chambers agravating the situation.

Try cooler heat range non projected tip spark plugs.
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Old 11-07-2009, 09:07 AM
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I believe your compression information is wrong
Increase your street machine's gas mileage and powerband performance!
COMP Cams High Energy cam and lifter kits are designed for street engines with an 8:1 to 9:1 compression ratio. They also work well with all stock components, including intakes, exhaust manifolds, and carburetors. These kits offer good increases in both gas mileage and power throughout the rpm range for most street applications.

Warranty


COMP Cams warrants its products to be free from defects in material and workmanship for one year from the date of purchase. This warranty is valid on camshafts only where new lifters and proper valve springs are used. This warranty is voided on any part used in racing applications or any cam part ending in -5, -6, -7, -8, or -9.
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Old 11-07-2009, 09:12 AM
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#8 cylinder is fine and no vacuum leaks. PCV to carb base and trans vac to manifold. Some of the rattle may be PCV chattering. Is there a quiet PCV without a moving valve?
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Old 11-07-2009, 09:21 AM
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Have you tried backing the timining off alittle maybe? I think you need to tweek the timing. I would go to 14 degrees at 800 at idle and go from there and see what you have then. But those heads are going to restrict the engine IMO.Just courious what rocker arms are you running?............Cole
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Old 11-07-2009, 09:55 AM
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Could be that the vacuum advance is adding too much advance, is it
adjustable? Did you try running it with the canister disconnected?

Another consideration is it might be too lean at that point, may be the
power valve closes leaning t too far.
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Old 11-07-2009, 09:56 AM
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On a o decked, flat top piston 350 your cr could be as high as 11.2:1 with a 58cc "power pack head" Even higher if such head have been shaved down.

Too high for pump gas.

38deg mechanical timing is too much. use 34deg total mechanical and 10deg vacuum adv. the rate in/out of that 10deg vac adv may need tweeking. (spring tension)
600 holley carb jetting. try 67 pri and 74 sec. \make sure the idle and high speed air bleeds are open (blow out with compressed air).
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Old 11-07-2009, 10:52 AM
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Good suggestions about the timing, but I thought the same thing and I have already tried those suggestions. Remember this is a VERY light car with a VERY torquey, low-rev engine and it can tolerate enormous amounts of advance without knocking. Retarding the timing and using a short, stiff advance curve gives horrible results, while a very aggressive curve gives excellent results all the way into low 50s. Cam is pro installed and degreed, so it's not cam timing.

Carb has been painstakingly set up too, and runs best on stock jets and power valves with a 25 shooter (31 is stock). Fresh bowl job, float adjust and idle mixture adjust - all good. It did respond very well to the lightest secondary spring.

There may not be an ideal solution to this (admittedly minor) problem. Below 60, the engine is pushing weight and above 60 it's pushing air. It's just the little butterflies at the top of the rollercoaster that makes the valves a little ticklish.
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Old 11-07-2009, 11:09 AM
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When this type of noise doesn't respond to a reduction in ignition advance,
higher octane and or more fuel, I look for another source of the noise other
than detonation. Although what you describe sounds just like spark knock.
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