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  #1 (permalink)  
Old 06-10-2004, 04:41 PM
dion's Avatar
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need help from the experts

some of you may remember a while back I was having trouble with some odd noise with my 350, well it's still there and I was hoping for some freash ideas, cuz I'm out of them.

Here's the story, built 350 from top to bottom, 9.5-1 dished pistons, .60 bore, large cam, vic jr intake, Q jet, HEI etc.. well the truck ran fine but I wanted a little more power so we swaped the rebuilt stocker 76CC heads for a set of 64CC 2.02 1.60 double humps with roller rockers, a lot more power but now at 2500 to 3500 RPM there's this ugly kinda slapping noise coming from the exhaust, it's only at that RPM and only when the truck is being driven. I figured it was dentonation from CR jumping up, I ran the timing at all sorts of different settings and none of them made a difference.

I've changed the stock HEI to an Accel with supercoil, no change. thought it might be valve float so changed the springs, nada, still there.
A local speed shop told me they didn't think it was mechanical and that it was a baffle in my muffler, well my 40's were pretty old so I bought new ones, still there.

I've been told by three different hot rod mechanics that it's just high compression and that's how it sounds. I don't buy it, there's something I'm missing here. I've checked everthing I can think of.

Any ideas will be helpfull, thanks.

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Old 06-10-2004, 05:06 PM
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No expert here by no means, but did you check the roller rockers clearence? they can hit several things, we had to grind them in a few cases to get the clearence needed. also at one time, I had this strange tap with my car, drove me nuts a long time, never did find it until I pulled the engine, the motor mount plate to the block was loose, letting it rock back and forth. another time I was checking over everything trying to find a tap and found the power steering bracket bolts that are behind the pulley were loose. Sometimes noises can be hard to track down, but that`s what I use the broom stick to the ear for. Good Luck Dion.
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Old 06-10-2004, 05:22 PM
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thanks DV, haven't checked for clearance but have re-adjusted them a few times thinking they were out.

strange thing is that you can't hear the noise coming from the engine itself just from the pipes while driving.

I've been chasing this noise for a year now probably, it's really getting old.
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Old 06-10-2004, 05:49 PM
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Did you check valve keeper to guide clearance for these heads in reference to this cam? (Valve lift) If it's close then there's a possibility that the lift on the cam might be high enough to cause the valve keepers to bottom out on the tops of the valve guides, causing the valves to react kinda strangely at certain RPM's. Wouldn't notice it at idle or around there, but only when the RPM's started to pick up.

Just a thought.
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Old 06-10-2004, 05:56 PM
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Dion, try uncorking the exhaust....could be a something with the mufflers...is it coming from both sides?
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Old 06-10-2004, 06:02 PM
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james, good idea, I did talk with my buddy who put the heads on and he said the clearance was good, but that doesn't mean it's not hitting.

step, I do believe it's coming from both side, what do you mean by uncorking? I recently put new mufflers on and the noise is still there.
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Old 06-10-2004, 06:20 PM
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Kind of hard to do while driving but if the sound happens when you rev it in the shop, have you tried the good old rubber hose "stethoscope" trick? Hold one end of a 1/2" hose to your ear then the other end to the various possible offending engine locations. That usually very quickly finds the problem.

Oops, I see you specified that it only happens while driving. Try re-curving your distributor, bring total advance in slower w/ stronger springs and/or put on an adjustable vacuum pot and adjust it according to the enclosed directions. If that doesn't make any difference I am inclined to agree w/ your mechanics that what you are hearing is just the high compression report.
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Old 06-10-2004, 06:22 PM
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willies, did that, couldn't even find a vac leak.

could it just be high compression? I've heard 12.5-1 before and it didn't dound like this and I'm nowhere near that CR.

total timing is 36, I did put on the heavy springs, no change.
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Old 06-10-2004, 06:27 PM
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Try a tank of aviation +100 octane. If this doesn't make a change, you probably don't have detonation. If there are no clearance problems, probably nothing wrong.
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Old 06-10-2004, 06:28 PM
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thanks willies, I'll give it a shot.
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Old 06-10-2004, 07:17 PM
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It's sounds like you're having resonating in the exhaust. Get someone to drive the truck, and you drive next to them with the widow down, see what you hear.
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Old 06-10-2004, 08:51 PM
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I'll give it a try ghetto, thanks. maybe my headers are too small for this much compression?
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Old 06-10-2004, 09:06 PM
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Many things can make ugly noises. Look closely at ALL the exhaust connections for carbon spots (flange, ports etc). Make sure the bolts on the flywheel, bellhousing and converter are all tight. These things can all make noise. Check the clearance on the driveshaft and U joints, make sure your engine and tranny mount rubber is all nice and tight, if these shift getting torqued, clearances change and things can hit.

Have you tried just putting it in gear and standing on the brakes, then reving it to 2000 (or wherever the problem occurs) and try listening to it? Get a friend and pop the hood, don't hurt anything, and be careful! But I think that might be the best way to find out where the noise is coming from under load-

K
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Old 06-10-2004, 11:50 PM
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my car was doing that to......... thought the motor was lean......i came to find that the stock ignition module couldnt handle it...i converted to an msd module and its gone ..............make sure you have something better then a stock ignition module....good luck
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Old 06-22-2004, 10:45 AM
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Made some progress, changed my plugs to a colder NGK plug and the noise much less now.

I also found out the my Accel HEI has a slight miss in it so I'm planning on getting an MSD set up and see what happens.
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