Vaccuum Readings
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Pro
Joined: Apr 2001
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From: Waukesha WI
Vaccuum Readings
I might have finally determined what my "carb backfire" sound is. Have been having the sound onver about 3000 rmps seems to lose power and sounds pretty rough. Put a vaccume gage on it today. Normal readings at idle (20ish) anything over 2500 rpms and the gage drops slightly and does a real quick 5hg swing over and over. From what i have read this is a weak valve spring, or other valve problem. Does this sound right??? How else could this be verrified. I was thinking it was a carb (rich) problem for a long time. But I think its valve related now.
Melting Slicks
Joined: Dec 2000
Posts: 3,407
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From: Carmichael CA
Re: Vaccuum Readings (DollaGreen)
When you open the throttle the vacuum will drop...not sure what the exact readings would be, but I'd figure about 1inHg every 1,000RPM might be OK. But are you saying that the vacuum fluctuates about 4inHg at 3,000RPM? If so then yes as I recall my mechanics vacuum gauge indicates that problem would be valve adjustment, valve timing, or some other problem which effects only a one or two cylinders and not the others. The fluctuation is due to unbalanced firing. Other causes could be leaking valve stem seals, fouled plug, and other common nuisances.
The recommendation is to pull the plugs and inspect them first before even thinking about pulling the heads. Also check your timing at that RPM to be sure it's not timing....at 3,000 it should be at around 40degrees BTDC. Also, you may have a loose distributor shaft or mechanical advance weights that have unbalanced springs on them.
The recommendation is to pull the plugs and inspect them first before even thinking about pulling the heads. Also check your timing at that RPM to be sure it's not timing....at 3,000 it should be at around 40degrees BTDC. Also, you may have a loose distributor shaft or mechanical advance weights that have unbalanced springs on them.
Melting Slicks
Joined: Apr 1999
Posts: 2,010
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From: Moon Twp. PA USA
Re: Vaccuum Readings (Rockn-Roll)
..... Other causes could be leaking valve stem seals, fouled plug, and other common nuisances.
The recommendation is to pull the plugs and inspect them first before even thinking about pulling the heads. Also check your timing at that RPM to be sure it's not timing....at 3,000 it should be at around 40degrees BTDC. Also, you may have a loose distributor shaft or mechanical advance weights that have unbalanced springs on them.
The recommendation is to pull the plugs and inspect them first before even thinking about pulling the heads. Also check your timing at that RPM to be sure it's not timing....at 3,000 it should be at around 40degrees BTDC. Also, you may have a loose distributor shaft or mechanical advance weights that have unbalanced springs on them.
I am running unbalanced springs on my mechanical advance weights. That is, one stiffer than the other. This is how you set the advance curve to come in when you want it.
BTW, I run 36 degrees of mechanical advance in by 2500 rpm + 12 - 14 degrees vacuum advance.
Thread Starter
Pro
Joined: Apr 2001
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From: Waukesha WI
Re: Vaccuum Readings (glen242)
Thanks for the replies...here are a few things to add to the info...
All tune up items done recently and I am sure are correct...
dist is about 30 miles new, redone by Lars, and car is set to his timing specs...
Not so much concerned with the hg drop, as I am with the variance...have heard this is a valve problem...
I am sure the car is not running even to its rated 190 hp potential...sounds bad at 3k+ rpms. Popping, da da da da loping sound (for lack of a better way to describe it) almost sounds like a backfire, but not as loud... GM mechanic said it sounded like backup through carb, suggested valves.
Planning on rebuilding anyway, but thought i might be able to put a bandaid on tis somehow for a while.
All tune up items done recently and I am sure are correct...
dist is about 30 miles new, redone by Lars, and car is set to his timing specs...
Not so much concerned with the hg drop, as I am with the variance...have heard this is a valve problem...
I am sure the car is not running even to its rated 190 hp potential...sounds bad at 3k+ rpms. Popping, da da da da loping sound (for lack of a better way to describe it) almost sounds like a backfire, but not as loud... GM mechanic said it sounded like backup through carb, suggested valves.
Planning on rebuilding anyway, but thought i might be able to put a bandaid on tis somehow for a while.
Le Mans Master
Joined: Aug 2000
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From: Semper ubi, sub ubi
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Re: Vaccuum Readings (DollaGreen)
Check your fuel filters, your L-48 should have two filters. One is a canister type filter and the other is either a sintered bronze or small paper element that is right in the fuel inlet on your Q-jet. Low fuel flow and the resulting lean condition will give exactly the type of performance (and backfire) that you are experiencing. I've seen it several times, and I've even seen it on my own Corvette. I thought I had a flat cam or completely dead valve springs because of very similar performace. I changed out both filters and it fixed my engine immeadiately. New filters are a cheap and easy fix.
The vacuum swings are a sign that you probably have a worn valve train, possible weak springs. Another possibility is that your timing chain and/or gears are worn. A stretched chain or worn gears can cause your valve timing to walk back and forth causing uneven performance.
The vacuum swings are a sign that you probably have a worn valve train, possible weak springs. Another possibility is that your timing chain and/or gears are worn. A stretched chain or worn gears can cause your valve timing to walk back and forth causing uneven performance.
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Joined: Aug 2000
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From: Toledo,Ohio,USA
Re: Vaccuum Readings (DollaGreen)
Backfiring backout the carb is what happens when the exhaust lobes, 1 or more start wearing down on the camshaft. The car will run at low speeds but once the rpms start picking up the exhaust valve isn't opening up long or high enough to get rid of all the exhuast pressure so the remaining exhaust exists out the intake valve and up the carb. Very common on small block GMs.
Take the valve covers and turn over the motor and watch to see if all the exhaust valves are moving the same amount.This would explain the readings.
Take the valve covers and turn over the motor and watch to see if all the exhaust valves are moving the same amount.This would explain the readings.
Safety Car
Joined: Feb 1999
Posts: 3,631
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From: Fountain Hills AZ
Re: Vaccuum Readings (DollaGreen)
As others have posted, it could be something other than a bad vavle spring but your symptoms are indicative of a weak or broken spring. Being that the popping is from the carb, it would likely be an intake valve spring. If that does turn out to be the case, replace all of the springs, not just one or some of them. If one has failed, the others are probably not far behind.
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Joined: Apr 2001
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From: Waukesha WI
Re: Vaccuum Readings (Vetterodder)
Just want to clarify, this "popping from the carb" is not nearly as loud as a backfire (im thinking of the gunshot variety backfire) more of a puh puh puh, like the sound sometimes heard when engine breaking a car.
