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Senior Chief · 10-15-2011, 10:05 AM

Two probs, maybe related. At 45-50 mph my vacuum reads slightly lower than normal (14 vs 20). When accelerating, vacuum goes HIGHER to 25, instead of dropping! Just started this.
When at idle, it drops to 0, and quits running. ?????????

Also, it has (1 year or so, since we bought it) backfired through the carb when taking off from a stop, or climbing a hill. ???????

Any ideas? Any advice gladly received!!

Senior Chief

IC2 · 10-15-2011, 12:01 PM

Quote:

Originally Posted by Senior Chief

Two probs, maybe related. At 45-50 mph my vacuum reads slightly lower than normal (14 vs 20). When accelerating, vacuum goes HIGHER to 25, instead of dropping! Just started this.
When at idle, it drops to 0, and quits running. ?????????

Also, it has (1 year or so, since we bought it) backfired through the carb when taking off from a stop, or climbing a hill. ???????

Any ideas? Any advice gladly received!!

Senior Chief

Nasty problems to diagnose from afar

- Check the spark plug wiring routing first of all - two of them on the wrong distributor terminals will cause this

- Check the condition of the plug wires. Cross firing between wires can be difficult to diagnose. If those wires are 5-6 years old or an "off brand" they can be 'toast'

- Check the inside of the distributor cap for carbon tracking. Also, check the rotor for damage or wear. Is the initial timing correct? Are both springs under the rotor attached? Does the advance plate work easily?

- Is the distributor vacuum advance ruptured? Is the hose hooked to intake vacuum (below the carb butterflies - I assume it's a carbureted engine)

- Check for vacuum leaks anywhere along any vacuum hose including the master cylinder and PCV hoses

- Do a compression check. A burned intake valve will cause a carb backfire.

- Then - there is that big block engine, especially Chevy, age old problem - a camshaft that has lost a lobe. You need to pull the valve covers to do this check.

Let us know what you find

Midniteoyl · 10-15-2011, 12:09 PM

If a Holley, theres also the Power Valve.. A single backfire could blow it.

jimbo16720 · 10-15-2011, 12:44 PM

Sounds like a vacum leak. Pull a hose that has vacum all the time when running and see if the rpm changes. Could be a bad hose anywhere or even an air leak where the carb bolts to the intake.

Kurmudgeon · 10-16-2011, 12:53 AM

Vacuum dropping to 0 at idle definitely sounds like a massive vacuum leak. That's when vacuum should be highest. Two things that could cause that big a leak are the power brake booster and the EGR valve.

The vacuum booster is the easiest to check, so you might want to check that first. If you have one, that is. You may have Hydro-boost brakes, which means you would not have a vacuum booster.

When an EGR valve gets old it will frequently stick and quit functioning. Since it spends its life exposed to hot exhaust gases it can get pretty grungy and corroded. Depending on what position it's in when it sticks, it is a potentially massive vacuum leak.

There are certainly other possibilities, but these would be the most common. Unless someone has changed it, you almost certainly have a Quadrajet carburetor, so there is no power valve issue like the Holleys have. But even with a blown power valve, I wouldn't expect to see vacuum drop all the way to 0 at idle.

Vacuum leaks in the engine bay are pretty easy to locate. But some of the heating/AC controls behind the dash are vacuum operated, and leaks there can be a real bear to find and deal with.

Good luck.

Ragingbull · 10-16-2011, 07:35 AM

How many miles are on your motor at this time? Its very possible that you have had a timing chain jump a tooth or more. This could cause any or most of the symptoms you stated.

bicklebok · 10-16-2011, 05:44 PM

Hey Chief! I would start with the basics and start looking for a vacuum leak.

Poptop · 10-18-2011, 08:50 PM

A few years age I had a Chevy 454 in a MH that would backfire at highway speeds when going up a steep hill. Turms out the RV shop that tuned it up set the timing at 6 degrees after TDC when it should have been 8 degrees before TDC (14 degrees difference). If you have electronic spark advance instead of vaccum make sure you disconnect the brown wire to the distributor before setting the timing. Also, there are two timing marks, one on the top (difficult to access) and one on the bottom. Make sure you use the correct spark plug wire for the timing mark you use. Unfortunately I don't remember which one is which anymore.

Iron Man · 10-19-2011, 08:12 AM

look at this on how to use a vacuum guage
How to Use and Interpret a Vacuum Gauge

ga traveler · 10-19-2011, 08:42 AM

Almost for sure a vacuum leak. Check the large line to the accumulator first. These crack at the ends very often. Check the accumulator for cracks. These are plastic and tend to crack.If these are OK then go to the rest of the hoses.

bigskymt · 10-19-2011, 08:49 AM

Unless your plug wires were recently replaced I'd start with them. You may also inspect the inside of the distributor cap for a carbon trail between two towers. Failing plug wires are most evident under load. HEI is 30-50kV. An open wire or high resistance will allow the spark off the rotor to search for a connection to ground. If it jumps to another tower on a cylinder that has an open intake valve, you have a backfire.
If your vacuum drops to zero at idle that would be main, not ported vacuum leak. On the rear of the intake manifold behind the carb is a 3/8" or so pipe with several lines running from it, there would be the first place to look. The carb base plate is another source of main vacuum. The carb body is ported vacuum. We used to spray WD-40 at a suspected vacuum leak and listen for RPM change. Now we use something that's not so combustible. Water sprayed at vacuum leak will provide a noticeable change in RPMs.
Quadrajet carbs have an accelerator pump with a leather or neoprene seal thats prone to leakage. Failure of the accelerator pump to operate properly will result in a hesitation, not a miss, when trying to accelerate.

JimElliott · 10-19-2011, 10:40 AM

Quote:

Originally Posted by Iron Man

look at this on how to use a vacuum guage
How to Use and Interpret a Vacuum Gauge

Iron man.....That is indeed a great chart to be used constantly and thats why a small 2 1/8 gauge installed in your dash is such a perfect instrument to use......

Jim

JimElliott · 10-19-2011, 10:49 AM

Bigsky.....
Sometimes I will use unlit propane IF the air leak is "tuff" to find....

Mapp gas is really good.

Jim

Senior Chief · 10-20-2011, 10:25 AM

Thanks a million!! You hit the perverbial nail on the head I, found the vacuum leak(actually leakS). But, since you seem to be very good at this, let me ask further. The backfire has been with us since we bought this critter 2 years ago, but we've really only been driving it for about a year.
Plug wires, plugs, HEI coil all brand new high quality stuff. Firing order right, no wires crossed.
At random times, when you turn off engine, a column of gas spray erupts from the top of the carb, Not very often, just sometimes.
There are no valve or rocker noises heard with mechanics stethoscope on valve covers. The 'rotational' noise of the valve train is a lot noisier on one end of one cover than all other places, but can't be heard with out the stethoscope.
At first, and sometimes now the 'backfire' is just a little cough. While I still had the vacuum leak, it was actually an explosion. A real backfire in the carb.
Can't think of anything else.
Any ideas?
Again, thank you so much for curing my vac.problem.

Bill