The Stovebolt Site Forums The Shop Area The HiPo Shop Vacuum test results/intake backfire

I did a vacuum test on my newly rebuilt 1964 Chevy 327 V8. It has a Holley 1850-C 4 barrel on an Edelbrock Performer intake. The cam is a “.218/.218. At 750 RPM idle I get a steady 10” of vacuum. I’m at 7300 feet in altitude and I’ve read that I lose 1” of vacuum per 1000 feet of altitude. So adding back in 7” puts me in the middle of the good range. Or does it? I’m doing some trouble shooting for a backfire through the intake and I am trying to be systematic about it. Any thoughts or questions?
Thank you,
Larry

You are correct that altitude affects idle vacuum, but I don't think you are in the middle of the good range.

Several of my trucks live at 6250 ft elevation and they are all tuned between 16 and 17 inches at idle (as opposed to normal 20-21 at sea level). Based on that, I would think you should be closer to 15-16.

At 1000 ft elevation you may lose close to 1 inch of vacuum (compared to sea level), but as you go up in elevation each 1000 feet of change will be less and less. At 8000 ft you are above about 1/4 of the earth's atmosphere and the difference between 10,000 ft and 11,000 ft will be relatively little. My engineer friends tell me that for each 1000 ft you lose 3% of power due to less air pressure=less oxygen for combustion.

Kent

Not enough information- - - -like when does the backfire happen? Idle, acceleration, steady cruise, etc. Is there any fluctuation in the vacuum gauge? Is the .218 a lobe lift specification? What about lift at the valve, and duration in degrees? With a 1.5:1 rocker arm ratio, a 218 lift at the cam only shows .327 lift at the valves. That's a very mild cam grind, so your vacuum at idle is WAY low for some reason. I'd expect to see 19" or more at a 750 RPM idle. What's your initial ignition timing setting? Have you checked to see if the cam is installed correctly with the crankshaft and cam timing marks aligned?

A Holley 1850 is a 600 CFM, so you're not over-carbureted, but it's a huge step backwards from the Rochester Quadrajet that's supposed to be on there.
Jerry

Jerry, I think the cam is 218/218 duration, which could be the lower vacuum reading. The Performer manifold should be fine and the Holley would be a direct bolt-on, I think the Q-Jet was introduced in '65? My '57 283 2 bbl. with 2 degrees more static timing than stock will idle at 560 rpm with 18", I'm about 500-600 ft. elevation. LAJones, do you trust your vacuum gauge, do you have something to test or compare it to?

Yes. The duration is 218 @ .050, Valve lift is 458. Intake separation is 105 deg and exhaust is 115 deg.

The backfire occurs as soon as I accelerate from about 20 mph on the flat or when I start up a hill.

Initial timing is 12 deg BTDC

Thanks for your answer. My vacuum gauge is very old so we’ll check against a new one

On a newly rebuilt engine I’d suspect a wiped exhaust lobe. Pull the valve covers & have someone spin the engine while you see if you have equal movement of the rocker arms (Pull the coil wire to keep it from starting). I’m assuming that it’s a flat tappet cam.

RLB has a good point, pull the valve covers, idle the engine and watch each rocker arm, they should all rock the same distance, also watch each push rod, they should spin 1/4-1/2 rotation each time. If the push rods aren't spinning, those lifters/cam lobes are highly suspect. You can buy a set of oil deflectors to keep the oil in the rocker when doing this test.

Thank you. I’ve done that test and it appears that the cam is okay.

Have you checked for a restricted exhaust system? Low manifold vacuum and backfiring on acceleration are both symptoms of exhaust gas that can't find its way out of the system easily. One of my customers was turning around on a narrow road and backed into a muddy shoulder berm- - - - -shoved about a foot of tightly packed dirt up the tailpipe. The car barely made it to my shop, and it took us a while to figure out why it was sluggish and wanted to backfire.

I have checked the exhaust and both sides appear to be clear. Right now the exhaust is pretty simple: 2 1/2” down pipes under the cab to cherry bombs then angled pipes exit in front of the rear tires. I’ll get a proper exhaust after I solve the backfire issue.

The truck has about 50 miles on it. I did the cam break in with no problem and started driving it around close to home. It ran fine until recently.

Once, my COE started to lose power and it was because the distributor clamp was a bit loose and the timing became retarded. I would check your timing. I know this is not ideal but one way to set your timing is with a vacuum gauge. At high altitude like you are, this will adjust for the altitude change since timing is assumed you are at sea level. Set the timing with the highest vacuum and you will be close. You can advance it slightly from there and you will be closer. If this doesn't get your vacuum near 15 inches then you have another problem.
Kent

Did you set the initial timing with the vacuum hose to the distributor disconnected and plugged? Is the vacuum hose connected to manifold vacuum or ported vacuum?

Holley's instructions say to use ported vacuum but that is not always the correct choice.

Your carb has has backfire protection to save the power valve. That's nice to have.

Thanks for the tips. I’m on my way to see what I can figure out.

Yes on the distributor vacuum line. It’s hooked up to ported. And I did see the info on the power valve, but I did buy an extra one in case.

UPDATE:
I might have accidentally solved my problem. I have been going through all the engine systems and checking for any obvious problems. I’m running a Holley mechanical fuel pump and I thought I’d do a quick pressure test. I disconnected the line at carburetor side of the pump for reasons I can’t remember now, but instead of collecting a small amount of fuel from the supply line, I found that it was continuing to pour out of the pump. Bad diaphragm making it lean out and backfire when accelerating? What do you guys think.
Thank you,
Larry

If the gas tank is still behind the seat it will be higher that the fuel pump. Liquids do tend to run downhill without much help.

When you are accelerating, does the engine lose power like you are running out of gas? you can check the pump pressure and volume to confirm any issue there. Small block Chevy standard pump should be somewhere in the 5.5 - 7.5 psi range, volume probably a pint in 30 seconds of idling?

Thank you.

I didn’t think that fuel would flow though the pump unless it was running.

I was on my way to check the pressure when I found the fuel running out of the pump outlet.

When I accelerate, I immediately have backfiring through the intake/carb

The tank is behind the seat.

I have wasted a few billion electrons describing the right way to test a fuel pump. Everybody ignores me, so I'm not interested in doing it again. The nice thing about banging one's head against a wall is that it feels so good when you stop! Have you checked the distributor cap for cracks and/or carbon tracks? Are you certain that the firing order is correct? 1-8-4-3-6-5-7-2.
Jerry

I’ll look up your fuel pump testing process

Distributor cap checks out okay

Firing order is as it should be

Thank you

HRL,
I found your post about fuel pump testing from 11/10/2022. Good reading.
Thanks,
Larry

Why isn't that "fuel pump testing" procedure in the Tech Tip section?!

That's where I intended it to go- - - -several years ago. It was only the latest edition of the same test I've been preaching for 20+ years. Guess it never got there!
LOL!
Jerry

Need some politicin to git-r-done. Sweet talkin, grease the wheels, bribery, etc.

Thank you all for your ideas and suggestions. Once I work my way down the list, I’ll let you know how it turns out. It might take a while.

Larry

If it leans out and backfires when you step on it, how's the accelerator pump on that Holley? Did you bolt it on new? If not (or maybe even if so) does it have the throttle shaft cam that actuates the shot pump rocker? Is that rocker moving up and down, if you look in the carb throats, do you see a solid squirt of fuel when you work the throttle? If its an old carb, the pump diaphragm could be dried up and not providing enough fuel, just hanging up halfway activated all the time.

Good thoughts. No problem there.

However, plugs number 1, 4, 6, and 7 are black, not oily

Sooty plugs indicate a rich condition.

You probably have a vacuum leak that is making a few cylinders run lean. Does the engine have a PCV valve? Try removing and plugging the PCV, and putting a little Propane into the crankcase through the oil filler or the PCV grommet in a valve cover. (don't overdo it!) If the engine speeds up or smooths out, especially if the vacuum goes up, look for a leak at the bottom of an intake passage where it meets a cylinder head that's drawing air from the crankcase. That was a favorite trick of the NASCAR guys who had to run restrictor plates- - - -create a deliberate vacuum leak into the crankcase, and rely on the big reservoir of oil in the dry sump oil system to let the engine suck up a little oil on pace and caution laps without losing oil pressure. Those cars smoked heavily during a caution, and cleared up as soon as they got back up to racing speed. I watched that happen at Talladega on several occasions.
Jerry

I’ll try that when I get it running again. I do have the 1964 style PCV setup with the air intake at the rear of the block and the closed oil filler cap connected to a PCV valve at the rear of the carb.

What was the advantage of the induced vacuum leak on the NASCAR rigs? I see the part about keeping oil pressure up, but just wondering about fouling plugs.

Thank you

A vacuum leak from the crankcase to the intake bypassed the restrictor plate- - - -more air- - - -higher RPM. I did something similar on the claimer engines I built- - - -1/2" diameter countersunk Allen bolts holding the carburetor adapter to the manifold- - - -with a 1/4" hole drilled down the middle of the bolts. The Holley #7448 350 CFM 2 barrel the local rules required was an attempt to hold engine building costs down- - - -no point in building a big bucks engine if it can't breathe. No matter what the gear ratio was, a 355 cubic inch small block Chevy couldn't wind tighter than 6100 RPM- - - -no matter what cam, valves, heads or other go fast stuff we tried to use. (A big air leak past the carb- - - - - -however- - - - -) all the race was going to decide was who would run second, half a lap behind the guy running one of my engines!
Jerry

I call that “cheat’n fair and square”….
When I first got into racing I was getting dropped big time out of the corners, when I asked a seasoned driver what’s the hell was going on his reply was… you ain’t cheat’n good enough….. got it…

Chuck

Another sneaky trick I used was to build a very well prepared and balanced 305 engine, and gear it to wind up a lot tighter than the 350+ cubic inch engines my competitors were running. Feeding 50 less cubic inches gave us an RPM advantage through the small carburetor, and allowed my driver to run deeper into the turns at full throttle, and climb onto the loud pedal sooner coming off the turns. The cheater carb bolts just added another sneaky trick to the total package. My engines sounded like a bumblebee on steroids!
Jerry

Although my NAPCO truck will never see a racetrack except maybe as a push truck, I like hearing about what goes on there. I’ve been lucky enough to go to Bonneville with a group that supercharges vintage motorcycles and then sets records with anything from a 50cc Honda to a 1000cc Harley. The knowledge gained through racing filters down to our daily drivers and our hobby cars.
Now I need to get back to work on my 327 so I can let y’all know what caused that backfire.
Larry