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Car won't rev

I have a 1979 Jeep CJ5 with a 290 V8 swapped in. I replaced the carb and the spark plugs but now the engine wont rev. I put a vacuum gauge to measure the intake vacuum and it was at 20 pounds of negative pressure but the needle bounced around a lot. It is currently being fueled out of a bottle because I am not sure how well the fuel pump is working. Could this be a vacuum leak or something else?

I’d go after the valvetrain. A needle bouncing all over at idle is generally indicative of a sticking valve, a broken valvespring, a valvetrain simply worn out (weak springs, bad lifters, etc,) or something of that nature. Remember that the vacuum is created by the pistons puling air past the intake valves, and if the intake valves aren’t operating evenly vacuum will be all over the place.

But yeah, a vacuum leak is possible. If the engine’s age is near the Jeep’s age, you could try replacing all the vacuum lines… one at a time so you don’t reconnect them incorrectly. Vacuum line by the foot is dirt-cheap, and replacing the lines is easy.

So what makes you think the fuel pump isn’t working? And does the bottle smooth out the idle?

At what point did the fails to rev problem appear? After you swapped the engine? Or did it run fine after the engine swap, but the problem started after you replace the carb and the spark plugs? If the latter, I expect there’s a problem (block, wrong jet, etc) with the carb’s main fuel circuit.

I bought the car with the engine already in it so and I replaced the carb and plugs after I found that the engine wasnt reving and the problem still is there.

Could it be a timing issue? The car seems to idle fairly normally though.

And I wanted to make sure that the carb was getting enough fuel so I am running the engine off of a bottle.

And should I pull off the valve covers to see if the lifters and springs are good?

Thank you!

Thanks for the help!

Do you have a sufficient fuel supply to the carburetor with this bottle? Is it connected to the fuel inlet connection of the carburetor and elevated at least a foot above the carb?

What does the engine sound like at 1/4 throttle?

The car revs at about 1/4 throttle but not anymore than that.

The car revs to about 1/4 throttle but not anymore than that

Carburetors require very little fuel pressure, just enough to keep the float bowl full as the throttle body pulls the fuel out. Typically you’d be talking about 3 psi or less. The bottle should be able to provide sufficient flow IF there’s the ability for air to enter the bottle while the fuel is draining out. If there isn’t, a vacuum will quickly develop in the bottle that’ll prevent it from draining freely into the carb. So, if you can rig a way to allow air to enter the airspace in the bottle while the gas drains out, you could try running the engine with the bottle as a test only. If the engine still won’t rev, it isn’t a fuel supply problem.

How to do this? I’d use a bottle with a cap, tip it upside down and insert into a hole a sheetmetal screw with a fuel-resistant O-ring, then flip it right-side-up and fill it partially with fuel. Then you can plumb it into the carb, tip it upside down again, and remove the screw. You can then reverse the process to remove it from the carb. OUTSIDE! CAREFULLY! With a tri-rated extinguisher handy. NEVER use water on a gas fire.

Okay, thanks! I’ll try that and get back to you later.

Okay, thanks! I’ll try that and get back to you later!

One idea, rev it to the point it won’t rev any more, then shut the engine off and peek at the carb’s fuel bowl, taking note of the fuel level. If the fuel level is the same as it is at idle, you’ve got enough fuel flow to the carb input at least. Fuel flow from the tank to the carb is not the reason if there’s no drop at all in the fuel bowl level at high rpms.

Just curious why you are running the carb fuel supply from a bottle? It’s fairly easy to test a mechanical fuel pump. Just disable the ignition system, unplug the hose from the carb inlet, insert the hose in a jar, and crank the engine 30 seconds. Then think what’s the maxiumum amount of gas you could ever use in one hours of driving? 10 gallons? So if the pump delivers gas at that rate, the pump is ok. 10 gallons per hour == 2.7 cups per minute. So a little over a cup in 30 seconds of cranking, you are good to go. You’ll probably get a little less than that b/c cranking rpm is slower than driving rpm, and fuel flow is related to the rpm.

You’ve already installed a new fuel filter, right? If not, something to try.

Could it be a vacuum leak? Anythings possible, but the symptom of a vacuum leak is usually poor idle, but runs much better the faster the rpm. That’s the opposite of your symptom, so unlikely to be the normal type of vacuum leak. You could have a problem with the EGR I suppose, it is opening up too much as the vacuum lowers with increasing rpm. Seems unlikely tho.

I continue to think you’ve got a problem with the carb’s main fuel circuit.

I took the valve covers off and all the valves seem to be moving properly. There were 3 pistons that seem to all be on the compression stroke at the same time (exhaust and intake valves didn’t have any tension on them). Does this indicate bad timing or is that normal?

While running off the bottle the carb seems to be getting plenty of gas. I will say that the engine chokes and shakes very violently when you try to raise the throttle. Do you know why this happens?

That sounds like it could be too lean or too rich condition occurs as the throttle valve starts to open beyond idle. You’ll get the symptoms of a misfire either way, worse if it is too lean, which is my guess. As the throttle valve opens, that’s when the main fuel circuit in the carb gets activated and allows gasoline to move up from the fuel bowl jet & pass up the main fuel well and directly into the carb’s venturi area where it is supposed to spray out. But if that path is clogged, no spray, and runs lean and misfires.

Another possibility, there’s often a vacuum port set up for that area of the carb, above the throttle plate. For powering vacuum devices that need to be activated at off-idle. If one of those vacuum operated devices were leaking, this could be the symptom. On my truck one of those devices powered from that port is the vacuum timing advance canister.

You should see about 3000 RPMs at 1/4 throttle in neutral, are you trying to reach the red line?

Are you holding the throttle steady or snapping it open? If the engine coughs or falls flat when you snap the throttle open the accelerator pump in not working.