Carb manifold vacuum port problem

Teal68 · Feb 17, 2018

Some info: 68 Bronco, 289, original 2 barrel carb, np435

Problem: running rough at idle. Smooths out some at higher rpm, but really it just mask the problem. This problem started right after a steep ledge climb.

Stuff I have checked:
- all plugs look great and I have spark all plugs
- for some odd reason I have about 3psi pressure at the carb manifold vacuum port instead of vacuum!
- I do have vacuum at the manifold, but only about 15 because it's running rough.

So, why would/could I have pressure at that carb manifold port instead of vacuum?

Thanks,
Tyler

B RON CO · Feb 17, 2018

Hi, my guess is, if you have a high mileage motor, it could be the timing chain. Check it by looking at the rotor. Turn the balancer bolt clockwise (direction of engine rotation) to make sure there is no slack. Reverse the ratchet and see how long it takes for the rotor to turn backwards. If it takes @ 10 degrees ( you can use the timing tab as a guide) I would say the chain is stretched. The nylon cam gears also wear out. Good luck

Teal68 · Feb 17, 2018

Would that explain why I have pressure at my carb manifold vacuum port instead of vacuum?

B RON CO · Feb 17, 2018

I don't know why you would have pressure at the ported vacuum (I think) port. It should be 0 at idle.
I just suggest you look at the rotor to get free and easy information about your timing chain.
How many miles on the motor?
Good luck

Teal68 · Feb 17, 2018

I'll check for slack in the chain in the am and report back. Unknown mileage, but I do know it's a rebuilt motor from the medallions glued to the heads and block. Burns no oil and has plenty of power.

I have pressure at the port that should have vacuum at idle.

jagbucket · Feb 17, 2018

I would look at cleaning the carb possible that dirt was in the bottom of the float bowl and got sucked up on the incline. no clue how you are getting pressure unless its some egr type system idk

Justafordguy · Feb 17, 2018

You shouldn't have pressure on any carb port. One port should only have vacuum above idle for your distributor and the others should have manifold vacuum. If you trust your gauge I say it's time to clean/rebuild the carb.

Teal68 · Feb 17, 2018

Justafordguy said:
You shouldn't have pressure on any carb port. One port should only have vacuum above idle for your distributor and the others should have manifold vacuum. If you trust your gauge I say it's time to clean/rebuild the carb.

Just to be sure about the gauge I checked it on my Wife's Bronco and it proved to be good. That, and I get vacuum when plugged directly to the manifold on my Bronco. I did pick up a rebuild kit for the carb today while I was out and about.

Teal68 · Feb 19, 2018

Messed with it some today. B Ron Co, slack timing chain does appear to be the problem. Only take few degrees to start moving it. I also tired another carb to see if I had the same pressure problem. Sure enough about 3 psi instead of vacuum, and runs poorly at low RPM. Just odd!

Even though the plugs look great I'm smelling a lot of unspent gas on the driver side. BTW, no crossover tube in the exhaust. So....I bought one of those IR thermometer and the number 5 and number 8 cylinders are about 100 degrees cooler than the other 6. Is that normal??? I also pulled the 5 and 8 plugs again to look them over, and they look great. Maybe pull a rocker cover to see if a stud lifted some???

DirtDonk · Feb 20, 2018

That would be another good step to take.
Not sure how it's possible to see pressure unless the gauge is wonky. Or the carb is seeing some strange goings-on in the venturi.
Or maybe it's the valve issue you're now thinking of checking. Perhaps a valve is not closing all the way and letting some momentary pressure up the pipes. More than one would be more likely for you to see it up at the carburetor. If at all.

So, when you are connected to this port and you see the pressure, is this only at idle? What happens when you open up the throttle a little?

And no, 100° difference does not sound good.
It's hard to know without direct comparison just what some expected variables could be between different cylinders. They do have different flow characteristics in the intake and exhaust systems, so not all 8 cylinders are always going to be the same. Especially at the four corners. And number 8 is kind of known for being a little off.
Do you have power brakes? Which runner is the vacuum line connected to? If going to #8 cylinder, perhaps temporarily capping it off will let you see if there is anything going on there.

If you have not dialed-in your idle mixtures lately, give that a try. But I would certainly pull the valve covers off to make sure everything is doing what it's supposed to under the lid.
A bad spark plug or wire could give cooler readings. Check with a timing light on each wire at least to tell you if you're getting good and consistent sparks through each wire. Anything off rhythm or not sparking is suspect.
A worn out distributor could add to your inconsistent readings.

Was this a long uphill climb you were on when it happened? Or just a bumpy short jump? A big bump can dislodge all sorts of stuff in the carb, as was mentioned. In the tank too.
But a hard pull can start the process of pulling rocker arm studs out as well. If the engine got hot during this process, things like that are even more likely.

So yeah, plenty of things still yet to check.

Good luck.

Paul

B RON CO · Feb 20, 2018

Hi, I was thinking along the lines of... a stretched timing chain or one that jumped a tooth will let the intake valves stay open during the compression stroke, letting the positive compression pressure up to the carb, and that is what the vacuum gauge is seeing.
I would bring the balancer up to the top dead center mark and mark the distributor body with a sharpie directly over the #! cylinder, and carefully check were the rotor is. This will also let you gauge timing chain stretch unless you recently adjusted the timing.
Try to judge how much play you have when you turn the crankshaft bolt from clockwise to counter clockwise. I would guess 5 degrees or less is nothing to worry about. High mileage motors will see a lot more slack in the chain. I wouldn't say just pull the timing chain cover and look because it is a big job.
With the new information about the 2 rear cylinders ( the ones you would expect to run hotter then the others) I would do a compression test next.
Post that and your vacuum gauge reading, at idle from a direct vacuum source, like the tree on the back of the intake mainifold.
Good luck

Teal68 · Feb 20, 2018

Paul,
Gauge is good. Reads fine hooked directly to the manifold, and I checked it on my Wife's Bronco just to be sure. When hooked to the carb it reads a steady 3 psi and does not fluctuate no matter what I do with the throttle. Weird! I also swapped an extra carb on my Bronco and it did the same thing. That tells me the carb is not the problem.

I kind of figured the front and back cylinders could be a little cooler, but on the passenger side all of them are within 10 degrees of each other. For that matter, 1,2,3,4,6 and 7 are all within 10 degrees of each other.

I do have power brakes and they are connected to the main port on the back of the intake manifold which may very well be the runner for 8.

It was just a short climb. Really just about a 4' or 5' ledge.

B Ron Co,
I had a few minutes this morning so I did compression testing. All cylinders were 145 to 151. I have a really healthy motor! Vacuum gauge hooked directly to the intake manifold yields about 15 to 17 depending on how rough it's running at the time. It does not flicker badly.

Also, I have about 12 degrees of timing at idle.

That's all I got for now....I'll have to dig into it some tonight if I get a chance.

Thank you all for the suggestions. I'm determined to figure this out.

Tyler

B RON CO · Feb 22, 2018

Hi, I see you have a 68 289 which is early, but if there is an EGR valve under the carb I would take a look at that. Later engines are know for running rough when the EGR pintle gets carbon fouled. I don't know if the other early emmision systems will be let exhaust pressure into the intake manifold. Good luck

Teal68 · Apr 27, 2018

Little update here, but I'm still baffled and need Bronco Guru help....

I can pull the spark plug wires off of 2, 3, and 5 and it has zero impact on how the motor runs. Basically I'm running on 5 cylinders. I do have spark at all the plugs so I pulled the valve covers hoping the rockers for those cylinders were loose. Nope, they are all tight as they should be. Which makes sense since the vacuum gauge was not flickering.
What the heck is going on????

B Ron Co, I do not a EGR or spacer under the carb. Just the simple spacer with one vacuum port at the rear.

68ford · Apr 27, 2018

Intake valve seats burnt. Compression blowing back up into the intake. Been through the same thing.

68ford · Apr 27, 2018

I just read where you didn't compression test and all were even. Has me thinking you may have lost the cam lobes on the exhaust side. If the exhaust valxes do not open enough, residual cylinder pressure that did not get out of the cylinders will blow back up the intake when the intake valve opens.pull the coil wire so it won't start and crank it over. You will probably hear the air coming back into the manifold along with a very inconsistent cranking speed. I have worked on half a dozen diesel engines that spit out a exhaust push rod. Basically did exactly what you are describing.

Rustytruck · Apr 27, 2018

Do a leak down test. I'm guessing that you blew the head gasket between 2/3. Check if when 2 or 3 is on compression if the other cylinder has its intake valve open.

Before tearing anything apart Make sure you check your plug wires. Ohm check them or pull a wire and swap it with another one and see if the miss follows the wire to a new location. How do your plugs look?

Teal68 · Apr 27, 2018

Thanks for the suggestions I'll dig in a bit deeper this weekend. Wouldn't bad valve seats, wiped lobes, or blown head gasket show up on my vacuum gauge?

Rusty, I have swapped plugs and wires just to be sure. It's kind of weird, but the plugs looks fine too!

Rustytruck · Apr 27, 2018

Put compressed air in each cylinder through the spark plug hole while that cylinder is at the top of the compression stroke and see if the air is leaking up the intake.

DirtDonk · Apr 28, 2018

Teal68 said:
Wouldn't bad valve seats, wiped lobes, or blown head gasket show up on my vacuum gauge?

Yes. Normally.
But for now I wouldn't rule anything out just yet. At least the steady needle is a good sign, but I would not consider it a guarantee just yet.

You said you had spark to all cylinders, even though certain ones made no difference when pulling the wires off (a bad sign in itself of course) but were they all good looking sparks? Not yellow or red, but nice and crispy blue/white with a nice snap to them?
If so, then it would seem an internal issue. Maybe...

It's just strange that all cylinders check with nice high and very even pressure numbers, if something inside them is jogging combustion. You would think it would show up with a compression test.
What about the intake design as a clue. Are all the bum cylinders on the same plane of the manifold?

And here's another oddity. If they all have such even pressures, and all have good spark, but those three don't seem to make any difference when you remove the spark, then how were they within 10 degrees of each other in the exhaust temps? Only #5 cylinder has both issues showing up during tests.
What were those temps by the way? If lower or higher than normal, maybe it will lead us to another potential issue.
Maybe they're all getting a bad mixture or the timing is farther off than you think. Could the timing marks be wonky? Did you check the timing during all this messing about? Or were you quoting numbers from a previous tune?

Good luck.

Paul

Carb manifold vacuum port problem

Bronco Guru

Bronco Guru

Bronco Guru

Bronco Guru

Bronco Guru

Full Member

Bronco Guru

Bronco Guru

Bronco Guru

Bronco Guru

Bronco Guru

Bronco Guru

Bronco Guru

Bronco Guru

Bronco Guru

Bronco Guru

Bronco Guru

Bronco Guru

Bronco Guru

Bronco Guru

Similar threads