Chasing the vacuum leak

[jstone4646]

Ok so quick recap, truck starts and runs fine, just idles really high. Even when the carb mixture screws are fully closed it still runs high. Been chasing the leak and found at least one culprit on the rear of the carb, maybe more. So let me detail out what is going on at that end. Coming off the manifold I have a 5 way splitter. 4 are small lines, one is the larger brake booster line. So the 4 small go like this. 2 are capped off. 1 runs to the carb (front passenger side manifold vacuum port). 1 connects to a hard line that runs down into the transmission tunnel....I have not chased to see what that is yet.

So when I spray that area with starter fluid the engine begins to stall (it's already high idle so I guess it's flooding out...or....the liquid is temporarily plugging the leak and the idle is being corrected.

Here is the carb layout. Top front side port goes to the manifold, bottom front side port goes to the distributor. Larger lower rear port is capped. Larger lower front port runs to the pcv valve.

Based on this info, can anyone clue me in to if this is proper at all? 84 F150 automatic 351. No A/C.

[Gary Lewis]

I'm very sure that line from the vacuum tree/splitter should not go to the carb. And for testing purposes I wouldn't have a line to the distributor. So I would pull both of those hoses and cap the tree/splitter for your test.

And, by the way, the line that goes to the hard line and down the transmission tunnel goes to the transmission. It is possible that the vacuum modulator on the tranny has a hole in it, so I'd pull that line and cap it at the tree/splitter. Then, once you get the idle down to where it should be then hook it up.

In other words, get it down to basics.

As for the engine stalling when you spray starter fluid, you are getting close to the problem. I'm going to guess that there's a leak at the carb base. Did the engine originally have EGR and you left the adapter off? If so, that will cause a leak.

[jstone4646]

I'm very sure that line from the vacuum tree/splitter should not go to the carb. And for testing purposes I wouldn't have a line to the distributor. So I would pull both of those hoses and cap the tree/splitter for your test.

And, by the way, the line that goes to the hard line and down the transmission tunnel goes to the transmission. It is possible that the vacuum modulator on the tranny has a hole in it, so I'd pull that line and cap it at the tree/splitter. Then, once you get the idle down to where it should be then hook it up.

In other words, get it down to basics.

As for the engine stalling when you spray starter fluid, you are getting close to the problem. I'm going to guess that there's a leak at the carb base. Did the engine originally have EGR and you left the adapter off? If so, that will cause a leak.

Well here is a question (remember I am a carburetor noobie). Does the carb absolutely need a vacuum connection or can I just plug them all for testing? I know vacuum pressure is good because if I pull a cap off and stick my finger to the hole (this was a manifold hole) it pulls my finger in pretty good.

[Gary Lewis]

Well here is a question (remember I am a carburetor noobie). Does the carb absolutely need a vacuum connection or can I just plug them all for testing? I know vacuum pressure is good because if I pull a cap off and stick my finger to the hole (this was a manifold hole) it pulls my finger in pretty good.

The carb is a source of vacuum, as is that tree/spreader in the manifold. So it makes no sense to connect them. None. So you should pull that hose and cap both connections. And for that matter, you could cap the PCV connection and the brake booster connection.

As long as you aren't going to be driving it, just getting it to idle correctly, you don't need any vacuum devices connected at all. But, all connections on the engine and carb must be plugged.

[jstone4646]

The carb is a source of vacuum, as is that tree/spreader in the manifold. So it makes no sense to connect them. None. So you should pull that hose and cap both connections. And for that matter, you could cap the PCV connection and the brake booster connection.

As long as you aren't going to be driving it, just getting it to idle correctly, you don't need any vacuum devices connected at all. But, all connections on the engine and carb must be plugged.

Ok great, I will pluck all the vacuum and plug them all tomorrow to get this idle sorted. And to answer your earlier question about egr, I don't know. I never removed any egr stuff so I can't answer that. My guess is none of it is there. My only experience with an egr system was on my 93 blazer when I was younger. I deleted that whole system and put a block plate over the engine. How a CPI 262 Chevy and a carbd 351 Ford compare as far as egr goes, I am clueless.

[Gary Lewis]

Ok great, I will pluck all the vacuum and plug them all tomorrow to get this idle sorted. And to answer your earlier question about egr, I don't know. I never removed any egr stuff so I can't answer that. My guess is none of it is there. My only experience with an egr system was on my 93 blazer when I was younger. I deleted that whole system and put a block plate over the engine. How a CPI 262 Chevy and a carbd 351 Ford compare as far as egr goes, I am clueless.

What intake manifold is on the truck? The stock 4bbl manifold requires the EGR plate. And Edelbrock makes two manifolds - one with and one w/o EGR.

[ArdWrknTrk]

What intake manifold is on the truck? The stock 4bbl manifold requires the EGR plate. And Edelbrock makes two manifolds - one with and one w/o EGR.

I agree with Gary.

A mismatched 4bbl with an egr intake but no plate (or block off) will cause a big leak.

Still shouldn't run if the choke is open and the idle screws are in.

Maybe it's time for a smoke test?

It doesn't even need to run for that.

[Gary Lewis]

I agree with Gary.

A mismatched 4bbl with an egr intake but no plate (or block off) will cause a big leak.

Still shouldn't run if the choke is open and the idle screws are in.

Maybe it's time for a smoke test?

It doesn't even need to run for that.

Jim - The smoke test is a good idea. David's writeup on that is here: http://forum.garysgaragemahal.com/Vacuum-leak-detection-effective-and-inexpensive-td11511.html

As for running with the screws closed, I'm wondering if it is pulling fuel through the vacuum line that runs from the carb to the vacuum tree on the back of the intake.

[ArdWrknTrk]

Jim - The smoke test is a good idea. David's writeup on that is here: http://forum.garysgaragemahal.com/Vacuum-leak-detection-effective-and-inexpensive-td11511.html

As for running with the screws closed, I'm wondering if it is pulling fuel through the vacuum line that runs from the carb to the vacuum tree on the back of the intake.

Hard to imagine fuel being pulled *from* a vacuum port, but without that line there's no chance.

Picking away at one failure mode after another is just peeling the onion.

Most carburetors aren't very complex but they are the equivalent of a bridle and bit at this point.

Ask someone from an urban environment to saddle up your horse!

In another generation ICE's will be as esoteric as a steam engine.

[Gary Lewis]

Hard to imagine fuel being pulled *from* a vacuum port, but without that line there's no chance.

Picking away at one failure mode after another is just peeling the onion.

Most carburetors aren't very complex but they are the equivalent of a bridle and bit at this point.

Ask someone from an urban environment to saddle up your horse!

In another generation ICE's will be as esoteric as a steam engine.

Yes, it is hard to imagine that line pulling fuel. But with it off and the ports capped we will know it isn't. Hence my suggestion to get things down to the absolute minimum.

And I agree, carbs are going to be very passé, which is part of my quest to put EFI on both Big Blue and Dad's truck. But, I think there are enough aficionados of the ICE to keep them alive for a bit more than one generation. I hope!