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Chasing the vacuum leak

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jstone4646

Chasing the vacuum leak

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.

Project "Red" (not really named yet): '84 F150 4x2, Holley 4bbl 351 w/302 cam shaft and side exit pipes. Still learning what else is there but in progress of putting her back together and on the road.

Gary Lewis

Re: Chasing the vacuum leak

Administrator

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.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

jstone4646

Re: Chasing the vacuum 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

Re: Chasing the vacuum leak

Administrator

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.

Gary, AKA "Gary fellow": Profile

jstone4646

Re: Chasing the vacuum leak

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

Re: Chasing the vacuum leak

Administrator

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.

Gary, AKA "Gary fellow": Profile

ArdWrknTrk

Re: Chasing the vacuum leak

Administrator

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,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

Gary Lewis

Re: Chasing the vacuum leak

Administrator

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.

Gary, AKA "Gary fellow": Profile

ArdWrknTrk

Re: Chasing the vacuum leak

Administrator

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.

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

Gary Lewis

Re: Chasing the vacuum leak

Administrator

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!

Gary, AKA "Gary fellow": Profile

ArdWrknTrk

Re: Chasing the vacuum leak

Administrator

We've had ICE powered personal transportation ubiquitous for a century.
~4 generations, imo.
25 years from now????

Battery electric and fuel cell vehicles eliminate ignition, exhaust, fuel metering and much cooling and braking.
Never a tune-up, oil change, filter, belt or exhaust replacement nor concern about emissions.
This is going to be devastating to the aftermarket parts sector and petroindustrial complex.

*They already win unquestionably in performance and equivalent fuel economy*

Our electric infrastructure has a long way to go though.
Investment here will be huge and painful.

Sorry for the thread tangent.

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

jstone4646

Re: Chasing the vacuum leak

Update, removed and plugged all vacuum ports on the carb as well as unecessary Intake ports...cranked it up. Still a high idle, I sprayed more starter fluid along the rear (firewall side) of the carb and the idle dropped down to normal...I pulled the carb and you can visibly see on the gasket where it's leaking pretty good.

So before I just replace the gasket, what do you guys think about spacers between the carb and intake? I have one brand new still in it's packaging that the previous owner left with the truck. Worth Installing, or stay away? Its not an overly expensive spacer, looks like an auto zone straight off the shelf, has a 4 port insert, and a large square insert for various manifold setups I suppose, comes with new gaskets as well.

ArdWrknTrk

Re: Chasing the vacuum leak

Administrator

What intake is on the truck now?

Will the new gasket cover it?
Or do you*need* the spacer?
How is the throttle cable alignment?
Would the spacer help or hurt?

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

ArdWrknTrk

Re: Chasing the vacuum leak

Administrator

In reply to this post by jstone4646

If your current manifold has EGR (a notch that wraps around the back of the carb inlet) then you need to close it off.
To both the inlet and the atmosphere.

Most aftermarket spacers don't do this.
You need a block off plate, available from Edelbrock or Transdapt.

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

jstone4646

Re: Chasing the vacuum leak

In reply to this post by ArdWrknTrk

I don't know what Intake is on it, need to figure that out and get some number off of it. As for the spacer, that is what I am asking. The packaging claims "increase power!" but that is just an advertising gimmick in my opinion because I personally do not know if they really do or not....which is why I am asking! I can say for sure my manifold has zero egr items on it. No valve, nada.

ArdWrknTrk

Re: Chasing the vacuum leak

Administrator

Adding intake length (spacer) can help power in some instances....

You need to look at the manifold to figure out why your carb doesn't seal.

Take the new thick gasket and hold it to the bottom of the carb.
Drop the gasket over the mounting studs of the manifold.
Is there somewhere that it doesn't line up?

The EGR would have been in the factory spacer, but the port would also be in a factory manifold.

Is the manifold iron or aluminum?

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

Steve83

Re: Chasing the vacuum leak

Banned User

In reply to this post by jstone4646

This is the cheapest, safest, most-effective way to pinpoint vacuum leaks:

jstone4646 wrote

I don't know what Intake is on it, need to figure that out...

Post lots of pics.

jstone4646 wrote

The packaging claims "increase power!" but that is just an advertising gimmick...

Right - it's dead weight. No more useful than magnets around the fuel lines, or a mason jar with electric wires connected to the intake. Sell it or toss it.

jstone4646

Re: Chasing the vacuum leak

Update: well i finally got somewhere, my local parts store did not have the gasket for my 4160 Holley so I decided to use the one that was included with that spacer kit (I did not install the spacer, just used that gasket that was part of the kit). She cranked up very briefly and died out because the mixture screws were closed, so I backed them out 1.5 turns on each side and she runs somewhat decent. I need to go purchase a vacuum gauge so that will be something I do tomorrow but for now it appears that the carb/manifold gasket was the first issue. I figure now I can dial in the carb then start attaching the various vacuum lines one by one and see how it goes from there. Thank you all for the assistance and knowledge sharing! I can spray the back end of the carb now with starter fluid and there is no change to idle, so that is also a good thing and I should be on the right track now.

Gary Lewis

Re: Chasing the vacuum leak

Administrator

Yippee!

Congrat's!

But, before you dial the idle mix screws in perfectly you should at least connect the PCV hose. That's because that is a metered vacuum leak, and you need to account for that air in your idle mix setting.

Also, when you connect the vacuum line to the distributor you need to think about which port on the carb to use. If you use the one that has manifold vacuum your idle speed will shoot up due to the extra advance. I recommend you use the one that doesn't have vacuum at idle but does any place above idle.

And, I really do not think you should have a vacuum line coming out of the carb and going to that vacuum tree. I am very sure that is not correct for this carb. I'd leave that fitting on the tree capped and if the one on the carb isn't the ported one for the vacuum advance then cap it as well.

Gary, AKA "Gary fellow": Profile

jstone4646

Re: Chasing the vacuum leak

This post was updated on .

Well I spoke too soon, she's still high....better, a lot better but still high. Right around 900-920 high. Where before the gasket and vacuum plugging she was about 1250-1350. If I pull on the throttle linkage towards the front of the truck the idle will drop a little but it takes a significant amount of force. My throttle linkage is not resting on the idle adjust either, it just takes some significant force that no return spring would hold unless it was seriously heavy duty. I also found that the electric choke is NOT wired up. There is not a hit wire going to the choke assembly, only the ground, so my choke plate is staying at maybe 1/4" open. Video to follow shortly.

Video update: my phone won't let me link an embedded video so until I can get to my PC here is the link
https://youtu.be/34QMtQWfvUg

Ignore my stupidity in the video...35T...35...7...duh 351

Update: manifold is an Edelbrock Performer 351 without egr