Vacuum leak or carb problem?

[Stoney]

Hey all,

I've been trying to figure out the stalling issue on my 79 J-10 with AMC 360 V8 here lately and am trying to narrow it down.
So, after replacing the aftermarket ignition system that was in the (not running) truck when I bought it, it now starts right up and I drive maybe once a week. Unfortunately the truck likes to stall unless I keep some pressure on the gas pedal. I have to drive with both feet . Sometimes it will staying running without me having to keep my foot on the gas, but not always. At first it seemed like it stalled more when it was warmed up, but now im not too sure, it doesn't really seem to have a pattern. Also if its warm and I let it stall it wont start back up till it cools off.

Carburetor is a 4 Barrell Holly, about 2 or 3 years old. previous owner had it installed. Since I know almost nothing about them I just made an appointment for it to be tuned in mid-August. That way either it'll fix the problem or I can definitely rule that out as the problem. I talked to the Shop today and the "Carb guy" told me that a vacuum leak will cause a higher idle. I don't have a RPM gauge, but the idle sounds fine to me. Could the issue be the vacuum line instead? should I got do a smoke test with it? or is there another way to check?

Any other ideas or feedback would be great.

[letank]

Spray carb cleaner or WD40 at the base and around the base of the carb... any vacuum leak with create a idle speed change. Look for loose vacuum lines... or questionnable base carb gasket

As for the warm restart... usually it is vapor lock or a weak fuel pump, our mechanical fuel pump leaning against a warm engine is not the best set up with the reformulated gas containing ethanol...

Post edit: as said below, the vacuum gauge is the best tool... I have one install permanently inside on my dash... as for values for the 2150, my sea level is a steady 18" and 20" when the choke is on!

[rocklaurence]

Buy a vacuum gauge. You should have 10-14 " of vacuum at idle. The mention above about locating a Vac' leak is good. If you dont have Smog inspections in your county, remove and plug as much of the Vac' Crap as possible--Im only running PCV, Brake-booster and Timing circuits. Once your certain the leaks are corrected and there aren't leaks, check the carb'. I had this exact issue with an Edelbrock on my J10. It was the idle circuit. It wouldnt run with the throttle plates closed and I had to constantly have throttle applied to get any gas into the motor.
The Vacuum gauge will help you tone the Idle of the Carb'. There are plenty of carb tooling videos on YouTube.

[Stoney]

Spray carb cleaner or WD40 at the base and around the base of the carb... any vacuum leak with create a idle speed change. Look for loose vacuum lines... or questionable base carb gasket

As for the warm restart... usually it is vapor lock or a weak fuel pump, our mechanical fuel pump leaning against a warm engine is not the best set up with the reformulated gas containing ethanol...

Post edit: as said below, the vacuum gauge is the best tool... I have one install permanently inside on my dash... as for values for the 2150, my sea level is a steady 18" and 20" when the choke is on!

so I've sprayed carb cleaner around the base of the carb before without hearing any throttle increase, ill try spraying in more places. I replaced the mechanical fuel pump a couple months ago as well.
I don't know much about carbs at all, I've heard of vapor lock and have a basic understanding of it, is there any way to prevent it? though im not sure what you mean by 2150, is that the carb?

[tgreese]

Sorry if this seems negative. When fitting an aftermarket carburetor, it's your responsibility to select a carburetor that can be tuned to the application, and then to tune it to your application. The manufacturer assumes some basic level of competence by the purchaser. However, the carb maker will have an array of parts available that can be used to change the characteristics of the carburetor. Holley has lots of technical information on their site to help you.

if you had the factory carburetor, you could read and follow the TSM for your year. This assumes that you leave all the emissions devices in place, and work to achieve the performance the factory intended. You start removing all the emissions devices, or even worse substitute a different carburetor, and you need to understand what you are doing.

This is a Holley, and there is lots of information online and in the library or bookstore about tuning them. I'd suggest you buy a Holley book and learn what you need to make this carburetor work well. Holleys are well-liked because the tune-up parts are widely available and the tuning process is fairly easy. However, you need to start somewhere, and some book learning would be a beginning.

[letank]

so I've sprayed carb cleaner around the base of the carb before without hearing any throttle increase, ill try spraying in more places. I replaced the mechanical fuel pump a couple months ago as well.
I don't know much about carbs at all, I've heard of vapor lock and have a basic understanding of it, is there any way to prevent it? though im not sure what you mean by 2150, is that the carb?

Yes the 2150 is the stock motorcraft carburetor.

As for your Holley try to find the model number, it is stamped on the front or around the fuel bowl. I have a couple sitting on the shelf, but never bothered with them. Then go to Holley website and download the owner's manual... Sorry I cannot be more specific about your carb.

https://www.mortec.com/carbs.htm

Otherwise if you have all the emission control system, make sure that it is plumbed correctly, such as EGR , PCV ...

May be increase the idle speed a tad

[Stoney]

So just wanted to say I didn't install this Carburetor, the previous owner did. So that's why im not sure about the "background" of it, if you will. I'll definitely get some books on it asap and try to solve this mystery.

another thing to note, the truck REALLY likes to stall when I put it in reverse, but it doesn't do that with any other gear. It has a TH400 tranny in it that was swapped by the previous owner as well. could that mean anything or narrow it down somehow?
sorry if that's a stupid question.

[devildog80]

The TH400 has a vacuum Modulator.

Are you sure that vac connection is good?

[letank]

the truck REALLY likes to stall when I put it in reverse, but it doesn't do that with any other gear. It has a TH400 tranny in it that was swapped by the previous owner as well. could that mean anything or narrow it down somehow?

There are no stupid questions! We all... at least I have done all the mistakes!

yes the trans modulator can leak a bit, it will show as a large exhaust white smog upon start up.... as the trans fluid is sucked up the carburetor by the vacuum control hose!

the stalling in reverse could be an indicator -COULD- of a lean mixture, I had that happened on a project... on cold morning it was impossible to drive in reverse... and indeed, it had main jets of 50, stock is 55 for the motorcraft 2150... so you have a Holley... you can always fiddle with the mixture screw(s) and enrich the mix by 1/4 to 1/2 turn... yes it is not the proper way... but I am the shortcut guy

[Stoney]

The TH400 has a vacuum Modulator.

Are you sure that vac connection is good?

no, im not sure at all. I don't know much about those Trannys. I'll look some stuff up, but any links would be greatly appreciated.

[Stoney]

the truck REALLY likes to stall when I put it in reverse, but it doesn't do that with any other gear. It has a TH400 tranny in it that was swapped by the previous owner as well. could that mean anything or narrow it down somehow?

There are no stupid questions! We all... at least I have done all the mistakes!

yes the trans modulator can leak a bit, it will show as a large exhaust white smog upon start up.... as the trans fluid is sucked up the carburetor by the vacuum control hose!

the stalling in reverse could be an indicator -COULD- of a lean mixture, I had that happened on a project... on cold morning it was impossible to drive in reverse... and indeed, it had main jets of 50, stock is 55 for the motorcraft 2150... so you have a Holley... you can always fiddle with the mixture screw(s) and enrich the mix by 1/4 to 1/2 turn... yes it is not the proper way... but I am the shortcut guy

Also I've noticed that if im accelerating hard, like when im coming onto the highway, the transmission sounds and feels like it goes into neutral for like, half a second, then I get power back. it'll do this a couple times until I get to speed or stop accelerating so much, could that also be from a bad transmission modulator?

[devildog80]

Searched the web and found some info-

TH400 transmission modulator trouble shooting

One of the sites that came up-

https://www.chevyhardcore.com/tech-stor ... smissions/

[Stoney]

so I just went out in the rain and checked out my transmission modulator and there was oil or tranny fluid all around it, so I ordered a new one. Do those leak like that or could it be coming from somewhere else possibly?
now that I think of it I did see a plumb of white smoke come from the passenger side of the truck when I was sitting at a red light just down the road from where i started it

[devildog80]

If you have oil around it, might only need a new O-ring, and not the complete Modulator.

Check that link I posted as it talks about that very issue....and more!

[letank]

Also I've noticed that if im accelerating hard, like when im coming onto the highway, the transmission sounds and feels like it goes into neutral for like, half a second, then I get power back. it'll do this a couple times until I get to speed or stop accelerating so much, could that also be from a bad transmission modulator?

check your transmission fluid level, wheel choked, on level ground, trans in neutral and at operating temperature - which is about 5 to 10 miles of driving depending on outside temp- and having shifted to all gears including reverse, and back to neutral, engine at idle speed.

[1978J10REDWHITE]

I would isolate everything that is connected to the intake and get it down to the carb only, and see what they produces such as smooth adjustable idle etc.
This is a lot of TS type work to verify things Like was said the TH400 Modulator , or the EGR or a poor intake gasket etc. your rig has way too much on t to do it piece by piece, SHOTGUN it~!

A healthy engine should have 14~18 Intake vacuum (Hg) around or at near idle!
The Carb air fuel mix screws and the timing will affect this both in a certain combo.

Need to First get those two things to achieve the best highest idle vacuum numbers obtainable.
Your PCV / EGR network hose-tubes connections is a "controlled "Vacuum LEAK" system so it is possible a poor seal on the vlv covers and the hoses connected somewhere will allow extra air to get into the intake making this lean High idle condition.
Definitely need to VERIFY every single hose in your entire engines connections and any intake switching port control devices, etc, they are NOT cracked leaking.
Hose clamps etc.
All three of my rigs , 76,78,79, I shotgunned ALL the rubber tubes, hoses in the entire cab then under dash, 100% Everything!
There were plenty of bad pieces and so to TS this piece by piece was a complete waste of time.
Once you get the best #'s from Carb idle screw Adj and timing, then introduce things one by one.
Including verify the small 7/32 I think is the size, metal vacuum tubes that might be on Passenger side , which branch down to the Transmission.
Those fail and get clogged too!

[Stoney]

Also I've noticed that if im accelerating hard, like when im coming onto the highway, the transmission sounds and feels like it goes into neutral for like, half a second, then I get power back. it'll do this a couple times until I get to speed or stop accelerating so much, could that also be from a bad transmission modulator?

check your transmission fluid level, wheel choked, on level ground, trans in neutral and at operating temperature - which is about 5 to 10 miles of driving depending on outside temp- and having shifted to all gears including reverse, and back to neutral, engine at idle speed.

i just replaced the transmission gasket, so im 99.9% sure that the fluid level is fine, ill check again though

[Stoney]

I would isolate everything that is connected to the intake and get it down to the carb only, and see what they produces such as smooth adjustable idle etc.
This is a lot of TS type work to verify things Like was said the TH400 Modulator , or the EGR or a poor intake gasket etc. your rig has way too much on t to do it piece by piece, SHOTGUN it~!

A healthy engine should have 14~18 Intake vacuum (Hg) around or at near idle!
The Carb air fuel mix screws and the timing will affect this both in a certain combo.

Need to First get those two things to achieve the best highest idle vacuum numbers obtainable.
Your PCV / EGR network hose-tubes connections is a "controlled "Vacuum LEAK" system so it is possible a poor seal on the vlv covers and the hoses connected somewhere will allow extra air to get into the intake making this lean High idle condition.
Definitely need to VERIFY every single hose in your entire engines connections and any intake switching port control devices, etc, they are NOT cracked leaking.
Hose clamps etc.
All three of my rigs , 76,78,79, I shotgunned ALL the rubber tubes, hoses in the entire cab then under dash, 100% Everything!
There were plenty of bad pieces and so to TS this piece by piece was a complete waste of time.
Once you get the best #'s from Carb idle screw Adj and timing, then introduce things one by one.
Including verify the small 7/32 I think is the size, metal vacuum tubes that might be on Passenger side , which branch down to the Transmission.
Those fail and get clogged too!

I was considering just replacing all those hoses anyways and now that you're saying it it definitely sounds like the way to go. Do they have vacuum hose kits? or do I just buy so many feet to cover the whole truck?

[tgreese]

No vacuum hose kits. Go to the parts store and buy a length of what you need. Measure/guess and buy more than you need. Most of the hoses are 5/32" vacuum hose. Get that exact size or it won't fit. There are some places on the engine with odd sizes - measure or take samples of what you need when you buy the hose.

Vacuum hose has a comparatively thick wall and no fiber reinforcement. Fuel hose is fiber reinforced (positive pressure) and marked "for fuel." You can use fuel hose for vacuum (ie power brakes) but not vice-versa. Do not use coolant hose (heater hose) for anything on the fuel system - it will soften and come apart.

[Triumph215]

Pull a few spark plugs. Are they sooty or white? Generally speaking white would be lean carb jetting, sooty is rich jetting. There are other factors involved but at the plug this would be a fair indicator of what's happening there. At least if they're black and sooty you could eliminate the lean factor and work on other things.