won't idle, looking for expert advice.

[shorty402]

Afternoon all. I have a 69 swb 402/t400. Cant get it to run at idle. If have to hold throttle to about 2k and hold choke almost completely closed to keep it running. Accelerator pump works fine. Im thinking idle circuit not working or possibly a huge vacuum leak. I am going to take carb apart and clean and look for any obstructions and plug all vacuum ports on carb and intake. Any other suggestions would be greatly appreciated.

[Steeveedee]

Plug all the vacuum ports before you tear into the carb. That would include the ones on the intake manifold. If it runs OK, swap the lines back on until it goes crappy again. If not, you know what to do.

[shorty402]

Thanks for the reply. Already have carb off. Its been sitting for quite awhile with old gas and should be gone through anyway. Will plug everything before trying to start it again. Can't even time it like it is now.

[oneshotkyle]

Vacuum leak

[1967GMCC20]

Agree - Vacuum leak... clean carb, inspect mounting flatness.

WD40 spray around carb base usually can find.

[jabborabbo]

Carbs are built by elves that live in the Black Forest of Germany ((right next to the Keebler Cookie Company)…I would check EVERY orifice/passage in the idle circuit for obstruction… I have personal experience with clogged orifices/passages…and ain’t NONE of them been pleasant….

[54blackhornet]

I had a Stihl chainsaw that would not run properly. Had the carb cleaned by an ultrasonic parts cleaner. Cured the problem.

[shorty402]

Will be taking carb apart tomorrow, it will be raining all day anyway.
I will report back with any findings. Thanks

[1967GMCC20]

Working on, and resolving carb issues is good therapy. And when your good at it, you'll be the guy to fix everyone else's.

My buddies junk carb, he gave me, now runs like a top on my 402BBC.
All I did was scrape out the chalk from dried out fuel remnants.
Elec choke is nice.

[Dashman]

It's getting additional air from somewhere, that's why you have to use the choke circuit to close off air and add fuel. I would look at the throttle shafts and bores, and also the carburetor to manifold gasket.

[shorty402]

Update on possible vacuum leak. Removed carb, disassembled, cleaned, checked all passages, and checked flatness of base. Everything looked fine (this is a almost new holley 670). Put a small amount of sealer on gasket, installed back on engine, and plugged all vac ports on carb and intake. Restarted and still the same problem. Today i removed the intake and cleaned everything. Next used a straightedge with a light and see a slight gap along the bottom edge of the ports in the middle half of the edge ( maybe 2-3 thousands).
Can i use some kind of sealer around the intake ports to help this. I will be re-torquing from the center out. Any thoughts or recommendations appreciated.

[Rust_never_sleeps]

I'm no expert, but you can shoot some carb cleaner into the gap you suspect and you'll hear the revs pick up if it's being sucked in and enriching the mixture.

I've used Versachem #2 sealant to fill gaps, but I doubt it's any more or less effective than what you're already using

[Dashman]

As I've learned, aftermarket intakes require custom fitting to the block and the heads, and may require milling the surfaces. I recently had a problem with a Holley 600, 4160, where it would run with the idle enrichment screws turn all the way in. Turned out, the power valve wasn't tight enough. Although compared to your issue, it would idle in park smoothly without any choke, but it's stock heads on its original block. Has your engine experienced any overheating issues? Just thinking that the heads could have wrapped a bit.

I have a Ford 2N tractor that experienced some corrosion at the intake port, and it leaks enough air that it requires nearly full choke to keep it idling. Similar to what your experiencing.

You could remove the heads and have the surfaces checked and milled, then check the fit of the intake and mill that if needed. Good luck!

[shorty402]

Thanks for the input. I think the problem is on the bottom of the ports in the valley area. I put the intake on dry after gasket cleanup and could'nt see any gaps along the top where it meets the heads and fits flat at front and back. I did check the heads for flatness in this area and seems fine. Will report back after install with new gaskets. Thanks again.

[shorty402]

Another dumb question. I have an Edelbrock air-gap intake and the big block heads have the openings for a crossover. Do I need intake gaskets with the block off plate?

[RustyPile]

Originally Posted by shorty402

Another dumb question. I have an Edelbrock air-gap intake and the big block heads have the openings for a crossover. Do I need intake gaskets with the block off plate?

More importantly... Do the runners in that manifold match the size and location of the intake runners in the head(s).. Peanut port (small runners) heads have very small intake runners, and that manifold probably has larger runners.. Closely check the gaskets against the heads and manifold to be sure you have sufficient gasket surface on both. Even if you don't have the small runner heads, it's still a good idea to check these items.