Vacuum leak caused by choke removal

dcdoug · #1 (**permalink**) 05-22-2013, 08:16 AM

Many of you experts out there may already know this, but I learned something last week that I thought I'd share for others.

I was recently watching a video on carburators and learned that if you remove the choke on a Holley 4150 carb, you need to replace one of the mounting screws and use thread sealer or you will likely have caused a vacuum leak.

I removed the choke on my 3255-1 quite a while ago (car always ran fine) but decided to investigate the claim from the video last night and found that indeed the center screw hole that was one of the choke mounting points is not plugged and goes somewhere into the center of the carb. So it seems to be true. My engine is out of the car right now and I couldn't run it to test if it actually is a vacuum leak, but it sure seems likely.

Here is a pic of the carb before the choke came off. It's the center hole of the three in this area when the choke is removed.

Jerry Clayton · #2 (**permalink**) 05-22-2013, 08:22 AM

thats the source for the vacume to suck the warmed air thru from the exhaust crossover heat----the later electric chokes didn't require that--------

dcdoug · #3 (**permalink**) 05-22-2013, 10:51 AM

Ah, makes sense. I'll plug it. Wonder how much of an effect that has had on my secondaries opening?

patrickt · #4 (**permalink**) 05-22-2013, 12:01 PM

Quote:

Originally Posted by dcdoug

Ah, makes sense. I'll plug it. Wonder how much of an effect that has had on my secondaries opening?

There are vacuum passageways behind the choke housing. Obviously, you don't want them just left open sucking in air.

Here is a site with a wealth of pics of them. https://sites.google.com/site/mokmos...ing-adjustment What you're really doing, or should be doing, when you remove an automatic choke is to convert it to a manual choke, but with no manual choke there; you just use the manual choke pieces so there's no leaks. When you look at the pics, you'll see it's pretty straight forward.

mickmate · #5 (**permalink**) 05-22-2013, 12:12 PM

Good call Doug, just FYI the hole takes a 10-32 tap nicely. I used a button head with a nylon washer under it to seal. I probably could have taken the little gasket off that was on there and used that. You can see I have the choke next to it to show both parts in the same picture. You'll notice the choke fell completely off that one.

Jerry Clayton · #6 (**permalink**) 05-22-2013, 12:16 PM

You can also use a piece of lead shot or bb

patrickt · #7 (**permalink**) 05-22-2013, 12:28 PM

Just stick a glob of JB Weld in there with your finger.

dcdoug · #8 (**permalink**) 05-22-2013, 03:52 PM

I had it set up for a manual choke at one point, but decided that was for sissies and more of a hassle than it was worth.

Thanks for the tips.

Question: Nick, what are those two little holes for at the base of the carb (top of your pic since it's upside down)? Same size small little holes centered on the primary and secondary barrels.

patrickt · #9 (**permalink**) 05-22-2013, 05:13 PM

Quote:

Originally Posted by dcdoug

...what are those two little holes for at the base of the carb (top of your pic since it's upside down)? Same size small little holes centered on the primary and secondary barrels.

You see this little diagram of the back of the choke assembly housing? It fits in those three spots on the body and allows fresh air to be sucked in from the outside and circulated around the choke so it doesn't burn up. It has a little brass filter and screen on the other side to keep from sucking crap up. I couldn't find a better pic than this... but I didn't try all that hard considering the sissy remark.