Carb not pulling vacuum at idle

slimjimandtherats

Active member
So I'm looking for hints to what i should try next about solving my vacuum leak :banghead: . The carb is new to the car. It's a weber model 1940 stuck on a 61 170cid. At idle, the carb is barely pulling any vacuum, but it is enough above idle to allow me to drive her around town somewhat decent. The diaphragm in the distributor is newer, and holds vacuum when i apply it. I can physically hear a whistle coming from the carb as well but it's hard to pinpoint it. It is coming from the within venturi.
Tonight i took the carb off the base plate, and replaced the gasket for good measures, but she still isn't doing anything different to suggest any improvement.
Also, all fittings from the carb to distributor have been checked, and hold a good seal.
Cheers
 
I am not familiar with a Weber 1940, Is it a Holley 1940 by chance? Do you have the hot water plate that the heater hose attaches to still in place between the base of the carb and intake log? If so have you checked the bottom gasket? Have you checked the timing? Is the car a manual or automatic? If it's an automatic is it shifting properly? If not check the connection at the vacuum modulator on the transmission. If all else fails and you can't find a vacuum leak you can spray carb cleaner around all of your mating surfaces and fittings one at a time and see if the engine idle raises. If it does, that is the source of your leak.

Good Luck,

Kevin
 
Howdy Jim:

I've had success in locating vacuum leaks by using a piece of rubber hose as a stethoscope, by holding one end to my ear and moving the other end around the carb/manifold to pinpoint the leak. It ain't pretty but it works. Good luck and keep us posted on your progress.

adios, David
 
Rad, so ya my bad, holley 1940**. I'm going to take up that route of making a gasket for the hot water plate after work. I tried the carb cleaner trick early off, but had no luck with any type of idle change. It's an auto, timing is anywhere between 8-10 degrees btdc. The transmission has been shifting (what i feel) slower lately, I'd say a delay of about 5-6 seconds from when you let up on the throttle, to the gear change. I just remember it being a little faster, but at slow speeds i don't see much change. It's probably no different, I just havn't driven her much over the past 6 months.
I'll crack the heater plate to manifold gasket and check the gasket. I just replaced it with a fresh one, but ive gotta give it a peak.
And rightfully noted, I'll try and pick up a stethoscope from a buddy, but that vacuum line trick should help if that falls through.
This time of year up in the pnw is frustrating because the light fades so quickly. I think were down to 1640 sunsets.
Now, i replaced the gasket (between the hotwater plate and butterfly side) with the same pattern. could i have the wrong gasket? the kit comes with several different patterns. It has the 4 holes around the center venturi, and two for the mounting screws. Which gaskets should i use between the manifold, and hot water plate, and between the hot water plate and carb? Cheers
 
Check out the vids on "TV Cable" adjustment.
That may B all the transmis needs.
 
K, so tonight I checked the gasket between the manifold and water heater(good to go) then swapped the 4 hole gasket between the carb and water heater to a solid gasket with only the venturi cutout. Didn't work so well so it's back to the 4 hole + venturi. i used one of the thicker gaskets that came off it originally and its back to square one.
I poked around within the carb while it was running to try and feel for the leak but i couldn't locate it. I'm gonna run and grab a vacuum tester so i can get a good grasp of the true vacuum im pulling and grab some fuel line and poke around with that guy. Thanks for all the advise. I don't expect anyone to solve my problem, but i like the directions you guys send me in. Cheers!!!
 
Just got back from getting some numbers.
The most vacuum i'm getting is about 5 in hg at full throttle. sounds pretty weak to me. At idle, up to around 1500-2000rpm (guesstimate) im not reading any vacuum. I tore apart the carb again to inspect the bowl gasket for anything crazy but found nothing. I'm kinda high centered right now on the problem. I'm not quite sure where to continue. Anyways, I messed with the idle mixture screw a bit but nothing came from that.
What else could be checked? To recap, I've checked the gaskets between the manifold, and water heater plate, as well as the one between the HW plate and carb. The gasket on the manifold side is solid with one large carburetor hole. The gasket between the carb and HW plate has 4 small holes surrounding the main carburetor hole. I have confirmed both gaskets on the carb (bowl, and throttle) are not torn or distorted (just replaced both recently). The high pitch leaking sounds is coming from the center of the venturi where you have the two cast "metal crosshairs"(if you will).
My goal was to get this carb running solid so i could slap her on my project 200cid engine and have a good running carb to work with. I do have a spare 1100 autolite carb from the 200cid that I may do a partial rebuild to and slap on her. We'll see. Cheers
 
:unsure: Have you done a compression test on your engine yet? If so what kind of readings did you get. You might also check the condistion of your timing chain set. You can do this by turning the engine until the Distribitors rotor moves make a chalk Mark on the Distribitors body and the crankshaft dampener then turn the engine the opposite direction until the rotor moves and mark it their too. Let us know what you find. Good luck :nod:
 
I checked compression about 3 months ago. It was in the the neighborhood of 120-130ish dry test. I'll get another set of readings but I believe the previous owner put fresh rings in her.
Timing set was swapped out when i had the pan off and clearanced the rod bearings like 3 months ago. The slop in it was unreal(~1.5inch of play). I barred over the motor till TDC, and confirmed both markers on the cam and crank were facing each other when the new chain and gear set when on. Seemed fairly foolproof of a transaction.
I'm gonna go through and valve lash everything again along with another wet/dry compression test to see if anything else changed.
I don't know why it would help, but just to make the information known, the #5 or 6 cylinder is knocking a fair bit. That's why i had the pan off and checked rod bearing surfaces and continued to plasti-gauge them. They all passed with flying colors. This leads me to figure the wrist pin must be worn and allowing the piston to bounce around the cylinder walls a lil bit.This only began a few months ago and the motor has barely been run since it began. Hope any of this helps. Cheers
 
Hi, where are you checking your vacuum. You should be checking it at a fully open vacuum source below the throttle or on the intake manifold. If you are using the distributor vacuum source your readings will be off. Good luck
 
B RON CO":khn5y6gn said:
Hi, where are you checking your vacuum. You should be checking it at a fully open vacuum source below the throttle or on the intake manifold. If you are using the distributor vacuum source your readings will be off. Good luck

That was my thought as well. It sounds to me like the OP is using the vacuum port for the distributor, which will definitely give incorrect readings.

It wouldn't even run at all with zero manifold vacuum at idle.
Joe
 
Are you checking a fitting that actually sees full vacuum or a ported vacuum fitting? In case you're unaware, a ported vacuum fitting won't provide ANY vacuum at idle (when you'd actually want your timing advanced by vacuum. This is an emissions control), but it does ramp up to full manifold vacuum with engine speed.
 
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