69 tri-power, can you test secondaries with a MityVac

[Jack O.]

Sorry, I know this is a common topic but I've read everything and am still not sure.

I know my secondaries are not opening as I can both feel it when I drive and I did the paper clip test. So I wanted to check if one or both had a leaking diaphragm (carbs we just rebuilt). I disconnected the rear carb secondary closing mechanism from the throttle and verified the secondariness move freely. I then connected a MityVac to the diaphragm vacuum port and it does not move. But, I did same to the front carb and got the same results which makes me think perhaps my test is invalid as I wouldn't expect both diaphragms to be bad, although possible. So is this a valid way to test the secondary diaphragm? Or is there some other place where the vacuum is leaking because of the way I'm trying to test? I also validated that my MityVac setup holds vacuum by placing my finger over the end of the hose.

I was fairly sure I had tested in this manner before and located one bad diaphram which is what I was expecting here.

Thanks.

[Patrick B.]

Re: 69 tri-power, can you test secondaries with a MityVac

I think the secondary diaphragms are exposed to Venturi vacuum of the secondary carbs as well as the primary carb. Therefore they could not be operated by a mityvac unless the hole in the vacuum Venturi were covered. Look for a vacuum hole in the Venturi of the carb you want to test. If it is present, it must be covered.

[Chris H.]

[Tim G.]

Re: 69 tri-power, can you test secondaries with a MityVac

I installed the three Holley carbs back on my '67 435 yesterday and checked the secondaries after everything was buttoned up. I took my shop vac and covered the hose inlet with masking tape and put a small hole in it to insert the vacuum hose off of the primary carb. If you turn the shop vac on the suction should open the secondaries when you manually open the primaries. Mine worked fine, today I will start and drive before it gets too hot in Texas.

[Patrick B.]

Re: 69 tri-power, can you test secondaries with a MityVac

The shop vac has so much continuous air flow capacity under vacuum that it can let air in from the venturi hole and still generate enough vacuum to operate the diaphragm. The mityvac has zero air flow capacity while holding a vacuum so the hole must be sealed. I wasn't sure it could be accessed with a pinkie. That is good to know.

However, the shop vac may be powerful enough to operate the diaphragm even if the diaphragm is leaky so it may not be a sensitive test.

[Jack O.]

Re: 69 tri-power, can you test secondaries with a MityVac

Thanks a lot everyone! I will give it another try today. I figured I was missing something, no way both diaphragms went bad at the same time.

[Dan M.]

Re: 69 tri-power, can you test secondaries with a MityVac

If anyone is interested I have a video of the actual tripower operation of my 1969 L71 stock motor on a dyno. Happy to share with folks just send me a private email. Please note its a 23 Meg file.

Dan

[Duke W.]

Re: 69 tri-power, can you test secondaries with a MityVac

I think the secondary diaphragms are exposed to Venturi vacuum of the secondary carbs as well as the primary carb. Therefore they could not be operated by a mityvac unless the hole in the vacuum Venturi were covered. Look for a vacuum hole in the Venturi of the carb you want to test. If it is present, it must be covered.

Center carb venturi vacuum is what is applied to the end carb diaphragms to open them up. Therefore, with the engine off disconnect the signal hose from the vacuum pot and use a piece of test hose connected to a vacuum pump and pump them down. I don't know when they should start to open, but will guess at around 6-8" Hg.

You can also test them by blipping the throttle real hard and you should see them begin to open.

You can also test by driving. They should begin to open by about 2000-2500 at WOT, you will heard loud induction noise, and the engine will be pulling very strong by 4000.

If they are jammed shut there will be little to no induction noise, and the engine will start falling off at 4000 due to having only a two-barrel carburetor.

Duke

[Timothy B.]

Re: 69 tri-power, can you test secondaries with a MityVac

I think the secondary diaphragms are exposed to Venturi vacuum of the secondary carbs as well as the primary carb. Therefore they could not be operated by a mityvac unless the hole in the vacuum Venturi were covered. Look for a vacuum hole in the Venturi of the carb you want to test. If it is present, it must be covered.

I agree with you, Patrick. The small pickup hole in the passenger side venturi (all three carbs) needs to be covered to check the diaphragm's with a miti-vac.

I have never seen a vacuum secondary throttle plate open by blipping the throttle.

When the primary throttle blades are opened, venturi vacuum creates a low pressure (vacuum) at the small pickup hole in the primary carburetor. The vacuum signal is further bled off by the small pickup holes in the secondary carburetors until the vacuum is strong enough to overcome the secondary bleed and the spring above the diaphragm. At that point the secondary blades start to open and as air rushes through the secondary venturi the low pressure that's created changes the function of the secondary bleed holes and they further add to the vacuum applied to the diaphragm.

[Duke W.]

Re: 69 tri-power, can you test secondaries with a MityVac

I have never seen a vacuum secondary throttle plate open by blipping the throttle.

You didn't blip it hard enough. It also works on any four barrel carb.

Duke

[Jack O.]

Re: 69 tri-power, can you test secondaries with a MityVac

So sure enough, I plugged that hole and testing with the Mityvac lead to the rear carb. Leak was between diaphragm pod and carb so i removed and replaced the gasket and cleaned-up the surfaces a bit. But I didn't see anything that would obviously cause a leak. It's possible that tightening the screws may have been all it needed, not sure. So everything was working, took it out a couple of days ago and paper clip test passed and I could feel the difference.

Just did another test drive and things are back to not working. I know it's the rear carb again but haven't diagnosed why yet. What a pain.

[Chris H.]

Re: 69 tri-power, can you test secondaries with a MityVac

Lots of potential vacuum leak sources. Need to be methodical and check them all.

[Jack O.]

Re: 69 tri-power, can you test secondaries with a MityVac

Thought I'd post the resolution of my secondaries issue since I doubt it is well-known or common.

Once I loosened the carb to remove it the shaft moved freely and secondaries worked fine. I noticed same thing on other carb as well. The base plates had to be replaced. Not sure whether they were warped or too thin from being machined but secondaries work fine once they were replaced,

[Danny P.]

Re: 69 tri-power, can you test secondaries with a MityVac

Sorry, I know this is a common topic but I've read everything and am still not sure.

I know my secondaries are not opening as I can both feel it when I drive and I did the paper clip test. So I wanted to check if one or both had a leaking diaphragm (carbs we just rebuilt). I disconnected the rear carb secondary closing mechanism from the throttle and verified the secondariness move freely. I then connected a MityVac to the diaphragm vacuum port and it does not move. But, I did same to the front carb and got the same results which makes me think perhaps my test is invalid as I wouldn't expect both diaphragms to be bad, although possible. So is this a valid way to test the secondary diaphragm? Or is there some other place where the vacuum is leaking because of the way I'm trying to test? I also validated that my MityVac setup holds vacuum by placing my finger over the end of the hose.

I was fairly sure I had tested in this manner before and located one bad diaphram which is what I was expecting here.

Thanks.

if you just rebuild the 3 Carbs. maybe use the wrong housing gasket to the base plate and block the passage that happens a few times , or maybe your secondariness springs are to stiff i use the yellow colour code they work great, black and brown are too stiff .

[Jack O.]

Re: 69 tri-power, can you test secondaries with a MityVac

Turned out to be the vase plates which the rebuilder replaced. Like I said, when the carb was not torqued down everything worked fine.