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Discussion Starter · #1 ·
Hi all - been scratching my head on how I could determine if a motor has an intake leak.. best I could come up with is cranking it over with plugs in and see if the crank case develops a negative pressure? I have a sbc 350 here that I put 193 heads on last year for my 22 searay fishing boat, it idled ok (little rough) until i noticed the qjet secondaries were slightly cracked open.. after bending the tang back to have them closed and only have the primaries open it simply will not idle.. I tried 3 other carbs and the sucker just wont idle with any of them. At a loss here but was wondering what tricks you guys can come up with. I have to crack it and crack it (squirter) to keep it running. If i do open the secondaries it will prob idle again but not perform properly..

Option 2 is idle it high using the secondaries and compare vac drop by disabling one cylinder at a time..

Option 3 is use my iginition analyzer and compare rpms cylinder to cylinder straight off the plug wire to see if one has a lower or higher rpm (my guess for a cylinder leaking would be raise rpms/run lean/suck oil)..

What are your thoughts?

What about a block off plate on the manifold top (no lash/valves closed) and 5-10 psi in with a gauge to show if the intake can hold pressure? Maybe add a psi gauge on the crankcase (block the rest of the ports) to see if it increases when intake is pressurized?

Seen the smoke machine but dont have one handy, but i guess that could work too..
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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You can spray carburetor cleaner where the intake meets to cylinder head, carburetor base plate and the vacuum connections with the engine running. As soon as the carburetor cleaner goes into the vacuum leak, you'll hear the engine RPM go up briefly. I do this on a cold start, if I can. You don't need to soak it down, just quick spray bursts. The carburetor cleaner evaporates and doesn't leave a mess. You're not going to burn down the boat doing this either.
 

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You can spray carburetor cleaner where the intake meets to cylinder head, carburetor base plate and the vacuum connections with the engine running. As soon as the carburetor cleaner goes into the vacuum leak, you'll hear the engine RPM go up briefly. I do this on a cold start, if I can. You don't need to soak it down, just quick spray bursts. The carburetor cleaner evaporates and doesn't leave a mess. You're not going to burn down the boat doing this either.
That won't test for a leaking intake gasket that is leaking bottom of intake ports into the lifter valley though.
You would need a way to spray into the lifter valley.

To Fasta..
Why did you close the secondaries completely at idle??
I'm no Q-jet guy, but virtually every other 4 barrel carb out there has some amount of airflow through the secondaries at idle. It keeps the metering system clean and active and helps with mixture balance.

It won't idle now because it can't get enough airflow.
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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That won't test for a leaking intake gasket that is leaking bottom of intake ports into the lifter valley though.
You would need a way to spray into the lifter valley.
That's correct. I gave advice and forgot to answer his actual question.

Fasta,

Engine off. This would only work if you don't have oil leaks. Get a loaner vacuum pump from the auto parts store and connect it to the vent on one of the valve covers. Block the PCV and the other vacuum lines going to the engine. Attach a vacuum gauge to the oil dipstick tube. Block the throttle open to some degree. Turn on the pump and watch the vacuum gauge. If you have an internal intake manifold vacuum leak of significance that pump (usually used to vacuum air conditioning systems) shouldn't be able to draw a vacuum. If you can draw a vacuum I would think you're ok and would proceed to the external vacuum leak check.

Some folks modify an engine pulling plate that bolts to the carburetor flange. One port for pressure and one for a gauge. Or one port if you have an inline gauge on your pressure source. You would close all of the valves, as you suggested. It should hold the 5-10 psi you mentioned.

From what you described (essentially ruling out a carburetor issue) and what was not said (oil consumption, fouled spark plugs), I went straight to the advice on finding an external vacuum leak as the culprit.
 

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Discussion Starter · #5 ·
1979 thats exactly what I did except I just used a vac port, drilled/tapped the plug and screwed in my outdrive testing kit (one way valve and psi gauge).. so i could monitor bleed down - all valves were shut/no lash and it showed leakage. As a backup I did pressurize it and sprayed some oil on top and also noticed air bubbles.. These were enw felpro gaskets - ive had 4 motors do this in th elast year, none ever before - maybe a bad streak but for what I do at the shop im putting a tiny bead of RTV around each port now- the time and frustration is not worth it. Fwiw I pulled the intake gaskets off the Volvo Penta 5.0 i replaced and hey presto- RTV around each port as well.. not 100% if that was factory or not but im sure it was.. who knows-

Some folks modify an engine pulling plate that bolts to the carburetor flange. One port for pressure and one for a gauge.
 

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Discussion Starter · #6 ·
Thanks - wasnt aware it required 2ndary flow- i had a carb off my original 350 you helped me out with big time almost 10 years ago and the secondaries were shut far as i remember- ill check though..either way i did fond intake leaks on this motor.. even with the secondaries a little cracked it still ran a little odd warmed up, mid/wot was good- plus a cam bearing went all crooked on me- its been one of those never ending stories! hopefully this is the last stint and i can get my boat ready for fishing season :)

Why did you close the secondaries completely at idle??
I'm no Q-jet guy, but virtually every other 4 barrel carb out there has some amount of airflow through the secondaries at idle. It keeps the metering system clean and active and helps with mixture balance.

It won't idle now because it can't get enough airflow.
 

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Vacuum leaks tend to result in a high idle rather than a not idle. The leak has to be really big to stop the engine.

The fact that it idled with the secondary’s open and won’t with them closed shows it doesn’t get enough air.

The 193 is the swirl port truck head with a 64cc chamber a decent to moderate RPM head should work pretty well in a boat not expected to be a racer.

Need to know more about the set up and whether your talking an in neutral idle or if the engine is turning the prop. Info on cam and compression is useful as is ignition timing from base through what the automatic systems are doing.

What about the intake, the 193 head uses an odd bolt angle on the those alongside the plenum, Not just any 12 bolt intake allows these 4 bolts boxing the plenum to be used?

Bogie
 

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Discussion Starter · #8 ·
Yes I modified the intake (ghetto style) - the motor was ripping fast, i think the 193 are excellent stock boat heads. I was wildly surprised by the torque.. ppl throw 193s away, im very happy with them. Stock for the rest of it.. believe the motor originally had smog heads and lower compression - just swapping to these heads made a considerable difference..

vortecs are so crack prone and not worth spending money on aftermarket heads on this setup anyway-

it was a year ago so memory fades.. Ill test run it today and see how it goes- I would have liked to port these according to that guy on youtube who has like 3-4 hours of porting these (willy?).. maybe just half of what he does- that would have been fun if i had the time -

having my own share of lifter issues to deal with as well plus my oTC cam tool has finally been confirmed as problematic, been fighting cam bearing / lifter / vacuum issues for a while now - finally ironing out the issues one by one- plus i got a shop to run!

Vacuum leaks tend to result in a high idle rather than a not idle. The leak has to be really big to stop the engine.

The fact that it idled with the secondary’s open and won’t with them closed shows it doesn’t get enough air.

The 193 is the swirl port truck head with a 64cc chamber a decent to moderate RPM head should work pretty well in a boat not expected to be a racer.

Need to know more about the set up and whether your talking an in neutral idle or if the engine is turning the prop. Info on cam and compression is useful as is ignition timing from base through what the automatic systems are doing.

What about the intake, the 193 head uses an odd bolt angle on the those alongside the plenum, Not just any 12 bolt intake allows these 4 bolts boxing the plenum to be used?

Bogie
 

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Check out this video at the 10:00 mark and he shows you how to find an internal vacuum leak. On this vehicle it showed about ten inches of vacuum on his gauge from the pcv port on the valve cover.

 
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