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Vacuum Leak - Can't Find It

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Old 01-23-2017, 01:25 PM   #21
plaidside
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With both idle adjustment screws turned all the way closed, I drove out to his shop. He put his hand over the primaries nearly covering them up and the RPMs came up.
First, if the idle mixture screws are turned all the way in then where is the fuel coming from to keep the car running?
So IMHO I do not think you have a vacuum leak.
Qjets are very sensitive to airflow down the triple primary venturies, so if you have a large cam or the timing is not advanced enough at idle the throttle plates will have to be opened enough to allow more air for idle. But by doing this, more air is passing the sensitive venturies and they will drip and the idle screws become ineffective.
With the engine idling shine a bright light into the primaries and see if you can see fuel dripping.
GM uses two systems that I know of to overcome this and if you would like I will go into detail.
But first I would check what your base timing is w/o the vacuum advance connected and report back.

The second issue:
It will idle all day at 700 RPM but when driving and lift foot it will be at 1200-1300 and eventually get back to 700.
Try and disconnecting the vacuum advance line and see if it stops doing it.
I have found if the vacuum advance line is connected to the ported or timed port of the carburetor and if the throttle plates are opened too much at idle then the vacuum bleeds off very slowly which keeps the vacuum advance from returning to zero the minute you release the throttle. So the timing that the vacuum unit has adding when you accelerate, because the port is uncovered fully, is slowly going back to zero and the idle speed follows the timing down to base idle.
If you are using ported advance then move the vacuum line to a port that has full vacuum advance at idle and see it after resetting the idle speed and mixture if it cures your problem.
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Old 01-25-2017, 07:51 AM   #22
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Originally Posted by plaidside View Post
First, if the idle mixture screws are turned all the way in then where is the fuel coming from to keep the car running?
So IMHO I do not think you have a vacuum leak.
Qjets are very sensitive to airflow down the triple primary venturies, so if you have a large cam or the timing is not advanced enough at idle the throttle plates will have to be opened enough to allow more air for idle. But by doing this, more air is passing the sensitive venturies and they will drip and the idle screws become ineffective.
With the engine idling shine a bright light into the primaries and see if you can see fuel dripping.
GM uses two systems that I know of to overcome this and if you would like I will go into detail.
But first I would check what your base timing is w/o the vacuum advance connected and report back.

The second issue:
It will idle all day at 700 RPM but when driving and lift foot it will be at 1200-1300 and eventually get back to 700.
Try and disconnecting the vacuum advance line and see if it stops doing it.
I have found if the vacuum advance line is connected to the ported or timed port of the carburetor and if the throttle plates are opened too much at idle then the vacuum bleeds off very slowly which keeps the vacuum advance from returning to zero the minute you release the throttle. So the timing that the vacuum unit has adding when you accelerate, because the port is uncovered fully, is slowly going back to zero and the idle speed follows the timing down to base idle.
If you are using ported advance then move the vacuum line to a port that has full vacuum advance at idle and see it after resetting the idle speed and mixture if it cures your problem.
Found my vacuum leak. Air is being sucked from the lifter galley. The last time the intake was off I drilled and threaded 3/4" openings to be able to monitor cam wear by simply taking a threaded plug out. By shoving air down the primaries I am getting air coming out of that opening. Unless I am wrong, the only way for air to come out of that opening is either a gasket leak or a crack in the manifold.
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Old 01-25-2017, 07:58 AM   #23
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You must have some serious, serious concerns about cam lobe wear is all I can say..
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Old 01-25-2017, 09:13 AM   #24
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Originally Posted by rahaul View Post
Found my vacuum leak. Air is being sucked from the lifter galley. The last time the intake was off I drilled and threaded 3/4" openings to be able to monitor cam wear by simply taking a threaded plug out. By shoving air down the primaries I am getting air coming out of that opening. Unless I am wrong, the only way for air to come out of that opening is either a gasket leak or a crack in the manifold.
That is a new one one me!
Where did you put these "openings" and how many did you do?
could you post a picture.
Joe
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Old 01-25-2017, 09:23 AM   #25
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You must have some serious, serious concerns about cam lobe wear is all I can say..
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Old 01-25-2017, 10:44 AM   #26
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Glad you found the issue, I have never heard of any small block needing inspection plugs.
A picture would be nice. A new manifold may be in order?
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Old 01-25-2017, 06:32 PM   #27
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You must have some serious, serious concerns about cam lobe wear is all I can say..
This.

Totally.

I am pretty intrigued by what concerns would result in taking such extreme measures.

I am not in the business of building engines full time, but I've been tinkering with building engines, as well as working on them, for over 45 years, and I have never heard of anything remotely resembling this thought.

A far easier way to check lobe wear is to remove the valve covers (4 bolts) and measure pushrod lift, plus it's an exact measurement instead of a visual.

Sounds to me that in this case that the medicine is far worse than the disease.

YMMV.

Last edited by Easy Rhino; 01-25-2017 at 06:35 PM.
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Old 01-26-2017, 08:07 AM   #28
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This.

Totally.

I am pretty intrigued by what concerns would result in taking such extreme measures.

I am not in the business of building engines full time, but I've been tinkering with building engines, as well as working on them, for over 45 years, and I have never heard of anything remotely resembling this thought.

A far easier way to check lobe wear is to remove the valve covers (4 bolts) and measure pushrod lift, plus it's an exact measurement instead of a visual.

Sounds to me that in this case that the medicine is far worse than the disease.

YMMV.
The guy that built my camshaft said it would self destruct because of the valve springs I was using. I know this will make zero sense w/o the whole story. All I will say right now is harmonic vibrations were breaking valve springs (according to him) and his answer was to go up on spring pressure and my reasoning was not to. I won't mention any names because the guy is brilliant on camshaft design. When a BBC makes consistent 24-25 MPG and as high as 27, he did what I asked him to do - build a cam for fuel mileage. By removing a threaded plug and using a digital inspection camera, I can see if this cam is "self destructing". It hasn't yet.
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Old 01-26-2017, 10:33 AM   #29
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Had a customer who was an engineer about 30 years ago. Came into my shop with a Dodge Omni, with a cyclone fence eyelet bolted through the dead center of the roof, with the D part of the eye inside the car. Connected to this were about 8 braided cables, all drawn banjo-string tight with turnbuckles, anchored to the corners of the inside of the car: kick panel areas, Bi-pillar bottoms, rear seat bottoms, etc. It looked like a giant umbrella inside the car without any canvas on it. You had to thread yourself into the car to get into the drivers seat, to get around the cables, that were basically a gigantic cheese slicer if you got into a wreck, you being the cheese. I asked him what was up, and he stated that he did this to tame a vibration issue. When I had the car in the air for its oil change, I let him know that his blown rear struts had caused his rear tires to cup severely, which was causing his vibration. He politely thanked me, declined my offer to repair his car, and drove off on his octagonal tires. Lots of strange stuff out there!
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Old 01-26-2017, 10:50 PM   #30
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Had a customer who was an engineer about 30 years ago. Came into my shop with a Dodge Omni, with a cyclone fence eyelet bolted through the dead center of the roof, with the D part of the eye inside the car. Connected to this were about 8 braided cables, all drawn banjo-string tight with turnbuckles, anchored to the corners of the inside of the car: kick panel areas, Bi-pillar bottoms, rear seat bottoms, etc. It looked like a giant umbrella inside the car without any canvas on it. You had to thread yourself into the car to get into the drivers seat, to get around the cables, that were basically a gigantic cheese slicer if you got into a wreck, you being the cheese. I asked him what was up, and he stated that he did this to tame a vibration issue. When I had the car in the air for its oil change, I let him know that his blown rear struts had caused his rear tires to cup severely, which was causing his vibration. He politely thanked me, declined my offer to repair his car, and drove off on his octagonal tires. Lots of strange stuff out there!
That is about the weirdest thing I've ever heard of!
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Old 01-27-2017, 02:01 AM   #31
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Engineer's can zero in on the oddest things.
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Old 01-27-2017, 06:55 AM   #32
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Engineer's can zero in on the oddest things.
I agree and they always will come up with some explanation even if it makes no sense. In my case I tend to believe the harmonic vibration theory. They were quality valve springs(Beehive if I recall) and Morel lifters. Cam builder said it would take purchasing a multi million dollar machine to recreate the same scenario to see exactly why these valve springs are breaking. I ended up calling Comp Cams and went with their recommendation of 120 lb seat pressure valves. I was up to 375 lbs with the cam builder.
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Old 01-27-2017, 07:02 AM   #33
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I was up to 375 lbs with the cam builder.


Just Wow. 375 pounds on a street engine????

Just how much lift and what rpms were you running at?
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Old 01-27-2017, 09:09 AM   #34
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Just Wow. 375 pounds on a street engine????

Just how much lift and what rpms were you running at?
Good question. I stressed over and over that fuel mileage was priority #1 and HP was secondary. I really think he was building this cam for both as much as he could. Very aggressive valve lift which was the reason I spent big money on Morel roller lifters. Gross intake lift .628 / exhaust .585. I know he was concerned about valve float which over time the valve is slamming the lifter hence the big springs to eliminate float. In my book the less valve seat pressure the easier everything is on wear.

What RPMs am I running? Very few. 3:08 with T-56 6 speed/ Final drive 1:54 / 1600 RPMs at 80 MPH and an occasional 5K in 1st and 2nd.
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Old 01-27-2017, 10:38 AM   #35
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That is about the weirdest thing I've ever heard of!
That's why, 30-odd years later, I remember it. Like yesterday. The wire was super thick, braided cable that you could darn near tow a car with, too. And, I forgot to add, the reason the roof didn't buckle was that he had a cyclone fence pole mounted to the floor with a flange and it was attached to the roof hardware. I had forgotten that when I posed..so it was even more intrusive on the car. Without the post, the roof would have been caved completely in. When he drove off, you could hear the tires hopping as he went down the street....like a monster truck with mud tires.
As for fuel economy and camshaft profiles, although no engineer, I do know that the more valve lift, the more fuel and exhuast, which usually means more horsepower at the expense of fuel mileage. An economy cam would have well under .500 lift, and be most likely around .400 FWIW.......
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Old 01-27-2017, 12:46 PM   #36
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Sounds like far too much camshaft for your application. GM uses cams with under .500 lift for stock and performance grinds. I think GM may have an edge over Ronnie Racer in his garage when it comes to street engines. I would change the cam for a milder design if you want reliability. If you plan to live at the race track you are set up well but with a 3.08 rear gear you would be better served with a high torque cam design.
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Old 01-27-2017, 01:05 PM   #37
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Seems as if all the time spent on cam development, valve springs, etc., would have cost much more that any savings obtained through fuel economy. In any case, the important question is how the engine runs now that you have discovered (and presumably fixed) the vacuum leak. If that didn't do it, my money remains on the distributor advance. By the way, we would all like to see some pictures of the inspection holes and their plugs.
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Old 01-27-2017, 01:29 PM   #38
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X2! Photos of the inspection holes and their plugs would be most appreciated!
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Old 01-27-2017, 07:33 PM   #39
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Seems as if all the time spent on cam development, valve springs, etc., would have cost much more that any savings obtained through fuel economy. In any case, the important question is how the engine runs now that you have discovered (and presumably fixed) the vacuum leak. If that didn't do it, my money remains on the distributor advance. By the way, we would all like to see some pictures of the inspection holes and their plugs.
Yes, I will oblige on the pics if i can figure out how to post them.

I didn't spend anything on cam development, only the cost of parts. The fuel savings have been well worth the initial issues. The cam makes tons of torque on the low end. It is quite incredible. When everything is dialed in I can actually shift into 6th at 35 MPH/ 800 RPMs and throttle up w/o hesitation. The LS-6 in factory form was 475 ft lb but I can promise it is easily 525.

Intake may go back on sometime this weekend.
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Old 01-27-2017, 08:37 PM   #40
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Wow, I missed that engine note. LS6, as in 1970 454 with 450 HP?

Thought you had a 327. The LS6 must be a lot of fun. The big blocks had just as many if not more issues with cams going flat and high lift high load springs exaggerate that issue.
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