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Discussion Starter · #1 ·
My '83 RC 318 is running poorly.  Recently, it would not even idle anymore.  I tested and found awful vacuum.  A compression test showed a problem with the #1 cylinder which turned out to be a pushrod through the rocker.  I replaced the pushrod and rocker so at least it now idles but roughly and vacuum is still poor.  At idle I get a fluctuating 8-13 in Hg.  I can richen the mixture by placing my hand over the carb and it will idle much faster so I believe I've got a vacuum leak.  Timing is good at 16 BTDC. 

I disconnected and plugged every vacuum hose with no improvement.  I tried the propane test around the intake but didn't notice any change.  Next, I sprayed carb cleaner around every possible source of the leak.  I think I got it to ingest some carb cleaner at the back of the intake.  It's tough to spray back there so I'm not sure I found the leak or the carb was sucking in the fumes.

Is there any other test I should try?  Is the back of the intake manifold a common place for a leak?

- Rick
 
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Well if there was a vacuum leak at the manifold it would show up with really low pressure like 3-5 Hg, but 16 degrees BTDC, that sounds a little suspicious....

the readings that you showed from 8-13 Hg, according to my class notes is a carb/injector adjustment and/or burnt/leaking valves...

use propane where the carb/throttle body meets the intake manifold...

 

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Discussion Starter · #3 ·
The first place I checked for a leak was at the base of the carb.  I checked with propane and carb cleaner and didn't find a leak.  Wouldn't burned or leaking valves show up with a compression test?  All cylinders tested between 110 and 130 psi so I figured the valves weren't the problem.

I just read that vacuum readings will be approximately 1 in. Hg lower for every 1000 ft. above sea level.  I'm at 6500 feet so maybe my 8-13 Hg is not that bad.  If 18-20 Hg is normal at closed throttle at sea level, then adjusting for altitude mine should be about 12-14 Hg.  Is it unreasonable to hope for a steady vacuum gauge needle on an engine with 80K miles?
 

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I'm at about 7500 ft, the best vacuum I typically get is about 14. So yours seems low for a stock or near stock engine. The fluctuation is a problem, expectiong good vacuum is not unreasonable for 80k.

Maybe a leak down test will reveal more?
 

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Discussion Starter · #5 ·
A leak down test would be helpful.  Unfortunately, I'm not equipped to perform one.  I don't have a compressor or the tester.  Maybe I should find a shop to perform the test.
 

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Discussion Starter · #6 ·
I have been using the vacuum gauge on a new vacuum pump.  As a test, I also connected an old cheap vacuum gauge I hadn't used in years and amazingly it showed an almost steady reading.  The reading was much higher so I used the vacuum pump to calibrate the gauge and now I get around 13 Hg at idle.  With both connected, the cheap gauge has a steady needle while the vacuum pump gauge fluctuates.  Why?
 
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R_C said:
I have been using the vacuum gauge on a new vacuum pump. As a test, I also connected an old cheap vacuum gauge I hadn't used in years and amazingly it showed an almost steady reading. The reading was much higher so I used the vacuum pump to calibrate the gauge and now I get around 13 Hg at idle. With both connected, the cheap gauge has a steady needle while the vacuum pump gauge fluctuates. Why?
13 Hg at an idle with a steady needle, that sounds more like it....
 
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