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340 mystery build - low vacuum

rt-man

rt-man

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Engine bought last summer, just fired up yesterday. Only 5 inches of vacuum at idle. :(

What I know:
'72 block
steel crank
new pistons and rods, shiny walls (as seen through boroscope)
'72 360 heads, valves opened to 2.02 intake
Mopar purple shaft cam. Unknown specs.
New MP distributor
Edelbrock Performer 318/360 intake
Carter AVS 750 carb

Compression test and leak down test to follow...
26734460_1913512831993306_6587686173579150969_n.jpg
 
any vacuum leaks? no carb ports not capped allowing a vacuum leak? Bad brake booster valve? Leak at the intake manifold? Possibly late ignition, valve timing? Do you have adjustable rockers, they could be to tight? Is the vacuum steady or does it fluctuate?
 
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If it's a Mopar .509 hydraulic, they don't hold much Idle vacuum. Especially with stock carb and distributor calibrations. Needs about 20 degrees initial with roughly 35 total. Could have lean idle feed calibration as well.
Doug
 
You know, as Doug mentioned, I was only getting 2"-4" vacuum at idle with a 292/509 purple shaft cam in a big block...granted I had some carburetor issues, but REALLY low vacuum for a hydraulic cam.
 
rt-man said:
Engine bought last summer, just fired up yesterday. Only 5 inches of vacuum at idle. :(

What I know:
'72 block
steel crank
new pistons and rods, shiny walls (as seen through boroscope)
'72 360 heads, valves opened to 2.02 intake
Mopar purple shaft cam. Unknown specs.
New MP distributor
Edelbrock Performer 318/360 intake
Carter AVS 750 carb

Compression test and leak down test to follow...View attachment 574151
Click to expand...

Compression check done, they were all low but equal. Could be that the rings aren't broken in yet...? Also, all spark plugs were fouled and wet.
 
Gas fouled, oil fouled? What is the idle rpm? It can be pretty easy to gas foul the plugs on a new start up if the carb isn't set up right and low vacuum and rough idle (because of cam?) doesn't help much....
 
When I bought my Road Runner, it had a wild cam in it. I forget the spec's, but it was .509 lift as I recall with a lot of overlap and I think 108 LSA and had only 2-3" of vacuum. Needed something like 2000 RPM before you let the clutch out or it would stall. I replaced the cam with a much milder one because it just wasn't any fun to drive. Vacuum went up to 17 or 18". The cam could very well be your problem, but without the spec's, it'll be hard to tell for sure.
 
More information is available.

The carb I gave the installer was either warped or whatever. The right bank was running super lean (exhaust pipe close to red), while the left side plus were all wet and fouled. He put on his test carb and it ran much better. However, still low vacuum at idle (long duration cam?) with no gasket leaks, and compression low. He says 80s all around where he expected 140. Hoping the rings need to seal. Plan is to put in a brandy new carb (to not mess with the plumbing, go with an Eddy AVS Thunder 650), let it run for an hour outside and recheck.
 
Can you drive it? Letting it idle for an hour outside in a no load condition isn't something I care to do.....
 
Right drive it easy till it warms up then accelerate hard up to the illegal limit then coast down to slow- do it 10 times
acceleration forces the rings against the cylinder walls
doing it your way will glaze the walls
 
Thanks for the advice.
I'm as equally concerned that a valve is sticking, or a rod is too short, or something else is keeping the compression down SO low.

Any thought to misadjusted valves...?

Mystery motor, ugh.
 
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okay, more information. A leakdown test was done on all cylinders, 90 psi in, 20 psi leaking out. My mechanic says no more than 8psi leakdown is where it should be.
 
Sorry for the short answer....I'm in the middle of a major allergy attack and just doing good to be sitting at my desk. :(
 
rt-man said:
okay, more information. A leakdown test was done on all cylinders, 90 psi in, 20 psi leaking out. My mechanic says no more than 8psi leakdown is where it should be.
Click to expand...
How long did it take to leak down the 20 psi?
 
Okay, I'm back. After test driving it Sunday, I've decided to pull the motor and tear it down. 90psi in all cylinders, plus no power assisted brakes. They worked fine with the old low compression 318 in there. Now the pedal feels like a brick at any speed. I even had to roll through an intersection as the car wouldn't stop. And that's after putting in an auxiliary vacuum canister hooked up to the brake booster. The car had no snap whatsoever, nothing under my right foot. Carb is a new Summit 600cfm vacuum secondary. I don't have the timing but its very advanced.

Let's find out what's going on. I'll report back.
32958024_2108287065849214_2832625987928195072_n.jpg
 
An update: when communicating with the engine seller, this is his response:

"It simply has "too much" cam. It would have been perfect with a single plane, headers, etc. - probably 400-450HP.

The 90 Psi in ALL cylinders proves that the short block and heads are flawless. Just swap in a stock 340 cam, or something on that order. DON'T "TEAR IT DOWN". You can even do this with the intake manifold on with the factory lifter tool, which I would loan you.

When you "de-tune" an engine, all components must have the same 'prox RPM range. You have now an engine that has mostly stock components, but way too radical a bumpstick for the other mid-range-application components."
 
Maybe some of the engine guru guys will chime back in because I may be wrong here but... that seems like a really low cranking compression, and more beyond what I would think even a really radical cam would bleed the pressure down to.. I know the guy said don't tear it down, but why spend money and go through all the effort to change the cam only to find out there's still problems.. at this point I would not take anybody's word for it check things out for yourself. It just seems like something isn't right. You can gain a lot of insight by simply pulling the valve covers and intake manifold.
If it is as he says a "mostly stock motor with just a huge cam" it is a very good possibility that valves and pistons have hit each other and would explain your symptoms
 
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