15 inches of vac at idle

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71Swingerman

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Ran compression test today here is the results 1. 130 PSI. 2. 140, 3. 145 , 4. 140, 5 120, 6 140, 7. 115, 8. 125. Does anyone have any ideas how to cure it. myself just looking I might think rings, except it don`t smoke any. Headgasket maybe. the engine is a 78 360 with indy Xheads with 62 cc chamber however have been cut .20 to true them up. Thanks in advance. I can use any and all help. Forgot to add the valves are 2,02 and 1.60 and the cam is mopar 268/272 duration
 
How did you perform the test?

And are you trying to cure a low vacuum reading?

What cam are you running?
 
What kind of symptoms does it exhibit?

Almost seems like a gross vacuum leak.

Do a leak down test, it will be more accurate.
 
15" vacuum does not seem abnormal for a 268/272 duration cam...?? Any symptoms of poor running? Any excess oil consumption? How many miles on the engine, and are they street miles or ??

Repeat the compression readings with carb propped open and all plugs out; make sure the engine cranks over the same 4 or 5 times per cylinder reading. Make sure the threads on the tester seal well each time. If still low, add a teaspoon of motor oil to the low cylinder(s) and repeat test and record the readings. If the readings increase then ring, piston, or bore issues are likely. If no increase, then it could still be rings (i.e. bad top, 2nd AND oil rings, letting the cylinder oil kinda/sorta seal up the top rings all the time), but it is becoming more likely valves or guides: worn, sticking, etc. Put compressed air in each cylinder at a moderate pressure and listen for where the leaks are.
 
Seems normal to me.
 
nm9stheham said:
15" vacuum does not seem abnormal for a 268/272 duration cam...??
Click to expand...

Where do you see he has 15" of vacuum? Are you looking at the difference between cylinders on the compression test? The test numbers show 5 cylinders to be pretty low which sort of indicates a 'gross' problem.

A good way to check to see if the rings are not sealing is to do the compression test 'dry' like mentioned above and get your number. Then, squirt some oil down the hole and do the test again. If the numbers on the gauge increase, then its the rings. If not, then you have other problems.

If you had a blown head gasket, it likely would manifest itself by overheating and white smoke out the tailpipe. You can do a rudimentary check for that when it's cold. Take the rad cap off and start the car. Let it warm up a little and look down the radiator filler neck for bubbles. If there are bubbles, you likely have a head gasket leak but it's
not a definitive test. You need to confirm with a coolant pressure test at least or with a chemical head gasket test.

If possible, a leak down test would be best here. You would be able to pin point the source of the lost cylinder pressure much more easily.

In lieu of that, pull some plugs and take some pics and post them here. Shine a light down the spark plug hole and look for damage. Check your oil for anything odd. Basic stuff first.

You didn't mention any timing stuff either. Do you have that handy?
 
rmchrgr said:
Where do you see he has 15" of vacuum?
Click to expand...



usa.gif
15 inches of vac at idle
Click to expand...

From the OP, and the thread title.
 
Test was done dry with the throttle open and cranked 46 times. the vacume was found using a vac guage on the non timed vac port on the front of the 650 elde carb. Timming is set at 10 before
tdc
 
Bad Sport said:
From the OP, and the thread title.
Click to expand...

Woops, sorry! I feel pretty stupid. Just read the post and missed the title altogether. I was all pumped up to show off my internet diagnostic prowess.

I agree, 15" of vacuum at idle is not bad.

So back to my initial question then - are there other symptoms? One thing I can think of here is that the cam is not degreed correctly.
 
71Swingerman said:
Test was done dry with the throttle open and cranked 46 times. the vacume was found using a vac guage on the non timed vac port on the front of the 650 elde carb. Timming is set at 10 before
tdc
Click to expand...

Assuming 4 to 6 times, lol.

Did you do a wet test?

Recorded the highest reading each time.

Battery charger on the battery?

With your setup you could stand some more initial I would think, around 16* maybe.
 
Try running the compression test wet and dry back to back.

Test it dry and record the readings. (2 or 3 times is good).

Then get an oil can and squirt 2-3 shots of oil in the cylinder and rerun the test. The oil will help seal any leaks in the rings.

If the readings jump up with the wet test vs the dry test, then there is a piston/ring/bore sealing issue...

If the results stay pretty much the same, then if it is a low cylinder, the problem is in the valves, get a valve job....
 
There is 4 clyinders with less than 140 PSI which is what the Motors Manual says is correct for a 69 340. And yea the power brakes really suck advancing the timming past 10 degrees does increase vac to 16 inches. but with a 4 row classic ind. radiator with 15 psi cap and a jag fan clutch it does get hot on 90 degree days and yes I am running a correct fan shroud. was going back to direct drive fan but thought a good idea to check vac and do a comp test for anything that stood out.
 
71Swingerman said:
Test was done dry with the throttle open and cranked 46 times. the vacume was found using a vac guage on the non timed vac port on the front of the 650 elde carb. Timming is set at 10 before
tdc
Click to expand...

15" of vacuum at what RPM? If it's at 750-800rpm that's exceptional for this camshaft and timing events. If it's at 1200+ not so much!!!

Not NEAR enough initial timing for that camshaft.

Cam may be installed retarded as well complicating matters via the line up the dots method.

DO NOT DRIVE THE CAR AFTER DOING THIS

Start car and get it warmed up, turn distributor so you have 18-20 initial timing and turn the idle speed down to 800-850. See if it drops any of the timing you just put in. If it does, add it back to the 18-20 range. Car should idle easily at 800 rpm with that camshaft.

Find the most initial timing the starter will handle while cranking OK, no serious dragging or kickback. Subtract that number from the total number you hit and limit the mechanical advance in your distributor.
 
71Swingerman said:
There is 4 clyinders with less than 140 PSI which is what the Motors Manual says is correct for a 69 340. And yea the power brakes really suck advancing the timming past 10 degrees does increase vac to 16 inches. but with a 4 row classic ind. radiator with 15 psi cap and a jag fan clutch it does get hot on 90 degree days and yes I am running a correct fan shroud. was going back to direct drive fan but thought a good idea to check vac and do a comp test for anything that stood out.
Click to expand...

If you advance the initial, you need to limit the mechanical in the distributor. It will run better, performance wise anyway.

Mine is set @ 16* initial, with the dizzy limited to 16* for a total of 32*. No issues at all.
 
I put together a 72 318 short block with TRW forged 10.5 pistons, then put the stock upper end of my 68 318 on top (heads, intake, carb) with a stock 340 cam (original cam from my brother's 78 Lil Red Express which is the same as a stock 340 auto cam) and Rhoades lifters.

I was pulling 24" hg at idle with that set-up, but had too much compression and would overheat when it got above 80° outside...

I found a cheap set of 77 360 heads with a fresh valve job and installed them with a 69 stock 340 cast iron intake and the vacuum only dropped to 22" at idle.

I have been flamed for running Rhoades lifters and some have called them "snake oyl"...

But they work!

If you want more vacuum, drop a set of Rhaodes 2018 lifters in there. You will get more vacuum at idle, more MPG, stronger low and mid range, all without sacrificing top end...

They are great for a street car. I love them. Call me what you will, but my engine worked well and I have been using them for over 25 years now.

Read the second article here to learn more about how they work:

http://www.rhoadslifters.com/Pages/Articles.html


I recommend using them, it's your choice.
 
crackedback said:
15" of vacuum at what RPM? If it's at 750-800rpm that's exceptional for this camshaft and timing events. If it's at 1200+ not so much!!!

Not NEAR enough initial timing for that camshaft.

Cam may be installed retarded as well complicating matters via the line up the dots method.

DO NOT DRIVE THE CAR AFTER DOING THIS

Start car and get it warmed up, turn distributor so you have 18-20 initial timing and turn the idle speed down. See if it drops any of the timing you just put in. If it does, add it back to the 18-20 range. Car should idle easily at 800 rpm with that camshaft.

Find the most initial timing the starter will handle while cranking OK, no serious dragging or kickback. Subtract that number from the total number you hit and limit the mechanical advance in your distributor.
Click to expand...

Pretty much............
 
I have a 318 with the Comp Cams XE268H.
Had to tweak and adjust all over the place to get up to 15".
I am happy with that and my power brakes work well.
 

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Do you have a lot of miles on the engine?
Actually pretty good vacuum for cam.
There are some cylinders a little low,if its an old build/lot of miles a de-carb would help.
Do it when next oil change is due though.
As you will want to change the oil after the de-carb is done.
I prefer to warm engine up and pull all the plugs and buy a gallon of pb rust blaster,about 4-5 ounces per hole and let it sit overnight.
Bump it over the next day before reinstalling the plugs just incase it didn't all leak by the rings.
Put plugs in and make some serious smoke,about 1700 rpm until it quotes smoking,change the oil and filter.
Good to go,bet you will feel the difference.
 
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