Thread: Low engine vacuum but no vacuum leaks.

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Need to know year of the engine, type of ignition and if computer controlled including and crank or cam sensors, throttle position sensor, etc, what manifold, carb, fuel pressure while cranking and running, type of gas being used, age of gas, type/age of all fuel lines-----------

In addition to what every one else has offered.
Don't hesitate to open the exhaust at the manifolds and re-do a vacuum test. It's not seen much anymore but exhaust pipe sometimes had a interior pipe that would swell and kink closing off the pipe. This would effectively choke the motor airflow/vacuum. Like I said, you don't see this anymore but it should be easy (relatively speaking) to open the headers/manifolds and double check the vacuum.

I bought a Dodge D-100 for 200 bucks 'cause after throwing some 1200 bucks at a local garage this thing wouldn't run! It had new ignition, carb, pcv etc. etc. and it would barely idle. After getting it to my shop - I dropped the pipe at the manifold and it ran perfect. I determined the cat converter had plugged 'cause they were running leaded fuel in a unleaded vehicle. swapped in a cat from a wreck and sold the truck for 1500 2 days after I bought it. There's other examples of course, it does happen and is easy to check. Ya never know.

Originally Posted by DennyW

I myself didn't say more, because no one has any clue about his setup. He did not explain anything in detail...

47Fordcoupe recently purchased this car which has a 305 SBC in it. ??? hahaha. Lotys of if's to this...

What else should I explain? This is a pretty stock engine aside from a Edelbrock carb and intake, and I also explained this: "I already ruled out a harmonic balancer ring slip. Found tdc with a piston stop and made a new mark on the balancer. Installed a 1" wood carb spacer, , heat shielded the starter and solenoid, fuel pump and fuel line, installed new spark plugs/wires, new distributor rotor,coil and cap. Idle timing is set at 30* and it won't decrease without stalling."

Thank you all for your help

Bone stock to moderately cammed carbureted motor;
5 - 7 inches of vacuum is too low for this motor, something is wrong, PERIOD. 30° advance at idle is too much for this motor, something is wrong, PERIOD.

If the motor is running well, check the following;
Check and be sure you have your timing light hooked up correctly to number 1 cylinder and advance feature set correctly.
Check and be sure your timing light is compatible with your ignition system (not all are compatible with all ignitions).
Check and be sure your vacuum gauge is connected to manifold vacuum and not ported source, ported can read no vacuum at a low idle speed to very low vacuum at high idle speeds.
Check TDC again, be sure marks are accurate. DO NOT GUESS, be POSITIVE and make your timing marks accurate.
Check timing with idle under 800rpm, too high of idle speed will bring in mechanical advance and make timing read high.
Check timing with vacuum advance disconnected, vacuum advance will make timing read high.

If the motor is running poorly check the following;
Check firing order to plug order, incorrect order will cause low vacuum and poor performance.
Check compression, low compression will cause low vacuum and poor performance.
Check carb adjustments, poorly adjusted carburetor will cause low vacuum and poor performance.
Check valve lash adjustment, out of adjustment valve lash will cause low vacuum and poor performance.
Check exhaust pressure, as mentioned exhaust restriction will cause low vacuum and poor performance.
Watch timing marks at idle to see if timing is fluctuating, up/down, timing mark fluctuation means there is too much movement at cam or distributor. Cam or distributor fluctuation will cause poor performance.
Set timing by TOTAL timing method, incorrect timing will cause all sorts poor performance.

Go to work on it and tell us what you find...

something is wrong, PERIOD.

^^^Yeah, what he said.
Here's a tutorial I wrote several years ago that may help.....
https://www.crankshaftcoalition.com/...op_dead_center

.

The motor is running poorly. It doesn't feel like it's misfiring but it feels very sluggish when accelerating from a stop, no torque.

Originally Posted by 36 sedan

If the motor is running poorly check the following;
The Check firing order to plug order, incorrect order will cause low vacuum and poor performance.
Check compression, low compression will cause low vacuum and poor performance.
Check carb adjustments, poorly adjusted carburetor will cause low vacuum and poor performance.
Check valve lash adjustment, out of adjustment valve lash will cause low vacuum and poor performance.
Check exhaust pressure, as mentioned exhaust restriction will cause low vacuum and poor performance.
Watch timing marks at idle to see if timing is fluctuating, up/down, timing mark fluctuation means there is too much movement at cam or distributor. Cam or distributor fluctuation will cause poor performance.
Set timing by TOTAL timing method, incorrect timing will cause all sorts poor performance.

Go to work on it and tell us what you find...

I suspect any of these suggestions might be the issue... except the exhaust as the car has an open dual exhaust (no cats or muffler, besides 2 glass pack)

Did someone pack the "glass packs"? Been done many times with steel wool to quiet them. If so, it can choke off airflow.
When searching for the problem(s), you must verify and not assume. A simple potato was a favorite trick when I was a kid!

Originally Posted by 34_40

Did someone pack the "glass packs"? Been done many times with steel wool to quiet them. If so, it can choke off airflow.
When searching for the problem(s), you must verify and not assume. A simple potato was a favorite trick when I was a kid!

My vacuum gauge is not indicating an exhaust restriction.

Welllllllllllllll--------------you seem to not fully answer questions we have asked and protest comments made--------If you really, really want to chase down the problem, we must start with the basic functions------------knowing that 3 things are required----air, fuel, ignition-----evidently you do have all three-however , it seems as tho many variables that are required to be in proper order aren't--------you need to start by varifying that the camshaft is properly timed to crankshaft-------do this by accurately degreeing the crank/tdc with the positive stop method which you did not answer about hoe EXACTLY you did it--------then you need to find where the lobe split is by doing the check for lobe c/l at max lift of both the intake/exhaust lobes------if this isn't close to being exactly at tdc on the valve overlap cycle it needs to be corrected----if it is late it will cause low vacume and dieseling----------
You need to prove that your ignition is working correctly and in the right order, timing advance is defective because of faulty weights, springs, rotor shaft advance /retard, worn cam gears, crossfeed from faulty wire install--------take the cap off and by hand twist the rotor to see if it rotates on the main shaft and then falls back . You may need to remove the gear and dismantle the shaft to clean , lube and free up the entire mechanical parts.

What is the fuel your using and how fresh is it??? How about the hoses ??? Which manifold are you using--the answer that its an Edelbrock is correct?? which model and type? standard, vortec, etc?? how about the intake gaskets?? all those sbc stuff isn't the same and wrong choice of parts means trouble. One inch wood block? get real--------

I'll be gone a few days to a couple shows but will try to follow up-----

Originally Posted by 47Fordcoupe

My vacuum gauge is not indicating an exhaust restriction.

Aaaaaaaah?????? low vacuum!!, I don't want to come off as the bad guy again, so I'll butt the h-ll out........

Originally Posted by 36 sedan

Aaaaaaaah?????? low vacuum!!, I don't want to come off as the bad guy again, so I'll butt the h-ll out........

No Sinsei, young grasshopper needs all the help he can get. And you have the way of addressing a member with respect. Some of us lose patience and can become gruff.

Perhaps now is the time for the air pump example? So a Low vacuum signal can mean air cannot get out so it won't get air in. Equals low power!

I appreciate everyone's input on this issue. Just a disclaimer, I by no means whatsoever consider myself to be an expert mechanic, as I'm sure many of you are.. i have very basic engine knowledge which is why I came here for help. However, I've seen replies on here suggesting I do things I have already done. Just in case some of you have missed what I've already done, I'm going to post it here once more. Things that I've done trying to solve the problem prior to my post:

1-Checked idle timing multiple times. (It's showing 30* and I know I should be around 4-12* at idle)
2- established tdc using a piston stop 3 times to confirm, yes marked the center of the 2 points)
3- verified that the harmonic balancer did not slip from the inner ring by marking between the 2 rings.
4- checked for vacuum leaks using propane gas, as well as with a smoke machine. No leaks found.
5- replaced spark plugs, wires, HEI distributor rotor and cap. (Installed the new spark plugs wires in the same order they were in, which COULD be problem)
6- adjusted the Edelbrock 500 cfm carburetor by turning the idle mixture screws all the way in (clockwise) and then 1 1/2 turn counterclockwise (I know that's a starting base)
7- removed valve covers and verified that no valves were sticking with the engine idling.

Thanks again to all who have tried to help and please keep the suggestions coming ��

On page 1, post 13, I outlined two procedures to help you determine the problem(s). Apparently you ignored them, because I have not seen any results. All of us are grabbing at straws trying to help you, but if you're not going to follow our suggestions, then you're probably not going to find the problem and that's OK because I'll still sleep well tonight regardless. I didn't start this post to be snotty, but damnit, I'm not getting any input and I'm beginning to think I'm wasting my time trying to help. Now get your sheets together or I'm outa here and I suspect the rest of the gang will be too.

Turn out the lights, the partys over............................