Technical Could cam timing cause low vacumn at idle

I have this mid 80s 351-W I had built that never had the highest va***n at idle,I have to rev it up a few hundred RPMs to get the highest va***n readings. It was supposed to get a RV cam but who knows what was put in it and all I know it has a 302 firing order,when I first put it in the car to get the engine to idle the smoothest the timing really had to be advanced but it pinged and would not start so it could not be kept there. I had 3 automatic trans in this car and they all have third gear issues at very light throttle where this engine seems to have the lowest va***n so I am thinking about taking the front of the engine off and see if the cam is timed correctly and not a tooth or two off.

 

If was indeed a tooth off , if it " ran" at all it would be badly !

 

Not necessarily. I know an engine that had poor vacuum and ran like a dog. It was off three teeth. Hook a vacuum gauge on it and look at a vacuum chart, available on the internet. It will tell you if the timing is off. http://www.earlycuda.org/tech/vacuum2.htm Here is one to use. Vacuum gauge is a very good tool for diagnosis.

 

It does not run bad but never had a smooth idle and slightly lower then normal va***n at idle,there is just something not right and has been that way since day one. It has about 15 to 16 inches of va***n at idle and if I slightly rev it up it goes to over 20.

 

Al***ude can affect vacuum. 15/16 is a tad low. What’s the idle rpms?

 

15 is not out of the range of something with a mild cam. Without knowing the cam specs and the carb and ignition setup, most of this is a guessing game.

 

what carb used? - carb or distributor may need some changes - guess already have double checked TDC and where distrib is pointing - sometimes rubber vacuum hoses/rubber caps look good but, have aged cracks in them

 

Holley carb and not much rubber va***n lines on this car,this engine has been this way since it was installed.

 

If you changed the cam from stock, you will need to make the distributor not stock to work with it.

 

I am thinking its definately not a RV grind like I ordered but it wants to make more power higher then I want it,thinking about getting a stock early 70s 351 cam and see how it does. At idle at around 800 RPMs the va***n reads between 15 and 16 but when I speed it up it jumps to around 21,I think I am going to hook up a va***n gauge and see what it does when I drive.

 

I have been fighting light throttle 3 gear shifting issues and that is when the va***n should be the highest but not with this engine,I am guessing it makes the highest va***n around 1000 to 1200 RPMs.

 

I struggle to get 13" @ 900 rpm, 18 "@ 2500 going down the road .You say the vacuum is low at cruise , how low ?

 

That 15-16" of vacuum at idle isn't that radical a cam at all. But if you put a vacuum gauge on it, try tweaking the distributor a little and see if the vacuum gets any better? A low vacuum isn't all about duration, and might be more about lobe separation. A narrow LSA on a cam will really lower the vacuum.

 

Went out and put the tach and va***n gauge on it,it idles at 600 in gear and 13 inches of va***n. Out of gear it goes up to 16 and to get it up to 20 I have to rev the engine up to around 1600,on other engines I had around 20 at idle and this was supposed to be a stock engine with a RV cam but something is not right. Tomorrow I hook up the va***n gauge and take it for a spin and see what I get,I am curious on how much va***n it has when it wants to slide into third.

 

I was told a modulator valve likes to see around 15 to work the best.

 

An "RV Cam" typically has 272 Degrees of valve overlap which will not be that bad of a vacuum sense to affect a transmission modulator but develop low end torque. Lift is probably around or below .400 total lift which will not affect idle anyway. Typically vacuum diminishes with a 290 Degree cam so there is a possibility they installed a multi slot timing set and degreed the cam for another application. Without knowing the parameters of the cam and how it is installed you are shooting blind.

 

Did not get a cam card and the builder could not tell me what was installed,it seems like the engine really starts to wake up over 2000 so I know its not a RV grind. Idle has never been smooth.

 

Some timing chain sets have 3 different keyways on crank gear to advance or retard cam, When installing a new chain set while engine was still in the car made a mistake and lined up with the wrong dot on crank gear, it didn't run well so pulled it apart and degreed the cam, decreeing all my cams ever since.
One of those thinks I had to learn the hard way

 

Probably time to take it apart.

 

Degreeing and following cam manufacturers installation instructions defenetly can be beneficial, curious to hear the vacuum readings during your test drive.

 

Three different trans and all had third gear issues which I think is va***n related,this engine has not been right since day one and since there are not external va***n leaks it has to be cam related.

 

351 with 302 firing order? aren't they different?

 

There is more than one 302 firing order.

 

The 302 firing order is 1-5-4-2-6-3-7-8 and the 351 order is 1-3-7-2-6-5-4-8. Both will have counter-clockwise distributor rotation. All of the motors except 351 and the 302 H.O. will have the first firing order, the 302 H.O. and the 351 will have the other.

 

It has a cam for a 302,some says it makes a difference and some say it does not but there are 49 cubic inches difference.

 

You can make a positive I.D. of the cam & gears

 

I am thinking the cam timing could be retarded since I really had to advance the timing to get it to idle smooth,this 351 s in my 55 sunliner and do have most of the parts to put a Y block back in and might just start but want to get the trans working better since I was planning on doing it next winter.

 

One other test you can perform before dis***embly. Run a cranking compression test. Just in case you are unaware, have the carb wide open and crank 6 revolutions per cylinder. As I remember, you lose 5lbs. per thousand feet of elevation. Low compression goes in hand with low vacuum. However, if the cylinders had excessive taper, it could also reflect in low vacuum due to poor cylinder to ring seal.

 

I have done a compression test in the past and I think all were normal but will do it again,this engine would always blow some oil out the oil cap no matter what I done.

 

Could just be that someone installed the cam retarded which will change everything the cam was designed by the grinder. A three position timing gear will allow one of three possible settings' retarded, straight up, or advanced. Any one of which makes the cam more applicable to the builders intentions. Installing retarded with different ratio rocker arms will turn it into a great turbo application. IE 1:73 ratio on the exhaust with 1:60 ratio on the intakes will help eliminate turbo lag while increasing low end torque. Reverse the cam orientation to advanced will produce the opposite effect. Before blaming the transmissions you need to find out what you have, how it was built by the original owner or builder. Sounds like a mis-applied application from what the engine was intended.