Need input from big cam users.

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spinman_1949

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Still working on getting the 340 to run decent.

I have it running much better than when I bought the car, but still the idle is just about impossible to get stable. So here is my basic question.

When I start the engine and let it warm up, I then hook my vacuum guage to the manifold port. The engine is in neutral. Right now initial is 22 degrees. Total is a bit high at around 38. Full in at 3000 RPM. No mechanical at idle though. Put in a second spring to cure that. But I start getting some mechanical at around 1400. Vacuum is not to port and I have an adjustable can so not involved at idle.

What I see at any point in the engine RPM is a huge swing in vacuum. The needle is bouncing from 5 to 10 inches at idle. It continues to bounce even as I increase the RPM's. Not as bad but enough to make it impossible to read. Unless I guess somewhere in the middle. Now we know the cam is a solid cam and based on some very crude measuring I think the lift is in the 480 range.

So anyone that runs a solid high lift cam. Is this bounce in vaccum normal, or I am guessing I have some intake valve bleeding going on. That is the only thing I can figure that would cause this. A vacuum leak would just lower the reading but not amke it fluctuate so much right?

Thx for offering your experience.

Oh and the engine is kinda stumbly until it reaches around 3000 RPM.
 
i guess you dont know the duration on the cam...and what carb are you using?
 
70aarcuda said:
i guess you dont know the duration on the cam...and what carb are you using?
Click to expand...


No I do not know the duration. The carb is a holley 670 avenger. The model with the brass float plugs. The stock intake and stock exhaust manifolds. Oh and as luck would have it the stock torgue converter. Which makes the idle in gear all that much harder.
 
Really that's a pretty small cam but you may still need a looser converter to get it to idle in gear. You probably need to ditch any vacuum advance, limit the mechanical advance so you can keep about 22-24 degrees at idle with a max of about 34. Next I would suggest reducing the size of the idle air bleeds which are probably about .120 or .080 in that carb to about .040. You may find it necessary to crack the secondary blades a little so the primaries aren't open too far and passing the transfer slot. If that doesn't work reduce the idle air bleed size in the secondaries, after that drill a .125 hole in the primary blades. One step at a time though or you may go too far.
 
Yup

Still playin. Just can not get it to settle down. I think the cam is not that radical TBQH. I am beginning to wonder if I do not have some burned valves to deal with. Lets not forget how this all started with a seriously overtimed engine.

I may have to get some prof help. At the very least do a leakdown test to eliminate valve issues. Plus I need to check the timing chain. Could be bad causing the odd fluctuation in timing I see at times.

Ahh the joys of buying a car. The fact that I can not identify the cam is driving me nuts.
 
Back in 1971 I installed a Stewart Warner vacuum gauge in a new 1971 Roadrunner with a stock 383 and it always bounced around at idle. It could be in my case and in your case the vacuum gauge it just too sensitive or the port that it reads from is not a stable source. I can't remember what source my Roadrunner used but my best guess is that it was just simple manifold vacuum. I have read in the past that some vacuum gauges require damping in order to stabilize them.
 
What I am looking for is just what people have seen re vacuum readings with cams. When I say the needle bounces I mean it really bounces !! Is this what everyone sees when putting a vacuum guage on these motors? While I understand that a big cam can cause this kind of fluctuation, I would think that above idle the vacuum should settle down, which it does so to a degree but it still bounces 3 inches or so.
 
My 280-474 purple cam has 6 inches of vacume@ idle, and the needle does bounce a lot and my 360 idles rough but does not stall ever and runs like a rape ape.
 
I had a fluctuation too, turned out to be a rocker locknut had loosened. If it's doing that constantly, you better pull the valve covers and make sure you're not hanging a valve open(or closed).
 
my Crane 226 - 230 @ .050 cam bounces the needle at idle 10 - 13 inches but steady's out as the rpm increases.

A leak down test will check for leaky valves or just pull the heads and get them re done
 
The vacuum gauge need to be connected to the carb baseplate or the manifold plenum. Connecting to the manifold port over a runner, on a dual plane especially, will read the pulsations from that runner and that's gonna be alot of the fluctuation in the gauge.


Meaning not to use where arrow is pointing....
 

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Rick,

Yes I have vacuum connected to carb plate manifold vacuum. Two ports provided. One is larger for PCV valve.
 
Terror,

I think I will pull the covers this weekend and go reset the lash on all the valves and check for a loose locknut. I think these rockers are the kind that hold their position without a locknut though.
 

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spinman1949 said:
Rick,

Yes I have vacuum connected to carb plate manifold vacuum. Two ports provided. One is larger for PCV valve.
Click to expand...

Gotcha.
Yea those adjusters are a interference fit and over the years can tend to change adjustment more easily. Couldn't hurt to check them out. Looks nice and clean though:thumblef:
 
Rick and Snake,

Thx for taking the time. Nothing like dealing with lost mechanics. LOL I did a little fiddling with some air hose fittings and my compression tester so I can do a listen for leakdown test this weekend. Oh and there is one other equation that I did not think of. The power valve. My guess is the one in the holly is stock. 6.5 which works fine with a motor running 13 inches of vacuum at idle, but this sucker is pulling less than 6 inches which makes me wonder if I am not bleeding some extra fuel from the power valve. Holly tech says take idle vacuum and divide in half and that is your power valve selection. So my guess is I should install a 3.5.
 
spinman1949 said:
Rick and Snake,

Thx for taking the time. Nothing like dealing with lost mechanics. LOL I did a little fiddling with some air hose fittings and my compression tester so I can do a listen for leakdown test this weekend. Oh and there is one other equation that I did not think of. The power valve. My guess is the one in the holly is stock. 6.5 which works fine with a motor running 13 inches of vacuum at idle, but this sucker is pulling less than 6 inches which makes me wonder if I am not bleeding some extra fuel from the power valve. Holly tech says take idle vacuum and divide in half and that is your power valve selection. So my guess is I should install a 3.5.
Click to expand...

Me and some others have gone back and forth on the PV issue. This is a issue i do have have personal experience with and Holley's recommendation is fine on moderate to high vacuum engines. When you get into the 12"ish range or less at idle you need to be just a few numbers under the idle vacuum. Problem is as a example a 8" idle number can become a 13-15" number at cruise rpm and if you used a 4.5" PV you need so much throttle movement to get there that you develop a flat spot under moderate acceleration. Idle to wide open is ok under there senario, but street cruising requires a different setup.

With that said, your vacuum readings are too low in my opinion and until you get the possible valvetrain and maybe timing issues squared away i would wait on messing with the power valve. 6" of vacuum is way to low for anything other than a full race cam or a retarded moderate street cam. You need to work on that first in my opinion.
 
If one of the exhaust valves were slightly stuck open or their rocker over tightened would that cause the sporadic Vacuum?
 
How about trying about 14 degrees initial, if it's (the cam) not that radical, you might find the motor likes that better. Total of 34 degrees would be great too.
 
Engine will barely run at 14 initial. It will run but it falls on its face off idle.
 
What settings are you gonna adjust the valves to. I would go to a loose generic setting like .022 int. and .028 ext. and see how it respondes. If you still have low vacuum at that point i think you need to look at the chain and possibly degreeing the cam in.

P.S. i've run [email protected] cams that i retarded a few degrees to take some bottom end out of them for street tires and they still pulled 10-11" of vacuum. And most .480" cams don't have more then that kind of duration.
 
Rick,

I figure something is not right. I do not think the cam is THAT radical. One thing I also have noticed. The exhaust manifold bolts are very rusty in appearance. So exactly how many miles are on this engine really. The valvetrain looks very clean though. I wonder if a valve job was done on this engine. Did hardened seats get installed.
 
I looked at one of your other threads and the rust i see looks more like a engine that was stored more then one with alot of miles. I don't believe hardened valve seats are a problem at this point. A compression test may help to reveal a problem and also help diagnosing the low vacuum readings. I would run the valves first and see how the engine vacuum picks up, if at all.
 
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