Low low vacuum at idle

[josehf34]

Guys my '74 fury is a factory 360 SB car but since I own the car I've always had a low vacuum problem at idle

Right now the engine is freshly rebuilt, has new timing gears and chain, XE262H comp cams camshaft, stock heads and pistons and a Street Demon 625 carb but with the engine fully warmed I only get between 10 - 12" of vacuum and other people with the same camshaft ion 318s and 360s claim to have between 17 - 19" of vacuum

My initial timing is 19ª BTDC (is the point with better vacuum signal while idling and without ping at WOT), already tested for vacuum leaks without results, the engine idles at 700 RPM but If I rev it to 1200 RPM the vacuum increases to 17". I don't know if has something to do but when I bought the car there was a vacuum amplifier installed in the intake, I removed it but still have it hanging around

any ideas?

 

[Big_John]

My initial timing is 19ª BTDC

Way too much initial timing.

 

[josehf34]

Way too much initial timing.

I tried with 15º initial but the vacuum is even lower between 7" and 9" and I feel the throttle response is not the best

 

[Dobalovr]

Your removing the vac advance hose during inital timing and plugging it right?

 

[mr. fix it]

how lumpy is that camshaft?

 

[josehf34]

Your removing the vac advance hose during inital timing and plugging it right?

Yes!, every time I adjust the carburetor idle mixture screws or adjust the initial advance I unplug the vac advance.

When I unplug the vac advance I can't notice any difference with engine idling but If I suck the vac hose I really notice a change in idle quality (sometimes better, sometimes worst, dependes on the initial advance). Is that normal?

how lumpy is that camshaft?

According to comp cams website these are the specs of that camshaft:

Duration @ 0.006": 262° / 270°
Duration @ 0.050": 218° / 224°
Max Lift w/ 1.5RR: .462" / .469"
Lobe Separation: 110°

I don't think this is a camshaft problem, I installed the same camshaft in a friend SBC 350 and the vac at idle is really good, about 18" of vacuum and I can't notice the car is idling until I rev it, but my car shakes a lot, vacuum gauge fluctuates a lot and It did the same with the stock camshaft. When I did this change I took the opportunity to change all lifters and inspect for bend push rods or even broken valve springs but everything seems to be fine

 

[rexus31]

How's your PCV Valve?

 

[mr. fix it]

checked for vacuum leaks?

 

[superfragl]

Way too much initial timing.

I don`t think so...
I am running 22 initial. I had to do this to get the vacuum up to 15". With initial at 16 I had only 11" of vacuum

 

[Dobalovr]

15-17" of vacuum is normal for a BB Mopar. Was your cam gear advanced on install? You say the engine shakes and vacuum gauge bounces? Burnt valve? There are smarter minds here than me who can help diagnose this issue I am sure they will chime in shortly...

 

[josehf34]

How's your PCV Valve?

is working fine, If I pull it out of the valve cover I can see it working when I rev the engine

checked for vacuum leaks?

yes and I didn't find any leak but I think I'll try again using some butane instead carb cleaner

15-17" of vacuum is normal for a BB Mopar. Was your cam gear advanced on install? You say the engine shakes and vacuum gauge bounces? Burnt valve? There are smarter minds here than me who can help diagnose this issue I am sure they will chime in shortly...

Valves are fine, when I installed the camshaft pulled off the heads and did a valve settlement job just to be sure each valve were ok. About the cam gear advance I replaced the entire timing kit and installed the new in the zero mark so I think isn't advanced

Yes, just when the engine shakes I can see the vacuum gauge bounces but If I left the initial advance something between 14º to 16º BTDC the engine doesn't shake too much and vacuum gauge is pretty stable but still bounce a bit, the real problem is that vacuum is really low, something between 11" and 13"

 

[Knebel]

I also have a 360, but with an unknown cam. My idle vacuum in park is about 14" and in drive its 10" to 11". I have messed with the timing a lot and tried 10, 12, 15° which will then only give me 5"HG vacuum. So then everything over 18° is fine for me, I have the initial set at 20° but it idles and vacuumes best at 26° when I plug in the vacuum advanced to manifold vaccuum, it idles way better and has more "pep" to it but that creates other issues between P and D my EFI cant seem to tune out. Thats why I have it connected to ported vacuum and just deal with it. The problem was that when put into park, the rpm would go up to 1600 and then slowly come down to my 800rpm idle as the efi adjusts. Dosent happen as bad when ported vacuum.

 

[Dobalovr]

MY 78 360 running a Purple Shaft cam with 4 degrees advanced on the cam gear everything else internal is stock, pulls 17" at idle with the Eddy. 12 degrees advanced so I am absolutely no help here I am afraid.... If the car runs well and you are happy with it but are just running low vacuum there are worse things to have to live with ...like a friend with a 350 SBC pulling 17".

 

[josehf34]

MY 78 360 running a Purple Shaft cam with 4 degrees advanced on the cam gear everything else internal is stock, pulls 17" at idle with the Eddy. 12 degrees advanced so I am absolutely no help here I am afraid.... If the car runs well and you are happy with it but are just running low vacuum there are worse things to have to live with ...like a friend with a 350 SBC pulling 17".

I feel like the car is not running completely well, I'm having poor gas milleage in town, around 9 - 10 mpg and the cam is not too much aggresive to have an erratic idle or have the engine constantly shaking, also I'm not sure if the car is performing like it should

Why is recommendable have the camshaft advanced?

 

[65sporty]

Are you sure it's running on all 8 cyl's? If you have spark plug pliers you can pull one wire at a time to see if you have a misfire. I have a plastic set of pliers just for this. If you pull a wire and it doesn't change that cylinder isn't firing and then check spark at the plug and compression.

 

[Dobalovr]

I feel like the car is not running completely well, I'm having poor gas milleage in town, around 9 - 10 mpg and the cam is not too much aggresive to have an erratic idle or have the engine constantly shaking, also I'm not sure if the car is performing like it should

Why is recommendable have the camshaft advanced?

It seems to me that the bigger problem here is the poor performance "shaking" engine troubles than the vacuum problem as you said the car ran like that before the cam replacement. How confident are you in your ignition components. Does the engine shake under load or whenever you rev it. Does it feel like it has power but just not producing it? What colour are the plugs? Have you tried pulling a plug wire off the cap one at a time and notice if one doesn't change the engine idle performance? Most carb problems are electrical I am told...there seems to be more going on and further diagnostic is necessary. Time for the experts to chime in here

 

[1970cat]

bent pushrods?

 

[josehf34]

bent pushrods?

Nop, all are fine

Are you sure it's running on all 8 cyl's? If you have spark plug pliers you can pull one wire at a time to see if you have a misfire. I have a plastic set of pliers just for this. If you pull a wire and it doesn't change that cylinder isn't firing and then check spark at the plug and compression.

I'm not sure if is running in 8 cyl, I've tried to unplug a cylinder and see if idle changes but I can't notice a big difference, even the vacuum signal doesn't drop too much doesn't matter which cyl I unplug

It seems to me that the bigger problem here is the poor performance "shaking" engine troubles than the vacuum problem as you said the car ran like that before the cam replacement. How confident are you in your ignition components. Does the engine shake under load or whenever you rev it. Does it feel like it has power but just not producing it? What colour are the plugs? Have you tried pulling a plug wire off the cap one at a time and notice if one doesn't change the engine idle performance? Most carb problems are electrical I am told...there seems to be more going on and further diagnostic is necessary. Time for the experts to chime in here

I know the ignition system is not the problem because I've tried with two mopar ECU, one GM HEI (my current ignition system) and one procomp multi spark and the best result was with the procomp multi spark but wasn't a huge difference

I feel like the car performs good but when I drive a Dart with a stock 318 or even my friend chevy 350 I really feel like they have a lot more power and a better response than my car, for example going up hill I feel like I've to give it so much throttle to have it going

 

[rexus31]

I had similar symptoms. My issue turned out to be a bad connection at the Ballast Resistor. Using a multimeter, check voltages at all the wires pertaining to the ignition system. You could have a poor connection somewhere. Is the motor grounded?

 

[65sporty]

Have someone sit with it idling in gear and try pulling the wires again one at a time and see if that makes a difference.