low vacuum + runs rich

67rcks · Jul 3, 2011

My 302 makes only 14 inches of vacuum (when warm, 10 when cold) while wideband shows 12.5-13 AFR. I am sure there isn't any vacuum leak, and the engine does not react to propane or starting spray. It has mild 218/228 .471" lift cam, holley DP, its power valve is not leaking. The engine runs somewhat rough, especially between 2000-2500 rpm. Is it normal for these engines to produce low vacuum with an aftermarket cam?

horseballz · Jul 4, 2011

How big is your Holley DP carb? I believe anything bigger than 650 or maybe even 600 could cause your symptoms. You might also think about your PCV valve. I switched my 68 to a late 80's PCV that has a large and a small nipple. I used the small one and capped the large one and it was good for another inch or so of vacuum.
My $.02,
Gene

PoppyMod · Jul 4, 2011

Awaiting reply, before I offer another .02 cents.

Loftus · Jul 5, 2011

Power Valve

67rcks said:
My 302 makes only 14 inches of vacuum (when warm, 10 when cold) while wideband shows 12.5-13 AFR. I am sure there isn't any vacuum leak, and the engine does not react to propane or starting spray. It has mild 218/228 .471" lift cam, holley DP, its power valve is not leaking. The engine runs somewhat rough, especially between 2000-2500 rpm. Is it normal for these engines to produce low vacuum with an aftermarket cam?

My 351c has 7" in park and 5" in drive.(112 lobe 520/540 lift)
For sure the cam will effect vacuum .Lobe center or overlap,duration can effect this.
The Stock power valves are 6.5 depending on list# of the carb.
I ended up with a 3.5 or 4.5 power valve. I played with it some until it ran the best.
Use the vacuum gauge to check the valve adjustments. If the needle bounces around the valve adjustment is incorrect/out of adjustment.
Let me know if it is a stick or auto.
No one ever talks about power valves or believes me that they need to be fitted individually to each application and Holley makes a dozen different increments.
The 6.5 means the valve is open blow 6.5 vac. It is the ONLY thing that helps the carb from idle(idle mixture screws in side of metering block) to the cruse circuit(the booster venturi) .Where you are having an issue.

PoppyMod · Jul 5, 2011

67rcks said:
My 302 makes only 14 inches of vacuum (when warm, 10 when cold) while wideband shows 12.5-13 AFR. I am sure there isn't any vacuum leak, and the engine does not react to propane or starting spray. It has mild 218/228 .471" lift cam, holley DP, its power valve is not leaking. The engine runs somewhat rough, especially between 2000-2500 rpm. Is it normal for these engines to produce low vacuum with an aftermarket cam?

Hi,
If you are running hydraulic lifters and your valves are too tight, this will reduce your vac. My cam is 220/234 with a gross lift of 570/562. My engine makes 16.5-17 at idle. So, check your valve settings. As noted, the vac gauge is a valuable tool for a number of tuning points.
Good luck!

horseballz · Jul 5, 2011

PoppyMod,
Are those duration numbers (220/234) at .050" or are they "advertised" duration. My Ford Racing B303 cam is 284/284 advertised duration and 224/224 at .050" with .300-I/.300-E lobe lift and .480/.480 valve lift and I can't get more than 15.25" vacuum at 900 idle. Of course my cheap vacuum gauge might not be very accurate, I use it more as an A/B reference. The vacuum DID come up from about 14" after advancing my cam timing from retarded 1.5 deg to advanced 4 deg. Standard For issue ting chain read at 1.5 retarded (maybe chain stretch/slop) with only 5000 miles on it.
Thanks,
Gene

PoppyMod · Jul 5, 2011

horseballz said:
PoppyMod,
Are those duration numbers (220/234) at .050" or are they "advertised" duration. My Ford Racing B303 cam is 284/284 advertised duration and 224/224 at .050" with .300-I/.300-E lobe lift and .480/.480 valve lift and I can't get more than 15.25" vacuum at 900 idle. Of course my cheap vacuum gauge might not be very accurate, I use it more as an A/B reference. The vacuum DID come up from about 14" after advancing my cam timing from retarded 1.5 deg to advanced 4 deg. Standard For issue ting chain read at 1.5 retarded (maybe chain stretch/slop) with only 5000 miles on it.
Thanks,
Gene

Hi Gene,
These numbers were taken @ .050 and simply noted as "duration". My cam is a custom "CI" billet. My lobe lift is 356/351. I'm running 1.6 ratio roller rocker. The cam card listed my vac as 16.9. I'm pretty much "spot on".
I would recheck the PR geometry and the pre-load. You, should be no greater than 1/2 turn at most. I'm running a heavy 1/4 turn.
If your are running 7/16 X 20 studs, a 1/4 turn while running 1.6 ratio rockers will give you about .040 preload. You want to be somewhere between .020 and .060.
Based on the math, 1/2 turn is approx. .080, a little tight.
If your rig is newly assembled, was the cam degreed, not simply "dot to dot"? Also, you should verify the balancer stampings to insure "0" TDC is really located as stamped.
Good Luck!

horseballz · Jul 5, 2011

PoppyMod,
I got the 5.0 Explorer engine with"P" heads, "pre-assembled" from a Ford MotorSports dealer 11 years ago and it sat on a stand, in a bag for 10 years. after getting it running last year, with 1.7 rockers, it was very fast/strong, but I started noticing a few issues and had suspicions. IE: Stinky rich idle, dark tan spark plugs, power band rpm too high for practicality and horrendous (11-ish mpg) fuel milage,even with a light foot. I swapped to 1.6 rockers and used a degree wheel and piston stop to find TDC. Cam was installed dot to dot with only one keyway on the crank sprocket and it came up almost 2 degrees retarded, after multiple checks. I put on a new Cloyes timing set and set it to 4 deg advanced at crank (2 deg at cam) and this lowered my power band rpm a bit, but I'm still pretty rich/stinky at idle and fuel milage went up to about 14 mpg. Before advancing the cam, I istalled the timing gears "straight up", dot to dot, for a test and got O deg advance or retard. Rockers are non adjustable ((except by shims) pedestal mount, roller rockers and were set according to torque spec while measuring preload with a dial indicator. They came out to a range of .039"-.041" at specified torque. I suppose I could put in a larger shim to "loosen up" the preload a tad but pretty much dead center of the .020"-.060" range seemed like it should be about right. Your thoughts are appreciated.
Thanks,
Gene

PoppyMod · Jul 5, 2011

horseballz said:
PoppyMod,
I got the 5.0 Explorer engine with"P" heads, "pre-assembled" from a Ford MotorSports dealer 11 years ago and it sat on a stand, in a bag for 10 years. after getting it running last year, with 1.7 rockers, it was very fast/strong, but I started noticing a few issues and had suspicions. IE: Stinky rich idle, dark tan spark plugs, power band rpm too high for practicality and horrendous (11-ish mpg) fuel milage,even with a light foot. I swapped to 1.6 rockers and used a degree wheel and piston stop to find TDC. Cam was installed dot to dot with only one keyway on the crank sprocket and it came up almost 2 degrees retarded, after multiple checks. I put on a new Cloyes timing set and set it to 4 deg advanced at crank (2 deg at cam) and this lowered my power band rpm a bit, but I'm still pretty rich/stinky at idle and fuel milage went up to about 14 mpg. Before advancing the cam, I istalled the timing gears "straight up", dot to dot, for a test and got O deg advance or retard. Rockers are non adjustable ((except by shims) pedestal mount, roller rockers and were set according to torque spec while measuring preload with a dial indicator. They came out to a range of .039"-.041" at specified torque. I suppose I could put in a larger shim to "loosen up" the preload a tad but pretty much dead center of the .020"-.060" range seemed like it should be about right. Your thoughts are appreciated.
Thanks,
Gene

Hey Gene,
OK, so it seems you have the cam under control. What compression, timing, size of the valves, carb, headers, intake?
Built for street, track, or street/track? Do you have way to check the AFR?
It appears your pre-load based on your numbers, is in the right range. Your vac should be a little higher though. What is your initial timing? Have you adjusted the carb using the vac gauge? BTW, get a good vac gauge, one of the best tools in your armory. BTW, the 1.7 rockers will multiply the pre-load settings. So, good to know you went back to the 1.6, at least, until you can sort out the low vac and the "fat" apparent AFR.
Happy Trails!

67rcks · Jul 6, 2011

I believe the valves are fully closing (the gauge's needle does not bounce), holley is 650 cfm, t5, stock 2.79 gears, PCV valve does not change vacuum levels. I even advanced cam timing by 3° hoping to get some more vacuum, but it is pretty much the same as it was before.
The problem is lousy+noisy acceleration, which starts at 2200-2300 rpm. I suspected it is sucking too little A/F mixture due to low vacuum yet as I am reading 14 in. is pretty much OK. Timing is set at 14° BTC at idle, duraspark+MSD off 85 stock curve, vacuum advance unknown, could be the problem there?

PoppyMod · Jul 6, 2011

67rcks said:
I believe the valves are fully closing (the gauge's needle does not bounce), holley is 650 cfm, t5, stock 2.79 gears, PCV valve does not change vacuum levels. I even advanced cam timing by 3° hoping to get some more vacuum, but it is pretty much the same as it was before.
The problem is lousy+noisy acceleration, which starts at 2200-2300 rpm. I suspected it is sucking too little A/F mixture due to low vacuum yet as I am reading 14 in. is pretty much OK. Timing is set at 14° BTC at idle, duraspark+MSD off 85 stock curve, vacuum advance unknown, could be the problem there?

Hi,
Regarding the vacuum advance, running ported or full? Running "full" with a leaking vac diaphram could add to a low reading, at idle. Running "ported" off idle would affect timing in acceleration, and engine vac, I'm thinking. Check this out. BTW, when you state "noisey", as in valves?
Good Luck!

Loftus · Jul 6, 2011

List #

67rcks said:
My 302 makes only 14 inches of vacuum (when warm, 10 when cold) while wideband shows 12.5-13 AFR. I am sure there isn't any vacuum leak, and the engine does not react to propane or starting spray. It has mild 218/228 .471" lift cam, holley DP, its power valve is not leaking. The engine runs somewhat rough, especially between 2000-2500 rpm. Is it normal for these engines to produce low vacuum with an aftermarket cam?

It is better you have the t5 and not an auto. It's interesting that 14 is low vacuum to you. What is the lobe center of the cam you have?
2.79 is a grunty gear for a DP. I was never happy untill I got Vac seccondaries.Even with a 3000 stall converter and 4.33 it was too much too soon.
If you ease into the throttle does it run better?
With the t5, 14 hg may require a 5.5,6.5 or 7.5 power valve.
Get the list # off the air horn on the carb and look to see what power valve is in their now. It is listed in the Holley book where the carb kit and gasket #’s are. Heck tell me and I will look it up for you.
The power valve requires a special tester unit so don't assume it's ok. I experimented with 2-3 before I was satisfied.

horseballz · Jul 6, 2011

I've already hi-jacked 67rcks thread enough, my apologies. I'll start a new thread with my engine specs, titled "Gene's Engine" and hopefully PoppyMod will see it and continue helping.
Thanx,
Gene

low vacuum + runs rich

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