Low vacuum
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Senior User
Joined: Dec 2007
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Low vacuum
Doing some fine tuning to my Bronco, its a (from what I know) stock 351M with a Weiand Stealth 4-bbl intake and new Holley Truck Avenger 650 carb. Timing is set at 14 degrees base, but I'm only able to pull 15-16lbs of vacuum. I have the idle mix screws adjusted for the highest vacuum reading, and the floats are set. The Bronco has always run pretty good for me, but it has absolutely no power until 2500-3000 then it really wakes up. Its still no power house by any means, but I was always under the assumption that you should be able to pull 19-20bs of vacuum with a stock motor? Vacuum readings where taken with the vacuum advance hooked up on the port on the metering block on the carb and the vacuum gauge hooked up on the port on the base of the carb on the front passenger side corner.
It drives on the road just fine, idles smooth, and cruises around just fine, lately its just been used as a mud bogger so I just lock it in 4-lo and let it wind out all the way to 5000-5500 and keep it there. Really seems to like RPM's.
So am I missing something? Is there reason to believe it has a cam causing low vacuum? Or...?
It drives on the road just fine, idles smooth, and cruises around just fine, lately its just been used as a mud bogger so I just lock it in 4-lo and let it wind out all the way to 5000-5500 and keep it there. Really seems to like RPM's.
So am I missing something? Is there reason to believe it has a cam causing low vacuum? Or...?
Cargo Master
Joined: Apr 2010
Posts: 2,600
Likes: 64
From: Billings, MT
If it doesn't have a lope at idle, it's not a bigger cam causing it. Could be cam lobes gone flat though. Check for vacuum leaks by spraying around the carb and intake with carb cleaner, then run a compression test.
Elder User
Joined: Jan 2011
Posts: 831
Likes: 1
From: Tanilba bay NSW Australia
Do you know what it has been historically? did it ever go better down low?
I have a street dom on a rebuilt engine 9.5 comp, holley 725 comp cam 264 DEH, 302 aussie heads and cast headers, and at idle in drive it is 10-11 hg. just remember if it has a holley and that manifold it isnt stock. Stock is the cast two barrel manifold and 400 (approx) cfm carby. From what i've just read, the stealth has a range of idle to 6000rpm so that shouldnt be an issue.
Has it had an aftermarket cam installed as that may have something to do with it? has it ever been rebuilt? heads/valves etc?
As an example mine is a 400/C6 in a 72 galaxie LTD, 2.7 nine inch on 26" dia tyres and well over 4000lbs. It starts off with a bit of a surge more than a snap and after 2500rpm it starts to wind up-I've got it to 120mph but it's just too floaty to hang onto on the test road so I chickened out-LOL.
going from that it seems strange that yours doesnt start a bit lower, but mayby the old 351m needs that amount of revs to "wind up".
I have a street dom on a rebuilt engine 9.5 comp, holley 725 comp cam 264 DEH, 302 aussie heads and cast headers, and at idle in drive it is 10-11 hg. just remember if it has a holley and that manifold it isnt stock. Stock is the cast two barrel manifold and 400 (approx) cfm carby. From what i've just read, the stealth has a range of idle to 6000rpm so that shouldnt be an issue.
Has it had an aftermarket cam installed as that may have something to do with it? has it ever been rebuilt? heads/valves etc?
As an example mine is a 400/C6 in a 72 galaxie LTD, 2.7 nine inch on 26" dia tyres and well over 4000lbs. It starts off with a bit of a surge more than a snap and after 2500rpm it starts to wind up-I've got it to 120mph but it's just too floaty to hang onto on the test road so I chickened out-LOL.
going from that it seems strange that yours doesnt start a bit lower, but mayby the old 351m needs that amount of revs to "wind up".
Lead Driver
Joined: Nov 2000
Posts: 6,619
Likes: 340
From: Albuquerque
I always use full vacuum for the advance vs ported vacuum. With full vacuum you get the advance anytime vacuum is high. This ads to your idle vacuum. You need (imho) at least 20 degrees at idle with the vacuum hooked up. Ported vacuum doesn't come in untill the throttle is opened a little. Also your idle vaccum readying expectation might be a little high.
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Senior User
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According to Holley's manual I am reading the vacuum from the "full vacuum" port, and the advance off the "timed spark vacuum" source. I do have a slight hesitation off idle if I go to WOT from a standstill. I might try switching the vacuum advance to the full port vacuum and see if throttle response improves.
Compression numbers are as follows:
1-125
2-138
3-135
4-135
5-140
6-135
7-140
8-125
I was told by the PO that the motor was originally out of 70,000 mile LTD.
Compression numbers are as follows:
1-125
2-138
3-135
4-135
5-140
6-135
7-140
8-125
I was told by the PO that the motor was originally out of 70,000 mile LTD.
Elder User
Joined: Jan 2011
Posts: 831
Likes: 1
From: Tanilba bay NSW Australia
dont hook up your advance vac from your distributor to anything other than the appropriate port on your carby, read a very heated argument on another forum about this. It is not the right signal to your dizzy. Stick to your Holley manual, it is correct. Is your dizzy ok, mayby the vac actuator on it is faulty and you are only seeing the benefit of the mechanical advance? try disconnecting the vac to the distributor (plug everything off) and see if it changes.
Cargo Master
Joined: Apr 2010
Posts: 2,600
Likes: 64
From: Billings, MT
According to Holley's manual I am reading the vacuum from the "full vacuum" port, and the advance off the "timed spark vacuum" source. I do have a slight hesitation off idle if I go to WOT from a standstill. I might try switching the vacuum advance to the full port vacuum and see if throttle response improves.
Compression numbers are as follows:
1-125
2-138
3-135
4-135
5-140
6-135
7-140
8-125
I was told by the PO that the motor was originally out of 70,000 mile LTD.
Compression numbers are as follows:
1-125
2-138
3-135
4-135
5-140
6-135
7-140
8-125
I was told by the PO that the motor was originally out of 70,000 mile LTD.
I'd check for vacuum leaks around the carb and manifold
Another thought is timing chain stretched.
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Lead Driver
Joined: Nov 2000
Posts: 6,619
Likes: 340
From: Albuquerque
Ported vacuum advance is baisically an emmissions crutch. For improved idle full vacuum adds advance.Low initial and limited centrifugal advance were also used to improve emmissions, not performance.
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I'd love to read more about this! Got any links to any authoritative sources for this?
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