426 Hemi Engine Vacuum
#2335131
07/11/17 01:59 AM
07/11/17 01:59 AM
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Joined: Jun 2008
Posts: 133 N. Ca.
floorit426
OP
member
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OP
member
Joined: Jun 2008
Posts: 133
N. Ca.
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Hi,
I just recently had my carbs rebuilt, by a reputable company. However, I put them on and I can't get the engine to idle below 950 rpm in gear (auto). I have set the timing at 35 degrees, total. The carb man said it sounds like a vacuum leak of some sort and I have checked for vacuum leaks and have found none. I put a vacuum gauge on it and it only pulls 10 inches. The cam has a lot of overlap and 300 degrees+ duration. My question is, how much vacuum do you think is typical for an engine, like this? BTW, although the engine ran well, prior to the carb rebuild, it has always been a "high idler", just not to this extent. I don't feel it is the carburetors, but perhaps a pre existing condition.
Thanks
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Re: 426 Hemi Engine Vacuum
[Re: floorit426]
#2335255
07/11/17 12:07 PM
07/11/17 12:07 PM
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Joined: Nov 2003
Posts: 36,040 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
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there was a preexisting condition that you suspected was carb related & after the carbs were restored the condition is worse? If no other changes/settings were altered then as said I would suspect the valley pan gasket leak (especially with the vac being lower (if I am reading that right).
live every 24 hour block of time like it's your last day on earth
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Re: 426 Hemi Engine Vacuum
[Re: floorit426]
#2335388
07/11/17 04:14 PM
07/11/17 04:14 PM
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Joined: Jan 2007
Posts: 10,136 A Red State
SNK-EYZ
I Live Here
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I Live Here
Joined: Jan 2007
Posts: 10,136
A Red State
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Back in the late 80's I had a 426 Hemi that wouldn't idle properly with the stock Street Hemi intake. A rough vacuum leak type of idle with a mild hesitation.
The engine was built to stock specs with a 70-70 stock Hemi hydraulic cam so it should have idled smoothly.
The carbs were freshly rebuilt (professionally) and it acted like it had a vacuum leak at idle.
I checked for all the normal gasket leaks and couldn't find any.
Someone told me to check the floor of the intake for possible small holes (porosity or corrosion) in the floor of the intake that would go into the exhaust crossover.
I pulled the carbs off and saw a dark spot like carbon on the intake floor. I pressed it lightly with an ice pick and it went straight through into the exhaust crossover. I wasn't running the heat tubes on the car.
I had the area welded up and the car idled perfectly after that. Vacuum leak fixed!
That may be your problem or it may not. You would just have to pull the carbs and to inspect the intake floor to know for sure.
Kayse can't keep up at all now. lol
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Re: 426 Hemi Engine Vacuum
[Re: floorit426]
#2335605
07/11/17 11:18 PM
07/11/17 11:18 PM
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Joined: Jan 2003
Posts: 14,499 So. Burlington, Vt.
fast68plymouth
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 14,499
So. Burlington, Vt.
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I think it needs to be cleared up whether it simply won't idle below 950rpm in gear(as in, that's as low as it can be made to go), or......it doesn't like to idle below 950rpm in gear.
Frankly, with a 300deg cam, 10" of idle vacuum seems really good to me. And, with 28deg initial timing......that will tend to make the motor idle at a higher rpm, and probably helps explain the (higher than I'd expect with a 300deg cam) vacuum reading at idle.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: 426 Hemi Engine Vacuum
[Re: floorit426]
#2335841
07/12/17 12:17 PM
07/12/17 12:17 PM
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Joined: Feb 2010
Posts: 1,645 Phila. Pa.
Mattax
top fuel
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top fuel
Joined: Feb 2010
Posts: 1,645
Phila. Pa.
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Floorit426, Yes you have several possibilities but regardless, as pointed out, with this cam, it will have significantly less vacuum at idle. In most basic terms, the overlap of the intake and exhaust valve open at the same time results in poor vacuum at idle. That doesn't mean there isn't a leak, but 10" at 950 rpm could easily be just due to the cam. Can it be tuned a little more finely? Maybe. There will be a learning curve which includes experimentation. Idle, off idle and part throttle are the hardest things to tune well - especially with a hot cam. Mechanical Tach drive distributor. Map the curve (by this I mean measure it). It's hard to improve on the DC curve that came on their magnetic pickup versions - at least the ones I'm familiar with. However, the portion of the curve that most people find frustrating is the idle. Why? Because they set up the distributor for high rpm performance first. Then easy starting second. Idle characterists last. Just about the opposite of a stock dizzy. The frustration: If yours is set up this way, at engine speeds under 950 rpm, the advance may be dropping like a rock. This lack of stabil timing at idle makes it harder to tune. If yours has the super heavy secondary spring, do not throw it away! This is the speed secret. I had a co-worker who ran in NHRA stock class in the 70's. He tried many combos of springs before discovering the DC secondary spring was key to best high end. Now with the internet I've learned this was a trick used by all the factories. Graph of Aluminum DC Tach Drive Distributor Curve Then go up several posts in that thread A-B-A-B testing. Of course you may find your distributor has been modified or was set up different... Tuning idle with a wild cam. Generally the engine will want more initial - which it seems like you have (28 is a lot). To run slower at idle, it may need a little less advance and a little richer mixture. The richer mix is needed because of the amount of exhaust reversion into the cylinder and intake. Less timing with richer is needed because richer burns quicker. In neutral an engine can be made to run very lean, because there is hardly any load on it. But it has little power when put in gear. This much more apparent with an auto trans where rpm will drop when put in gear.
Last edited by Mattax; 07/12/17 12:21 PM.
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Re: 426 Hemi Engine Vacuum
[Re: floorit426]
#2336200
07/12/17 10:37 PM
07/12/17 10:37 PM
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Joined: Nov 2003
Posts: 36,040 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
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Yes that $22 FBO plate is by far the best deal going for adjusting mech adv. Highly recommended. You can set 10-12-14-16-18 deg of mech adv (in crank degrees).
live every 24 hour block of time like it's your last day on earth
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Re: 426 Hemi Engine Vacuum
[Re: floorit426]
#2336243
07/12/17 11:36 PM
07/12/17 11:36 PM
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Joined: Feb 2010
Posts: 1,645 Phila. Pa.
Mattax
top fuel
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top fuel
Joined: Feb 2010
Posts: 1,645
Phila. Pa.
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... FBO Systems sells a limiter plate that looks like it will change the advance mapping to allow less initial timing, if needed. If I'm understanding you correctly, this might make the idle easier to tune. Maybe, depending on where the slot is positioned. It will probably mess up the rest of the curve because Don (and most people) use the slots as hard stop for the max advance. You could try buying one and modifying one set of slots so they are shorter on the inside. This will shorten the advance and put more initial tension on the primary spring; And it won't mess with the secondary spring controlling the max. It's been a long time since I've taken my tach drive distributor apart. I think the advance slots are slightly angled. If so, make sure the slots in the FBO plate are wide enough to accomodate that. If later on you need to adjust when the secondary spring engages, the spring perches are on eccentrics that can be rotated with a screwdriver.
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