426 Hemi Engine Vacuum

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

Hi Bill,

First of all thank you for being so polite about not naming the vendor who restored your carbs (me), I appreciate it.

As I mentioned to you before in our emails back and forth, it does sound like a vacuum leak to me, 10 inches of vacuum is quite low for a stockish Hemi, mid to high teens would be the norm. I can't diagnose your engine from here but I would look very hard at your timing (distributor and vacuum advance in particular) as well as your carb base gaskets and the intake manifold seal (not an easy one).

Hemi carburetors are pretty simple and not usually a source for a vacuum leak, plus they ran fine on the test engine before shipping to you. That said after you do your checking and you feel it might be the carbs please feel free to return them and I'll give them a thorough checking over at no charge.

Thank you for your business!
Scott

Is there such a thing as power brakes and a Hemi in the same car? Booster could be leaking vacuum. PCV valve?

Kevin

Scott,

I hope I was clear in stating that I think the carbs are NOT the problem. I do believe this is a pre-existing condition that I expected to go away with having the carbs re done. I'm sure you'll agree that they were pretty ratty and needed to be redone, anyway. Now, I believe the carbs can be checked off the list and I can move on. Next, I will pull the distributor and have it checked out. It is an old iron tach drive DC model that I put in at least 35 years ago. It has about 28 degrees initial lead and 8 degrees built in that come on at about 2k. I'm sure I can improve on that!

Again, thanks for your service and I am still thrilled with the results, I just wish that had taken care of the problem. I appreciate your input.

Bill

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).

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.

HEMI's are notorious for vacuum leaking on the bottom of the intake gasket at the intake port because that area is so narrow. Borrow a bore scope and run it down each runner. I'm betting you'll find one that has an oil trail. Thats your leak, it will also solve a problem of more than a normal oil use problem.

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.

The car will not idle lower than 950 rpm. It seems to have ample power at wot, though. Like I said, the distributor has been in the car a very long time and the curve is the curve that it came with. I have never bothered to recurve and quiet it down a bit, but now I think it may be part of the problem. I will also check out some of the other possibilities that were previously stated. Thanks, for all the input. There definitely seems to be more to look at than I thought of. I've gotta get this thing running right. Summer is flying by!

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.

What you say makes sense and, in looking at the graph mine definitely follows the curve of the race electronic, tach drive model. I bought it new and it never has been messed with. 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.

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).

I'm with Fast68 on this, the Hemi described is not a stockish Hemi, because it has a 300 degree cam with a lot of overlap.

10" sounds pretty typical.

Compression ratio also affects vacuum, more comp = higher vacuum.

R.

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.

OK. I wasn't sure if it was electronics or points. The points distributors have a different curve as they didnt have to worry about losing time in the electronics.

To realy know the curve on yours, you'll probably have to get a couple more rpm and and timing measurements.
Based on your 950 rpm and 2000 rpm measurement, if the distributor curve is similar as the later one, the initial timing is around 13 degrees. Great for easy starting.

I don't think shortening the slots alone will do the trick unless the engine will tolerate starting at 28 degrees. On the graph is a red line showing a more reasonable 22 degrees for starting. If this is what has to be done, then the primary spring will probably also need a slightly higher spring rate. The spring rate will change the slope. More initial tension will shift the curve to the right.

Have you check the torque and tightness of all the intake bolts? If not do that and let us know what you find either way

Here is a point of reference for you:

528hemi, 270advertised 235 @.050 solid flat tappet 108 LSA, 10.0:1 compression. Cam installed dot to dot. Factory intake with cloned Carter-brocks. 16in vacuum at 750 rpm idle.

Mine acted like the way you are describing when I first put it together, I had a massive vacuum leak. Good luck with your Hemi.