306 10-1 cast pistons svo gt40x aluminum heads that have been decked edelbrock 600cfm carb Trick flow stage 1 cam 1.7 roller rockers Mechanical fuel pump weiand xcelerator intake with 1" carb spacer Mallory hei distributor What should the idle rpm be with this cam, I have it set around 750 at idle which drops to 500 rpm in gear. It gets rough but does not stall and feels actually pretty decent aside from sounding like its got a cam from hell in it. Is this allright? Using the drivers side port on the edelbrock carb to check vacuum, i get around 9-10 at idle in neutral 750rpm. This sounds low to me. I have pinched the power brakes, and modulator line, the pcv valve seems to be working but honestly had not tried pinching that line. Nothing added vacuum. Also adjusted the idle screws and could not get anymore vacuum.... Timing I believe was 17 degrees at idle...I do not recall now it was 17 or 13 not sure which. Ideally once my rear end is done and I put the manual valve body and converter in the trans, I will be putting on a 750 4150 holley with no choke and mech secondaries, and will take it to a place to set timing on the dyno and carb jetting. So I just need to run it about another month and a half with the little edelbrick.
pcv valves are tuned for there intended application. the trick is to choose one for an application that is closest to yours. i would suggest trying one from a late 60s high performance car. any of the cars that had solid tappet cams. the wrong pcv valve can cause what is equivalent to a vacuum leak at idle. you need to see what your total timing is. rev the motor up to 3000 rpm and see what its at. usually 32 degrees is a good starting point. to dial it in you need a way of measuring the performance. a dyno is best. it can be done on the drag strip also. if it doesnt stall in gear at idle you should be ok. its just a matter of comfort at this point.
The 750 DP carb will be too much for what you've got. It'll be a far happier motor with a 600 or 650 DP. Me personally ? I'd go with a vacuum sec Holley in a 650 or 670 range and tune the secondaries to come on when you want them to.
I drove it around with the idle at 750 and holding 500 in drive and it did not stall once and seems like it has decent power at my timing spot and I cannot tell any knocks or missfires of any kind. What about the vacuum is it considered low for this setup? Around 8-10 at idle? Also on the carb, I will likely be paying someone to jet and tune it, so I figured a mechanical secondary 750dp would be a carb I can keep for a long time and tuned to the engine should give plenty of power.
You can tune it all you want, but the simple fact is that when you nail the go pedal, all four barrels open instantly and this allows the vacuum signal to drop to zero at the venturis. This in turn stops the flow of fuel to the venturis, forcing you to use more accellerator pump volume to compensate for the loss of fuel thru the venturis to keep the engine running. This is a carburetor, not EFI. The one thing that allows the fuel to move from the bowls to the venturis is the vacuum created by the movement of air thru the venturis. The bigger the CFM rating you go, the more vacuum you loose. If you have your heart set on a 750 DP carb then go for it. But don't be surprised to find it performs like crap at lower rpms. And be prepared for crappy fuel economy, this is a race carb, they're not intended for fuel economy. Bigger is not better when it comes to choosing a carb.
Luckily she is not my daily driver, so fuel economy is smiles per gallon over here I understand the carb can be too big just figured a 750 would be borderline for the setup, did not realize that this motor would flow so little. The intake ports on the heads were freaking huge, and it seems to rev to the moon. Maybe a 650 would be the ticket.
The 750 is more then fine for a good headed 306 with some compression no worries there It will idle happier around 950 in nuetral
bump that initial timing MUCH higher even if the carb leans out and runs out of mixture screw adjustment.. then and add some more fuel by changing the idle bleed restrction size(screw in bleed conversion is best). You'll obviously need to pull mechanical advance out to keep total where the motor likes it at high revs though. and IMO, baddad is spot on with the above as I too learned that lesson the hard way on a vanilla 69 fastback stang with a decent running high compression 302(with heavily milled early 68 heads). The Supershops salesman said that my larger Erson solid cammed motor would scream with that carb.. so I bought it on the spot. What a low rpm turd and tuning task that turned out to be! Once above 4,000rpm it came alive but it got pretty expensive to drive around in too low a gear all the time just to keep the carb happy with the cam. Cops could hear me comin from 10 blocks away. lol Live and learn. So, while that 750DP can certainly be made to work on small motors with some heavy footed "backroad tuning trickery".. or dyno tuning .. and preferably a progressive secondary linkage.. it WILL be noticeably softer down low(at LEAST 10 lb ft over a 1,500-2,500rpm spread on such a short stroke motor). Especially on a motor with such a low idle vacuum resulting from lack of timing. IMO, a heavily modded 650 with choke tower delete and a stubstack.. would be the best bet with noticeably better throttle response down low(where the engine spends much more time on average anyways). Even better would be to also install some race style stepped down leg boosters into that modded 650 main body. Plus.. peak power don't mean squat if you don't have the rest of the drivetrain tailored to the task of a narrow rpm banded short stroke smallblock. Pretty well known by now that 750's are much better suited to midsized smallblocks and above(350ci+) which move more air at low speeds to better deal with sudden/heavy throttle blade angle changes. Although drop leg boosters or even annulars would be the better bet on engines with lower signal strength.. such as having too large a carb for the engine size will introduce into the mix. But if you do go big?.. it might be best to put that car and driver on a diet.. than add some fairly short gears in the rear to keep it feeling snotty off the line. PS.. do you have a vacuum advance pot on the disty? I assume not with such low idle timing. I would also assume that's why the idle vacuum is so low. I would buy an adjustable vac advance pot and use a handheld vacuum pump to find what the engine likes best.. then adjust the pot to match that. Personally speaking, on most street motors I like a moderate to heavy initial timing setting(20+ degrees) combined with heavier springs(and advance limiter or modded weights to keep the total number under control).. with a light to moderate vac pot timing supplement at lower off idle rpm and light cruising speeds. Word of caution though.. it's pretty easy to lean out the idle and part throttle fuel curves with heavier timing if you don't tune the fuel back in. PSPS.. that motor should be pulling 14 lbs or even slightly better when the timing and fuel is dialed in correctly.
Just FYI, but all Holley DP carbs have progressive secondary linkages. It's when you suddenly nail the go pedal to the floor(at low rpms) that the progressive linkage there, becomes in effect a 1:1 linkage, it's at those times(low rpms) that you'll find out when a carb is "too big" for the engine it's trying to feed.
It is a vacuum advance mallory hei. Pulls good with no detonation so far. Vacuum seems low with the trick flow and 1.7 rockers? Also I raised the idle to 850-900 neutral and it seems to be much smoother. I have 2.79 gears and a c4, but tomorrow it gets 4.11 and a locker. Weight saving so far has been aluminum heads, radiator, eagle alloy wheels, disc brakes in the front, rear agm battery, headers, electric fan and waterpump. I also do not have any accessories no a/c power steering or that junk. I will be removing the heater motor and core, also going aluminum driveshaft and mini starter... What else is there on a 70 small bumper mav to remove? I would like to get the new carb and have it dialed in along with timing on a dyno. Let a pro really tune for maximum power out of this thing. I have never heard detonation would like to keep it that way.
You should set the idle after the initial timing. The idle speed should be as low as possible and still not die in gear. It's a performance cam, it should not have a smooth idle. If you wanted a smooth idle, then you should have put a stock cam in it. My 331 will idle in gear at 500 rpms with a Z303 cam and 1.7 rockers (228/228* @.050, lift is .587/.587) Does it have a smooth idle ? Hell No!
whatever the motor likes. there is no set rule and you tune to what the equipment will allow. Like I said earlier.. I like tons of timing down low and then pull it away up top(which is why the vac advance works so well for the street as it goes away under heavy/full throttle). Gives the best of both worlds and much better throttle response even from larger cams. My 385 Chev will easily pull 14 inches at idle with a larg'ish custom solid roller(248 degree @.050).. and upwards of 16 with a slightly smaller hydro roller(236 degree @.050).. so it's just a matter of tuning to broaden the power band. I also like the programmable ignition boxes since you can develop run/retard curves to really dial things in. oh yeah.. just wanted to mention that my Chev runs between 48 and 52 degrees of locked out lead up to about 1,800rpm. So yeah.. it's definately responsive and torquey for such a big cam and even some of the lope is gone due to such high vacuum. And after dozens of my own small block builds along with dozens of friends..and their friends friends builds.. have I ever met a small block Ford that did not respond with upwards of 44 degrees lead down low. Less efiicient iron heads need more.. and aluminum can often need less. The motor will tell you what it wants for fuel and timing and speculation has little to do with it in the end with so many possible build combinations.
I thought you want to start at low timing and then get more advance the higher the rpm? Does the vacuum advance with rpm?
