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383Scampman

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Hi guys , need help with my tune . My project is 98% done (thank goodness) but now gremlins are popping up . '72 Scamp, 383, 727. 9" with 4:10's . anyway, since I installed the 3" exhaust I am having a skip at idle . It seems to smooth out above 2000 rpm and the pipes are black and sooty . Holley #3310 vacuum secondaries with #72 jets front and rear . Vacuum advance distributor with 24* advance and 5* at the crank . Vacuum leak , float setting , power valve , acclerator pump . Ran great with open headers and/or flowmasters . Any help will be appreciated . Thank you in advance , Tom
 
383Scampman said:
Hi guys , need help with my tune . My project is 98% done (thank goodness) but now gremlins are popping up . '72 Scamp, 383, 727. 9" with 4:10's . anyway, since I installed the 3" exhaust I am having a skip at idle . It seems to smooth out above 2000 rpm and the pipes are black and sooty . Holley #3310 vacuum secondaries with #72 jets front and rear . Vacuum advance distributor with 24* advance and 5* at the crank . Vacuum leak , float setting , power valve , acclerator pump . Ran great with open headers and/or flowmasters . Any help will be appreciated . Thank you in advance , Tom
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What kind of Cam are you running? And also is that 29 degrees total timing?
 
how longs it been sitting - bad gas? improperly connected plug wire? wrong gap on plug(s)? loose ground? air leak at or near carb?
 
I've heard of "miss, pop, cough, sputter, stumble, hesitation, etc etc", but what is a skip?
If it's "off idle stumble", I'd say accelerator pump.
 
It's probably rich at idle. You need to either open up the idle air bleeds on the primary side a few thousandths or close down the idle feed restriction a few thou.

This is the problem with carbs that don't have 4 corner idle and changeable brass.
 
yellow rose said:
It's probably rich at idle. You need to either open up the idle air bleeds on the primary side a few thousandths or close down the idle feed restriction a few thou.

This is the problem with carbs that don't have 4 corner idle and changeable brass.
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Is four corner idle really a problem or a preference?
 
383Scampman said:
Holley #3310 vacuum secondaries with #72 jets front and rear
Click to expand...
That don't sound right, but may be unrelated; the 3310 usually has a 8 number split when a power valve is used, so 72/80 would be a better idea. With no powervalve used then 80/80 might be a better idea.
But with the 72 fronts and no PV; the engine should be lean when rolling into the power,(primaries only) and giving you fits like lean misfires, and severe stumbles. Worse when the too-lean secondaries come in................so I don't get the black and sooty pipes, unless your ignition system is crapping out.
And on that note your timing report is bizarre.
Your idle-timing is of lesser importance than your power-timing, so get the power timing where it needs to be first.
Your power timing is what counts; how much and at what rpm does it stop advancing. Forget what the V-can is doing for now, just defeat it. For reference, a 383 with iron heads might like 36*, and it might like it in, between 3000 and 3600 depending on the combo. Sometimes sooner, Sometimes more than 36*. But 34* at say 3400 is pretty safe.
 
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I'M thinking its rich at idle . Cam: Comp x-treme energy .500 lift , 292* dur., 244*[email protected]". Ran strong before 3" exhaust . It's just at idle , sounds like crap. Sputter, I guess . Yes 29* total timing . Thanks guys and keep up the comments . Later, Tom
 
4 corner idle lets you idle off of the secondaries and get to use the whole transfer port for acceleration. This is what is called squaring the primaries. Very important on large cam motors that need to idle at a higher RPM.

Squaring the primaries is done with the carb off of the motor. looking at the carb upside down there is a small rectangle slot in the in the horn just above the venturi plate. Adjust the plate using the primary idle screw until the slot showing is as long as it is wide. It should look square

This is your permanent primary idle position, Put the carburetor back on and start the car. Adjust the idle using the secondary idle screw. Do not ever turn the primary screw. If you cannot bring the idle up high enough with the secondaries. Put smaller air bleeds in the primaries this will add more fuel at idle but still give you the full usable length of the transfer port.

Once this is done you can work on the Accelerator pump cam.

Always worked for me. My car would not hesitate at all . it was like a light switch.

 
383Scampman said:
It seems to smooth out above 2000 rpm
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This is about where your vacuum peaks and so reversion has ended, ergo all the air entering the engine is now going in the same direction. If that air has the correct amount of fuel in it, then smoothness is the result.
I ran that 292/509 cam in my 367 SBM, and as I recall, the intake gets all messed up at idle. The 108LSA version that I had gets real dirty at 750/800 idle.
So you gotta start with a proper Transfer slot exposure and plenty of idle timing, and a loose convertor. On my 3310 I had to drill holes through each of the primary air-valves to give the engine the air it craved. But be careful, if the holes get too big, the idlespeed will be too high. The secondaries need to be closed up tight but not sticking.
My best guess is to make the exposure a little taller than wide, then don't move the idlespeed screw. After that, you will be setting the idle speed with idle air bypass and idle timing. I suggest one hole in each primary valve of about 5/64 or 6/64, that is .078 to .094. The hole should be on the transfer-slot side and about half way from the throttle shaft to the edge. I bias them towards the idle discharge ports.
And I suggest a start point of 14* idle timing with the 5/64 holes. If the idle speed is still too slow, with an automatic and hi-stall, you can increase the timing to say 18/20. If still too slow, you can go back to 14* and increase the holes to 6/64, and start over on the timing. My Eddie-headed, 11.3Scr, 367 cuber, I got a nice idle at 700/750/14*/ with 5/64 holes. If your TC drags the engine down more than 75/100 rpm, you will have to compensate for that. If your mixture screws are out more than 1.5 turns then increase the transfer exposure slightly and put the mixture screws back to 7/8 turn
The above assumes the V-can is hooked to the sparkport and the PCV is working off the front of the carb, and that the indicated TDC on the damper has been proven to be true TDC, and that the valves actually do close at some point.
After this is done, you may have to rework your power-timing and rate of advance.
If you drill holes that are too big, don't panic! you can chamfer the holes on both sides, and drop a solder plug in there, a little bulgy on both sides. Mine have stayed in there since year 2000, and well over 100,000 miles. Then move over to fresh metal and start over. .125 will be way too big. 7/64 (.109) will also probably be too big. So stick with the 5/64 as long as you can.
These mods should dry up your idle fuel, without introducing side-effects.
 
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383Scampman said:
and 5* at the crank
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Not a chance any car with that kind of performance camshaft is going to idle decent with so little initial timing. Needs around 20* is my guess at a minimum.

I bet it cranks forever or you need to mat the throttle pedal to get it to fire.

Add 15* at idle and DO NOT drive it if it helps the issue. Fix mechanical to get total in the ballpark.
 
Thanks , thanks , thanks again guys . Just went to the car and I have drilled primary butterflies and .035" squirters . top of carb is wet with gas. Also, I had the vacuum advance hooked to manifold vacuum . Stupid me ! More comments please , Tom
 
Not stupid at all.
Many guys run the Vcan off manifold with cams of this size, and bigger. You can get away with that, if you have an automatic and a hi-stall. Altho 24+5= 29 may be excessive ,it depends on the convertor.
However, this large amount of idle-timing may result in you backing out the speed screw very significantly... to slow it down, which closes up the transfer slot exposure, and to make it idle, the mixture screws get cranked waaay out. Of course with the transfers so far closed, as soon as you tip the throttle in, it could go lean, introducing a hesitation, which you promptly cure with pumpshot. The thing is, the mixture screws are still running at 100% so the result is that the entire slow-speed circuit is rich......... Never mind what the late-closing intake is doing to the AFR.
 
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AJ/FormS said:
Not stupid at all.
Many guys run the Vcan off manifold with cams of this size, and bigger. You can get away with that, if you have an automatic and a hi-stall. Altho 24+5= 29 may be excessive ,it depends on the convertor.
However, this large amount of idle-timing may result in you backing out the speed screw very significantly... to slow it down, which closes up the transfer slot exposure, and to make it idle, the mixture screws get cranked waaay out. Of course with the transfers so far closed, as soon as you tip the throttle in, it could go lean, introducing a hesitation, which you promptly cure with pumpshot. The thing is, the mixture screws are still running at 100% so the result is that the entire slow-speed circuit is rich......... Never mind what the late-closing intake is doing to the AFR.
Click to expand...

Good point A/J. ! I never considered the consequences of too much initial timing that way.
 
j par said:
Is four corner idle really a problem or a preference?
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It just makes things easier with a mild engine. With a fair amount of cam timing, it just makes it nearly impossible to get the idle correct without it. Many guys just live with a shitty idle but I hate that.
 
Drilling the blades was a bad idea. You may have to fill them to get the idle correct now with proper initial timing.

Drilling is a last resort type deal. I know it gets suggested all the time, it's a bunk shortcut when the tune up is off.
 
I just went to a AVS2 650 from a QF 750 VS.
It idles nicely but I still had to (and am) monkey with it. Overall HP so far has not recovered to the 750 level. That could be the CFM. Maybe shoulda ordered the 800.
 
318willrun said:
LOL... by lookin' at the picture of your plug you posted, you might have an issue or two yourself... eddy related?????? :D :D :D
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I don't think you're giving those plugs a fair Shake. They were a year old and had went through quite a bit. they were on when I had the old Cam and breaking in the new one. I just got the carburetors kind of tuned in and never change the plugs or clean them. And then even worse I started it all cold with the choke on and shut it off within 30 seconds and pulled the plug out, but I thought I had said all that along with the picture...
 
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