New Build, Won't Idle Under 1500...

Randystys

Randystys

Member
Hey guys, this is Randy, I have a 66 El Camino, 383 tunnel ram...
I have a post or three on the forum on it with pictures and all the specs, and you guys have real helpful in getting the twin 450s sorted out and driveable...

I just built a new 383 motor for it, all forged internals and clearances for nitrous. Comps NX nitrous Cam and the same tunnel and carb setup that ran just fine on the last motor... Has HEI ignition and I added a MDS 6A box so I could add a retard control... Other than that it's the same setup as my last motor.

Fired right up broke in the Cam at 2500 or so, but when it came time to idle it down... Idles smooth, 17-18" vacuum...right down to 1600...but soon as you got 1500 it drops out dead...boom... No stutter, flutter, choking...just drops dead...???

What might cause this?

I sprayed all around the manifold with starter fluid while it was running checking for vacuum leak no change...?

Thanks in advance for any help
 
well, without hands on thats rather difficult to easily diagnose,
I wish you were local, so I could drop by,
but Id check the ignition timing advance curve,
check your getting spark and fuel pressure,
it sounds like its ignition related, but you need to verify everything,
and verify TDC vs the timing marks on the damper/timing tabs match reality.
and did you degree in the cam ? or just use the factory dot-to-dot timing marks to install the cam?
and Id do a compression check on all the cylinders?
and verify the valve adjustment, back off all the rockers until they click at idle,
then slowly retighten the rocker nuts just to the point the clicking stops and no further as a test.
let us know the results
have you tried hooking up the hei without the msd box, as that control box is rather suspect at this point
whats the ignition advance setting the timing light shows at the lowest rpm it idles at?
whats the fuel pressure?
is there a chance the ignition is not getting 12 volt feed once the rpms drop below 1500 rpm,
the alternator may be failing to provide power at some rpm,
the issue definitely sounds like its ignition related.
 
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Appreciate it Grumpy, gives me a checklist to go through one at a time rather than circle around twisting the idle screws over and over wondering why it doesn't change...

As far as adjusting valve lash, will that still work at 1800rpm?
I did dot to dot on cam chain on TDC before I put the heads on.
Fuel pressure is steady at 7psi
I'll check the voltage to the msd I've heard that from other people as well...
Then work down the list...
 
I think what I'll do first is jump the distributor back to the coil with a hot lead bypassing the msd... That will cut my searching in half...or at least one third...
 
I think I'm on to something...
I ran the dist direct to coil (as it used to be) added a hot to coil and fired it up ...
It seemed to idle down to a reasonable level, at least by ear...
Later after work I'll put a tach on it and confirm... And then dive into msd...
Thinking maybe 12v feed is suspect cause the rest of rpm range runs smoothly.
Will post back
 
yeah, that msd control as previously stated is suspect
 
Well the msd is a new unit... I checked the basic wiring instructions and that seems correct, I did move main power from a circuit breaker to direct power, but that didn't make a difference.
There really aren't any settings for rpm on this thing, just the switch to drop X amount of timing when the boys kicks in but that whole system is off.
I guess next is call tech and ask for support before sending it back in...
 
if it idles and runs without the msd and has issues with the msd control connected I think you must consider the possibility the msd control has issues or is not set up correctly!
 
So the tech ran me thru a couple electric tests, thinking maybe distributor is not giving a solid signal that the box can pick up on... Can try either a $60ish msd tach adapter box that"might" straighten out the signal... Or a new distributor of known compatability
 
Have you verfied the ignition timing?
Do you use the magnetic pickup in the distributor or the amplifier as trigger?
Shifting the wires (polarity) from the pickup will move the ignition timing a lot.
 
Okay so I know everyone likes to have closure on these troubleshooting threads, what's the fix...
Turns out the msd tech was right, my distributor was dropping the signal below 1500... It was a Pertronix knock off...a real Pertronix should work, but they have had issue with no name knock offs...
So I bought a Karl's billet magnetic pickup distributor and cable (made for and recommend by msd) hooked it up, fired it up and after a minute warmup and a little tweaking idled right down to 800-900...
So thanks again Grumpy for guiding me down the path of logical progression and off the rabbit trails...
 
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