Won't Run Right - Argghh

[dss LT1]

My 72 LT1 just won't run right. It has new MSD 6A box and new MSD distributor coil & wires, also a new Edelbrock 1407 Carb. It is timed at 12 initial, mechanical is all in at 2500 and it tops out around 51 with the vac advance. It continues to stumble when you punch the gas from idle (900). The edelbrock manual says one of the solutions to a stumble is to adjust leaner - I tried leaning out the primaries on the carb with a different rod and jet combo- no change, maybe a little worse. It also runs-on when I shut it off. There is no stumble when the car is cold, only once it warms up...

I am out of answers - don't know what else to try. Should I lean out the secondary jets? Any other ideas???

[Guest Sidewinder]

Is the car hard to start when the engine is warm?

If the car diesels when you turn the key off you have a little too much timing, you should be at 32 degrees full advance with no vacuum adv. If you unhook the vacuum adv does it still stumble on acceleration? Make sure you plug the line before driving it around the block.

Have you adjusted your pump stroke (the white rod with the S shaped clip on the top passenger side) for the squirters on the carb? If not make them leaner or not as much stroke. The manual should tell you how to do it. If I remember correctly it's the hole closest to the radiator. Don't drop the little clip off the rod though, it is SMALL.

Do you have the light blue and light silver springs installed on the distributor?

Vacuum leak? Choke functioning properly?

The 1407 is a 750 cfm, a good rule of thumb is C.I.D. X RPM X VOLUMETRIC EFFICIENCY = CFM

That is 607.6 for your set-up on stock heads. A 650 CFM is plenty.

Other ideas guys?

[Guest XLR8TNC5]

I the car hard to start when the engine is warm?

If the car diesels when you turn the key off you have a little too much timing, you should be at 32 degrees full advance with no vacuum adv. If you unhook the vacuum adv does it still stumble on acceleration? Make sure you plug the line before driving it around the block.

Have you adjusted your pump stroke (the white rod with the S shaped clip on the top passenger side) for the squirters on the carb? If not make them leaner or not as much stroke. The manual should tell you how to do it. If I remember correctly it's the hole closest to the radiator. Don't drop the little clip off the rod though, it is SMALL.

Do you have the light blue and light silver springs installed on the distributor?

Vacuum leak? Choke functioning properly?

The 1407 is a 750 cfm, a good rule of thumb is C.I.D. X RPM X VOLUMETRIC EFFICIENCY = CFM

That is 607.6 for your set-up. A 650 CFM is plenty.

Other ideas guys?

Wow, thats impressive talking right there. I don't care who you are!

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So, when are you coming home babe? wink wink!!!

[dss LT1]

let's see, choke has bee removed (AZ), light blue springs are in the distributor, I'll check the timing and other stuff again tonite.

I picked the 1407 carb because it was closest to the original Holley's 780 cfm.

[Guest Sidewinder]

let's see, choke has bee removed (AZ), light blue springs are in the distributor, I'll check the timing and other stuff again tonite.

I picked the 1407 carb because it was closest to the original Holley's 780 cfm.

I would make sure the choke is open all the way and move the linkage for the squirters. Keep us updated

[Bigfoot]

Is the car hard to start when the engine is warm?

If the car diesels when you turn the key off you have a little too much timing, you should be at 32 degrees full advance with no vacuum adv. If you unhook the vacuum adv does it still stumble on acceleration? Make sure you plug the line before driving it around the block.

Have you adjusted your pump stroke (the white rod with the S shaped clip on the top passenger side) for the squirters on the carb? If not make them leaner or not as much stroke. The manual should tell you how to do it. If I remember correctly it's the hole closest to the radiator. Don't drop the little clip off the rod though, it is SMALL.

Do you have the light blue and light silver springs installed on the distributor?

Vacuum leak? Choke functioning properly?

The 1407 is a 750 cfm, a good rule of thumb is C.I.D. X RPM X VOLUMETRIC EFFICIENCY = CFM

That is 607.6 for your set-up on stock heads. A 650 CFM is plenty.

Other ideas guys?

Wow, way above my head.

[Guest 1FASTC4]

C.I.D. X RPM X VOLUMETRIC EFFICIENCY = CFM

Cool... tell me about volumetric efficiency... I understand the words but not what you are referring to.

I recently downsized from a 750CFM to a 650(albeit a double pumper) because several people told me that the 650 was plenty big... I took their word for it and have benefitted from a better runing motor(383) that is more vacuum friendly.

Dss, even though we are in AZ I found that my car runs better w/ a choke... I had Joe over at Classic add an electric choke and it really helps on cold starts.*

* Yes I could have easily added it myself, it is neither expesive nor difficult.... I don't even have enough time to sleep though so others must do my tinkering for me.

[dss LT1]

It is now at 9 intial and 32 with mechanical. It does seem to run a little better with the vac advance plugged.

It is running 17.5 Vacuum, I re-adjusted the idle mix and reset the idle speed to +-800. The pump arm is at the lowest of the three positions. The springs in the distrbutor are now one heavy silver and one light blue, pushes the curve out more. I left the carb at 8% lean (rod and primary jets)

It seems to be better overall, but still a stumble nonetheless. Could it be that the vacuum the carb produces is more than the vac advance needs or can use...?

Any other ideas?

[Eddie44]

Wow, thats impressive talking right there. I don't care who you are!

.

.

.

..

.

.

.

.

.

So, when are you coming home babe? wink wink!!!

You are one lucky guy Mark! All you've got to do is get out the Chilton books, read to her a little and.....oh well, now I'm getting excited!

[Guest Sidewinder]

No offense guys but can we keep this to the topic, somebody needs help

1FAST, let me rewind the brain about 12 years .. Volumetric efficiency is a ratio, or percentage, of what volume of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions. i.e. (EXAMPLE) My cylinder head can hold 49cc but on one stroke the induction on my car can only draw in 35cc (EXAMPLE). Do not attempt this at home .

dss, Does the car still "run on" after you shut it off?

If you put the old carb on it with the advance hooked up does it still stumble?

Is it hard to start when it's at operating temp?

(First suggestion) Did you take it around the block with the advance unplugged? If not, unplug the vac ad. plug the line so you don't have a leak, back out of your driveway and drop the hammer on it, tell me what happens. Remember the car doesn't alot of make vacuum at full throttle.

Do you still have the old carb?

Does the vacuum gauge swing wildly back n forth?

Just gee whiz, My '70 Mustang w/a very radical 351 Cleveland made 469 at the wheels with 650 CFM carb and 452 with a 750 on the desktop dyno. With the formula applied it called for a 635 CFM carb.

I wish I still had my 650 Edelbrock. We could try it out.

We WILL fix this!!!!!

Edited for spelling

[ChadC]

It is now at 9 intial and 32 with mechanical. It does seem to run a little better with the vac advance plugged.

It is running 17.5 Vacuum, I re-adjusted the idle mix and reset the idle speed to +-800. The pump arm is at the lowest of the three positions. The springs in the distrbutor are now one heavy silver and one light blue, pushes the curve out more. I left the carb at 8% lean (rod and primary jets)

It seems to be better overall, but still a stumble nonetheless. Could it be that the vacuum the carb produces is more than the vac advance needs or can use...?

Any other ideas?

Its been 2 years since I have worked on that carb with my 454. The thought that comes to my mind is the "Accelerator enrichment pump"

while opening from Idle the carb has an added 'squrit' of extra gas to prevent stalling. THe thinking is you will be moving from a dead stop and need the extra gas to prevent stalling.

It only 'squirts' from idle to part open. IF the car is running at say 2000 RPM and you gun it to WOT the 'squirt' does not happen.

Thank fully you have an elderbrock, not much to tune. My advice is the above stuff does not work then set the unit back to default.

The Idle air mix, Idle jet sizes, The other Jet sizes, the accelator pump (I think that is the arm with the 3 holes and rod with an R clip going through it), The meetering rods, and there are color springs ( red and blue if I recall).

Then take an after noon and follow the directions to set up the carb from scratch.

If you dont have one already buy a jet and meetering rod kit for your carb. THe best $35 you will spend and if will give you different sizes for your needs.

[dss LT1]

Still running-on but more intermittently. I haven't broken out the old carb yet. It is not hard to start - cold or hot. I drove it around the block after every adjustment. I drove it with the vac advance plugged and connected and it seemed to be better with it plugged. I dropped the hammer quite a few times (FUN :D :D ), and it just takes off like it's stolen, just stumbles from idle if you punch it. If you punch it between gears to keep the revs up it is just fine.

[Guest Sidewinder]

Still running-on but more intermittently. I haven't broken out the old carb yet. It is not hard to start - cold or hot. I drove it around the block after every adjustment. I drove it with the vac advance plugged and connected and it seemed to be better with it plugged. I dropped the hammer quite a few times (FUN :D :D ), and it just takes off like it's stolen, just stumbles from idle if you punch it. If you punch it between gears to keep the revs up it is just fine.

With that being said I would steer away from the distributor(maybe). Put the old carb on it and see what happens. Keep us informed. Does it pull better with the distributor modifications? If it "runs on" still it's the timing.

Ohh Damn, I just thought, did ya get the dist. one tooth off?

[dss LT1]

[Guest Sidewinder]

Ohh Damn, I just thought, did ya get the dist. one tooth off?

[anmracing2]

I don't have much else to add to this one, Mark has it covered, but test the diaphram on the dist too. That could be effecting the timing also. If it is richened up with the metering screws by a quarter turn, does it still bog when you gun it from idle?

Andrew

[P51tj]

Glad to see we have some "Piston Heads" around here!

[anmracing2]

Also check that accelerator pump. I was just reading where some one was having the same issue and the pump wasn't doing it's job.

Andrew

[dss LT1]

Just finished re-installing the old/new Holley and new lines to the fuel pump. The stumble is gone - hooray. It fired right up, I adjusted the idle mix to go a little leaner - still at max vacuum - 17-18, and adjusted the idle speed to +-800. I left the tiiming as-is at 9 initial and 32 with mechanical. Test drove it and got it real warm (had it up to 95 on Pecos Rd - oops did I say that...) and still no stumble. it is still running on but I think that is a malady specific to the MSD - it is known to cause run-on in these old Chevys - need a diode in the voltage regulator line out of the alternator to kill the power to the MSD when the key is off.

The real test will be if this carb maintains it's float levels - prior to the restoration the seconday float would stick high and flood terribly.

Thanks for all of the input - if nothing else I am learning more about this car every day.

I guess I'll store the edelbrock for a backup.

[anmracing2]

Good job... Stay with the Holley.

Andrew

[Guest Sidewinder]

Put some seafoam in it to de-carbon the tops of the pistons. Might have some gunk on the tops holding a little fuel.

[anmracing2]

Have you used the Seafoam stuff before??

http://www.hilltoprv.net/browseproducts/SE...OR-TUNE-UP.HTML

Andrew

Hey Mark,

Another question, would it be worth it for emission issues too??

Andrew

[Guest Sidewinder]

Andrew,

This stuff works GREAT. If you do the de-carbon it smokes like a mother after you let it sit in the cylinders. As far as emisions I don't think it would help much. My buddy couldn't get his vehicle to pass, he put some "emissions pass" in the tank and it passed. I think the main ingredient is denatured alcohol. Got it at Autozone.

Mark

[anmracing2]

Thanks Mark,

I'm having mad issues with emissions on the '71. It's called "Emissions Pass"?

Andrew

[Guest Sidewinder]

Maybe not "emissions pass" but it's along those lines. Pass emissions or your money back kinda deal. I have heard it work on several occasions.