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???

 

I'm having the same problem with the same set up you have...

YBnormal...drive a vette

 

Suggestion: Just for the heck of it - try switching the vacuum source (i.e. if you are using ported, try manifold or vice versa) and see if things are different.

 

Lean stumble from inadequate enrichment metering or you have your transition slot over exposed.

Check how much of your transition slot is exposed first. Edelbrock may have a specification declaired in the manual but a general guideline is that no more than .020 of the slot should be exposed on the primary throttle blades. If you are into the transition slot then this circuit is feeding the idle circuit prematurely, which causes a fat idle and a lean stumble on throttle transitions. This is more of a condition you experience when driving the car normally. This doesn't show up significantly if you "punch it." But it depends on how bad the stumble is.

You may be dealing with scenario number one where you don't have adequate pump shot or your primary metering rods are not coming up quickly enough. In that case, you want to check the position of the pump rod and that you are getting a full, strong pump shot. You might also step up the spring tension on the metering rods so that they lift sooner.

You may also have to look at the rod and jet sizes to correct the fuel curve.

Hard to say without being there since the severity of the stumble will lead you to different things.

 

I am not a carb expert but I am trying to do this myself and learn something along the way. So far I have learned that you can't take a brand new carb and put it on your car and expect it to work great right out of the box. Changing rods & jets is new to me, I don't know what the transition slot is. I can easily change the rod & spring though at this point I am guessing as to what to put in there if that is even the problem. Could it be the secondary jets? I am now running it 8% lean according to the manual - too much?

Anyone else?

Looks like the old Holley will be going back on if I can't figure this out. I had it restored (not just rebuilt) and was going to keep it until I was able to restore the whole engine...

 

Not to be too harsh but you need to either spend some time researching how your carb functions and how to tune it (might be some books out there) or pay someone to do it else you will be a frustrated mess and waste a lot of time

 

Already frustrated and already have a lot of time invested. And already had a real bad experience when I took it to a "professional" On suggestion of others on the forum I decided to try and figure this out myself.

I just want to drive my car!

 

How much vacuum do you have at idle?

It almost sounds like it is the accelerator pump or the step-up springs.

Can you list our your rods, jets, spring color and your vacuum at idle and WOT?

 

Main Jet 1431 (.107), and Rod 1445 (.065 x .047) and the blue spring that came with the initial configuration. I bought the tuning kit with all of the different jets, rods & springs. For the Vac, I'll have to look at that when I get home tonight. I think it was about 15lbs at idle before I changed the jets & rod.

 

check this thread on Transition Slot exposure
http://forums.corvetteforum.com/show...ransition+slot

 

Your current configuration is both cruise and power lean by 8% (depending on your secondary jet size).

The 1407 should have come configured this way:

Pri Jet - .113
Sec Jet - .107
Rod - .071" x .047"
Orange - step up spring
Pump Drive in the top hole.

I would restart with this configuration and then go from there.

Are you running a choke? Is it open all the way?

Verify you have WOT - 100%.


Have you adjusted (or verified) the float levels as per the manual?

From the manual: To properly adjust the floats in the EPS carburetor, two procedures must be followed. First, invert the airhorn cover (Figure 8) holding the airhorn gasket in place. There should be 7⁄16"
between the airhorn gasket and the top of the outer end of the float. To adjust the float level, bend the float lever until the recommended level is attained. DO NOT press the needle into the
seat when adjusting the float lever. Next, you should check the float drop (Figure 9). Hold the airhorn upright and let the floats hang down. There should be 15/16" to 1" between the
airhorn gasket and the top of the outer end of the float. To adjust the float drop, bend the tab on the back until the recommended float drop is attained. For the off-road float adjustment,
see page 9.

Then, I would install the Orange or Pink Step up springs as per the original set up.

Run the car and reset the idle if necessary - If the problem still exists, it may be the Pump.

From the Manual: If you encounter any hesitations or stumbles that do not seem to be related to the basic metering or have not responded to changes in the basic metering, move the
pump drive link to one of the holes closer to the carburetor body. This will increase the stroke length of the plunger and result in more pump delivery.

 

Choke has been removed (Arizona). I have not touched the floats. I have not touched the secondary jets.

I will try your suggestions when I get home tonite.

Can someone help me understand why the stumble would develop only after the car is warmed up (no choke).

 

You should also set your idle with the engine warmed up.

I'm thinking your car is rich during cold start up.

This is going to create a rich condition when warmed up. If the accelerator pump is in the incorrect position then when you go to WOT from idle it may be dumping too much fuel.

How does it run from a partial throttle stand point?

 

From partial throttle it is great - no stumble at all. It is now at 9 initial 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 (least travel) 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?

 

Hey Dave
Are you using the stock LT -1 Intake?

 

The factory LT-1 Intake manifold

 

Dave
Which model Holley did you have on it before switching to the Edelbrock? (And why did you switch to the Edelbrock)
Did it run well with the Holley before you installed the MSD ignition?

I believe the 72 LT-1 was a dual plenum intake.
Does it it accommodate a spreadbore as well as the square bore?

 

It was the original 6239 Holley - I switched because the Holley was worn out - leaked fuel through the linkage, float continually getting stuck even after a rebuild, etc. It would run good for a week until the float stuck and then it would flood terribly. I had it thoroughly restored (took 6 weeks) so now it should be usable. I have heard they are very difficult to use/tune properly so I have left it off. Also I was going to hold it until I was able to rebuild/restore the whole engine.

Putting it back on involved changing out all of the fuel line to the pump as well. I may try it this weekend.

 

Did you check the floats?

Are you still using the blue springs?

What position was the the acc pump in or was it already in the lowest position?

 

If I remember right, the 1407 is a 750?

That carb would have rather big primaries....and require much more tuning to make smooth in all driving conditions. It's probably also too big for most 350's.


Very few universal carbs run well out of the box, especially a 750 on a mild 350. If you pull that much vacuum at 900, I also assume the cam is rather docile.

After the idle mixture screws are adjusted properly, the accelerator pump shot is probably going to need to be tweaked....using the pump arm hole closest to the pivot and probably even a bigger nozzle.