71 LS5 454 all stock 58,000 miles, Auto Trans

Bought it a few months ago, slowly fixing it up. It has never ran good and smooth since I've had it. Here's what I have done to it:

New spark plugs, AC Delco Gapped to 0.35 (same before and after)
New plug wires, dist cap, and rotor (same before and after)
New Lectric Limited ignition module to replace points and condensor (same before and after)
Timing set at 10 Deg BTC, but I have tried it at 6, 7, 8, 9, and 10 deg
(timing was initally set at 7 deg, at 10 deg it has much better throttle response)

Tried 2 Carbs, a rebuilt Rochester Quadrajet and New Edelbrock 600cfm

Both carbs work about the same. Actually I can't get the rodchester adjusted right. No matter how I adjust timing and idle combinations I can't get the idle adjustment screws to have any affect. I put the Edelbrock carb on it tonight and it idles much better than with the Rochester (I know it has a different idle circuit design).

As long as i'm idling or sunday driving (slowly accelerating) it runs fine (no backfiring or popping, etc). When I put the accelerator to the floor the engine starts missing/cutting out really bad. With the rochester it cut out much worse than with the Edelbrock. Wondering if its because the Edelbrock is delivering less fuel for the coil/ignition to keep up with?

If I manually shift the car, and really wind it out, it starts to clear out above 4000rpms but never really runs smooth.

Car still has the original coil on it. Could a weak coil cause this problem? I ordered a new, slightly higher voltage coil for it. Should be in tomorrow. Checked resistance of the old coil and found the following readings, 2ohms between + and - posts, 9K ohms between - post and coil wire connection. 9K seems normal, but 2 ohms is near the edge of what's considered normal.


Thanks,

Roger

 

If the new coil doesn't fix it I would start to look at your fuel pump, fuel lines and give the vacuum lines a good look over.

 

I forgot to mention, I also put a new fuel tank and fuel pump in the car. The old tank was very rusted so I replaced both and also blew out the metal fuel lines.

I have checked the vacuum lines and don't find any significant leaks.

Thanks

 

Roger,

Have a look at the rubber fuel line between the steel line and the fuel pump. If it's been on there a while, it could have swelled internally, choking it and only allowing enough petrol to get through for low throttle driving conditions.

Just my $0.02 worth.

Regards from Down Under

aussiejohn

 

The 600 cfm carb seems a might small for an LS5. My LS4 has a 750.

If the coil change doesn't work you may want to borrow a bigger carb to some testing.

 

Well lets hope its the coil, you have pretty much done everything else. The quadrajet should have plenty of breath to run your car, so I doubt that the carbs are at fault.

 

I know you should trade that old big block to me for my perfectly running 72 corvette with a 350 and all your stress will be gone

 

You said you put in a new tank --- how about the pickup with a new "sock"? If not, that can be partially plugged and causing fuel starvation.

Also, if you are running a stock fuel pump there are 2 "S" hoses down there that age and swell or can crimp - fuel shut down at high demands.

 

if its a square bore carb and a stock 454 then 600 is plenty big enough according to all the carb calculators.

 

VT72 I can barely pass that deal up but I think I will try to work it out a little longer. LOL

Roger

 

I put new rubber fuel lines on when I replaced the tank and fuel pump. I also replaced the sock on the pickup tube in the tank. I made sure that the rubber lines are not kinked, pinched, etc.

I also replaced the metal fuel line between the pump and carb. I cut the old metal line open with a grinder and looked inside it to see if it was rusted. It was very clean so hopefully thats an indication that the metal lines that run along the frame are in decent shape. I cleaned the metal lines on the frame by blowing clean gas through them. I had a catch can on the far ends and blew them unitl nothing but clean gas came out. To be honest the I got very little rust/trash out of the metal lines. I believe the majority of my rust was coming from the fuel pump. When I shake the fuel pump it sounds like it has rockes in it. The old tank doesn't look that bad but still replaced it.

I really hope it the coil. After running it a while I pulled a couple of plugs and found them full of carbon. Seems to me the engine is getting too much gas. It actually sputtered less with the edelbrock 600cfm carb so less fuel seemed to make it better.

Can't wait to get the new coil and give it a try.

Thanks for the suggestions

Roger

 

Installed new coil tonight. Still have same problem. If I drive normal, engine accelerates fine, with car in neutral car runs and revs fine, but, with the car in gear, if I give it a lot of gas it starts cutting out from about 1500rpm to about 2500rpm then it clears out. Once I am rolling along at about 30mph or more and put the throttle to floor it does not cut out, it runs great.

What in the wide world of sports is causing this?

Could I have a problem with distributor advance springs, weights, etc?

Sparks plugs are still clean. The engine is not backfiring through the exhaust or carb. The engine is just cutting out.

Help, help, help

Roger

 

Roger, I would definitely go this direction. Also, are you sure the Vacuum going to the advance is working at all. I do not think this is a fuel problem. Normally with a fuel problem (not enough) it does not back fire (not loading up) I think you should concentraite on the ignition area. What you describe is the ignition just breaking down under load.
just my 02...and yes a weak coil can do this...have you noticed if it does it when the car is warmed up and not when it is cold??
c-ya
ToddG

 

Roger,

There is a sticky on timing at the top of the C3 tech section. If you have not already, I would suggest you read it and map out your advance curve. Make sure the advance is coming in properly.

The timing settings you state seem a bit low. Make sure you are setting the initial, or base timing, without the vacuum advance connected. Again, I refer you to the information it the sticky post. It is VERY GOOD info.

Also, based on your carb comments above, I would also suspect a possible fuel flow issue. Check the float levels. I would keep the Edlebrock on for now since it "seems" to run better.

You make the comment "Wondering if its because the Edelbrock is delivering less fuel for the coil/ignition to keep up with?". More then likely, the edlebrock is delivering MORE fuel, closer to what the engine requires for the proper air/fuel ratio. With less fuel and more air, the engine will break up.

I was also going to suggest checking your dwell, but then went back and re-read your initial post and saw that you have a module installed. I might be tempted to suspect that once you confirm the timing is set correctly.

Also check the voltage at the coil. Should be around 9 volts when running. Verify it stays steady when revving the engine.

Keep us updated on your progress (or lack thereof......)

BTW I have a 71 LS5, bone stock, runs great. So once you find the culprit, and it may be a combination of things, it will run like a top.

Tom

 

Weights/advance shaft mechanism in the dizzy are prolly rusted up...If the distributer is locked up, you are only running the 10* inital and it will run like a turd for sure....


Definetly check out the distributor next.

 

I recently took my distributor apart and replaced the tach drive gear. I cleaned all the internal parts. Everything was working fine. The bushings were fine with acceptable clearances. I shimed the drive gear so it has the correct gap.

Just checked centrifugal timing advance. It is only proiding about 24 degrees of advance so I increased my base timing to 12 degrees, so that I get a total timing of 36 degrees. Next I am going to check the vacuum advance and see how much it's advancing it. My car has the CEC which is supposed to disable the vacuum advance except for certain conditions. Right now I have the CEC disconnected. The vacuum advance is disconnected right now while I take check/adjust the timing.

When I rebuilt the distributor I replaced the condensor and points with Lectric Limited's electronic module. Earlier today I put the points/condensor back in and set the dwell to 30-deg. It didn't change anything.

Could I possibly have valves that are out of adjusment? Any thoughts. They would have to be too tight because I don't hear them rattling so I don't believe they are too loose.

Thanks,

Roger

 

When you reconnect your vacuum advance, connect it to a manifold vacuum port rather than the ported vacuum used by the CEC.


Probably not, but you may have a bent valve like another forum member did recently. He found an exhaust valve bent which was causing his cut out/sputtering issue.

cc

 

For the purposes of getting the car running right, forget the CEC and just use full manifiold vacuum. What RPM is your 36 degrees in by?


Tom

 

Currently I am bypassing the CEC. The vacuum advance is connected to one of the ports on the carb that has vacuum all the time.

I don't think I have a bent valve because my vacuum readings are pretty steady at 17in Hg. They continuously alternate +/- 1in while it's idling so I do think I have worn valve guides, but the car doesn't smoke at all so maybe they are not too bad.

If I had a bent valve wouldn't I see a dip in vacuum each time that cylinder hits? Would I have a continuous miss?

My engine reaches 36deg timing advance at 2500rpm. It comes in pretty quicky.

Roger

 

I really thought the coil would solve this problem, oh well.

If you posted this, I missed it, did you test your vacuum advance can? They can get a hole or crack in the diaghram and mess up your timing. Also, when you set the 10 degrees of initial timing, was this with the vacuum advance disconnected and plugged?