Let me start off by saying that there are several unknowns in this equation.

I have a new to me 71 LS5 4 speed. Engine was rebuilt by previous owner. The guy I got it from traded for it and sold it to me. I am trying to find out who rebuilt the engine and what exactly is in it. That is the biggest unknown. I have no idea where timing is set right now. Thats the other big unknown. That one I can probably figure out.

You can hear it run when the guy I bought it from loaded it on the rollback. Hopefully gives you and idea of the cam, etc in the engine.



When I bought the car it was running race fuel. Had that sweet smell. Guy I bought it from told me it didn't need to run race fuel but that the previous owner liked the smell, etc when he rolled into car shows.

I have since run the race fuel out of it and am probably on the first full tank of super unleaded from the local gas station.

I thought I heard the engine clatter tonight while running. Also, it has developed a nasty off idle stumble.

I would rather not have to run race fuel in it if I can keep from it. Is it possible to adjust the timing for it to run better on 93 octane fuel? Or do I just need to bite the bullet and get some race fuel? If you think adjusting timing will work, can you give me some initial idea of where to start?

How about octane boosters? Do they really boost the octane? Would that work in the short term until I can find some race fuel?

I know there are a lot of unknowns here, and I apologize in advance for that. Just looking for what direction I need to go with this.

Doug

 

Your biggest problem will be if the compression ratio has been changed. You should check compression for starters. Backing off on timing will help to a point.

 

What is the compression ratio of the motor? That will dictate whether or not you need race fuel. What kind of race fuel did the PO have in it? 110? c16? 104?
Even if you drive around on pump gas with high compression and advanced timing it really shouldnt give you a problem unless you are romping on it.

That car sounds friggin NICE!!!!!! Sounds like there may be a bit of compression in it. You need to ask the PO.

 

Your engine was originally 8.5:1 compression so that it could run on regular (87 octane) gas. No need for 91 or 93.

If the previous owner played around with the engine, then it's anybody's guess.

 

I sounds pretty crisp in the video which usually indicates a fair bit of compression. But it also has a big cam which will help bleed off compression and lower the dynamic compression (DCR). Have a compression check done on it. I believe the rule of thumb is 185psi and under will run on pump premium (might be a little lower with iron heads). You can back off on the timing a few degrees but big cams like lots of timing and if you retard too far it will run hot and cause detonation that way.

 

Love the sound.

Run a compression test on a couple cylinders. 200 psi cranking compression is about as much as you can tolerate without having to run plus octane fuel, and timing will be critical. Would be nice to know a little more about what's inside that raspy engine.

My SB runs fine on 91 octane@ 200 psi but you have to pay attention to the timing. It also has aluminum heads.

 

I agree with you all. I am trying to get in touch with the PO to see who built the engine and what was done to it. It sounds incredible.

I will see if I can run a compression test on a couple of cylinders.

Just need to try and get rid of the off idle stumble for now and make sure I am not hurting anything with the lower octane fuel.

Thanks for the answers so far. I really appreciate it.

Doug

 

If it's rattling a lot you should limit your driving until you know what is going on.

 

Really only clatters when I am getting on it.... and I am able to exercise restraint and keep my foot out of it.

Doug

 

While cruising, have you noticed any tendency to "trailer hitch", making it feel as though it has a slight miss? If so, your total timing is likely too high. You might throw a light on it and see what numbers it's giving you.

 

If you can get the info from the PO you'll be light years ahead, instead of having to guess about everything. That engine sounds cold but very healthy- nasty cam! If you have a dial back timing light, charting timing from idle on up at various RPMs until you reach max advance will tell you where you're starting. I'd be curious about what you come up with. You may be able to go less aggressive and still be happy without running race gas. Until you start sorting it out I'd drive it gently. Octane boosters really don't help much, and I really doubt they would do you any good. Apologies if you already know all this stuff already.

One question- did it just start stumbling off idle after you ran out of race gas?

 

I have noticed that. DOesn't happen too often but it does happen.

I will get a light on it tomorrow and find a compression tester and check that too. It's a little loud to fire up tonight... neighbors and all....

What should I be seeing as far as timing goes?

Doug

 

Please, don't assume I know anything. I appreciate all the info I can get.

The stumble started today. It had a full tank of race gas in it when I picked it up. I haven't let it get lower than a half tank before filling each time. I have filled it maybe twice from half. I keep smelling the race gas... until today. Took it out for a drive earlier with my son and noticed the clattering and the stumble started. Throttle response was instant before. Today, when I would hit the gas leaving a stop light it would act like it was going to die just before picking up. It has an Edlebrock carb on it. Looks like a Quadrajet but has an Edlebrock tag on it. Not that that makes any difference.

Doug

 

Claysmoker raises a really important point. That's pretty much where I was headed but I was waiting to see if you could chart the advance curve first. I had that problem with my 427 and I wrestled with it for awhile. Even with an adjustable vacuum unit on the distributor, I couldn't hit the sweet spot to get a good idle and good response off idle without surging or "trailer hitching", as Claysmoker describes it. Depending on what you find, I may have a solution for you which I finally got from a guy who had done a lot of dyno tuning on big block Chevys with big cams.

 

Until we see what he has, it's pretty much stabbing in the dark, but I know I ended up about 18 initial, 34 vac/centrifugal and 48 total to keep mine happy on 91 octane. Anything past 48 on the total would make the thing trailer hitch.

 

Okay... got some numbers for you guys....

Compression test shows 120.

Timing:
1000rpm 18
1700rpm 30
2000rpm 40
2500rpm 42

Ran it to 3000 and it was still at 42 degrees.

Didn't seem to want to start this morning. Usually it's couple of pumps of the gas pedal and it fires right up. This morning I had to crank it quite a bit. Not sure of that has anything to do with all of this. Also, the stumble off idle is still there. Pulled a plug wire to do the compression yes and the boot and wire came off... but not the clamp on the plug. I guess I am running down to pick up some new wires today too.

Not sure if it makes a difference but figured you ought to know.... it is running an MSD distributor and MSD 6A box.

I don't necessarily need a tire burning performer. I just want it to run good and solid. I would give up some performance for a good running car, if I have to.

Talked to the guy I got the car from. He knows the guy who built the engine for the PO. Hopefully I will have a phone number and get a call in to him later today.

Again, thanks for the help folks.

Doug

 

That's way too much. Back it off to 36-38 maximum.

 

Did you pull the vacuum advance and plug it when you checked timing?

 

No sir, I sure didn't. I used a dial back timing light and checked it with everything connected. I have checked them that way on other cars but I didn't this time.

As soon as I get some new plug wires I'll check that too.

Doug

 

Did you lock the carb wide open for the compression test? If not, go back and do it again. And pull the vacuum line like Ignatz said.