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I have a 69 L71, its very original, I've had it a good few years, but its unused as it runs like a pig. About 2 years ago I had in dynoed and as I suspected it was around 180-200hp.
The engine was professionally rebuilt in MD, tri carbs in MI, I bought the car internationally without driving it (its in the UK now) so assume it always had issues with the previous owner.
It drives fine if you like a 180HP 427! We took it down to Le Mans even, its cool but gutless. The viscous fan is louder than the engine!
After some lay up I've been back to it today, stubborn not to be defeated, but could do with some basic guidance on where to start. I desperately don't want to start taking the engine apart to check cam timing. I even bought a 4bbl set up but don't have the heart to fit it as that would be defeatist!!
I notice it has no vac advance on the distributor. I seem to recall checking manifold vacuum and it was very low indeed, then again I've read that the take off point may be 'ported' so not correct. The car has its correct air injection and cast manifolds with a working (moving freely) downpipe valve. I even tried converting the 3 carbs to fixed simultaneous operation, didn't help!
A basic guide as to logical steps to check in order would be really useful if anyone has time to drop me a note specific to this 3 carb set up on the 427.
I imagine the first thing someone more knowledgeable than me will ask you is "what timing advance degree are you running?" So you may as well start there and provide that information when an expert chimes in!
Also, your description of the distributor suggests to me that it is neither original nor appropriate for that engine.
Pull the spark plugs. Now pull the valve covers. Crank the engine on the starter and watch all 16 rockers and see if they all move up-and-down the same amount. Now bump the starter looking at the number one intake and exhaust rockers. Right at the point the exhaust is just closing and the intake is just opening, you should be at top dead center on the timeing mark. Now do a compression test. You didn't mention smoke so I will assume not a smoker. With 2 dead cylinders that 427 should make 300. Points ignition? Cleaned or replaced, gapped and timed?
I had a 69 400 horse a few years back......it too ran pretty gutless for a 440 ci engine (.060 over).
I set total timing to 38 degrees and that REALLY woke it up.......a few weeks later I cut off the 2" factory exhaust and replaced it with a chambered system from Wilcox 2 1/2" system......this freed the exhaust side up so much the idle decreased 200 rpm with no other changes.....
Now that the engine was happy....the end carbs would actually pull and it was a rip on the interstate.....3.08 gear.
So I advise you to replace your exhaust.....it may or maynot be the problem but the poor engine is just choked with the factory system. A pair of headers will reap even more benefit...but with it being so valuable, you may not want to go that far. The L-71 has a lot more cam timing than the 400 so it just exasperates the problem. Why GM did this is a mystery to me and probably the biggest flaw of these cars back then, even the L-88 had 2" exhaust! The factory 2" system can't support the 350/300 much less a 427/435 and it is basically the same system.....and as another bizarre fact....the 68's had 2 1/2"!!!
With the timing you need a good curve in it too.....install a curve kit from Moroso or Mr. Gasket.......if you have a dial back timing light you will be able to see if the timing comes "all in" by 3000 rpm. Set the dial for 38 and rev it to 3k......it should strobe zero on the balancer and fall off when you let off the throttle.
From there you need to verify that your end carbs are opening......you can put a paper clip on the end carbs vacuum pulloff rod.....go run it WOT and then inspect to see if the paper clip moved.
From there a typical compression test and leak down can rule things out too.......but if it runs smooth like you say and does not smoke.....this is probably ok.
So timing,exhaust,end carbs......
Let us know what you find....
Jebby
Last edited by Jebbysan; Feb 24, 2019 at 05:23 PM.
My 435 runs perfectly and the only suggestion I can add to the good advice you've already have been given is to check and make sure your gas pedal is giving you 100% throw. I do have Hooker super comp headers and side pipes.Hang in there it'll be worth the effort.
My 435 runs perfectly and the only suggestion I can add to the good advice you've already have been given is to check and make sure your gas pedal is giving you 100% throw. I do have Hooker super comp headers and side pipes.Hang in there it'll be worth the effort.
Great advice on the WOT......
I bet that 435 loves those headers.....I bet it sounds great too!
First check is make sure the end carbs are opening at WOT.... Second check would be the timing (total timing at 3500 or so).... THEN start digging into stuff. I would bet your secondary carbs are not opening all of the way.
We have a 69' 427/400 car and the tri-power end carbs only open about 1/2 way at wot, and they were "professionally" restored.... Ours has a nom bastard motor in it that floats valves by 5200...Its a total dog as well... I haven't dug into it too much yet though, my dad just creeps it around town and is content with it. LOL..
I did a Chassis Dyno on my L71 when I first got it (3 years ago), 281HP with factory timing (my cam lobes/lifters were worn 0.015, discovered later) and Super Comp side pipes. The other posts pretty much cover all of the easy checks, I actually used a USB Bore Scope Camera to see my Secondaries open (I went through the same steps as you will, it's just that my Motor was original and was down on power due to age). I ended up building it up as a 460 (now 481 RWHP) and found that the old Hooker Mufflers were a major choking point for the new Motor due to small diameter and rust).
To answer some of the points with what I do know...
Yes its a numbers matching authentic L71. Yes the fr/rr carbs open but I don't know how much and seemingly not until 4500-5000 rpm.
I never checked ignition timing but I will. Yes the pedal does achieve WOT. The exhaust riser is moving, maybe I should lock it open but I don't know which way open is (against the spring when cold?)
Can I ask some specific questions - the car has a 'warm' cam in it (I have the specs somewhere if thats important) otherwise its stock.
- What intake vacuum should I see and where should it be ported from to be accurately measured?
- My distributor is stock with an Ignitor 2, it has a vac advance can but its not hooked up (its blanked) - is this correct and if not where should it be ported from?
- What advance curve should I be looking for an L71 (generically would help at least)
- Can anyone confirm the correct take off position for the centre carb vacuum connection to the F/R carbs, and the correct take off position for the centre carb 'bellows' vacuum. (a photo of the underside would really help)
- Could the fact that the car has factory A.I.R be playing any part in this?
- Is there a solution to the ridiculously noisy viscous fan - I can't hear the engine under normal driving, just the fan.
Here is the 'Not so hot' rod in question - thanks
for your help!!
I had the same fan on my 427/400.......Car and Driver in 68' complained about the fan as well! I put the mentioned chambered exhaust on the car and it drowned out the fan......you must be running stock exhaust.
The A.I.R. is a bit of an issue with stock exhaust because it is pushing air into the already taxed exhaust system. My system was removed and I recommend it if you don't need to use it and put it in an air tight box....the A.I.R. systems on these are bringing HUGE money now.
Timing should be 36-38 degrees total....the vacuum advance NEEDS to be hooked up (disabled when checking timing) to constant manifold vacuum. Do not worry about initial timing. The curve should be 38 degrees total all in by 3000 rpm.
One thing to consider.....the L-71 was a high compression engine......maybe the timing was retarded to run on pump gas? Do a compression check on the unit to see where you are at.
Others will pop in about the vacuum ports as it has been a few years since I have had mine.
Good Luck!
I was not able to run pump premium successfully ) if timing was retarded enough to prevent full load pinging then it would diesel, I used a mixture of race gas (110) and pump premium. In stock form my secondaries opened around 4000 (I have manual and 4.11's)
I can post a photo of the Vac. lines this weekend if no one else posts up.
One thing to keep in mind on the Secondary operation, the line from the center carb only provides some of the Vac. signal, the rest comes from ports in the Secondary Bore so if you have an exhaust restriction or some other problem preventing the engine from moving enough air the Secondaries won't open as quickly or as much (I also believe a low restriction Air Cleaner can affect the operation, I'll be testing this once things warm up here, I run my foam dry rather than oiled as GM specs.)
I had the same fan on my 427/400.......Car and Driver in 68' complained about the fan as well! I put the mentioned chambered exhaust on the car and it drowned out the fan......you must be running stock exhaust.
The A.I.R. is a bit of an issue with stock exhaust because it is pushing air into the already taxed exhaust system. My system was removed and I recommend it if you don't need to use it and put it in an air tight box....the A.I.R. systems on these are bringing HUGE money now.
Timing should be 36-38 degrees total....the vacuum advance NEEDS to be hooked up (disabled when checking timing) to constant manifold vacuum. Do not worry about initial timing. The curve should be 38 degrees total all in by 3000 rpm.
One thing to consider.....the L-71 was a high compression engine......maybe the timing was retarded to run on pump gas? Do a compression check on the unit to see where you are at.
Others will pop in about the vacuum ports as it has been a few years since I have had mine.
Good Luck!
Jebby
When I got my 69 427/390 it ran OK but was slow and got about 8 mpg on a good day with a tail wind. Also, my wife could walk all over me with her 78 L82. It was an all original matching numbers car, heads, intake, cam, carb and distributor still with the MS360 vacuum advance (produces 12 degrees) and it was never rebuilt. It was also timed at the stock 4 degrees initial.... A bit of a pig to say the least. I rebuilt the engine and original heads close to stock config with good parts and forged pistons but wanted a bit better cam so used a Comp 270HR as I wanted the hydraulic roller. On tuning the distributor I have the mechanical starting at 900 and all in at 3000. I upped the initial timing to 18 degrees with mechanical advance limited at 18 so 36 total and used a B26 vacuum can which when I tested produced 15 degrees instead of the advertised 16 so I end up with 51 max. That really woke it up. I run it on 91 octane no ethanol with no problem and use the original Quadrajet carb and Winters intake. It still has the factory smog pump. On a "run what you brung" day a couple of years back I managed a 13.08 in stock form with the spare still in the carrier. Cool temps and low density altitude but really made me happy! Also, it now gets 12-13 mpg. Only on the highway of course but who really cares about that.
Last edited by CanadaGrant; Mar 1, 2019 at 11:47 PM.
Poorly L71...Help!
When I got my 69 427/390 it ran OK but was slow and got about 8 mpg on a good day with a tail wind. Also, my wife could walk all over me with her 78 L82.
It was an all original matching numbers car, heads, intake, cam, carb and distributor still with the MS360 vacuum advance (produces 12 degrees) and it was never rebuilt. It was also timed at the stock 4 degrees initial.... A bit of a pig to say the least. I rebuilt the engine and original heads close to stock config with good parts and forged pistons but wanted a bit better cam so used a Comp 270HR as I wanted the hydraulic roller. On tuning the distributor I have the mechanical starting at 900 and all in at 3000. I upped the initial timing to 18 degrees with mechanical advance limited at 18 so 36 total and used a B26 vacuum can which when I tested produced 15 degrees instead of the advertised 16 so I end up with 51 max. That really woke it up. I run it on 91 octane no ethanol with no problem and use the original Quadrajet carb and Winters intake. It still has the factory smog pump. On a "run what you brung" day a couple of years back I managed a 13.08 in stock form with the spare still in the carrier. Cool temps and low density altitude but really made me happy! Also, it now gets 12-13 mpg. Only on the highway of course but who really cares about that.
