L-72 tune up
 

Well my COPO runs!!but.........its very sluggish on the bottom side of the RPM range. I have 12 degrees intial advance but it doesnt seem to want to run at anything below 1200 RPM and as soon as any load is on it it want to stall!! Anyone have any tune up specifics I can use to help this monster run any better?? Jeyy Macniesh did the distributor and it is set up for a L-78 motor not a L-72 if that makes any difference. Cam is a solid roller .575 intake and .597 exhaust 112 degree lobe center..not real radical..I can get the cam card later if needed. Thanks

ALbert

Re: L-72 tune up
 

Al, I would disconnect the vac Adv and set the total timing to 38 degrees. Hook the VA back up and see where it is then.That is how Jerry had me do mine. I had to the change the Vac canister, but it runs good now.

Re: L-72 tune up
 

Sounds like my car when I first fired it up!

It's hard to diagnos remotely, but I had to set my idle high because the mixture screws were off - K.Marple fixed that up for me, big thanks. My car wanted to fall on it's face when I stabbed the gas because of the idle circuit, so I installed the white pump cam in hole #2 (I think), and that cleared up the off-idle stumble.

Make sure your plug wires are correctly routed, I made that mistake once too - a few guys from Kansas helped me correct that mistake at Englishtown https://www.yenko.net/ubbthreads/imag...mlins/grin.gif Also, simply make sure they are all connected, I had one drop off last year at SCR8 - couldn't figure out why I went from 14.2's to 15.5's!

I agree with Tony on the distributor, yank the vac adv hose, and set the timing for 38 degrees at around 2000 rpm. That should eliminate the distributor as a major culprit, and then you can focus on other items.

Re: L-72 tune up
 

Run nthe rpm up high enough to achieve max centrifugal advance, then check the timing and set for about 36 degrees without any vac advance. You'll need a decent dial back light to get an accurate reading. Test vthe car, pull a couple of plugs and check for any detonation, and finally make any minor timing adjustments to the total timing. Then connect the vacuum advance and test again for detonation.

Re: L-72 tune up
 

I like to work backwards, start the tune for good idle, tip in response, and midrange, then recurve the mechanical advance for the correct total timing.

Most of the cars I've tuned like a LOT of initial, assuming you're not ping limited due to too much compression. My car runs GREAT with 18 degrees initial and another 18 from vacaum on full manifold. That's right, idling with 36 degrees in it!

As long as it will crank hot, and not ping at part throttle with the vac advance unhooked, you don't have too much initial. If it pings at part throttle with vac advance, get an adjustable canister and tune it out.

Continually work the idle speed, idle mixture, and then power valve / primary metering rods as you do this.

Jim

Re: L-72 tune up
 

Post the cam card...being it's not the GM cam, and that it looks fairly larger than the original 143, it may just be the cam is weak down low because of the cam timing/duration and what not...

Re: L-72 tune up
 

albert.....all that work on a real car and you put in a solid roller???? its all about the combination. factory intake?? factory holley?? points distributor?? exhaust manifolds?? it may be a small roller, but its never gonna work well with all the factory L-72 stuff.you need a good curve in the dist.as much initial as possible,with total being at least 36-38 degrees.stick a MSD distributor in it and paint it black.nobody will ever notice,and forget the vacuum advance,not needed.are you running a correct holley?? if so put it on the shelf,and throw on a regular 4779 750 dp.drill the rear throttle blades and it should idle at 800 rpm's no problem.is the car an auto ?? stock converter??that could be a problem. post some more info on the combo.between craig and i we should have enough parts laying around to try. car looks great by the way!!!

Bob Gonier

Re: L-72 tune up
 

Rob, .575 intake .597 exhaust..110 lobe center....358, 364 @.050 duration.

Bob..come on..its a stocker!!:D factory intake, points, carb etc..got to make it work!. The key is I dont have alot of time playing with it yet..hopefully this week will generate some rainey dasy so I can play with it. I was also using a POS timing lite for the initial fire up and that will be changing as well. Just want to have some idea of where to look and what to play with. Lots of great ideas ..thanks to everyone who replied.

ALbert

Re: L-72 tune up
 

Al, what is the LSA (lobe seperation angle) on the cam? Also, recheck the duration, and if the card lists both, post both the advertised and @ .050" or .010" or whatever numbers. That roller has a lot more duration than the factory 143...I'm gonna guess it has a tight LSA, which is gonna hurt anything down around where you're wanting to put the idle...

And a word of advice, don't go swapping carbs/distributor just yet...I've heard *rumors* of folks getting those factory parts to work "ok"... https://www.yenko.net/ubbthreads/imag...ns/naughty.gif

Re: L-72 tune up
 

[ QUOTE ]
Al, what is the LSA (lobe seperation angle) on the cam? Also, recheck the duration, and if the card lists both, post both the advertised and @ .050" or .010" or whatever numbers. That roller has a lot more duration than the factory 143...I'm gonna guess it has a tight LSA, which is gonna hurt anything down around where you're wanting to put the idle...

And a word of advice, don't go swapping carbs/distributor just yet...I've heard *rumors* of folks getting those factory parts to work "ok"... https://www.yenko.net/ubbthreads/imag...ns/naughty.gif

[/ QUOTE ]

OK Rob, I have the cam card in front of me here are teh specs:

Lift at valve .575 intake .597 exhaust
lash .024 intake .026 exhaust
duration at .050 256 intake 264 exhaust
advertised duration 296 intake 300 exhaust
exhaust closes 18 Deg ATDC
exhaust opens 66 Deg BBDC
intake opens 18 Deg BTDC
intake closes 58 Deg ABDC
Lobe separation 112
Center line of 110 Deg will make the cam 2 Deg advanced

And no worries about the "stock" parts leaving anytime soon..I WILL make them work. I can build the car just have not alot of experience with tuning them..a new learning curve for me. Thanks for the help.

Albert

Re: L-72 tune up
 

I have to agree with Rob on the LSA thing - I too tried a 'better' cam when I built my engine, and kept all else stock. I could not get that thing to idle at less than 1300+/- RPM because of the tight cam.

I then took it out, put in the stock cam that I bought from my local dealership, and was able to get the engine tuned through conventional means - with the help of others https://www.yenko.net/ubbthreads/imag...mlins/grin.gif

Now, it's a solid 14.2 @ 99MPH runner, looking to get another tenth out of it at SCR9 https://www.yenko.net/ubbthreads/imag...iggthumpup.gif

Re: L-72 tune up
 

With that ET,what RPM does it idle at now Marlin?

Re: L-72 tune up
 

Cam was selected as it was suppoed to be close to the stock L-72 cam but be a roller..machinist is very knowledgable and has done alot of motors for me. Going to try the dialback method when I get some time to play. I have had bigger cams work with stock components..this one will just require some time..and I'm learning as I go https://www.yenko.net/ubbthreads/imag...iggthumpup.gif Thaks for all your inputs

ALbert

Re: L-72 tune up
 

[ QUOTE ]
With that ET,what RPM does it idle at now Marlin?

[/ QUOTE ]

The car idles around 900 - special thanks to K.Marple for that.

At SCR7 I was launching just off idle around 1200rpm, but last year at SCR8 I got my spring clamps installed - 2 per side, and continually raised my launch rpm until the final round where I think I was hitting the tires at around 2000 rpm. There was still a slight bog, without tire squeal, so that tells me that I could have raised it higher. I was a bit timid however, because my car has it's 'born-with' original drivetrain - and I do this for fun, not to get my best ET by breaking something.

Not sure who I'll banter with this year without Frankie https://www.yenko.net/ubbthreads/imag...s/confused.gif

Re: L-72 tune up
 

OK, had some time to play with the motor a bit..here are the results..set timing by ear and test drove..ran good but.....timing mark(flash) is at 12 o'clock instead of on the tabat 2 o'clock!!!!, almost 50 degress out of place!!..any ideas of what I have skrewed up!! Runs great otherwise..HELP!!!!!!

ALbert

Re: L-72 tune up
 

possibly wrong balancer or timing pointer??? if it runs well,doesnt ping, and turns over when hot........crank it up some more!!!! you'll know your close/gone too far when it doesnt crank very fast when hot. good luck....


bob

Re: L-72 tune up
 

If it is the original balancer, chances are the rubber has slipped some. Did you check where the timing marks are at TDC? That is where I would start checking things.

Re: L-72 tune up
 

Mine was the same way, the balancer was fine, it was just me over compensating for other items that were out of tune! The real test was a shot down the 1320, what felt fine on the street was like a dog on the strip https://www.yenko.net/ubbthreads/images/graemlins/no.gif

Re: L-72 tune up
 

well, checked the marks at TDC and the balancer mark is dead center of the tab..I rember this from assemble but double checked anyway..balancer mark is in direct line with the keyway (correct?)..gonna check the dwell next then pull out my hair https://www.yenko.net/ubbthreads/imag...mlins/grin.gif..have some time tomorrow PM and will go at it again. Thanks everyone.

ALbert

Re: L-72 tune up
 

Are you checking the timing with the vacuum advance disconnected?

Disconnect the vacuum advance and set the total timing to 36*-38* at 3000 rpm. Then jazz the throttle just a little more to make sure the timing doesn't continue to advance. If it doesn't, lock the distributor down, re-connect the vacuum advance and take it for a test drive. If it does, you may want to look into having your distributor re-curved to bring the advance in by 3K.

HTH.

Eric

Re: L-72 tune up
 

It is possible to have 18 degrees, or more initial, and drag another 15 degrees from the vacuum advance at idle, yielding 33 or more initial. This would move the timing mark right off the tab.