Low Vacuum Issue/Big cam

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68 Formula S383

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I'm an old school MOPAR guy - Pre-1970 with no experience with the Vacuum accessories.

1974 Dart Swinger, power assist disc brakes, 318 2-bbl motor, electronic ignition w/vac advance, 904 stock converter origninally.

Last weekend, I installed new Mopar 2x roller timing set, 284/484 purple shaft hyd. cam, (Degreed straight up) 360 J heads pocket ported, cut .060" comp face and .020 intake face (2.02/1.60), torquer 360 manifold and Edelbrock 625 carb. Timing set at 10* advanced with vac. adv. plugged, goes to 20* adv. when hooked up. I cannot get it to idle consistently below 1500 rpm. It will idle fine for about 30 seconds then starts to drag down and eventually dies. I replaced all vacuum lines, verified that there are no engine vacuum leaks. The vacuum amplifier is still in the system and holds vacuum. Is there a vacuum switch that I'm missing or is there a trick to getting enough vacuum to run?

I have not checked the actual vacuum reading as the car is at the muffler shop getting piped from headers to mufflers.

Thanks!

I've run this set up in several pre-70 BB and SB cars without problem.
 
Where do you have your vacuum advance line hooked up. Sound like it's direct vacuum. You need it on a ported advance nipple. It should have little or none in the way of vacuum through the line at idle. What will happen with a direct vacuum source, the vacuum signal drops and the advance goes away, it acts like a dog off the line

IMHO, 10* initial is not near enough with a 484 cam. Should be in the 15-20 range, maybe more. Take the vacuum advance line off and plug the port at the carb. Use a vacuum gauge to tell you what the engine wants. Picking a number and trying to make it work isn't the best way.

read this for setting initial with a vac gauge.
http://www.forabodiesonly.com/mopar/showthread.php?t=56694
 
take the vaccum advance off do not think you need it with this setup will not hurt anything....my opinon only i run a simular setup just fine have fun go mopar...
 
Vacume advance is for part throttle,gas milage.if you just put around not likly with your set up leave that vacume advance hook up.Do you have a stall converter in there a 3000 stall would take the load off your motor alow it to idel better.You dont want it to idel at 1500 it will run rich.Good luck.I hope your cam works out thats the lift I plan for my 318 than 360 next winter.
 
Did you degree the cam? What vacuum does it pull? What was done to the distributor? What is the cylinder pressures?

edit.. oops. read too fast..lol

Without knowing the cylinder pressures... It sounds like you have the vacuum can plugged on the wrong port on the carb. I would remove the line from the dist can and plug it for now. Add initial to get 16° initial. Slow the idle down to 800 or less and rebalance the mixture screws. You will need the strip kit for the carb, because the step up springs are probably wrong. You will need to curve the distributor and set up the vacuum can, but it should bea decent package, even if it's a little soft under 2K.
 
Good ideas from all of you. Thanks! I'll check vacuum with gauge as soon as I get the car back from Muffler shop tomorrow and report back.

Mopar or No Car!
 
Summit or any local speed shop should have them. Not cheap, but a required purchase if you have anything hopped up.
 
68 Formula S383 said:
I checked, the vacuum advance is plugged into the timed vacumm port on the carb/
Click to expand...

Ok, no vaccum leak just low vacuum (7.5") and an Edelbrock carb. Switched to the blue springs and it idles good and runs like a scalded dog. Thanks to all who took the time to respond!!

Mopar or No Car!
 
Check the plugs too. That carb may be a little lean for that combo. Congrats on the tuning bit.
 
68, back in around 1980 we used that cam in our stock 340s. It ruined everything about them. On top of screwing up vacuum and idle and driveablty at lower engine speeds, it slowed them down as we refused to gear them lower and shift them higher than 5800.
Hate to say it but I'd yank that cam out and use a stock 340 cam or a new generation cam from a reputable source, MOPAR has some I think now that work well. That cam is too big for any sort of low end even with a high compression 340 and even a 360. Hate to see how a 318 feels with one. Not saying it is bad, just saying it is not too good for street action or where you want to retain some semblance of driveability, idle, low end torque and not to mention, fuel economy.
My opinion, I am sure others have had better results.
 
VOETOM said:
68, back in around 1980 we used that cam in our stock 340s. It ruined everything about them. On top of screwing up vacuum and idle and driveablty at lower engine speeds, it slowed them down as we refused to gear them lower and shift them higher than 5800.
Hate to say it but I'd yank that cam out and use a stock 340 cam or a new generation cam from a reputable source, MOPAR has some I think now that work well. That cam is too big for any sort of low end even with a high compression 340 and even a 360. Hate to see how a 318 feels with one. Not saying it is bad, just saying it is not too good for street action or where you want to retain some semblance of driveability, idle, low end torque and not to mention, fuel economy.
My opinion, I am sure others have had better results.
Click to expand...

I agree, 284* of duration is WAY too much for any 318 that will be intended to see ANY street use. Max I'd go with a 318 would be 268* or so for a street car, which is still more than enough.
 
I agree with the two people who said that cam is too big. Too much duration @ .050", especially for the lift. Yank it out, sell it and put in a Lunati Voodoo 268 cam. It's time people started realizing that MP is resting on it's 30 year old laurels with regard to cam design.

While you're at it, get rid of the Edelbrock carb and put a Holley 670 street avenger on it.
 
68 Formula S383 said:
I checked, the vacuum advance is plugged into the timed vacumm port on the carb/
Click to expand...

Because you don't have enough initial timing you likely have opened the throttle blades enough to uncover the timed port and that's why you are getting a timing change with the advance hooked up.

The 15-20+ initial timing recommended previously will allow you to turn down the idle. But you will also need to limit the total mechanical at the top end by either using an adjustable distributor like the Mopar Performance unit or an aftermarket one or weld up the slots in the stocker.

First step in tuning a motor is to get the timing dialed in before touching the carb.

Also, the 74 318 is a low compression motor and even with shaving the heads you still may not have enough static compression to run vacuum accessories with that cam.
 
dgc333 said:
Because you don't have enough initial timing you likely have opened the throttle blades enough to uncover the timed port and that's why you are getting a timing change with the advance hooked up.

The 15-20+ initial timing recommended previously will allow you to turn down the idle. But you will also need to limit the total mechanical at the top end by either using an adjustable distributor like the Mopar Performance unit or an aftermarket one or weld up the slots in the stocker.

First step in tuning a motor is to get the timing dialed in before touching the carb.

Also, the 74 318 is a low compression motor and even with shaving the heads you still may not have enough static compression to run vacuum accessories with that cam.
Click to expand...
This is about the best answer so far. You need to run more initial, for a cam this big in a motor this small with relatively low compression, you will need about 18 deg. Alot of us talk about more initial on here, but no-one ever explains how to get it, we tend to assume you know, and that may not be fair. You have to disassemble the distributor, and add material to the outer ends of the advance slots. This is a trail & error deal. You can start by adding about 3/16-1/8" of material to the end of the slot. The idea is to add more than you need, then test it, grind a bit out, test again, till you get it where you want it. I recommend gas welding the end of the slot, as the weld will be a little softer & easier to work than mig. You need to have a timing tape or degreed balancer on the motor. Plug the distributor back in the motor,leave the vaccum advance off for now, set the total timing at 2500-3000 rpm to around 35 degrees and let the engine drop back to idle, check the initial timing. With the slots welded up, it will probably be over 20 degrees. Disassemble the distributor, grind a little material out of the slots you just welded up, put it back together & repeat the process until you can get 34-35 degrees total with around 18 degrees initial. At this point, go back and re-set the idle speed/mixture screws.
Next, you are probably gonna have to go into the carb. If you have the initial where it should be, but the engine is still unresponsive to the idle mixture screws(pretty likely with this cam in a 318) the throttle plates are still too far up into the transfer slots, and the engine is not idling on the idle circuit. At this point, you need to go in and drill a small hole in each primary throttle plate, just ahead of the the throttle shafts. Start around .080 (on this combo, you will probably be good to go at .080) and work your way up, until the engine is able to idle with the throttle plates almost closed.
By now, this thing should be idling pretty nicely. The 108 lobe separation is an ADVANTAGE with the 318. It will help increase cylinder pressure relative to the wider lobe center cam, and it will pump up the mid-range torque. You just have to know your tuning basics. Big cams are not for the "bubble-pak" crowd, you have to have the skills to tune them properly.

This stuff is ALL trail & error, DONT just go in and drill the holes in the carb, do it in the order suggested, work slowly and methodically, step by step, and not only will your car run better, but you will really be learning how to tune properly, which will put you way ahead of the vast majority of guys out there that are jsut bolting parts together & hoping for the best.
 
george said:
This is about the best answer so far. You need to run more initial, for a cam this big in a motor this small with relatively low compression, you will need about 18 deg. Alot of us talk about more initial on here, but no-one ever explains how to get it, we tend to assume you know, and that may not be fair. You have to disassemble the distributor, and add material to the outer ends of the advance slots. This is a trail & error deal. You can start by adding about 3/16-1/8" of material to the end of the slot. The idea is to add more than you need, then test it, grind a bit out, test again, till you get it where you want it. I recommend gas welding the end of the slot, as the weld will be a little softer & easier to work than mig. You need to have a timing tape or degreed balancer on the motor. Plug the distributor back in the motor,leave the vaccum advance off for now, set the total timing at 2500-3000 rpm to around 35 degrees and let the engine drop back to idle, check the initial timing. With the slots welded up, it will probably be over 20 degrees. Disassemble the distributor, grind a little material out of the slots you just welded up, put it back together & repeat the process until you can get 34-35 degrees total with around 18 degrees initial. At this point, go back and re-set the idle speed/mixture screws.
Next, you are probably gonna have to go into the carb. If you have the initial where it should be, but the engine is still unresponsive to the idle mixture screws(pretty likely with this cam in a 318) the throttle plates are still too far up into the transfer slots, and the engine is not idling on the idle circuit. At this point, you need to go in and drill a small hole in each primary throttle plate, just ahead of the the throttle shafts. Start around .080 (on this combo, you will probably be good to go at .080) and work your way up, until the engine is able to idle with the throttle plates almost closed.
By now, this thing should be idling pretty nicely. The 108 lobe separation is an ADVANTAGE with the 318. It will help increase cylinder pressure relative to the wider lobe center cam, and it will pump up the mid-range torque. You just have to know your tuning basics. Big cams are not for the "bubble-pak" crowd, you have to have the skills to tune them properly.

This stuff is ALL trail & error, DONT just go in and drill the holes in the carb, do it in the order suggested, work slowly and methodically, step by step, and not only will your car run better, but you will really be learning how to tune properly, which will put you way ahead of the vast majority of guys out there that are jsut bolting parts together & hoping for the best.
Click to expand...
Sorry about the stupid happy face thing, that SHOULD read "with this cam in a 318," close parentheses. Stupid computers!
 
68 Formula S383 said:
Ok, no vaccum leak just low vacuum (7.5") and an Edelbrock carb. Switched to the blue springs and it idles good and runs like a scalded dog. Thanks to all who took the time to respond!!

Mopar or No Car!
Click to expand...
Re: Mopers comments about the step-up spring. Do the other stuff first, as getting the initial right and getting the carb working right will have a BIG effect on the manifold vaccum. ONCE this is all done, if there is STILL not enough vaccum to hold the power piston down against the spring, THEN you need to look at messing with the step-up spring. Just swapping the spring out may get the metering rods down in the jets, but the manifold vaccum will go up, it'll have more bottom end, & get better gas mileage if its set up properly.
 
george, easiest way to find the initial timing your engine wants is with a vacuum gauge. Keep increasing the timing in small increments while keep the rpm constant until you find the maximum manifold vacuum.

The total mecahnical is a pretty much a function of the design of the cylinder heads and you can use rule of thumb numbers to get real close. Open chamber mopar sb heads like 34-36 total. From this number subtract the initial timing you determined and that is the amount of timing you want in the distributor.

Easier than continuely stabbing the distributor back into the engine and trying is to glue a dime store protractor to an old rotor (center of protractor over center of shaft, then tape an old piece of coat hanger to the housing for a pointer. You can now twist the rotor against the advance and measure the amount in the distributor. Remember that the can turns at 1/2 the speed of the crank so the reading on the protractor needs to be 1/2 of what you would read on the crank.

Ex. if your engine wants 18 degrees initial and you shoot for 35 total you need 17 in the distributor or 8.5 when measured with a protractor.

Also, if you do a search on the web there are folks that have calculated what the slot lengths need to be for a given amount of timing.

Don't forget if the distributor is a factory stock unit the springs are likely going to be too stiff to bring the timing in at the optimal rpm. Again a good rule of thumb for a mopar sb driven on the street is to have it all in around 2500 rpm. MP sells a two spring kit that is two very light weight springs. Replace the stiffer of the two stock springs with one of these light ones and you will be very close to the 2500 rpm point.

Now the vacuum advance; You need to measure what you get for manifold vacuum while cruising on level road without a head wind. You then use a vacuum pump to actually measure what vacuum is needed to stroke the advance mechanism. There is a small allen screw inside the port on the advance mechanism that you can adjust the preload on the spring that keeps the mechanism at its zero point. Adjust so all the advance is in at your cruising manifold vacuum. If you experience detonation on throttle tip in you may need to back off on the vacuum advance to elminate it.
 
The protractor tip is a good one. Thanks. Honestly, I have been doing this for a while, usually get it pretty close first try, but its nice to have a way to check before you put it all together. Good tip. Thanks
 
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