Rebuilt engine lacking power

[geezer#99]

I wouldn’t be changing any springs until I power timed it like in the instructions.
Then adjust your dizzy mechanical timing.
Who knows, maybe your motor will like 16 or 18 initial.

[chris989]

Vacuum advance,,,,,

Not sure about yours but with my MSD, the vacuum advance was 20`. I had to make a bushing to limit the vac adv to 10. (AMC motor)

Back to basics,,, as the engine speed increases, the timing must fire sooner and sooner to produce max power at ~TDC.

Do you have an adjustable timing light? If yes, set the dial to 32 and as you rev the engine, see if the mark comes back to TDC at the balancer.

Find out at what RPM your timing is at, 2000, 3000, 4000, etc

[andyh1956]

On your chart start with the left side.
I think those numbers are distributor numbers for different OEMs. Yours should be 31021. The different color springs for your dist are in the next column, gold, silver & black.
In the curve start column it lists the rpm which the springs start to stretch & the mechanical timing advances. The gold springs start to stretch at 400 rpm & silver & black start at 600. (Why the Hell they would want the springs to advance at a speed lower than idle??? Go figure)
In the last column it tells you at what rpm the timing stops advancing with the color spring you're using.
It's a trial & error procedure but completely doable.
However it's harder on a gm engine with the limited degree scale of the pointer.
It's better to set it up like a Ford w/ a simple pointer & a timing tape on the balancer.
I know you must be confused by now!

[MJN]

Sounds like I need to get a timing tape for the balancer and go from there. Also, the vacuum advance vacuum source on the carb is ported, so no vacuum at idle. Long weekend out of town coming up so won't be much progress after today. This stuff is new to me, never messed with adjustable vacuum advance or changing advance springs before, if you couldn't tell already. Thanks for the help!

[Desert1957]

Timing tape is a good idea, If you can find an older timing light with the adjuster knob makes it easier.



Using the Advance Timing Light to Measure Total Timing
The advance dial feature also allows you to check the total timing. Total timing is base timing added to the vacuum added advance or computer added advance. To calculate the total timing of your engine, rev the engine to 3,000 RPMs and maintain it there. Now, when you aim the timing light at the timing marks, you can no longer see them. Rotate the dial on the timing light until you see the timing mark begin to recede on the timing tab. The further you turn the dial, the closer the timing mark comes to the 0 degree mark. Advance the dial until the timing mark is sitting at 0 degrees and then look at the dial. The number you now see indicates total timing.

[chris989]

Originally Posted by MJN

Sounds like I need to get a timing tape for the balancer and go from there. Also, the vacuum advance vacuum source on the carb is ported, so no vacuum at idle. Long weekend out of town coming up so won't be much progress after today. This stuff is new to me, never messed with adjustable vacuum advance or changing advance springs before, if you couldn't tell already. Thanks for the help!

You want the vac hooked to manifold vac. Vac at idle, no vac at wot.

read this, it will help you understand the ins and outs,

http://www.superchevy.com/how-to/add...nition-timing/

[MJN]

Originally Posted by chris989

You want the vac hooked to manifold vac. Vac at idle, no vac at wot.

read this, it will help you understand the ins and outs,

http://www.superchevy.com/how-to/add...nition-timing/

So based on that article the advantage of constant vacuum advance at idle is running cooler at idle. The advance still does it's job plumbed to ported or constant vacuum during cruising.

[Dead Parrot]

Once the throttle is advanced enough to uncover the port source, pretty much the same.

Another benefit of manifold vac advance is lowered risk of running off the cruise part of the carb while at idle. A symptom of being on the cruise part of carb at idle is the idle mix adjustment has little effect on idle speed.

If you want to try manifold vacuum, tie in to the tree behind the carb that comes out of the intake. Be sure to plug the ported port during the experiment.

Ported vs manifold shouldn't make any difference to your originally posted problem as when you are at or near WOT, vacuum advance is out of the picture due to near zero manifold vacuum. That's where your initial + mechanical fully takes over.

[geezer#99]

He doesn’t need to tee into anything, the blue cap under his choke linkage is manifold vacuum.

[Tom]

Originally Posted by MJN

You don't have to understand, its the cam I chose because it fit my application and how I use it. Does it really matter how old the grind is if it works? I'm not racing it.

That wasn't the point. The factory stock cam fit the application and works. You choose an aftermarket grind for more power, torque, efficiancy. Why choose a less efficiant and powerful design compared to a modern one when your already paying for a new cam and doing the work to put it in? Racing has literally nothing to do with it.

[Jake Wade]

40+ year old lobe design or not, that MTC-1 Melling camshaft is proven and still works very well to this day. It is well suited for your application. The camshaft is not the problem and I would choose it over these modern fast ramp. lobe killing, noisy grinds of today.

[MJN]

Originally Posted by Jake Wade

40+ year old lobe design or not, that MTC-1 Melling camshaft is proven and still works very well to this day. It is well suited for your application. The camshaft is not the problem and I would choose it over these modern fast ramp. lobe killing, noisy grinds of today.

I agree

Originally Posted by Tom

That wasn't the point. The factory stock cam fit the application and works. You choose an aftermarket grind for more power, torque, efficiancy. Why choose a less efficiant and powerful design compared to a modern one when your already paying for a new cam and doing the work to put it in? Racing has literally nothing to do with it.

Nothing wrong with the cam I chose, it fits well with what I am using my truck for, the RPM range I am running and drivetrain, period. I'm not changing the cam, it is not the issue. I also chose to run a small block 350, iron heads, iron intake, iron exhaust manifolds, Quadrajet...all are less efficient than a modern LS or Vortec 350, doesn't make it wrong. I'd ask for actual data plots showing how much less efficient and powerful this cam is compared to some other comparable modern grind but at the end of the day it doesn't matter to me.

Anyway, enough about the cam, it is what it is and not changing. I had to order a timing tape for the balancer so waiting for that to arrive and then I can get back to determining what total timing is set at.

[MJN]

Ok, So got the timing tapes finally, they don't last long but of course I had to reposition it a few times to get it in the right spot. My initial timing at idle is 16-degrees BTDC. At 2000 rpm with vacuum advance connected the tape read 22, so 38 total. I unplugged the vacuum advance and reran it at 2000 rpm and the tape read 15. So, 16 initial, 15 mechanical and 7 vacuum, total of 38. Then the tape flew off and wouldn't stay on. Right now the truck seems to respond and have more power on the road than before, especially at partial throttle. I'll see if I can get my hands on an advance timing light next.

[geezer#99]

Yup!
You need a dial back light!
All the numbers you got there don’t jive.
Study this link (or ignore it) and learn about timing your motor.
http://www.badasscars.com/index.cfm/...d=76/prd76.htm

Or power time your motor like your instructions suggested.

[chris989]

Originally Posted by MJN

Ok, So got the timing tapes finally, they don't last long but of course I had to reposition it a few times to get it in the right spot. My initial timing at idle is 16-degrees BTDC. At 2000 rpm with vacuum advance connected the tape read 22, so 38 total. I unplugged the vacuum advance and reran it at 2000 rpm and the tape read 15. So, 16 initial, 15 mechanical and 7 vacuum, total of 38. Then the tape flew off and wouldn't stay on. Right now the truck seems to respond and have more power on the road than before, especially at partial throttle. I'll see if I can get my hands on an advance timing light next.

If you reran the test at 2000 and got the same ~16 BTDC then i would say you have zero mechanical advance. Either the dist is locked out anticipating a controller of some type to advance it or it is just frozen.

Chris

[MJN]

Originally Posted by chris989

If you reran the test at 2000 and got the same ~16 BTDC then i would say you have zero mechanical advance. Either the dist is locked out anticipating a controller of some type to advance it or it is just frozen.

Chris

I have the distributor set at 16 BTDC at idle. The weights are free, not hung up or locked out in any fashion.

[geezer#99]

At what idle rpm did you set your 16 degrees?

[MJN]

Originally Posted by geezer#99

At what idle rpm did you set your 16 degrees?

Idle is set at 800 RPM.

[MJN]

This is from the distributor instruction sheet on how to adjust the vacuum advance. This is where mine is set at currently.

[rpmerf]

Here is what I do.

Measure it at idle, without the vac advance. Sounds like it comes up to ~16*. This is your base timing.

With the vac advance still disconnected, measure it at ~3000 RPM (or whenever it stops advancing), it should read somewhere around 34*. No math, that is what it should read. This reading minus your base reading is what you get from mechanical.

Plug the vacuum advance in. Whatever it reads minus what is was without the vacuum advance is what you are getting from the vacuum advance. So if you are getting 16* at idle without the advance and 22* at idle with the advance, the vac advance is adding 6*.

I figured out my max vac advance by driving cold. When your engine is cold, your choke runs the engine rich, so you need less advance. Richer A/F mixture burns faster. I think I started with about 14* and backed it down to 12* because it was pinging. It's been running perfect this way for ~6 years.

It does sound like you are not getting any mechanical advance.

[geezer#99]

You need to get the idle a little lower to set your initial. You don’t want to see any mechanical timing kicking in. That can happen at 800 rpm.
Was your 16 set with your timing tape or on the timing tab?

[MJN]

Originally Posted by geezer#99

You need to get the idle a little lower to set your initial. You don’t want to see any mechanical timing kicking in. That can happen at 800 rpm.
Was your 16 set with your timing tape or on the timing tab?

Timing tab, just by moving the distributor.

[geezer#99]

I’ve never seen a timing tab that showed more than 12 degrees.
Where you just eyeballing the extra 4degrees?
You got a pic of yours?

[MJN]

This is a picture of the timing tab I have, I found a NOS one to use. Goes to 16 BTDC
GM #3991436
https://paceperformance.com/i-513484...al-finish.html

[geezer#99]

So it does.
Now the ultimate question!
Is it accurate?
Does zero on it align with the balancer mark when #1 is at true tdc?