Rebuilt engine lacking power

[cadillac_al]

Everything sounds perfectly fine to me. At this point i would be double checking compression (valve lash issue), fuel pressure and the fuel filter. I still think it is lack of fuel. Weak fuel pressure will show up in high gear first. I wonder if the builder changed the jets or needles. It is the original carb for the truck right?

[chris989]

andy is right, check your timing at idle, and through the rpm range and make sure it is advancing.


disconnect the vacuum advance for now and plug it until you solve this problem.

what is the CC volume of your heads? what is the static compression?

also, what is the valve separation on this cam. 214 @050 288 advertised is a long ramp. Maybe it just needs a lot more timing than your distributor can deliver? I would make sure you are in the ~35 degree range at 3000.

My lunati is Advertised Duration (Int/Exh): 262/268
Duration @ .050 (Int/Exh): 220/226

[MJN]

So, I did some digging and found out the distributor (Mallory 8548201C) came with an adjustable vacuum advance. The instructions that came with the distributor did not state this or how to set it, however I found a revised version online that stated how to adjust the vacuum advance. I also adjusted the secondary air valve spring tension and bumped timing to 12-degrees BTDC. This helped and feels much more responsive now. The carb is the correct one for the truck, I did have the builder set it up a bit differently than stock though. I went with a level 2 build, https://quadrajetpower.com/our-quadrajet-build-process/ Right now I only know base timing, not total timing.

These are the spec on the cam I'm running.
Basic Operating RPM Range: Idle-4,500
Intake Duration at 050 inch Lift: 204
Exhaust Duration at 050 inch Lift: 214
Duration at 050 inch Lift: 204 int./214 exh.
Advertised Intake Duration: 278
Advertised Exhaust Duration: 288
Advertised Duration: 278 int./288 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.420 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.433 in.
Valve Lift with Factory Rocker Arm Ratio: 0.420 int./0.433 exh.
Lobe Separation (degrees): 112

The distributor came with different springs, per the chart how do I know which ones to choose?

[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!

[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.

[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.

[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.

[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]

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.

[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?

[MJN]

Sure hope its accurate, thats how I assembled the engine, TDC with the balancer lined up on zero.

[geezer#99]

Before you put the heads on did you set #1 at tdc with a piston stop and then install or mock-up your balancer and timing tab?

[MJN]

Just followed the How-To Rebuild Your Small Block Chevy Book, step by step during assembly. I can't remember exactly off hand at the moment the sequence of events, I also had assembled it over a year ago now.