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Spark Advance, Timing and Air/Fuel mixture Ported or Full Manifold Vacuum

Chief Brody

Chief Brody

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Can't get it out of the driveway...sputtering so bad when i put it in gear. Idles fantastic though.
 
I've been there! Maybe you upset the fuel pressure. Did you accidentally move a fuel line and rupture it? "Undo" the flare at the end of a hard line to cause a poor seal? Any smell of too much fuel or visible leak?
 
When you set the timing, did you plug the vacuum advance hose? What's the timing set at?
 
  1. I hooked up a vacuum gauge to the intake manifold
  2. I tried to set the timing first before I messed with air / fuel screws and thought I had it
  3. I removed the spark advance from the carburetor and plugged the carburetor port. I did not plug the spark advance tube
  4. I had to guess at the RPM because I dont have a tachometer so I set it to 12* BTDC with the timing light at idle (whatever RPM that is)
  5. I could hear a miss with that, not bad but it was there and very rhythmic
  6. I decided to rev up the engine and try to get close to 2500 RPM to set a "total timing" to 36* BTDC, I did that and it seemed to run good. It sounds great at idle
  7. When it idled good and revved up smooth I went to dialing in the air / fuel mix screws. I dialed them all the way in and backed out till I got the best vacuum reading
When I was done the vacuum gauge is steady in the green (good) area. The engine has great vacuum.

Time to test drive and i put it in gear and it sputters badly. Out of gear I can floor it and it sounds pretty good.
 
I dropped the timing light after all this and I am ordering one with a builtin tachometer. I don't know how to get back to where it was because I have no idea what RPM it is idling at now.
 
GoGoGirl said:
I've been there! Maybe you upset the fuel pressure. Did you accidentally move a fuel line and rupture it? "Undo" the flare at the end of a hard line to cause a poor seal? Any smell of too much fuel or visible leak?
Click to expand...
No, it's not that, but thanks for the input. I got metal lines.
 
I'm guessing that RPM is pretty well tied to timing? I need to know that I at 750 RPM to set the timing to 12* BTDC?
 
Chief Brody said:
I plugged the carb port, not the vacuum advance hose...sheesh..maybe I should have
Click to expand...

Plugging the port on the carb is correct.

Sounds to me like it is really retarded(I have been told it is politically incorrect to say that). Also check that the TDC mark is correct with the number 1 piston location. I have seen cap loads of balancers installed with the incorrect locator position.
 
I watched video for a week on this...I thought I had it.

The carb adjusting tool kit I got , the socket isn't deep enough to really get a good fit...
ktmoutfront said:
Plugging the port on the carb is correct.

Sounds to me like it is really retarded(I have been told it is politically incorrect to say that). Also check that the TDC mark is correct with the number 1 piston location. I have seen cap loads of balancers installed with the incorrect locator position.
Click to expand...

I'm the one that set the timing on this engine originally...LOL...it run good for a long time. I didn't have a tach then, so I guess it was just a guess then. The balancer hasn't moved since then...warbles a little or the pulley does...but still in the same position
 
I would start with rotating the distributor the opposite direction you adjusted it. Drive it and see what happens.
 
RPM is directly tied to timing as timing increases as RPM goes up. If you guess and have the RPM too high, your base timing is probably off.

Somebody correct me here, but timing set with an auto needs to be done with the trans in gear, park brake set in addition to pulling the vacuum line off to the advance can and plugging the port on the carb. If you set it with the trans in park, you aren't taking into account the load of the torque converter. Ever notice the engine rpm drops slightly with the trans in gear?

I'm thinking your mixture screws are set based on the vacuum reading you got. But you need to go back and reset the timing. With a tach so you know. Some guys can go by ear and dial it in, but I prefer to set it with a tach and the timing light. I like my dial back timing light so I can dial it to the value I want and move the dizzy until the timing light shows the mark at the zero mark. Much easier to see at the zero mark as the stock tabs usually don't go past 10 or 12 degrees.
 
Chief Brody said:
No, it's not that, but thanks for the input. I got metal lines.
Click to expand...

As do I, and I must have overtightened the nut(s) at the carb and caused the end of the last piece to damage its little flare. Ended up leaking like a sieve.
 
If you have a factory distributor max advance can be as high as 3600+ rpm. I'd bet your idle timing is way advanced since you set it at 36° at 2500. Way advanced will make it pop and buck under load at low rpm. Set the idle at 10° advance and take it for a drive. Id bet it'll run much better.
 
What is the best idle RPM to do timing?

What is the best RPM to do "total timing?
 
I don't have a 74 manual, but I looked it up on Prodemand. Here's the specs:
Application Man. Trans. Auto. Trans.
350"2-Bbl. 0°TDC 8°BTDC
350" 4-Bbl. Fed. Light Duty
Suburban 6°BTDC 12°BTDC
G-20 & 30 Sportvan 8°BTDC 12°BTDC
All Others 8°BTDC 8°BTDC
 
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