r16678

Need some guidance I have a 1979 L48, I cleaned up my engine put a new timing chain on, new oil pump, new water pump, changed spark plug wires, rebuilt the carb. It will start but stalls as soon as I take my foot off the gas pedal. When it is running it will run a bit rough, any guidance will be greatly appreciated. I did verify on the distributor the rotor is pointing to #1 spark plug at top dead center. Thanks in advance for the guidance.

BKbroiler

Make certain your plug wires are set in the right firing order. Looking down at the distrib cap, clockwise from #1 - 1,8,4,3,6,5,7,2

r16678

Thank you I checked that and they are aligned as you described. I will verify that I am on TDC #1 when rotor is pointed at spark plug #1.

r16678

I forgot to mention that I get a back fire back through the carburetor also

r16678

I verified I am at TDC #1 with rotor point to #1

Bikespace

Is the vacuum advance hooked up to manifold vacuum? My 79 did that when I accidentally plugged it into ported vacuum. Once timed correctly (and a bunch of new ignition parts), it ran great again.

You might want to go through the timing steps (look for Lars' instructions) and make sure everything is correct.

r16678

How to check if the vacuum advance is hooked up to manifold vacuum, is the vacuum advance correctly goes from carb to distributor? If so then I did that correctly.

LB66383

What to me sticks out in your description is that you changed the timing chain. Are you absolutely sure you had the sprocket marks lined up when you put it all together? It could be off by one tooth. If that is OK, it sounds like a timing problem. Have you had someone start it and hold their foot on the pedal to keep it running while you checked the timing?

HeadsU.P.

Trace the number 5 & 7 plug wires from the cap to their respective plugs. At no point should those wires cross or run too close to each other in their looms. SBC are prone to crossfire on those two cylinders because of the firing order.

REELAV8R

The rotor should not be pointing at #1 wire when cylinder #1 is TDC if you have the timing advance dialed in.
You should have at least 14* of advance.

calwldlife

as said by others
timing
chain off a tooth

and i would not be surprised if the carb needs fine tuning after
rebuild and chain that is not stretched.

does it run smooth at higher rpms?
what timing did you set it to?
repeating that it is set to tdc is not an answer

Bikespace

On the chance that it is not a timing chain issue:

Manifold vacuum is always on (when the engine is running). Ported vacuum is only on off-idle. You can check with your finger, or a vacuum gauge, but a sure location for manifold vacuum is the port on top of the intake manifold between the carb and the distributor, which is likely running to your brake booster. Your carb should have both manifold and ported ports on it, and a Q-Jet diagram (if that's what you have), will show you which is which.

Please search the forum for an answer on timing that makes sense to you (I did a LOT of reading before diving in), and good luck!

av81

Recheck firing order to confirm , then run with a vacuum gauge on and notice if the needle is bouncing or holding steady at lowest rpm before it stalls. You most likely just have the firing order wrong .

r16678

Thanks I do remember getting the sprockets lined correctly, kit included both sprockets with identifier to line them up. Will try have someone else start it and then hold down the gas.

r16678

I will check that

r16678

Thanks do you know how to read the timing tab on a 1979, there is no numbers.

r16678

No it runs rough, I just followed the basic of setting 10-12 degrees before TDC when plug 1 at the distributor is pointing at plug 1.

r16678

I checked the firing order I have it correct. I will go back and check for vacuum leaks

calwldlife

thanks for the info.
at this point it is a hmmmm.
rough at all rpms
you both redid carb and chain.

checking the chain sprockets is a lot of work.
guess i would rule out carb.
vacuum line runs, specifically pcv.

my gut is pointing to timing aka chain since
it is rough at all rpms.

r16678

Going to check the vacuum lines. I am not sure if I am reading the timing tab correctly, please see attached image. Which marker represents 12 degrees before top dead center. I do not know how to read the increments and there are no numbers on the tab.