74 620 L18 timing /carb question

[Mydime]

Hey guys hope I might get a helping hand here. Have searched the internet for hours now and have not found a definitive awnser at this point.

 

I just converted my 74 L18 620 to a Weber 32/36. As well as a upgrade with the matchbox style Dizzy distributor. Changed out the plug wires, coil, and increased the plug gap to accomodate for the hei systems additional spark.

 

Problem: Backfire through carb is the issue. Aslo a slight hesitation while incresing RPM while in park, sometimes the backfire happens during this area of hesitation.

 

Looking at timing at this point but it looks to be dead on at 10 btdc

 

I have found two different procedures on setting timing and they only differ in the fact that one says to leave vaccum attached to the distributor canister while setting, the other states to remove from canister and plug the line....

 

Any help is greatly appreciated.

 

Thanks

[izzo]

i dont know a good answer to your question

 

but when i installed my weber, it wasn't quite seated all the way down even tho it looked like it and it would backfire thru the carb when i tried to run it. thanks to datto510 for helping me figure that out.

 

 

a test you can do is start the car, run it. and spray some brake cleaner around the carb. if it starts to die off then theres a leak. if not then i just wasted your time and brake cleaner :lol: and it will eliminate this being a problem.

Edited September 30, 2009 by h2theizzo

[Mydime]

Well one thing that might be causing this is if I have been reading carefully enough is that the vacuume port on the weber is full vacuume and not ported.

 

If this is true how should I proceed?

 

Cap off vacuume port on carb, set distributor to fire at 12 degrees, and run no vacuume advance?????

[datzenmike]

Check the firing order again 1342. Rotor turns counter clockwise.

 

All carbs before about '70 ran on manifold vacuum so the hose was unplugged to remove any vacuum advance so the static timing could be set. After '70 they were ported and had no vacuum advance at idle so there was no need to pull the hose off to set the static timing. Anything above idle and vacuum is applied to the dizzy. (it's an emissions thing)

 

The carb may be running lean which can cause a backfire.

[datsunaholic]

ehh, yes and no. It depends on where the idle speed screw is set, since on a Hitachi the idle speed screw cracks open the throttle. One of my trucks, the prior owner put 2 vacuum gauges in a pod on the column- one reads manifold vacuum and the other reads distributor vacuum (ported through the primary chamber). Even at idle, and particularly COLD, it always reads a little bit since the throttle plate is cracked open ever so slightly.

 

You can tell with a timing light how much difference it makes- usually not a lot.

[datzenmike]

The vacuum advance canister requires a minimum 3.94" mercury before any advance happens ('78 620 automatic all) and 2.76" mercury for all non California standards.

[Mydime]

I have double checked the firing order, it is correct.

 

The distributor is a 79 model, and the carb is the 32/36 dgav.

 

When I pull the vacuum line to set static timing to 12 degrees then re-attach the line the timing almost jumps off the tab, I will double check and see exactly how much it increases after being hooked back up.

 

Centrifugal Advance distributer Degrees @ Distributor RPM

Dist.Modle# Advance/Starts Advance/Full

D4K8-10 0 @ 600 11 @ 1950

 

Vacuum Advance "Distributor Degrees"

Inches of Vacuum to Start Plunger

3.90

Maximum Vacuum Advance

6.5 @ 9.8

[Mydime]

Is it possible that this is the cause? setting the static timing to 12 degrees with vacuume unhooked and then with the vacuum reconnected it is increasing timing to 20 degrees or more i beleive.

[datzenmike]

The L20B dizzy is calibrated to have no vacuum at idle so it should read 12 degrees. Try to find a ported vacuum outlet on the Weber. Other than that retard the static timing down from 12 degrees until the backfiring stops. This isn't optimum but you will be able to drive it.

[Mydime]

Exactly my problem, there is only one vacuum port on the weber 32/36 dgav and that is a full-time vacuum. So how much would i notice not running the vacuum advance?

 

Should I recurve the distributor to make up for no vacuum advance?

[datzenmike]

Gas mileage will suffer slightly and throttle response won't be as good.

 

Try without, then try with it connected. If it still backfires then the carb is too lean.

Edited October 2, 2009 by datzenmike

[Mydime]

So looks like it was a vaccum leak....my adapter plate was not flush with the manifold....the emissions block off plate was hitting the top left edge of the adapter, took the angle die grinder to it and put it back together and talk about a night and day difference. Thanks for everyone that had some input.

[izzo]

So looks like it was a vaccum leak....my adapter plate was not flush with the manifold....the emissions block off plate was hitting the top left edge of the adapter, took the angle die grinder to it and put it back together and talk about a night and day difference. Thanks for everyone that had some input.

 

thats what mine did too, i just loosened it and tightened the weber, then made a custom block off plate for the egr shit.

 

figured it was that :D

[banzai510(hainz)]

when you put the matchbox dizzy on did the matchbox have a mount with it?

ck to see if they are the same. If not then the dizzy might be pegs to one side where its enough to start but when you rev up it goes out of time.

 

there are different 2 mounts for the L20 matchbox that I know off.

 

make sure when at TDC the prongs inside the cap are pretty much lined up to tell it to fire.