Stromberg WW Main Jetting Recommendation

[Rddau]

I have a 68 GMC 305 V6 with the above mentioned carb. I've had the carb rebuilt, as is the motor, and the plugs are almost white after just idling in the drive. Just curious what you all are using for main jets. I live in Minnesota, so about 1200' elevation. Backfires through the carb when hitting the throttle. Timing is fine. Vacuum is fine. Distributor advances properly.

[geezer#99]

Backfire thru the carb is usually lean mixture and retarded timing.
What main jet is in it now?
What is your initial timing set at?
How much vacuum at idle?

[Sheepdip]

Backfire thru carb can also be a bad accelerator pump.

[Bob B.]

White plugs and backfire through the carb sure sounds like it's lean. Stock jets in those are .055" I believe. Could be an accelerator pump problem or maybe a low float level. If your vacuum was low I would say check the PCV valves.

[Rddau]

Originally Posted by geezer#99

Backfire thru the carb is usually lean mixture and retarded timing.
What main jet is in it now?
What is your initial timing set at?
How much vacuum at idle?

Don't know the main jet yet but it was fine before the rebuild.
Timing is at 10 BTDC with vac line plugged.
Mechanical advance works.
Vacuum advance works.
Vacuum is low, don't remember the number. Vac line to distributor is connected, there are no other ports.
Float is slightly lower than recommended. Confirmed by local carb guy who suggested the lower setting. At recommended setting it blew fuel through.
Dwell is at 33. All new points, condenser, plugs, wires. Still the old coil though.
Accelerator pump works.
There is no PVC valve. See pictures. The hose that is open goes into the air cleaner. I do not have anywhere to put one as I've seen in other pictures here at the bottom of the carb.

The strange thing to me is I've had this running really nice but I can't go more than a couple miles. I end up choking it and pumping the throttle just to limp back home.

[Steeveedee]

Positive Crankcase Ventilation (PCV) was installed on vehicles made starting in '64. Do you have pictures of what your engine looked like before the rebuild?

Maybe someone else with this vintage engine can provide some pics...

[Rddau]

Here's a before picture

[Rddau]

Can someone post a picture of your 305 pvc system?

[geezer#99]

Some info here.
Might help.
Lots more info on that site.


http://www.6066gmcguy.com/gmcv6a.html

[67km1590]

PCV valves screw into intake ports under the valve covers.

[Rddau]

Originally Posted by geezer#99

Some info here.
Might help.
Lots more info on that site.


http://www.6066gmcguy.com/gmcv6a.html

So am I to understand there is a replacement paper filter element on the right front of the engine? This engine does not have anything like that. Does / can it just pull through the main engine air filter? Does it have 2 pcv valves, one on each side?

[Rddau]

Is this the pcv valve? Middle of valve train, under the cover?

[Palf70Step]

I can't tell clearly from the original photo, but the area circle in yellow, the crank case vent or PCV system is missing or off by the looks of it. So like mentioned PCV could be some of the cause.

New parts are bad sometimes, so the stuff used to rebuild the carb could be off. Can you make it richer with teh idle screws to see if that helps. The back fires always have been due to a timing issue in my past. It is not uncommon for the marking on the damper to be slightly off . I cannot remember if the V6 is pressed on or bolted on damper.

My self, I would adjust the timing by ear to get it the smoothest, then adjust the mixture screws to see if that helps.

[geezer#99]

Inlet filter is behind the drivers side head.
Shows that on the flow diagram.
And your pic shows one of the two pcv valves.
One in each head.

[geezer#99]

Another possibility on the pcv is the newer v6 (your 68) uses one pcv in the drivers head and the breather on the valve cover, compared to the diagram in the link which shows 2 pcv’ s and rear filter.
Emmisions set ups were changing in the mid sixties.

[Rddau]

Any help on where to buy new pcv valves? Can't find online and O'Reilly doesn't show them available.

[cbeerman]

Have you rechecked your valve lash? V6s are solid cams.

What's your fuel supply like do you have pressure and flow?


The Stromberg WW has idle screws circuit screws and internal tubes that feed that circuit what happens when you turn those screws does the RPM or vacuum change?

https://www.carburetor-blog.com/know...e/ww-exploded/



Here is some more info on the WW carb

https://www.carburetor-blog.com/article-categories/ww/



BTW that is a breather in your valve cover that goes to the air cleaner it won't cause a vacuum leak.


https://6066gmcclub.com/showthread.php?t=48799

The PCV valves screw into the head on V6s
GM part number 6421979
AC Delco CV623C


Good luck and keep us posted

[Bob B.]

Originally Posted by Rddau

Is this the pcv valve? Middle of valve train, under the cover?

In this picture the PCV valve is right next to the spark plug wire holder under the valve cover. Looks like a little pipe fitting with a hex head and a small hole in the top. If a GMC V-6 does not have a breather hose going to the air cleaner from a cap on a valve cover or the back of the block, there is a small separate breather air filter in a small can behind the left (driver's side) head. Looks like you have a cap with a hose, so there would not be a separate breater air filter.

You should have a vented gas cap. When your truck stalls quickly pull the gas cap off and see if a vacuum is building up in your tank.