Stromberg WW Main Jetting Recommendation
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.
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Re: Stromberg WW Main Jetting Recommendation
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? |
Re: Stromberg WW Main Jetting Recommendation
Backfire thru carb can also be a bad accelerator pump.
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Re: Stromberg WW Main Jetting Recommendation
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.
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Re: Stromberg WW Main Jetting Recommendation
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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. |
Re: Stromberg WW Main Jetting Recommendation
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... |
Re: Stromberg WW Main Jetting Recommendation
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Here's a before picture
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Re: Stromberg WW Main Jetting Recommendation
Can someone post a picture of your 305 pvc system?
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Re: Stromberg WW Main Jetting Recommendation
PCV valves screw into intake ports under the valve covers.
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Re: Stromberg WW Main Jetting Recommendation
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Is this the pcv valve? Middle of valve train, under the cover?
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Re: Stromberg WW Main Jetting Recommendation
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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. |
Re: Stromberg WW Main Jetting Recommendation
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. |
Re: Stromberg WW Main Jetting Recommendation
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. |
Re: Stromberg WW Main Jetting Recommendation
Any help on where to buy new pcv valves? Can't find online and O'Reilly doesn't show them available.
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Re: Stromberg WW Main Jetting Recommendation
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 |
Re: Stromberg WW Main Jetting Recommendation
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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. |
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