305 V6 Vacuum Issue
 

I have a 1967 GMC C20 305 V6 with the NP435 4 speed that has a vacuum / backfire / stumbling issue. If you lightly press the accelerator, it wont stumble or backfire but if you try to take off normally it will. Some background information.

I bought the truck 3 weeks ago, brought it home on a trailer. Originally had the Stromberg WW carb on it and it had the same backfire / stumble issue.

Replaced the points & ignition coil with a Pertronix 1162A kit and Flamethrower Coil.
Replaced the Stromberg with a Holley 4412-3, 50 Powervalve with 74 Jets, idle adjustment set to max vacuum. Manual choke. Definitely still running rich after adjustment. Maybe too the jets are too large?
Installed new spark plug wires, cleaned cap & rotor. Also cleaned and re-gaped the AC Delco R44XLS plugs to .040
Ran a compression test last night. #1 125, #2 115, #3 115, #4 125, #5 125, #6 120.
Vacuum is 17 to 18"Hg but not steady, I have attached a YouTube link showing the fluctuations. I move the throttle and it's not backfiring but if I do it any faster, it will.
Listening to the exhaust, it appears to be misfiring as well - see the 2nd video link. Not sure how to embed them in the post.
Timing was set to about 2.5* with the distributor disconnect. Vacuum advance does function properly as it advances well over 10* when attached.

Any thoughts as to why its stumbling and backfiring thru the carb? I can adjust the distributor with enough advance that it doesn't backfire but it runs too rough due to pre-ignition.

I have searched for similar issues on the board here but don't really come up with such a small vacuum variation. The vacuum should be steady, correct?

I am thinking possibly an intake leak? The carb has a good seal, new gaskets on top and underneath the Mr Gasket adapter. I am thinking about doing the WD-40 intake leak test as well.

There is a tick on the driver's side but I don't know what effect it might have. See the 3rd video. Sounds like a lifter tick. Maybe I need to adjust valve lash?

My next step was to install a fresh set of plugs and possibly a new cap and rotor but I'll have to order them as no one stocks them for the 305 V6.


https://www.youtube.com/watch?v=-vzMDL5TCqY

https://www.youtube.com/watch?v=yJ9lnCy0eCs

https://www.youtube.com/watch?v=LODZtsji5VI

Re: 305 V6 Vacuum Issue
 

The back fire and the tick your hearing could be related. Check for an exhaust leak at the head.
I would also verify the timing by finding TDC (with a piston stop or other method) and then make sure the balancer indicates TDC. If this is OK then a leak down test is in order. This truck may have been parked long ago due to a jumped timing chain or burnt valve(s).
It looks as if you have covered the basics.
I hope this helps.
Good luck

Re: 305 V6 Vacuum Issue
 

Thanks for the tips. I'll check the exhaust this weekend.

I had already completed a leak down test when I did a compression check. None of the cylinders lost more than 3 or 4 psi after 10 minutes. But I'll pull the #1 plug and confirm the TDC

Re: 305 V6 Vacuum Issue
 

Backfiring through the carb is normally a timing issue. As HO455 said, I too would verify TDC with a piston stop first and foremost. Then I would advance the timing to around 12 degrees with the vacuum advance line disconnected from the distributor and plugged. Point gap should be around .018 and dwell at 32 degrees, depending on what your Pertronix instructions suggest of course.

Re: 305 V6 Vacuum Issue
 

The Pertronix unit that I installed is the magnetic pickup which does away with the points. It is properly gaped at 030. The engine does seem to run better with more advanced timing than the 2.5* it calls for. Thanks for the help.

Re: 305 V6 Vacuum Issue
 

Since you say it's running rich, it's probably that, for the slow vacuum variation. Here's a handy chart I ganked off of here. The bottom right case is what your symptoms most match.

Re: 305 V6 Vacuum Issue
 

Ok, so I've verified that the TDC #1 with piston stop and it is correct. I checked the orientation of the rotor in relation to the #1 plug wire and it matches up.

I pulled the distributor to get a better look at the stamped number and noticed a lot of play in the vertical movement of the shaft. .095 to be exact. With v8 distributors, this should be .015 to .020 - is this v6 distributor supposed to be the same way? If so, the excess gap will allow about 5* of timing variation...

I'm thinking of downsizing the power valve to a 35 from the current 50, that should help the rich condition.

I'm installing new spark platinum tipped spark plugs tonight and will set up an initial 12* of timing. We'll see how that runs. Cap and Rotor are on order to be here Wednesday.

Re: 305 V6 Vacuum Issue
 

The dealer may still sell the shims to take the slop out of the shaft. If that slop gets big enough, the rotor can pop up enough for the electrode to catch on the distributor cap electrodes and break the rotor electrode loose.

Re: 305 V6 Vacuum Issue
 

Thanks, I'll check with a local dealer. I do suspect that this is part of problem as the cap shows wear from the rotor hitting it. I installed new plugs and set the initial timing at 12*. Runs much better! Ordering new jets and smaller power valve.

Took it out for a quick drive and only got it up to 40mph before I found that I have a bad u-joint.