1967 Backfiring Through Carb

[Lee B.]

Re: 1967 Backfiring Through Carb

The dimple is on the same side as the rotor is pointing. My end play is about .005. I will put the distributor back in later tonight and take a pic.

[Lee B.]

Re: 1967 Backfiring Through Carb

IMG_4999.jpg
Still backfiring.

[Duke W.]

Re: 1967 Backfiring Through Carb

Good, but I didn't know the distributor was out.

Did you index the crankat 8-10 BTDC #1 before you removed the distributor. If not, suggest you do that. Rotate the engine CW with #1 plug removed. Once you feel pressure at the plug hole keep rotating the crank until the notch hits 8-10 on the timing tab. To rotate the engine you can put the shift lever in top gear and just keep bumping the car forward until the notch is in the proper range. If the notch gets a little past the target range, you can bump it backwards to line it up.

Look down the hole and note the orientation of the intermediate oil pump drive shaft slot. Now hold the dist over the hole with the VAC oriented about halfway between the manifold and coil bracket, and place the rotor tip about 20 degrees right of engine center line. Then rotate the bottom 90 degrees forward and note the orientation of the flat at the end of the shaft. If the slot is not oriented to engage the flat, tweak the slot as necessary with a paint mixing stick or similar tool so the slot is in the same orientation as the flat at the end of the shaft.

Once you think the slot and flat are properly oriented to engage, hold the distributor above the hole and orient the rotor tip to be parallel to engine centerline. Now lower the distributor and as the dist. gear engages the camshaft gear the rotor will rotate clockwise. If you aligned the slot properly the flat will engage and the distributor should seat on the manifold. If it hangs up about 1/4 inch above, the slot is not oriented correctly and you'll have to tweak it and try again.

The above can be a bit tricky. Even though I've been doing this for 60 years I don't do it that often, and it can sometimes take me two or three tries to get the slot aligned properly so the distributor seats. Take your time and be patient. It takes a little practice.

Once the distributor is seated it's time to static time the engine. Rotate the distributor housing CW to the limit then slowly rotate it CCW and when the points just barely start to open the timing will be very close to where the balancer notch is on the timing tab. Now snug the hold down bolt, but not more than allows you to move the distributor easily, and recheck the point opening.

Using an ohmmeter is more accurate and should get the timing withln a degree of what you set on the tab. Hook the ohmmeter between the black dist ground wire and a good engine ground. With the distributor base fully CW it should read zero ohms. At some point as you rotate the distributor base CW it will jump to infinity (open circuit) when the points open. This is when the coil discharges to the spark plug.

If the above is done properly, the engine should fire. The OE '67 300 HP curve starts at 700, but you should verify this so you can verify initial timing at less than whatever speed the centrifugal actually starts. Of course, setting timing must be done with the VAC disconnected and plugged with a golf tee or equivalent. Once the engine warms up so it will idle stably below the centrifugal start speed, check timing and adjust if necessary.

Once timing is verified as correct, secure the hold down bolt, but not too tight; 10-15 lb-ft is okay. Noe verify that timing is still correct, then connect the VAC, let the engine fully warm up and go through the idle speed-mixture adjustment procedure with a target final hot idle speed of 500.

Let us know how it turns out.

Duke

[Lee B.]

Re: 1967 Backfiring Through Carb

Thanks. I feel like I have done this 100x but I will follow your instructions tomorrow to see what I get.

[Duke W.]

Re: 1967 Backfiring Through Carb

I just saw the photo, and the distributor is NOT installed properly. If properly installed, the cap window should be approximately normal to engine centerline. Yours is about 20 degrees to the left (drivers side) and the VAC is against the inlet manifold or nearly so. It's probably one tooth off.

Remove the cap, then put the trans in top gear and bump the car until the balancer notch is at 8-10 on the tab. If the rotor tip is pointing in the direction of #1 cap tower, it's at 8-10 BTC #1. If the rotor tip is pointing 180 degrees opposite at #6 rotate the engine one revolution to 8-10 on the tab and the rotor is pointing at #1 tower.

Now review my previous post on how to orient the intermediate oil pump shaft slot and flat at the end of the dist. shaft and go from there. If after static timing, the plane on the cap widow near normal to engine centerline you should be good to go.

While you're at it, post the data stamped on the coil bracket and if you have a vacuum pump post the start and stop vacuum values of the VAC.

It's not clear what plug wires you have, but if they are the OE type, check resistance. There are lots of reports that the repros of these wired don't last long, and if resistance is more than about 5K ohms per foot they should be replaced.

Oh, and also verify that the wire immediately to the right (driver's side) of the cap window is on #1 and all others are in the firing order sequence 18436572. On a V8 two can be reversed and the engine will still run, but not behave very well.

Duke

[Leif A.]

Re: 1967 Backfiring Through Carb

Lee,
This is the orientation and wire sequence Duke is talking about. Can someone, please, flip this picture. I'm not sure why it orients this way on this site. Thanks.

[Timothy B.]

Re: 1967 Backfiring Through Carb

Lee,

After re-reading your first post you changed many of the secondary ignition parts and the back fire problem was still present. I know you may have done this but go over your work again just to double check things.

Looking at your engine picture and noting the engine was rebuilt when you restored the car, my feeling is that the camshaft used is not a GM part and the indexing of the drive gear at the back of the cam is not the same as the GM factory camshaft, that does not mean it's bad.

Try moving the distributor one tooth clockwise and reset the initial timing to 6* and if that puts the vacuum advance control up against the coil bracket then it's time to rotate the lower distributor lower gear 180* away from the factory installation. Doing that will allow correct timing of the engine with the V/A control ending up approx in the middle of it's travel space when you reinstall it.

At low RPM's the carburetor idle circuit is in play and I can't help but wonder if some jelled fuel could be restricting the idle feed restriction (idle jet). I agree to verify all the above suggestions mentioned but you may want to try some sea foam or some type of fuel system cleaner directly inside the carburetor and see if it helps or if you are so inclined remove the front bowl and probe the idle jet for a restriction.

Your carburetor looks like a 3810 and they are good carbs.

[Lee B.]

Re: 1967 Backfiring Through Carb

Here is a pic of where I ended up after re clocking the distributor. It runs great but still backfires. I am at 6 degrees static timing and the VAC is all the against the coil. IMG_5002.jpg

[Duke W.]

Re: 1967 Backfiring Through Carb

Do you have documentation for the engine rebuild with the manufacturer and part number of the installed camshaft?

Cases have been reported where proper distributor orientation could only be achieved with the dimple pointing in the opposite direction of the rotor tip, and this has been attributed to the camshaft gear not being properly oriented.

So I recommend you pull the distributor and rotate the gear 180 degrees if the camshaft source is unknown. Reinstalling the dist. one tooth CCW, which will move the dist. base 360-13 = 27.7 degrees and it will end up same as in your first photo with the the VAC butted up or nearly so against the manifold at the same initial timing. Rotating the gear 180 degrees will half this, so the VAC will end up about midway between the interference points.

The plane of the cap window looks near normal to engine centerline, in which case the VAC should not be butted against the coil bracket, but maybe that's just the camera angle or lens distortion.

Did you verify that the plug wire installation in the cap is per the firing order and that all the wires attach to the proper spark plug? It's easy to get either 1 and 3 or 2 and 4 reversed at the plug

I'd still like to know the data on the VAC bracket and the start/stop vacuum points.

Duke

[David Z.]

Re: 1967 Backfiring Through Carb

Flipped image.
Leif's distributor.jpg

[Lee B.]

[Duke W.]

Re: 1967 Backfiring Through Carb

Do you mean 11,000 ohms? That would be about 5069 ohms per foot, so they are marginal. You should check all. Are they "repro" OE RFI suppression wires? As I think I previously said, they have a bad reputation for longevity.

The AMA specs say the VAC max advance is 12 @ 15" and I recall that the ID is MS 355 12?

I don't recognize the name "Camcraft" as a camshaft manufacturer.

Duke

[Lee B.]

Re: 1967 Backfiring Through Carb

They are new AC Delco wires. And yes 11000 ohms. I will replace VAC before reinstalling dist

[Duke W.]

Re: 1967 Backfiring Through Carb

There's no need to replace the VAC. It may be original, and the start-stop points you measured are not too far off, but I'd like you to verify the ID data on the mounting bracket, MS 355 15 as you stated in post #26 or MS 355 12.

Your new AC Delco wires are not made by GM, but the third party who makes them are likely licensed by GM to manufacture them using the AC Delco name and theoretically they should be manufactured to the original GM specs, but who knows.

GM sold off the AC Delco plants back in the 1990s, so AC Delco is now just a marketing outfit within GM. They buy parts from third parties packaged in AC Delco logo boxes. If you ever need a new VAC, buy A NAPA VC1802, which is made by Standard Motor Products. It's stamped B22 and is available in many different brands like NAPA/Echlin, BWD, Niehoff, SMP's own brand, Standard, AND Delco... same SMP-manufactured part in ALL those boxes.

The B22 specs are slightly different, being 16* @ 15" max. This is best with a manual trans without A/C. The 12" is best for Powerglide since they pull less vacuum idling in Drive at 450-500. It's probably also best for manuals with A/C since the compressor load when it's engaged at 450-500 reduces manifold vacuum. But GM chose to use the same distributor for all 300 HP configurations.

I have no idea who manufactures "AC" spark plugs.

Duke

[Lee B.]

Re: 1967 Backfiring Through Carb

This is an A/C car. So are you recommending the VC1802?