1967 Backfiring Through Carb

[Larry E.]

Re: 1967 Backfiring Through Carb

Before going wild and changing everything and not knowing LET'S TRY PICKING THE LOW HANGING
FRUIT FIRST:

Here is my two cents: IF THIS IS THE ORIGINAL ENGINE AND MANIFOLD I would put a mirror behind
the distributor to see if the "factory" lines are are lined up. See picture below. There are reports that sometimes
the factory timing (which is the best) does not agree with the official specs. of the engine.

If OK I would try to get and use pure 100LL Av.Gas. For mine(66 Base Engine) it made a world of difference.
It ran so rough when I got it until the switch. Of Course JMHO>Larry

[Timothy B.]

[Lee B.]

Re: 1967 Backfiring Through Carb

Here is a pic after swapping the distributor gear 180 degrees. Not quite as smooth idle but runs fine. The VAC is roughly halfway between the manifold and the coil. I am blocked in right now so I have not had a chance to drive it yet. Will let you know in a little bit. IMG_5003.jpgIMG_5003.jpg

[Mark E.]

Re: 1967 Backfiring Through Carb

You apparently didn't read the thread I referenced in post #12. The dimple was correctly oriented but the wires were improperly indexed being one terminal CCW from correct, and I immediately spotted this. Once corrected along with proper distributor installation, which oriented the plane of the cap window to be near normal to engine centerline at the correct initial timing, the low speed "trailer hitching", which made the car virutually undriveable was eliminated.

I learned 60 years ago that of the 208 combinations of dimple orientation, wire indexing, and final distributor orientation there is only ONE that allows the correct initial timing to be achieved on a C2 Corvette either small or big block, especially SHP versions.

Duke

Duke- Clocking the distributor differently, installing the distributor gear 180 degrees or indexing the spark plug cables in different distributor cap holes does not cause or fix "trailer hitching." To your point, orienting these as per the manual does position the distributor housing in a way it can be turned to set timing to specs. I suspect your trailer hitching improved because static timing changed when you reinstalled the distributor.

Lee- It's not ideal the VAC hits the coil bracket with timing set at 6* BTDC, so you may want to reorient the distributor one tooth. But this isn't the cause of the problem. It should run fine with this static timing.

[Duke W.]

Re: 1967 Backfiring Through Carb

No, I recommend a 12" "B26" (NAPA VC1765 or equivalent in another brand) for A/C since idle vacuum at any given idle speed will be lower than the same speed without AC, and that's why GM replaced the earlier 15" VAC with a 12" beginning in 1966. Since you measured only 10" to pull your VAC to the limit, it should be okay.

Please verify the stamped ID. I believe it should be MS 355 12, not MS 355 15.

When doing the idle speed mixture adjustment the compressor should be engaged, and target as as low a speed as possible like 450. With the compressor off idle speed will be 100-150 more. If you set it at 500 with the compressor off, engaging the compressor might stall the engine.

Modern cars automatically maintain idle speed with the compressor engaged and might even "idle up" when it's engaged. With vintage cars you have to find a happy medium that will allow acceptable idle quality with the compressor engaged and hope it does jump too high when the compressor is off.

It's even more difficult to achieve with Powerglide and A/C.

Duke

[Lee B.]

[Duke W.]

Re: 1967 Backfiring Through Carb

There is always a solution to any problem. This one is perplexing, but it's not time to abandon ship yet.

One issue that I have is in your latest photo in post #33. If I understand correctly this is with the drive gear dimple pointing opposite the rotor tip, and you said that the VAC is about halfway between the interference points, yet the plane of the cap window is pointing decidedly to the left when if should be near normal to engine centerline as in the photo in post #25.

In the post #23 photo the plane of the cap window is oriented where I expect, but the VAC is butted up against the coil bracket. This is very strange.

I still think the basic problem is misalignment of the rotor tip with the proper cap tower, which is causing misfiring (or maybe cross firing). I'm at a loss right now, but there IS a solution.

Keep i mind that at a given timing value - wherever you set the balancer notch on the timing tab - moving the distributor one tooth either way will alter the distributor base position 360/13 = about 28 degrees and rotating the drive gear 180 degrees will alter the dist orientation by half that - 14 degrees

If you have a dwell meter and some test lead extensions take the car for a ride and watch the dwell when the missing occurs. If the dwell drops way off it could be the ground wire inside the distributor. See the following threads.

file:///home/chronos/u-581cf367b8d2d77a1152e5ef4d7d3f6f07e085ab/MyFiles/Downloads/Small%20Block%20Distirbutor%20Alignment%20-%20Duke%20Williams%20(1).pdf

It could also be a failing breaker plate ground wire. See the following thread.

https://www.forums.ncrs.org/showthre...print-overhaul

If it's misfire due to rotor misalignment I'm not sure if that will affect dwell.

I still need you to confirm the ID data on the VAC.

Duke

[Lee B.]

Re: 1967 Backfiring Through Carb

The VAC was correct with the 15 value not 12

[David H.]

Re: 1967 Backfiring Through Carb

Lee

Question from the peanut gallery.

Have you checked carburetor's power valve? Method below is from a John Hinckley post on CF.

Manifold vacuum at idle? Vacuum gauge indicating problems?

Dave

https://www.holley.com/blog/post/how..._vacuum_gauge/

---

To check the power valve, put your finger over the front bowl vent with the engine idling - if it takes several seconds for the engine to die, the PV is OK; if it dies almost immediately, the PV is leaking. Plugging the bowl vent forces fuel into the engine by way of the PV vacuum passage; if the PV is good, the fuel will be forced out of the boosters to stall the engine and this will take several seconds to occur.

[Larry E.]

Re: 1967 Backfiring Through Carb

Lee: Have you tried 100LL Av. Gas Yet? Larry

[Lee B.]

Re: 1967 Backfiring Through Carb

I use the 91 octane non-ethanol. Never seen 100 around here

[Larry E.]

Re: 1967 Backfiring Through Carb

Lee: Go to your nearest Airport with a 5 gal can. Fill with 100LL Av Gas and try it;you will be
surprised. For best results it will have to be done several times until you get out the the gas
you got now in the tank. I use straight Av gas and the cars run so much better and never gets gummed
out. BTW it is not just the Octane that makes it better; it is much more regulated then the gas
you are using. The gas you are using has a lot more "crap" added into it that we do not even
know what is in there. JMHO. Larry

[Lee B.]

Re: 1967 Backfiring Through Carb

After replacing a lot of parts and working with a friend who knows older corvettes really well, the problem turned out to be 3 bad spark plugs. I had installed a set of new AC Delco R45s spark plugs. Probably had 30 minutes of run time on them. Pulled them and 3 were really sooty while the rest had a light grey appearance. I installed a new set of champion plugs and the car started right up with a stable idle. Drove it for a while and no backfiring. As far as I am concerned the problem is fixed. As a bonus I rebuilt my distributor and it is sitting in the engine in the correct position. Thanks to everyone for all the help and suggestions.

[Mark E.]

Re: 1967 Backfiring Through Carb

After replacing a lot of parts and working with a friend who knows older corvettes really well, the problem turned out to be 3 bad spark plugs. I had installed a set of new AC Delco R45s spark plugs. Probably had 30 minutes of run time on them. Pulled them and 3 were really sooty while the rest had a light grey appearance. I installed a new set of champion plugs and the car started right up with a stable idle. Drove it for a while and no backfiring. As far as I am concerned the problem is fixed. As a bonus I rebuilt my distributor and it is sitting in the engine in the correct position. Thanks to everyone for all the help and suggestions.

Sooty (carbon fouled) or black with a more wet appearing residue (oil fouled)? These indicate different problems, especially after 30 miles.

[Lee B.]

Re: 1967 Backfiring Through Carb

They were black sooty carbon fouled. I am out of town now but will test them when I get back next week and post the results. I think they were intermittently firing. 5 of the plugs were light gray in appearance. The 3 were black sooty.