KR 428 Cobra Jet Spark plugs

[shelbydoug]

I think that you need to go back into the carb. I would say the issue is highly likely that you have a bad power valve.

You can't just install a new one "out of the box" and presume that it ok. These things are so bad and such crap even brand new that it is REQUIRED that you test them.

I had literally 12 brand new ones, never installed, and when I tested them, six were no good.

A bad power valve will give the symptoms that you are describing.

[GT350AUS]

Yes that was my initial thought but I had the carby rebuilt by a reputable builder and tested.

The problem began after sitting for 3 years in a workshop where the guys would start it when they needed to move it and it just idled and fouled itself.

Power valves dont go bad suddenly when not being driven but occasionally started so the diapghram which is the common cause of a bad pv is not the centre of the problem.

I have a mach 1 that suffered similar issues and after rebuilding that carby. The pro b lem continued so I elimated the car by as the problem at bolting on my gt350 715 to the mach 1 and the problem was still there. The cause was found to be fouled plugs.

So this path seems likely to me.

Once I've changed the plugs I'll eliminate that as an issue and get back to you all with my result.

[shelbydoug]

Yes that was my initial thought but I had the carby rebuilt by a reputable builder and tested.

The problem began after sitting for 3 years in a workshop where the guys would start it when they needed to move it and it just idled and fouled itself.

Power valves dont go bad suddenly when not being driven but occasionally started so the diapghram which is the common cause of a bad pv is not the centre of the problem.

I have a mach 1 that suffered similar issues and after rebuilding that carby. The pro b lem continued so I elimated the car by as the problem at bolting on my gt350 715 to the mach 1 and the problem was still there. The cause was found to be fouled plugs.

So this path seems likely to me.

Once I've changed the plugs I'll eliminate that as an issue and get back to you all with my result.

To me, the only one that is reputable is me. I WANT to see the results of testing it myself. I don't want to get this call from the Vet, "I'm sorry your puppy is dead". Dying has got very specific symtoms. If you ignore them, there is no fixing anything.

Your question to the carb rebuilder should be, "did you actually test the power valve WITH A POWER VALVE TESTER?" If he says no, I just put a new one in, then there you go. 

Just my perspective on having gone through this before and offering a likely cause. If you want to keep chasing a phantom that is right in front of you, I can't help. Best of luck on finding the culprit. 

[1967 eight barrel]

I agree with Doug. For 7.00 put a power valve in it. Remember that engine vacuum also plays in the selection of the correct one.  Also a point to note is the original Holley carbuerator does not have power valve protections incorporated, so one backfire through the carbuertor and the valve is usually blown.
                                                                   -Keith

[GT350AUS]

I do agree regarding the pv howvere I put a new one in before having the carby rebuilt and the problem hadn't changed so I decided to have it rebuilt.

I guess I've got nothing to lose by trying another pv again.

[GT350AUS]

Guys what size power valve should it be running just in case the wrong size is in it I want to be sure i replace it with the right one?

[shelbydoug]

6.5 would be stock.

[GT350AUS]

Guys, Just as a side step to the carby backfire issue let me throw something out there.....

I had the tilt steering looked at and the workshop got it working. Apparently it was a bad relay.

The backfire problem has started since the tilt was sorted. Now the tilt away doesn't draw the steering to tilt away for some reason yet to be looked at but the relay is working off the door because  I can hear it switching when I push the switch in and out.

If there is a leak in a hose or if a hose is not connected to the vacuum canister on the tilt steering can this cause the manifold to leak so much that it would lean out causing the backfire I'm getting under load??

[shelbydoug]

Guys, Just as a side step to the carby backfire issue let me throw something out there.....

I had the tilt steering looked at and the workshop got it working. Apparently it was a bad relay.

The backfire problem has started since the tilt was sorted. Now the tilt away doesn't draw the steering to tilt away for some reason yet to be looked at but the relay is working off the door because  I can hear it switching when I push the switch in and out.

If there is a leak in a hose or if a hose is not connected to the vacuum canister on the tilt steering can this cause the manifold to leak so much that it would lean out causing the backfire I'm getting under load??

The vacuum hoses in that system tend to be there forever. Some of the orginals on my car were on the engine when it was painted. What happens with that is it makes them inflexible and when you are working near them or try to bend them, the rubber in them along with the paint cracks.

So yes, it is possible and the percentage of likeliness of broken but not obviously has risen .

In addition, the vacuum can, under the battery also has the tendency of rotting out from the interior, without any or at least any significant outward signs. The thing is just made of "tin" and because of the material it uses, does not have necessarily much longevity.

Put a vacuum gauge in the car. It will help. Your issue seems to be vacuum related. The tilt mechanism can be one of the sources of failure, sure.

[GT350AUS]

Doug by using my vacuum gauge would I be looking for a drop in vacuum reading and where do you suggest the best place to connect it would be....base of the carby?

[shelbydoug]

Connect it to the intake. Back of the carb where the existing vacuum fitting is now.

1) you want to see what you have at idle now. With a stock KR I'd presume 15 at idle. Maybe 17.
2) you want to drive the car. See what the vacuum normalizes at. 3) what it reads when the car backfires.

There are other places you could have a vacuum leak from. The brake booster. The heater control, etc.
These cars are not fun to find the leak at. Lots are hidden under the dash.

I personally permanently installed one in my dash in place of the clock (never worked anyway). I just find it a necessary animal to be there all of the time.

[Harris Speedster]

OK,
Learned a lot about cold plug issues and hot plug issues.
With that said>>  A std for concours judging is;.

A BF 42,
star 
autolite emblem,
Correct?

Bob,  not to shift gears, but what is correct for a late oct build 67 gt 350?
Going to sell some nos plugs we have here, and would tear me up if I sold a set I truly need.
Respectfully submitted,
John

[Bob Gaines]

OK,
Learned a lot about cold plug issues and hot plug issues.
With that said>>  A std for concours judging is;.

A BF 42,
star 
autolite emblem,
Correct?

Bob,  not to shift gears, but what is correct for a late oct build 67 gt 350?
Going to sell some nos plugs we have here, and would tear me up if I sold a set I truly need.
Respectfully submitted,
John

A BF 42 with the star would be for a 67 GT500 according to the owners manual relative to Shelby. A 67 GT350 would use the BF 32 with the stars. I believe the plug graphic design changed eliminating the star sometime in very late 67 production ,probably after Shelby production ceased. At least that is my best guess until more compelling evidence is uncovered.

[Harris Speedster]

OK,
Learned a lot about cold plug issues and hot plug issues.
With that said>>  A std for concours judging is;.

A BF 42,
star 
autolite emblem,
Correct?

Bob,  not to shift gears, but what is correct for a late oct build 67 gt 350?
Going to sell some nos plugs we have here, and would tear me up if I sold a set I truly need.
Respectfully submitted,
John

A BF 42 with the star would be for a 67 GT500 according to the owners manual relative to Shelby. A 67 GT350 would use the BF 32 with the stars. I believe the plug graphic design changed eliminating the star sometime in very late 67 production ,probably after Shelby production ceased. At least that is my best guess until more compelling evidence is uncovered.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Thank you Bob.
John

[GT350AUS]

Doug and Keith

I will start a new thread in regards to the backfire because I think this thread has changed direction from spark plugs to carby to vacuum leak