[DogMeister]

Trying to dial in my Holley Sniper Quadrajet EFI but that's another story. #4 cylinder is misfiring. The motor is a 1969 400 YB block that was the 8.6:1 2bbl big car engine. I installed a 461 stroker kit 10 years ago, and did not change out the Melling SPC-5 cam which looks to be between the 9779066 and -068 cams. The cam is running 4 degrees advanced. I know it's too small for the motor (RAIII cam on order) but it did put out 460 lb-ft at the rear wheels and only 260hp but it ran perfectly. Calculated CR is 10:1 with the #45 heads. Tremec TKO-600 and 3.55 posi.

The motor has less than 5k miles on it before pulling it from my old '67 GTO. It did sit for 2 years while I finished the new one. Brand new Autolite platinum FP86 plugs which are supposedly equivalent to the RS45. New Taylor 8mm wires, new cap and rotor on the Summit HEI distributor that I've been running for 8-9 years.

I swapped a new plug into #4, still not firing. I'm getting a spark from the wire at the plug end. Compression on all cylinders is 205-215 including #4. Realizing that I'll be putting the new cam and lifters in soon, I'd still like to get rid of this misfire first.

I'm open to suggestions.

[steve25]

The first thing I would double check would be the firing order.


If everything else was normal with cylinder 4 then the plug not firing should mean you would be seeing it getting wet with fuel, and you did not post such.
It would not be the first time a shop rag was found in a intake runner!

If you have a adjustable valvetrain then loosen up both valves on #4 such that you can set a .008” feeler gauge in there then start the motor and see if you still have the miss.

One thing that is for sure, 205 psi showing up on a cranking compression test with iron heads is too much even for 94 octane if you want my 2 cents worth .

You might have good valve seal during a cranking compression test, but when the motor is running have a leaky exh valve.

If a intake was bad you wound have back fires coming out the carb which it seems you do not.

[DogMeister]

Yeah the cylinder compression is high because of the advance on the cam. When I put the RAIII cam in I'll run it straight up. Anyway, when I put it back together tomorrow I'm going to check the firing order. Again. Without relying on my memory this time.

[Sirrotica]

A flat cam lobe, broken push rod depending if it's exhaust, or intake, will still show compression, but won't fire. I had one that broke a pushrod that showed compression, but wouldn't fire. It's been 40 years ago, I believe it was an exhaust pushrod. Replaced pushrod, and all was good.

Run it with the valvecover off to check cam lobes, of course a pushrod should be obvious, the rocker arm was off on mine indicating a broken pushrod. Mine had screw in studs, but your heads I believe are press in studs, might be a stud coming out too.

[steve25]

The firing order is right on the intake if it’s a stock one.

[HWYSTR455]

Wiggle the plug wire off the car with an ohm meter on it, bet you find it's dropping connection. I switched from taylor wires a while ago, they are not what they used to be.


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[DogMeister]

Originally Posted by steve25

The firing order is right on the intake if it’s a stock one.

Yes but you need to look at it lol

[Mr Twister]

I had a misfire on a car that was difficult to find. Turned out that the plug wire felt like it was clipping on the plug when it was not. I put on anew plug wire and all was good.

[HWYSTR455]

Google 'taylor spark plug wire problems' and you will never buy them again.


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