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BJ8 blew out intake manifold plug

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H

Hoghead

Jedi Trainee
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Two truly impressive gun shot size backfires and then would not start.
After a lot of tail chasing I find a freeze plug sized hole in the rear of the intake manifold log. Naturally it could not be at the front so I can simply hammer in a new one!

My local parts house only has a rubber style plug and I wonder if this will hold up?

Carbs done by Joe Curio 5000 km ago. Never backfired before.

Distributor done at the same time by Advance Distributors and I fitted a Pertronix, matching coil, cap, and rotor, yesterday as part of the tail chasing

Timing is correct, valves seem to be operating properly, 180 psi in each cylinder, fuel at the jet needles, spark at the plugs, plug gap .025.

Is this a known issue?
 
My guess is the plug started to leak air, then the intake charge went very lean and caused the explosive pop in the intake. 80's Porsche's used to have this problem with a lean charge on warm up. The pop from the lean charge would crack the intake air box on the Porsche. The aftermarket fix or prevention for it was a spring loaded blow off valve that could open under a high pressure situation like an intake backfire. My guess is nothing is wrong just that the plug started to leak air, if you can replace and seal the system you will probably be back to running well.
 
My BJ8 did the exact same thing years ago. It used to backfire through the carbs occasionally, usually when trying to accelerate on a cold engine, but sometimes after I'd been coasting for a while and punched it. This is usually attributed to a lean mixture, but I'm sure mine's wasn't. I had a nearly dead cylinder--60PSI--and since an overhaul with good compression on all 6 it hasn't done it in over 5K miles.

I don't think a rubber style would a) survive a fuel-rich environment or b) survive another backfire. When mine blew, I was able to get a proper-sized brass freeze plug--I bought a spare, too--but it seems inch-sized plugs have become mostly extinct. I found a website that would make any size plug, but I'm pretty sure the cost would be high for a one-off order. I bought a set of steel plugs for my engine builder, but he refused to use them, and was able to find metric-sized brass ones that were close but oversized, and he cut them down to fit on a lathe. I replaced mine in situ, using--IIRC--a small hammer to pound it in. This messed-up the heat shield on the firewall--asbestos!--but it worked. I thought afterwards that using the male socket holder on a 1/2" breaker bar might have made a better hammer. I smeared some JB Weld around the periphery of the plug, and it's held for many thousands of miles.
 
I think just keeping the rubber plug will give you a safety valve, keeping the other plugs in place, and since its in the back it isn't easily seen.....sound like you have it solved!!
 
Spitting back through the carbs is caused by weak mixture (cold engine?) and can be worse if the timing is retarded. Static timing is critical.
 
Static timed it to get running

Nice bit of engineering not to put timing marks on the pulley. 15 degrees is 20.2 mm past the notch, and since impossible to measure I had to guess.
Runs great
 
This exact thing happened to one of our club members (Austin Healey Association of Southern California)last year. We were on a ten day road trip and when arriving in Santa Cruz on a brutally hot day, stuck in traffic, the cars all got quite hot. My friend pulled into the hotel with the car running fine. Parked it and went into register and came back out and the car just wouldn't start, despite hours of effort from a small army of shade tree mechanics.

As a final desperate act we got some starter fluid and gave it a try. When he cranked up the car a brilliant flame erupted from the rear of the manifold. Viola, mystery solved, but what to do.

After a little horse trading a fellow club member graciously allowed him to use his trailer to get it home. Once home he took to Russ Thompson's British Car Service. Russ russeled (pun intended) up a metal plug that was designed to spring into place. Russ has been doing this for about a century and had never seen this before.

You might want to look up Russ he would be happy to steer you in the right direction.

https://www.buzzfile.com/business/Thompson-British-Car-818-256-0693

Good luck,

Jeff
 
Agreed, I have a higher spec, after market vibration damper with comprehensive timing markings. The originals crack where the keyway is and the rubber softens, they also groove where the oil seal is and leak.
 
I once lost the casting plug on the back-end of an MGB manifold__why does the front one not go first; a mystery of life I suppose?

In my case, it was a not too old valve job, and the #4 intake seat had pounded itself lose. Driveability was gradually deteriorating on my hour-long commute home, and I was darn close to being within coasting distance when it would run no more (if not for a STOP sign and a couple of cars...).

As the seat was reducing the valve/lifter clearance, it eventually reached the point of not sealing off the combustion chamber, allowing the controlled explosions to reach the intake manifold. When I slowed for the STOP sign and closed the throttles. the disc went bye-bye.

Lucky for you, the solution was easier, but I do recommend you carry a spare rubber expansion plug, for the reason Bob cites above.
 
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