backfire on liftoff

[phaetn]

Installed a new head. It was a lot of work. All the long-winded details in another post with plenty of pics and vids.

Now the car really backfires on liftoff especially, with a puff of blue smoke. I don't mean a nice burble, I mean a really loud pop that makes dignitaries's security details nervous.

Compression is good; valve clearances are all within spec. Carb is the same as I had on with the old head. Mixture scew has been adjusted. It was so bad that when I really reved with the throttle linkage in a parking lot by hand and let off there was a big bang, then the engine actually struggled to recover and quit. Something's not right.

What could cause this? I'm thinking either a lean condition, or I also still have a leak on the exhaust where the stainless steel header meets the downtube.

Any advice?

I haven't checked timing since the rebuild, maybe that's a place to start. It's the distrubutorless igntion and crank position and valve pulleys are identical to before. Also, I guess I could look for an air leak by spraying carb cleaner. It's only on lift off though -- no stumbling when under power or at idle.

Thanks and cheers,
phaetn

[RRoller123]

I am betting cam timing is off, I had a similar issue, but not as severe.

[phaetn]

Ok, it's getting quite bad. It can even backfire (out the tailpipe, not spitting up through carb) between gears shifts going up from 1-2-3. Just that time after a lot of throttle then lifting off for the change can cause it.

Tonight I had to make a sudden stop on the freeway because of a poorly indicated lane closure; I had a backfire gearing down in almost every gear from 5-4-3-2. Then a huge amount of smoke and the engine really wanted to quit, but it stayed alive (barely). Then started going just fine. To relieve this I stopped gearing down when decelerating and it's fine that way.

The backfire out the pipe only happens when lifting off, never under full or balanced throttle, though lifting off after full throttle makes it worse than otherwise (makes sense - there's more of a charge of fuel and air in there left to burn off).

The carb and intake is the same as before, though the head, valves, and cam boxes are new to the car. I used my old cam pulleys and they are lined up with the pointers. Valve clearance is perfect. The exhaust is the same as before -- it's new this year but ran fine before the head exchange.

I double checked igition timing with a strobe and it's the same as before.

Should I try retarding the exhaust cam by one tooth so it's closed later? That way it will still be closed rather than allowing combusting gasses out through the tailpipe.

Cheers,
phaetn

[RRoller123]

Maybe just running far too rich and unburned fuel is igniting in the tailpipe? Check jets and emulsion tubes?

[phaetn]

Before messing around with pulleys, timing, etc. I decided to do things methodically. I knew the exhaust leaked, so I went with that first.

I went to a crazy welder who is quite eccentric and a very salty character but he really knows his stuff. I showed up at 8am with a coffee for him and he asked, "What the f%*k is this? Can I weld it?" and went off on a diatribe. We actually get along very well as I am a "good customer". I detached the header above his pit (a death trap, no doubt, as he doesn't have a lift, but it's grandfathered) so we could get a better angle on the joins. He got all the way around and even filled a bit of a gap on the downtube flange.

I understand why Vick's makes it as two pieces, so it can be fitted without undoing a tie rod and it's easier to ship, but in my case it was hard to get enough clamping force without leaving gaps. Now it's welded and should never let go again.

For CA$68 (a little over US$50) he game me about two and a half hours of his time as we had to wait for the header to cool before I could take it off; then he cut the pipe behind the downtube so it was loose on the back end; then he welded the join from below, then we repositioned so he could weld it from above; then we waited for it to cool so I could put it back on again and torque it with the gasket and Permatex copper sealer, then he welded the back pipe again, even giving me a bit more clearance because the angle was a bit better this time around.




Best $70 I ever spent! We talked about everything from health of wives, numbers in the universe, string theory, society today, an MGB Midget he used to have, terrorism, what's right with the world, and everything else in between. A couple of other customers showed up and he did a few other quick jobs while we waited for my pipes to cool.

On the drive home I could already hear that it was much better without the exhaust leak so no backfire, but still a burble.

Once home, and before a 2hr freeway test drive in preparation for FFO (I was getting bits from an Allison's shipment), I listened to the engine again; now that it was much quieter at the header I could hear a sucking from the carb. A squirt from the carb cleaner confirmed the location of the leak -- where the choke assembly attaches. It's finicky work with next to no space because the master cylinder interferes, but I managed to squeeze in an O-ring to stop the leak. Video of success!.

Car is much, much better now. Yay!

It still burns oil. That's another story.

Cheers,
phaetn

[AriK]

Those old school mom&pop specialists are few and far between. God bless them. Little steps. Seems like things are in order. What a project! Bet you're relieved it's over.
I also cringe when i'm working near the carb with those small bits. The worst design ever. Those tiny hold-down nuts/studs securing the air cleaner to the carb....! there must be tons of stories working themselves loose and getting sucked into the intake. My general rule is i always stuff the venturis with paper towels when i work near there and always use loctite on those nuts since its a disaster waiting to happen. Trouble is i weaken the threads everytime i try to get them off. Never win.
Counting down less than a week. Gonna have a blast!

[phaetn]

Counting down, too! Hope we have good weather!

Yeah, I always put a rag over the open choke butterfly valves to help prevent anything dropping down in there. This time the engine was running with no air filter on it so I just kept everything away.

On my test run today the car cruised totally comfortably at 75mph, 4000RPM indicated, for about an hour each way. Still lots of power available if need be, but right in the zone for a long haul.

I had a slight hesitation when applying the throttle past 3/4, not unlike what it used to do a year or two ago. Amazing how half a turn of the mixture screw can change that. Now it pulls cleanly pretty much throughout the band; top end performance is way better!

Cheers,
phaetn

[spiderdan]

Glad to hear it's running well buddy
Still planning on escorting you to the border and met up with arik
After all I am only just a "Local Driver"