Backfire through Carb after head gasket replacement…….

[David Taylor]

Good evening all……I’ve just replaced no the head gaskets on the Stag……heads were skimmed……new payen thick gaskets, strobed the timing in to 12deg, lumenition ignition, checked the mixture……all is well. The car starts and runs, however when revving it I’m get the odd backfire through both carbs, but not the exhaust…..stupidly perhaps the machine shop didn’t lap the valves in upon rebuild, perhaps some of the valves aren’t closely completely?! Maybe if I run it for a bit the situation will improve……,.just thought I would ask in case there is any previous knowledge of this…….many thanks, David

[DJT]

Welcome.

Have you timed it using plug #2? It could be that you have timed the engine 180 degrees out. Strange though it may seem, the Stag engine will start if timed 180 degrees out, but will not run properly.

Another David Taylor.

[DZP]

Hi David,
I would not run the car until you find the problem, it could be expensive.
Is the backfire on acceleration (under load) or on the overrun?
Check the easy stuff first.
Are the plug leads in the correct firing order?
Is there any tracking in the distributor cap or rotor arm?
Is the distributor bolted down tightly,
Is the base plate in the disti connected to the vacuum advance unit.
Ensure the plug leads are spread-out as far as possible in case they are tracking.
Disconnect the vacuum advance.
Next I would check the valve timing, one of the cams could have jumped a tooth.
Are the timing chains under tension?
Are there any signs of the valves sticking open e.g. uneven look to the valve springs or dry valve stems.
Have you got oil pressure, is there oil around the valves, if they are dry they could stick open.
Run a compression test to see if all cylinders are roughly the same PSI.
Have a look in each cylinder using a boroscope for bent valves or something out of the ordinary.
Regards,
Den

[David Taylor]

Thanks for the reply……I’m a DJT as well…..
Yes I’ve experienced the cams being 180 degrees out but don’t have this here…….yes timed from compression stroke TDC number2……dizzy pointing to number2.

[David Taylor]

Thanks Den……the back fire is only when revving, I haven’t actually driven the car yet since the rebuild.
It’s crossed my mind that the valves have got bent although I dint tighten the cam down tight until the last minute to avoid open valves getting damaged……I think a compression test is needed to see if inlet valves are leaking badly! Thanks for the reply.
Sorry for some reason when I post unusual characters appear!

[dasadrew]

There are indeed some unusual characters on the forum, but none have shown up on this thread yet.

Welcome to the Forum, anyway

[DJT]

Sorry for some reason when I post unusual characters appear!

If you are using an Apple mobile device, do not use apostrophes or emojis. The just come out as gobbledegook. For example, I typed do not rather than don’t.

[richardthestag]

would the engine even run with cams 180degree out?

It is possible to get the RH chain one tooth out, especially so if the heads have been skimmed. worth checking that all the markers line up after rotating the engine.

Check compressions, don't assume bent valves until you have checked the tappet clearances.

Be sure that your TDC marks on the crank pulley are actually representing TDC on piston Forum

Worth checking for air leaks around the carbs as an excessively weak mixture might cause an inlet backfire.

[richardthestag]

There are indeed some unusual characters on the forum, but none have shown up on this thread yet.

Welcome to the Forum, anyway

[David Taylor]

Thank you all for your considered replies……now it’s time to admit that I discovered the simple error of my ways this afternoon. So simple it’s utterly embarrassing to share it with you, but yes I got HT leads 1&3, adjacent plugs and adjacent in firing order, round the wrong way. I’m sorry for anyone’s time I wasted…….I thought I had checked these 3times already…….what an idiot I am.

[mole42]

Thank you all for your considered repliesâ¦â¦now itâs time to admit that I discovered the simple error of my ways this afternoon. So simple itâs utterly embarrassing to share it with you, but yes I got HT leads 1&3, adjacent plugs and adjacent in firing order, round the wrong way. Iâm sorry for anyoneâs time I wastedâ¦â¦.I thought I had checked these 3times alreadyâ¦â¦.what an idiot I am.

It happens to us all…..we recover :-)

[KOY 23]

Don’t be embarrassed, interesting thread, I learnt that a Stag engine can start with timing 180 degrees out. Maybe they can run backwards as well?

[dasadrew]

My regular underbonnet check: Check oil, check coolant, check clip-on numbers on HT leads are all still there....

[richardthestag]

Maybe someone could invent a 2 stroke stag v8 KOY 23

[richardthestag]

Thank you all for your considered repliesâ¦â¦now itâs time to admit that I discovered the simple error of my ways this afternoon. So simple itâs utterly embarrassing to share it with you, but yes I got HT leads 1&3, adjacent plugs and adjacent in firing order, round the wrong way. Iâm sorry for anyoneâs time I wastedâ¦â¦.I thought I had checked these 3times alreadyâ¦â¦.what an idiot I am.

I use the 4 way HT lead clips on the cam covers to order the leads 1-3-5-7 and 2-4-6-8, makes it easier to visually check that the workshop pixies have not cheekily swapped a lead on the cap but then it also makes routing the leads down each side of the engine visually easier also.

You must have a much smoother idle now also

Everyday is a school day, you wouldn't be an idiot even if you did it again but your experience might lead you to the resolution quicker.

Have a "ham fisted gibbon" award though for your efforts