I'm not an engine expert by any stretch of the imagination. But your suggestion does make sense to me. If it is a sticky valve/loose valve, then would that allow some of the combustion to seep past the valve, up the inlet and inite some of the fuel between the carb and valve creating the popping/backfiring that can be heard in the video? If it's not the valve, then I'm stumped on any other explanation. Discounting any other possible explanation for the minute, what's the process for fixing a sticky valve/loose valve guide @Zed ? As an aside, it's getting worse.
IF the inlet valve is held open when it fires it will pop back through the carb. It's usually a loose moving valve guide holding it open. Head off, fit new oversized guide. Check/replace the others. Look for cracks or other problems before you shell out on repair. There are other reasons why engines backfire through the carbs though. A lean condition caused by a leak that opens up when it's hot. Ignition erratic when warmed up... there will be more.
It's only happening on the left side of the engine (cylinders 3 and 4) if that helps narrow it down. Ignition - distributor has been replaced with a 123Ignition (didn't resolve the problem). Plugs have also been replaced. If it was ignition related, why would it only affect one side of the engine? Carbs have been overhauled with new spindle bushes fitted along with a few other bits. New gaskets fitted when the carbs were put back in. Inlet manifolds are CB Peformance and fitted about 12 months ago. I've checked the bolts for the inlet manifold, but for completeness, will check again. Any other possible areas I can look at for a leak?
Seems you're ahead of me! I didn't want you taking the head off and finding nothing on my account. Just being cautious, but it appears you've already narrowed it down to that head.
I was afraid you'd say it was the head Is there anything I can flush through the engine to see if it will help unstick it (long shot I know - but you never know).
No, because if it's valve related you could bet 99.9 times out of 100 the guide is moving... on these VW aircooled engines. Most likely it moves with the valve when it's cold, then the head grabs to guide when it warms up. The valve spring will cause the valve to push the guide back up it's hole to a degree, but sometimes it doesn't and the hotter it gets... This is excessive but you get the idea.
Is this an engine out job or can the head be removed with the engine in situ? So...any recommendations on who to use to get this fixed as it sounds quite a bit outside of my comfort zone skills wise. I'm based in Bedfordshire.
Tell you what - take the carb and manifold off and have a look down the hole. The guides should look like this -ish - yours will have more metal around the guide which has been chopped off on mine. If a guide is slid down to the valve head, or there are marks on the valve stem to show it has been down there you'll know.
You can do 3/4 in situ, I had to a couple of years ago. IIRC I jacked the van up a bit for more access an also swung the engine/box down on it's front mount, again for better access and much easier to sort the tinware out of the way. No need to empty the oil or disconnect anything unnecessary. Part way...
Hmmm...think this job might just be a bit too much for me. I have a garage (big tent), but no hard standing underneath it so working on it in there is possible, but jacking it up etc. is not an option. Hopefully someone will be able to recommend a local garage who can look at it for me.
You can look at the exhaust guides from underneath too. I doubt it would be these though, unless one wasn't opening and that would be kinda permanent, not a warming up thing. If an exhaust valve was held open it would pop in the exhaust which it isn't. Here's mine anyway just to show the view you get with the exhaust off... gotta come off anyway to get the head off.
You know best. I do mine in a sloping stony car park. It's quite handy being able to leave the engine in there out of the rain and it's basically trapped and safe. This is where it was when I took the head off. The ground is pretty solid, I might not attempt if I had 4" of pea shingle! I'd find a way though purely because I wouldn't trust anyone else and I wanted a look and measure up to check deck, cc head etc. Would I believe a mechanic? I might, but would I know?
It's been nearly 40 years since I took the head off an engine...and that was on a 1973 Range Rover where I was able to get in the engine bay to work on it. I'll google a few videos on the removal process and see If it's something I could tackle. If the heads coming off, I can look at fixing the oil weep on that side whilst I'm at it.
Part of the trick/luck is getting the head off without disturbing the barrels and if you do, do not turn the engine in case it pushes one away from the block. If I saw the merest crack appear here or even suspected that barrel/block joint was compromised, I know from experience you'd then need to pull the barrels off and reseal them. If it was out of the bus I'd do that anyway, but in those conditions it's a godsend when the barrels stay put. It's absolutely as common as a common thing on an engine that hasn't been apart for a long time (unlike mine) that the heads will be well and truly stuck onto the barrels. You have to look ahead and be ready for all these things and know you can deal with them, you don't want to get stuck with it dismantled. say that happened and while the barrels were off you peeped past the piston and thought the cam looked a bit worn. Then what? I'd still go for it but that's me. I don't trust very many people to work on my bus and the engine... I don't really trust anyone apart from the chap who built it but it's easier TBH to just deal with it myself. They are very simple.
The engine came from Elite Engines in Essex a few years ago (before I bought it). It went back to them in Aug 2021 as the engine filled up with fuel. Not sure what was done during the time it was with them. Hypothetical question. If the heads off, is it worth doing all valves or just the one that needs attention? I fully understand what you're saying about working on the engine. If I had the space and tools, I would tackle it myself.
Well, You need to have look. The other possibility is a loose valve seat. There will be a point where a new head is the best option.
This is familiar. https://thelatebay.com/index.php?threads/weird-plug-problem.97085/page-3. I eventually chickened out and sent it off to the VW Engine Company.
How did you get on with the VW Engine Company? What was the end diagnosis and cost (if you don't mind me asking)
