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MG MGB Technical - Do 2 & 3 Exhaust valves always run hot?

I've got the Burgess modified head off the engine and it looks like this. What does the appearance of the valves show to any experts out there. To my mind I might have expected all the exhaust valves to be pinky-brown. I haven't seen inside this head since the middle 1990s when it looked like the second photo!





Mike Howlett

Mike
She's looking a bit chaffy there buddy
first thought would be a touch on the lean side, but it depends what running it had been doing just prior to disassembly, even 1-4 appear a touch on the light side looking through the chaff and there's plenty of that going on
If it had been on a higher speed,cruise, light throttle run then you'd expect it to be on the light coloured side like that--but it's borderline
Thoughts are -
maybe the plugs are too cold for the running it's doing
maybe sports air cleaners running std needles could be an issue
fuel with too much ethanol for the mixture settings
Bit hard to say really without knowing the setup-
Looks like there's been some weeping going on in places past that headgasket as well
IF it's got a bigcam in it then the centre cylinders suffer a bit from port robbing especially at mid speed
Can I ask why you pulled the head off
William Revit

Thanks Willy. I don't know what sort of use the car has had over the last 15 years - I only bought it back in 2020, having owned it before from 1994 to 2005. However, having some knowledge of the only owner during that period I suspect it had light use and not much maintenance.

When I last had it it had a standard camshaft, a Burgess fast road head, K&N filters and richer needles - I can't remember what type, and I haven't stripped the carbs yet to find out.

The car was in need of major attention and having done the necessary welding and sent the body off to be painted, I am tackling the oily (very oily) bits, principally to assess the work that will be needed to get it shipshape again. It seemed strange to me that cylinders 2 & 3 look weak, so it can't be down to one carb not performing properly.

Anyway I'll have to see whether a rebore is necessary, although it is already +30 thou. What is the max for these motors? And I'll take a look at the crank and cam shafts.
Mike Howlett

--If it's been running as is for years then it must be something recent otherwise it would have burnt valves by now----
I'm tempted to think it's been running fuel with ethanol then,
If it's had the needles changed,as you suggest, If you have to run it on E5-E10 then a slightly richer needle again will probably be needed
One thing that I just thought of is a blocked fuel filter or pickup screen in the tank--If either of these are choked up then it 'could' be starving for fuel under load giving you those light coloured valves but have normal mixture when not under load
There's heaps of possibilities really
If it does have a modded cam in it, that will be why 2-3 are leaner looking---and the bigger the cam the bigger the port robbing issue---As an example I ran a monster cam in my old hillclimber with around 325 degree duration. It idled on two cylinders and at idle you could pull 2-3 plug leads off and it made no difference at all-

+30 thou is nothing to these engines, I've done heaps at .125" (83.5mm) without an issue but any more than that and sleeves(liners) will be required
I'm on 85mm in mine with thin excuses for liners and it's been bulletproof
William Revit

Good info Willy. When I had it you could still buy unleaded with no ethanol, but now all regular fuel here has up to 10% and the super grades are up to 5% ethanol, although a few super grade brands claim to be ethanol free. Wow, plus 0.125" bore - that's incredible.
Mike Howlett

To be honest 2 and 3 look fine and 1 and 4 a little sooty/carboned up, you can still see the 'nicer' brown under the carbon/soot on 1 and 4, so sooting seems recent. What do the plugs look like chamber to chamber?
From experience no 3 has the hottest chamber under normal conditions and no 2 ex can get very very very hot under race conditions (siamesed 2 and 3 ex valves)
Peter Burgess Tuning

Hi Peter
Yeah ,if I tilt my screen about the valve colours change a bit, so tilted till the red on the gasket looked about right and yeah the valve colours are better at that---There's plenty of carbon buildup though, maybe the breather system needs looking at. or it's just a case of a car that's been off the road for a bit and hasn't had a run to clear it out
Can't blow the pic. up enough to get a real good look at the plugs but they do look dark/chaffy as well from what can be seen.
It'd be interesting to see the backs of the ex. valves to get more of an idea of it's longer term running-
As you say the sooting seems recent-
Nice piece of history there with Mike's before and after head pics.

Stay safe guys
willy
William Revit

Thanks for butting in Peter. I have to decide whether the engine needs a rebore. Is that something I can assess myself, and if so how?

What should I do with your lovely cylinder head? Obviously clean everything up, but should I remove the valves and lap them in to the seats? Or if I do a leakdown test on each chamber with petrol or paraffin and they are tight, can I just leave them alone?

I have notes of work done during my previous ownership and I well remember being at your premises on the 11th November 1999 (was it really 22 years ago - yikes!). The car had a new standard camshaft, No.6 carb needles and K&N filters and NGK BP7ES plugs, but was otherwise standard. According to my notes you made sure the balance weights were working and cleaned the carbs. With the new head you achieved 90 BHP at the rear wheels, a considerable improvement.

I note that you and I talked then about the possibility of oil blowing past the rings, and nothing has been done about that since then. According to the speedo, the current mileage is 75,334. When you had the car on your dyno in 1999 it was 45,850, so the head has been on the car for 29,484 miles.
Mike Howlett

With today's fuels I'd think that with K&N filters, No6's would be a touch lean --AAA seem to be the needle that do the trick there for most road cars
and the plugs would be better being 6s, 7's are borderline too cold for a roadcar now
Anyway, Peter knows way better than me what would suit your car best and what the latest trend is over there to suit your fuels, I'll leave you to it--
Cheers
willy
William Revit

Mike,
you can check the bores yourself but you'll need either an internal micrometre or a set of these
https://www.ebay.co.uk/itm/323879284487?epid=1358766016&hash=item4b68b5af07:g:nvoAAOSwICZhj8sz

and an external micrometre/Vernier calliper.

I have a set somewhere if you want a borrow, just send me a PM and I'll post them out to you.

Best of....
MGmike

M McAndrew

Thanks Mike, that's kind. Since the telescopic gauges are quite cheap I might as well buy a set. The problem is the large micrometer won't be cheap, unless you are offering to lend one of those too? I do have a digital caliper - would that do? At my age I reckon this will be last car I renovate so it's the wrong time to start buying yet more tools, especially expensive ones.
Mike Howlett

I meant to ask. What are the max tolerances for bore wear on these engines?
Mike Howlett

Mike, yep those too.. I have a 0-1, 2-3 & 3-4 inch sets I picked up on eBay just because I could! If you want a borrow let me know. I can either post them down to you or we can get out for a run and meet somewhere in the middle (although I won't be in an MG :O( ). However, a digital caliper with XX.xx display should be okay and probably easier to handle but you're welcome to the micrometre sets.

I'm sure Willy or Peter will be along to give you the definitive answer on the wear tolerances but until then... anything over 3-4 thou and I would be looking at doing a +10 rebore. The piston clearance when new is 2-3 thou depending on where measured and I'm guessing double that will start to give the rings a bit of trouble.


Best of...
MGmike
M McAndrew

correction 0-1, 1-2 & 2-4 (I think!!!)
M McAndrew

Mike, I think I have sent you a PM. I hope you get it.
Mike Howlett

Hi Mike McAndrew. The BBS moderator can't pass my PM on to you as he only has an out of date email for you. This was my message:

Hi Mike. I hope this gets to you. Where in Lothian are you based? I am in Troon, Ayrshire. I would like to borrow your bore measuring tools and will happily meet you somewhere between us, but it may not be for a day or two. Lots to do here. Are you free during the day, ie. retired like me? I suggest we communicate via ordinary email rather than here. My email is mikehpersonal AT gmail DOT com. Thanks, Mike.
Mike Howlett

Mike, email on it's way to you...
M McAndrew

Happy to help if you need me Mike.
A lot of water under the bridge since 1999! I cannot believe I have been working at that unit since Sept 1989!!!!!
I reckon strip the head down and polish up the valves and some scothbrite on a split rod to polish the head up again.

Willy. The no6 seem pretty good over here for good fuelling. AAA can be too rich. We try to tame them with blue springs to weaken the mixture. Sometimes AAU are just ok. I have had some no6 converted to biased needles, I think the length is safe so the needles will not pop out of the jets. I am looking forward to having a play with these.
No 6 plugs for town work and 7s where the engine can be used more spiritedly.
Peter Burgess Tuning

Cheers Peter
William Revit

This thread was discussed between 10/11/2021 and 16/11/2021

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