The Question
I have recently bought an MGB V8 roadster conversion that has been laid up for nearly three decades after its owner suffered ill health, but only recently accepted that he isn’t going to get the car back on the road and sold it to me.
It’s actually in very good condition as it went into a dry integral garage in good order so it’s a bit of a time warp from when it went in. I have been progressing through the various jobs to get the car back into a running state and it’s with the engine that I have encountered a really odd problem.
Having changed oil and coolant, etc. plus also drained off the remaining old fuel, cleaned the tank, fitted a new fuel pump and cleaned the carb out, I was very pleased that the engine started and ran quite well. Initially, I didn’t run the engine for any length of time but the first time I ran it up to temp, all was well. However, having switched the engine off to check various aspects, I then came to restart it about half an hour later and it didn’t start and cranking sounded quite different initially with a tapping sound for a few seconds. I gave up trying to start after the battery nearly went flat, so I put a charger on it and left it until the following day, and then went to try and start the engine and it fired up normally.
All was well for a number of weeks as I didn’t run the engine for very long as I only moved the car around to get access to do various jobs, but the next time I ran the engine up for a longer period until it got warm and then switched off for a period of time the non-start and tapping returned when I came to move the car back in the garage. I pushed it back in and shut the door on it as I was now thinking it was going to be a replacement engine. Have you any idea what might be wrong?
The Answer
This sounds remarkably similar to an issue I experienced on my MGB GTV8 conversion after buying from a friend after a nearly twenty year layup. Having got the engine running reasonably and allowed it to warm up, I switched off and carried on with other jobs for a couple of hours before wanting to start the engine again to move the car, and it didn’t! In cranking the engine there was clearly a variation in compressions from the way the engine was turning and a light tapping sound as the engine turned over which sounded just like piston to valve contact. Then the engine started and the noise went away, but it wasn’t anywhere near as smooth as it was two hours earlier, but smoothed noticeably as it warmed.
The following day, the engine started and ran very smoothly from cold and it warmed up normally, but after a short period of being off, it wouldn’t restart and the tapping returned. I did a compression test and to my horror found one cylinder reading zero, one with 5psi, one with 50psi, others with around 150 and only two with good readings of 170 psi. Oddly when I repeated the test after three hours of further cooling, the one cylinder on zero was now at 175psi, the one at 5psi was now 160psi and the one at 50 was 170psi. All other cylinders were above 160 psi and one was 180psi, (it was a raised compression engine). The engine started and ran sweetly with those compressions so I warmed the engine then switched off and came back half an hour later to find the ticking noise and non-start had returned.
This sticky coating has affected all eight combustion chambers…
To me this was clearly sticking valves and initially I was thinking of taking the inlet off and spraying the ports and the valves with carburettor or choke cleaner, turning the engine over to get each valve to open so there was more visible valve stem to clean may well have been enough to relieve this problem. I also considered one of the on the car injector cleaning processes which replaces the fuel supply with very strong cleaning agent which also acts as fuel to run the engine for about half an hour. Having seen these used on several engines and witnessed their ‘on the car decoking ability’, the relative simplicity of the treatment was attractive, but the cost of around £150, less so.
…and all the inlet ports of both heads.
BCC500 - Aerosol Carburettor Cleaner & A177 - Aerosol Brake Cleaner
I also wanted to take the opportunity to check out the valve and seat condition, so I chose to remove and strip the heads, but for your engine I think that the relatively simple and cheap removal of the inlet manifold and clean with a couple of cans of carb cleaner is well worth a trial. If it fails then you have lost little money and can follow the route I went.
Copious spraying and cleaning with Carb and Brake cleaner followed by lapping in the valves restored clean and efficient operation.
On my engine there were minor contact marks between several inlet valves and the tops of pistons, but no damage as the ‘kiss’ was gentle and seeing as the valve operation was in the same direction as the piston movement, contact would only encourage the valve to close. However, the inside of the ports and chambers were a real mess with a horrible sticky brown fuel/oil deposit coating everything. It was very easy to wash off using aerosol carb and choke cleaner but I ran out before finishing, so completed the cleaning with brake cleaner which worked just as well. Once clean, I lapped the valves in, reassembled and refitted the heads which was followed by an instant engine start and ran really well. A subsequent compression check showed all eight cylinders delivered between 170 and 180psi.
The inside of the ports also benefitted from the same cleaning.
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Please Note: The information provided in this blog is intended as a general guide only. While we aim to ensure accuracy, classic vehicles can vary, and procedures may differ depending on model and condition. Always consult your vehicle’s official workshop manual before carrying out any work. If in doubt, seek the services of a qualified professional.
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