33 replies to this topic

[PaulColeman]

I took my Daughter's 1976 Mini 1000 for its MOT the other day and it was very lumpy. It passed with an advisory that it was only running on 3 cylinders, so I investigated a bit today. I thought it might be the dizzy cap so I cleaned all the contacts inside but it made no difference. So then I removed all plugs. 1 (nearest the fan) and 2 were a nice golden brown colour but 3 was black and wet. 4 was black but dry. So I then did a compression test and got 145, 150 and 140 on 1, 2 & 4 but 3 has virtually nothing at all. The oil is clear and looks clean and there is no white mayonnaise under the filler cap. I spoke to a friend of mine who suggested it could be a stuck valve so I pulled the rocker cover and had a look but both valves on number 3 were moving up and down correctly. So then I checked the clearances of that cylinder and they were only slightly out. So I adjusted them up and checked the compression again but it had made no difference.

 

What are the collective thoughts please? Anybody had a similar experience and solved it?

 

It's gotta be ready for the London To Brighton or she'll never forgive me!!

 

Thanks, Paul.

[Swift_General]

Head gasket, or could be something more serious. Either way the head needs to come off to investigate.

Edited by Swift_General, 25 March 2017 - 02:50 PM.

[sonikk4]

Hold fire, try adding a drop of oil down that pot first. Redo the compression test. If it goes up then the rings are nadged, if it still stays the same then either got a burnt or bent valve or blown hg. 

[PaulColeman]

Thanks for the replies. We put a new head gasket in it not too long ago - a couple of years. Why would it have gone so suddenly and where would the break likely be? I expected water in the oil if it were a head gasket?

[Northernpower]

The quickest way to sort it is to whip the head off. Its either a burnt valve of blown gasket. Either way, you and us can speculate as much as we want but you won't find out until the head comes off.

[PaulColeman]

Ok. Will have to wait until I'm in the right frame of mind!!

[sonikk4]

Thanks for the replies. We put a new head gasket in it not too long ago - a couple of years. Why would it have gone so suddenly and where would the break likely be? I expected water in the oil if it were a head gasket?

 

It can go between cylinders however that tends to predetonate the next cylinder making some horrendous noises. Had that with an old SRI Cav of mine. Sounded like the bottom end had failed but it was just the hg. BUT as you are getting compression on the cylinder next to it make methinks it could be a burnt or bent valve or even a hole in the piston. Try the test with oil first then go from there.

[PaulColeman]

Ok. Last year (800 miles ago) the MOT man failed it on emissions - the CO was 10% and couldn't be adjusted down to the max allowable. So I fitted a new jet to the carb and took it back and the MOT man set it to 2.5% but my Daughter then mentioned after a while that it was a bit gutless and not as responsive as it had been. So this year I asked him to increase it to 4% but as I drove it to the MOT station it was lumpy. I don't know how long it had been like that as I only ever drive it once a year when it needs the MOT!!

[sonikk4]

Could be too lean which means then the burnt valve is a possibility.

[PaulColeman]

Yes sounds like it. Do you know what percentage the CO should ideally be to avoid this?

[sonikk4]

Yes sounds like it. Do you know what percentage the CO should ideally be to avoid this?

 

Sorry Paul i do not but i'm sure someone more knowledgeable will come along shortly.

[tiger99]

It is amazing that it passed the MOT on emissions with a cylinder missing. That suggests that it was not passing lots of unburnt fuel down the exhaust, so no fuel being sucked into the failed cylinder, so most likely a really badly damaged exhaust valve. Not a big problem but it's seat may need to be recut, based on similar experience with a very different engine.

I don't think you can relate exhaust CO level to valve temperature as it is dependent on so many different things. On modern engines and light aircraft there are invariably exhaust gas temperature (EGT) probes in the exhaust ports, and these tell you A LOT about what is going on. You can safely weaken the mixture on an aircraft up to the point where the EGT gets to a certain value, and no more. EGT probes could be fitted if you were doing some major engine development, but are more commonly found on the latest low emissions engines as the ECU needs that information.

I would suggest taking an hour, if you are not used to it (we can get it down to maybe 10 minutes with experience) and whip the head off for a look.

Drain the cool down to below head level, typically by undoing the top end of the bottom hose. Manifold unbolted. (If you have substituted bolts for the studs it is much easier). Wires out of the way. Mark plug leads for correct fitment afterwards. You will not be disturbing the timing. Top hose off (thermostat housing can stay on head for now). Rocker cover off (2 nuts, I think 11/16" AF spanner or socket but am not quite sure). Undo the 5/16" UNF nuts (1/2" AF socket) on the rocker pillars. Loosen the 3/8" nuts in the reverse order to tightening (9/16" AF socket) and remove. Lift off the rocker shaft assembly, and then take out the push rods, keeping them in strict numerical sequence for refitting. Give tyem a twirl as you lift to break any suction between rod and tappet, because you do not want to lift a tappet out of its place. A few quick blows around the sides with a softish hammer (copper or hard plastic) to break the seal and it will be ready to lift, usually.

I think I could do it quicker than typing that out on a phone! It is not hard. The forum will help if you run into any snags.

A revised sequence leaves the rocker shaft held by the 5/16" studs only and a quick blast with the starter will break any adhesion of the head. Rocker shaft and pushrods are removed last, immediately before lifting the head. Takes no longer and may save aggro.

[PaulColeman]

Thanks for the reply/suggestions. If too lean a mixture has caused the problem I need to get it setup so that it won't happen again. What level should I be aiming at to do that?

Thanks, Paul.

[gazza82]

If gasket had gone between 3 & 4 both cylinders would be down. Either rings or a valve or valve seat probably.

[PaulColeman]

Yes, the last time the head gasket went it went between 3 & 4 and as you say, both were down. I think the cause of the last head gasket failure was that some of the head studs were loose.