missfire at high rpm

[ratrodmog]

Hello everyone,
We have one Morris off the road while I'm replacing the cylinder head gasket so we're using our other Morris for a few days. It seems to have developed a missfire but only at high 'er' revs. We don't rev it up at all, but it stutters as you go up th rev range. Most noticable is when we're driving around 50mph, as soon as you try and get to 55-60 it stutters very badly. This is a fairly new problem as we drove to Cornwall a while ago at 60-70mph the whole way pretty much. It idles fine, starts on the button and driving around town is no problem. Any ideas?
Cheers, Ben

[bmcecosse]

Probably fuel starvation - throw away any filters you have fitted. And check the carb piston rises and falls easily. Could also be tight points gap - check it at 15 thou.

[oliver90owner]

Could also be a problem of dwell angle - usually only a nuisance with V8s but can be caused by a worn distributor . Either a badly worn shaft or a worn out cam lobe. Check points opening for each cylinder and for play in the bearings.

You could check the plug electrode colour, for mixture setting, after a decent run at, or close to, the problem rev range. Seems like an easy fix once the diagnosis is carried out.

[ratrodmog]

Thanks for the quick reply guys . Should have mentioned that it has an electronic ignition kit fitted. It does seem like it could be fuel starvation but it seems odd that it has just changed from running fine all through the rev range to now struggling at higher rpm's.

[amgrave]

If you have got the crankcase breather going to the back of the air filter housing it might be the air filter needs changing due to oil contamination, especially as you have run it at high speed for a distance before the problem appeared.

[liammonty]

I'd be checking the electronic ignition I reckon. If you've got points and a condenser you can put in temporarily, that would be a good way of checking it definitively.

[IslipMinor]

What sort of misfire? A hard 'dropped a cylinder' or soft like running out of petrol? Very easy to chase the wrong things - don't ask me why I changed the complete fuel system many years ago for a 'hard' misfire cause by a overheated, sticking No. 1 exhaust valve!

I fitted an Aldon 'Ignitor' electronic ignition module around 18 years and 50,000+ miles ago and had absolutely no problems with it until last year, when we started to get a high speed misfire, with no obvious cause.

Earlier this year we were on the MSA Spring Classic run and due for a few hours on the Castle Combe Circuit, when the misfire got worse and I traced it to a broken wire inside the red lead that feeds the module. The break was inside the outer plastic covering and inside the distributor body.

I can only think that the constant movement of the base plate with the centrifugal advance has eventually fretted the wire core and created an intermittent fault. When I pulled the red wire it came apart into 2 pieces!

We carried a set of points so re-fitted them as a temporary measure and all worked well for the rest of the run.

Would be worth checking the 2 wires feeding the module for continuity and possible internal break(s).

[ratrodmog]

Finally managed to track down the source of the misfire! Replaced the coil, leads, went back to points, checked timing....nothing. Put everything back on as this made no difference. Turned out to be the +ve wire going to the coil was loose in its crimp connector and finally fell out resulting in no spark. Wouldn't have seen it otherwise! Running like a dream now, hit 75 last night. Result.

[bmcecosse]

Well done - as I've said ad nauseum -it's NEVER the coli, but it can be the coil connections. Always worth checking.

[liammonty]

Well done - as I've said ad nauseum -it's NEVER the coli, but it can be the coil connections. Always worth checking.

It's lucky I didn't pay too much attention to that advice on at least 2 occasions, else I'd still be broken down

[bmcecosse]

Ah but as we have discussed many times - it's only OLD coils that never fail. Moderns are mostly junk made in far away tin huts - and often sold with the wrong impedance - 2.3 ohms instead of the correct 3.2 ohms........ And as you well know - I put this statement on regularly just to remind everyone to check the connections/other parts before automatically going for a new coil - which may be one of the cr@p ones I mention above.... It's my 'catch phrase' if you haven't twigged that already!