Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 Violent misfiring on full throttle at 3-4000 RPM

This morning a new featurette on my M3W. It is misfiring violently on full throttle above about 3000 RPM, where it feels a bit like an old fashioned distributor rotor hard rev limiter. I am thinking fuel starvation. I wonder if this might be a symptom of incipient fuel pump failure given I am still on the original LR pump after 8 years but only around 7,000 miles. Anyone else had this symptom? I am about 1/3rd of the way through a tank of 98 octane, bought from the same reliable source in France, where I have bought all my 98 for the last 16 years.

The last time I had something like this around 50 years ago on a racing Healey 3000, it took us months to find the cause but only after we had changed the fuel lines, fuel pumps, carburettors etc. We took the fuel tank out to get it steam cleaned, rumbled and coated. When we collected it from the fuel tank people, they handed us a 6 inch square of very smelly chamois leather and said: "do you think this might have been your problem?" On hard acceleration, it must have drifted back in the tank and covered the fuel pick up. I don't think this can be the cause in the M3W!

Wilson

We had similar sounding mis-fire on a long trip in France a couple of years ago...repeated fiddling with plug leads at the roadside stopped it for a short periods...on return home new leads have meant no recurrence...original leads looked/measured ok so no idea what was going on

The fuel pressure seems fine at 48PSI but of course I cannot check to see if it is dropping under full throttle load. I am suspicious that it might be plug leads or the coil pack. I will phone Marcassus in Toulouse to see if they have spares. Does anyone know if it is possible to fit a Harley Davidson coil pack? I have an H-D Dealer at Roquebrune sur Argens about 40 km away.

Wilson

The standard plug leads are pretty basic and aren't great for longevity. If you can manage to borrow a set of good ones to do a swap test it could at least rule them out. Definitely sounds like you need to hook up to ProTune and check for any stored error codes too.

Calum,

I only have Mac computers, which from memory don't work with Pro Tune. I am ordering a new set of plug leads, plugs and coil pack from Brands Hatch Morgans. My last set of leads if that is the issue have lasted 8 years, so if the next set also last 8 years, that will about take me through to a Zimmer frame. I don't see myself climbing in and out of an M3W in my mid 80's. Indicative of a plug lead problem is that I had to spray them one slightly damp morning last month with WD40 to get the M3W to start. The plug tops are quite corroded. I did change them about 4000 miles and 4 years ago but another change will do no harm. I have asked for one grade cooler.

Wilson

The fuel pressure should be 58 psi, so that could be the problem if it is only 48 psi. Have a check that the spark plug screw caps are fully tight as they sometimes work loose and cause ignition issues. NGK Iridium DCPR7EIX are a good alternative to the standard Champion RA8HC spark plugs. There is an equivalent Harley Davidson ignition pack p/n 31655-99 that may be easier to obtain. The standard Morgan fit is S&S 55-1576. Magnecor do some very nice replacement KV85 ignition leads p/n 2547, if you find you do need some new leads.

Wilson
As ever, lots of ways for an engine to end up with a misfire but I'd be suspicious of a combination of plugs and leads both being past their best. The connectors on the plug ends of the leads aren't exactly brilliant when new so throw a bit of deterioration of the leads and plugs into the mix and it could be the culprit. Don't know if you can use one of the Windows emulators on a Mac to get access to ProTune, I'm sure somebody must have tried
Calum

Calum,

I used a Windows emulator years ago on one of my Macs to access some Windows only program but it caused a myriad of problems with my Mac to the extent I had to do a reinstall of the base operating system, so I would be very reluctant to try that again. I was running NGK Iridium plugs, whose caps were tight. I have just changed to a used but good condition Champion RA8HC plugs with no apparent change. The whole thing came on suddenly this morning, so I would doubt that it is fuel, as you would expect that to creep up on you. The Iridium plugs did not have the whitish appearance of fuel starvation. Brands Hatch said that given the symptoms, the first thing they would change would be coil pack and leads, so that is what I will try. Interestingly the tops had vibrated off the Champion plugs in their boxes but the NGK's were still totally tight. To check the fuel pressure, I am using an old Vintage Tyres pressure gauge, that came as a "freebee" when I bought a set of 4 Michelin TB15 race tyres for my 911, as I don't want to get petrol in my good tyre gauge. I know it under-reads a bit on tyres so the rear pressure may well be more than 48PSI.

Wilson

I too had only Mac, until I purchased a M3W, then I added a Windows laptop that now resides in the garage specifically for ProTune. It is just another Morgan specific cost.

I did the same. There are a bunch of nice “executive” style refurbished dell laptops that get sold on eBay for a couple hundred. Also check the windows version as there might be compatibility issues.