Misfire when hot

[SimonBrown]

My 1969 E Type drives well when cold but after about 10 miles running starts to misfire on part throttle. I can drive through it but am reluctant to rev it over 3000rpm as the engine has just been rebuilt. It is on SUs and electronic ignition. I have checked the usual things to no avail. Any ideas as I cant trust it far from home?

[Peter27]

Hi Simon. The fact that you can drive through it probably eliminates a couple of suspects. What 'usual things' have you checked and what type of electronic ignition have you fitted? When did the problem start happening - or has it been a problem ever since the engine rebuild?

[cactusman]

Almost certain to be electrical....at least that is where I would start. Heat causes things on the high tension side to break down more easily so things like the distributor cap, rotor arm (get a red one from the distributor doctor). Check all the plug leads are secure in the caps and the cap. Also check the plug caps. You will probably have caps with built in resistors. The resistance is not critical but will be in the 5000 to 10000 ohm range. The repro champion caps use crap film resistors and these can fail open circuit or high resistance so any cap that is beyond 10000 ohms dump. Original caps used a carbon rod slug resistance that almost never failed....there is progress! The electronic ignition module is unlikely to be the cause as it will either work or not but check for loose wires etc on both the low tension and high tension sides.

If all the electrical bits are ok then I might suspect either too rich or too lean on one carb maybe. A check of the plug colour is a rough guide. Glazed appearances suggest lean and black or sooty suggest rich. Check the jet heights and floats in the float chambers assuming you have SU carburettors.

Lastly if it only happens at high revs and load it might be lack of fuel.... check the pump, filters etc

[christopher storey]

Well, I agree it may well be electrical, but the fact that it only happens on part throttle and that you " can drive through it" presumably using wider throttle openings and/or higher rpm also suggest fuelling possibilities such as sticking carb pistons on 1 or more carbs, faulty carb damper ditto , or ( just the faintest possibility ) a defective piston spring . I know this is not for the purists, but have you tried richening it with choke when it happens, to see if it makes it better ( which suggests a weakness problem such as an air leak ) or worse ( which suggests rich mixture cut) ?

Incidentally, on the electrical side a further suggestion I would make is to see that the advance weights are not jammed

[SimonBrown]

Thanks for your advice. It is fitted with a Petronix Electronic Distributor module and the misfire has been there since the engine rebuild. Plugs, leads, distributor cap and rotor arm have all been replaced and a conversion from Stromberg to SU's. It happens at low engine speed as soon as it is under load so I am inclined to think it is electrical. I haven't tried larger throttle openings as the engine has done less than 100 miles.
I shall try a red rotor arm and then swap the plug leads with the old ones first I think.

[mgcjag]

Hi Simon...had exactly the same thing....took me months to find after i had changed out all the ignition parts and checked the fuel/ carbs etc........turned out to be the ignition wire to the coil was not fully crimped in the terminal to the coil.......i had unpluged it and checked for 12v lots of times but on one occasion i just pulled the wire and it pulled out......re crimped it and no more mis fire.........if i had this type of fault again i would remake all the circuit connections again.......

[SimonBrown]

Thank you, I do think it is in this area as my car has also been rewired.
A few hours tightening connections will be my first call.

[caveman]

If you are confident that your rotor arm is a quality one, (Distributor Doctor Red are best), it could be your electronic ignition unit playing up.

I had similar, a few years back now, under light acceleration at around 3000rpm.
Remove or disconnect electronic ignition and re-fit points and condenser and try again. Its a low cost way of checking the electronic ignition out and you can keep the points & condenser in your boot tool kit incase it happens out on the road and away from home.

[Hugo]

At first glance this sounds like an HT problem (if it happens at increased throttle openings at low revs). Modern rotor arms are often a cause of trouble, so It might be an idea to replace it again, with a quailty red one. The trouble with HT issues is that you can often 'cure' the problem by replacing an item which is not actually the cause of it. For instance, if your plug leads are breaking down & causing a misfire, you can often get rid of the misfire for a while by replacing the plugs, as the new plugs will place less of a load on the leads. And vice versa.
One thing you do NOT need is a good connection between the plug leads and the distributor cap or the plugs themselves. Quite the reverse in fact. I would try pulling the plug caps off & just laying them loosely on top of the plugs. An old trick with motorcycles was to chop the HT in half and insert a plastic button in the circuit - strip each cut end of the HT lead & twist the wires through diaginally opposite button holes. The spark then has to jump this gap before it gets to the plug, so you get a 'fatter' spark at the plug, rather than one which just 'dribbles' across the plug. Of course you don't want to do that to a 6 cyl Jag unless you enjoy sewing, but that is the principle at work.

[SimonBrown]

Took the E Type out for a run on Saturday and was convinced I had fixed the misfire having found a few loose connections.
After about 40 miles it started to misfire again and finally ground to a halt.
Called the AA who gave a fantastic service and were so careful with my car delivering us both home.
A new rotor arm from the Distributor Doctor and all is well again!

[cactusman]

Good news....I carry a spare arm in a bag under the passenger seat among other bits like fuses and the all important length of wire and a few wire ties

[Marcus2571]

I had something similar which ultimately led to total ignition failure. Checked all leads, replaced rotor arm (red for red!) and still nothing. Thought was failed electronic dizzy but dropped sump & transpired to be failed/teeth sheered off helical gear that drives distributor drive shaft off the crank . Of all the things!

Engine had also been rebuilt very recently so surprising it went - concerned that the gear had been misaligned during rebuild but can't be sure. Faulty part? Chewed up by steel gear that drives it? Who knows but slightly unsettling... Any ideas gratefully received!

Part 11 on attached image.

[Mikael B]

Had similar problem with pentronix and original coil, not a good combination. Misfire on idle and at very high revs. If you have this combo I suggest you bin the pentronix or change coil to one that can handle pentronix ignition.
Did go back to standard configuration for awhile but now moved to Megajolt and wow that is a miracle change.

[Hugo]

Sheared the teeth off the skew gear? Blimey, that's not something you'd expect. You'd drive yourself crazy trying to check the ignition timing! What was the oil pressure reading? If the pressure relief valve is stuck shut it will put an extra load on the drive mechanism, but I would expect the skew gears to tolerate anything the plumbing can cope with. I once saw a Transit explode the oil filter because of a similar problem. What a mess!

[Marcus2571]

Was going up an incline on motorway, Oil pressure was OK until the persistent misfire for c300 yards. Eased off accelerator and realising was about to stall made my way as quickly as possible to the hard shoulder!

Didn't get a chance to check pressure at the vital moment as was otherwise engaged - any ideas on why the gear teeth would have been chewed up?

[Hugo]

In a word, no! Never seen such a thing.

[christopher storey]

My guess would be that something ( chewed up washers or split pins are a favourite ) has got into the oil pump and partially stalled the rotors. I would pull the cover off the pump and look inside for signs of debris/damage

[chrisfell]

My guess would be that something ( chewed up washers or split pins are a favourite ) has got into the oil pump and partially stalled the rotors. I would pull the cover off the pump and look inside for signs of debris/damage

If it was a split pin that got into the oil pump, it'll look like this.







I found the culprit in the oil filter.




In this case the oil pump survived, but was slightly less effective, so I replaced it. What I hadn't bargained for was very tiny pieces of swarf getting through the oil filter and into the oil galleries. Two small pieces got into the cam feed pipes, reducing the flow to the intake cam. Increased wear over the next two years cost me the cam and several followers.




And all because a PO (who shall remain nameless but once claimed to be the worlds largest retailer of E-Types) had installed undersized split pins in the big ends. When I pulled the sump I found the remains of more pins in the sump, and several more pins were loose in their nuts. All were replaced with the correct size.

[Marcus2571]

Oil pump all clear thankfully and the only debris in the sump mesh filter was the chewed gears from this:

[Hugo]

That thrust face looks like it's been under some load. Did you have negative end-float?