M Turbo misfire

[Ron McC]

Hi All,

For the past few weeks my M turbo has started to misfire when under acceleration. It's also started to backfire now and again when sitting at idle. So far I've done the following

New Iridium sparkplugs,
New ignition leads,
New distributor cap and rotor arm.

Is there any possibility that the distributor itself is worn or breaking down internally? Can the timing be altered slightly if the internals are worn.

The vacuum advance/retard side of the dissy is working okay.

Any feedback/ideas more than welcome.

Cheers

Ron McC BOF246T

[DavidY]

Ron

The distributor drive is known to wear, but I would talk to Adrian Venn before doing anything else, he knows these cars inside out.

davidy

[TVR_owner]

Ron,
If the misfire is only on when the car starts to boost, it maybe a plenum chamber fuel leak? (car cannot maintain Fuel pressure obver bosost pressure, runs very lean and develops misfires).

[Ron McC]

Any info on how the timing should be set up bearing in mind the car has Luminition fitted. The timing marks on the cover 1 lower, 1 upper, which one is tdc.

Thanks for the replys, still scratching my head though.

Ron McCubbin

[thegamekeeper]

Screwdriver !!!!!!!!!!!!!!!! surely you mean a dial gauge, less than £19 now at machine mart. Adrian I hope you dont use a screwdriver to set valve timing!!

Edited by thegamekeeper on Monday 1st September 19:44

Edited by thegamekeeper on Monday 1st September 19:47

[Ron McC]

Had another look at the car this afternoon, have discovered that if I disconnect the HT lead from the No6 (left hand bank, rear) spark plug the backfire dissappears. The car still runs fine and idles ok,however no backfire. Reconnect the HT lead and the backfire makes an instant reappearance. Any thoughts!!!! I did start to think of bent valve or valve gear gap needs readjusting.

Edited by Ron McC on Monday 1st September 20:05

[Ron McC]

Compression test done.

R/H bank front 130 psi R/H bank centre 120 psi R/H bank rear 130 psi

L/H bank front 130 psi L/H bank centre 120 psi L/H bank rear 120 psi

Disconnected L/H centre and the backfire was still evident,reconnected that up then disconnected L/H rear HT lead,no back fire, went for a quick trip and it runs smoother when that particular plug is not firing. Could I possibly be looking at a burnt out valve/seat or just an adjustment of the tappets.

Ron McC

[Ron McC]

Yes,the running smoother with plug No6 disconnected has me a bit baffled as well. At the present moment it's still on the original cylinder heads and I use one of those "bullets" to convert the unleaded to leaded. Fuel passes through a chamber filled with lead pellets which "leads" the fuel before it goes into the carb. Meant to get about 50,000 miles out of it before it needs changed.I had it fitted to the car when I bought it in 03. So far I've done just over 6500 miles in it.


No unusual clattering or horrible noises coming from underneath the rocker cover. Will investigate further on Sunday.

[oliverb205]

As I recall it Practical Classics did buy one and tested it on an old engine of a car they had bought, and they found that it had vitually no benefit. The big test was done by the Federation of British Vehicle Clubs, and Fuel Cat or any of the other similar companies refused to let their products be tested. The FBVC tests found just 4 additives to work, and another 3 or 4 have been approved since.
Although not a scientist, I cannot see that lead pellets would ever dissolve in petrol to anything like the required amount.

Oliver.