Missfire at top end

[kelvinwannell]

Can anyone help with my 4.0 chim missfire, heads skimmed, lifters all good and cleaned, cam is good, valves had a light clean, all ok. Engine back in the car, starts drives etc. good on the bottom end, but as the revs get up, is starts to miss, very unhappy at the high revs.

New coil this week, leads, cap, rota, were last year, just one lead failed, now replaced. Timing seems ok on the strobe 8 BTDC at tickover and it ticks over fine. Any help geatly appreciated.

Kelvin

[bobfather]

I struggled to find a misfire like you describe, slowly changed things one at a time. Turned out to be the ignition amp on the coil bracket

[QBee]

Amazing how many of these threads turn out to be the ignition amp.....

[kelvinwannell]

The one thing I have not changed as yet, thanks for the reply. I will order one now.

I have checked all my leads, cap and rota, they are good (not very old and no Beru, just heat sheild, which is far better). What you have suggested makes perfect sense.

Thanks for your help.

[bobfather]

Ignition amp suffers from overheating, be sure to buy some thermal transfer paste to put between the amp and its bracket. You also need to check which one you have, there are different wiring configurations

[Belle427]

Plus the aftermarket ones available are all junk.
Some old stock comes up on eBay from time to time.

[Zener]

Belle427 said:

Plus the aftermarket ones available are all junk.
Some old stock comes up on eBay from time to time.

This ^ to the point I would rather fit a used OE Lucas (no not the green and white boxed generic crap) than currently available

[blitzracing]

Keep an eye on your tacho when its misfiring, if its erratic its missing ignition pulses from the amp. If its stable the problem may HT related.

[spitfire4v8]

Favourite thing I find for high rpm misfires is actually the coil .. and for some reason the worse culprit is the nice new shiny bosch units .. if i have a car on the dyno and it has a new bosch coil i know to expect issues. A cheap and cheerful local parts factors replacement cures it every time.

[Belle427]

It seems to be a big problem on these cars now, just a pity you can’t buy quality ignition components anymore.
Bosch were always quality and got recommended 100% of the time if a coil was needed so that seems to be a problem now too.

[QBee]

There's a lot of fake Bosch components around on the net these days. I have gone back to going to my local auto-spares place.
You know, one of those places where you have to get off your arse, get in the car and talk to a guy who knows what he is talking about.