Misfire after 20 minutes

[DaveGT6]

Any one got any thoughts on this little problem that has raised its head?

The car is a 2l straight six Triumph GT6. No changes have been made since it was running clean but it has started to misfire after about 20 minutes of hard racing. The misfire comes in at 6000rpm and continues up to red line at 7000. It isn't as agressive as a rev limiter but the engine sort of fluffs and won't rev cleanly. It stops as soon as you ease off the throttle. We thought it might have been plugs fouled by Millers so reduced the dosage but its still there. Can't be points bounce as its on Aldon Ignitor and its certainly not valve bounce.

I'm going to change the plugs again as a matter of course and will try it on harder plugs to see if that does anything - although it has run on NGK B7EVX for ages without issues, I'll experiment with 8s.

Anyone any ideas?

[Al Weyman]

Valve float caused by weak valve springs? I would first look at the coil if it has one though as they can start playing up when they get hot. What plug gap are you running as I had a similiar problem incorrectly reading that with the HEI ignition system they should be set at 60 thou (stock on a Chevy with electronic HEI) however with the MSD module MSD recommend 36thou as any bigger causes a misfire at top end, closed them down to this and it was fine, you can hear the misfire at Thruxton on my YouTube vid.

[COLIN STUBBS]

does it misfire in any gear when it starts to do it? Dont discount lack of fuel pressure/flow if its just a full power top geary type problem. Or even sumut as daft as a bit of evaporation in the carbs if they are near the exhaust.
Does sound more like loss of ignition power though....

[Tim Falce]

When it's hot will it run at tick over OK, maybe valve clearances ar getting on the small side.

[terence]

Fueling?

[ian_w]

A high speed / throttle misfire is typically ignition related. What do the plugs look like? Be careful about going for harder plugs as these are much more likely to foul and give misfires. In the past I've fixed a misfire by switching to softer plugs ( in that case going from an NGK 8 to a 7 ).

In my experience, most people fit plugs that are too hard ( too high a number in NGK terms ), this leads to the plugs running too cold and looking very black - ideally the plugs should be very clean with the centre electrode looking virtually like new. In the old days, the lead deposits gave you a good indication on your AFR, its much more difficult on unleaded and plugs these days run much cleaner.

Here is a link to the NGK website with some pictures:
http://www.ngkntk.co.uk/working/RunS...=ASP\Pg170.asp

[Al Weyman]

I agree with that Ian as I used to do the same myself, now I get equally good results on a road car type NGK without the expense of some fancy racing plug.

[Peter Mallett]

Check you aren't getting oil up the distributor.

[tristancliffe]

As well as the above, check the earth of the Ignitor, and try and check your battery voltage whilst at high revs. If your battery is dying it might not be able to cope at higher revs.

Also, double check that your rev limiter hasn't reset (if you use one), and the soft limiter is cutting in early.

[DaveGT6]

Hey, thanks for all your responses - there's some really useful stuff here.

Valve float caused by weak valve springs? - No don't think so, non broken and top spec. and recent.

I would first look at the coil if it has one though as they can start playing up when they get hot - yup I'm gonna put a spare on to see.


Dont discount lack of fuel pressure/flow if its just a full power top geary type problem. Or even sumut as daft as a bit of evaporation in the carbs if they are near the exhaust. - fuel pressure a little low at 3 PSI (I'm on triple Webers) so on advice from Chris C. I'm going to up it slightly. It could be evaporation as the carbs are VERY near the exhaust.

When it's hot will it run at tick over OK, maybe valve clearances ar getting on the small side. - valve clearances checked and adjusted so got that one covered.

Fueling? - Could be. Funny that it's OK for about 20 mins or so though. As said I'll tweak the pressure up a bit and look to some insulation. Mixture is bang on - Chris had it on the rollers to check.

A high speed / throttle misfire is typically ignition related. What do the plugs look like? - plugs look good. Light brown, no glazing, no sign of overheating and all dry. I'll try the 8s instead of the 7s but note your comments re-black and fouling.

Check you aren't getting oil up the distributor. Presumably resulting in arcing? Checked inside the cap last time and it was clean but I'll look again. If I've misinterpreted your comment please correct me!

As well as the above, check the earth of the Ignitor, and try and check your battery voltage whilst at high revs. If your battery is dying it might not be able to cope at higher revs. Mmm interesting thought. I'll check the earth. I run an alternator and battery has plenty of cranking power.

Also, double check that your rev limiter hasn't reset (if you use one), and the soft limiter is cutting in early - It's the type with a dial on it and it hasn't been reset. Also it only starts missing after 20 mins of hard running so I don't think that's it although it DOES feel like a soft cut.

I think my money is still on an ignition problem. The plug leads have been on the car a good number of years so I'm going to change those and the coil before the next race. I'll blast the plugs and refit (7s) as consensus seems to be against fitting harder ones and Chris (engine builder) fits 7s. It doesn't feel like fuelling as it doesn't cough and splutter and goes away the instant you lift off throttle when a fueling issue is usually delayed response as the flow catches up.I'll also run without Millers to see if it's coating the plugs.

Cheers!

[Al Weyman]

Watch you dont get in to detonation problems without the Millers especially if it is a leaning out problem. I aim for an absolute minimum 4 psi at max revs and about 6 to 8 ticking over (Big Holley). A fuel pressure guage is a very useful instrument if you dont have one especially when trying to eliminate problems like this.

[DaveGT6]

Originally Posted by Al Weyman

Watch you dont get in to detonation problems without the Millers especially if it is a leaning out problem. I aim for an absolute minimum 4 psi at max revs and about 6 to 8 ticking over (Big Holley). A fuel pressure guage is a very useful instrument if you dont have one especially when trying to eliminate problems like this.

Good point about the detonation - I'll keep an eye (and ear) on that. I have a fuel pressure gauge - a couple of years ago I was chasing an erratic and intermittent misfire which at the time I put down to electrical. Turned out that the fuel pump kept momentarily cutting out at random. Only sussed it by pure chance when I turned the pump on and for once it didn't start clicking straight away. Would have been a lot easier with the pressure gauge that I subsequently fitted - you live and learn!

[tristancliffe]

A dying battery can show good voltage at low load, but a bigger voltage drop at higher load than a healthy battery... I don't know what type of battery you use, but a "drop-test" can show up problems...

I agree it's probably unlikely, but surely worth a look if you can't find anything else wrong.

Ignitor's website suggests that 90% of ignition problems with their stuff are caused by poor earths...

Neither help with the 20 minute thing though - that's the tricky bit!

[GORDON STREETER]

I suppose the fuel tank is breathing properly ? (seeing that its time related)

[bdwoody]

dave is the 6 running a dizzy if so check the rotor arm my tr6 did something similar a few years ago.try holding the king lead above the rotor after it has run for a while if it "cracks" its going to earth and it shouldn,t or just change it and try it.

[Tim Falce]

Head gasket?

[bdwoody]

after 20 mins tim ? then ok when cold ....

[Tim Falce]

Originally Posted by bdwoody

after 20 mins tim ? then ok when cold ....

Yes, I've had similar on my Jaguar, mine didn't even start using water for a quite a while, or it could even be a cracked liner they can open up when hot then close when cold.
Hopefully it wouldn't be that though but they are all things to rule out.

[JohnD]

Damn!
Gordon got in before I could!
"misfiring after 20 mins" is a well-known Triumph foible, due to a blocked breather on the fuel tank.
Stop as soon as it happens and asap take off the filler cap. A hiss of air being sucked in as you loosen the cap will confirm it.

John

[GORDON STREETER]

DaveGT6 if you can't cure it just do shorter races
The fuel tank thing is as old as the hills but I assume you checked that first ?
I had a misfire that I couldn't get to the bottom of and booked a track day midweek and took loads of spares. After spending most of the day driving round and changing things it was still there !
I was doing a long race that weekend but the problem wasn't there in qualifying nor the race, and touch wood never since ! so I'll never know.

[DaveGT6]

Originally Posted by GORDON STREETER

DaveGT6 if you can't cure it just do shorter races

Ah the voice of reason!

I'm replacing the leads, dizzy cap, rotor arm, coil and plugs as a matter of course as I'm pretty certain it's an electrical gremlin. I'll check the installation of the Ignitor to ensure that the earth is good. Trouble is if this lot cures it I won't know what was at fault!! Still cheaper than arranging a test day though (assuming I fix it?!).

I've checked the fuel tank non-return valve and it is breathing fine. The fact the misfire stops immediately that you back off from full to partial throttle implies to me an electrical rather than fueling issue as fuel shortage doesn't recover instantaneously.

Pretty sure the headgasket is fine. It's not losing water or pressurising the cooling system plus there's no tell tale white vapour from the exhaust and the plugs look ok.

Keep up the ideas though it's certainly making me go through a big list of possibilities.

[GORDON STREETER]

Rotor arms are the bain of my life. One put me out of a 40 min race on lap 3 at the A1 GP Heritage support earlier this year. The blue ones seem to be better if you can get hold of genuine ones. We had a thread on just this, along with electronic dizzies.

The fact that it disappears after backing off makes me think it could be a fuel/plug related problem.
But its this after 20 mins time thing that's puzzling ?

[Austinspace75]

[QUOTE=DaveGT6]
Valve float caused by weak valve springs? - No don't think so, non broken and top spec. and recent.
[QUOTE]

Are these 'recent' springs different to the ones you were running previously? Does the change of them coincide with the emergence of the problem?

The only reason I ask is due to some experience my Dad had with a racing Imp in the early 90's where he fitted some new top spec and apparently 'correct' (the right colour) carb valve springs and then had loads of trouble with the engine 'going off' after 15/20 mins... only to find much later on that the springs were not actually the rate that they were stated to be. He refitted the old ones and it cured the problem immediately.
The only difference I can see between your problem and his is that the Imp would also be very difficult to start when hot with these erroneous springs fitted. Doesn't sound like yours does this however.

Its just a thought.

I do hate these sorts of problems, its always something small, but could be any one of such a long list of possiblilities. Best of luck finding it.

[COLIN STUBBS]

Just to say a little more about my previous reply ages ago I raced a Vauxhall VX4/90 in classics and we fitted an old Blydenstein 4 branch manifold which was very close to the twin carbs. After about 20minutes of running the engine would splutter and die. We fitted insulation and god knows what to it and in the end had a small fan and some ducting which sorted it. Iv heard of this happening also when a tubular exhaust gets a bit thin due to age so could be worth a thought.....

[DaveGT6]

Originally Posted by GORDON STREETER

Rotor arms are the bain of my life. One put me out of a 40 min race on lap 3 at the A1 GP Heritage support earlier this year.

The fact that it disappears after backing off makes me think it could be a fuel/plug related problem.
But its this after 20 mins time thing that's puzzling ?

Yes the quality of repro rotor arms is quite appalling. I've had problems with them as have many people I know. A friend ground to a halt at Copse corner on first lap of a 40 minute race a couple of years ago due to rotor arm failure!!

Re. comment on valve springs - the engine was rebuilt last year and ran all year faultlesley. Nothing has been changed this season. It appears to have acquired this +20 min misfire whilst sitting in the garage over the winter (warm, insulated and carpeted -yes honestly!!) so I'm sure it's not a mechanical issue.

Of all the responses I still like Gordon's observation that a sprint race career would eliminate the issue!!! However, we'll get to the bottom of it yet. Any more thoughts?