Misfire - an interesting story...

[John Clancy]

It may be an interesting story this one but it's driving me crackers!

A misfire began some time ago that led me to call the good ol' RAC having established that nothing obvious (plugs, leads, condensor, coil etc. etc. etc.) would cure the problem. As the engine got hotter the problem got worse until it virtually stalled. If I didn't keep revving the engine it would have cut out altogether.

The conclusion of the RAC man was the head gasket. This was not my opinion and his conclusion rather shocked me as it was the last thing I would have thought of. I think the TR7 still suffers from the reputation it gained very early on.

The head was removed and taken to a local engine specialist who employ a chap who spent about 40 years on Triumph engines. New TriumphTune valves and guides were fitted and the tappet clearances set.

After reassembly there was a 'ticking' when the car was moving but at rest it was probably quieter than it has ever been. Going up inclines it was obviously down on power as I could feel it surging or misfiring (but not audibly). The timing and points were spot on.

I've been taking advice and playing around a bit today with a view to changing the timing chain but apparently a loose chain would not have caused the lack of power unless it had slipped - it hadn't. Then I risked taking it up the road again and it was misfiring badly. By the time I'd completely the few hundred yards trip the engine was misfiring exactly as it was when the RAC man diagnozed the cylinder head gasket.

I've just renewed the condenser again to prove it isn't that. Anyone got any ideas? Need any more information?

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[jclay (RIP 2018)]

John,

Trying pulling the distributer cap and check the inside for cracks or carbon arcing lines. Start the engine in a closed garage with the lights off and see if there is arcing around the spark plug wires.

My daughter's Mustang has a similar problem and it proved to be a rubber mount on a ballast had perished and the ballast would short out on the fender wall if you turned just right.

If it was a US car, I would tell you to pull the air pump and block the air injection ports. Hey, check the exhaust manifold for cracks.

[jclay (RIP 2018)]

Wild A$$ed guess, you could have an air leak in the intake system that is leaning out your mixture. It doesn't leak when the car is cold, but as it heats up, the metal expands and lets air leak into the system somewhere. Could be carb mounts. It is definitely something that was not touched when the head was off.

[ponyboyone]

Have you checked the fuel pump? Check the plugs right after one of the missing/stalling incidents, how is the burn? Very clean white looking plugs means fuel starvation.

[John Clancy]

They're all a nice biscuit colour Pony. Haven't checked the fuel pump - any recommendation on how I should check it? Perhaps just check there is still fuel in the pipes after it's started misfiring?

I've been looking for arcing JClay but can't find any evidence.

One thing that occured to me whilst just speaking to 'Uncle' Rex, TR8 registrar of the TRDC was during the clean up of the engine bay prior to the head going back on I had the carbs and manifold just hoiked out of the way at right angles. Could this have shifted something which once back in place has just taken a few days to settle. I think the covers are coming off those Webers in the morning.

Keep the suggestions comin' fellas, however ridiculous.

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[jclay (RIP 2018)]

Pony has a good point. Bad pump or blocked fuel filter. Even water in the lines.

When does it backfire? All the time when it running? What about at idle?
When you are accelerating? When you let off the gas?

If it idles, but backfires when driving, it does sound like the fuel pump. I was driving down the high-way when mine went out. Car just went to a stop. I could start it but not drive over 2500 rpms. Drove home at 20 mph.

[bmcecosse]

Check/change all the ignition system bit by bit. Dizzy cap/rotor arm/HT leads/coil/condenser/points (or is it electronic?) - maybe even rig a bypass wire to eliminate broken wire/poor connections. It's almost certainly electrical.

[John Clancy]

I too thought it was electrical to start with but having changed every component involved (except points so far) and still have the same problem I'm now thinking along the lines of fuel. Martin at Robsport suggested bypassing the tank so I shall get a length of fuel pipe and give that a try.

Prior to that though, I'll take the top off the back Weber as the misfire seems to start on number 3 cylinder and then gets worse a short while after - so I'm currently thinking it's misfiring on 3 and 4 (a bit on conjecture here admittedly but bear with me) which denotes a problem coming from the back carb. I've had water in the tank before and know what that feels like - this is slighly different so I'll have to see how it pans out. Perhaps there's a valve in these carbs somewhere or a clogged jet. Perhaps it's just filling up with water from the tank after all the rain we endured a while back.

Probably the first thing I'll do is take the top off the back carb. I had the carbs and inlet manifold at right angles for a while during the cylinder head overhaul so if there's something in there that would have shaken it about a bit answering why the problem appeared to have been solved when the head initially went back on. 10 miles later and it's back.

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[John Clancy]

Prior to setting about the latest test I ran the TR at standstill until the misfire commenced. I then pulled off number 3 lead - made no difference denoting that cylinder was misfiring. I then pulled off number 4 and that made no difference either. Conclusion = back carburettor/fuel problem.

I took Robsport's advice and run a great length of petrol pipe from the pump into a jerry can in the cockpit. This sort of thing always makes me nervous as I don't like playing around with petrol. <b>BUT</b> I've just driven the TR up and down the road and even on the steepest incline there is no misfire.

Prior to this after a short period where it had a bit of a misfire I stopped, let the engine cool down for a couple of minutes and restarted. There was then no evidence of a misfire. I switched off and restarted a couple more times and probably had it running on the drive for over 20 minutes before driving out. I couldn't detect any semblence of power loss.

I haven't had the petrol tank filler neck cover off yet but either all my water proofing work there over the years has been for nothing or I picked up a tank of dirty fuel. Or, there is some sort of vacuum between the tank and the back carb.

I haven't quite been in the current position before with this ongoing problem but I have been close so I won't get carried away and will try again tomorrow. If it misfires again tomorrow today's success has just been throwing me a googly. Whatever happens surely I'm on the right path now.

Question: How long is the breather hose from the fuel tank?

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[simon hubbard]

I wonder if it is possible to let me know what the outcome of this was problem was? i seem to be having similar problems. Thank you. Simon.


TRR676R Speke Built 2.0L Tahiti Blue TR7

[John Clancy]

The cause appears to have been dodgy fuel. I sucked everything out of the tank and I've kept the worst of it in a jar for posterity. It has a waxy, greasy look and feel to it. I suspect silicon but whatever it is there was plenty of water too and that rotted the most forward base part of the tank. This was causing rusty sediment and that appeared to cause a blockage to the back carburettor. Strangely, the front carburettor was always okay hence why no one could diagnose the real cause.

The car has now done over a thousand miles since the tank was changed for a brand spanking new one which was the only way out of this problem.

Let me know your own symptoms Simon and I may have some good suggestions for you. The length of fuel pipe from a jerry can to the petrol pump was a brilliant recommendation from Robsport that ultimately pinpointed the culprit. Something I will remember to always try with similar future problems.

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[simon hubbard]

Thank you John. The contents of the fuel tank are now low. So maybe there is some muck in there, i might fill it up at lunchtime. Recently had the fuel sender unit replaced so the gauge now works. But from cold the car drives ok, as it warms up it gets worse - however sometimes it is ok?!. I need to remove the plugs etc and check them all. But if something was out then should it not happen all the time?

I also have heating/water loss problem. i dont kow if the water pump is faulty, Robsport said it maybe working intermitantly, or if the head gasket has gone.

The car heats up and if it gets hot - 3/4 on the gauge it seems to drive fine, if like like this morning (using the car to drive to work, about 9 miles) it stays just below 1/2 on temp gauge its seems to misfire, progressively getting worse until it stalls (unless revs are kept up).

my skill and knowledge is limited, dont want to let it beat me but it is driving me mad.

Simon


TRR676R Speke Built 2.0L Tahiti Blue TR7

[simon hubbard]

I have the car booked in at my local garage to try and ascertain if the gasket has gone and to get the water loss/running hot problem looked at - prssure test the cooling system? i might have a go at changing the waterpump myself, but it seems alittle daunting.


TRR676R Speke Built 2.0L Tahiti Blue TR7

[John Clancy]

I wouldn't recommend attempting the water pump yourself. I would recommend taking the car into Robsport where they deal with these things all the time.

The simple test to check the petrol is to get a jerry can of fuel and a 4 metre length of fuel pipe. Disconnect the feed from the tank at the petrol pump and connect the new length of fuel pipe to the pump. Then insert the other end of the pipe into the jerry can and stick it through the driver's side window into the footwell. Go for a drive and see what happens. If you have the same symptoms you can eliminate fuel. Don't forget to block off the disconnected main fuel pipe so you don't lose any petrol

3/4 on the temperature gauge is too hot. However, many senders and gauges are faulty to start with so it may actually be okay. If it's reading true though then the chances are the engine has been overheating which would quite likely point to a blown head gasket. The compression test should give a value in excess of 150psi per cylinder. Anything under this and I think you've found your cause.

Another possibility is a leaking inlet manifold. Spray WD40 all around it whilst the engine is running and if you see any bubbling anywhere then that could point to the possible cause.

If you have points rather than electronic ignition ensure the dwell angle is set to 39 degrees (43.33%) and if it's vastly out from this you'll find the car will run like a dog. Very slight misfiring up and down hills is how my own car tells me it needs a tune. When mine had suddenly gone way out one time without warning overheating did ensue.

Keep us appraised of how you get on and dependent on your evidence I'll doubtless come up with hundreds of other possibilities.

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[Rblackadar]

I know there were vapor lock issues with the early Speke builds. Apparently the pump mounting location wasn't the best for heat dispersion, hence the spacer they added later.