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No 4 not firing at idle


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Hi Guys

I've a 1972 US import converted to RHD running SU carbs. At idle no 4 isn't firing as the revs pick up it chimes in and then as the revs drop it stops firing again. I can see this happening using my colortune plug. The car is running standard points any ideas what might cause this would be appreciated.

Cheers Stephen

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Sounds like it must be an ignition problem to affect a single cylinder, or possibly a sticking valve.

Try changing spark plugs and HT leads. Also consider the distributor cap and look for uneven marks on the internal contacts, or tracking between plug leads. I found an intermittent misfire on one cylinder of my try a few years ago was due to tracking on the cap between HT leads connections.

A compression test should show a sticking valve. Alternatively try removing the rocker cover then turn the engine over by hand and look for a valve on cylinder 4 that didn't move as expected.

Nigel

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Hi Nigel

I've just fitted new HT leads a couple of weeks ago as the lead on no 6 broke. With the colortune I don't see a spark on no 4 at idle. With the normal spark plug when I pull the HT lead off no4 I don't notice a difference to tick over, whilst I do if I pull the HT lead off the other cylinders. So my initial thought is an ignition problem but at the moment It doesn't feel like the plug or lead are at fault. I'll check the valve clearance tomorrow when she's cooled down. I do have spare rotor, caps, coils, condenser etc in the boot so will try swapping a few components around tomorrow. 

I think I'll buy a compression tester as well.

Cheers Stephen

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Could you not swop one the ignition wires and see if it follows, please only do one thing at a time, and put that part back on again. That's the only way to find what is actually going on.

John

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14 minutes ago, John L said:

Could you not swop one the ignition wires and see if it follows, please only do one thing at a time, and put that part back on again. That's the only way to find what is actually going on.

John

Hi John

Sure thing I'll give that a try.

Cheers Stephen

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Hi Guys

I checked a few things today. With a new spark plug in No4 HT lead held against the block I get no spark at idle, I start to get spark at 2000rpm. I've tried this with different HT leads, new plugs, a new cap and new rotor arm and seems to be consistent. I get no spark on No4 below 2000 rpm. 

There is some small amount of play on the rotor arm but I'm not sure how much play is normal. I'm starting to wonder if this is related to the distributor.

Any thoughts appreciated.

Thanks Stephen

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You could check the points gap at the number 4 lobe in the distributor compared to some of the others perhaps.

Do you have a dwell meter, this is far better for setting points gap with a worn distributor, but I cannot remember what the dwell should be for the 6 cylinder, could somebody else advise the value needed pls.

This is where an accuspark electronic unit will get over this problem. but general advise would be to send it off to the Distributor Doctor for a proper long term fix,

You should still get a spark at idle even is the compression is wrong/low.

John

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3 minutes ago, John L said:

You could check the points gap at the number 4 lobe in the distributor compared to some of the others perhaps.

Do you have a dwell meter, this is far better for setting points gap with a worn distributor, but I cannot remember what the dwell should be for the 6 cylinder, could somebody else advise the value needed pls.

This is where an accuspark electronic unit will get over this problem. but general advise would be to send it off to the Distributor Doctor for a proper long term fix,

You should still get a spark at idle even is the compression is wrong/low.

John

Hi John

Thanks I have already emailed the Distributor Doctor to see about getting it rebuilt. I'd just like to be reasonably confident that it's the problem before I do so.  I'll have a look at the points gap on lobe 4 as you suggested. 

I don't have a dwell meter it's not a process I'm familiar with so looks like something to do some research on. 

Many thanks

Stephen

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Try changing the distributor cap in case there's tracking between HT leads. As mentioned above, this happened on my TR6 some years ago. It took me ages to track down the misfire but a new cap cured it instantly - a cheap fix.

Nigel

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3 minutes ago, Nigel Triumph said:

Try changing the distributor cap in case there's tracking between HT leads. As mentioned above, this happened on my TR6 some years ago. It took me ages to track down the misfire but a new cap cured it instantly - a cheap fix.

Nigel

Hi Nigel

I have tried changing the cap I get the same results with my old cap and a brand new one.

Cheers Stephen

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2 hours ago, Stephen H said:

I checked a few things today. With a new spark plug in No4 HT lead held against the block I get no spark at idle, I start to get spark at 2000rpm. I've tried this with different HT leads, new plugs, a new cap and new rotor arm and seems to be consistent. I get no spark on No4 below 2000 rpm.

Seems pretty conclusive that you have an ignition problem (but beware - there may be others!). Frustrating though it may be, I suggest you go through it again. As John says above, do one thing at a time. I'd identify a known-good combination of plug & lead, and then watch for spark when holding against the block. Swap the cap, and the rotor arm, in turn. Get another cap, if you can bear it, and try that. As the problem is only on one cylinder, I can't think of anything except the cap.

One thing that does change with RPM is the distributor advance, but again - why one cylinder? I suppose it's conceivable that a very worn shaft might cause the problem, but I can't picture how it would only affect one cylinder. Maybe a combination of poor timing/ dodgy advance springs/ worn drive shaft could conspire?

If you have a strobe light you could have a look at the timing. Not to see if it's correct (but check that anyway); more to see if it wanders. That's a sign of a worn distributor. Mine was doing that until Martin Jay reconditioned it, and now the timing marks are rock-solid under the strobe.

Keep us posted. These engines are pretty simple. But sneaky when getting on in years. Much like their owners!

John

Edited by JohnC
typo
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I would go with John L on that one. You might just try cleaning up the points and opening them up to 15/20 thou or a bit wider just to rule it in/out?

 If the distributor cams are worn differently or the shaft is slightly off centre when rotating this could well be a cause.

Might be worth cleaning or fitting new points and setting a gap before going to the expense of distributor doc? 

Kevin

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I followed John's advice and checked the points and adjusted to 15 thou. The gap was almost non existent and on lobe 4 there was no gap at all but just enough play in the shaft to create a gap. So after adjusting the points the car is running on all 6 and so much smother. So looks like a combination of worn bearings in the distributor shaft, worn lobe on plug 4 and badly adjusted points was the cause. 

I think I'll enjoy what's left of the weather and book a slot with the distributor doctor as an over winter job. 

Many thanks for all the advice much appreciated. 

Cheers Stephen

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