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AGR 441 B

Mysterious Missfire

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My 4A engine adopted intermittent misfiring and loss of power eight weeks ago. All electrics definitely OK, (3 coils, many new plugs, rebuilt dizzy, swapped electronic for points, several condensers, several red rotor arms, 2 complete HT sets including caps, new dizzy base plates). Despite good compression test, total engine rebuild to reset liner heights, new head gasket, figure of 8 etc. No damage to valves or head faults detected. Carbs rebuilt, new needle valves, floats OK, pistons free. This week misfire became permanent, No. 2 cylinder went out, only running on 3. (Helpful but not helpful I suppose, now I know where but not what !)

Compression test shows between 11.5 and 12 bar and no significant difference to the other 3. Several plugs and illuminating plug tester still shows strong spark, all tappets set and checked normal, all valves and springs operating normal and in correct sequence, no loose pushrods. Oil clean as when it went in at Easter, water clear, no bubbles, no overheating, no waxy stuff anywhere, no mechanical noises to indicate a problem.

HAS ANYONE ANY IDEAS PLEASE?

All suggestions gratefully received,

 

Desperate John

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Hi John, check sticking rocker on worn shaft with ridge.

Or leaking inlet manifold to head joint.

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Thanks Chris, ran for a few minutes with rocker cover off to watch for valve problems and all on 2 seemed to be operating OK. I`ll check the inlet manifold, perhaps I haven't tightened those bally butterfly clamps enough when I rebuilt it. Was a new gasket in there though.

 

Dave, should have mentioned it runs smooth for a few seconds when I start the engine before it drops to No 2 totally off. so I think valve timing is OK. I`ll check valve positions on No 1 against timing marks to be sure.

 

Niall, timing chain wasn't changed in the rebuild but there`s no noises or valve inconsistency apparent.

 

Alan, blown through carb jets from both directions several times and all fuel lines, took tank out on Thursday and renewed all hose connections. A jet would surely put two cylinders out as well do you think?

 

Thanks for your thoughts chaps. I`ll let you know when I get there.

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Hi John,

have you got a new fabricated exhaust manifold. These are usualy thinner than original and the manifold nuts have trouble clamoping down.

The clamp nuts need building up on one side.

Roger

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John,

 

I don't see that you have changed the distributor cap? A cracked cap can still provide a good spark to #2 but not at the right time.

 

Cheers

Graeme

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just taken carbs and inlet manifold off. Nothing unusual to see. Ports all clear in manifold and head. Nothing hanging around on top of the valves so far as I can see. Manifold to head gasket shows nice clamping impression and no leaks round edges.

Roger, its the original cast iron manifold with thick flange and clamps OK. No sign of leakage and even colour on all four ports.

 

Graeme, got two different dizzy caps, one MOSS unit with all push on five silicon HT leads, one new original Lucas style with copper leads screw held in cap. New NGK plug caps fitted to this one in this exercise and coil link and plug links swapped around from plug to plug and plug to coil to make sure its not a lead. Inside caps both clean as a whistle. Going to start stripping head off my spare engine tomorrow as I fear its a head off job again.

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What engine breathing do you have, is the PCV valve still in place try disconnecting it,

vent rocker to atmosphere and block off outlet on manifold.

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If it runs for a few seconds OK and then drops away...could it be fuel flow? When starts to misfire can you recover the situation by pulling choke out further or squirting easy-start down the intake?

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John

Is #2 plug wet with fuel after it dies? f so check that the SU piston lifts and drops suddenly - with a clunk - with the dashpot rod removed. If the piston is not lifitng the mixture will go very rich and would tend to foul #2 , and not #1, due to liquid fuel running down the manifold wall. Because the engine slopes down at the rear.

Peter

Edited by Peter Cobbold

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Hello again chaps,

 

I haven't got the PCV system on. I had the single pipe from the rocker cover into the Y fire trap and a pipe to each of two fat mesh filters as on early 4s. Shown on page 47 of latest MOSS catalogue. During the past few weeks I`ve taken it off and fed the rocker cover to a long pipe down to open air. At present I`ve taken everything off to watch movement of the carb pistons, (which move in parallel showing even suction), and plugged the manifold balance pipe outlet that supplies the brake servo so that is out of play.

 

Yes, tried the pulling choke out technique which sometimes enables you to catch it when cold but its running so bad now it doesn't seem to make any difference.

 

Took N0. 2 plug out after a few minutes running to see if it was wet but wasn't. to be fair we`ve got a big fat spark there even when the cylinder is off so the fuel might be burnt off anyway. To compare we took lead off No.3 to stop that one for comparison but it was dry as well so we didn't get any help from that test.

 

Taking the head off spare engine today while I do some more thinking but I think its going to be a head swap.

 

Thanks so far,

John

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Bent disy shaft ? Is the points gap the same on all four. Bent shaft could make #2 run too retarded, and it will fire but not make torque.

Peter

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Have you checked the fuel pump for flow, remove fuel pipe from carbs and place in Jamjar, crank the engine for just a couple of seconds and the Jamjar should be nearly half full.

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Hi. Have you checked valve adjustment clearance and are the valves fully opening? ie. have you got a badly worn cam lobe that’s not letting in full fuel mixture in or exhaust gas out.

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So the situation has got worse since the initial misfire.....that is a puzzle....suggests that it is not a mechanical problem.

Pardon me for suggesting to check the dizzy cap, ignition leads, coil and condensor again.

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Just a faint possibility and rather basic but in all your fiddling with replacing leads and plugs etc - are you sure the leads are still connected in the right order?

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As Rob says I began to doubt that this could be so elusive so popped inlet manifold back this morning and situation same. With push on leads swapped lead off No. 2 onto No. 3 and 3 lead onto 2. Pulling the lead of and on 2 its still dead. Took the whole set off and put second dizzy cap and five different leads on. Still same. Changed the plug for one from another cylinder that is firing. Still same. This engine has been running OK for several years. Did Scottish 500 including Applecross pass last year. When trouble started about 8 weeks ago it was on Accuspark electronic. Took that off and tried second Accuspark that I carry as spare. No luck so I put points back. No luck. Then I changed condensers twice then changed rotor arms, the red ones from Dist Doc. Got new base plates from Dist Doc in case it was the base earth that flexes with advance. No luck.

have had all rocker gear off and on several times during Compression testing, so clearances set before and checked again this morning. Nothing apparent. Push rods are in same position in tubes as other cylinders don`t look bent, movement vertically seems same and valve sequence, induction, compression, power, exhaust seems OK. Thinking may be broken valve spring but can`t see anything, broken cam follower perhaps but no noises or bits in oil.

 

Gradually coming round to head off again.

 

Desperately, desperate John now. (I`ve missed 4 show club runs now and another coming up on Sunday.)

Enjoy your TRing chaps

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Hi

 

Try popping some neat petrol into the carb serving number 2 cylinder - take off the air cleaner to do this and check, by lifting the the carb piston up, that there is nothing blocking the inlet port to no 2 cylinder, and if you can, try and get some petrol in there.

 

If you have compression and a spark, the only reason why it cannot fire is lack of fuel, so follow that angle - bear in mind you can have compression but if the inlet valve isn't opening for any reason, then it can't fire - if you try the petrol squirting in bit ( only a little and be careful!) and it doesn't fire, take out plug and if it isn't wet, then clearly the fuel isn't getting into the cylinder and that could only be because of the valve not opening or a blockage.

 

Good luck

 

cheers

Rich

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I once had a hard-to-trace misfire problem which eventually turned out to be a broken vacuum pipe from inlet manifold to distributor.

 

Worth a check - and much cheaper than a head rebuild ;-)

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It sounds a littler like the problem I had a few years ago. Eventually cured it by turning the engine over on the starter with the float chamber and needle valve removed from the carburettor. I was only trying to check the flow from the fuel pump, but it seems there must have been a bit of crud in the fuel duct between the needle valve and the fuel pipe. I'd already blown through the pipes several times, but this must have been just downstream of the pipe into the carb.

 

Pete

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As they say, if you have compression, a spark and fuel, almost regardless of ignition timing (like +/- 20 shouldn't make a difference) it should pop!

 

So you have checked compression cold, and have some, you have checked spark and have good ones (and at the right time?) which leaves fuel, but as #1 is running fine there has to be a question about #2 not getting any.

 

I'd try a quick test get the engine running and then pull #1 and #2 plug leads, with a bit of luck and maybe a bit of throttle work you should be able to keep the engine running on two, then shut down and look at #1 & #2 plugs, they should both be wet by then. If #1 is and #2 isn't then I don't know how, but #2 is being starved. If both wet, then we are looking for something that after a bit if initial running fails and drops #2, like compression loss as soon as something gets warm.

 

I see you say you have watched the rocker gear working, but have you actually measured valve stem movement on #2?

 

If you just remove the rocker assy, and just take a hammer and tap on the top of each valve should should easily detect a different note and valve spring failure.

 

Out of interest have you had hardened unleaded seat inserts fitted, if one goes loose you can get some very strange results.

 

Interesting will be interested in the answer when its found.

 

Alan

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Took the head off today which revealed nothing amiss. Valves are moving freely, seating faces are good and even. Springs aren't broken, no seating issues, cam follower faces in good order. Looking down after removing the followers the lobes on the camshaft for the misbehaving cylinder are smooth and no scrapes or chips out of them. Nothing on the head, no cracks, exhaust valve seats seem still tight. No sign that there may have been recent slippage. Guides still tight and nothing in the ports that could cause trouble. In removing all the fuel lines etc found no blockages. I remain totally mystified. Time to pause, accept some downtime and do some strategy thinking on where to go next.

 

May all your TR troubles be little ones, (this one probably is but its finding the b-gger !

Regards,

John

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