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Mysterious Missfire


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#1 AGR 441 B

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Posted 04 August 2018 - 10:51 AM

      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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#2 ChrisR-4A

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Posted 04 August 2018 - 11:02 AM

Hi John, check sticking rocker on worn shaft with ridge.
Or leaking inlet manifold to head joint.
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#3 ctc77965o

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Posted 04 August 2018 - 11:03 AM

Cam timing?
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Dave Lacey
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#4 TR NIALL

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Posted 04 August 2018 - 11:16 AM

Was Timing Chain New maybe it’s the problem.
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#5 AlanG

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Posted 04 August 2018 - 11:37 AM

Blocked jet?

Alan.


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#6 RogerH

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Posted 04 August 2018 - 01:02 PM

Hi John,

           if the bad running is at low speed then check the oil level in the carb dash pots.

 

Roger


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Time is a drug - Too much will kill you
Forgive your enemy, but remember the ass-hole's name.
never surrender, never give up and always balance your butterflies

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TR4A - July 1967 daily driver         2010 RBRR No.22  2014 RBRR No.69
TR4 - March 1962 transforming like the proverbial caterpillar - it's now Royal Blue and now with the chromy bits.
 
 
 
 
 

#7 AGR 441 B

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Posted 04 August 2018 - 01:02 PM

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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#8 RogerH

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Posted 04 August 2018 - 01:04 PM

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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Time is a drug - Too much will kill you
Forgive your enemy, but remember the ass-hole's name.
never surrender, never give up and always balance your butterflies

Veni Vidi Flati
TR4A - July 1967 daily driver         2010 RBRR No.22  2014 RBRR No.69
TR4 - March 1962 transforming like the proverbial caterpillar - it's now Royal Blue and now with the chromy bits.
 
 
 
 
 

#9 graeme

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Posted 04 August 2018 - 03:45 PM

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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#10 AGR 441 B

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Posted 04 August 2018 - 07:45 PM

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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#11 ChrisR-4A

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Posted 04 August 2018 - 07:58 PM

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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#12 ctc77965o

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Posted 04 August 2018 - 10:52 PM

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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Dave Lacey
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#13 Peter Cobbold

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Posted 05 August 2018 - 12:03 AM

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, 05 August 2018 - 12:04 AM.

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#14 AGR 441 B

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Posted 05 August 2018 - 08:58 AM

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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#15 Peter Cobbold

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Posted 05 August 2018 - 10:32 AM

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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#16 ChrisR-4A

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Posted 05 August 2018 - 04:51 PM

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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#17 Ian U

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Posted 05 August 2018 - 08:43 PM

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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#18 Malbaby

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Posted 05 August 2018 - 10:50 PM

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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#19 RobH

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Posted 06 August 2018 - 07:36 AM

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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#20 AGR 441 B

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Posted 06 August 2018 - 11:29 AM

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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