Gt6 mk3 misfire different now

[Dolomitejohn]

Hi all. Thank you all so much for all the advice re all the issues on my gt6. 

Re the misfire. I put the original coil back on and the random misfire when hot after a soak has gone.  However...... I now have a misfire at 4000 rpm plus when thoroughly hot. 

I have checked compression. Ranges from 155 to 168 psi when cold. 

I have also fitted 6 new plugs. Ngk from chis witor. 

I have not yet changed the condensor. 

I also stupidly did not check the gaps on the new plugs. 

Ideas for high rpm misfires? 

Managed to drive 20 miles to and from the TSSC Oxford meet tonight.  What a great bunch of nice people. 

Cheers

John

[johny]

so to recap, it ran ok to start with so you put a new coil on and it random misfired? You then put the old coil back and changed the plugs and it now misfire at high revs?

If so its now pointing to a problem with the plugs or you disturbed something with the ignition leads when changing plugs/coil. I would go back to the old plugs and have a close check of the leads and dizzy cap.....

[Dolomitejohn]

Hi johnny

Sort of yes. Trouble is with the old original set up I didn't really drive the car in any further than a mile here and there. And certainly I didn't rev it much as didn't want to be seen as anti social in my village... Noisy exhaust. 

The plugs I removed were mixed manufacturers. Colours of tips were light brown on 3 and slightly blacker on the other 3. Nothing that concerned me. 

I also broke one plug upon removal. Doh... 

The car doesn't pink at all and the misfire is behaving like a rev limiter of sorts, (it will rev past this point but not comfortably). Also it misfires at 4000 whist in neutral so no load.  

Difficult problems to fix on line I know.... 

John 

[dave.vitesse]

If its mis-firing in neutral then it does sound like an ignition problem. You could look for arcing, get someone to rev the engine and have a look around the dizzy, coil and plug leads. Maybe try in the dark e.g. At night.

Dave 

[NonMember]

Remind me... do you have electronic ignition? I guess not as you mention the condenser, but refusing to rev past 4000 is a known failure mode of some electronic units.

[johny]

I took it the misfire was when driving in which case one test I always do is pull out the choke slightly when the engine does it. This is to discount the possibility thats its fuel related as weak mixture can cause symptoms similar to yours.....

[Colin Lindsay]

Have you checked the timing, particularly the advance?

I was able to overcome poor running on one of my cars years ago by pulling the choke out when it started to mess about on the road, but it was only masking the problem and nothing at all to do with the fuel. I had the timing set spot on by the timing marks, but eventually found that they were way off and the car ran superbly at an indicated 23 degrees, not the stated 7 or even 13. It used to idle very well but on the road it used to run out of revs at higher speeds because the timing advanced so far off the marks that it was miles away from any kind of sparking position.

 

[johny]

What was the actual problem in the end Colin?

[Pete Lewis]

Its not common but on the 6cyl the front pulley,damper ring is bonded in place this breaks down and looses adhesion the timing/damper  ring will now rotate about the pulley hub and hence tdc is Not.where it should be

Thoughts about high demand misfire can be lack of fuel supply  thinking rubber slivers, naf fuel pump supply pipe air leaking in from poor connections,

Pete

 

 

[dougbgt6]

24 minutes ago, Pete Lewis said:

Its not common but on the 6cyl the front pulley,damper ring is bonded in place this breaks down and looses adhesion the timing/damper

Oh yes, JohnD was doing a paper on that, wonder how it's going?!!

Doug

[Dolomitejohn]

I think I concluded my damper was not slipped, but can;t remember how I came to this conclusion.

If I was to simply run engine at idle and adjust timing by rotating dizzy, what would I be looking for to get the optimum position ?  High engine speed etc ?  I have read that finding the highest engine speed and then retarding it a nats whisker is the way to go ?  Is this true ? Which way is retard ?   It does not have a vernier adjuster.

How do I check the advance bob weights are working ? I know the vacuum module is holding pressure .   Dizzy base plate seems tight but does move (plenty of lubrication in there).

Fuel hoses all new, pump seemed ok and in good condition, tank cleaned out and treated with Acid to convert the rust (will take another look inside just to check).    

I feel that the misfire saga I am suffering from is temperature related.  i.e. when its hot......

John

[dougbgt6]

13 minutes ago, Dolomitejohn said:

I have read that finding the highest engine speed and then retarding it a nats whisker is the way to go ?

Spot on!

The timing chain and other mechanicals can be worn and setting with a strobe puts the ignition in the wrong place. I think my timing  may be set around 7 degrees, I don't know, I don't use it.

Start the engine loosen the dizzy, turn anti clockwise a gnats and the revs should go down, this tells you the dizzy is around the right place. Turn clockwise until you reach maximum, it's a plateau so don't go too far, then back a gnats. Take it for a run, if it's still not right turn back a bit more.

You can also do this with a vacuum gauge, turn for maximum vacuum and back 1 division. Same thing, ears or eyes! 

Doug

 

[johny]

you need a strobe to check the dizzy operation so that, with the vacuum pipe disconnected, when reving you should see the timing advance from the action of the weights (I think the WSM even has the correct advance at various revs). Also as Pete says a partially working fuel pump can produce fuel starvation when theres high demand and the only way Ive found of testing its two non return valves is by reversing blowing through each of them individually.....

[Pete Lewis]

  To test mechanical advance discon the vac pipe   with a strobe  handy 

you  should get 29-31  crank degrees advance at 3000 rpm

of you read the manuals all dizzy testing is done  deccelerating

so build revs to 3500 and back off

pete

[Dolomitejohn]

Hi

OK been out in gt6 again it does not misfire untill really hot. 10 miles plus. 

Once it was misfiring I stopped and tried adjusting the dizzy as you suggested. I concluded was in correct place. Its now pinking a little so I shall reset using static method as I did before once cooled. 

I also fitted hose laser flashers to the plugs. They flash when plug sparks.  They all still spark when misfiring. 

I also tried the choke check. No change. 

So where now? Air leak? Sticky valve?  Lack of fuel?   

I can't get my head around why with dolomite sports coil it misfired much Lower on rev range, and with what looks like original lucas coil it misfires at 4000 plus? Maybe it's faulty also? 

To be fair I don't need to rev it that high, but I want to..... 

Help..... 

[Colin Lindsay]

9 hours ago, johny said:

What was the actual problem in the end Colin?

No idea! It's still at the same position, and still runs perfectly. I just marked the new position with white paint and now set the timing to that. 
I tried moving the distributor drive gear back or forward a tooth but that made it even worse, so I left it the way it is, and won't tempt fate again. 

John - have you replaced the distributor cap and rotor arm? 

[Dolomitejohn]

Hi 

No not replaced these yet. They look new. 

Just been out again after a 30 soak. 

Misfire at lower revs again and lasted the 6 miles home. 

It has to be something tat is affected by heat and once really hot doesn't Realy recover unti the car is cold. 

John 

 

[Pete Lewis]

There are some diabolical condensers on sale , nice looking tin can with nowt inside

If its lucas get a red rotor arm dont use anything with a rivet inn the sweep plate

Delco make sure the sprung contact does meet the brush in the cap

Check there is not wear allows the spindle to move  about , give it a good shove sideways., does it move

This really upsets the points gap.

Pete

[Dolomitejohn]

Hi pete

It s a black rotor arm with a small rivet holding the spring to the brass contactor.  The spindle of dizzy is solid.  I can only turn the rotor arm anticlockwise against dizzy springs. It returns itself OK. But I can't turn it clockwise. Is this correct? 

The dizzy cap has made in England on the inside and looks new. 

I will post some photos in a few mins. 

John 

[Dolomitejohn]

Hi pete. See photos attached. Does this look good to you? 

[Dolomitejohn]

And 2 more photos. 

[Nick Jones]

I'm confused...... your original problem was heat related, then after a coil change it was rpm related with 4000 rpm being the problem area....... and now it seems to be heat related......?

RPM related could be due to a "coil charging" issue.  That is, in order to generate the energy  for a strong spark, the coil needs to be fed enough electricity to charge it up between sparks.  As rpm increases the available time for coil charging goes down and problems emerge.

One possible cause is that you are using a 12v coil in a ballasted system so the charging voltage is low (not quite as straightforward as that but available energy is reduced) A poor connection anywhere in the LT circuit can have a similar effect but often brings poor starting with it.

Another is the points gap is wrong (and I always struggle with which way, too large or too small, has the greater effect) reducing charging time.

Finally, duff condenser, which could also be affected by heat.  I note that some are  now taking to fitting Bosch condensers intended for more more modern cars outside the distributor as the quality of many of the "correct" items is so poor.

Good luck.....  Not that the later Mk3 GT6 engines are all that willing after 4000 rpm anyway......

Nick

[Pete Lewis]

Ok delco rotors dont suffer (generally) with rivet earthing problems , its more on  lucas do make sure the centre sprung is making contacting the  delco cap brush ,  

Not keen on the points adjust screw is fully one end of its slot, wonder why  thats run out of available adjustment

Turning the rotor spindle  move a bit opening the weights the other way will just bottom the weights and stop.

Are you still on pancake filters    

Which comes to needle specs and the whole set up is just too lean

Do  check the coil is 12v , some ballasted coils are marked 12v as the base vehicle voltage but remember you had 3 ohms ???

Pete

Whilst it shows a cable drive to the tacho ,, its not been fitted up with a electronic tacho has it 

 

[poppyman]

Have you tried the bosch condensor i mentioned yet John?

Tony.

[Dolomitejohn]

Hi tony

No not yet. I purchased a new original type from td fitchet a few weeks ago. It's un branded and came in a plastic bag. 

Where do I get a bosch condensor from. What is part number? 

Yesterday when i was driving and playing with it I noticed the coil (original lucas) was so hot to touch. Even with nitrile gloves on I could not have held it in my hands. Do gt6 owners move the coil from engine to keep it cooler? 

I probably need to buy a new dizzy cap, ht leads and ballast coil plus rotor arm to eliminate them. Got new points to fit also. 

Getting a bit fed up with it now. Just want to drive...