Low rev misfire

Low rev misfire

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Jhonno

Original Poster:

3,459 posts

82 months

Monday 13th May
quotequote all
spitfire4v8 said:
Sounds like youve done pretty much everything reasonably possible so far
I'd like to think I try and exhaust the "obvious" before I start bothering others with these things.. laugh

I should have brought it in to work, I could have played with it at lunchtime! Not that I'm impatient to sort it..

Jhonno

Original Poster:

3,459 posts

82 months

Monday 13th May
quotequote all
Right, I can't get Evo Olli's software to work on my laptop, well I thought it would work with a USB to Serial Adapter but no luck.

So.. I held throttle part open on the idle stop so it would sit about 2k.. The banks were out of balance again, at this point at least. So I've balanced them the best I could. I have 2 cylinders that flow more. No.1 and No.8.

No.1 has a hollow screw in it. So if someone (Joolz? laugh ) could tell me the screws fitted (allen heads), I will replace the hollow screw.

I need to take it for a run now and see how it behaves.


Jhonno

Original Poster:

3,459 posts

82 months

Tuesday 14th May
quotequote all
Driven it twice now.. It is currently holding a slightly fast idle, and the stutter is still there, but much less, if it is doing it you can back the throttle off slightly and it will go through it. It also gets better as it warms up. When I first drove it after setting it up it was a bit of a dog, however it seems to have settled into something driveable. I will tweak the throttles again, to get the correct idle and see what occurs.

I think to be anal about this, I need to get hold of the 5 hollow bolts, to up the flow of the other 3 cylinders on the bank with the highest flowing cylinder.

Thom

1,672 posts

188 months

Tuesday 14th May
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Not sure I understand how it seems to go out of tune rather quickly after each time you have adjusted the throttles.
Isn't there just some excessive play in the linkage(s)?

Jhonno

Original Poster:

3,459 posts

82 months

Tuesday 14th May
quotequote all
Thom said:
Not sure I understand how it seems to go out of tune rather quickly after each time you have adjusted the throttles.
Isn't there just some excessive play in the linkage(s)?
Perhaps there is something I am doing wrong.. BUT.. I have done this before very successfully! I am the closest I've been currently, however there was always a couple of higher flowing banks, which I hadn't realised could be such an issue until Joolz pointed the potential lean misfire issue out. It is a high comp engine. My linkages are pretty tight. The banks stay in sync (to how they are set) across the throttle range.

Perhaps it is partly as the newly built engine evolves and beds in..? Grasping at straws perhaps.

Edited by Jhonno on Tuesday 14th May 10:57

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Thom

1,672 posts

188 months

Tuesday 14th May
quotequote all
Jhonno said:
It also gets better as it warms up. When I first drove it after setting it up it was a bit of a dog, however it seems to have settled into something driveable
Perhaps this is stating the obvious but it's important to adjust the linkages with the engine fully warmed up. I feel that quite a bit of thermal expansion may take place in the very hot engine bay of a Cerbera, and most classics using several synchronised carbs should always be adjusted when warm, in my experience.

Jhonno

Original Poster:

3,459 posts

82 months

Tuesday 14th May
quotequote all
Thom said:
Perhaps this is stating the obvious but it's important to adjust the linkages with the engine fully warmed up. I feel that quite a bit of thermal expansion may take place in the very hot engine bay of a Cerbera, and most classics using several synchronised carbs should always be adjusted when warm, in my experience.
Yeah, it is being done whilst warm!

Jhonno

Original Poster:

3,459 posts

82 months

Tuesday 14th May
quotequote all
https://photos.app.goo.gl/gbwKNLePrBwBJFHs6

A video of diagnostics whilst running..

I have noticed the odd bank TPS tends to hunt a bit, also, not fully shown in the video, but the adaptive hunting is!

FarmyardPants

3,446 posts

159 months

Tuesday 14th May
quotequote all
From my limited knowledge if it runs ok after resetting the adaptives but then “learns” to run poorly, it must be a defective lambda sensor, since that is what the learning process uses for input. Even if you have a throttle or two flowing more air, it should learn to adjust for the bank overall. The fact that it runs ok over 3k rpm and wider throttle openings corroborates this. Bad lambda sensors can also cause high idle problems.

Jhonno

Original Poster:

3,459 posts

82 months

Tuesday 14th May
quotequote all
FarmyardPants said:
From my limited knowledge if it runs ok after resetting the adaptives but then “learns” to run poorly, it must be a defective lambda sensor, since that is what the learning process uses for input. Even if you have a throttle or two flowing more air, it should learn to adjust for the bank overall. The fact that it runs ok over 3k rpm and wider throttle openings corroborates this. Bad lambda sensors can also cause high idle problems.
Yes, I am lead to believe this also, and have found others with the same symptoms/issues. However, mine seem to switch as they should! Hence asking if they can give wrong/misleading readings..

Jhonno

Original Poster:

3,459 posts

82 months

Tuesday 14th May
quotequote all
Right.. Yet more tweaking and tuning after work, and for the 6 mile trip home it holds an idle in or around 1100rpm, and very little hesitation from low rpm.. Some but not that much, it could be those 2 high flow banks throwing it slightly.. I might let it work things out for a minute before any more tweaking.

I think I have found a source for the hollow bolts M4x6 I believe. So will order those up and go from there if it continues to run as is. I'm thinking it should be running lean (as in adding fuel in the adaptives) under lambda control as it is running 4.2 injectors still, so maybe I have been running the throttles too far advanced for the rpm.

Edited by Jhonno on Tuesday 14th May 17:50

Jhonno

Original Poster:

3,459 posts

82 months

Tuesday 14th May
quotequote all
2nd drive update..

Still much more driveable, and holding idle after a few seconds of throttle when starting, and sometimes it will pull hard from low revs, however it is still missing. It has a inconsistent miss you can feel if you put your hand on the end of the driver's bank exhaust, which is also the bank with the highest flowing cylinder/greatest flow difference.

Thom

1,672 posts

188 months

Wednesday 15th May
quotequote all
I doubt that a runner or two flowing a bit more (how many % are we talking?) than all the others would cause the engine to run so badly rich, according to the video, which also suggests the throttle pots are functional.
Anything richer than around 0.80 to 0.85-ish on a cold start up should not be needed for such an engine.

Edited to add : does the rear exhaust runner you mention feel cold compared with the others? A fouled plug could cause the low rpm misses you describe. Does the MBE ECU feature some "limp home" mode enabled when detecting faulty sensors, and then deliberately injecting more fuel than under normal running conditions to protect the engine?

Edited by Thom on Wednesday 15th May 09:40

Jhonno

Original Poster:

3,459 posts

82 months

Wednesday 15th May
quotequote all
Thom said:
I doubt that a runner or two flowing a bit more (how many % are we talking?) than all the others would cause the engine to run so badly rich, according to the video, which also suggests the throttle pots are functional.
Anything richer than around 0.80 to 0.85-ish on a cold start up should not be needed for such an engine.

Edited to add : does the rear exhaust runner you mention feel cold compared with the others? A fouled plug could cause the low rpm misses you describe. Does the MBE ECU feature some "limp home" mode enabled when detecting faulty sensors, and then deliberately injecting more fuel than under normal running conditions to protect the engine?

Edited by Thom on Wednesday 15th May 09:40
The fueling now runs leaner than the video, and adds fuel in, in it's new fairly driveable state. I perhaps had too little air, too much fuel at idle before.. It is a 4.5 running 4.2 injectors currently, so under lambda control you would expect it to be adding some fuel in..

I tried to get to the runner last night, but it was late and would have involved taking the airbox off. Front 2 felt even. The high flow cylinder does run a significant amount more than the rest. If it was a little bit I wouldn't be that worried about it. I will pull the spark plug tonight to see what it looks like. Of course it has to be the most awkward one.

spitfire4v8

2,840 posts

122 months

Wednesday 15th May
quotequote all
Thom said:
I doubt that a runner or two flowing a bit more (how many % are we talking?) than all the others would cause the engine to run so badly rich, according to the video, which also suggests the throttle pots are functional.
Anything richer than around 0.80 to 0.85-ish on a cold start up should not be needed for such an engine.

Edited to add : does the rear exhaust runner you mention feel cold compared with the others? A fouled plug could cause the low rpm misses you describe. Does the MBE ECU feature some "limp home" mode enabled when detecting faulty sensors, and then deliberately injecting more fuel than under normal running conditions to protect the engine?

Edited by Thom on Wednesday 15th May 09:40
Sadly the video tells us little about the running of the engine, the warm up timer won't have cut out and the adaptive values will just be what was stored from the last time it was run. Also the constant moving of the throttles/revving doesn't allow us to see anything long enough to make any kind of judgement anyway.

Airflows : it's not uncommon for one cyl to flow say 5kg/hr and the neighbour 6 .. that's a 20 percent airflow difference. When you consider that a poor combustion chamber design already struggles on lambda=1 to have smooth combustion then it doesn't take anything like 20 percent weaking of the mixture to have one cylinder not firing at all. In fact even a good chamber design would struggle with that.
Just a 5 percent mixture change weaker has noticeable effects on the running smoothness, it's only the fact we have 8 cyinders that means it's as good as it is.

Jhonno

Original Poster:

3,459 posts

82 months

Wednesday 15th May
quotequote all
spitfire4v8 said:
Sadly the video tells us little about the running of the engine, the warm up timer won't have cut out and the adaptive values will just be what was stored from the last time it was run. Also the constant moving of the throttles/revving doesn't allow us to see anything long enough to make any kind of judgement anyway.

Airflows : it's not uncommon for one cyl to flow say 5kg/hr and the neighbour 6 .. that's a 20 percent airflow difference. When you consider that a poor combustion chamber design already struggles on lambda=1 to have smooth combustion then it doesn't take anything like 20 percent weaking of the mixture to have one cylinder not firing at all. In fact even a good chamber design would struggle with that.
Just a 5 percent mixture change weaker has noticeable effects on the running smoothness, it's only the fact we have 8 cyinders that means it's as good as it is.
Sorry, it kept dying, so I was having to manually keep it running..

Yeah, my 2 high flow cylinders are more than 1kg out, so definitely worth trying to bring the others in line!

spitfire4v8

2,840 posts

122 months

Wednesday 15th May
quotequote all
^^ also worth pointing out that the effects of drilled screws is at its greatest on closed throttle .. as soon as you start to open the throttles then that percentage in-balance decreases, so the influence of mixture in-balance decreases with throttle opening, but it makes sense in your case to at least try and get them somewhere near. It eliminates the closed throttle and very-close-to-closed-throttle airflow variable at least.

Thom

1,672 posts

188 months

Wednesday 15th May
quotequote all
Could drilled screws then cause the throttles to keep going as much as 20% of airflow out of sync after each adjustment?

Edited by Thom on Wednesday 15th May 15:42

Jhonno

Original Poster:

3,459 posts

82 months

Wednesday 15th May
quotequote all
spitfire4v8 said:
^^ also worth pointing out that the effects of drilled screws is at its greatest on closed throttle .. as soon as you start to open the throttles then that percentage in-balance decreases, so the influence of mixture in-balance decreases with throttle opening, but it makes sense in your case to at least try and get them somewhere near. It eliminates the closed throttle and very-close-to-closed-throttle airflow variable at least.
Yeah I did see somewhere you said about 0.5kg per screw at best.. Seeing as my issue seems to be at idle/smallest throttle openings it is worth a try! And I will be happier with them closer anyway.

Jhonno

Original Poster:

3,459 posts

82 months

Wednesday 15th May
quotequote all
Plugs from the bank with a miss..

https://photos.app.goo.gl/RuoeCBLmgMRr9hfSA