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Rhythmic misfire
http://international2cvfriends.com/forum/viewtopic.php?f=7&t=5997		 Page 1 of 3
Author:  Roger V [ May 13th, 2017, 12:04 am ]
Post subject:  Rhythmic misfire
Evening all

The Shedouin has over the last few weeks, developed a rhythmic misfire at idle. Suspect it has lost power through the range.
Running 602 - 8.5:1
Lumintion
Black coil but not standard.
Coil is mounted on gearbox and consequently using longer HT leads. I believe from a range rover.
Plugs (and leads) cleaned, but of unknown history
No air leaks when tested with brake cleaner.
Compression test when cold with throttle wide open = 150 and 155 psi

I've been looking at this for to long!

So guys, what should I look at , and in what order?

Any suggestions appreciated. ;)

Edit .... Electric fuel pump

Edit 2 .... It starts well
Author:  ken [ May 13th, 2017, 1:42 am ]
Post subject:  Re: Rhythmic misfire
Roger,
why do the compression test 'cold', when the whole point of such a test is to determine how the engine is performing under working conditions?

Anyway, how long ago/how many miles was it since you've checked the valve clearances?
If you can't remember, I'd recommend doing them now, setting the exhausts to 0.25mm and the inlets to 0.20 and then repeat the compression test.

p.s. Just noticed that you mentioned an electric fuel pump.
Any idea what its delivery pressure is?
If it's higher than the normal A series pump, it may be defeating the carb's needle valve at idle.


Roger V wrote:
Evening all

The Shedouin has over the last few weeks, developed a rhythmic misfire at idle. Suspect it has lost power through the range.
Running 602 - 8.5:1
Lumintion
Black coil but not standard.
Coil is mounted on gearbox and consequently using longer HT leads. I believe from a range rover.
Plugs (and leads) cleaned, but of unknown history
No air leaks when tested with brake cleaner.
Compression test when cold with throttle wide open = 150 and 155 psi

I've been looking at this for to long!

So guys, what should I look at , and in what order?

Any suggestions appreciated. ;)

Edit .... Electric fuel pump

Edit 2 .... It starts well
Author:  lpgo [ May 13th, 2017, 8:05 am ]
Post subject:  Re: Rhythmic misfire
Roger V wrote:
Evening all

So guys, what should I look at , and in what order?

Any suggestions appreciated. ;)

Edit .... Electric fuel pump

Edit 2 .... It starts well


Running on Luminition there is almost no current running trough your points so no self-cleaning. Probably they will have collected some oil or grease. Replace or at least sandpaper them .. and look for improvement.. Also take a good look at all the connectors especialy at your pointbox maybee theres oil or grease in them too...

Good luck. This can be a tricky one..


Ps. If you still can try to let it run without luminiton.. You never know..
Author:  ken [ May 13th, 2017, 10:23 am ]
Post subject:  Re: Rhythmic misfire
Lpgo,
no points with Lumenition, since it's an optical switch which uses an infrared emitter and detector pair.
Probably not the best plan to attack those with sandpaper... ;)



lpgo wrote:
Roger V wrote:
Evening all

So guys, what should I look at , and in what order?

Any suggestions appreciated. ;)

Edit .... Electric fuel pump

Edit 2 .... It starts well


Running on Luminition there is almost no current running trough your points so no self-cleaning. Probably they will have collected some oil or grease. Replace or at least sandpaper them .. and look for improvement.. Also take a good look at all the connectors especialy at your pointbox maybee theres oil or grease in them too...

Good luck. This can be a tricky one..


Ps. If you still can try to let it run without luminiton.. You never know..
Author:  Roger V [ May 13th, 2017, 10:26 am ]
Post subject:  Re: Rhythmic misfire
Thanks Guys

Geo, lumenition does not have points, at least not this model. Through it does retain the advance weights. I know you know this. :lol:

Ken, I researched the fuel pump and from memory its about 2.5 psi.

I reported the compression cold, because I was going to read both hot and cold, but when I went to unscrew the gauge, the pipe started to turn but not the union. I'll look into it again. :)

Tappets! I did think that as I was going to sleep last night.

Could it be timing to far retarded? Would this match the symptoms?
Author:  Roger V [ May 13th, 2017, 12:31 pm ]
Post subject:  Re: Rhythmic misfire
Update

I checked the timing. It was ok at full advance, but very retarded at idle. About 4 teeth before the datum.
I have advanced the timing to about 1 tooth before the datum, but this puts the full advance about 3 teeth past the 7 teeth mark for full advance.

The car idles and pulls better now, but I'm concerned it may pink.

Is this wear causing the broad timing span?

Appreciate your advice and comments. :)
Author:  Derek [ May 13th, 2017, 6:05 pm ]
Post subject:  Re: Rhythmic misfire
Check the bob weights in the A/R unit for serviceability. Clearly the unit is advancing, but is it too much through damage/wear?

I'd pop a new set of plugs in there first though if only to eliminate a faulty one.
Author:  Roger V [ May 14th, 2017, 11:00 am ]
Post subject:  Re: Rhythmic misfire
Thanks Derek

Plugs is something tend to get a bit neglected by me. I'll treat it to some new ones.

I'll have a look at the weights.

It wouldn't be a bad idea to start again and remark the datum etc. :D
Author:  ken [ May 14th, 2017, 11:17 pm ]
Post subject:  Re: Rhythmic misfire
Roger,
there are 'stops' on the centrifugal advance mechanism which can be bent inwards to limit the total amount of advance.
I found that the 'business end' of a 1/2" extension bar was ideal for this purpose; just be careful not to overdo things.
.
Plugs which are 'borderline' would be most likely to give problems at high cylinder pressures, such as under heavy load/wide open throttle during acceleration, I'd have thought...



Roger V wrote:
Thanks Derek

Plugs is something tend to get a bit neglected by me. I'll treat it to some new ones.

I'll have a look at the weights.

It wouldn't be a bad idea to start again and remark the datum etc. :D
Author:  Roger V [ May 14th, 2017, 11:53 pm ]
Post subject:  Re: Rhythmic misfire
Thanks ken (and Geo and Derek)

I remember Geo recommending that in another thread, and I will probably end up doing that. However, I am curious as to the cause. I prefer to resolve the cause than treat the symptoms

I had a run out in it today and it felt better, but I would be happier knowing it is correct.

Something else to consider is, I'm in the process of converting to LPG. I've fitted a mixer ring on the carburetor and its possible its affecting the mixture. Though it wasn't running correctly before. Think I'll change it back until I get it running well on petrol. As I changed to many variables at once and have lost the plot.
I've still to check valve clearances. Difficulty there is I have to drive to my lockup to do that kind of stuff and the engine will be hot. Could do the compression test though. :D

Cheers
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