Date: 29-03-24  Time: 14:07 pm

Author Topic: Cylinder 3 misfire at low revs  (Read 3165 times)

Whomightyoube

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Cylinder 3 misfire at low revs
« on: 09 November 2018, 05:29:42 pm »
Hi everyone. I am a new member. I have a cylinder misfire at low revs. Exhaust is stone cold on 3 on tick over. After cleaning the carbs seemed to be the same on starting. I reved up a bit and then went for a run, seemed ok till stopped at lights and when pulling away down to 3 cylinders again.


[size=78%]Spark is good, new plugs, swapped 1 and 2 and 3 and 4 HT leads and swapped the LT connections to the coils, still same so I don't think it is electrical problem. Compression is down 17% on 3 but still 12 Barg. 1,2&4 are 14.5 Barg. I can't remember if I did compression test after warming up by idling so may do a retest after a run. I thought that 12 Barg should still get ignition. All 4 exhausts are hot after a blast on the road. I have had the carbs off twice and cleaned them out with carb cleaner and air. Floats and needle valves al look good but I need to check the fuel levels. I thought it might be the choke plunger so swapped  1 and 3, but no. I also swapped 2 and 3 diaphragms and 3 and 4 pilot jets and pilot screws. I haven't replaced any of the bits.[/size]



I thought that the compression being down might be due to that piston / cylinder being cooler. Put some oil in 3 and rechecked compression but I think I out too much in as it went way up to 18 Barg and oil came out the pressure release valve on the tester. Not checked the valve clearances yet.


No 3 plug does appear to be fouled up a but.


Anyone got any ideas about what this could be?

Whomightyoube

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Re: Cylinder 3 misfire at low revs
« Reply #1 on: 09 November 2018, 05:59:01 pm »
Oh also.


I reset all pilot screws to 2 turns out. They were 2 & 5/8 , 1. & 5/8, 2&1/8, 2&1/8. I did this during second carb removal as the original plugs were all a bit dark.


 The bike which I bought new in 2002, has never required choke to start which I was told is fairly normal, but does seem odd.


It had been sitting in the garage for sometime so carbs did seem the most likely. I possibly should have ditched the old fuel and cleaned out the tank fully.

deeteefifty

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Re: Cylinder 3 misfire at low revs
« Reply #2 on: 09 November 2018, 06:57:26 pm »
Can you swap a carb to eliminate, see if it moves the fault to another cylinder?

Whomightyoube

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Re: Cylinder 3 misfire at low revs
« Reply #3 on: 09 November 2018, 08:12:21 pm »
Not sure. The manual has instructions for splitting them which suggests marking which one is which. Not clear if it is possible to reorder them. All sounds a bit of a fiddle to put them back together.


It's a nice idea because it would determine whether or not it is definitely a fuel problem. Can't help thinking that there must be an easier way.

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Re: Cylinder 3 misfire at low revs
« Reply #4 on: 09 November 2018, 08:29:16 pm »
Just got myself an ultrasonic cleaner for a recommissioning of an old XJR400. Cleaned an amazing amount of shite out the carbs, look good as new! Just got to rebuild them and try them. Carb cleaner just would t remove the varnish

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Re: Cylinder 3 misfire at low revs
« Reply #5 on: 09 November 2018, 08:35:15 pm »
When you say you swamped diaphragms I presume you mean with the carb pistons as well, and checked they are moving freely. I can’t see that the butterfly valves would have got out of sync but might be worth making sure they are all moving the same amount. With one cylinder not firing a vacuum test won’t be much use .Good idea to do a compression test after it has run with all cylinders firing so as to clear any unburned fuel from the cylinder, even if you let it cool down, but without letting it tick over again. If compression still down on no. 3 I would first check valve clearances. Can’t think of anything else at the moment. Hope you get it sorted and would be interested to know the result.
By coincidence I measured the temperature of the exhaust pipes on my 600 the other day and at tickover from cold no3 cyl. was 100C and the other three were 135C. A few minutes at 2000 rpm and all four pipes showed 200C. Bike runs fine and sounds good at tickover but I’d like to find out why one cylinder runs cooler when I get time to investigate.  If I hadn’t been playing about with my infra red heat gun I would never have known and just enjoyed the bike.

Whomightyoube

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Re: Cylinder 3 misfire at low revs
« Reply #6 on: 09 November 2018, 09:05:43 pm »
Swapped Pistons as well. All move as they should.


Butterfly valves all look to move together. So the fuel reduces the compression because it delubes the rings? Makes sense.


On idle 4 doesn't get as hot as 1 and 2 but 3 is just ambient. With yours, could it be the pilot screw adjustment?

Whomightyoube

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Re: Cylinder 3 misfire at low revs
« Reply #7 on: 09 November 2018, 09:16:32 pm »
Just got myself an ultrasonic cleaner for a recommissioning of an old XJR400. Cleaned an amazing amount of shite out the carbs, look good as new! Just got to rebuild them and try them. Carb cleaner just would t remove the varnish


So do you recon that I just didn't clean the carb out fully? I was wondering if I should have ditched all the old petrol and completely flushed the tank. I sprayed cleaner into all the orifces and blew out with air. Same with all the jets. Pilot jets are very small. If there is crap in the petrol then wouldn't all the carbs be affected, not just no 3 all the time?

deeteefifty

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Re: Cylinder 3 misfire at low revs
« Reply #8 on: 09 November 2018, 10:17:10 pm »
This won't be the answer to your problem, I found a lens shaped gauze filter at the lower end of the petrol pipe where it attaches to the T piece that splits to the 4 carbs. Worth a clean if you haven't already.

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Re: Cylinder 3 misfire at low revs
« Reply #9 on: 09 November 2018, 10:34:50 pm »
To me it sound like the pilot circuit is blocked on carb three. To test if the pilot circuit is blocked try starting the bike on choke. Don't touch the throttle just leave the choke on and let the bike idle for a couple of minutes. Check does cylinder three start to get warm when you let it run on choke. The choke circuit is a different circuit to the idle circuit so if it does get warm on the choke but it stays cold off choke it suggest the idle circuit is blocked. You can test if cylinder three is getting warm up by throwing some water on the headers and see if the water dries.

His Dudeness

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Re: Cylinder 3 misfire at low revs
« Reply #10 on: 09 November 2018, 10:42:44 pm »
I had a similar problem once. The bike would idle normally but when you went to pull away cylinder 3 would misfire at low revs but then run normally again up the rev range and the problem got worse when it rained. It was caused by arcing through the spark plug caps. I think you're symptoms are a bit different though. It sounds like cylinder 3 is totally dead at idle and then comes good up the revs so to me that sounds like a problem with the idle circuit in the carb but checking the spark plug caps and trimming the ht lead is easy to do so I'd start with that

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Re: Cylinder 3 misfire at low revs
« Reply #11 on: 09 November 2018, 11:02:13 pm »
exactly what his dudeness said. I did up a zzr1100 that had old petrol in it in storage for eight years. I cleaned the carbs four times before it ran perfectly. each time I thought id done it but had the same problem as you, a couple of cylinders only running on choke so blocked pilot circuit somewhere. its amazing how tiny the hole can be thats blocked and then causes massive problems. I coudnt even see the last bit of the blockage I cleared it was that small, but I knew id finally got it because the bike finally ran perfectly.

His Dudeness

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Re: Cylinder 3 misfire at low revs
« Reply #12 on: 09 November 2018, 11:18:01 pm »
Yeah it can be a right pain getting all the passages clean. The thing to remember is that you have to clean more than just the pilot fuel jet in the bowl. The idle circuit is the pilot air jet, the pilot fuel jet, the transfer ports and the mixture screw port. This diagram shows it well




At idle the butterfly valve is closed so air is drawn in through the pilot air jet (PAJ in the diagram), the air then goes to the holes in the side of the pilot fuel jet (PJ). Fuel is drawn up through the centre of the pilot fuel jet and mixes with the air from the pilot air jet. It then comes out the transfer ports just after the butterfly valve and also comes out the port at the mixture screw. So all of those passages need to be clear. If you just clean the pilot fuel jet in the bowl and don't clean the pilot air jet, transfer ports and idle screw port you can still have problems.

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Re: Cylinder 3 misfire at low revs
« Reply #13 on: 10 November 2018, 12:29:25 am »
Don't rule out a moody plug, you may well get a spark, but i had a similar scenario and a brand new plug was faulty.
I changed the lot and it fixed everything.
In fact, a second plug failed two weeks after the first.

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tommyardin

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Re: Cylinder 3 misfire at low revs
« Reply #14 on: 10 November 2018, 12:33:11 am »
Yeah it can be a right pain getting all the passages clean. The thing to remember is that you have to clean more than just the pilot fuel jet in the bowl. The idle circuit is the pilot air jet, the pilot fuel jet, the transfer ports and the mixture screw port. This diagram shows it well




At idle the butterfly valve is closed so air is drawn in through the pilot air jet (PAJ in the diagram), the air then goes to the holes in the side of the pilot fuel jet (PJ). Fuel is drawn up through the centre of the pilot fuel jet and mixes with the air from the pilot air jet. It then comes out the transfer ports just after the butterfly valve and also comes out the port at the mixture screw. So all of those passages need to be clear. If you just clean the pilot fuel jet in the bowl and don't clean the pilot air jet, transfer ports and idle screw port you can ustill have problems.


Hey Dude,
Thank you for posting this it is very, very helpful. In fact I have printed it out and shoved it in my Haynes manual.
Well done  :thumbup :guitar

His Dudeness

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Re: Cylinder 3 misfire at low revs
« Reply #15 on: 10 November 2018, 01:03:33 am »
No problem ;) I didn't draw it just found it on a google search. I'll explain it a bit more. With the butterfly valve closed the vacuum of the engine is drawing fuel and air through the idle circuit as I explained in the last post. Once you start to open the butterfly valve by opening the throttle the effect of engine vacuum moves past the butterfly valve and moves further back along the carb to the bottom of the slide. See the hole in the bottom of the slide marked SVP in the diagram well the vacuum from the engine goes up through that hole, up the centre of the slide and it acts on the top of the rubber diaphragm. So now there is vacuum acting on the top of the diaphragm. It's not shown in that diagram but there's a slot cut in the body of the carb so that atmospheric pressure acts on the bottom of the rubber diaphragm. The top and the bottom of the diaphragm are sealed from each other so with low pressure acting on top of the diaphragm and relatively high pressure from the atmosphere acting on the bottom of the diaphragm the diaphragm deflects up. As it deflects up it lifts the jet needle out of the main jet. So now the main fuel circuit can come on. Air can now flow through the main air jet (maj), through the holes in the emulsion tube and fuel gets draw up through the centre of the main fuel jet and mixes with the air from the main air jet and the mixture flows past the jet needle and is drawn into the engine. And as you open the throttle more, more vacuum can act on the top of the diaphragm so the diaphragm lifts more and the jet needle lifts more and more fuel flow. That's my understanding of it anyway

deeteefifty

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Re: Cylinder 3 misfire at low revs
« Reply #16 on: 10 November 2018, 08:01:18 am »
That's my understanding of it too. Now. 😊👍 Ta Dudeness.
My mate had tight valve clearance cause the same issue on a Kawasaki, I don't know how the cylinder managed to join in as the revs increased though.
« Last Edit: 10 November 2018, 08:09:34 am by deeteefifty »

tommyardin

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Re: Cylinder 3 misfire at low revs
« Reply #17 on: 10 November 2018, 08:03:47 am »
Brilliant  :thumbup
I knew that it was essential that the diaphragm should be in perfect condition and that  even a pinhole in it would render it useless, but did not understand how it all worked together.
Thanks again Dude for a very helpful post. 
  :woot

Dynspud

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Re: Cylinder 3 misfire at low revs
« Reply #18 on: 10 November 2018, 08:58:26 am »
Does the diagram and description apply to the FZS 1000 carbs as well?
Take a deep breath, coz it all starts now, when you pull the foccin' pin

Whomightyoube

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Re: Cylinder 3 misfire at low revs
« Reply #19 on: 10 November 2018, 09:28:59 am »
Yeah it can be a right pain getting all the passages clean. The thing to remember is that you have to clean more than just the pilot fuel jet in the bowl. The idle circuit is the pilot air jet, the pilot fuel jet, the transfer ports and the mixture screw port. This diagram shows it well




At idle the butterfly valve is closed so air is drawn in through the pilot air jet (PAJ in the diagram), the air then goes to the holes in the side of the pilot fuel jet (PJ). Fuel is drawn up through the centre of the pilot fuel jet and mixes with the air from the pilot air jet. It then comes out the transfer ports just after the butterfly valve and also comes out the port at the mixture screw. So all of those passages need to be clear. If you just clean the pilot fuel jet in the bowl and don't clean the pilot air jet, transfer ports and idle screw port you can still have problems.


Great stuff. Is this a generic diagram or specific to the Fazer?


There is also another screw which looks like a jet but has no hole in it. Any idea what that is?


I took all jets and screws out in the bowl and carb cleaner and blew out with air. I guess the TP would be better to seat the pilot screw to force air through it. Are the MAJ and PAJ removeable?

[/size]There is also a small orifice next to the jets. Any ideas what this is?[size=78%]

[/size]I am wondering why it ran well for a short time after reving up to the lights when there were a couple of minutes idle, then lumpy again.[size=78%]

Whomightyoube

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Re: Cylinder 3 misfire at low revs
« Reply #20 on: 10 November 2018, 09:33:38 am »
To me it sound like the pilot circuit is blocked on carb three. To test if the pilot circuit is blocked try starting the bike on choke. Don't touch the throttle just leave the choke on and let the bike idle for a couple of minutes. Check does cylinder three start to get warm when you let it run on choke. The choke circuit is a different circuit to the idle circuit so if it does get warm on the choke but it stays cold off choke it suggest the idle circuit is blocked. You can test if cylinder three is getting warm up by throwing some water on the headers and see if the water dries.


Will try that, next week now as I have a busy weekend. My experience is that the bike dies not start at all on choke although when this trouble started I could only start it on choke.

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Re: Cylinder 3 misfire at low revs
« Reply #21 on: 10 November 2018, 09:54:31 am »
Brilliant  :thumbup
I knew that it was essential that the diaphragm should be in perfect condition and that  even a pinhole in it would render it useless, but did not understand how it all worked together.
Thanks again Dude for a very helpful post. 
  :woot
yeah if there's a hole in the diaphragm the pressure on both sides of the diaphragm becomes the same so the diaphragm won't deflect up properly so the jet needle won't lift up and the fuel mixture won't flow through the main jet 

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Re: Cylinder 3 misfire at low revs
« Reply #22 on: 10 November 2018, 10:02:21 am »
Does the diagram and description apply to the FZS 1000 carbs as well?

It's not specific to any bike. It's a diagram and description of a Constant Velocity carb. I've looked at the 400 carb but the 600 and 1000 should be more a less the same or any CV carb should be basically the same. Here's a video that shows it well



Whomightyoube

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Re: Cylinder 3 misfire at low revs
« Reply #23 on: 10 November 2018, 10:07:17 am »
I have just started on full choke which was not as easy as starting without any. 3 exhaust got hot but not as quickly as the others. When I took choke off, the engine stopped. I then went for a ride. All four smooth for several miles. Short stop at junction OK. Another slightly longer stop at lights where I let it idle  30 seconds and down to 3 and lumpy at low revs all the way home.


My bike don't like red lights. How does it know ?

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Re: Cylinder 3 misfire at low revs
« Reply #24 on: 10 November 2018, 10:22:02 am »
I have just started on full choke which was not as easy as starting without any. 3 exhaust got hot but not as quickly as the others. When I took choke off, the engine stopped. I then went for a ride. All four smooth for several miles. Short stop at junction OK. Another slightly longer stop at lights where I let it idle  30 seconds and down to 3 and lumpy at low revs all the way home.


My bike don't like red lights. How does it know ?
That proves it then. It's not getting enough fuel through the idle circuit. It's running on the choke circuit but not on the idle circuit. I think you will have to clean the carbs again. Before you do that have you tried turning up the idle speed screw up a bit? It might just be too low.