misfire: what is happening here?

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wetering
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misfire: what is happening here?

#1

Post by wetering »

I have been struggling with intermittent engine misfire since quite a while. In vain as you will understand. After endless checks and experiments I ended up with this:

https://www.youtube.com/watch?v=dZlYf-y2qGM

You probably won’t understand dutch but I am telling you that I am going to disconnect plug wire 4 (the one that intermittently loses vacuum) and you see what happens.
Interestingly, repeated inspection of the spark plugs did not reveal any abnormalities or differences between the plugs. However during a last experiment I let the engine idle for several minutes, all plugs were OK, except plug 4 which was carbon fouled.
Can anyone help me? Feel that it looks like an intake valve failure, but seems to be an unusual way of diagnosing ?

Thanks,

Jeroen.
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Whiskerfish
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Re: misfire: what is happening here?

#2

Post by Whiskerfish »

There are many possible causes. To have it consistent to one cylinder is actually helpful in my mind. First thing I would do is swap the # 3 and the # 4 Plug wires and plugs to see if it moves. They run off the same coil so then can be moved for troubleshooting. If it stays on # 4 then check the Compression, belt setup, and then Carburation.
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wetering
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Re: misfire: what is happening here?

#3

Post by wetering »

Will do, but wonder whether this makes sense: 1: there is a spark at all plugs, 2: as you can see in the movie: disconnecting plug wire 4, shows a considerable drop in rpm and bad idle which in my view strongly suggests that ignition is OK. As stated, I have posted this issue after endless checks, so I am quite confident that compression and carburetion are not the culprit. I am particularly interested in your feedback on the vacuum gauge reading ......

like this: http://www.secondchancegarage.com/public/186.cfm
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Re: misfire: what is happening here?

#4

Post by BlueThunder »

wetering wrote:Will do, but wonder whether this makes sense: 1: there is a spark at all plugs, 2: as you can see in the movie: disconnecting plug wire 4, shows a considerable drop in rpm and bad idle which in my view strongly suggests that ignition is OK. As stated, I have posted this issue after endless checks, so I am quite confident that compression and carburetion are not the culprit. I am particularly interested in your feedback on the vacuum gauge reading ......

like this: http://www.secondchancegarage.com/public/186.cfm
Its either electrical or fuel. These are the only two possibilities. Fuel is more difficult so lets eliminate electrical as the possibility. Are these stock coils? If yes, snip 1/4" from the spark plug end. You will need to unscrew the plug cap. Look at the plug cap. Is it clean and bright? Or dull and full of gunk. Personally, I would replace all of them as the caps are only $3-$4 each. There is a 5K resister inside the caps and sometimes they break like a filament in a light bulb. It sometimes connects and sometimes doesn't - which explains why #4 doesn't fire consistently.

While you're at it, pull the coils and make sure that the grounding point is clean and bright. Do the same coil ground ring connector. Do the same with the pigtail connectors to the coils. I would replace with new ones and solder the wires to the new connectors. Again, they are not expensive to replace. And check the connections from the right control (ignition off-on-off) switch. Again, make sure every connection is bright and shiny. Any of these points can create problems.

Once done, then you can go on to fuel - i.e. carbs.
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Re: misfire: what is happening here?

#5

Post by jdvorchak »

that is weird the way it drops vacuum intermittently. I don't think it's ignition or fuel. If it were just a misfire it would still be pulling some vacuum. The way it drops all the way out tells me your intake valve on that cylinder is sticking open and that loud pop you hear is actually the compression pushing back up into you intake manifold. You could have a bent valve or a broken valve spring. Or maybe just a lot of carbon build up on the valve stem. In any event take the valve cover off and take a look. I'd take out all of the plugs and slowly turn the engine over by hand and keep a close eye on that intake valve while the cam pushes it down and the spring tries to seat the valve.

Then run a compression check and a leak down test to see if that valve is leaking all the time or just when it sticks.
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Re: misfire: what is happening here?

#6

Post by jdvorchak »

The more I think of it. If that really is #4 (left side rear) then you might also find rust under the valve cover. Since when it's on the side stand that is the lowest cylinder. Condensation or even rain water likes to collect there. One note of caution. When putting the valve cover back on do NOT over tighten the bolts holding the cover down. Those bolts break easily. They are designed to stop against the head and the rubber grommet under the bolt head is what pushes down on the valve cover. Good and snug is all they have to be.

I didn't notice your second url until now, but you definitely have symptoms of scenario #5.
New users please visit our "Shop Talk" for common tips and help:
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Don't fix it 'til it's broke...
The person with the most toys wins!
If it has wheels or a skirt you can't afford it.
Amateur Radio Station WW9L

71 CL350 with Bingham side car
2021 HD Triglide
2008 HD Dyna Low Rider
83 KZ1100L1 Son totaled it
83 KZ550LTD Shaft sold

Previous bikes: 84 GL1200 STD, 83 KZ550LTD, 83 CB1000, 78 CB400A, 82 CB900F, 79 CB750K, 2001 GL1800, 2000 Dyna Super Glide, 1972 CB350 K4, 1985 GL1200A, 1997 1200 Sportster, 84 GL1200I, 82 Honda CM400E, 81 Suzuki GS650L, 72 Triumph Bonneville 750, 72 Honda CB350, 66(?) Honda 305 Scrambler, 6? Yamaha yz250, 62 650 Matchless (Norton-Villers).
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Re: misfire: what is happening here?

#7

Post by wetering »

Blue thunder: thank you so much for your recommendations. I have done it all. The ignition is new, both low tension and high tension circuit (Dyna electronic ignition, coils and plug wires + plug caps + plugs). Nevertheless, I Checked everything. I took off the carbs, totally overhauled 3 years ago using randakk’s carb kit, inspection revealed nothing that could explain the misfiring, reinstalled it, no change and in my despair I tried another newly revised one: no change either.
Jdvorchak: ‘if there is misfire it would still be pulling some vacuum’: that’s right and that is what you see on the video, misfire as a result of an ingnition failure: disconnection of the plug wire does not result in any change, except for a fall in rpm. Upon inspection valves and valve springs are OK. Valve clearances also, exactly 0.010. I did a compression test, no evidence of loss of compression, all 4 cylinders more or less equal. I did not perform a leakdown test since my tester broke down, but you can wonder if this is useful in the absence of compression issues. There is no rust under the valve cover.
Meanwhile I googled ‘vacuum gauge readings’: you will be directed to the websites of several professional engine rebuilders who elaborate on additional diagnostic possibilities of vacuum gauges, other than what they are intended for, i.e. carb synchronization. Very interesting.
I am afraid I will have to take off the cylinder heads and save money for a rebuild ……
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Re: misfire: what is happening here?

#8

Post by Whiskerfish »

One more quick test to do is try repeating it with a battery charger hooked up.
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Re: misfire: what is happening here?

#9

Post by wetering »

I did a leakdown test: the engine is like new. I took off the cylinder heads: the cylinders looked like new, cylinder heads also looked OK. I took the cylinder heads to an expert who in spite of the more than reasonable condition 'refurbished' the cylinder heads (just using the old parts, no new parts). I meticulously reassembled the engine and the result turned out to be very disappointing: runs and starts good but after about 15 km misfiring occured again, again that intermittent vacuum drop of cylinder 4 and slow return to idle. I am in despair. Anyone any thoughts? I have no idea how to proceed ......

Thanks,

Jeroen.
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Re: misfire: what is happening here?

#10

Post by Whiskerfish »

The slow return to idle is a good clue. I would focus on that and odds are high if you resolve or find the cause you will be on the right track. With it effecting a single cylinder concentrate on vacuum leaks at the Intake runner boots and the exhaust down pipe ( I think you guys call them over there) to head connection. A minuscule amount of false air at either place will cause those symptoms.
"Agreement is not a requirement for Respect" CDR Michael Smith USN (Ret) 2017
"The book is wrong, this whole Conclusion is Fallacious" River Tam
2008 GL1800 IIIA "TH3DOG"
1975/6/7/8/9 Arthur Fulmer Dressed Road bike
1975 Naked Noisy and Nasty in town bike
and a whole garage full of possibilities!!

Psst. oh and by the way CHANGE YOUR BELTS!!!!
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Re: misfire: what is happening here?

#11

Post by wetering »

Thanks.

Indeed 4 factors seem to be of importance in establishing a diagnosis:

1: the engine must be warm
2: misfiring occurs at low throttle in particular
3: there is an intermittent 'vacuum leak' in one cylinder only
4: slow return to idle

Seems quite straight forward but with regard to a possible cause: after having considered and excluded numerous possible scenarios I still have not the remotest idea .........

I would be happy with any helpful suggestions !
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Re: misfire: what is happening here?

#12

Post by Whiskerfish »

Going back to your video and seeing that still happen after you pull the plug wire from # 4 is perplexing. It does make it seem to be a pure mechanical issue but that would not explain the slow return to idle or the fact that the misfire only happens once warm. To me that is false air. When things happen as it warms up usually that is associated with materials expanding and leaks developing due to that expansion, or at least that has been most of my experience.
Is there another set of carbs around you can swap in for troubleshooting?
"Agreement is not a requirement for Respect" CDR Michael Smith USN (Ret) 2017
"The book is wrong, this whole Conclusion is Fallacious" River Tam
2008 GL1800 IIIA "TH3DOG"
1975/6/7/8/9 Arthur Fulmer Dressed Road bike
1975 Naked Noisy and Nasty in town bike
and a whole garage full of possibilities!!

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Re: misfire: what is happening here?

#13

Post by chrisz »

I've looked at the video that was posted at the start of this thread. Going only by the sound that is made when the vacuum drop is noticed is very similar to what I hear at first startup after a long sit/stay (no ride for several days). I've always assumed that a cylinder or two was perhaps low on fuel, as after a minute or two the bike recovers and I don't have this sound again unless the bike sits for several days unused. (Maybe it's telling me something,... to ride more.) But if your plug on #4 was showing rich, perhaps it is flooding. Just saying it might be fuel related, but my advice and a buck won't buy you a coffee in this country.
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Re: misfire: what is happening here?

#14

Post by wetering »

Dear Chrisz: it is definitely not fuel related: the plugs have always been equal, light tan coloured as it should be.

Dear Whiskerfish: as said, I have tried everything you can imagine in order to find out what is wrong, including another carb, newly and meticulously overhauled with excellent Randakk's stuff, with no changes at all.

I agree with you that everything points to false air. The only entrance of false air I can imagine is the rubber of the intake runner, though it looks perfect, but maybe an intermittent leak occurs when the engine warms up. I will have a look at it. (the only thing I did not have a look at, as far as I am aware, though I skimped a little with WD40 and propane without obvious results ...... ). On the other hand, I can hardly imagine that an intermittent vacuum leak in one cylinder only would cause that vigorous engine stumbling and the engine generally runs well at idle.
Last edited by wetering on Thu Aug 25, 2016 8:46 am, edited 2 times in total.
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Re: misfire: what is happening here?

#15

Post by robin1731 »

When the shop did your heads did they check for cracks? A crack in an intake port can be hard to see with the naked eye. Or a crack in an intake elbow. Or even the carb body. Cracks in any of these places can cause issues. Normally they would get better when warm but not always.

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