misfire: what is happening here?

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wetering
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Re: misfire: what is happening here?

#16

Post by wetering »

Thanks for your reply Robin. I checked, they checked: naked eye method. I know that experts have more sophisticated tools than the naked eye and in retrospect it would have been better to check not only with the naked eye since after all what I have done and the lasting vacuum drop on cylinder 4 it seems that the possibility of issues has been narrowed to a hidden cylinder head crack indeed, and vacuum leak of the intake runner. A crack in the carb body or any other carb issue is unlikely since I swapped carbs without change in symptoms. Do'nt know if I used the same intake manifold on cylinder 4 ......
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Re: misfire: what is happening here?

#17

Post by jdvorchak »

You could pull the head and rest it on the workbench with the combustion chamber facing up. Then pour solvent or gasoline to fill the chamber or at least cover the valve faces. Let it sit like that for 10 minutes or so and see if it leaks down past the valves onto the valve stems. A little seeping is probably normal but generally it should not leak. If it pours out pretty fast there is something wrong. Fill both combustion chambers and compare 2 and 4 for leak rate.

Without a magnaflux I think this is your best bet.
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Re: misfire: what is happening here?

#18

Post by robin1731 »

And most shops wouldn't check for cracks unless you asked them too. But a good/real machine shop would have the equipment to do it. Many/most times cracks can't be seen with the eye. I did find some on a set of heads for someone on here one time. I almost missed those.

They would need to look at the area where the manifold bolts to the head. That is also not the usual place they would look for cracks. They would look for them more around combustion area and around coolant flow areas. The most common places you would have a crack.

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Re: misfire: what is happening here?

#19

Post by robin1731 »

jdvorchak wrote:You could pull the head and rest it on the workbench with the combustion chamber facing up. Then pour solvent or gasoline to fill the chamber or at least cover the valve faces. Let it sit like that for 10 minutes or so and see if it leaks down past the valves onto the valve stems. A little seeping is probably normal but generally it should not leak. If it pours out pretty fast there is something wrong. Fill both combustion chambers and compare 2 and 4 for leak rate.

Without a magnaflux I think this is your best bet.

That would work normally. But I'm thinking a crack where the manifold bolts to the head. And most of his issues occur after the engine is warm.

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Re: misfire: what is happening here?

#20

Post by jdvorchak »

Thanks Robin. My thinking is that intermittent "blow back" into the intake. I still think it's a bad valve spring or sticking valve stem. Or it could also be that the left side timing belt is one tooth off and combined with rough idle from not being in sync, could cause that intake valve to open prematurely or not close in time. My suggestion to fluid check for a crack was to prove that the valve seat was good and he should look elsewhere.
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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?

#21

Post by jdvorchak »

I just thought of something he could check by only taking off the valve cover. Spin the engine until the cam lobe is pointing away from the valve and try to spin the valve spring by hand. Also try to manually push those valves in and if you can, it's a worn valve spring. If it's really bad enough the spring will spin by hand.
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.
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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?

#22

Post by wetering »

jdvorchak wrote:Thanks Robin. My thinking is that intermittent "blow back" into the intake. I still think it's a bad valve spring or sticking valve stem. Or it could also be that the left side timing belt is one tooth off and combined with rough idle from not being in sync, could cause that intake valve to open prematurely or not close in time. My suggestion to fluid check for a crack was to prove that the valve seat was good and he should look elsewhere.
Gentlemen,

All my experiences made me quite uncertain, however I know for sure that:

-valve springs are not the most likely culprit: all have been checked and were well within the limits as specified by the manufacturer (with regard to length)
- sticking valve stems neither as this would definitely leave traces: they really look 'virgin' and there is no doubt about valve stem to guide clearance.
- you may chop my head off the rest of my body if the valve timing turns out not to be right
- the engine is not perfectly in sync, I agree, but still well within the limits as specified by the manufacturer
- adequate valve closure seems to be beyond discussion: valves and valve seats were refurbished by the shop ..........
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Re: misfire: what is happening here?

#23

Post by robin1731 »

Sorry but checking valve spring length really does no good. You have to check valve spring pressure at installed height. Those limits are in the manual.

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wetering
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Re: misfire: what is happening here?

#24

Post by wetering »

This was the last test before re-dismantling the cylinder heads:

https://www.youtube.com/watch?v=5PB8vGYdDvU

It is not an intermittent vacuum drop, instead, hot air is expelled from the cylinders, see the picture what happens if it hits my finger.

All parts look fine. I took the head to the shop that 'refurbished' it and they claimed that either I failed to correctly adjust the valves or the clearance as specified in the Honda shop manual is wrong :IDTS: I am not really inclined to believe that explanation. I took the heads to another shop for a second opinion: they had no idea why it does not work and their 'advice' was: 'good luck' :lol:

I don't know what to do next ......
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Re: misfire: what is happening here?

#25

Post by elwood440 »

The intake valve isn't closing every time. It has to be what's happening.

There's only a couple ways your combustion charge can get out of the intake side. Either the valve didn't close all the way, there's a crack somewhere, or the plug fired at the wrong time. I'm leaning toward valve not closing.

More things to check:
-Is the spring pressure within spec? As Robin said, a correct height spring can be weak. It may be cracked, or just tired.
-Does the rocker move freely on the shaft? You would need to pull the cam carrier to check. Is there evidence of wear or galling between shaft and rocker? If so, use Old Fogey's fix here
-Are the oiling holes in the rocker shaft pointed in the correct direction?
-Is there strange wear on the tip of the valve adjuster, or on the end of the valve stem?
-Is the valve adjuster in good shape? solid, one piece, crisp threads, slightly domed tip?
-finally, have the heads been off since the problem started? Could there be a screw or something in the cylinder that sometimes gets caught in the valve??
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Re: misfire: what is happening here?

#26

Post by gltriker »

Betcha you had your now badly blistered fingertips on an open synchronizer fitting when that happened. Correct? :shock:
Couldn't believe it just happened, may not have processed how badly your first fingertip was burned, and unwittingly sacrificed the other fingertip soon afterwards?

Last summer, I had the same thing happen to 2 of my fingertips, too!

I leave 4 compact 90 degree synchronizer fittings permanently in place on the stock oem intake manifolds on my GL1000 trike's engine, and each of their barbs is sealed with a slip-on rubber cap when they aren't being used to synchronize the carburetors.

Unbeknownst to me at the time I cooked my fingerprints, one of those rubber sealing caps had dismounted off its barb and I noticed the bike was not idling smoothly with an intermittent hint of a slight misfire, too.
When I eventually noticed the sealing cap was missing, I placed my fingertip on the open barb to feel how much vacuum was present there at idle. Very Sorry I Did. :IDTS:
Apparently, with the undesirable extra air entering that particular intake manifold's unplugged barb, that circumstance was causing a lean idle fuel misfire event, which backfired invisible to the eye hot gases out through the opened barb of the still installed synchronizer fitting.

As far as I can remember now, I utilized some black tape to block off the 90 degree barb fitting until I could buy some more caps. As soon as that fitting's barb was again plugged, the intermittent misfire disappeared and the engine idled smoothly again.

Try introducing an unlighted propane torch into the area of the intake manifold, again. ;)

First, though,, enrichen the pilot fuel delivery screw an extra turn ccw. :oldies

That's all.
Cliff (74yrs ;) )

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Re: misfire: what is happening here?

#27

Post by ericheath »

Really not following to closely, but could a partially plugged exhaust tube cause this? Fires three or four times then gets a buildup and comes back through the intake valve during the overlap period??

Perhaps peeking into the headers for this
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Re: misfire: what is happening here?

#28

Post by Whiskerfish »

When in doubt always go back to the basics. Clean fresh plugs , ignition timing, OEM air filter and intake setup, Carb idle drop settings, carb synch, proper throttle freeplay and operation, valve adjustment......

Have you resolved the slow return to idle issue? Does your idle decrease as you perform the idle drop on each carb? How is you idle voltage at the battery?

I hate to disagree with some of the others but I really think this is a tuning issue of some kind and not mechanical. I have seen this before. Using mercury sticks it shows up as a complete loss of vacuum when it happens for just that split second. I can not remember specifically but it was resolved that day and was ignition or carb related. I did not pull the heads I do remember that.
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Re: misfire: what is happening here?

#29

Post by jdvorchak »

whisker might be on to something. If there was an induced spark, or really advanced ignition timing on the cylinder just opposite this one, during the intake stroke it could light the mixture and result in what you're seeing. remember these are wasted spark. But if that were the case I would expect to see that on every revolution. Perhaps a hot spot inside that cylinder is causing the random mis-fire and blow back. Does it do this on start up or after it's been running for a few seconds?

If it does it even with a cool engine I'd be looking at new plug wires. Perhaps your old wires are leaking and actually inducing a spark from an adjacent plug wire? Try running the bike in the dark and see if you can see any sparks.
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?

#30

Post by wetering »

Dear all,

Thank you all for your interesting suggestions. To be short: all possible issues have been addressed already, checked, not only once, not twice but several times and where applicable or where in any doubt appropriate measures have been taken. The only thing that I did not check meticulously is a clogged exhaust as suggested by Eric. I will have a look at that ……….
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