Honda-SOHC
SOHC.co.uk Forums => CB500/550 => Topic started by: mickwinf on March 07, 2011, 09:33:50 PM
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hi guys would like some tips to get my k3 running better. After 28 years in bits the engine now runs, but only on three cyls, no 4 is not firing. I have swapped plugs and checked timing, also opened gap on valves which seemed to improve matters slightly.New plugs points and coils fitted but only occasional spit back through no 4 carb, if plug lead taken off with engine running it makes no difference.When checked against earth plug has strong spark.Is it likely that it is a valve problem, the head and barrels were rebuilt by po, and i already discovered cam timing was well out!! Any ideas welcome, thanks,Mick
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If you have a spark on #4 it sounds like fuel starvation.
Is the plug wet when you remove it?
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steve it dosn't seem that wet when i remove the plug, so i tried squirting carb cleaner in the carb and a bit in the cylinder, expecting it to fire but no luck!
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+1 on fuel starvation... float stuck shut? Pop the bowl off if you can and see if there is any fuel feed to that carb?
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what do you mean by cam timing well out. It might have bent a valve if its that far out. Check compression on that cylinder...compare to number 1.
If you haven't got a compression tester take the plugs out and put your thumb over the hole. crank it on the starter motor. Compare the difference between 1 and 4. Should blow your thumb off if OK, wont do much if you have a bent valve.
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thanks for quick response guys will try again. Ref cam timing i reckon it was 90 degrees out, some "expert" did it before, luckily i only turned it over slowly on the kickstarter and stopped as soon as it went solid! hope i have not got a bent valve, i have just ordered a 12mm converter for the compression gauge which should throw some light on it!
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I'd go with a compression check as well just to help you eliminate it if nothing else.
Have always used lab bottle with long plastic spout to squirt a small amount of fuel down the suspect carb when running and see if fires then.
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Well I hope your OK but it doesn't take much to ding a valve. They bend at the head very easy. I was mechanicing for a mate at the IOM tt in the very early 80's he was in the F2 race on a 460 Kirby Honda. A young german lad approached us in the paddock with his cylinder head from his 400 four. His cam chain had snapped as he was kicking the bike over to start it - it bent 4 valves badly. To get him home I took the valves to some engineering place by the harbor and put them in a lathe at high speed and just tapped them with a lump of wood until they spun true. Worked very well..I lapped them in and he put the head back on with a spare camchain we had. Got him round the Island all week and back to Germany. Nice lad he was, kept comming back every night to see how my mate had got on, we had some good sessions on the beer with him.
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It doesn't take much at all to bend a valve, I was turning over a CB450 engine on the bench by hand and just kissed a valve, it bent, and had to be replaced.
Cheers
Den
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My first check would be to have it idle in the (pitch)dark and watch for any fireworks (sparks between HTleads/plugcaps and engine).
K3's sometimes have a problem with their carburetor venttubes. Check if they're all (4) open and as a precaution you may cut a few inches where they end (close to the centerstand).
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thanks for advice guys i had it running again tonight, it seems to fire on all four when the throttle is blipped but on lower throttle openings still on three, so maybe carbs are the problem after all! petrol is getting to carb ok and vent pipes are clear, no extra sparks in dark.
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If the fuel line is blocked or kinked or has a poor flow from a filter then I've found number 4 can go out of play. On my bike it was intermittent and due to a cheapo filter that wasn't allowing enough flow, also it was quite tight to fit a filter in so the pipe was bent at a funny angle and was restricted.
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If the fuel line is blocked or kinked or has a poor flow from a filter then I've found number 4 can go out of play. On my bike it was intermittent and due to a cheapo filter that wasn't allowing enough flow, also it was quite tight to fit a filter in so the pipe was bent at a funny angle and was restricted.
i have to agree with this, as today on my way to work the bike started to run lean (i had just filled up)... oh great i thought im back to having a miss fire.
but turning to reserve cured the problem.
I will have a look tommorow but im sure i will find a partially blocked fuel tap.
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bad news.today i did a compression test,less than 50psi on cyl 4, so looks like valve trouble after all, really depressed now!
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Dont be.. at least you found the problem more than half the battle.
they are good fun enjoyable bikes.
take your time and reap the rewards.
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What were numbers 1,2, and 3...?
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other cyls 125 psi so should be ok, will keep you informed, wont be able to take head off for a week or so, have to make a living to pay for this money pit!!
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well i had a bit of time today so decided to take off the head. i got all the bolts and nuts off, but the head refuses to come apart from the cyl barrels, the whole assembly is loose from the crankcase. Any ideas chaps? I have tried tapping with a block of wood and gently levering but no luck. I dont realy want to take engine out again!!
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You have taken out the 10mm headed bolts front and back and the tensioner top bolt havent you? Only ask 'cos i have made the same mistake in the past!!
There ought to be two pry points, one on each end
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yes i took out the bolts you mentioned Bryan, where are the pry points? I think the PO stuck the head gasket with sealer/ cement!
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Head gaskets are always stuck like the proverbial and usually take a lot of time and care to remove on these bikes. the pry points are I think - been a while - at each corner at the front and Honda actually recommend using a srewdriver! there is actually even a recess for this if you look carefully
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thanks for advice i found the pry points, one screwdriver and one hammer and the head is off!! took out the no 4 exhaust valve and it is bent, i think i have a spare so should be an easy fix ( i hope!!)
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Mickwinf,
When you said "after years in bits" did you mean your bike OR the motor...
Just had a thought, If the motor was in one lump before you took it apart to find the bent valve.....,
It could well be worthwhile, to try and find out what caused the valve to bend??
Just a thought. To bend, it must have clouted the piston, I would want to know why..
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both bike and engine have been in bits engine had been assembled when i got it, supposedly by a mechanic but the cam timing was wrong, i think when i turned the engine over it hit the piston.Moral of the story, always check yourself, dont take someone elses word for it!!
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well after a strip down and replacement valve i now have a four cylinder 550 again! Got it back together and fitted my rebuilt wheels, had a ride down the drive,probably the first time it has moved under its own power since about 1982!! Feels great!!
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That's great news, nice when a plan comes together.
Cheers
Den
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bad news guys, it has gone back to 3 cyls! this time no 2 cyl is not firing properly, the engine sounds ok but a little flat, the no 2 exhaust is not getting hot, but it runs worse if i remove no 2 plug cap so it is doing something.Oh no i thought, another valve, but compression is 140 psi. I tried another plug which seemed to help a little but still not right.Anyone had similar probs? Mick
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Bad plug cap or low battery voltage?
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hi bryan, battery is charged, bike starts first kick,coils and caps are new,but i will try another cap in case i have a duff one
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I seem to have had a few dud spark plugs in the past few years which were dud streight from the box, and one that the threads parted company from the rest of the plug - not overtighened as on inspection there was the yellow passivate plating inside the broken metal of the plug so new isn't always known good.
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Have you ridden it to see if it runs on all 4 when brought up quite a bit from idle jet circuit or is it still not getting the pipe on #2 hot? if it runs out ok this would suggest idle jet circuit part blocked.
On some bikes you can swop the wiring and HT leads round to see if the problem goes with one coil and at least isolate 50% of the resoning. May have to unbolt the coils to make things reach.
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i am thinking it is fuel rather than electrics, but can't ride it yet as not on the road,will probably have to take carbs off again and clean out, again!