Honda-SOHC
SOHC.co.uk Forums => CB500/550 => Topic started by: mickwinf on May 22, 2011, 06:22:39 PM
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hi guys,today i had a play with the k3, took the carbs off to clean out again and fit the new carb to airbox rubbers which arrived from America last week.Once they were refitted i set them with vaccum gauges,timing and valve settings etc and also fitted another twistgrip as the old one was sticking.Now for a quick roadtest before dinner.It started up first kick and sounded great, but when out on the road it seems a bit flat about 5000 rpm it dosn`t seem to want to go above that.Any ideas from you experts out there?? thanks,Mick
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You might want to take a look at this thread with similar symptoms http://www.sohc.co.uk/index.php?topic=2647.0 (http://www.sohc.co.uk/index.php?topic=2647.0)
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Obviously somethings very wrong...5,000 revs, blimey my mates got a 13,000 mile from new K3 that he dug out with the intention of a similar rebuild to my 500 - I think he reckons I'm going to be keen to help. We had a quick spin round the block on it. I hit 9.500 revs easy and it pulled from nothing in 5th all the way. I was very surprised how good it was. Its a very tidy bike anyway, but the stock exhausts are a bit pitted and with alot of use I can see them falling apart...a shame. Lovely bike to ride though.
Not sure what your problem is...What happens at 5,000 exactly.....what do the plugs look like. Is the advance and retard mechanism OK, are you absolutely sure you got the valve timing right. Is the throttle fully opening, is the choke closing. Are point and condensors OK, does it rev high and freely on the stand. It really is imposible to pinpoint something definate...if I heard and rode it I'd know pretty much instantly....its one of those sort of problems that require you go over everything again...have you got a strobe..?? have you got good fuel supply...??
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thanks for ideas, i have checked valve timing and clearances,static timing spot on,will check with strobe for advance/retard.Carbs have been cleaned thorougly several times with carb cleaner and compressed air, but i have not checked float hight.I do have a in line filter, will try without to see if that makes a difference, i do have a motad fitted.At low speed it seems to be ok but dosn't want to rev above about 5000 rpm, the exhaust sounds flat, as if it was on 3 cyls, but no misfire, seems to rev ok on the stand.Compressions were ok when checked before( see my other threads)I remember my old 500/4 reving right through the red line and out the other side once when i tried to outrun a gold wing!!
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Does it also come to life above 6,000 if you down change to get it there?
Does the effect get less severe if you give it a longer run out?
I richened the needles 1 notch which made a significant improvement but haven't gotten around to adjusting the jetting further. In my case the flat spot is present regardless of throttle position.
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had another go at it today,rechecked valves and timing with strobe, it seems to be advancing ok.plugs were checked and found to be the wrong grade so swapped them with spares and checked fuel filter was not blocked.When stood it revs straight uo to 8000 revs no problem but when on the road it is a bit sluggish and is reluctant to rev, if i drop a gear or two it will maybe struggle up to 6000 rpm but is not happy.it does seem to be a fuel type problem but the carbs are clean and the tank vent hole clear.Thanks for all the advice, i feel a bit isolated here in Scunthorpe, z1100r i wish you could hear it and help me cos i am a bit fed up!! Totty i read the thread and you have my sympathy!
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Can you try it without the fuel filter in the system?
The engine uses a lot more fuel under load - reving in neutral will not necessarily show up the problem (which sounds like fuel starvation to me)
I had a 750 with that problem. Cleaning the (two) fuel filters didn't help so I removed them completely which fixed the problem.
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yes Steve will try that next, didn't have any spare pipe today!
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How old is the fuel?
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totty fuel is all new tank has been sealed
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Air cleaner blocked or eahaust bunged, or if you are old enough to carry waterproof leggings DO NOT put them under the seat!!!
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Bryan air filter is new no obstructions did wonder about motad exhaust will try another system see if that improves it
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Exhaust would have to be pracitically blocked up for it to knock that much power off. You are talking about only being able to rev this bike to 6 grand on the road....theres something serious somewhere and I wouldn't ride it about too much until you find it. This serious a fault and power sap can end up in mechanical failure like a holed piston or seize.
What sort of speed are you doing at 5,000 revs.
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Should be about 60 mph
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Some more Ideas...Are the carb slides the right way round (can you get them wrong i cant remember), You definately only bent 1 valve didnt you..??, It sounds bad enough to have a brake stuck on.
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As the others say I'd try without the fuel filter to help understand it.
Also if you sit with it ticking over and then pull away and rev it in first will it run to the red line? if it does this on full float chambers then it would point to fuel supply volume being too low as well.
If you run it at 5000ish where it stops revving, then pull the clutch and use the kill switch, roll it to a stop and check the plug colour (otherwise known as a plug chop) it will give you an indicator as to to mixture at this point.
At the point at which it falters, try pulling the choke to see if it then revs higher and find out if it needs more fuel (wont work if there is insufficient fuel supply as above) but would help in analysis.
Try it without the air filter just to make sure it's not running you round in circles, it's not unknown.
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this afternoon i tried without filter,at first i thought i had cracked it, it seemed to rev better and respond to the throttle more eagerly,but again when i gave it some stick it falters at 5000 rpm at about 60 ish mph.So it does seem like fuel.Next i thought it could be the tap so i swapped it over, no change,removed air filter,the same in fact on the way home it started to run sicker at lower revs.it will rev a little more in first but reluctantly.I have checked brakes they are not seized i hope valves are ok compressions are good if choke on it runs worse.This is a real puzzle hope someone can help!!
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Very puzzling.
Presume you've got the tap on main tank? if so try it on reserve as it usually bypasses the longer feed pipe in the tank and if that is giving a restriction it would show.
Can you easily blow through the taps with your mouth as you should be able to.
You could try to set the static ignition timing to as much advance as you can get to assess that on a run out.
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tried on reserve no difference will try altering timing
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Something daft that I did once on 2-stroke was leave my bits of rag in the exhaust ports when put the exhausts back on. The rags got blown to the back of the pipes and caused me a fair bit of head scratching. That far back they dont burn away. Had to fish them out with very long lengths of welding wire. Caused a similar problem to yours.
clutching at straws now..!!
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Outside chance of kinked fuel line - a mate of mine who knows a lot more about bikes than me got foxed by the fuel line being just slightly kinked when the tank was fitted right - the problem was a right pain as in the garage it went away and only manifested itself out on the road where fuel demand got high enough for long enough. Lift the tank and all cured, put tank back on and only after a short drive would it go wrong again.
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Are the throttle slides opening fully?
Are the choke flaps opening fully?
What colour are the plugs?
Are all the downtubes hot after running?
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matthewmosse, will check but have tried different pipes, Florence, yes yes dont know yet, yes. Next thing i will swap the carbs for a spare set and see if that makes a difference.I still think it is a fuel issue, but i may have to start again with valve timing and compression checks,blast it !!
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I dont think its carbs...unless you have assembled them wrong in some way and the only thing that can cause this much of a problem is the slides...not opening or the wrong way round - cut out not at the back - or choke butterflys sticking. If your jets and everything are all std, and float heights let at least some petrol in and there is a reasonble flow from the tank it will certainly rev OK.
This is a fundamental problem, posibly cam timing or ignition I reckon...most likely ignition. Is it std points and condensor..?? have you done the dark test..no sparks or ticking anywhere. Dodgy coils, bypass ignition and kill switch - just earth the black and white coil wires direct.
try another ignition back plate with different point/condensors if you have one....look for lots of big sparks at one or other sets of points - little sparks OK.
If you have to do cam timing again...take off lid turn engine to TDC1-4 and middle of slot in end of camshaft should be exactly parallel with the head face - with the tensioner working. Can be pointing either way, just needs to be parallel when at 1-4 on adv unit.
After engine has run check tappets. I give up with feelers, all I do is TDC-1-4 and set 1(in/ex), 2(ex), 3(in), turn 360 to 1-4 again and set, 4(in/ex), 3(ex), 2(in). I think thats the right way round...but you get what I mean.
Job done.
I set them by undoing locknut and twiddling screw to touch valve, unwind 1/8 of a turn and lock-up. Thats good enough for me and how I've always done it for all 8 valves on my road engines or race engine.
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bad news, i tried another set of carbs but their condition is unknown although it did seem to rev better in first gear but ran badly so not conclusive.I thought i would recheck compressions and one cyl is down to 50 psi another is only 100 ish. Bugger
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That comment you made when you said "I thought I'd cracked it" what made you think the revs were better?
I ask because of this scenario:- if you start the motor, warm up and go on a ride at normal use then, IF the fuel supply is restricted it's possible that you've depleted the volume of petrol in the float chambers by doing this so that when you wind it up you get no revs increasing as fuel demand can't be met.
Soooo if you for example stop at roadside and turn it off, wait say 10mins (for the float chambers to fill)then restart and straight away nail it in first in theory while the carbs are full, then if it will get past 5000ish this would confirm supply problem and not timing of cam / ignition if that makes sense. If that's the case you need to strip and examine the whole supply pipes unions junctions etc to find it.
If you can't supply fuel at high enough rate then the simptoms can mimic very low float levels as the chambers will not be maintained at the expected level anyway.
It goes without saying that if you're not cnfident of cam timing you should verfy this as a priority.
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i have checked fuel supply and is all clear,i thought it was fuel partly because it sometimes runs better than others.very fed up now will try again in a day or two thanks for all the advice guys,if anyone wants to discuss i can be contacted on 07870112945,cheers all, Mick
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one cyl is down to 50 psi another is only 100 ish.
Are you sure? Can't have been finger trouble with the compression tester can it?
50 psi is on the lower side of bad.
Try again. Kill switch off, throttle wide open.
If low, pour a little oil into the plug hole and try again. A different (higher) reading indicates broken or stuck rings.
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Aah, bit of a bummer! If it's cam timing I wonder what could be leading you the wrong way.
I think I was writing my last post as you were, so some of it's a bit out of sequence.
You should be able to measure an increase in valve clearance if you set them when they were all straight a bent one should now show a big gap to the tappet.
I'm sure you'll get there and eventually be able to get it going well.
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bad news, i tried another set of carbs but their condition is unknown although it did seem to rev better in first gear but ran badly so not conclusive.I thought i would recheck compressions and one cyl is down to 50 psi another is only 100 ish. Bugger
Well the thing is ....there is never just one valve down....if you clattered one you just have to have clattered the others. We have been talking fine tuning to resolve your problem, but you just have to have a more serious problem than that. I know how serious it can be... I've been there with a German guy and a 400 four in teh IOM. He was just trying to start his bike on the Kickstart and bent four valves seriously badly when his cam chain snapped whilst he was kicking it over. I put his four valves in a lathe borrowed in the IOM by the quay and straightened them for him just to get him home. He wrote mke a letter a year later thanking me and to say the bike was still going on the same valves.
The guy that built your engine originally needs shooting.
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z1100r i agree, if you want a job doing.....
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Get yourself a nice evening with an early start and get the head off again. Turn it upside down and pour petrol into the combustion chambers, the level will go down fairly quick on the bad ones and fuel will be in the inlet or exhaust tracts of the dodgy valves. 50psi is awful and definately signifies a major problem, I'm not going to speculate what the problem is anymore, firstly you need to satisfy yourself that the base engine is sound.
Personally I would get all the valves out and roll them on something flat - surface plate, kitchen worktop. Once satisfied they were al straight I would put them back in 1 at a time turning them and watching the head for wobble. Then give them a light lapping in with paste just until i could see complete grey rings on valve and seat. I would then be happy with the head.
So moving on, this previous engine builder bloke sounds like he aint got a clue. I would definately at this stage whip off the barrel and make sure I had a full complement of rings and that their end gaps were roughly correct.
Its upto you how far you go but I think you need to get certain things straight like...RIGHT THE BASIC BLOODY ENGINE IS NOW OK.......!!!! Thats the bores the pistons, the rings , the valves, the head gasket, the cam timing.
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well... good news and bad, i had a spare hour this am so rechecked the compressions, i had not screwed the tester in properly!! now all 4 cyls are showing 140-150 psi.hmm take rocker cover off and check cam timing,it is spot on.So i am sure basic engine is ok. plan now is to re-assemble and check and recheck clearances clean and check carbs.I am thinking which jets would be operating at 5000 rpm, could it be wrong size main jet or needles set wrong?? will keep trying!
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well if your happy thats good. To get the lack of revs/power that your getting something has to be not working, not be just out of tune. You've swapped too much stuff for 2 sets of everything to be that wrong and have the same thing wrong it. I'm lost, willing you to find the problem quickly.
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i am stumped also,but i checked pistons etc when i replaced valve so am sure its not a big mechanical problem, but what the hell is it?!!
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Well that is good news that the motor seems ok.
At that revs and with throttle wide open it's mostly pure main jet running as I understand it with the height of the needle controlling when it gets to max flow as you open the slides more, i.e. if you raise the needles then it gets to max flow a little earlier but doesn't increase the total. If you raised the needles (and this was the fault) you'd expect to see some increase in the faltering revs, if it gets better then you'd have to consider going up in main jet size and bringing the needles back to the central position.
One thing we haven't gone over is the breathers into the float chambers, if these are blocked at all then it will run lean at high demand as you need free air to get in for the venturi vacuum to draw fuel out at
the correct rate.
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k2-k6 this is one thing i had not considered where are the breathers?
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You are setting the carbs up to the right manual aren't you...not the Haynes manual with the 500 carbs in.
http://data.sohc4.net/SM500/HSM500550_16.pdf (http://data.sohc4.net/SM500/HSM500550_16.pdf)
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well guys i have found the problem and sorted it!! Believe it or not all it was was a float needle! the little sprung rod that contacts the float was stuck in place on no 2 cyl, all that trouble for that!! now it revs like a good un, thanks for help everyone.
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Sometimes it's something small like that that's missed, it just shows when discussing jetting etc how such an insignificant looking device can have a huge effect.
Good you found it though, should get some decent time out on it during the summer now.
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I'm glad its sorted but still a little dubious that thats what was causing it to only rev to 5000rpm and be as sluggish as you say. I would of expected fuel to be leaking out all over the place and the plug on that cylinder to at least have been very black if the problem was that power sapping.
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I think what you're saying is that the needle valve was stuck closed, is that correct? if so then that cylinder would not have been running at all in that case.
If so you could have found it when you started it by putting your hand or a wet rag (before it got too hot) onto each header pipe as that tells you if one isn't firing.
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yes k2-k6 i think it was sticking shut.thats why it felt like it was running on three cyls,exhaust didnt get as hot.Iam sure this was the problem all along,will go for a blast at weekend.
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Hope it goes well anyway as it's nice to get a decent runout on a bike.
The question you made about breathers, they are just the tubes that come ot of the bottom af the carbs and dangle down the back of the engine. They obviously direct fuel away from the motor if the float sticks so neat petrol just goes to the floor but they also keep the pressure equal to normal so that the jetting is not unduly affected as the main jet demand rises during max throttle demand so that the carbs work as designed (obviously not the case here but we were cluthching at straws to help out).