Honda-SOHC
SOHC.co.uk Forums => CB500/550 => Topic started by: Steris on March 15, 2021, 09:40:37 AM
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Morning all.
I am having slow running problems, bike runs fine above about 3000 rpm, but wont tick over and has no power when pulling away. The plugs are all sooted up. When i set the float height to 22mm
the fuel over flows on all 4 carbs. Needles are on the third groove down. I have got the carbs off and stripped and ultrasonically cleaned them and all the jet etc. I have set each carb up on the bench with a sight tube so i can see the fuel level. At 26 mm float height the fuel level is 10 mm below the float bowl lip, and when drained into a beaker there is 50 ml in the float chamber. I seem to remember reading that this is right, can anyone shed any more light on this for me please.. Its driving me mad ::)
Many thanks Andrew
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Did you set the float heights correct Andrew. I'm sure you did but I'm just checking 😊
http://www.sohc.co.uk/index.php/topic,23233.0.html
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Hello Julie
Yep, I have set them just like you describe, this time I've set them individually ie off the bracket. I'm wondering if the springs in the needles have become weak, hence the need to make the float height greater, from 22 to 25/26. This would allow the float to exert slightly more pressure and seal the fuel flow. I have know read that the fuel should be 3-4mm from the top of the float bowl so i will adjust again tonight and see what i get.
Thanks for your reply :)
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I have seen needle with "sticky" pins, i cleane them in acetone and it improved them
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Clean / polish the taper seat of the inlet by using a small dowel sharpened like a pencil with something like t-cut to make sure it promotes a good seal. You can either spin it with you hands or in a drill slowly to improve any surface imperfections.
It usually helps the valve shut off easily with just the minimum of pressure.
If the moving valve tips aren't rubber (brass) you can also polish these the same.
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Hi oddjob
No they are the original's. I bought 4 repair kits but the needles are slightly longer than the original's and with them in you can't get the fuel level high enough.
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I've set one carb at 22mm and one at 26mm both with sight tubes attached, and there is only a slight difference in fuel level. I left them over night with a good head of fuel feeding them and the level was still the same this morning :D so i will go back to the 22mm float height and fit them back to the bike, but I'm sure they will leak again once I've refitted them :(. If the level is too high will this make the bike run rich on tick over (on the pilot jet)?
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Hi Andrew
I had similar problems when I restored my KH, just couldn't get it to idle or run properly even with the carbs having been blasted in an aqua jet and then ultrasonically cleaned. After the 2nd or 3rd ultrasonic cleaning and out of desperation, I put them back into the cleaner again for an hour each turning them half way through at 75 degrees, I was amazed at how much muck came out but after this the bike would idle and pull cleanly. I would try cleaning them again. I came to the conclusion that some of the airways in the carb must have been blocked with old fuel.
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"If the level is too high will this make the bike run rich on tick over (on the pilot jet)?" Yes but it's not a drastic change and more of an influence than absolute. Obviously it's ideal to be as close as possible to judge.
Picking up on what bruxby-clive noted, the idle circuit is drawing mixture from the float bowls by vacuum drop in venturi. This is modulated by the air adjustment screw to change the mixture ratio, but if the air passage leading to the screw is impaired then the vacuum pulls "full" fuel through the pilot jet. So in effect it's easily too rich if they are not fully clean.
One way to give an indication is with it running, then try each airscrew individually to see if that cylinder will range from rich (fully closed by turning inwards) to essentially too lean at about 3 full turns out. If you can't hear the cylinder changing much then that's an indication of the air passage still being partially blocked. You're dealing with very small volumes in these carbs, with any impairment making quite a % difference to the mixture.
You could end up with different settings on each airscrew and different from manual spec too. If you give each cylinder what it needs then discuss if they look really wonky as to what may be affecting it.
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I think I will give them all another hour in the ultrasonic Bath then try again, thanks for all your help so far, I will let you know how I get on over the weekend 🙂🙂
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Well all 4 carbs have had 2 hours in the ultrasonic bath. I have checked and re checked all of the jets and ports and everything is clear. I have blasted acetone through the slow pilot ports and its shooting through great, out the little jet in front of the slide and out the jet port and air intake port. Rubbers all replaced and float height set to 22mm checked and rechecked. I have balanced the carbs and rechecked the timing and valve gaps, it know ticks over at 900 with the air bleed screws wound 3 turns out but has no guts below 3000 rpm it just dies if you don't rev the nuts of it which suggests its only running on the main jets. I am now at a loss. Might have to spend some money on a another set of carbs :( Any advice would be greatly received
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Turn the air screws to 1 1/4 out and try it as straight comparison with nothing else altered to see what it does.
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Hi and thanks for the reply
I have had the air bleed screws set from 0 to 3 turns out and 3 turns out seems to be the best, it even runs with the air bleed screws completely removed :)
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Have you verified all air "vent" passages into the float bowls?
Try blowing into the overflow tubes to see if they are clear.
Something's odd, but must become apparent through inspection.
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Ignition advance and retard operation verified as ok?
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The vents and over flows are all clear. The advance retard seems ok, with a strobe it sits on the F mark on tick over and starts to advance about 1200-1500 rpm full advance about 3000rpm
Its driving me nuts :)
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I know that feeling. At least you are getting lots of practice removing and refitting the carbs. If you manage to fix it, do let us know. :)
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Have you tried spraying the intake rubbers from carb to head with something like wd 40 while running to check for air leaks?
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Check that the tips of the mixing screws are fine, they must have a conical tip, you turn until reaching the stop without forcing it, and then in a counterclockwise direction unscrew about 2 turns, the 4 equal, with rolling 100 meters You will notice if it drowns or lack of gasoline, if when placing the carburetors they lose their blows, a float may be hooked and not cut the valve, what jets do you have? You say that the needles are at their maximum height, that will only influence after 1 / 2 throttle up to the wot. mark the stroke of the fist in 1/8, 1/4, 1/2 and 3/4 and check how the motorcycle behaves in each position
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guidelines
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Would low compression cause slow running issues? I'm struggling to think of anything else that's causing my problem. It runs lovely over 3000rpm sits at 60 70 mph great, it also ticks over at 900rpm when cold, but once hot either dies or revs itself up to 3500 rpm. 🤔🤔🤔
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http://www.sohc.co.uk/index.php/topic,25429.0.html
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Would low compression cause slow running issues? I'm struggling to think of anything else that's causing my problem. It runs lovely over 3000rpm sits at 60 70 mph great, it also ticks over at 900rpm when cold, but once hot either dies or revs itself up to 3500 rpm. 🤔🤔🤔
Have you any compression readings for it?
Runaway idle speed is always excess air however it gets the supply to the combustion chamber.
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That the air screw rotates 3 turns and there are no changes in idle is not the right thing to do, I think you have the problem in the slow jets, I think you should reduce, which jets do you have at 38, 40?
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I had the same with a hanging idle and found one or two carbs needed balancing.
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Hi Merdoc
Everything in the carbs are original so 40 pilot jets. I'm going to remove the carbs again this weekend and drill out all the brass plugs in the pilot passage ways and make sure they are 100% clear..
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I wouldn't drill out anything. If yours is a CB500, realise that a common mistake is trying to achieve a highest possible idle rpm by turning out the airscrews. No doubt the engine will idle nicely and pur like a kitten, but it comes at a price: driveability is gone and you'll experience a poor acceleration. Once the engine is hot, rpm will not drop. Realise these are quite simple carbs that lack acceleration jets. Therefore airscrews need to be set as is specified in the manual, even when you need to adjust the big idle knob to arrive at an acceptable idle rpm. Mixture will be quite rich and I wouldn't be surprised an exhaust gas meter would show 5% CO. I recommend an idle rpm of about 1100-1150. Also realise that after a cold start, you will need to maintain a raised idle to prevent stalling. Some do it by keeping the throttle grip twisted a bit (as described in the Owner's Manual), others use the big idle adjuster knob, which they will reset at a traffic light, once the engine has warmed up sufficiently.
Verify fuel supply is constant without hesitations. Abstain from 'extra' inline fuel filters; I had nothing but trouble with them. If float valves and needles are somewhat sticky, a fuel additive like Redex may help.
Realise that todays gas is not the same as in the 70s.
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Tell us more, what air filters do you carry?
Have you checked the choke?
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http://forums.sohc4.net/index.php/topic,5410.0.html
I hope you find the answer here, don't give up !!!
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Thanks for the link Murdoc lots of good info on here, I'm running a standard air filter. I'll read through all this and try again. I'm not going to give up 🤣🤣
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Thanks for the link Murdoc lots of good info on here, I'm running a standard air filter. I'll read through all this and try again. I'm not going to give up 🤣🤣
Don't give up, the solution is out there and the results are so worthwhile
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At risk of covering checks already made, it doesn't appear to be an "adjustment" issue as there's no response to turning the air screws.
With size 40 idle jets this should be happening and clearly it's not. That's the issue, seems that idle jets or circuit is impaired (that's fairly normal if left for years etc) making any refinement redundant until you've got flow going.
In effect the elevated idle is having air that doesn't get paired with any fuel to give correct running when you shut the throttle. If it just gets enough revs going it will start to gradually draw through the main jet supply just sufficient to keep it running lean at those revs.
If the idle jet delivered fuel competently it wouldn't rise and also give you a response to air screw too.
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Thanks everyone for all your support and advice. I think there going to have to come off again and more time in the ultrasonic bath 🙄🙄 I think it's definitely down to blocked pilot ways... the saga continues 🤣🤣
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You may read this: http://forums.sohc4.net/index.php/topic,184299.msg2133957.html#msg2133957 Not saying it is the cause, but it wouldn't hurt checking the initial set up is still there. It's quickly done.