Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: adrian58 on May 05, 2013, 07:42:24 PM
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Nice day and took my K2 out for a 60 mile round trip, the first 'outing' since I bought her. All seemed pretty good other than it seems to be running too rich. Smoking a bit,(viewed by my mate behind me) white not blue. I got home switched off straight away and let it cool. I whipped out No1 and No4 plugs and sure enough they were a bit wet with petrol and electrodes sooty. I'm sure the chokes are ok and the air filter is new. I suppose the answer is to change all the jets just wondered if there was anything I could try first.(Not much change from £100 for the D/S overhaul kit). Was thinking about lowering the throttle needle a notch or would a hotter plug help ? Any thoughts ?
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"wet with petrol" sounds like more than a bit rich, worth checking needle position and float heights first to see what you have before commiting any cash.
Also see what number mains they have fitted as well.
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Was wondering about the float heights. I know if I leave the petrol tap on, one of the carbs will drip a bit from the overflow tube after a while. Still not dead sure how to set them. Would this cause a rich mixture ? Any tips would be welcome.
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I would address float height first and check your float needle valves as it could easily be the carbs flooding. Any good manual will show you setting up the float level ( a picture is worth a thousand words! ) If you leave the petrol on with bike on the sidestand you can get fuel drip when you straighten bike up but generally they shouldn't leak. Although my F1 can occasionally, could be when its hot etc.
As for jets, remember that the main jet doesn't really have any affect until 3/4 throttle! before that its mainly idle jet and slide cutaway up to about 1/4 throttle then the rest is needle jet, so think how you ride before shelling out on jets etc
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Is the throttle needle adjustable and if so what notch is the standard setting. I remember the previous owner told me he thought it was running a bit rich and that he had set the float heights with a vernier caliper. He did give the impression at least that he knew what he was doing. Took it out again today and it does seem to be a bit lumpy at about 1/4 throttle. Tickover fine and if you wind it up it's ok. Could be a plug a bit soot-ed up.Thinking of trying a slightly hotter plug any suggestions what number ?
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The plugs won't change the mixture even if you change the heat range.
Info for plugs if using NGK is usually 8 as standard....if you go for example to 9, then this switch is for the engine running hotter as the plug tranfers more heat away from the tip so that it won't start to overheat the electrode. This may be applicable if you were to use the bike for extended high speed work.
If you were to go for example to a 7, then this would be more applicable if you were running the bike in lower ambient temperatures so the electrode would stay hotter and give less risk of fouling ang giving poor ignition....it's unlikely for normal road use in this country to need anything other than 8 grade NGK.
I'd take a guess, as you say there is some leaking, that you first need to look at the fuel valves controlled by the floats.....this has a big influence over the observations you've made so it would be wise to verify that they are ok before going any further.
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+1 on what K2 said. If a carb is flooding it will affect all settings so you need to address that first.
Yes, the needle position is adjustable by setting which groove the holding clip is in, higher groove = weaker mixture and vice versa
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Just spent 2 hrs getting nowhere in the garage. Wanted to drop the throttle needle a notch but there was no way the two little screws inside the the throttle slide were going to budge to get to the needle. Any ideas how to shift them. There appears no signs of the carbs flooding which kind of steers me away from the float needles being the problem. I'm thinking that it may be a better option to buy the carb kit and fit it all rather than just the float needles and seats. Still will need to get at those throttle needles though.
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Hi Adrian,
When back in the old days I was a mechanic owners used to say can we watch you fix our bikes while they wait.As this was not allowed because when you see some gorilla banging the crap out of your desire it did not go down well.To get those 2 stupid little screws out first soak them with wd40 then protecting your slides from getting damaged hit the top of the screws with a blunt T bar if you like swiftly and they will come undone bit like a impact driver.I have done it all my life that way and have got out all the bast++d cross head screws I have encountered.Like I said protect your slide before you do this.I would really double check your float heights first and clear every orifice you can find.
Hope you have success.
Bitsa
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Thanks Bitsa, going to try that thing with the clear plastic tube on the drain plugs to see if I can see if the level is too high. I recon (if my research is right) level should be about 2mm to 4mm below float bowl joint. Don't mind spending the money on parts but being a mean bugger don't want to waste money either.
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Adrian.
I am all for new ways for dealing with various measurements but easy for me I have a Honda device but I do remember someone made a float height tool out of a floppydisc and if I recall I thought that was bloody good idea if you have some old discs.Like me I do not like spending money for no results so you have my vote.Needle heights in my opion do not change over night you get my drift?
Best of luck
Bitsa
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Annuver little tip with any stuck screws / bolts, use heat. I use a blowlamp, have seen people use welding torch but I don't recommend that as its too easy to damage the part(s) if you don't know what you are doing.
+1 with Bitsa on using a T- handle driver ( or similar )
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Float height tool
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Seamus that the one I remember seeing
Cheers
Bitsa
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BRILLIANT ! ! thanks :)
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If #1 & #2 are wetting the plugs, I would be inclined to look at the points gap & timing. It may be that the points have closed up & the plugs arn't producing a spark. As the carbs are more difficult to access, check the ignition first...with a timing light preferably.
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Hi
been reading this and have similar problem the plugs are like a black burnt biscuit wonder if anyone knows which way to turn the mixture screws to weaken off a touch ? it doesn't like running on choke at all so have to take it off as soon as it starts
cheers
Amos ;D
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If I remember right Amos its in to make it richer & out to make it weaker ;) cheers Mick.
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Ok checked float heights and if anything a bit low. Looked at all 4 plugs and Nos 1+2 showing sooty and a bit wet 2+3 pretty much ok. I'm now thinking have I got an intermittent/ weak spark as 1+2 share the same coil ? If I'm right, maybe that could also be the reason she's a bugger to start sometimes. It's on a Boyer so no points etc and all the plugs are new. Any thoughts help much appreciated . ps wouldn't start today then stripped 6 teeth of the starter reduction gear. Grrrrrrrrrrrrrrrrrrrrrrrrr >:(
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Almost certainly NOT ignition related, 1and 4 share the same coil, 2 and 3 share a coil, so if 1 and 2 are sooting up, it's fuel related :(
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Sorry meant to say 1and 4 share the same coil. :P
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Sorry screwed todays first post up. It's 1+4 plugs wet and sooty 2+3 ok and as they use the same coil thinking that could be cause. Brain fried after the started gear saga. :-*
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Okay, so it probably IS ignition related. I have no experience of boyer ignitions other than taking them off and binning them and refitting points plates, but you may want to have a look at your coils/leads before blaming the boyer.
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Wondering if maybe it's the plug caps. The No4 one has been changed at some time. Anyone else had probs with the coils or plug caps ? Am I right in guessing they are resistor caps ?
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If you suspect the ht caps, (unlikely 2 would fail together) try replacing them, but I'm more inclined to suspect the coil, or the wiring to it. Pop the tank off and have a good look at the coils themselves (looking for cracks or anything leaking from them) the security of the ht leads into the coils, and particularly the low tension wiring to the coil (corrosion in the bullet connectors can cause a lot of difficult to trace hassle, and check the connection is tight) If you don't see anything obvious, try swapping the low tension leads to the opposie coils (the blue wire to the yellow and vice versa, and use the ht leads that are currently on cyl 2+3 on 1+4 and vice versa. If the problem moves to cyls 2+3 it's got to be an ignition system fault,so put the wiring back to how you started, then you need to think about borrowing a coil to try, if the problem goes away, happy days, you need a replacement coil, if not, then start to suspect the boyer :( (I'm assuming you've already checked the wiring fron the points cover to the coils for continuity, particularly the join at the back of the motor, that joint is well known for being a pain in the arse!)
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Will the H/T leads be long enough to swop ? Didn't think about L/T leads and connectors. When you say the one at the back of the motor where is that ? Can get a new aftermarket coil for £20 but will look at all the things you said first. Just seems odd to me that the previous owner (who seemed to know his stuff) stripped and set the carbs with 2+3 being pretty much spot on and 1+4 way out. As for the plug caps, one has been changed at some time and thinking that 1+4 are more exposed to the elements. Can the resistors break down in them ? Thanks again for all your help. Adrian.
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I had Boyer-Bransden electronic ignition on my CB750F2 & the only time that I ever had problems with it was when I binned the bike on the right side & the Rickman engine bars bent & partially severed one of the two wires going from the points side up to the wiring loom by the removeable engine mount by the clutch cover. I never suspected that as a fault until one day it wouldn't run on all four at all. I buzzed through from the ignition back plate to the end of either the yellow or the blue wire...I forget which as it was back in the late '70s, & lo & behold I found the break. I bought a second hand shrouded pair from a breakers & that was it...sorted.
At one time I visited the small factory of Boyer-Bransden when they were situated near to my work at Elmers End. They said that in the vast majority of situations it was a connector problem with their system & not a fault of the black boxes themselves. In all of the years that I had it fitted to my 750 it was faultless.
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Found at least one very dodgy connection (fell apart when I took the tape off). About 4 others with crap connectors so going to renew the lot. Can't understand why previous owner spent £100 + on a Boyer then for the sake of about £2 fitted crap connectors. Pretty sure it's either a connector or the coil not the Boyer.