Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: billy192 on June 22, 2014, 05:40:42 PM
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well i have done 600+ miles on mostly A roads. today I went on the Mway only to find at 3.8K i get a miss it keeps missing all the way up to 5K, not taken it passed 5K as it will not rev passed this under load.
when you slow done to 45 ish its fine.
I thought it was fuel starvation but fuel is flowing OK. checked the points, plug gaps all spot on.
a great pity as everything is great but for this miss.
Billy
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Hi Billy, I had very similar symptoms on a ZXR400, it turned out to be the float heights, i.e when the engine demanded more fuel at higher revs the level in the bowls was too low because the floats were cutting off the fuel supply before they should, trial and error re-setting the float heights fixed it, I couldn't use the factory settings because the engine/intake and exhaust were non standard.
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Billy
Pull yer plugs and see what the colour is like white weak black rich then you will know if its carb problem
Cheers
Bitsa
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I pulled the plugs (burning hands) and they were white ish, i did try opening the choke but it did not make a difference. I am going to check the float level AGAIN. should i not lift the needle in each carb to rich top end?
Billy
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Are you certain that the ignition advance is working correctly, if no advance then it would struggle to get much above 5000 at all.
Lifting the needles will richen mid range (throttle not fully open), top end is main jet size but I don't think you'd be finding that out if you are not getting the slides fully open anyway....I'm making the assumption that you have not got the throttles fully open as you've not exceeded 5thou, though if you are saying that it won't go higher AND the throttles are fully open then that would suggest something else assuming the carbs are near enough right.
White plugs do suggest lean running......I'd make sure of the ignition advance and timing first as it's easy to do, before taking the carbs down again.
Also are you sure of your method for setting float hieghts?
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Yes I did them one , number 3 empties a bit on the floor from time to time so that needs adjusting.
The advance sounds as though it needs looking at
Will try Friday when I get back home
Thanks
Billy
Sent from my iPhone using Tapatalk
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Just to thought to add to my last post, it won't hurt to raise the needles if it's running a bit lean but one notch is considered a relatively small adjustment to trim an engine in to optimum so you shouldn't get a huge effect. As you probably know, the centre notch position is the starting point with room to trim it up or down depending on running conditions. if you get to either end notch and not get what you want, then it would indicate that the main jet was the wrong size.
Let us know how you get on with your checks on friday.
Nigel.
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another question.
does it make a difference which way the petrol feed pipes route?
mine go under the float bowles (under the carbs) and up to the cross over feed pipes.
should they go above the carbs? like feed from the top?
Billy
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Billy
Mine are routed like yours.From the tap straight down in front of carbs up under the bowls through a filter then to the splitter.
Fuel flow is no problem.
Cheers
Bitsa
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Bitsa,
I had an inline filter, but,,, the new tap and gauze in the tank look as if they would trap any unwanted debris.
is the tank strainer OEM part not up to it?
Niel
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Billy
All depends on how much rust and crap is floating around, my tank is rust free and yes the tap filter is also new but being over cautious wont hurt.I even have little filters on the float valves,
Cheers
Bitsa
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went out today between showers and crap still missing. so I thought at 60-65 which is about 3.5 k it starts hesitating so i pulled up the choke halfway and boom no missing it flew along at 70-80 no problem.
so off with the carbs again an check the main jet and the needle clip position. clip is in the middle one (correct for std) the main is 105 and according to the book it should be 120. would this make it miss? i assume by pulling the choke its running lean.
the bike before i stripped had a four into one would they have had to re jet lean for that?
so next step 4 new main jets 120.
Billy
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Looks like you are getting there billy, as when you choke it you obviously richen it at all revs and the fact if took off ok then shows it needs more fuel. I'd go with the spec and return the mains to std.
I was slightly confused as with your second post you said the choke made no difference but it's always worth a play with at different revs to see what affect you get and can give a cheap and quick diagosis.
Just to cover a bit more of what is happening here, an engine will run at a steady state (continous revs and load conditions) with quite a lean mixture but it then needs quite a bit of a kick with extra fuel to get a nice clean acceleration (they do this on later F2 engines with an additional pump as you open the throttles) but older carbs like the K6 will be set as a compromise of steady running AND enough fuel to accelerate without going too lean and causing a missfire, yours appears to be jetted right at the bottom or even less than required to achieve that balance. If you jet as per spec then any small adjust should be possible with the needle clips if it needs any trimming at all, but it should be much better if you change the mains.
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Silly question Billy, but are they the correct carbs for your bike? maybe some previous owner swapped them over, as 105 is the main jet size for a 750F1 and possibly they have been replaced at some time with F1 carbs (probably thought F1 has 4 into 1 as standard, easier than changing jets, and cheaper which it was years ago!)
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original carbs
billy
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Billy
My k6 has 105 main jets and needle postion is fourth one down.According to a carb list off the sister site your carbs should be 657a,657b or 086a with these settings and air screw 1 turn out give or take a bit.The list I have is for the states, altitudes etc.
Cheers
Bitsa
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Bitsa
i have ordered a set of115 and 120. i am going to try the 115 first and chop at 60-70 ish to see plugs. i have done everything else it has to be worth a try.
Billy
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had the carbs off again and the clip was not in the centre as i first thought (reading glasses on) it was on the forth one down.
my plan, clip in the middle (done, what a pain) and fit the new 115 main. go for a run Friday next. fingers crossed.
I will take a plug spanner with me and chop at 4K and 5K to check
Billy
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;D ;D ;D ;D ;D great news, fitted the 120 main went out for a test ride runs great. i will still do a plug test at 60-70 to make sure but first impressions are its good.
thanks for all the advise
Billy
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Billy
Good for you forgot to ask standard air filter etc?
Looking forward to your plug colour with interest
Cheers
Bitsa
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:'( :'( went on a longer run and it seems when it get hot and you roll the throttle in top then open barrr barr miss miss then of it goes. not sure now what to do
Billy
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Billy
What are your points like are they pattern or genuine.
Talking to my dad his old Ascona developed a miss when hot a bit like what your describing and after changing the points it still ran the same it turned out to be faulty condensers.It took a while to discover because faulty condensers normally take out the battery as well.
Mick
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both points and condensers are genuine Honda
its silly, up to the first 3-6 miles its great then as soon as you roll the throttle closed fast then try to pull away miss miss
i will get to the bottom of it
Billy
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Billy
Sounds rich to me as it is happy until running temperature.Its a bit like having the choke half on great when cold but crap when hot
Post a pic of your plugs mate.On the parts fiche the biggest main jet used was 110 and changed to 105 after a certain serial number.This is for the k4 model.So as regulations changed I am pretty sure the k6 was the same.
Cheers
Bitsa
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just finished putting back to STD
clip in the 2nd from bottom
105 main
points checked
new clips on rubber boots
new petrol pipe
test drive tomorrow with plug chop
Billy
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I'm following this thread to Billy as my bike is doing a similar thing, runs great for first few miles then starts acting up ?,
mine started as a flat spot as you rolled the throttle back on and stuttering at cruising speed on only a bit of throttle,
but ok when you wound it on, I've adjusted the float heights which made a difference and jetted up to 115's which made it a lot better,
but still starts playing up after a few miles, I'm happy with float heights and fuel flow, mine starts easily without choke (Rich ?),
and I also noticed no4 plug was sooty so I have swapped all the carb internals off no4 in to no1 to see if that plug goes sooty as it was ok ish before,
my next plan of attack is give it a longer run see how it performs and maybe try swapping condensers just to see if there's any difference ?, cheers Mick ;)
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These 750 4's are very temperamental, and hard to diagnose problems
pete
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Mick
Does sound a bit rich.I start mine on full choke then within 10 seconds down to half then right off.It took me ages to get her to run properly mainly adjusting air screws a bit here a bit there.
If she pops out the back I found it to be those screws dont do it now
Hope this helps
Cheers
Bitsa
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I'll keep chipping away like Billy get there in the end I hope ::) cheers Mick.
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Strange but good day to day. went to do my test run and went about 12 miles to tescos with her in doors on the back and sur enough it starts spluttering at 4K. i was miffed to say the least.
when pulling away from lights i took it to 7K in each gear and boom no miss, the min i roll the throttle back comes the miss.
when she was getting a few things i took off the overflow pipes to each float bowl, not sure why but I am at the point of tiring anything.
on the way home a went on a little de tour. i went through maisemore to the M50 and down to a place called Newent which was 15 miles on the M50 and drove at a steady 60 -85 all the way down the M50.
brill. great runs sweet.
now i am going to go out with the tubes back on to see if it comes back.
Billy
not sure what difference these over flow pipes off make but hey its running goooooood
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Sounds a bit strange was one pipe pinched?
Mick
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I was wondering before you posted if you'd checked those overflow pipes, and as mick says they could be pinched at the bottom end of them.
As a point of interest they are named "overflow" pipes and we all now that with a sticking float/valve then the fuel of course comes out of them and helpfully drips onto the floor without covering the bike itself with petrol.
What they are not described as is a more critical function, they leave the float bowl at atmosheric pressure if they are clear. The difference between the vacuum in the carb throat and the status in the float bowl causes the fuel to be pulled into the engine at the rate dictated by the jet sizes, but if you artificially alter the the float bowl pressure then the change in difference between the two areas causes an unwanted change to the jetting calibration so the engine no longer runs as intended......happens if you change altitude as well, which is why carbs struggle to adapt.
Hopefully billy, you'll have got to the bottom of it.....tenacity rewarded i think, well done if it's definitely ok.
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i can blow through them but not freely as there are several zip ties. fingers crossed
Billy
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Strange but good day to day. went to do my test run and went about 12 miles to tescos with her in doors on the back and sur enough it starts spluttering at 4K. i was miffed to say the least.
when pulling away from lights i took it to 7K in each gear and boom no miss, the min i roll the throttle back comes the miss.
when she was getting a few things i took off the overflow pipes to each float bowl, not sure why but I am at the point of tiring anything.
on the way home a went on a little de tour. i went through maisemore to the M50 and down to a place called Newent which was 15 miles on the M50 and drove at a steady 60 -85 all the way down the M50.
brill. great runs sweet.
now i am going to go out with the tubes back on to see if it comes back.
Billy
not sure what difference these over flow pipes off make but hey its running goooooood
hope you have sorted it,I live close,and use those roads between Hereford,Newent,Glos,and Ledbury,
these are perfect for sohc s
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They are until I come round the corner with an artic full of 96,000 glass bottles going to UBL at ledbury!!!
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Have my doubts on that but I do hope its cured Billy's bike.I hope Mick gives it a go you never know.I still think its down to modern shit fuels bring back good old leaded.If its not raining tommorrow I will take mine off and see if there is any difference.
Cheers
Bitsa
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If yours are clear Bitsa, it should make no difference to the running I guess. Billy's seemed to have the problem and then with just the tubes removed but exactly the same fuel supply it appears to be ok, guess we'll have to wait for his final verdict on that to see what he gets.
I know what you mean about the fuel though Bitsa, I've got quite a few small motors and the fuel left in the carbs deteriorates quite quikly and seems to go "crusty" in small jets if left. Doesn't seem to happen on motors using pre-mixed two stroke fuel though, somehow it seems more stable with oil in it.
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Well did a bit of trouble shooting. connected up #3 and #4 overflows and after about 6 miles started to miss. put my hand down an disconnected #4 (only one i could reach) went smooth all through 4-5-6 K connected it back in a lay buy miss at around 4K. so now i have all but #4 overflows connected with no missing. i did a plug chop at just over 4K and this was the result.
(http://i1209.photobucket.com/albums/cc393/njwill1/CB500T/IMG_8259_zpsd83ed512.jpg) (http://s1209.photobucket.com/user/njwill1/media/CB500T/IMG_8259_zpsd83ed512.jpg.html)
number 1 looking in my humble opinion as OK
(http://i1209.photobucket.com/albums/cc393/njwill1/CB500T/IMG_8252_zpsee16ac96.jpg) (http://s1209.photobucket.com/user/njwill1/media/CB500T/IMG_8252_zpsee16ac96.jpg.html)
(http://i1209.photobucket.com/albums/cc393/njwill1/CB500T/IMG_8245_zpsb3cff31d.jpg) (http://s1209.photobucket.com/user/njwill1/media/CB500T/IMG_8245_zpsb3cff31d.jpg.html)
number 4 looking rich again in my opinion a bit rich but not much.
this coupled with the fact that i am unable to connect the overflow on #4 suggests something is not right with this carb.
Billy
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Billy there is a vent to atmosphere at the top have you checked this/these?
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Bryanj do you know where the vent is?
Billy
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Billy
I think Bryan means the vents between carbs 2 and 3 they should have short pipes attached which hang over the airbox.Have you checked the overflow tube is not blocked on the carb you think has problems?If the carb is over filling and this tube is blocked the extra fuel has got to go somewhere.It can flood into the airbox or get sucked in by the engine and probably cause miss fire and rich.Maybe worth a look.
Cheers
Bitsa
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when i had-my carbs back from the guy that cleaned and rebuilt them there was a tube joining #3 and #2 sealing them should these vet out to the air box?
Billy
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http://www.cmsnl.com/honda-cb750k6-four-general-export-dkdm_model14382/partslist/E19.html#results
Hi Billy the two pipes, see link numbered 31 should be over the top of the air box ,not joined together.
Rich
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fantastic news, well to me it is. fond the vents and as i described there was a clear pipe joining them (making it air tight)
removed the pipe and connected up the 4 over flows and bingo great acceleration all the way to 90ish
just went to Swindon and back its like a different bike it idles smoothly also.
billy, happy chappy
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Nice one Billy now enjoy her.Might have a word with your carb builder though,
Cheers
Bitsa
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It's always really nice when you get something like this sorted out and they are really good to ride when running properly, hopefully you'll have some more of these summer days to enjoy it.
I think yours and green1 (mick's) dark colours looked really good in the recent pictures as these dark bikes come alive in bright sun, it's good to see some more of this era back on the road.
Nigel.
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Great news :)
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Glad you got it sorted mate, it took a while to remember about the vents as they are so big they never (dubious word) get blocked
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thanks Guys, you all have helped make me smile. 8) ;D
Billy