Honda-SOHC
General => New Member Introductions => Topic started by: Harriec on March 28, 2024, 02:08:35 PM
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Hi Guys,
Just thought I would introduce myself. I bought a CB70 F2 supersport a few years ago...I have now joined the forum as its letting me down and I can't get my head around the carbs so I thought I would tap into the wealth of knowledge that will be on here! I am based in Sussex so if anyone knows someone that likes to tinker with F2 carbs this way then please raise your hand! I was loving the bike until the end of last summer when it started to run like a bag of nails...now I can't even get it started! Two trips to two different garages (and about £600 later) and its still not running! Anyway...rain has stopped play today as I work outside...so im off to tinker with it and probably get more frustrated! :-\
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Hello Harriec
Welcome here, you have certainly found the right place for a new owner. Don't panic over your problems, there is always a solution.
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Hello and welcome to the forum,
Were you actually running it when it started to play up? How did it manifest itself? Down on a couple of cylinders? Cut out completely? Or had you put it away and left it? As the others will no doubt say, if it had some sort of fault and then two different garages had a look, it might now have more than the original fault! Don't despair though, there are plenty on here to help. I'm not a 750 man but plenty are, so help will no doubt come. You say carbs but do you have good recent fresh fuel in now? Is there a spark? I think Ash may have posted a good flow chart for fault finding, rather than just checking random areas.
Many others will soon chime in with suggestions,
Ian
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Welcome to the forum.
As Ian has previoulsy mentioned what fuel were you using last summer prior to the trouble? These old bikes like only the very best in terms of fuel, i.e expensive V-Power or suchlike. If you used bog standard supermarket fuel and left it sitting for a while it will probably have gone off especially in the carb bowls making it very hard if not impossible to start the next time you want to use it.
Do you know anything about its previous history, hjad it been rebuilt at any point etc? If it had been sitting around for a while prior to you buying it the carbs could have become blocked in the pilot jet circuits. Before taking the carbs off though go through as much of the bike as you can such as checking the plugs actually all fire and are clean (and are any sooty indicating a rich mixture. Is the choke working correctly with the correct settings? Check the ignition timing as well and if still on points check the gaps and condensors. I suspect you may be taking the carbs off to give them a good overhaul and clean in an ultrasonic bath. It's all fun and you'll get to know the bike and they are easy to work on.
Good Luck :)
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Welcome to the sohc Forum - I know nothing about the 750 models.
My limited experience is on the 400 & 500 only - many others here know these later two models much better than I do.
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Wow...Thanks for the quick and welcoming replys guys...Very re assuring...I was beginning to feel alone on this one!
Anyway, I just spent a few hours in the garage and managed to get it running ok. I removed the tank and made up a temporary tank to feed fuel. it now runs. I took the carbs of and checked floats etc, they all seem good but the bolts to the bowls were a bit loose. Turns out the petcock is not working in the 'on' position but it does on the reserve..do you have any recommendations on where to get a new one?
Also it wont idle well at all. if I get it idling nicely, it will for a few mins and then the bike will change revs on its own, sometimes it goes up quite dramatically and sometimes it will bog down and die. having done some research I think it would be worth syncing them. Does anyone Sussex way have a kit to do so or is it worth buying a kit? These 4 carbs are a bit new to me...I take apart small engine carbs on garden tools but nothing like this. The bike is running rich too, im getting a bit of blue smoke when revving.
On a side note, the guy I bought it from said it had all just been re built...although he had no paperwork to back it up. The exterior of the bike is very clean and when I took the carbs off they seemed in good internal condition.
Thanks in advance!
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Sounds like you are not fazed by removing carbs, so that's a big plus! You will be used to blocked pilot circuits and the need to make sure all jets and passages are clear. Sudden changes in revs with no external input from you could be an air leak downstream of the carbs when things get warm. When it's running you could try spraying easy start near the intake rubbers and see if the revs rise. Don't like the sound of the blue smoke - are you sure it's blue and not black? Might need to do a compression test. Did it burn any oil when you were using it before these problems started? At least it's running again!
Ian
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Someone more talented than me will comment about balancing carbs but can't you sync them by eye first...so get them roughly right without special tools - and it ought to then run ok-ish?
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Turns out the petcock is not working in the 'on' position but it does on the reserve..do you have any recommendations on where to get a new one?
Hi Harriec
Here is a link to that item on the David Silver Spares site, a site you may find very useful in future, not that there are not always options. https://www.davidsilverspares.co.uk/CB750F2-(Single-overhead-cam)/part_335380/
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Thanks Athame57 new petcock now ordered!
I'm a bit worried about trying to sync them when I don't really know what I am doing..I am abit worried about messing with the carb settings and making it worse! However it may come to that. I would really like to enjoy the bike this summer. Its my only bike as we can't afford a modern too and I miss getting out over the south downs at the moment.
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The F2 has 2 BIG carb problems
1 they are pd carbs and a swine to clean
2 they have an accelerator pump which frequently fails with standing.
They do not like standing for 4 weeks or more without being run.
In Ash's dropbox should be the complete K7/F2 Honda manual, the carb bit pictures are not the best but better than other manuals, the pilot jets are pressed in but can be removed with care and do need to come out to clean and clean the passages.
DO NOT attempt to strip the carbs off the rack unless you are a watch/clockmaker as there are gazillions of tiny bits that have to go back exactly right , find somebody with an ultrasonic that will hold the complete bank.
Finally DO NOT buy aftermarket rebuild kits the quality of the brass is crap
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I actually just tried as Orcade-Ian suggested. Sprayed some easy start down stream of the carbs and it did raise the revs quite dramatically. So I'm going to take the carb off and check the rubbers etc. It could be sucking in air? Worth half an hour's work of checking!
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Welcome to the forum Harriec.
Nice bike the F2 and sounds at least like you're making a little progress.
Echoing not stripping the carbs initially, especially if the rubber intake are suspect and need to be competent to give sensible evaluation.
The rubbers to head are unique to this installation F2 / K7 as far as I'm aware, and maybe difficult in supply. If that's the case there maybe a temporary way around to get up and running if you need that.
Initially though, non leaking and fully warmed assessment should allow you to then plan which way you need to react.
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Hello again,
I have successfully used PU windscreen fitting adhesive on a supercharger inlet system which was leaking - as a temporary measure until the correct seals arrived. They had a habit of being sucked in slightly and caused 'limp home' problems. You need to be scrupulously clean before application to be sure of a good result but it certainly worked, especially in your case if they are made from unobtainium.
Ian
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Welcome to the forum.
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So I took the rubbers off and they were all ok (in fact they look brand new). I do think they may not have been seated properly. To be honest I think I've been dealing with more than one issue.
So with those things sorted...it now runs well. It starts in the button and chokes all good. It gets warm and I have it idling nicely at about 1500rpm but as soon as I throttle it gets stuck at around 3000rpm then if I leave it it will come back down and eventually cut out. I constantly have to choke or change the idle. I'm pretty sure it's not sticky carbs as I had them out and they looked good. And the cables are aok. I replaced and set them up myself. Any ideas what else it could be?
Sorry for all the questions...you've all been very helpful!
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Some considerations.
The throttle cables need to have a prescribed amount of slack, usually the "pull" one is adjusted only. A rudimentary test is to turn steering from lock to lock and blip throttle (while running) at each extreme .... any change at either end, then it needs more room to play with. I can get the spec from manual at twistgrip if you need it.
There's concise method to setup idle circuit jetting if they're been apart before, that's in manual, to get them on aim for mixture. This can get them "hanging " as described if not fully understood and implemented.
Choke should give up to about 2,750 rpm fully out, but obviously when fully home shouldn't be lifting the throttle slides at all. Also data in manual to set that correctly.
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Thanks for this. I will test the throttle tomorrow and start there. I'll also look into the idle circuit set up...
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As to choke the F2 is very "cold blooded" and takes quite a while before it will run off choke, again the pilot ways are notorious for clogging
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Welcome to the forum
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Thanks for this. I will test the throttle tomorrow and start there. I'll also look into the idle circuit set up...
They are similar to your garden machinery in some effect. The adjustment screw on these carb is engine side of throttle slide and controls fuel (unlike predecessor, with air adjustment) winding out/unscrewing should bring more fuel to low speed.
Changing each one in isolation should bring about obvious shift in running for that cylinder, no response suggests impairment as way of assessment.
The aim in Honda setup routine is to bring them to borderline lean point to give combustion reference, then lift the mixture a little toward richer ( they indicate 100 rpm drop from that richer state) to get them as good for further evaluation.
If left too lean on this circuit, that forces you to then run the slides lower than ideal in controlling the tickover aim point. That can give some of the inconsistencies you've currently got.