Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: RGP750 on August 24, 2014, 06:01:03 PM
-
Great ride today around Kent and E Sussex, just short of 500 miles since re build.
With fuel above reserve every thing is great BUT, and i have spoken about this before,
as soon as the fuel reaches reserve she cuts out on the two cylinders fed by the front fuel pipe
from the tap.
the rear pipe feeds straight down and the front pipe does a right angle bend within the tap,
so least resistance /easiest flow is straight down so that's the way the fuel seems to want to go,i'm totally
puzzled by this and can see no answer except pass on £50 to DS for a new tap.
any ideas guys?
Rich
-
Do both pipes coming out of the tap vertical ? cheers Mick ;)
-
;) why you parking the bike up ???
its going to be a Indian summer you know
carry on regardless , I don't see that you have any real problem with your tap as far as I can tell I think all my CB750's do that and I got five. although some are better than others at least one of mine will run on two cylinders for ages until the reserve fuel fills up the float bowls and then they all chime in again , but like you say yes it could be the tap being blocked,I try not to leave the fuel get that low, as once you switch to reserve you are getting all the crap that's in the bottom of the tank come with it and that will only add to the problem ,I can do about 115 miles then its on to reserve, let us know if a new tap cures it, but I can live with it I need £50 more than Dave silvers ;)
BTW my fuel pipes run straight down and up under the t junctions I don't think they make any difference to fuel flow and I've only done this to make it easier to remove the fuel lines from the tank
-
Cheers Guys
I'm busy the next 3 weekends, and expect weather to be rotten by then,if it's not i'll get
in another ride.
Pipes are vertical, no blockages and if i put my finger over the rear pipe the fuel flows freely from the
front pipe. Even stopping to give the fuel time to fill up the bowls it dosen't happen, God knows why
i'm baffled by it.
Fill the tank all is ok.
:-[
-
Rich,
Presumably you've checked the #7 bit "Fuel Valve packing" is actually the right part / spec... ie is not homemade or off a similar, but different (eg single feed) petcock?
Have you had it all to bits in your hand & 'traced' the fuel run with the the cock in the 3 positions?
Cheers,
Simon
-
Simon
Yes to all ,probably 6 or 7 times.
With the tank in the bench ,turn tap on to reserve fuel flows freely from the
rear pipe ,the straight flow, nothing flows from the front, angled flow.
If i put my finger over the rear pipe fuel flows freely from the front pipe.
I think i may turn up a restrictor to resist the rear flow slightly.
to see if it balances out the flow.
I know i shouldn't have to but i don't like to be beaten by things like this.
Just thought ,i may contact the previous owner,he owned it from new.
interested to see if it has always done it.
Rich
-
Bizarre...
You mentioned use of a 'restrictor'.... am amazed the 'rear' pipe carb float valves don't restrict the flow enough to thence allow the forward pipe to feed.
Look forward to hearing the answer, when you fettle it.
Good luck.
(just though, mine might even do the same.... never ridden it down to reserve!)
-
Interesting problem, never ridden mine down to reserve.
Let us know how you get on.
Alan
-
Rich
Have I said this before to you cant remember?Save your £50 and set it up like a k6.Blank off the one that stops flowing and fit a £2.50 splitter.All depends on wether you want to keep it authentic
Cheers
Bitsa
-
Hi Bitsa
Yes you have, I just don't like being beaten.
I have emailed the previous owner to see it it has always done it.
Back in the 70s i used to run on reserve all the time,short of money so had to make it last.
Got caught out yesterday re filling up,they must run on reserve i'm sure otherwise what is the point.
Just a thought when was the 2 pipe tap superseded by the single pipe?? was it a problem!
Rich
-
Rich
Have to say I am with you on the not be beaten bit but I just cant remember my UK K2 having double tap.It must of had but do remember, like you not well off and switching to reserve loads of times I don't remember what your having happen.Do remember while racing with a Jag the open cable snapped shutting down dead nearly went over the bars.From new the shop I worked at sent all new bikes out on reserve with just enough fuel in to get you to the nearest garage.You know this will turn out to be a real silly thing.Keep plugging mate you have all winter to sort it.
Cheers
Bitsa
-
I haven't tried another tap ,anyone got one i could borrow? i'll happily pay your postage,
must be cheaper than buying a new one at the mo.
Just to try the pipe route i swapped them over on the tap last time i tried to sort it
and either pair connected to the front pipe were starved of fuel.
Rich
PS just had answer from previous owner never had a problem with reserve.
it must be something daft.
-
I've run both K2 and K6 reliably down through reserve and can't ever remember any supply problems. Also run them so low of fuel that it's emptied one side of the tank to the point where I've stood them on a curb and lent it over far enough to get what's left of the fuel over to the tap side of the tank and would then run up to about 10 miles further.
I'd go through all of the carb breathers to make sure that there is no restrictions to carb filling at all, you are right that it shouldn't do what you describe.
-
It has to be a tap problem ,tank on bench,attach a length of tube 6", to both pipes
on the tap,put tap on reserve,fuel will flow from rear pipe but not front,put finger
over rear pipe and fuel flows from the front pipe.
As Lobo says you would think when the two carbs being filled by the rear pipe are full the fuel would flow from the front
but this dosen't happen for some reason.
as for breather pipes the carbs were rebuilt this year and if i swap the fuel pipes over whichever pair is fed by the front pipe is starved. weird!!!
-
Rich,
A possible explanation...
The tank fuel enters a common chamber within the petcock, and from here exits straight out the 'aft' nipple, and via a couple of 90 degs bends the forward nipple. With the petcock ON all is well...
When set to RES, fuel only flows out the Aft nipple (easiest route), but not out the forward. However, if the Aft is restricted, it'll then flow normally out the Fwd nipple...
So... this implies (to me) that there is a restriction in the reserve tank feed, ie it is unable to supply enough fuel to BOTH forward & aft nipples simultaneously.
Ie... The reserve filter is likely clogged, or it's fuel gallery clogged prior to the common fuel bowl.
Perhaps a new filter, sonic clean or air line?
Simon
-
I would agree with Simon that the flow to the forward outlet must be in some way restricted (analogous to an electrical resistance) and the fuel then (only) takes the easiest route. The fact that full carbs then does not seem to promote the restricted flow is certainly odd and not easily explained. So we'll park that
Don't know about the 750 but the reserve mechanism on the 400 is very simple. You doubtless know all this I guess but anyway... the tap has two inlets - one for main tank and one for the reserve. The reserve level is achieved by the fact that the inlet pipe is about three inches long sticking upwards from where it mates with the tap. There is a gauze that envelopes this as the fuel filter. When the level drops below the pipe the reserve opening (being at the bottom) allows the remaining fuel out. The pressure will be that much lower as the level in the tank drops - may be it's not really at reserve - can you tell?
If it is then one assumes the reserve opening in the tap is partially blocked. I guess you have had the tap apart and cleaned it/checked the orifices.
-
...found this helpful pic. Hope it helps...am sticking to the above theory!
(ie the reserve feed (one of the red arrows) is partially clogged)
Simon
-
Hi Guys
Thanks for all of your input.
I'll take the ruddy thing off again.
I have blown out tap tubes with an air line and blown down the brass tubes with my mouth
resistance seems the same with both.
I could be a bit technical with this and try the flow by measuring the amount of fuel
flowing from the rear pipe over a measured time and then try the front pipe and see if they are the same.
maybe I feel a new tap coming on.
cheers Rich
-
Thanks Odd job I'll pm my address.
Be good to give that a try
-
Hi OddJob,
That pic shamelessly 'stolen' off the net.... I'm not near my bike (or inclined!) to dismantle it to this extent despite being very interested in the problem. (sorry Rich!)
If you're prepared to strip it all down again why not simply boil (the metal parts) for a few hours with a dishwasher tablet (whatever) - there's likely 40 years of varnish furring its arteries. Elsewhere, and recently, on this forum is a thread dedicated to such cleaning techniques.
Treat it to a new fuel strainer.... or better still leave this part out for the various bench tests.
Keep us informed...
-
Morning LoBo
Strainer is new, I'll try the boiling technique tonight.
The misses is away house sitting so shouldn't get grief ;D
Rich Looking at your post time have you got insomnia?
-
.... yea posting times... not 100% sad! just work abroad (Asia) & 7 hrs ahead of the UK... which often gives me plenty of time to alter my dafter posts before making a complete arse of myself...