32 replies to this topic

[Bjørge]

I serviced my MkI carbs the other day and everything seemed (almost) fine. A bit too much hesitation in the dreaded 2,300~3,000 rpms, maybe, but my optimism kinda helped me supress it.

Now, further riding told me something is wrong, it really didn't work at all except at idle and 3,200+ rpm.

 

I checked fuel level today, and it seems it's ~10mm too low !

 

Is something like this what you would expect with new float valves ? I will measure the new valves against the old ones, considering refitting the old ones (although I haven't kept track on which goes on which side).

 

Any ideas ? Is something wrong with the new valves ?

[Bjørge]

Investigating a bit, the biggest difference seems to be spring tension - the new one being maybe twice as stiff as the old one. This may play a role, or what ? I really don't know if fuel is cut at an instant when valve touches seat, or if it may requires a slight force... ?

 

I think I'll refit the old ones for now, needing the bike to run ~properly...

 

BTW, I used a Keyster kit for rebuild. 

 

Anyone succeeded swapping float valves for those ones ?

[richarda]

I think that it is highly likely that the 'plungers' on your new valves are marginally a different length compared with your original ones.  I have had this before on replacement valves (probably not Keyster though) on a number of different bikes and have needed to adjust the float heights to suit each time. Therefore I don't think this is an unusual situation so don't worry.  Trouble can arise though if the amount of bend needed of the tang is too great you seem to end up past an optimum setting and then struggle to get the settings right.  

[Bjørge]

Thanks, good to know !

 

I've put it all together with the old valves, and . . . . still the same problem. Think reason for wrong fuel level is somewhere else.

Having started thinking about fuel pump - a well known problem with a 23 years old MkI.

 

One fact supporting the theory that it's not about my carb rebuild, is that after the first 100kms after carb rebuild, everything seemed fine (almost, at least). Then, during my next ride a few days later - suddenly a totally unrideable bike < 3.000rpms. Thinking about disassembling the fuel pump, but need to read a bit more to find out whether this is a good idea.

[Bjørge]

Ahhh ! Carpe ! Found this:

 

http://www.carpe-tdm...&hl=+fuel +pump

 

This may be the problem.....runs good at first but as fuel tank empties fuel pressure is too low.....explaining why my test trip with a filled up tank went OK....

[Studley Ramrod]

Have you checked the lickle inline fuel filter Bjorge ? 

[fixitsan]

Mine didn't run wwell last week...found a tiny bit of debris in one of the pump valve seats. If the pump emptied it could not pull fuel because of the seal failure. I managed to get it to clear without having to remove the membranes, only the outer covers

[dapleb]

Its worth doing the arsey tate test bjorge. Still have mine in for the last few thousand miles. Or be really positamd git a new wan. New pumps are only 30 squid odd and 20 plus years out of a nice reliable mechanical wan aint bad goin!

[bfb1150gs]

this sounds like the problem i had with my mk1 after rebuilding the carbs it was a nightmare ...i rebuilt and cleaned the carbs about 10 times and it still wouldnt run properly   it would tickover no problem and would run perfect after 3000 rpm but below that it would backfire though the carbs and cut out and not start when hot  ...i replaced all the fuel lines and fuel pump but i didnt get anywhere until i found an old half full can of carb cleaner that had a large nozzle so it sealed the pilot jet casing so it could blast out the pilot jet ...i had to fit and inline filter to the fuel line as there was no tiny filter in the carb fuel inlet and had paint the inside of the petrol tank to stop the rust getting through and blocking it back up...since i done all that and set the fuel levels the proper way with a clear bit of tube and its been running sweet and hasn't missed a beat

Edited by bfb1150gs, 02 May 2016 - 04:51 pm.

[Bjørge]

Have you checked the lickle inline fuel filter Bjorge ? 

 

Did it when rebuilding carbs - worth another look for sure

 

Mine didn't run wwell last week...found a tiny bit of debris in one of the pump valve seats. If the pump emptied it could not pull fuel because of the seal failure. I managed to get it to clear without having to remove the membranes, only the outer covers

 

Opening the pump this evening.....excited....

 

Its worth doing the arsey tate test bjorge. Still have mine in for the last few thousand miles. Or be really positamd git a new wan. New pumps are only 30 squid odd and 20 plus years out of a nice reliable mechanical wan aint bad goin!

 

I've seen this mentioned a few times, but I'm not certain how to get acetate - overhead foils ? Do I have to find a hobby shop ? Would I be able to see whether diaphragm has had it ?

 

this sounds like the problem i had with my mk1 after rebuilding the carbs it was a nightmare ...i rebuilt and cleaned the carbs about 10 times and it still wouldnt run properly   it would tickover no problem and would run perfect after 3000 rpm but below that it would backfire though the carbs and cut out and not start when hot  ...i replaced all the fuel lines and fuel pump but i didnt get anywhere until i found an old half full can of carb cleaner that had a large nozzle so it sealed the pilot jet casing so it could blast out the pilot jet ...i had to fit and inline filter to the fuel line as there was no tiny filter in the carb fuel inlet and had paint the inside of the petrol tank to stop the rust getting through and blocking it back up...since i done all that and set the fuel levels the proper way with a clear bit of tube and its been running sweet and hasn't missed a beat

 

Just started to think about a missing o-ring when replacing pilot screws....it may be in there somewhere....

If so, I believe I have two problems; An o-ring and a weak fuel pump. Thanks for sharing this !

[Bjørge]

Sh&%¤!

I managed to crack the float bowl (the drain valve part, that is, not the outer casing) by overtightening the drain screw! Really, really stupid. But I don't think that was the original problem.....just lost a few mls of petrol on my way today. Think I just have to plug that drain tube for now and go on - put a new bowl on "needs"-list.

 

- I guess there is no easy fix ?  

 

Opened the fuel pump, and the acetate membrane looked really tired. The membrane almost touched the upper side of the vacuum cavity, the once flat acetate sheet had the shape of half a nespresso coffee pad   - probably explains why last 3 years of spring startup has been a nightmare - and why I have a problem with low fuel levels.

Dunno if I try and fix or install the leccy chinese pump instead. 

 

Now I have to fix my girlfriend's CB500 for thursday's ride, I fear....a bit humiliating as she picked up her brand new NC750 today and I will look like an elefant on a stepbike aside her....

[dapleb]

Seems yo have already answered the diagram question. It gets all flappy.

The mech pump is so reliable. 20 years of service is a good run.

The arsey tate can easily be 2ut to shape and holes punched as a trial. I have some spare sheets if ewe cannae find any. Would offer to make ya one (think I did churn some out at some point) but most of me stuff is packed up so might struggle to find template and punch.

[Hombre]

If you put enough fuel in so that the level in the tank is above the level in the floats then gravity does the pumping. As the level goes down and poor running cuts in then it is a pump problem.

[Bjørge]

If you put enough fuel in so that the level in the tank is above the level in the floats then gravity does the pumping. As the level goes down and poor running cuts in then it is a pump problem.

 

Yes, I believe I experienced this today - a longish downhill at modearte speed and it ran like nothing was wrong for a few hundred meters. Also, it runs quite happily from start after been left alone for a while. 

Seems yo have already answered the diagram question. It gets all flappy.

The mech pump is so reliable. 20 years of service is a good run.

The arsey tate can easily be 2ut to shape and holes punched as a trial. I have some spare sheets if ewe cannae find any. Would offer to make ya one (think I did churn some out at some point) but most of me stuff is packed up so might struggle to find template and punch.

 

You're right - prolly no reason to rely on cheap chinese pump when a simple acetate sheet can do the job 

Nice offer, by the way !

I'd might ask you for a sheet, but hope I can sort it out within a few days from a local dealer... Think I actually have a few old hole punches, but a bit uncertain about size range. 

[Naur]

Hi Bjørge!

During Easter holidays I did the carb rebuild with the Keyster kit. 
I did reposition the needle jet ring, (it had been tampered with previously) to the original position the fourth slot from the top (the flat end).
And I have had a huge improvement in performance and no problems at all.

 

I tell you this to support the hypothesis that your problems do not come from the carb rebuild, but that they merely coincide with it, which adds to the confusion. 

I hope you can figure it out, and get it running soon!
 

4. maj hilsen fra Peter i KBH (Danmark blev befriet den 4. maj )

Ohh, 
and I didn't check the fuel level.. I couldn't figure out how to. 
Maybe I've just been lucky, but no probs so far.

[Bjørge]

Hi Bjørge!

During Easter holidays I did the carb rebuild with the Keyster kit. 
I did reposition the needle jet ring, (it had been tampered with previously) to the original position the fourth slot from the top (the flat end).
And I have had a huge improvement in performance and no problems at all.

 

I tell you this to support the hypothesis that your problems do not come from the carb rebuild, but that they merely coincide with it, which adds to the confusion. 

I hope you can figure it out, and get it running soon!
 

4. maj hilsen fra Peter i KBH (Danmark blev befriet den 4. maj )

Ohh, 
and I didn't check the fuel level.. I couldn't figure out how to. 
Maybe I've just been lucky, but no probs so far.

 

Hi ! Happy 4th of May ! 

 

I have now attempted to fix the fuel pump, and it kinda seems like it's working. But it still doesn't run properly. Better, but not good. A bit different symptoms - now it runs good from start untill I back off the throttle at about 3,500 rpms in 5th. Then it starts studdering, and somehow keeps on doing so.

 

Again I thought everything was good, but no....

Will now check:

- Fuel filter

- Fuel pump "output" (do I recognise a good pump squirt when I see one ?)

- Fuel height (at least in the left carb with an intact bowl)

 

....start getting tired of this now......

[Studley Ramrod]

Sounds like you might have some crapola in your main jet/s.  Difficult to say exactly what the problem is after yer bike's been stood for quite a while.

[Bjørge]

Next round:

- Checked pump output; I haven't seen one before (vaacum operated one, that is), but it spew out some decilitre in ~10 seconds. I guess that's sufficient (although gravity deffo does part of the job).

 

- Plumbing on cracked float bowl seems to keep tight for now.

 

- Fuel filter at carb intake: OK.

 

- Came to think about what was said about the pilot screws earlier (also, Heynes warns about this but I read afterwards): Ensure that you get the little o-ring out. Well, I didn't, but thought little more about it. Right now I picked it out again, and what found I ? No o-ring ! That is, making a very specialized tool I got it out...along with an extra pair of brand old o-ring and washer ! Meaning I had spring-washer-oring-washer-oring installed.

 

My question: Does an extra o-ring and washer upset mixture to a degree that the bike could have an issue with running cleanly at a steady 60mph? It runs quite happily with a fair bit of throttle.

 

Yeah, I know I'm an idiot every now and then.   Quite often, actually; two days ago I dropped the tank on paving stones. Luckily it was already dented from an attempt to run-open the disk lock ten years ago 

Edited by Bjørge, 06 May 2016 - 05:21 pm.

[Studley Ramrod]

Only you can answer that one matey.

[Bjørge]

Only you can answer that one matey.

 

You mean I am the only one having run with an extra set of washer/o-ring on pilot screw ? Ok, kinda see what you mean.

 

I can report: It probably had a minor influence on idle/low throttle/low rpm running. Not much, though. Something for knowledge base ? 

 

I believe I've sorted out the fuel pump, though. It now runs ~90%, just a slight uneven running at low throttle 3,000-5,000rpms. Otherwise it's really good. I could live with it, actually, if fuel consumption is back to normal. Think I'll make an attempt on the fuel levels tomorrow. 

 

What a drag dealing with two problems simultaneously. Why did that fuel pump give me problems just as I overhauled carbs ?