SOHC.co.uk Forums > CB650
Carb issues
M0WBY:
Hi
They are between 1 and 2 carbs and 3 and 4 carbs. They seem to be just vents...
I have had the carbs off and checked every thing even again. Replaced all the o rings again. There does not seem to be any air leaks.
I took the air screws out and put a tube over their hole and blew out the slow running jets but they all seemed to be clear.
It has intermittent running on cyl 3 and will not run at all on cyl 4. It only runs on 3 when on full choke. It will now tick over, sort of, off choke but as soon as you try and rev it up it dies as it is only running on two cylinders. It is sparking I checked that and the timing. I checked the rocker gaps.
I think I am going to have to pull the top end off the engine and see what is going on... :-(
Booooo!
Bitsa (Ralph Wright - RIP):
Hi Mowby,
Not that I have done much work on the CB650 but they can not be much different to all the other fours.The things I would do is first set everything to the book air screws etc.Before trying to start the engine check all the plugs are sparking as you crank her over by the crankshaft.Point gaps are correct open carb bowel screws to make sure fuel flows out freely from all four.The compression to me sounds OK and if she only runs on choke definitely sounds to me fuel starvation.Open each bowel screw turn the fuel on and if no steady flow open the tank filler cap and if flow increases you now the filler cap vent is blocked.All the time you are messing with fuel lots of rag and no smoking please.Once you determine all four spark and all plugs are getting wet she should start OK.Once all four headers warm up then you can fine tune with balance and air screws adjust.After all this and no go I would then start to think about naff valves etc.I like the sound of you and only wished I lived near to you as I would have liked to help you out I could do with the company.Do not be to hasty to strip her down yet.With these old classics you must consider Rome was not built in a day some guys I have met have taken at least two years to get perfect.Keep your chin up you will get there.The carbs I sent you did you strip them,use carb cleaner everywhere blow compressed air through every orifice and reassemble to the best of your knowledge.I do not mean to underestimate your cababilities but I had to do mine several times as I took for granted that a shop that ultra sound cleaned them was it.No it was not.I wish you luck and keep going as I said you will get there and when you do you will think I fracking sorted that.
All the best
Bitsa
K2-K6:
As Bitsa says, be careful of the fire risk and if you are getting petrol all over the place then do it outside as you only have to save the bike and not your garage as well.
Just some basics to add to the others: switch the plugs about to make sure that the problem isn't one or two of them.
Compressions sound as if they would be ok to run without any particular problems.
If you can get it running at a fast idle then blow some petrol down the #4 carb (you can just use a long thin tube in you mouth to suck a small amount up and blow it into the carb mouth) to see if it will actually run on that cylinder, this would verify the carb if it did.
Check to see if the valves are opening on number 4 as well.
As Bitsa says I wouldn't pull it apart just yet as it may well be outside the engine and you'll do the other work for nothing.
Try running it in the dark to see if you have any sprks coming out the HT leads and not reaching the plug when in normal running configuration, specifically where the plug leads go past the head castings.
Try switching the 1 and 4 HT leads over (they are both fired at the same time) to see if #1 stops and #4 starts.
Let us know how you get on.
M0WBY:
Hi
The float heights on the CV (VB44 and 54) carbs are not adjustable oddjob.
I have fuel in all the carbs so I don't think the petrol tap could be a the issue but I am not ruling it out. I will do the test Bitsa suggested and/or remove and clean it out anyway just to take it out of the equation.
I suspect fuel starvation is part of my problem but I think there is another issue as well.
I have no garage so I have to do all my spannering outside but I am looking at renting a garage from a friend just as soon as he gets a roof on it!
I am going to investigate the sparking side of things again. On the 650 it has an electronic ignition as per http://www.jasonkent.ca/manuals/79%20CB650%20Service%20017.pdf (which is 79 manual - but the manual is the same for mine for this section). I will perform all the tests starting at the top and working my way down to see if I have missed something. It has new plugs so they should be OK but I can swap them over it is no big issue. I may replace the HT leads whilst I am at it.
I am thinking weak spark. It sparks at the plug on cyl 4 when I took it out and tested it but what it is doing under compression is another matter! I am thinking perhaps a pulsar or CDI unt is going bad or even a coil? But it is sparking fine on the other cyl, and it uses the same system for cyl 1 and 4, so the only thing it should be if it is a weak spark is the HT lead surely? Is there any reason why I can't just chop up and use a 7mm silicone car HT lead?
I just wish I had someone local who could look over it and give an alternative perspective on things.
Cheers
John
M0WBY:
Hi
Had a good session on the bike today checked the ignition system working from the spark plugs back. I checked the timing and it is spot on. Replaced the HT leads anyway. I can defo say it is not for lack of spark that it is not running on cyl 4. Also cyl 3 is still only running intermittently and does not really kick in unless it is either on full choke or when warmed up only really kicks in past 5K rpm. So I think I can rule out the ignition system! Straws have all been clutched at and been found wanting! It is not the ignition that is to blame!
I double treble checked the tappets again. I can confirm they are all correct. I checked that they were turning over with the covers off and can say that this side of the engine seems to be pukka!
So I have two options. Either it is a sticky valve and/or piston rings on cyl 4 and it also has a bit of fuel starvation.
So I checked the fuel side of things. I noticed that the fuel pipe was a bit too long and went upwards after it left the tap and the fuel delivery is not strong enough as it has only gravity allowing the petrol to flow. The slightest uphill bit in the pipe effectively slows the flow down to a dribble! So I shortened the pipe as much as I could. On my American VB44 carbs they have this part that runs off the vacuum in cyl 2 by an attachment screwed into the balancing screw hole. This has a pipe to something that is attached to the carb rail. This has one tube to the petcock and another that attached to the petrol intake on the carb and another that vents to fresh air. There now follows some more straw clutching... :o
I am wondering if the american petcock is the wrong one for my carb system. The CB650 night hawk has a vacuum operated petcock http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=221129886560&fromMakeTrack=true&ssPageName=VIP:watchlink:top:en. I am thinking I may need a vacuum operated petcock on mine. That one of the pipes could possibly be for a vacuum system all thought neither seems to suck or blow as far as I can tell... Bloody things doing my swede... thoughts please gents! I am thinking I may have to try reinstating the american system and block off the two vent pipes just to see if that is the issue causing my fuel starvation issues!
Anyway whilst it still is not running right it is better than it was. It is not back firing so much or coughing as much as it did! When fully warmed up it will run at about 6-7K rpm without the choke but it needs the choke 1/4 out to tick over and run below 5K rpm. No matter what I do it won't run at all on cyl 4 despite having a good spark. The plug comes out wet suggesting that fuel is getting through. I think I will have to rip this engine apart! That is the way I am leaning towards.
Ho hum...
Cheers lads for reading my moaning!
John
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version