Cv carbs won't settle and losing patience!

[Andyb]

Hi guys,  after 2 1/2 years rebuilding my GS550, my carbs have decided to start mess me around.  I have cleaned and rebuilt them (ultra sonically). I have colour tuned them to get them as close as possible, but now for some reason the bike willl either not start without turning the  idle screw in more and full choke, or with full choke that revs to about 3k, settles to 2k, then will suddenly burst back up to 3-4k. 

I know most most will say it's air getting in somewhere, but how can they be perfectly fine one minute, then do the above,

Slowly losing patience,  any decent / recommended shops around Bodmin, Cornwall, UK who can sort this out and then MOT it?

Dont want to change to slide carbs, trying to keep as original as possible. 

Edited August 30, 2015 by Andyb

[vizman]

how are the rubber mounts from carb to head? if they have a hole/rip or are not fastened up air will get in.

[Andyb]

 

@vizman They look pretty good and pliable. a bit of paint missing from the front on a couple. I have replaced the 'O' rings and replaced the screws with Allen bolts.

I replaced all the clamps for new ones, because the old ones looked stretched and didn't feel as if they were tight enough.

 

 

[vizman]

I would double check your throttle twist grip and cable. make sure the throttle is allowing the carbs to fully shut, make sure the cable is in the correct hole on the tube and that the twist grip is moving freely...even check the cable is not being fouled or obstructed in ant way .....it might sound like 'sucking eggs' but it's worth double checking.

[chunky]

Give the intake rubbers a blast with wd40 when it's running, if the revs rise they are the problem

[Andyb]

@vizman. If learning to suck eggs again fixes I will gladly do so.  I will check the throttle and run tomorrow if I get to the garage.  

[Andyb]

@chunky. Will try tomorrow if I get the garage.  

 

[chunky]

If all that checks our, I would check the diaphragms for cracks or tiny splits

[jamie k]

Checks for older bikes in general Starting from the top basically 

1 tank - make sure no rust and if cap has a vent it is clear

2 pet cock -  make sure not dry/blocked or worn / especially vac operated ones

3 fuel lines - everyone ultrasonics carbs that have lay for two years etc but who really cleans fuel lines etc ?

4 valves - some motorcycle engines refuse to start if valves are incorrect / this isn't just a make of carbs etc it can come down to a specific bike , suppose it depends what else has worn wear etc 

5 compression - again best carbs in the world don't work right without something sucking through them

6 exhaust / air filter - some bikes do some crazy stuff if you think of playing with these and don't spend the money and time to get it all properly set up jetted etc 

7 ignition - just check its not firing with any cylinders down then they are kicking in etc changing the revs etc 

And last but not least is the carbs I don't know  the specific ones fitted to a gs 550 - here's a few checks that fit basically any old bike resto 

1 rubber o rings and gaskets - this seems to be cropping up more and more everywhere , maybe it's their age , maybe it's the new fuel who knows but it's probably best just to change them if you strip your carbs again 

2 float heights and rubber tips , make sure they are set correctly and the tips aren't dried hard or damaged with laying up for so long

3 diaphragms if lay or really old get pin holes , hairline cracks etc also make sure caps have secured it uniformly around the top in the likes of mikinis etc 

4 float needles can be bent/worn and some have multiple settings at the top for the cirlips etc make sure they are all correct and if shims/ washers are there they are fitted correctly Also make sure the seat it goes into doesn't hang it up or snag

5 Jets some have washers or are supposed to be so many turns out etc just double check all this 

6 butterfly's - different carbs different specs but Check them as well and adjust as necessary 

jsut a a few ideas fella hopefully it helps either way post back and let us know how you get on

my own gs 500 twin has similar problem ideals at 3k sometimes and 1k at others ( makes for interesting commuting , never really know what's coming if you ping the clutch to fast ) anyway it has slightly different sized diaphragms and a few o rings missing - hasn't broke down or let me down so leaving as is just now what does work though is when it's at 3 k I hold it at biting point and this changes the revs which in turn sorts the carbs to around the 1k idle more food for thought 

 the list above is just some ideas for you to think through also do regular plug chops during all works and once running correctly it may be best to change your oil if you suspect petrol could have gotten in through continued flooding etc 

 

 

 

[Andyb]

@jamie k.  Wow great list of things to check. The bike came with a 4-1 but I didn't know if it was a runner. I replaced all the jets / rubber seals etc but I didn't check if the previous owner had re-jetted for it. 

I do have a 4-2 I can put on and check if it runs any better. 

I'll go through the list and check what I can 

[Ben Slabby]

Daft question, have you fitted the rubber plugs in the jet tubes? You know the ones i mean? 

[Andyb]

@Ben Slabby. Yep, and even double checked. They were new so won't fall out. 

[Ben Slabby]

Fair enough, I had problems with them in mine iirc.   

[Andyb]

Thanks for the suggestion though. 

[jamie k]

No probs fella

[TommyTucker]

Jets, Jets and more Jets, I had the same problem with my GS400 it had been in a barn for 19 years and was still running the standard jet sizes, modern fuel is shit now, full of chemicals and ethinol not like it was when these bikes were new, just keep jetting up till it settles. My GS400 is now running on nearly double the main jet size and the pilot is now 45 (the std was 22.5) she runs as sweet as nut now

 

[Andyb]

@TommyTucker.  Thanks I'll check the jets and try upsizing them.   Think I may still have an issue with the coils, or at least one of them. I have tested the primary side and they are reading 3.2. - 3.5 ohms, however when I connect to the secondary I only get open circuit, must not be  doing it right though.

[Screwriverracing]

Does sound like it could be coils, can you borrow a set so you can eliminate them.

 Cheers SRR

[Ben Slabby]

Depends on the range of your multimeter. I've found the one I normally use won't read high enough to measure the secondary resistance. 

Ive had to use a Megger type insulation tester in the past.  

 

[Andyb]

Does sound like it could be coils, can you borrow a set so you can eliminate them.

 Cheers SRR

i have found how how to test the coils and I get the following readings:  Primary 3.5 ohms. 

Secondary 23.5k ohms, with caps on.  Just need to find the specs to see if this is correct. 

These readings are are very similar both sides.

Edited August 31, 2015 by Andyb

[teltwosheds]

You may as well check your advance retard springs while your at it 

[Andyb]

Right I have fully tested the coils, and the readings are within spec. However when I started the bike plug 1 / header 1 was cold until I moved the cable. Is it possible that the cable is physically breaking  down under load?

Think I'm just going to replace both coils with some Chinese copies, unless someone has a good working pair with leads going cheap!

 

[Screwriverracing]

Try replacing the leads and caps first, or at least swap them around and see if the problem moves.

cheers SRR

[Andyb]

They are new caps, but the leads are moulded into the coil itself. Wouldn't know how to change them out. 

[teltwosheds]

Where the leads go into the coils, cut top and bottom along the length with jnr hacksaw or dremell,  remove the strip of casing to get to the lead.remove lead  and replace with new piece. Smear araldite over the new piece to seal it into the casing and put the strip back in with araldite whack a bit of tape round it till it sets job done ;-)