Carbs overflow

[DaveN]

Hi All,

I've only put 450 miles on the 400 since the rebuild but it is regularly overflowing the carbs and depositing lots of petrol on the ground if it's been standing for longer than a few days.

First time the overflow stopped after a long run and all was well.
Second time it did not and I had to take off the carbs and give the float valves and bowls a good clean out (some reddish deposits in all of them).
Not ridden it for 10 days and again, most of the petrol is ending up on the floor even after a 60 mile run. I've tried knocking the float bowls with a screwdriver handle but the overflow persists.

Looks like I have to have the carbs off again and was just wondering if this is a known issue with the 400/4 or have I a rogue set of carbs? They all benefitted from a DS carb repair kit during the rebuild. The bike runs well with no noticeable flat spots so I presume they are set up OK and the inside of the tank looks fairly clean.

Any suggestions or advice welcome.

[Johnwebley]

It sounds like light corrosion inside the tank coming off as it's being used.

Check the tap filter.

Maybe put some Redex carb/injection cleaner in and see how it goes

Sent from my SM-A750FN using Tapatalk

[Laverda Dave]

I feel your pain as a lot of 400/4 owners do!
I think it is widely accepted the brasswear of the rebuild kits is not as good as the originals. If you can clean the old brass in an ultrasonic cleaner and they are spotless try to re-use them with new gaskets.
A lot of the overflowing issues are caused by the fuel inlet needle not seating correctly (especially with the new ones).
One tip is to remove the needle valve itself and to use a cotton bud in a drill and polish the brass seat of the valve with some Solvol or similar on the cotton bud. Strip the cotton off the other end as you need to hold it in the drill. Once polished give the seat a really good clean to remove any polishing residue.
Also, make sure the float heights are 100% spot on, make up a gauge to set the height, using a ruler is very hit and miss especially for the middle two carbs if they are mounted on the rail.
You'll find being a 400/4 owner you will soon become an expert at removing the carbs!
Good luck 🤞.

[taysidedragon]

As above, plus always turn the fuel tap off when parked.
It's easy to forget if used to modern bikes with no tap.

[Nurse Julie]

As above but turn fuel off ¼ mile from home so the float bowls are almost empty when you get home which avoids the floats pushing on the float valve springs when the bike isn't in use.

[DaveN]

Thanks for the advice so far folks.

I have been using the fuel tap dodge when approaching home, but unfortunately, when it's open, it's pouring out at every traffic stop (traffic lights, junctions etc.) and I'm not at all convinced that it's not overflowing whilst driving too - my last full tank of petrol lasted approximately 75 miles before I had to go to reserve! I've also tried running the carbs dry during a run (petrol tap off until the engine cuts) in the hope it might dislodge any crud but no luck there.

[Nurse Julie]

Thanks for the advice so far folks.

I have been using the fuel tap dodge when approaching home, but unfortunately, when it's open, it's pouring out at every traffic stop (traffic lights, junctions etc.) and I'm not at all convinced that it's not overflowing whilst driving too - my last full tank of petrol lasted approximately 75 miles before I had to go to reserve! I've also tried running the carbs dry during a run (petrol tap off until the engine cuts) in the hope it might dislodge any crud but no luck there.

And you are 100% certain you have set the floats correctly at 21mm with the tang just touching the needle and not depressing it? . I'm sure you have but there really is only 2 possibilities, float valves not seating and floats set incorrectly. With only 75 miles to reserve, it does sound like it's pissing out as you are riding.

[K2-K6]

This recent thread may help out

http://www.sohc.co.uk/index.php/topic,22451.0.html

It seems fairly common with many of the kits to run into this problem currently,  should be ok after more intervention.

[DaveN]

No Julie, I'm not at all certain as I did not do the rebuild
I'll check all settings when I've got them off again. The new bits are in there, so I'd be surprised (nay, disappointed) if they were not set up properly, but stranger things have happened.
Looks like I'll be joining 'Carb 101' 

[TrickyMicky]

I know that I've mentioned this before, but, after a short non-riding period, (11 Years!!) I suffered on many occasions with incontinent carburettors. In my case it turned out to be the little pivot pins that the floats are fixed to. Modern fuels allied to non use resulted in them being very cruddy. They were given a very good polish with the finest grade wet and dry I could find, all replaced and rewarded with nice floppy floats, and from then on, yes, fuel tap off well before coming to end of the last ride of the day. Best of luck, you owe it to your knuckles if nothing else! Regards, Michael.

[AndyD]

Oddjob,
Interesting test - I've tried almost everything else with my carbs and they still leak like a sieve as soon as petrol is turned on.
As they're back on the bench I gave your idea a go and the pressure drops steadily with 'dry' float needles but as soon as I spray a little WD40 over them they seal and hold pressure.
This is with new DSS float valves fitted - tomorrow I'll put the old ones back in and see if they are any different.
What I can't understand is that the flow is so strong when the fuel is turned on that it seems hard to believe it's just leakage past the needle.
I did wonder if there was maybe something that held the floats down and I'll have a good look at this possibility as well.
Also when trawling t'interweb there are some comments on setting fuel height rather than float height and I wondered if this was an option on the 400. Certainly on the GS and GSX I've got that is the way they are set, using clear tubes from the drain plug. I did see some comments saying this could be done on the 400, set at around 3-4mm below the float bowl joint.
The saga continues
Cheers,
Andy

[AndyD]

Had another play with the carbs on the bench today and tried the original valve seats / needles but these let pressure drop significantly quicker than the Keyster ones despite repeated cleaning and polishing etc.
One thing I did notice was that without the bottom bowl, the floats could occasionally stick open when turned over and then drop down with any movement. Nothing seems to be rubbing or catching and I think the spring thingy in the bottom bowl would also limit how far the floats can drop when fitted.
I plugged in the float height gauges I made up for the GS1000 a while ago to see if this could shed any light on the problem and they certainly help see what's happening.
Sure enough as soon as fuel is turned on the carbs fill and then overflow at a quick rate.
Turned the fuel off, gave the whole assembly a good shake and when turned back on the fuel level stayed at the right level.
Left them for an hour or so and no leaks.
Drained them and then turned fuel on and once again the levels go too high and overflow. And again, turning off the fuel and shaking them stops the leaking.
So it looks like for some reason the floats on initial filling don't shut off the supply but once they are 'encouraged' with a shake they work fine.
Floats and pins etc. all look fine and move very easily so I can't see any reason for this behaviour so may play a bit more on the bench before wrestling with the airbox etc. to put on the bike again.
Cheers,
Andy

[Bryanj]

Slightest bid of crud or roughness on the pivot pin or iside the pivot tube can cause all sorts of troubles

[AndyD]

Thanks Bryan,
I'll pop the bowls off one last time and give the pivots a final check / clean then I think I'll try them on the bike and keep fingers crossed.

Cheers,
Andy

[DaveN]

Does anyone know if you can buy replacement pivot pins?

One of mine has been mangled and is shorter than the others.