Help...anyone?! Carb identification & general chaos...

Hi guys,
I brought an early x400 last year but never really got it running right and i'm kind of losing the will to live!

Pic here for attention...



The bike has never been able to start without a healthy squirt of easy start. So I know theres something wrong there. These are the carbs that its running - which to my research I dont think are factor fitted? Anyone know what they might be?



Iv also noticed that (I assume) is the fill for prime is just wide open and will be sucking in air. Indicated here as ref 1 - am I correct? The petcock fitted on the bike doesnt have a prime.





I would be so grateful is some XS Wizard out there could confirm my suspicions and let me know if you think its worth trying to get a petcock that will accommodate the prime on the carbs or if I should just block that up to stop the air messing with the mixture.

A million thanks!

Definitely not original carbs. #1 is probably a vent/overflow, but without knowing the exact model, that is a guess on my part. It also could be the suction for a crankcase vent.

What sort of intake do you have (I'm guessing that the original system is long gone)?

For what it is worth, these look like Suzuki GS500 carbs (Mikuni BSR-34). If this is so, then port #1 is a vent line to atmosphere, port #2 is the fuel inlet and there might also be a vacuum port on these carbs that will need to be capped off or it will cause a lot of issues.

Oh yeah, never mind about the intake system, I can now see that it has pod filters.

Thank you so much for the reply. Just looking at the top of the carbs and it looks like you’re dead right. Missing a cap? Looks like one is capped and the other isn’t.

Bike seems to run lean which I thought was a result of the open pipes but assume that vacuum would cause that? Sucking in too much air?

I don't know if that is the vacuum line or not, but an open vacuum will definitely cause a lean condition. Make sure you look over the carb bodies carefully and cap anything that you are not using. Go over the intake boots, vacuum hoses to the petcock, petcock, etc and address/replace anything that looks like it might leak. Once the bike starts, I would even put my finger over the vent, just to make sure that It is not doing something unexpected to the mixture.

Also, download a copy of the GS500 manual and it should tell you how to check/set the fuel level in the float bowl for these carbs. That will eliminate one more variable.

One question if you know - do these fit up directly to XS400 carb holders, or is something else being used to mate these carbs to the engine? Curious minds want to know because if stock, ganged up GS500 carbs fit right up, that opens a whole new world of cheap replacement carbs for the XS400 (and the other way around also.)

Massive thanks for the help. Ive done some digging and yes you're right they seem to be GS500 carbs.

I've just stripped them down and cleaned them up while they were off the bike. Following some other GS guides I can see that the vent indicated in my original post is as you say: just an breather so nothing to worry about there. I also found a few images and it appears the below IS for a vacuum line and both should be blocked up when not in use.


Before I put them on I will inspect all the rubbers, hoses and other bits to make sure its not sucking further air in there.

In regards to if these are a simple swap for factor fitted XS carbs, I'll take some photos of the carb holders and check diameter. Other than that unfortunately I wouldn't know if it was a straight swap. From memory they seem to modified in any way.

The vent and fuel rail fittings between the carbs look original as do the mounting rails. I suppose the carbs could have the bodies milled so they fit closer together while retaining the original fittings, along with new mounting rails. I'd bet on them being in original condition- a strong family resemblance to the carbs on my old Bandit. If you find the black fitting rotten at all its easily replaced. It should have a hose run up under the seat perhaps, to get it out of the ambient airstream; protect it from rain & debris etc.

You could put a lawnmower type fuel filter on that vent hose to keep out debris too.

UPDATE!!

Well, I cleaned out the carbs throughly, flushed the tank and then did the electrolysis technique to make sure it was all clean. Then I blocked up that top that vacuum line, replaced the fuel filter with a bigger one, put it all back together… first press and it burst into life! It’s never started that easy since Iv owned it. Wahoo!!

I think I’ve done too many turns on the idle screw because it’s revving like crazy but I’ll get a stubby screwdriver and adjust them on the bike.

Im chalking that down as a success - thanks alot guys!

Hi Guys, saga continues. Bike seems to be starting ok but running super lean once its going. I pulled the intake rubbers off to inspect and found them hooked up like this:


Now I understand these need to be capped off but is it common to see them with a bit of rubber between them?!

Theres no cracks in the rubbers but did find they are missing any kind of gasket..

Really confused!

That's interesting. Usually you mount those so each vac nipple points straight up and you cap them, or hook one to a vacuum petcock. Hooking them together could cause some strange cross signaling in the carbs, or could do nothing. Not really sure to be honest. I'd install them back how Yamaha designed it though.

Mjerrold said:

Theres no cracks in the rubbers but did find they are missing any kind of gasket..

Click to expand...

Definitely put a gasket between the carb holders and the engine, even if you have to make one yourself.

tstidham said:

I'd install them back how Yamaha designed it though.

Click to expand...

Yup, I agree. Mount them with the vacuum nipples pointing straight up and cap them individually.

BTW, those look like stock carb holders, so the GS carbs look like they fit up without modification to the Suzuki rails! That is some info worth putting away for a later date.

concur with above- if nothing else you can't balance the carbs with them hooked up like that. I did spend the time to build a multiport manometer to balance my old Bandit;




Later modified to add valves between the 4 tubes.

I've used it successfully with my Bandit a cb750 and the XS400. With it all hooked up, all valves opened and carbs balanced, there is 0 net flow of the water (I tried oil early on- bad idea). I found the same when balancing the XS400- so bridging the two boots is not a big problem IF the carbs are well balanced. If they aren't it will amount to a substantial vacuum leak on one carb or the other.

In a out of balance scenario, with all valves open on my manometer then generally one or sometimes several carbs (on a 4 cyl bike) will start sucking air thru the manometer from the other(s). If its way off enough water will aspirate thru to stop combustion- oil was a bad idea because the same thing would happen and the oil would foul the cylinder for a while.. quite annoying. So until balance is close, valves associated with adjacent pairs of carbs are only cracked open to avoid the air sucking thru too quickly. Once each adjacent pair of carbs is balanced (meaning valve open and water column is not moving up in one and down in the other) then the pairs are balanced together and ultimately the final balance is evaluated with all valves open.

I found once balance is off it tends to also skew the idle adjust, so I go for balancing the carbs then adjusting idle to its final position. Usually as I bring balance in I have to adjust idle down (or up) to keep the engine running at idle. If one AF screw is way out that can affect balance to some degree, so I'd suggest get both screws to the same place before balancing.

The XS400 carbs balanced very easily, my Bandit was quite fussy in comparison. I found the ultimate affect of getting the carbs balanced was improved idle and nicer starting/warmup.