B400 choke/starter circuit not working

[bdouvill]

Hi there,

It's been a long time ;-)

I had my B400 for about 10 years now. End of 2018, it started giving me headaches so I stopped using it. At the time, I switched to my resurrected 1993 VFR 750 (that was flooded but that's another story) then also bought a 2009 ZX-10R (as a daily).

I worked in parallel on the Bandit to get it back to work. One coil failed and I replaced it with a cheap one from Ebay. Also changed the spark plugs and changed all seals of the carbs because I started getting some leaks. I previously put a Litetek kit and that was fine, but I changed again all seals with Suzuki ones plus all one sealing the carbs together. I spend around 250€ in original Suzuki carb parts, that is crazy.

Back in September 2019, I was able to restart it, used it only once and stopped since then because of what follows:
- bike does not start when battery is fully charged and fuel is obviously going to the carbs
- choke/starter system does not work at all, apparently only gives more air to the engine and no fuel - let's talk about that later
- when spraying anything in the airbox, like break cleaner, engines start and then everything is fine.
I mean:
- idle is smooth and engine is starting gently
- throttle response is good
- bike works like a charm.
BUT if I stop and have to restart it, again, it starts very hardly.
Last point:
if it runs and I start using the choke, then it dies.

During last winter, I checked the valve adjustements -there's nothing wrong here after 101000km ;-)
Carbs are well balanced - I use a Carbtune Pro II, this is fantastic.

So it's so hard to start that I basically stopped using it after running it only twice in 2019...

I can't live like with that. In fact since I have it, I rode it for 35000km approx and the choke/starter circuit never worked.
I even changed all 4 carb started seals - item #42 here:
https://images.cmsnl.com/img/partslists/suzuki-gsf400-1992-n-e02-e04-e16-e21-e22-e25-e30-e34-e53-carburetor_bigsue0047fig-11_7e6e.gif

The more I think about it, the more I believe the choke circuit is clogged. My problem is that apparently, you cannot dismantle the circuit to clean it, I found no info, no picture of that.

The closer I found is there:
https://www.bikechatforums.com/viewtopic.php?t=191020
2 posts from the end:
"If you take the bottom off one the carbs you'll find a plastic tubular housing the plugs into the main body and seals with little o-rings. there's a small brass jet set in the housing that meters fuel to the choke (fuel enrichment circuit actually). If this jet is blocked, you'll get no fuel when trying to start the bike on choke. My sister's bike was so clogged up there was no evidence of the jet at all and I found it by digging all the crap from the end of the plastic tube. There's one in each of the four carbs."

On mine, I am 100% sure the float assembly is not clogged at all but I have no clue for the rest of the carb(s). There's the external line on the side of the carb but I don't know if it's possible to remove it to clean it. Otherwise, all I can do is to find compressed air and blow inside each side and check it's blocked or not.

According to the service manual (page 4-9), there's a crossing between fuel that comes from the bottom and air that comes from behind the diaphragm where the starter plunger goes. From there, it goes through the starter outlet into the main bore.

Will try to remove the carbs from the bike as soon as I can and find for evidence of clogged circuit. I think the end of the choke circuit (between diaphragm, starter plunger and main bore) should be easy to check but what about the rest (from the float chamber to the starter plunger)?

Anyone can help here? I googled like crazy before asking here.
Is there anything I can do apart from getting the carbs cleaned with ultrasound?

Thanks a lot in advance for your help.
Regards.

[Mechannibalism]

I think your right that the choke circuit is plugged up.  It could be the Jet/emulsion tube  or the outside transfer tube. I would blow it out backwards (from the port in the top of the carb by the vacuum diaphragm).  Squirt some carb cleaner in it and soak it over night.

[ventYl]

My Bandit carbs are sitting dismantled in garage right now because of opposite problem... My bike runs only using choke and when I turn it off it dies. It is also not possible to turn throttle - bike will die immediately. I have dismantled all four of them and found something what looks like dead worm in main jets. After cleaning them, not much of a change. Next there is green substance on almost every brass part inside carb. Those little brass jets for choke circuit are really small and it is possible these are clogged in your case.

I would try to get a syringe and firm hose which you can put on that brass extension and push something through it. You can start with plain air but something non-compressable as gasoline with some additive may be more useful as it will serve as lubricant for potential dirt. If you are not able to push fuel through that jet using syringe then the circuit is definitely clogged.

[bdouvill]

Hello again and thanks a lot for your answers.

So I dismantled the carbs - again, sounds like I did it a million times...

I did not find how to integrate the pictures in attachment in the text (...) but here it is. When spraying WD40 under pressure with the long tube coming with the WD40 can directly on top into the tube coming from the bottom - where the starter plunger sits (RED Arrow in the pic), WD40 comes out of the other side (YELLOW pic) so I guess it is not clogged. Same for all 4 carbs.

I let WD40 inside the circuit for 24h.

So all I can do is to mount everything back with great care and cross fingers. Will try to put at least 3 to 3.5 turns on the pilot screws because everyone knows factory setting is a joke. Changing needle position should not help in my situation (this has always been the default position) because it is not supposed to affect closed throttle position.

I keep you posted later.

[bdouvill]

Hi again,

I like life because there's always a way to learn something new everyday ;-)

Since my carbs don't seem to be clogged - see my previous post - I started to ask myself if I was 100% sure about how to use choke. For years, my bike slept in an underground garage where the temperature never went under 12degC (around 54degF). So everyday for years, I started my bike with no choke and the throttle slightly open (like 10 to 20%). Now, all my 3 bikes sleep outside under covers and it's regularly freezing (slightly) these days, even on the French Riviera.

So I googled this morning and found out that:
"Lots of more modern carburetors have an “enrichment circuit.” This is a special circuit within the carburetor that is only opened for starting. In order for the fuel in this type of carburetor to be drawn into the cylinders, leave the throttle shut completely when starting, or this circuit will be bypassed totally."
Full source is here:
https://www.revzilla.com/common-tread/how-to-start-a-carbureted-motorcycle

I am pretty sure that during my last attempts to use this bike back to last summer, I tried mostly with choke on+ throttle wide open but never with choke on and throttle closed so I just learned that this was pretty dumb :-(

But you learn more valuable lessons from your errors than from easy wins.

This still has to be validated with real experiment - which will happen as soon as time permits. The only time I really used it was back in September when I used it to go to work. Outside temperature was 25degC at least (77degF). So I parked it a 02:00PM and when I tried to start it again at 06:00PM, I had a hard time starting it again. In comparison, it always started days in, days out for 3.5 years prior to that, which is still a mystery to me...

Also in comparison, my VFR750 starts without any problem everyday, either with choke on and no throttle or with throttle slightly open.

I'll pay close attention to the pilot jet(s) settings to start with. I understand this will have an impact on cold start as well. Then I can still use my Colortune to fine tune them individually.

I'll keep you posted on my progress.

[tubular]

Hello Benoit,

What is your carb jetting and float height?

In my '89 GSF400 the standard factory jetting (PJ: 32.5 / MJ: 100) was simply too rich for where I live (location: Athens, Greece) even with aftermarket exhaust and K&N air filter. Currently I am using #32.5 PJ and #95 MJ (PS: 1.5 turns out float height 14.6mm) and the starting issues are greatly reduced. My feeling is that with #30 PJ there will be no issues at all. But i am very rarely riding the Bandit now days so I have not verified this to be honest.
Depending on your bike setup you might need to experiment with carb jetting. Running rich (or lean) will affect the engine starting behavior (among many other things, but hey... let's get the engine running first...  ).

TIP: before cold starting the bike, you may find turning the petcock to PRI (for a minute or so) to get the carb bowls filled, is making your life easier (especially on cold winter mornings). Just make sure you don't leave it in PRI position after the engine has fired up...