Help 1989 Gsxr250 starting problem

Inspect the fuel tank for water or rust. Check the outlet for blocked, damaged or missing filter screens. If it has one, disassemble and inspect the fuel tap. If it is a vacuum fuel tap, check the vacuum line for fuel, indicating a broken diaphragm.

Check the fuel filter if it has one. If it doesn't, consider fitting an appropriate filter.

You will also want to check the underside of the float valve seats to see if they have screens. Clean and blow out the fuel passageways.

If they slow or starter jets were clogged previously, consider replacing them all with brand new ones. Quite often I have had pilot or starter jets that have hidden crud inside them and are impossible to clean.


When you've checked the compression, was it hot or cold?

Have you looked at the valve clearances? Failure to stay running when hot is often a symptom of tight clearances, and you will lose compression. On bikes with aluminium frames, they will also tend to get too hot to touch.

 

No choke required on cold start, and misbehaviour if you do use it, says running too rich.

These carbs are fickle, OEM settings are basically where it's at.
So fuel height/float level is important to get set right.

Also, something worth checking as there's a separate idle circuit it's got both fuel and air jets, if the air jets are clogged/blocked it will run rich.

Air jets [there will be one for idle and one for the main jet] are in the intake of the carb throat.

Soft copper wire is good for checking/clearing things like jets, although for small ones, as @Linkin suggests replacement is likely easier.

 

Best to set clearances to maximum when cold in my experience. If they are borderline when cold, at running temp they will be below spec. Usually need a carb balance and mixture adjustment after clearances as well.

Does the bike have an aftermarket exhaust on it? Most common experience is they turn carbed bikes rich down low and lean up top, so rejetting may be on the cards if that is the case. I was unable to find suitable pilot jets when I ran into this with my GSF250V Bandit. Needed 3ish turns on the mixtures indicating a larger jet was required, but none were available. Standard setting was 1.25 or 1.5 turns.

How do the plugs look? Had a new set in there recently? Correct heat range and type?

You might also need to go run it in properly... get it warm and go give it a light thrashing... 2/3rds of max revs in each gear, letting it decelerate down before the next gear... helps to bed in the rings. Worst thing you can do is idle it forever. The iron liners on these types of engines are pretty bomb proof.

Regarding Mikuni BDST carbs, standard settings are almost always best. When they are not, the adjustments made are hiding a problem. Either air leaks, fuel flow, assembly problem, wear problem, or other miscellaneous issues on the bike which I doubt you have.

 

You're very welcome Bago, it's always disheartening when you run out of areas to investigate, know the feeling...

By all means keep posting what you learn, it helps the collective knowledge here and we all learn from it and can share it with others.

Basics are float height/fuel level affects mixture of all circuits.

Air jets are as essential as fuel jets as otherwise the fuel comes out in lumps and doesn't burn well - I learned this one from an Army of Darkness blog post - they had an R6 that wouldn't rev out and it turned out to be a blocked main air jet in one carb, meaning that the fuel from one carb was basically only that, unemulsified fuel in 'lumps' that wouldn't burn properly.

Mixture screws have a range as @Linkin noted, I believe from reading I have done that is between 1.5 - 2.5 turns IIRC and anything below or above that range should mean a smaller or larger jet is needed.

BUT with all of that said, if all else is stock standard: airbox and exhaust then OEM settings are desirable.

We haven't asked, is the airbox securely fitted?
The internet is littered with stories of failed attempts to install pod filters on CV carbs, the airbox is important.

 

Sounds like Guru @Linkin was using his psychic powers when he mentioned the aftermarket exhaust jetting.

I did a quick google, and I mean really quick, #40 idle jet came up and #105 main [please check these numbers]
EDIT it looks like #40 is the starter jet [choke]
So smaller idle jet and larger mains, IIRC the steps are 2.5.

Lots of valuable information in this tread re: jets and types of jets
https://www.2fiftycc.com/index.php?threads/mikuni-bsw27-carburetter-rebuild-kit.919/

No experience with vacuum operated fuel taps, but the pulses from manifolds when I sync carbs are very distinct, they need to be trimmed right down with screw valves otherwise gauge needles really bounce, so I think it's likely normal.

 

Try going down in size on the pilot jet - it's the rich mixture from the idle circuit causing issues with hot (re)starts.

Choke jets only come into play when the choke cable is pulled to the on position.

Unless you're fanging it 100% of the time, the main jet isn't causing plug fouling from the rich mixture

 

As it so happens I got bored last night [the TV is broken and to be honest I don't much miss it right now] and started to trawl aliexpress for you for to see if there are carb kits - unfortunately items no longer available for the most part.
I've now taken to using google to search aliexpress, their search just gives so many irrelevant hits and I find myself getting sidetracked easily, far too easily.

OK, with that said the kits may actually be available on aliexpress, it can just be random, between their search and google [which is deteriorating IMO] it's worth trawling, switching up the keywords etc, but I digress...there's a reason for this which will become clear, promise.

Shameless plug - Litetek seal kits, the rest just aren't in the game IMO.

I believe potentially two things are in play - the aftermarket exhaust, meaning you likely require smaller pilot jets as @Linkin likely, rightly, correctly identified. I think there's a simple solution in there for the first element - smaller pilot jets; AND

Because these are Mikuni BDST carbs, there's a possibility that there is needle jet/emulsion tube wear. [OH NO not the damned needle jet wear I bang on about so often - YEP THAT! - my obsession because navel gazing just doesn't engage the way that it used to for long hours].

I have also cross referenced the part numbers of the 250 bandit needle jets and the GJ73a needle jets - different part number and most importantly different orifice GJ73a; O-0 OH-Zero -v- GJ74a O-1 OH-One - bugger! -keep this in mind though!

Needle jet wear; The bug bear of the FZR250's!
Note: Needle jets/emulsion tubes do wear, they are a service part and Mikunis seem to be worse for that...Mr Mikuni -v- Mr Keihin, Mr Keihin wins every time IMO!

I am the other forum user @maelstrom mentioned that purchased some Chinese carb kits from aliexpress - all parts are OK, [also as @maelstrom mentioned I reuse jets they don't wear]

I purchased the kits purely for the needle jets/emulsion tubes to see what's what, plus I lost 4 Keyster kits off of the back of the ute and it sux to potentially have to shell out ~$200 for just 4 needles jets, which I had - still silently suffering at the consequence of my own stupidity...tissues please someone.

Anyway, being inquisitive I also purchased some 0.0010 accurate pin gauges [checked and measured as accurate] to check both the OEM tubes and the aliexpress ones..

Long story short, the Chinese kit emulsion tubes were 2.52mm orifice [ @maelstrom will grizzle at me, bigger than 2.520mm and less than or equal to, up to 2.529mm - a 2.530mm pin wouldn't go]
The orifice should have been 2.590mm [N-8 - I'll explain this later] - this is actually a good thing, too big and I cannot cure that, too small, that meant a 2.59mm reamer - that was very well priced on aliexpress sorted!

Now, for your carbs - looking at this page on Impex for a GJ73a
https://en.impex-jp.com/catalogs/moto/suzuki/9900b-68027.html
It also give the needle jet size in the schematic O-0 - that's OH-ZERO
The increments in Needle jets are 0.005mm, the counting starts from zero
N-8 is 2.59mm, N-9 would be 2.595mm, which means O-0 is 2.6mm

Impex lists the part number for the emulsion tubes 09494-00643

Good news, the part is listed as available, one caveat, that's not always the case, it can say it is when it isn't...DRATS!

https://en.impex-jp.com/parts/new/search.html?partNo=09494-00643 $14.55USD - when you account for exchange and shipping, is reasonable to expect them to arrive at close to the locally available price below.

https://www.mickhone.com.au/part/suzuki/0949400643#content - $27.40AUD - actual availabity TBD

Not so bad, well not as bad as some other parts prices that make you want to howl and shriek, squarely apportioning blame for your pending poverty on the cost of bike parts, which must be purchased as they're life essentials [give me the parts or life will not be worth living and I will choose death in preference] - we all know this, however few are willing to say it out aloud as a form of confession as I just did.

So, my suggestion would be to remove and inspect one of the needle jets from the carbs, check the series, it will be 3 digits and take a viewing loupe [magnifying glass will do in a pinch] and peer down the needle jet orifice, until you become dizzy and need to stop and gaze outside, to see if there's any wear.

It's like one of the 3D seeing eye puzzles, if you've even encountered one, it takes time to see it, but once you've seen it you cannot unsee it.

Further, potentially that series of needle jet may be available from Mikunioz in the correct size even if the other potential sources come up empty.
https://mikunioz.com/product-category/jets/needle-jet-emulsion-tube/?v=13b249c5dfa9.

Now as a final backup if all other supply sources fail, the same 'error' in the orifice size could be in play as I encountered [needs to be too small] IF the GJ74a needles jets are the same series, they very may well be.
So potentially, if all else fails you could take a punt on a set of Bandit 250 GJ74a aliexpress carb kits and get a 2.6mm reamer from aliexpress also

I just got a 6.990mm reamer from this store and it's 6.990mm, I measured and measured and measured.
https://www.aliexpress.com/item/4001208486423.html

This listing actually has the 2.6mm straight flute reamer which is what you'd want, spirals are for material removal, straight are for final pass, if you do end up going down the path of aliexpress carb kits, as I've already got the pin gauges up to 2.600mm, happy for you to send aliexpress needle jets to me to measure so you know what you've got to begin with - no cost.

Note I wouldn't just leap at the needle jets unless you're fairly confident about actual wear and I'd go down on the idle jets as a first step.

Throwing another set of carbs at it when that jetting may not be correct anyway, due to the exhaust, isn't really a solution as you may have to exactly the same type of jet changes on another set of carbs to get them to work correctly.
It also introduces a whole lot of other variables and you'd probably need to start from your current set of jets and tune from there.

hope this helps

Apologies for the long message, coffee is kickin' me hard today it seems...I can hear the thoughts of any readers that made it this far, please for the love of Dog, get the TV fixed man, that's 15 minutes of my life I can never recover.

Glenn

 

Exactly!!! In many cases, people with malfunctioning carbs (for whatever reason), think that "new" set of carbs will fix their issues. But unless you get brand-new Mikuni carbs from authorised Suzuki [Honda, Yamaha, etc] dealer, you're most likely getting unknown condition used set of carbs that's most likely worse off than ones you have now.

Like with electrical issues, testing and measuring to come up with numbers is key. Because those numbers can be compared to standards in manual to determine if part you measured is within spec. Then if not, you'll know which way to adjust the spec to improve bike's behavior.

I suspect in this case, even though numbers on jets say they are certain size, they may be victim of more-is-better mentality that causes many people to hand-drill their jets to hugely larger sizes! Only way to confirm these jets really are the sizes they claim is to measure them.