Hi all big problems after gsxr400 carb strip

[hazynut]

Hi all bit new to the forum but here's my problem I have owned a 1990 grey gsxr400rr for many years I had the carbs dyno set up a good 15 years ago to suit an art carbon exhaust can it ran very well making 60hp at the rear, not to long after (a few months) I found the bike wouldn't start, after much fiddling I put a new set of plugs in and it ran ok all day, but next morning it was a no go again, Long story but I had no time to mess so stored the bike away until last year I pulled it out, the tank had completely dried out so I sent some clean fuel though carb bowls were empty so sent some clean into them and emptied them and refilled and to my surprise the bike started straight up first kick even on plugs I thought to be fouled, it ran really well I put it away for the night and then it failed to start next day. Much head scratching brought me to rebuild the petcock which I believed to be flooding it over night, I changed plugs and the bike was back up and would start every day great I thought but after a couple of weeks I was back to a dead bike, I then proceeded to strip and clean the carbs I rebuilt with new orings all through the carbs. I refitted them put fuel in them made sure the bowls were full and tried to start the bike, it was a total no go I pulled the plugs and they are completely bone dry if I put a few ml of petrol straight into the cylinders replace the plugs I get it trying to start and burbling for a second or two then back to dead. on rebuild I set the mix screws back to factory set and hadn't messed with the needles height clips, I am at a bit of a dead end what to do next and any help would be much thanked re Alistair

[SevenThreeSeven]

Three thoughts:

(not saying any of them are absolutely correct and applicable to your situation, but maybe...)

1. the idle air and fuel jet circuit in these Mikuni BST carbs is notorious for getting clogged/fouled. It's the longest, narrowest passageway in these carbs. Often owners will clean their carbs and miss getting this passage completely clean. Snaking it through with soft metal wire (copper wire) from every end (both air supply and fuel supply), along with cleaning fluid and finally blowing it out with compressed air helps.

2. your comment (early in the post) about replacing the plugs and getting the bike to run but then finding that it wouldn't run the next day points toward an over-rich situation (quickly depositing a lot of carbon on the new plugs). the emulsion tubes in the BST carbs are viewed by Mikuni as "consumable items" and they do wear down (soft brass against the harder metal of the needles) into a subtle oval-shape that's rather hard to see with just the naked eye (from their original perfectly circular shape). This results in an over-rich condition for every engine condition (throttle position) beyond idle (any time the engine isn't using only idle jet fuel).

3. totally forgot the third thought while posting the first two.

[Variablevalves suck]

If you have fuel in the float bowl its going to be the pilot system needs a clean.
Are you 100% sure you put the pilot screw back in correctly as its easy to get the o,ring and washer back to front, should be spring onto the screw followed by the washer and finally the o,ring so as o,ring is seated on the carb body.

Also going back to the first time it happened ive had similar problem from old battery and also the pick up going off, this can be checked with a ohm meter and should read around about 100 ohms.

[hazynut]

Just read your replies thanks everyone. I did put mix srews back together ok spring, washer, then oring I took the basic setting off a web posting and if I can remember set them at about one and a half turns out. The bike had a brand new battery and previous to stripping the carbs I could always get it started on new plugs so it swung me away from the electrics, the needles looked ok and nothing obvious looked a miss. I just cant understand how every plug looks and smells completely dry, looks like carbs out again, hope I haven't messed something up by fiddling with them.
Thanks for all the help Re Alistair

[Variablevalves suck]

You can check fuel is getting past the fuel tap by setting it to prime, this is done by removing the small screw on the front of the tap, this then allows you to turn the tap another 1/4 turn switching the tap to on without vacuum.
If all good here then you going have to check the pilot system over.

[slabbygk71b]

If you get a 1 litre portable workshop fuel tank, you can work on the carbs without the bike's fuel tank getting in the way. Best £20.00 you can spend to make life easier. The other advantage is the fuel tap issue will be highlighted straight away as it wont be connected to the fuel lines, if the bike starts and runs you can suspect the fuel tap.