Monday, February 29, 2016

Mikuni CV Carbs — Great Tip For Seized Pilot Screws!

The GS550 needed carbs...
A previous post of mine described the making up of a bank of carbs from a spare set I bought. My originals had been broken by a previous owner and laid out in bits in a cardboard box. The carbs I got hold of were from a GSX750 — same bodies, but the pilot system was slightly different and the jets, obviously, were all the wrong sizes. But no matter, it proved that the GS would run... just not very well.

Because I want to put it back to standard, using the original airbox and filter, a good set of carbs was still on the wish list. And a couple of weeks ago, a bargain on eBay saved the day. For seventy bucks delivered, I had a very clean set for a GS650 on the way to me. Could not believe my luck!

It got better too, because once apart, the jets tell me that they were off the 550 after all. A strip, clean, new o-rings and we could be in business. 


Very impressed with overall condition, and they came apart quite easily. Usual varnish/staining and that beautiful smell of old, stale petrol. Am I the only one that loves that smell? Comes second to two-stroke fumes for me.

Removal of the float bowls, and this is where I realised the jets all corresponded to the smaller engined bike. Happy days!


Pretty clean inside, and nothing broken.



Everything was coming to pieces way too easily. This can only mean one thing in the world of Murphy — something will go tits up very soon.


Good old Murphy! Two pilot/mixture screws were rounded off and seized in place. I warmed them gently with a torch, filled them with WD and left them for a few days. Not a hint of movement. 
I headed off to the GS Resources forum for a browse through the carb section, and I soon found a great tip.

Break out the Dremel.


Put the carb in a vice first. Now, using a small cutting disc, line up the middle of the screw and slowly cut slots down the carb tube until you reach the screw. Continue cutting until you have a nice slot for the screwdriver. Next get a screwdriver that is a really good fit (file one to fit if you have to).

Now I used a miniature blow lamp to warm up the tube, the good folks at GS Resources said to keep heating until it moves. I was wary about putting too much heat into it but, sure enough, as it got to a certain temperature, the screw began to move. Keep heat on it while you undo the screw. 


Hey presto, both screws came out relatively easy using this method and, luckily, no threads were damaged. Just keep washing the swarf out if the threads 'pick up' when you fit a new screw. This worked perfectly and these are now ready to be put back together.

The diaphragms on the slides still look good. I used washing up liquid and plenty of water to clean them gently and blew any droplets of water out of them with an airline. I also rubbed the top of the carb bodies on a flat plate, with 180 wet & dry, to ensure a good seal when they're refitted.







Bargains do still exist on eBay, and hopefully the little GS550 should run well enough to give it a quick blast when they're done. More when I get the o-rings.