1979 XS250 Rich idle issue


XS400 New Member
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South Wales
Hello there everyone.

I picked up a 1979 XS250 from eBay that had been sitting in a farm outbuilding for at least ten years.

The bike has been stripped right down and built back up.
The engine was seized, so that was eventually freed up, the bores honed and a new set of rings fitted.
Unfortunately I am having issues with a very rich idle.
The bike seems to pick up and go once the throttle is cracked open, but I can't seem to solve the idle issue.
Below is a list of the things tried so far:
Cleaned the stripped carbs in an ultrasonic cleaner three times.
Confirmed the pilot jet is a 20 (main jet and needle were confirmed as standard too)
Tried various pilot screw settings - none make any real difference, although wound right in seems slightly better
Bought another set of carbs from ebay that were not complete, so built up using my existing carbs.
The parts that were retained from my original carbs are:
Both float bowls, complete with pilot and main jets
Main nozzle jets for both carbs
Both slide assemblies, diaphragms and needles

Unfortunately the problem remains with the newer carb bodies, I have tried pilot screw settings from fully in to 1.5 turns out - no perceived difference. I have also tried lowering the needle from the second slot from the top, to the very top slot. This seemed to drop the idle, but did not solve the overly rich mixture at idle.

The original air filter and pipework is still in place. The exhaust is a 2-1 system.
I have fitted fresh points, and was happy that they were timed correctly, although admittedly I have no checked them for a little while. The bike seems to run well once off idle, so I don't suspect ignition timing to be the cause at the moment.

One point of note is that the bike will start up almost straight away with no choke added at all - South Wales isn't the warmest of places!

At this point I am thinking that either a component that I have transferred across to these newer carbs is the culprit or the rich idle is being caused by something other than the carbs.
My next suspicion is that someone has drilled out the pilot jets. This would seem to fit all of my symptoms.

Would anyone be able to offer some advice please? The plugs from both cylinders are very similar in colour - very black, so I don't beleive it is a problem with just one of the cylinders.

Thanks very much, and any help would be much appreciated.