Mikuni Carb Selection
I have a 1987 Polaris 400 SKS. The previous owner installed a 488 overbore kit in it and it was ported to the max. I also found out that he had honed out the 38mm Mikunis to 39.5mm. It ran fine like this until one of the piston skirts broke apart. While inspecting my engine parts, I noticed that the web between two of the ports in the bad cylinder had a crack in it; not sure if the piston failure caused it, or if it was weakened from the heavy porting job. Either way I considered it scrap and purchased new 500 cylinder, pistons, and head.
I now have about 1500 miles on this "new" engine and it has yet to run right. It is fine once the clutch engages and the track starts spinning; however, it severely hesitates in trying to do so, especially when it is warmed up. It makes it real difficult to get out of holes and tight spots. I installed the lowest possible gears in the chaincase and a heavier spring in the clutch. This helped some, but only hides the problem.
Starting the engine is also difficult when it is warmed up. It often requires multiple quick pulls with the throttle held at about 1/4. Once it does start spinning on her own, it takes about 5-10 seconds (while maintaining the 1/4 throtle) before the RPM's get high enough for the clutch to grab the belt. I have had it to the dealer for "pre-season tuneups", and I have adjusted the pilot screws, but nothing has changed. I tore apart both carbs and found nothing wrong, no clogged ports or passages.
I am now thinking that the carbs; having been honed-out to 39.5mm may have been fine for the old ported-out engine, but too much for the new stock 500. Not enough airflow velocity at low RPM's to pull the fuel through the carb jets?
If I need to buy new 38mm carbs, should I consider the more expensive "flat valve" style or stay with the "spigot" design? Are they easily interchangeable or do they require different cables and mounting flanges? Do they require the same size jets for a given altitude and temperature? (3000-6000 ft./ 10-30 degrees) What is the difference between the TM and TMX? Both are listed under the "flat valve" category, and both are 38mm. I have a Dennis Kirk catalog in front of me, but I have seen better prices on Mikuni advertised somewhere in SnoWest magazine; who else sells these carbs at good prices?
Thanks for any help you can give me.