So after working for the last 6 months, or so, on resurrecting a bike which had spent the previous 20 years sitting with fuel in the tank I finally got it running this past week. The problem is that, while I can get it to start on choke and hold a decent 1000 rpm idle off choke once its warmed, I consistently get a high idle, (2500 rpm or so) with no return to normal, every time I blip the throttle beyond 1500 rpm, or so. I've repeatedly tried mixture screw adjustment in 180 degree increments from one turn out to 5 1/2 turns out with virtually no difference in either idle speed or "hanging throttle" response. The only way to break the "hanging throttle" is to shut it off. I'd previously disassembled the stock Mikuni carbs as a pair (did not break apart), removed the jets, ultrasonically cleaned everything, and blown out all drillings with compressed air. Carbs were then rebuilt. Needles and jets appeared to be stock. I assumed I'd be making adjustments due to my high altitude of 9,000 feet. Throttle cables are properly adjusted and are not hanging up. Choke mechanism was cleaned, adjusted, and seems to be operating correctly. New air filter installed and no intake leaks. The bike has 3,500 miles on it. I'd really appreciate any advice you could provide. Thanks. Ed
Your problem could be down to several things but if you are happy that there are no air leaks, main jets are not worn and standard or suitable for climate, diaphragms are not perforated and corresponding 'O' rings are in place, float levels are adjusted to factory spec, needle valves sealing ok and not causing float bowl flooding, idle mixture screws working and adjusted to approximate std settings/corresponding galleries not blocked and throttle slides performing correctly and not jamming, then I would spend some time on balancing both bodies as symptoms fit quite well.
If you are at 9000 feet the air is thinner (effective oxygen density 14.8% against 20.9% at sea level) so you will be running rich on standard settings - bit like having the choke half on - which would account for a hanging throttle. I think I would do a number of plug chops - find a quiet road where you can run under load at full throttle for 30 sec or so (obviously does not have to be in top gear) and cut the ignition, clutch in and coast to a halt and then "read" the plug colour do the same at half and quarter throttle, adjust the main jet, needle setting and pilot jet settings leaner by stages.
Simple point........make sure the throttle grip rubber flange isn't binding against the actual plastic housing, especially if it is all OEM. It can cause exactly what you describe.
Thanks to all for your well reasoned replies (and apologies for taking so long to get back with you - a day's worth of medical tests). I thought I'd done a pretty thorough job on stripping, cleaning and rebuilding the Mikuni's but it's troubling to run the mixture screws from one to five + turns and not see any significant change in response. The one thing I can't, unfortunately, do is the plug chop test - I live a couple of miles back a badly wash-boarded, dirt road and I'm afraid I'd drop the bike with the run-away throttle problem. Looks like tomorrow I'll pull the carbs, tear them down and clean them again. I don't remember how rigorous a check I did on the float height and, armed with some better drawings showing the passages, I'll do a better job confirming the passageways are free. If there's anything else I should do please let me know. BTW I didn't mention earlier that I balanced the carbs, at idle, with a Twinmax. Thank you all, again.
Well, I've got the carbs disassembled once again since the bike had the same 2500 - 3000 rpm runaway with choke. Wouldn't shut down with choke off and no throttle. Had to shut down with cutoff switch. Throttle and choke cables were free and properly actuating - no binding anywhere. I've ordered replacement rubber sleeves ($80 !!) to mount carbs to intake runners, not because they appeared to be leaking or deteriorated from the outside, but because the ribs on the inside at the intakes seemed to be breaking down. I'd cleaned the tank with vinegar, letting it soak for a couple of weeks - seemed to clean the tank of 15 - 20 years of goo. Installed new pump, inlet sock, and filter. Would you suspect any problem with the fuel supply to the carbs, given the 'hanging throttle' symptom? I found no obvious signs or residue in the jets. Any downside to adding an additional external filter to the carb inlet? Thanks. Ed
Hi Ed, if you still have your carbs off the bike you should be able to check for any blockage in the fuel feed lines as there is a small filter in the feed line at the carb inlet, or there should be unless it’s already been removed, it’s a small sock type filter that just sits inside the carb inlet tee joint,
Thank you, bumpstart. I'd previously cleaned the small filter during the first disassembly and have checked it at each of the subsequent tear-downs. I've not picked up much or anything at that filter. Ed
I’m no expert but balancing the carbs at idle will surely be rather pointless as only the idle circuit will be in play.
The butterflies need to be balanced so they open at the same time (that's open at the same time in relation to how each cylinder is performing). That's why I used to balance mine at about 1600rpm and then at 2000rpm. Above 2000rpm if the butterflies are balanced, each slide will lift in conjunction with each butterfly opening. That's assuming everything is in good condition.
My SS over ran like your suggesting I would pull up to traffic lights and it would rev embarrassingly mine turned out to be valve stem seals
Once the engine has warmed up, have you tried turning the throttle stop screw down a bit to see if that drops the high idle speed? If it does, try balancing at the lowest idle speed you can achieve. As Chris says, the carbs being a bit out of balance can cause what you describe, because one cylinder will be trying to pull the other, which in turn makes it run faster a bit more. There is definitely something odd about your set-up because with the choke fully on, I would expect it to run at about 2000rpm when cold and when off and warm, idle at about 1000-1200rpm (mine would idle at 1050rpm).
If you've removed or changed the throttle cable, ensure it is located correctly and not too taught Loosen off all adjustment and start from scratch (all too easy to overlook the basics and focus on the science) Check for any specks of dirt or fine grit in the seating groove for, or on the diaphragms themselves Acetone/nail varnish remover is the best soak for jets, leaving them at least overnight, or 8 hours A compressor-driven air gun is a must for blasting all passageways After such a long time, most gaskets will have turned brittle and may not be fully sealing after having been disturbed You say you've been thorough cleaning the carbs, but if you haven't done so, something like this is well worth the money https://www.ebay.co.uk/itm/2-x-Carb...e=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649 Good luck!
Hello Ed,I've been going through similar problems with my newly acquired 1997 900ss it had been sitting for 12 years .The Mikuni carburaters were in need of a good cleaning,I thought they were perfect and I put them back on the bike . It would run but idle was poor.I tried adjusting those pilot screws but only one worked on the rear cylinder , adjustments to the front cylinder made no difference I could ride the bike down the road but at idle only one cylinder was getting fuel. I pulled the carbs one more time I concentrated on the carb where the adjustments made no difference and I discovered a plugged jet in the idle circuit the hole was so small I thought it wasn't a jet but a brass plug,I was able to soak in carb cleaner then blow the gunk out of that tiny hole. After that the bike idles and runs beautifully and I was about to set the pilot screw and balance both carbs to perfection.I have a new problem however,for some reason when I reach 5000rpm in any gear the bike looses power like its starving for fuel.If I add some choke the engine takes off again.Trying to figure this out,have you any ideas?Randy
I finally discovered why the bike was running out of steam at 5000 rpm,the previous owner removed the air box cover.The air box with snorkels slows down the air volume to the carburetors in effect enriching the mixture to the cylinders .I bought and installed the cover with snorkels and it completely transformed the bike.It starts easy,runs beautifully right through to red line in any gear.What a terrific bike,I ride it every day.
Congratulations, Randy, that's great news. I see my surgeon this week to hopefully have physical restrictions lifted, following last month's back surgery. I need to get on a bike!
