| Author | Message | ||
     General_ulysses |
Hey everybody. I have a stock 1997 S1 with a stock exhaust/muffler and I'm almost certain completely stock engine. It has a Mikuni HSR42 carb on it that backfires through the carb quite frequently. I'll describe the problems and things I've done to the bike so far to give you the scenario: 1. When I first bought the bike, it had a Vance and Hines exhaust on it. It backfired through the carb with this setup too. I disassembled the carb at the time, cleaned it out, got fresh gas, and it still popped through the carb. After the bike is warmed up, it occurs when you crack the throttle from idle to higher power. Once the engine starts to pull, it pulls strong and runs good. 2. I removed the V&H and put the stock pipe on. I noticed more low end torque, but the popping through the carb persisted. Again, the worst driver for the popping was accelerating/revving it, backing off to idle and then opening the throttle again causes the backfire through the carb throat. 3. After a while I noticed fuel was spilling out of the carb overflow hose when the bike was not running. This often left a big fuel puddle under the bike after leaving it parked. I figured it was either the float needle valve was trashed (carb is almost 20 years old now, soaking in ethanol gas all that time) and/or the float bowl was mis-adjusted. Ordered the Mikuni rebuild kit, removed carb this past weekend, cleaned again and installed rebuild kit. Also swapped out the stock idle jet (size #20) with a size #25 (as per Al Lighton's suggestion). Slightly adjusted the carb float bowl to spec. Left the needle clip at second from top setting (I believe stock?). I forgot to check, but I believe the main jet is a stock 160. Adjusted the accel pump actuation to 2mm as per spec. Set idle air screw to stock 2 turns out from closed (as per Mikuni manual). 4. Put 'er all back together. Bike ran the same, essentially. Maybe even a bit worse. Popping through carb as usual. Although I haven't seen the fuel leaking out of the overflow anymore - although I did see a few specks of fuel dribbling out when it was running. Very minor and nothing like it was, but I'm wondering if maybe I should have set the float bowl even a bit lower than I did. 5. I also replaced the old plugs and wires, which were both original (bike has about 8300 miles on it). The old plugs were pretty sooty, although they weren't completely black or anything. I almost thought they were a healthy richness level of sootiness, but not that optimum tan color you see in books. So, according to what the plugs looked like, the bike appears to be running rich. Also, the exhaust smells rich in an eye burning kind of way. And the mileage was usually no better than 40 mpg. All signs of an overly rich scenario. 6. I noticed there was this vacuum gizmo thing that plugged into the carb with two electrical wires coming out of it and going into the wiring harness. I'm guessing it's a vacuum switch that retards or advances timing under certain throttle/power settings. Is this something I should be concerned about or test? If so, how do I test it? I think I've seen it in the archives mentioned as a VOES, which I'm guessing means vacuum operated electrical switch or something like that? 7. I noticed the bike burbles and backfires lightly as I decelerate from speed with the throttle closed. I also noticed I don't have any exhaust donut gaskets between the heads and header inlet pipes. I have them but need to install them. Think that might have something to do with carb backfire? I'm pretty sure it'll contribute to burbling popping in exhaust on throttle off decel. 8. Lastly, I tried adjusting the idle air/screw today per Mikuni manual. My results seem to indicate a rich situation. i.e. I turned the screw all the way out and the bike just never started running rough, which according to the Mikuni manual means the idle pilot jet is too big. (remember it had a size 20 and now has a size 25). Anyway. What do you guys think? I'd like to dial this carb in to get the most out of my stock S1 performance. Seems like I'm Robin Leach rich, but I dunno. Any advice appreciated. | ||
| Two_seasons |
My first thought was it's probably VOES related. You may have an intake leak or an exhaust leak. Best way to check that is MAPP or Propane gas (unlit of course) around your intake/exhaust seals. FSM (factory service manual) spells out issues and what to check, in order of importance, and the settings. I think on the VOES you'll need some HD service help though! | ||
| Akbuell |
I will let folks with a lot more experience on these motors than I advise on the carb issue. Mine is all iron head sportster with a Tillotson carb. As to the vacuum gizmo, VOES sounds right. They are either open or closed, and easy to check. Multimeter on continuity, then check with and without vacuum. Should show no continuity or continuity. If the switch shows open with no vacuum, should show no ohms with vacuum on. Or the other way around; just so what you get with vacuum is the opposite of no vacuum. Hope this helps, Dave (Message edited by Akbuell on March 30, 2015) | ||
| General_ulysses |
When I bought the bike last year, first thing I did was go through the intake system and replace all of the rubber seals and gaskets (got all the stuff at American Sportbike). So all of that stuff is good. I'll check the VOES in the meantime. Anyone know what the trigger vacuum is? | ||
| Cyclonecharlie |
As long as the vacuum line is connected, forget about the VOES. Even with new intake and carb, seals, it still sounds like an intake leak.And a 20 pilot is rich enough,with everything right you could run a 17.5 pilot. popping on decel.sounds like an exhaust leak.Two seperate problems. | ||
| S1owner |
Well above you state there are no donuts in the exhaust at the heads correct? Even the small sceaming eagles are in my opinion to small get stock ones thats where the exhaust leak is | ||
| Steeleagle |
I vote VOES. My alleged thinking: Popping thru the intake implies fire entering the intake system. Bad timing (or intake valve) would cause this. A stuck VOES (stuck CLOSED) would advance timing at low RPM and might cause the symptom of carb-popping. Leaking gaskets: On the intake side a leak would make it run like crap from the lean condition, but not make it pop. An exhaust leak would make it pop out the back from hot, unburned fuel mixing with the air from the leak. Not my 1st choice, but all pieces have to fit properly. Just my two cents. | ||
| D_adams |
Just for reference. http://www.wildwestcycle.com/f_voes.html | ||
| General_ulysses |
Thanks everyone, really appreciate the comments and especially the link to the VOES site, very helpful. I think I have a good gameplan on my troubleshooting now. After reading more about the VOES gizmo, I'm suspicious it may be my culprit - but with my unsealed header pipes as a contributing factor. If the VOES unit was stuck and keeping my ignition retarded, I can see why that might cause backfiring through the carb when I crack the throttle. If the ignition didn't light off the intake charge until late, that could very well mean it's not completely burned/consumed by the time the exhaust cycle was complete and the intake cycle started. If there was still a burning charge present in the combustion chamber when the intake valve started to open, it would force its way straight up the intake runner and right out of the carb throat. If the carb is set rich and I have an exhaust leak at the header, I could see where that might also contribute to the problem. The rich fuel/air mixture wouldn't get completely burned because there's not enough oxygen in the intake charge, but then when it comes into contact with air (oxygen) that's seeping in from the leaky exhaust, that may be helping to cause the continued burn within the combustion chamber, even as the intake valve opens. Anyway, that's my $0.02 theory right now. I'll put it into practice in the next few days and see what happens. Will let you guys know how it works out. Thanks again for all the help! |
