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engine misses at lower rpm's

  • Thread starter Thread starter Anonymous
  • Start date Start date
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Anonymous

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bike is an 82' gs850glz with 13k. during my recent tranny rebuild i also disassembled & cleaned carbs(which were almost spotless to begin with) and also cleaned up grounds & some connections. i installed an inline gas filter and adjusted the tamperproof low speed mixture screws to between 3 & 3 1/2 turns out. i had the bike running like a top and it still does except now & then the engine feels like it's running on 3 cylinders or its starving for fuel up to about 4000 rpm's or so. bike accelerates strong at higher rpm's & when opening throttle. when it's missing at low rpm's and i engage the choke about halfway, the bike smoothes out. however, the carbs were spotless when i reassembled them and all jets & oriffices were clean. the o-rings are fine. this problem happens at ramdom whether bike is hot or cold, and had cleared up once when i pulled the tank off, ran the bowls low on gas, reinstalled the tank and drove off with no problems. the petcock and vacume(diaphram) are working perfect. when the problem occurs the fuel filter and lines are full or gas and show no evidence of starvation. i may be wrong but the #2 exaust pipe may, i say may, feel slightly cooler than the others. dont hold me to that because it feels very close to the others. i know the #2 cylnder is the petcock vacume cylinder, but the petcock is ok & the problem occurs even when the petcock is set to allow free flowing fuel(not dependent on the vacume diaphram). float level is ok & air filter fine. any help appreciated.
 
engine misses additional info

engine misses additional info

i forgot to add that the problem dosen't seem to happen during a ride but happens after the bike has been off either overnight or a couple hours and then started and ran. thanks again.
 
Well, I'm not a real expert with the CV carbs and there are many things that can cause a lean condition. If your bike is stock, I don't understand why you decided to turn your mixture screws to 3 1/2 turns out. I assume this is to richen the mixture as you would with the pilot screws on VM carbs? Since that did'nt help, I would first put them back where they were.
So your bike feels like it's lean up to 4,000 rpm and it happens at any engine temp.
You say the petcock and vacuum line are good. Have you made sure the gas cap vent and gas tank filler vent hole are clear? Is your fuel line kinked? This is an easy/quick check. If those are good, then we're down to the carbs.
I know you've checked some of these but I'll just go down a list of some (lean) carb problems.
Loose clamps on the filter boots or maniflolds, bad o-ring(s) in manifolds,
Poor carb synch', just 1 being out from the others can cause this,
mixture screws out of adjustment, these are set by the factory and are nearly impossible to re-set if disturbed,
a damaged diaphragm, because it is made of thin rubber, the diaphragm can fail at one engine temp, but be OK at another engine temp,
a clogged fuel passage feeding the carb(s),
a clogged jet, incorrect jet size,
float height out of adjustment, damaged float, float needle valve(s) hanging up in their seat not allowing full fuel flow,
a dirty needle jet circuit.
I may have missed something, like I said, I don't have much CV carb experience.
I would also check your ignition timing and make sure it's advancing correctly too. Also re-gap the plugs. And check all connections at the coils, leads and plug caps. A poor connection can fail when cold, but work when hot, and vice-versa.
Sorry for so many possibilities, but after reading this topic and the other related topic, your problem could be almost anything. Let us know how it goes.
 
engine misses

engine misses

yesterday & today i rode the bike for about 15 minutes and it ran great, but like i said it's intermittent so tomorrow it may act up. the bike is stock other than a UNI air filter in the original air-box that the prior owners installed. running ngk B8ES plugs. before my tranny rebuild (about 2 weeks ago-fluke problem), the plugs were whitish along with the inside of the headers. the UNI filter probably contributed to that along with factory lean settings. i've only owned the bike for about 5 weeks. bike had an easy life and looks it. before the tranny rebuild, the engine ran great(although lean a bit). after the tranny rebuild the bike ran rough and wouldn't idle (keep in mind i had the carbs off and disassembled to clean, had the signal generator & plate off, airbox off,petcock cleaned,tank flushed with gas, cam chain tensioner cleaned,fuel filter added,etc, during rebuild. troubleshooting the rough running, i removed the tank, tinkered with the newly installed fuel filter & hoses, vacume line, and adjusted the fuel metering(EPA) screws. the bike then ran like new. the plugs looked much better afterwards also. than the above problem started. i rode one day for 7 hours and no problem. i did clean the gas cap vent y'day, although i didn't check the vent hose. i just had the signal generator off but didn't know to check the timing advance. do i just run the bike and watch the arms expand? keep in mind it's running like new now too. the floats were checked & left at 23.4mm (high end of range). it's hard to tell exactly where on the float to take the measurement as the float is inclined. i need to get a carb sync tool also. i also wonder if a coil or other ignition part is going. strange because the bike starts the instant i hit the starter button, idles so smooth, runs strong & quiet, yet acted up those few times whether hot or cold. i'm leaning more toward fuel delivery than electric but am open to suggestions. i should mention that i did see a crack on the intake boot(engine side), but it dosen't look like it's deep enough to matter. also, the clamps on the same boots, (all 4), when tightened all the way down, could still be rotated around the boots by hand with slight force (during reassembly after tranny rebuild). the rubber may have shrunk while the carbs were removed for 2 weeks. now that the bike is back in use, i can't budge the clamps and all seems tight (hot or cold).
 
Like I said, it could be so many things. All you can do is re-check things.
I'm not familiar with your ignition but if it's at all like mine, you hook up a timing light and you should see the advance marks move in line with the timing mark when you rev' it to 4,000 rpm's.
Synching the carbs should be done.
Does the stock air box still have the lid on? If it's off, this would cause a lean condition.
Check all plug/coil/lead connections.
 
engine misses

engine misses

yes the air box lid is still on. i'm going to price digital multimeters today. i'll take a short ride later to see how bike runs today.
 
You know, carburetion problems are sometimes confused with electrical problems, and vice-versa. Sometimes, an electrical problem results in a weak spark and the plug(s) will run "rich''. Not because the mixture is rich, but because there's not enough spark to have good combustion.
But in your case, you are apparently running lean. This would make me think the problem is the carbs too. The funny thing is, carburetion is usually not so intermittent. If something is out of adjustment or dirty, etc, the problem usually stays. We all know electrical problems are famous for coming and going. I know I'm not helping much here, just "thinking out loud". If carbs, you would have to check everything on our list. I would try the carb synch' first. If electrical, the easiest things to check are the plug cap/leads/coil connections.
If I can come up with any ideas, I'll let you know.
 
Ok, one idea. Are your floatbowl VENT tubes clear? I'm NOT talking about the 4 tubes under the bowls. A blocked/kinked vent tube or vent passage in the carb will interfere with the free flow of gas to the jets. Good venting is very important for the bowls or fuel cannot be drawn easily into the jets, resulting in a lean/fuel starvation/hesitating condition.
 
engine misses

engine misses

think out loud all you want, i appreciate the help. it's been raining here and i havn't been able to put on any serious miles to emulate the problem. the vent hoses were clear when i reassembled the bike a couple weeks ago. i'll recheck when i get the tank off again. i bought a digital multimeter y'day and may take some readings today. as far as carb sync, i was all set to order the "carbtune II" the other day until i read a couple testimonies about the "twinmax". now i'm tossed between the two. any suggestions. thanks again.
 
engine misses

engine misses

just ran the bike for 1/2 hour and it runs great. so, i checked the resistance on my signal generator and it's about 410 ohms. my clymer manual says it should be between 290-360 ohms. the bikes running great and the charging system test was ok. could this (410 ohms) be part of my problem? suzuki wants $185 for a new one.
 
I don't have any experience regarding your signal generator, so I can't help you there. We have members that can probably answer your question. Maybe Hap or ? If I can think of anything, I'll let you know.
Any other "clues" regarding the problem?
 
engine misses at lower rpm

engine misses at lower rpm

when the bike cooled down the reading fell within spec. i ran a seperate topic post "signal generator" earlier. i also did a search and found another post where his readings were the same as mine(depending if bike was hot or cold). i believe my sig-generator is fine but you might want to follow the other post also as someone else may have more info on this. i just ran the bike for about 1 hour with a long break in between and all is good still. tomorrow will be a long ride and a better test- details to follow.
 
engine misses

engine misses

i rode for 220 miles today and the problem didn't show up until about 2/3rds into the ride. it started missing/stumbling at lower rpm's as before only not as bad. it never really interferred with my ride but something is defiently going on. i ordered the "morgan carb tune II" today and should see it in about 1 week. i don't think carb sync'ing is the problem because it's so intermittent, but i'll sync carbs anyway. i may search this site to see what my coils resistance should be. i'll probably clean the coil connections also, even though they look clean. the slight stumbling clears up when i engage choke about half way, but wont any bike rev slightly with a shot of partial choke(not full choke of course)? maybe my newly added briggs & stratton fuel filter is restrictive intermittently. it sits horizontally with its output side up at a slight angle, but the gas line is always full of gas(on both sides of filter).
 
Hi. When a motor is cold or has not reached full operating temp', the choke will improve combustion and the rpm's will pick up. If the motor is at operating temp' and recieving a good mixture, pulling up the choke "1/2" way will stall the motor (mine for example) or certainly decrease the rpm's. The only way the motor would run well in this situation is if it is running lean to begin with. The way you describe it, your bike needs the richer choke to run well, but not ALL the time.
If your problem is a lean mixture/carburetion, and the problem comes and goes, then it would have to be either dirt/rust particles (not likely by your info), a moving part that is intermittently faulty,(float(s)/float needle valve(s), or an intermittent loss of vacuum or floatbowl venting. I'm not that familiar with the diaphragm in your carbs, but I know it controls vacuum, and if it is damaged this can also result in a lean condition. As for floatbowl venting, many people are'nt even aware of it. Any partial blockage of the venting passages will make it too hard for the jets to draw fuel from the bowls, resulting in fuel starvation. Even a slightly kinked vent tube can cause a loss of venting. A venting problem does not usually come and go, but it CAN seem intermittent by not being noticable when you are constantly changing throttle position, but then being very noticable at fairly steady speeds and especially steady speeds of 40 mph and above. I once had a very small piece of rubber that partially blocked the venting to 2 of my carbs and I had problems very similar to yours, except it was always noticable, but ONLY at steady speeds.
I wish I could ride your bike and see how the problem feels. Describing a problem can cause confusion sometimes. "Missing" and "Stumbling' can be taken a little differently by two people. We are most likely on the same page. Because it could be so many things, it's hard to decide where to start. Your approach may be different than mine. Like you said, a carb synch' will probably not fix things, but it is something that should be done anyways. I wish I had more experience with your type of carbs, maybe I'm missing something?
If I had your bike, at this point I would make sure all the coil and plug connections were good, simply because it does'nt take much time to re-check. Bad connections can wait to show themselves on a long ride. Then, even though the carbs SHOULD be OK, I would take them apart and re-check everything. Float heights,needle valves, everything. I have no way of knowing your abilities with carbs. You may know more than me. But there are only a certain amount of things that can cause your problem and it is sometimes possible to miss something. Trust me, just a simple "blow test" of the vent passages can reveal something that only one of us would catch.
If any part in your carbs is even slightly suspect, you may have to end up replacing it. Some problems can be very frustrating and you just have to eliminate all possibilities, one at a time. Your problem may be in just one carb, but if it's a "sometimes" sticking float valve, which would cause exactly your problem, it may never "stick" when you are inspecting it. I would pay special attention to the floats, valves, diaphragm and venting. Let us know how things go and I'll keep trying to help if needed.
 
engine misses

engine misses

the stumbling/misses effect i'm describing is similiar to driving away while the bike hasn't warmed up fully yet (only mine happens wheter fully hot or cold). i always questioned if a small piece of my new blue clear gas hose fell into the main "T" fuel inlet between the middle carbs. i had to cut the hose off with a razor prior and i seen a sliver or hose (about the size of a needle tip 2-3mm's long) fall around OR IN the "T" inlet. because it's gas hose i dont see it melting any time soon even though it's tiny. i already have 2 of my fuel metering(EPA preset) screws turned 3 1/2 turns & the other 2 at 3 turns out. the ceramic on the plugs is white, while the electrodes look very good in color. the plug shell has some dry black carbon but looks good. like you mentioned earlier, my intake boots could be a problem.
 
Hi. I don't think a small piece of fuel line would give you an intermittent problem. If the fuel passage was clogged by it, it would most likely stay clogged. When going through the carbs, I would blow out all the passages with compressed air, 200 lbs if you have it.
To ride your bike 150 miles with no problem and then have the problem show up the next 75 miles is interesting. Did it do this while cruising? What rpm? Did it happen suddenly with no change in throttle, or did it happen when you changed throttle position or came to a stop? Does it happen with petcock at "on" and also reserve? A problem that comes and goes like this may never show itself.
Rather than spending time riding/testing, I would take that time and re-check the carbs. You said they were clean, etc, but you also said the problem started after the carbs were serviced. There are a lot of things you can do wrong or miss when doing carb work. I would focus on the moving parts like the throttle valves, the floats and float needles. The carb synch' may help. Check the throttle valves for any possible "sticking". I would also put the mixture screws back to where they were. Adjusting these richer as you said you did, is just trying to compensate for the real problem and may make it harder to find the real problem. It may help to temporarily remove the fuel filter and just run a straight line for now and see what happens. Do you have the correct size fuel line? I use 5/16".
By the way, about your mentioning your rubber boots/manifolds, if you had an intake leak, your idle would be higher than normal once warmed up. A serious leak would result in an rpm increase of 500 or more.
Well, let us know how things go.
I'm really kind of out of ideas. I would rip into those carbs.
 
engine misses

engine misses

hi kieth. i may be onto something. yesterday after about an hour of riding the bike acted up again at about 3000-4000 rpm's. i believe it did as i slowed down coming into a town(using 3-4k rpm range). i really dont notice it when cruisng above about 4000rpm's. happens regardless of petcock position. i started pulling spark plug wires and listening/running bike. finally i twisted all the plug wires in the opposite direction that they were and drove away. i sware the bike accelerated harder and the misses/stumbling cleared up. when i hammered it i could feel the fork tubes top out- never felt that before. possibly 1 or more wires is shorting or internally a wire is breaking and i have it bent such that it's joined for now. i'll have to let the next couple rides determine if it's temporarily fixed and then decide on the dyna coils. i read that with the dyna coils("dyna s" i believe), that you don't need the ignitor.is that true? i'll probably ride today and post results later. thanks again for all your help kieth.
 
engine misses

engine misses

hi kieth. i may be onto something. yesterday after about an hour of riding the bike acted up again at about 3000-4000 rpm's. i believe it did as i slowed down coming into a town(using 3-4k rpm range). i really dont notice it when cruisng above about 4000rpm's. happens regardless of petcock position. i started pulling spark plug wires and listening/running bike. finally i twisted all the plug wires in the opposite direction that they were and drove away. i sware the bike accelerated harder and the misses/stumbling cleared up. when i hammered it i could feel the fork tubes top out- never felt that before. possibly 1 or more wires is shorting or internally a wire is breaking and i have it bent such that it's joined for now. i'll have to let the next couple rides determine if it's temporarily fixed and then decide on the dyna coils. i read that with the dyna coils("dyna s" i believe), that you don't need the ignitor.is that true? i'll probably ride today and post results later. thanks again for all your help kieth.
 
Hi. Hope you have found the problem. I can't answer your question about the Dyna coils and getting rid of the ignitor box. I think I have heard the same thing though. Dynatek is right out here where I live. They are probably on the web, but if you need their phone # I have it in the garage.
 
engine misses

engine misses

i did about 6 hours today and the problem didn't show up. i'll be convinced after 2 or 3 more rides. my carb tune II should be here in about a week also. i'll keeping posting updates in the meanwhile. the full dress tourers are catching my eye again(had an 88' goldwing in 88'). what could this mean for the "gs" !!
 
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