My GS850 Bogs down when accelerated quickly

[Bob Gibson]

Symptoms are:
MPG is around 20 to 25.
Whenever I have the bike running smoothly between 1500 and 3000 rpm and then try to accelerate quickly the bike bogs down. However, if I slowly accelerate, the bike will continue to accelerate until rpms are at about 5000 to 6000, then it starts to bog down regardless of what I try. Climbing moderate freeway grades or freeway on-ramps is just about impossible now. I’ve put only about 1000 miles on the bike.

History:
I bought the bike about a year ago. It ran well but bogged down slightly above 7500 rpm. No problem climbing onto the freeway or climbing grades back then. After filling up the tank a second time (the same day I bought it) I realized it was getting only 25 mpg.
When the rpms are high and I let off the throttle, I get a lot of backfiring through the mufflers. When revving the engine, both exhausts appear to put out similar pressure.
The mufflers appear to be stock along with rest of the bike, but the owner that I bought the GS850 off of, stated that a previous owner had done lots of top end work to race it (different carb jets?). Again, it “appears” all stock, however idler air screw caps were removed sometime in the past.

Observations:
Bike is in good shape with about 40,000 original miles.
Cold or warmed up, bike starts quickly (within a second!) when the choke is pulled out. When it’s warm the choke is really not needed, just a habit, but the bike runs better at low speeds when choke is pulled out slightly (can’t get to high speeds!).
Accelerating from zero to 30mph, in first gear, is rapid, but then the problems (bogging down) start and continue through all gears. 60mph is about my max speed right now, if I’m slow to get up to that speed. Note that I had the bike up to 75mph (passing another vehicle) on the way home from buying it with more power to go.
Standard engine compression test show cylinders 1 through 4 to be 115, 105, 90, 90. I did a wet test and the compression changed to 115, 110, 100, 115. I guess cylinder #4 may have worn rings, but I have not checked the torque on head bolts or ruled out any other possibility yet. 90 psia on both chambers left me thinking I need to do more diagnostics on that side of the engine.
There is a very slight amount of smoke when I first turn it on, but I mean slight. It’s not visible unless I start the bike inside my garage backed up to a wall.
Previous owner replaced the original air box with pods (Darn, I would like to get back to the original air box).
Brand new spark plugs out of cylinder 3 and 4 appear slightly Black with only about 100 miles of use. Plugs 1 and 2 appear clean.
I cleaned out the air pods with KN cleaner and re-oiled the pods about 5 months ago when problem first started. But my air filter cleaning and oiling seemed to just make it worse.
I don’t know what the fuel jet sizes are.
Help!
Bob Gibson

[ktp1598]

Tank vent clogged? Does it hiss when you pull the gas cap off?

[hampshirehog]

Hi Bob,
your compression figures aren't wonderful but she should still run ok. Have you checked your valve clearances? That's a must on these old gals and until there A OK you could be chasing shadows.

If your bike is running on pods you need to check that the carbs are set up right. Do search on here for 'plug chop' (if you've never done one before) and it'll tell you how it's done.

Plus what year is your bike? 1980 onwards had CV carbs, 79 had VMs and they function in very different ways.

[almarconi]

Sounds to me from your mpg figures like your having some carb issues.

When was the last time the carbs were completely dissassembled and cleaned? You said the bike runs better with the choke on is probably due to a dirty pilot circuit.

Since the bike has pods it will need to be rejetted. The pods flow way more air than the stock airbox. You really need to take the carbs apart to see what size jets you have in there now. If it was my bike I'd look for a stock airbox. From what I've read on this forum, the 850 is hard to tune with pods.

The back firing of deceleration could be due to valve gap and ignition timing issues. Have you checked your valves and verified ignition timing?

I would rather start off modifying a stock engine that was running well than trying to get a modified engine to run well because you don't know whats been done to it.

[GavinJuice]

First off you should state the year of your bike.

well i can tell you right out that your running lean. the backfire that you're experiencing is called a lean pop when you let off the idle at high rpm's. when your choking the engine to keep it running smooth your closing down the air supply to match your lack of fuel flow.

SO, starting with fuel flow you could have a petcock that isn't flowing enough, the rubber begins to break down. You can attempt to run engine in pri on petcock and see if that makes a difference, or remove the line and watch the fuel flow. Vacuum actuated petcocks you will have to remove the vacuum tube and suck on it when the engine isn't running in the on position. But you might not have one? Look for an inline fuel filter causing an obstruction also.

Now your issue with air flow makes things worse since air pods flow MORE air. Running air pods you have to richen up on the carbs to accomodate the increased air being burnt. You have temporarily done that with closing down the choke to bring that to a balance.

Your plugs indicate more blowby on cyl 3&4 along with burning rich. Whats hard about this situation is you don't know what kind of "race work" was done on this engine, whether is be valves cam or porting for head work. Any way you look at it, your engine is running lean in all areas, fuel flow could well be affecting that. This might also be a guessing game with jets to richen up your fuel mixture. What I would do is pull the carbs and see what has changed with jets.

It seems to me your carbs are clean since your condition seems to be affected by all the carbs. I think if you read equal cylinder pressures when you did your compression test your plugs would also indicate that.

I imagine a whole slew of more info is on the way.......

[Big T]

First, go back, pull the pods off and clean them again. It sound like you over oiled the pods and that's choking the air flow. Just put a little bit of oil on each pleat, it should take a minute or so to soak across the pleat. It's easier to put on less oil and add some rather than too much and start over.

Then, adjust your valves. That will help, if not cure, your compression issues. See the BassCliff website.

Find a stock airbox - if you want to keep pulling the carbs off multiple times, jetting for your pods will keep you busy doing that. With the stock air box, you can set your carb jets to stock and actually get some time to ride your bike. Check the For Sale section or ebay or local wrecking yards

You should check out the Carb Clean up page. Order carb O rings and heat to boot O rings from cycleorings.com and pull those carbs off and clean them thoroughly.

You don't mention what year 850 you have - early ones have different carbs.

Make the assumption that every one who worked on the bike prior to you is clueless. The prior owners assertion that it was raced is a good indication. Racing a shaft drive touring bike is beyond futile.

Post up some pictures and Keep posting up your progress here, there's a lot of 850 gurus on the board.

[Dave8338]

First off you should state the year of your bike.

well i can tell you right out that your running lean. the backfire that you're experiencing is called a lean pop when you let off the idle at high rpm's. when your choking the engine to keep it running smooth your closing down the air supply to match your lack of fuel flow.

SO, starting with fuel flow you could have a petcock that isn't flowing enough, the rubber begins to break down. You can attempt to run engine in pri on petcock and see if that makes a difference, or remove the line and watch the fuel flow. Vacuum actuated petcocks you will have to remove the vacuum tube and suck on it when the engine isn't running in the on position. But you might not have one? Look for an inline fuel filter causing an obstruction also.

Now your issue with air flow makes things worse since air pods flow MORE air. Running air pods you have to richen up on the carbs to accomodate the increased air being burnt. You have temporarily done that with closing down the choke to bring that to a balance.

Your plugs indicate more blowby on cyl 3&4 along with burning rich. Whats hard about this situation is you don't know what kind of "race work" was done on this engine, whether is be valves cam or porting for head work. Any way you look at it, your engine is running lean in all areas, fuel flow could well be affecting that. This might also be a guessing game with jets to richen up your fuel mixture. What I would do is pull the carbs and see what has changed with jets.

It seems to me your carbs are clean since your condition seems to be affected by all the carbs. I think if you read equal cylinder pressures when you did your compression test your plugs would also indicate that.

I imagine a whole slew of more info is on the way.......

Though I agree that a lean condition is a likely cause of the bogging/popping condition, with the CV carbs you are enriching the fuel circuit on choke, not closing the airflow, as you would do on a conventional carb set-up. Just a heads up.

[49er]

Firstly, welcome to the GSR.

Your #1 priority should be to check the valve clearances. The compression variations that you have can sometimes indicate tight/non existant valve clearances.

Next, check that you have no air leaks on the induction and exhaust systems. The intake boots become hard and crack, and the mount flange o-rings need replacing every 10 years or so.

Save yourself a lot of agro by finding a stock airbox and tuning the carbs to stock jetting and settings. The symptoms that you describe under certain throttle loads indicate that the main jet is too lean with the pods set up, and maybe the needle position is lean as well. But, at 20 mpg, the engine is consuming double the fuel of a stock 850. That would suggest that you may have radical cam timing /cam lifts/porting as well. Your plug reads are also inconsistent.

If your exhaust pressures are consistent, your pilot circuit is not likely to be clogged. It should stumble/bog down when coming off idle for a pilot jet problem.

If it was mine, I'd check the valve clearances, and strip and clean the carbs, taking care with cleaning all the small passages and jets. Take a note of all the jet sizes so that you know exactly what you're working with.

It should be an interesting project. You'll get it sorted, but you may need to think outside the square, if it's been radiacaly tuned.

[BassCliff]

Greetings and Salutations!

Hi Mr. Bob Gibson,

Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. There's lots of GS850G lovin' there.

Don't forget, we like pictures! Not you, your bike!

Follow all the great advice you've received so far; get the carbs straightened out - cleaned, float height adjusted, insure there are no intake air leaks (carb boots and intake o-rings), adjust the valves, clean up the electrical, etc. There's a checklist in the mega-welcome. Thanks for joining us. Keep us informed.

Note: If you dump a can of Sea Foam in your next couple of tanks of gas, it might help clean the carbs. But it's always best to take them apart, dip them, replace the o-rings, bench sync, then adjust the valve clearances before you perform a vacuum sync on the carbs. Run a dedicated ground wire from your regulator/recrifier directly to the negative terminal of your battery. CLEAN every electrical connection and ground from the headlight bucket to the tail light. Click here for information about stock GS850G CV carbs. If you plan on keeping the pod filters, you'll have to use larger jets to compensate for the increased air flow. The carb gurus can help you with that. I'm just the welcome wagon.

Thank you for your indulgence,

BassCliff

[Bob Gibson]

Thanks for your help - more info

Thanks so much for all of the advice and information/links so far.



Sorry for not having the year of the bike on the original note. It’s a 1981 so they are CVs carbs. I did notice a detailed photo-by-photo on CV carb rebuilding on this site (Thanks to whoever did this), but wanted to ask all of you for your opinions and advice prior to rebuilding carbs, just in case that was not the problem.

Now to answer some more of your questions, to get me closer to a possible solution and plan:
Yes, sometimes when I take off the gas cap it does hiss (like it’s pulling a slight vacuum). I have not removed and checked the pepcock, but the fuel lines and vacuum line “looked” in good condition a couple of months ago when I checked. Previously, during the holidays, I did remove the fuel lines and turn the valve through its positions. With the amount of fuel that came out, I didn’t think there was a blockage out of the tank. I’ll investigate further (pepcock, hoses, hose connections, etc...).

I also figured I needed to adjust the valves, but thought that could wait until I got the bike back to the condition when I bought it (1,000 miles/year ago), since I haven’t adjusted these at all. I just didn’t want to work on something that I didn’t think caused the recent problem. I was kinda surprised when the compression for 3 and 4 were exactly the same. I checked this a few times and actually thought my guage went bad. I wonder if the gasket may be leaking pressure between the two chambers, but not knowing the firing order and when the valves open and close, led me to work on this potential, later, however gaskets do fail sometimes. What are your thoughts? There’s a small Suzuki dealership near by, so I’ll order the gasket(s) next week to prepare for this job.

As for going back to the stock air box, absolutely! Actually from the day I bought it I’ve wanted to go back to the stock air box. I’ve looked around some local and some not so local bike part shops, but have not found one, though I’m not real sure what the dimensions are supposed to be (except for carb intake diameter). There was one advertised on e-bay back during the summer, but one of the bike shop mechanics I was talking to told me that there are a few different air boxes for the GS850, and so I decided to hold off until I knew what I was doing. Those pods just worry me.

Now, as for the air intake system, Could I do a quick diagnostic by spraying “ether” around the intake boots to see if the idle picks up? Or is that just for vacuum leaks on autos. Visually I don’t see any cracks, but my eyes have deceived me before.

As for the “racing” modifications, I sure don’t think so, but I figured I’d mention it since I’m not a bike mechanic. My automotive experience is from teenage years (long ago) when I use to build and race V8s. With stock exhausts on this bike, compression that does not indicate higher than normal compression (yes this may be misleading “if” ALL valves are set improperly), and no additional equipment that would indicate it’s been beefed up, not to mention, as some of you did, why would you race a drive shaft bike, led me to just assume this was a seller trying to make a sell. Once again, it sure looks stock to me which is a reason why I bought it.

As for those darn carburetor pods, yes the main reason I cleaned them previously, is that they had a lot of dirt on them (they looked fine when I bought the bike) and I did not want to find out one had a crack in the sponge (yes, I stated sponge, not pleat) that was going to plug the carb and then I'd be into big problems, like rebuilding carbs.. My secondary reason is that I was starting to lose some power at the higher speeds. Keep in mind my “higher speeds” means 75 to 80mph, not 100 plus mph. I thought the slight decrease in power was attributed to the dirt build up. When cleaning them, I did inspect them and all parts looked in good shape. I originally sprayed them slightly and the bike did not run real well, but this was just up and down the street on a cold bike (first stupid move). I then figured I didn’t have enough oil on the pods and read the KN kit instructions that stated to spray the pods until the red oil indicates a “lightly soaked” filter. So like a true idiot . . . Since then I sopped up all of the excess red oil over that first month time. Cleaning these pods is quite easy. Maybe I should have gone with my gut instinct and not sprayed them at all. Lesson learned. And yes again, they are sponge pods.


Note that the bike did run well, OK, good, when I first bought it. That’s the data point I’m working from, actually it’s my only good data point. Since I bought it I’ve added gasoline, changed the engine oil once, cleaned those darn pods once, and replaced the old plugs (old and new are NGK-B8ES) once.

Thanks again to all of you for your help.
Bob Gibson

[82Shafty]

Bob,

I too have raced many years with all sorts of vehicles, if it ran, I ran it was the mentality I had.

My story was I went through, did a bunch of PM to the bike, got it out the next season and now all of a sudden it was like she was losing power on the top end. Cutting out, popping, darn near felt electrical to me.

What I ended up doing was pulling the air filter out of the box (K&N) and ran it and WOW it came alive. Those CV carbs are so finicky its not even funny, it was actually too rich due to my oiling of the K&N. I've ran K&N for years and have never had any MAF problems, always have used a light oiling, you name it.

My suggestion is not to fret on it at this point. Spend the time, do the valve shim check. Then take the time to dip the carbs, check for any mutilations inside, do all new o-rings, and GET A STOCK BOX.

I'd still would like to throw mine on the rollers and dial in a 12.5 AFR under WOT.

[Bob Gibson]

Thanks for the incite. I took some pics to post on the site later (others were asking) and then took off the gas tank. All appears OK. Like you, I really think my major problem are those air pods. Since it's so easy to do, I'm going to clean the air pods later today and add very little to no oil and see if there's an improvement and then next week I need to better shim cylinders 3 and 4 (of course, I'll check 1 and 2 while I'm at it).

My only hesitation for going to the next step (rebuild carbs), is that I think it will lead to more "good to do" stuff.

Thanks again for your help.

[Suzuki_Don]

Hey Bob, how about an update on how things are going with the bike project.

[kilog55]

Firstly, welcome to the GSR.

Your #1 priority should be to check the valve clearances. The compression variations that you have can sometimes indicate tight/non existant valve clearances.

Next, check that you have no air leaks on the induction and exhaust systems. The intake boots become hard and crack, and the mount flange o-rings need replacing every 10 years or so.

Save yourself a lot of agro by finding a stock airbox and tuning the carbs to stock jetting and settings. The symptoms that you describe under certain throttle loads indicate that the main jet is too lean with the pods set up, and maybe the needle position is lean as well. But, at 20 mpg, the engine is consuming double the fuel of a stock 850. That would suggest that you may have radical cam timing /cam lifts/porting as well. Your plug reads are also inconsistent.

If your exhaust pressures are consistent, your pilot circuit is not likely to be clogged. It should stumble/bog down when coming off idle for a pilot jet problem.

If it was mine, I'd check the valve clearances, and strip and clean the carbs, taking care with cleaning all the small passages and jets. Take a note of all the jet sizes so that you know exactly what you're working with.

It should be an interesting project. You'll get it sorted, but you may need to think outside the square, if it's been radiacaly tuned.

so your saying when your are coming off idle taking off and you have a bog or stumble it is more then likely a pilot situation? cause my bike does this sometimes until it is warmed up from riding around for a bit.

[SqDancerLynn1]

I don't think you over oiled the pods. Your bike was probable never rejetted to work with the pods. Takes some duck tape & cover 1/2 of the filter see if it runs better if so it is too lean!! When the pods were dirty they flowed less air making the mixture richer.