Newbie: Stutter at WOT above 5Krpm/backfires when trailing.

[Anonymous]

I love my "new" '82 GS850G with 13k mi. She is such a sweet girl. However, when I wanna go fast she does this quick stutter, almost an extreme vibration in the front, with wide open throttle when I get anywhere above 5000 rpm. When this happens she also looses most of the power. I can coax her up through the rpms at less than WOT> As long as I don't go wide open, all is fine. It really is a rapid stutter, not a surging at all.
Also, when I close off the throttle at anything above, say, 4k rpm, she back-fires a bit.
She has some kind of after-market pipes on her, which are too loud for my taste, so I am saving up for something more like original.
Otherwise I think (!) she is stock.

Please help!!

Sincerely,
Soeren

[Anonymous]

Soeren Backfires

If the carbs have not been cleaned in a long time this should be done.Make sure you change all the O-Rings.Next take off the intake boots and if they are hard and dry change them and put new O-Rings on them.Make sure you sync the carbs. If you have a stock air box this should be cleaned up also.The most important thing is to have no air leaks.This I have found out the hard way.I also would say unless you know alot about bikes stay with stock parts.Our bikes are fast enough without change anything on them. hope this helps. One last thing TAKE YOUR TIME.

[tfb]

G'day Soeren,

The 'backfiring' you describe is probably just due to the temporarily lean running when you back off the throttle; you can read about the theory of why this happens at http://www.mikuni.com/tg_backfires_in_exhaust.html, but the important thing to notice that it isn't a sinister thing necessarily. If you have a throaty after-market exhaust, you can expect this lean-sounding 'popping' to happen.

Re. the bike being choked over 5K, yes, clean the carbies. You probably have filthy main jets or something like that.

Mike.

[Anonymous]

soeren backfires indeed...

Thanks to fogey and sfb.
I am reluctant to clean the carbs as I've realized, over years of ruining things, that I am rather ham-fisted with auto and mc repairs. One would think there'd be a gas additive that would fix slightly dirty carbs, much like there is for injection systems?
Maybe I'll start with doing the easier stuff, like plugs, plug-wires, clean the filters, then maybe graduate into changing those carb boots and o-rings.
Does this make sense?

Also, I took her out for some superslab speeding the other night and once she gets over that stumble/shutter at 5-6k rpms, she opens up wide and clear, and really takes off.
:twisted:

[bjrehill]

Hi Soeren,

It does sound like the carbs to me. The RPM's are less important than throttle position for diagnosing carb problems. As far as skill is concerned you just need to take your time. The carb cleaning series here is great. Just let the carb cleaning spray do its job before trying to use strength unscrewing all those screws. Also, if you don't have one already get an impact driver. You'll need it if you have any phillips head screws still on the gang plates.

Just review the carb cleaning series and have it in front of you as you work.

[Anonymous]

stumble

Thank you, I guess I'll try doing the carb thing. Is there a standard set of o-rings and/or gaskets I should get before starting? Will I need nything else than an impact screw-driver and your usual basic set of tools?
Also, I took the spark plugs off yesterday, and three of them have a lean condition with light grey dusting, while the fourth was rather wet and dark. What's up with that?

Thanks,
Soeren

[tfb]

1. If the plug is wet and dark immediately after the engine has been running, then it's wet because of excess fuel and dark (sooty) because of a rich mixture. In this case you will simply need to attend to carby jetting and cleanup.

2. However, if it is wet and dark after the engine has cooled down, then it's wet because of oil and dark (sooty) because of burning oil.

If it's the second problem, then it could have a number of causes: (i) cracked piston ring, (ii) split valve stem seal(s) and/or worn valve guide(s) for that cylinder, (iii) scored bore, (iv) incorrectly installed oil expander ring.

If it is indeed excess oil consumption for that cylinder, the best thing to do would be to pull the head and cylinder block off, and take things from there. Not a great scenario, but it may be what you have to do.

But first things first: get your carbies cleaned and maybe fiddle with the jetting to correct any lean-ness, and see how the plugs look then.

All the best,
Mike.

[Anonymous]

burning oil? nooooooo

Well, TFB, your response fills me with worries about oil burning. I think it was still wet'ish after having sat for a while. Something else I just noticed, there is too much oil in her Why the h^#%$ would the PO put too much oil in there It's not way too high, but well above the top of the little window down low on the clutch cover.

Anyway, where do I get the carb kits, etc., that I'll need to clean the carbs?

Thanks everyone! \/

[Anonymous]

For my purposes I used www.bikebandit.com and just bought the gaskets and o-rings needed. I also soaked the floats, jets, and bowls in a chem dip similar to Berrymans, the Gunk brand, found at your local Advanced auto parts.

But again this is what I did and I am sure someone else on hear will have some more sound advice!

Good luck man! :mrgreen:

[tfb]

G'day Soeren,

Mate, believe me, I hate to be the bearer of bad tidings... I remember the day only last year, when my heart sank as I looked down the spark plug holes to piston crowns with so much oil on them, that I could see my reflection , no joke.

The best way forward is to be brave, bite the bullet, etc. etc. but not to jump hastily to conclusions. Whatever happens, it sounds like you need to sort the carburettors/ fuel jetting. So, do that first. Then see how the bike runs. I mean, I would hate to see you rip your whole top end apart, and then find out that it didn't really need it. :roll:

Having things too full of oil definitely won't help. So let's do first things first.
1. Clean your air filter.
2. Get the oil to the correct level.

Then see how she goes. If still symptoms of trouble, then get into the carby cleaning etc.

If still looking dodgy, then go for pulling the top end apart and systematically checking bores, pistons, rings, valve seals, valve stems, etc.

All the best for sorting her out.

Mike.

[argonsagas]

Yes, there is a simple technique, very similar to adding fuel injection cleaner to other engines.

In fact, that's it!

Add fuel injection cleaner (NOT carburetor cleaner or gas conditioner) to your fuel tank and run the bike until it nears empty. Repeat if desired.


This is nothing near as good as a full carb clean-up, but it may make enough difference to keep you going until you feel ready to tackle the full job.

[Anonymous]

new pic!

Thanks all, youa re the best!! 8O

So, I'll clean and re-oil the air-filter, change oil and oil filter (I am starting to worry about the PO, since he had too much oil in there, who knows what oil is in there!). Then see if that helps at all. If not, maybe I'll try the "easy way out" and add some injection cleaner to the fuel and run a while like that.

The thing is, she runs real smooth all of the time except when I try to gun her. I guess I should also worry about the lean condition, however I can do serious damage that way, no?

Thanks. I am slightly more heartened(!) now. 8)

Oh, and I get a new rear tire tomorrow!

[argonsagas]

Re: new pic!

Thanks all, youa re the best!! 8O

Thanks. I am slightly more heartened(!) now. 8)

Oh, and I get a new rear tire tomorrow!

WASH THE TIRE!


Better still, SCRUB IT with a brush, using lots of soap and water.

I may be incorrect, but you appear to be still new to motorcycling. Tires are VERY slippery when new. This is due to the release compund used when the tire is made, and still on the tire when you buy it.

Many shops will do it for you, but some do not.

New tires must be scrubbed before riding.

[Anonymous]

update

Well,
I got the new tire installed, but they screwed up my rear master cylinder, so now I am waiting on the little rubber thingie that should fix that problem. I can't imagine how they messed up my brakes by putting on a new tire, they oughtn't even come close the master cylinder, or?
Oh well, I should most likely do the front one as well, I'd guess.

I siphoned off motor-oil to see how much the PO had overfilled it. About a quart!
So I changed the three oils, and I feel better. Though the bike still runs not completely smooth, and does that hesitation thing at around 5-6Krpm.
Next I'll clean the air filter.
Oh, and I installed new plugs. The PO had 7es in there, adn I put 8es in instead, as prescribed by the manual. Made no difference to how she runs.

In any case, I am enjoying the great weather, and riding when I can.
More later,
soeren! 8)

[Anonymous]

Re: update

I siphoned off motor-oil to see how much the PO had overfilled it. About a quart!

Sounds like your PO checked and added oil with the bike on the side stand instead of the center. :?