diagosis - junk

Sal Paradise

Sal Paradise

Hooligan
I have an old bike (1981 GS250) with 5k miles which has been a totally excellent runner. Bike starts up and idles perfect always. Low end carb performance feels good.
You start out and the bike runs awesome,pulls hard for a 250. But warm up and get up to 55 mph and it starts to buck and hesitate.Slow down ( to 35) and she smooths out.I rebuilt and adjusted the carbs 10 times. Upsized the main jets. No effect

I suspected that the float valve needles might be worn out because the bowls were really full. But raising and lowering the float leavels has no effect. carbs are shiny clean inside. I put an in line filter in and its clear. .Plugs look great. No smoke. Tested for intake leaks by spraying with wd 40 on the boots. - negative. Needles and tubes look good. Diaphragms look good. Synched carbs last night.

At 60 at WOT its like someone is turning the key off and on.Back off the throttle and it smooths out.
Bike overal feels like its got even less power than usual.

Any ideas what else to check?
 
Oxblood

Oxblood

750cc
I had the same bike that did the same thing around 40 I sold it to a buddy who got it running. I'll see if I can get a hold of him and see what he did to fix it.

--Oxblood

Damn this is bugging me, I knew what was wrong but for the life of me I can't remember what.

Another side note: This place is good for these bikes and will probably be able to answer your question if I can't get ahold of my buddy. http://www.thegsresources.com/_forum/login.php?do=login
 
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sweatmachine

sweatmachine

moped
what about downsizing the main jets? Idle mix too rich? Check all elec connections, cover with dielectric grease...the checklist is long!
 
Nick Morey

Nick Morey

Rocker
This may be off the wall but, you mentioned float bowls being really full, seems if I've heard of floats getting saturated or "fuel logged" so they don't float right. Now if there's some vent to move a diaphragm ie. accelerator pump or whatever in the float bowl that is getting plugged with fuel... Seems as though the floats can be checked to see if they stay bouyant in fuel to at least the halfway point of the float. What the hell, its worth a shot.
 
B

Blackston

750cc
This may sound really dumb but trust me it happens...have you checked the position / condition of the air cleaner element?...sometimes depending on the element, they can be reversed in the airbox and you will get enough air for the bike to run at low speed but as soon as you open the throttle to feed more air to the carbs it over fuels and boggs down. Just a thought, like I said it does happen and the same thing can happen when the element is chokkers with crap...I'd put your floats back to standard height level with the carb bowl mating surface and return your jetting and needle and clip settings to standard so that you have a common reference point. Then only change things one at a time to see what effect you are having. Hope this helps...

Cheers...

Bret....:)
 
Sal Paradise

Sal Paradise

Hooligan
Thanks. I am going to try and order a rebuild kit for the carbs with all new gaskets, and float needle valves, etc.

I do change only one thing at a time, good advice there - but extremely time consuming.My neighbors are sick of seeing me going up and down our road, it takes 15 minutes just to get it to start bogging.

I opened the gas tank vent already.Just went out to my garage and I see the bike is now leaking gas (probably bowl gaskets or sticky float valves) so the carbs are coming off again tonight after work. hopefully I find something obvious, like the needle valves are shot...and changing fuel levels are fugging up the mixture......
 
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Sal Paradise

Sal Paradise

Hooligan
Driving in to the office, it occurred to me the leaking float bowl and high fuel level in the bowl might be making the bike run super rich.

I noted the left bowl leaking, checked and its tight so will check the float levels again.

I think high fuel level in the bowls is consistent with the bike running really well for a few miles and then getting worse. I think running on pilot circuit and a small amount of main needle its okay but when I really open the throttle the needle comes up and it is sucking up excess fuel.

At least thats my current theory.

Electrical is good on this bike, but this is definately the time to check connections and I will.



Oxblood - was it the vacum petcock? Those are noted as being PITA
 
D

drlapo

Hooligan
have you checked the sparkplug color following the "misfire"?
unfortunatly a lean mixture will aslo result in blackened plugs aa the mixture (too lean) will not burn
but at least this will indicate an electrical or fuel problem: sooty dark plugs= mixture problem, wet plugs= electric
 
Sal Paradise

Sal Paradise

Hooligan
plugs

yeah, plugs look good. I did a plug chop and got one perfect plug and one slightly white or lean. Thats why I went up one jet size and synched the carbs.

I found out with this bike at least, good looking plugs dont equal good preformance.

I have to say, this GS does not seem to have the electrical problems that others have.

Maybe because its never been stored outside and it has low mileage.
 
Sal Paradise

Sal Paradise

Hooligan
I have not. Thanks i will check it after i lower the float level.

seems overall the ignition is good. always starts and idles great.
 
D

drlapo

Hooligan
weak plug cap resistance will show up at higher RPM: it screws up the advance

does this bike still have points? if so, do you have a broken or weak advance spring? or maybe the cam is stuck(points cam)
 
B

Blackston

750cc
drlapo said:
weak plug cap resistance will show up at higher RPM: it screws up the advance

does this bike still have points? if so, do you have a broken or weak advance spring? or maybe the cam is stuck(points cam)
Click to expand...

Points...Mmmm...Condensor

Cheers,

Bret...:)
 
Sal Paradise

Sal Paradise

Hooligan
Thanks.

This bike has electronic ignition.Last night I looked at the advance unit, which is esentially a pair of weights and springs. It was in perfect condition, like just about everything this bike. I gave it some WD40.

Did a plug chop again - perfect of course.

I was able to download the service manual for the bike for free. Today I will test the coils and check for 12v power to them .At the same time Iwill check the plug wire resistence.

If I don't find anything obvious like a bad coil I will rip the carbs off for the 100th time and put in new pilot jets, readjust the floats again and blow everything out, again. I lowered the float level and the problem is now more in the transition between pilot jet and needle. Starts off good, then stumbles and hesitates at about 1/4 throttle, then its off to the races. The tank and carbs are super clean already but when you have an oldy like this, you always tear the carbs apart whenever it doesn't run right.
 
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wolfie

wolfie

Two Stroke
From your description, it sounds like you may be dead on with the idea of it being an inlet valve, or bad float height.

Ditto on the advise to ask at www.thegsresources.com . There are a lot of very knowledgeable folks over there who know all the ins and outs of just about every gs. There is also an excellent photo tutorial on how to rebuild mikuni carbs.

I wonder what kind of shape your carb o-rings are in. Specifically the inlet o-ring on the side with the high fuel level.

If you haven't replaced them, http://www.cycleorings.com/ is an excellent source for carb o-rings. I've purchased from him probably half a dozen times, and always been satisfied.

Do your inlet needles have rubber tips, or are they steel? I've seen both on the GS's. Either way, I would think that any excessive wear should be visible.

Something else to consider, is that if you are having fuel height issues, you likely are dumping gasoline in your crankcase. It wouldn't cause your issue, but it's something to check. You'll probably want to change the oil once you have it sorted.
 
Sal Paradise

Sal Paradise

Hooligan
leftside plug cap resistance 247,000 ohms.

Final verdict on this - the floats were too low causing a low speed bog. I was over correcting, adjusting by eye.

The whole thing started with the bad plug cap.It was a hot day, it must have fried. That was the high speed miss. That caused me to start chasing ghosts in my carbs, and i lowered the floats...too low .....which caused the low bog.

All fixed. Thanks. drlapo I owe you a beer..
 
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