I want to correct hanging idle and bogging at 1/4 throttle

[lucas]

Hey all,

I recently got a '91 GS500 running (my first motorcycle!) and I've tested it around my neighborhood.  There is a lot to do, I'm sure, before I have it running smooth like a top.  I have done a fair amount of reading on both of these topics on the forums and I figured I should make a thread about my case.  There are so many variables... if you have experience on out how to solve these issues I'd be grateful for your help.

Issue #1, the rpm's will remain high after revving the engine or when putting the clutch in while slowing for a stop etc.  The revs will stay at 3000 or more for I-don't-know-how-long, forever I guess, at least 15 seconds.  I can apply the clutch to drag the engine down to about 1800rpm and the revs with then drop to 1400.  If the revs are between 1800 and 3000 the engine will promptly speed up to 3000.  Blipping the throttle just adds rpm, a small blip will make it rev at 3300 a big blip will make it hang at 4000 or more.

Issue #2, the bike will bog when I open the throttle more than about 1/4.  The tone lowers and that means.... running rich, right?  The engine does not recover and accelerate normally but will bog for, again, forever I guess.  I have a long gradual slope in my neighborhood and only having 1/4 of the throttle to work with meant the engine was actively or on the verge of bogging the entire way. 

What I have done so far: Rebuilt the engine completely so literally everything is suspect, checked for air leaks using WD-40, checked the float bowl levels using clear tubing, gave the carbs a second thorough cleaning, checked the little O-rings were in place in the plastic carb caps, checked the slides drop with the same speed when lifted manually, bench sync, turned idle screw down,

What I should do: check the valve shim clearance, check compression, check the timing, finish syncing carbs, adjust idle screw some more, check the carb diaphragms for tiny holes or tears

Other things that might be relevant: The idle mixture screw covers haven't been drilled out, so those haven't been adjusted by hand.  The exhaust I got with the bike had some like rocks and dead grass and spiders in it.  The carbs I bought for this bike had lots of sticky gross old gas or oil covering it - I got as much as possible but there are areas that are not accessible by hand.

Lucas

[Blueknyt]

it sounds like you still have induction/ mixture problems,  this could be intake boots and their O-rings, could be boots themselves, could be carbs are outa sinc or all the above,  i might have missed you saying if airbox as there or not. but if you took the airbox off you need to rejet, if you changed exhaust you deff need to rejet.

stock jets for your year IRC is 37.5 pilots and 127.5 mains,  bumping the pilots to 40's will be a great help even with airbox,  boging past 1/4 could be a sign of massively over rich (black smoke out exhaust) or kinda lean (but gen has some coughing through the carbs)  check to see if a main jet didnt Fall off (it happens) with airbox go to a 130 main, lacking airbox about 140 mains, aftermarket exhaust and airfilter about 150 mains

try this for starts, i dont sync my carbs with tubes and water like so many do, and being as it only effects upto about 1/8 throttle if even that far.  I use a large sewing needle as a feeler gauge.  i open the throttle up with idle screw till the needle just slips in between throttle plate and carb body, then adjust the other side to match with the needle. bump the throttle open and let it gently snap close a few times and recheck. 

[lucas]

After the most recent carb cleaning i was running it without the stock air box... I was thinking it would be a waste of time, that I was just going to have to take it out again.  Last last week I had the air box on and it behaved about the same.  I'll replace the box and try again.

I will check for the exhaust color and listen for the coughing.

I have sprayed the intake with wd40 and I can't detect a change in the idle, is there another test to check for leaks?  I hesitate to use carb cleaner because I am under the impression that it deteriorates rubber parts.  The intake boots do show cracking around the tab area.

Thanks for your time and advice.

[Blueknyt]

choose how your gonna run it, with stock airbox and filter, airbox and drop in aftermarket (K&N emgo, V&H) or straight POD/sep filters.   stock airbox and filter actually richen the mix. so runing without the airbox or without the filter your running lean for both jets.   40 pilots should be installed anyway, faster warmup and i feel better response just off idle.  they can be used with the stock box and filter.   

set your petcock to prime, and block off the vacume hose to the petcock just to make sure thats not a Vac leak or sucking fuel past a ruptured petcock diaphram.  WD ive never found to work that well, ive used soapy water , starting fluid, and even propane from a torch,  torch worked the best specialy with tubeing pushed onto the end, other end gets into tight places (just in case, NO you do NOT light the torch to do this)

[lucas]

Cool thanks for the tip about the jets.  I'm still learning about carbs, I'm still kind of baffled by how they work! 

I have a propane torch, I like the advice.  Thanks for reminding me not to light it lol.

I have a sickening feeling that the vacuum line to the petcock might be the problem... see, I don't remember installing it at all.  I'm going to check it when I get off work tonight.

[Blueknyt]

you should check to make sure the carbs are properly sync'ed up. if one is open more then the other you wont be able to pin down some of the other issues that could cause simular symptoms that seem to change for no reason.

so in short
Sync carbs: manually is fine
figure out what your perm induction filtration is going to be, jet accordingly (this includes stock exhaust or slipon or aftermarket). 
plug all vacuum lines, run on prime check for vac leaks with propane (as less wind as possible).
if you want a break down of jetting i can give you ballpark sizes for setup, but there is always alittle wiggle room 

[lucas]

OK, I checked the vacuum line to the frame-mounted petcock and it is in place and doesn't appear to be cracked.

Carbs are bench sync'd, stock airbox is back on, 37.5 pilot, stock exhaust

Please explain a little more about the testing you are recommending.  Should I take the petcock vacuum line and bend it back on itself?  When I block it off what should I look for should the engine die or run better or something else.  Maybe there is a thread that has the instructions for this laid out but I'm not really sure what keywords to search for.

[Blueknyt]

2 problems with the vacuum line for petcock, one could be a source for a vacuum leak causeing one carb to run leaner then its buddy. second is the diaphram inside the petcock is actually a vacuum pulse activated fuel control valve. Meaning vacuum is applied on the back side of a spring thats holding the valve in a closed position. when the vacuum forces the spring to collapse the valve moves and fuel flows, when the vacuum is removed (I.E. shut the engine down) the valve closes so fuel doesnt continue to flow.  sometimes the diaphram will get worn out and leak fuel to the vacuum side and get sucked into the carb via the vacuum line makeing it richer then its buddy.  so, one way to get both of those possible issues out, disconnect the vacuum line from the petcock and stick a screw/nail, screw driver or even a real vacuum port block off cap (auto parts stores have em in help isle) something in it to block it off. now the propane torch, i like to push a bit of clear vinyl hose onto the torch, open the valve and snake the hose end around the carbs and anything else that might suck air. (give it a count of 5 mississippi for the hose to purge the air before fishing for vacuum leaks) do this while the engine is running.  If there is a vacuum leak, the propane will find it and the engine will change tempo. propane almost always finds my leaks where spray liquids did not. it just can't be breazy out and like i said, the tubing really helps to get in close to potential leak area's that a torch body cant get.   

just a keep an eye out tip, when you run the bike in prime, look at the vacuum port of the petcock (hose removed) and see if it weeps fuel   BTW this is the frame petcock not the tank shut off.