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Bike runs flawlessly on full choke only

  • Thread starter Thread starter jakeh
  • Start date Start date
J

jakeh

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You guys may remember me as the guy who swapped in a set of keihin slide carbs into my GS650 a while back, I never had it fully operational until now because of very leaky intake boot which I has since replaced. Now I have an odd issue with tuning. First, let me say I know Im starting way out of the ballpark most likely by just bolting up these carbs from a 750cc bike, but Im making headway.

Heres where Im confused: The carbs are super clean, theve been run through one of those ultrasonic cleaners and all internals replaced. The old bike was then apparently jetted to run on pods and open headers. So my thought train is this: Bigger displacement on old bike = im getting too much fuel, pod jetting on old bike = getting way too much fuel, open header jetting = holy crap its a Niagra fuel falls coming into my engine. Never the less, I bolted the untouched carbs on the bike and fired it up on full choke. it idles great, throttle response great, even rode it and it blew my mind how fast it was, running GREAT. But, as soon as you let the choke off even a little, it dies.

I believe this indicates a lean condition right? BUT HOW THE HELL?????

P.S. Currently running pods and stock pipes. I will drill baffles if yall think it may help
 
If it runs fine with choke then, yes, this indicates to me that you are running lean. Sounds weird to me as well. Maybe some of the gurus can better explain. Do you know the jet sizing in your carbs?
 
The main jets are 115s also, I asume the screw I have pulled out in this pic is the mixture? Is that where I should begin to tinker?
 
fitting these carbs from a larger CC bike to a smaller CC bike you would still need to up the jets. the larger CC bike creates more vacuum on the inlet stroke , which draws more air in, which in turn pulls more fuel in through the jets. therefore your smaller capacity engine is not "sucking" in as much air/fuel as the larger CC bike.

upjet it.
 
jakeh said:
The main jets are 115s also, I asume the screw I have pulled out in this pic is the mixture? Is that where I should begin to tinker?
Click to expand...

I can't tell from that picture, is that "screw" inside the bowl? If it is then it's just another jet. If it's in a spot that you can access it when the carb is assembled then it's the mixture screw.
That's exactly where you should start. If it's already out a lot of turns then you'll want to change out your idle jet to start. Then report back.
 
Yes you can access them with the bowls on, they are all exactly 1 and 1/4 turns from seated. It seems theres plenty of thread to back out further...
 
jakeh said:
Yes you can access them with the bowls on, they are all exactly 1 and 1/4 turns from seated. It seems theres plenty of thread to back out further...
Click to expand...

Ok. Start backing it out, and making the jets bigger like Agemax said. I personally wouldn't go more than 3 or 4 turns from seated without changing out the idle jet.
 
It looks like in the pic supplied that the pilot jet in that particular carb doesn't screw out. But it is pressed in and non removable. The pic is a little blurry, but does it have a slot to remove it? If not I have taken a small vice grip down at the base of the jet and clamp it lightly enough to twist it back and forth enough to remove them. Even applied a little heat at the carb body to expand it a little. Then lightly ream them a little and insure that they are properly cleaned enough since the actual jet is so far up into it makes it hard to get it cleaned. If it needs to go bigger ummm you might could ream it a bit but risky with going too big. Are the pilot air jet removable if they are you can possibly reduce the pilot air jet size a little and therefore richening up the pilot circuit enough then you can use the mixture screws to get it where you need it.
 
UPDATE: backed out the mixture screws to 3 turns from shouldered, reassembled and test rode. Bike still only ran on full choke, but still had gobs of smooth power, did a plug chop, definitely running lean, white tipped plugs.

Yes, the pilots do not seem to be removable, If I remove them the way you said, how do I reseat them?

I some what understand carbs, but Im really at a loss here. Do I go with bigger pilot? Or bigger main? Both?

Ive never played with air jets at all, does anyone have any info on how to tinker with them?
 
jakeh said:
UPDATE: backed out the mixture screws to 3 turns from shouldered, reassembled and test rode. Bike still only ran on full choke, but still had gobs of smooth power, did a plug chop, definitely running lean, white tipped plugs.

Yes, the pilots do not seem to be removable, If I remove them the way you said, how do I reseat them?

I some what understand carbs, but Im really at a loss here. Do I go with bigger pilot? Or bigger main? Both?

Ive never played with air jets at all, does anyone have any info on how to tinker with them?
Click to expand...

You can reseat them by putting the jet in the freezer (or among dry ice, which works better) and heating up the carb a bit (make it hot enough that water sizzles) and you should be able to drop it right in without any resistance. Once they go back to room temperature they'll be a perfect press fit. You can press them in, but it does kinda wear out the aluminum if you do it over again. The dry ice method is better.

You'll want to start with the lower end of the spectrum first. If you're sure that at WOT it's lean then change out the main jet, otherwise start with the pilot and air mixture which control about the first 25% of the throttle. Then add or removed shims under the needle which will affect around 50% open throttle, then mess with the main jet which affects the mixture at WOT.
 
The jets that are in the float bowl submerged in fuel are obviously fuel jets and then there is air jets that are exposed to the atmosphere that are air jets. Both the main and pilot and sometimes intermittent circuits have them. The pilot air jets depending on the carb are located in the inlet side (air box or air filter side) or sometimes under the slide diaphragm up in the top if it is a constant velocity (CV) type. Spray some carb cleaner through them to locate witch one is for the pilot circuit. If it is removable larger or smaller air jets can sometimes be found to replace them with. Richening or leaning out that particular circuit.
 
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Is then idle knob set so it will actually idle or "attempt" to stay running at least? Look with a flashlight and once the end touches the linkage, give it 2 full turns..may be a little high Rpms if this fixes it, but hey..do what you gotta do.
 
My guess is that the carbs aren't clean yet. :-k

Yeah, you said:
jakeh said:
The carbs are super clean, theve been run through one of those ultrasonic cleaners and all internals replaced.
Click to expand...
However, it's not the replaceable "internals" that get clogged and block the flow of fuel, it's the passages between them.

It sounds like the passages for your pilot circuit are still plugged keeping the gas from flowing through all your new "internals".

Not sure how many here will know details on Kehin carbs, but clogged passages will happen on ANY brand of carbs.

.
 
in the morning I am going to pull them off again and try and squirt carb cleaner up through the pilot jet and try to watch it come into the venturi. That should let me know if at least the passage is allowing flow right? If so, needs larger pilot jet, if not, needs to be cleared.

Hopefully that gets me somewhere.
 
I have to agree with Steve on this, the carbs need dipping. Also with these carbs , the float adjustment is very critical as you have to set them with fuel level tubes, it is very hard to get them correct trying to measure the float height when most of the float is within the carb body not the bowl.
 
You need to make sure the paths from the air jets to the plot jets & passages are clean and the hidden path from below the pilot jet to the idle mixture screws are all clear. I learned that the hard way with about 8 carb R&Rs & finally sawing an old carb in half & finding the hidden passage. The air screw passage was blocked thus no air to let the fuel flow
 
Hey guys! Awesome news, yall were right, the pilot fuel passages were completely clogged. I got jipped on the "professioal cleaning". it took several hours but we finaly got the passages and jets clear. After reinstalling, we started the bike on choke as usual and slowly took it to open choke. Idled great! pilot circuits working!

New problem. When throttle is applied two bad things happen. One: I get a serious lag/bog before it winds up. Two: it takes FOREVER for it to wind back down to idle.

My hypothesis is that ironically now that were running with correct choking, we have a rich scenario? would that explain the bog as too much fuel to burn and the slow wind down as residual fuel in the venturies?
 
Thanks for being humble enough to follow suggestions AND being man enough to admit what you found.
icon_thumbsup.gif


Now for your new "problem". A slow return to idle is typical of a LEAN idle (pilot) mixture, not rich.

I am not going back to previous threads to see what carbs you put on there, so please remind us ... what size are they? If they are much larger than stock, you will have some bogging when applying throttle, just because they are too big.

.
 
They are keihin PD round slide carbs. same size throat and same stock intake boots were used. the only difference in diameter is where the filters attach it is only 42 mm. So the slow return to idle would be caused by the pilot? Huh, learn something new all the time. in the morning I will try and plug chop it during a bog out to learn more on the fuel flow condition.
 
So knock on wood everybody.... this morning fired up the bike to do plug chops, fired up and idled nicely, no choke (its also 104 degrees here in Houston today) first few neutral rev attempts resulted in slight bog, after it warmed up. It revved freely and smoothly. Hopped on and took her around the block, rode smooth with good power. It still takes a little longer to come down from free revs than I would expect, but then again Im used to my CRF250R with basicly no flywheel to speak of. It is probably reasonable. Plug chop showed greatly improved but not yet ideal conditions, they are no longer bright white, but not quite to a target brown. Im sure I can jet and tune that out.

My only concern now, when free revving it in neutral at lower rpms, when the revs are released and the bike comes back down to idle, it makes a notable *gargle* sound out the intakes for lack of a better description. I assume that is noise that it always makes but can only be heard now that the pod cones are in place? I hope so. And sometimes when idling, theres a light click about every second I cant seem to find. I figure it cant be anything very severe be it such a slow click. Something anyone has ever heard of?


Thank you all for all of the help. I would not be able to do this without it
 
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