off idle stumble fixed but

[petejw]

hey guys,
been mucking around with the carbs,
there the keihin carbs fitted with pod filters
they have fuel vent hoses that are supposed to fit to the air box,
similar to the 78/79 mikuni carbs.
ive also been experiencing off idle stumble,
from what ive read these hoses need to be connected to the airbox so they can sense air pressure in the air box, having these hoses vented to atmospheric pressure creates the lean mixture from off idle, depending on severity this can cause the near stalling of the engine.
heres a pic of wat ive tried to achieve by attaching these hoses to the ram tubes like this.



the bike starts nicely, and with no load freely revs and returns
to idle quickly without the nortorious floating or breaking down, great
i take it 4 a ride, no more stumble and the bike pulls much harder
BUT, when the slides are suppose to lift the bike immediately
dies in the arse. The slides arent rising, This set up has solved the off idle stumble but
with this set up disconnected the bike has the off idle stumble, slower pick up off idle, but rides normally thruout rev range and throttle position.

anyone hav any thoughts or suggestions,

ive also restricted the hoses, no difference, i disconected one hose to let it vent, also tried restricting this hose, the more the hose was vented the stumble became worse but the slides would rise further.
thought id put it out here

cheers

[81xsproject]

If I remember from pggg's old post, he said he clipped off a number of turns from the slide springs. It's been a while, so you should check it out for yourself.

[petejw]

hey 81
pggs got the air box fitted to his,
i have tried that, made it worse,
i ended up extending them, made it better.

[Crazcnuk]

Not sure, but if you extended them, and it got better, does that not mean that the slides may be lifting off too early, rather than too late?

The spring keeps the slide down, so making it longer, or stronger would be holding them down.

[egsols]

I don't know if I'm reading you right, you're say the vent hoses are changing your mixture depending on how you route them?

As I understand the vents are there to allow float bowls to balance with the outside pressure. If they weren't vented you could create a situation similar to vapour lock in a gas tank and the bike would quit.

The 78/79 miks vented to the airbox. The 80/81 did away with the external vents and vent through the back of the carb into the velocity stack.

My buddy's KLR650 vents through a tube running down to the bottom of the bike.

I don't think that the vent's should have any effect on the carbs unless they were blocked.

[petejw]

hi craz, ive gota spare set of springs that were the original
height, they made no difference.

hi egsols,i kno wat u mean but wen u think about it, when those hoses are connected to the air tight air box, once the engine is running its actually creating a vaccuum in those hoses, if that sounds incorrect some one correct me.
ive got the 81 carbs, the problem doesnt seem relevant with these carbs, i also recall tc saying he doesnt have any problems with his pod filters i think this is because of the vent at the back of the carb,
ive noticed the problem of off idle stumble seems to exist only on the carbs with fuel vent hoses like the 78/79.

i was reading on the factrory pro site, that to get cv carbs 2 work properly they needed to be attached to the airbox but didnt state how 2 attach them with pods.

thanks for the thoughts

[egsols]

Hi petejw,

I know what you're saying. On the 78/79s the nozzle that the vent hose attaches to goes well into the air box but it does not go as far as the inlet for the velocity stack. The vents are therefore not in direct line with the airflow. I believe it was xschop that I was discussing this with about putting the vents directly in the flow path and creating a bigger vacuum than normal.

I don't know how much the vacuum from the vents would effect the carbs other then help the fuel fill faster. On the other hand if they aren't vented the pressure will prevent the bowls from filling.

But it sounds like the vents are effecting your idle circuit and messing up your main circuit. At what rpms is it dying?

[petejw]

hi egsols,
yeah ur right, but do u agree its not atmospheric pressure in the box that these hoses r connected to, compared to having them vented?


this just made such a huge difference down low, that i wanna
stick to try and getn it rite.

i can also definately tell the slides arent rising, thats when its bogging down. i did a chop test, plugs were dry.
even messed around with the needle heights made no difference,
with the hoses disconnected the bike ran flat and had the stumble
but ran to redline.

i did go overboard with all the hoses and t pieces cause i thought it mite make a difference,. as in it would b 'balanced' but this made the same effect with the 2 hoses going into the stacks.
like this.


much neater looking and had same affect.
my nxt step is 2 move the t pieces
to the foam part of the filter,


thanx for the replys
wanted 2 get a bit of a discussion going about it.

[egsols]

Now you got me thinking.

Could it be possible that since you have the vents so close to the carb intake that they are actually drawing fuel from the bowls and introducing it back into the carbs, thus making it run slightly rich? It that were the case then perhaps your idle mix is way too lean. Under load and when the flow increases you might be running way too rich (vents now drawing lots of fuel) and causing the bike to bog.

Just a few thoughts.

It might not hurt to richen up the idle mixture and move the vents away from the bells to see if that makes a difference up top and down low.

[randy]

Egsols,

I was thinking along similar lines. With a high vacuum on the vents, you may be pulling up the fuel high in the bowls, same enrichening effect as Egsols described. If I were you, Pete, I'd vent the bowls to atmospheric pressure and concentrate on adjusting the base mixture richer with jets and or screw adjustments, if equipped.

[petejw]

dnt get me wrong, in the past 4 weeks ive spent most days trying every combination of pilot jets, needle washers and main jet, unhooked to atmospheric pressure it runs ok down low but should run better, other than that its fine but reading thru the internet its only seems 2 b a problem with cv carbs with fuel vent hoses with pod filters/velocity stax.
on the factory pro site, they state that u can never get rid of the lean mixture transition unless there plugged into the air box.
so i thought id try it, theres just such a difference.
but im wondering i kno they vent fuel,
it doesnt seem to b a fuel delivery problem,
could they also have anything to do with how the slides open and close?
i bevelled an angle on the t pieces inside the stax so that it would either face the incoming air or away from it. no difference.
i also restricted the hoses to the diameter of the holes that they would enter in the air box. no difference.
also 2 it just doesnt bog down if i leave my hand on the throttle the engine dies.
theres a reason the slides arent raising
i just dnt kno wat that is. lol

my last resort at this stage will be moving the t pieces to the foam part of the filter, im figuring bout an inch or 2 from the inlet?

thanks 4 the input

[psycoreefer]

My understanding is that the slides rising is dependent on:
1. butterfly's opening and allowing more volume of air to flow through the carb.

2. The strength of the springs that go between the top of the slide and the top of the carb.

3. A small hole/passage from the throat of the carb up into the area between the the top of the carb and above the rubber slide diaphragm. On stock Mikuni carbs the hole is "in" the bottom of the slide, not sure where it is on your carbs.

4. A usually large hole/passage from the area below the rubber slide diaphragm to outside. This is a half moon shaped port on the front of the stock carbs.

The way it works is when the butterfly opens, more air flows through the small area between the bottom of the slide and the carb throat. This increase in velocity means lower pressure (vacuum). This vacuum is then transfered to the area above the diaphragm through an air passage and then the (greater) atmospheric pressure on the underside of the diaphragm causes the slide to raise.

If moving your vents hoses causes the slides to not rise then I'd suspect that perhaps the bowl vent and the upper diaphragm vent are some how connected, and where you have them attached to the filter has a bit to much vacuum. Remember that air quickly moving over a small hole also creates a vacuum.

You could test that theory by blowing into the vent hose and seeing if you can feel air coming out any of the holes in the top of the carb under the rubber slide diaphragm.

You may also want to try and figure out how much if any vacuum is on the vent hose when you hook it to your filters, maybe a clear tube stuck in some water substituted for the vent line into the filter and see how high the water rises in the tube.

[red bandit]

When I have my ZRX carbs on I have the bowls vented to atmosphere, no stumble. With the stock velocity stacks things are resticted. I can run bigger jets and it runs much better without the stock stacks. I also have stock exhaust. At this point my stock XS carbs out perform the ZRX carbs when the stock stacks are on. I'm not done playing with the jets on the ZRX carbs. These are my results, yours my vary.

[petejw]

hi redbandit,
i found the stacks to be better at top end, at wot
wat jets r u running at?
ive all but fixed the stumble only there slightly now under load
but doesnt pull as hard as it should down low,
gredat up top.

[red bandit]

Top end (5-6k and up) with stock stacks is were my lack of power is. Right now I've got 130's and they run good with stacks but stock xs carbs run better. I have 132.5 & 135's on the way. I tried 137.5, ran like crapola with stacks, screamed without but probably rich. Didn't want to run without air cleaner. What pipes do you have? I'm trying to find some hose w/ larger I.D. to replace the stock stacks. Maybe radiator hose?? I want to keep the stock air cleaner, I like the looks. Mine is modified XSessively.