Excessive Fuel Consumption: Carbs' Overflows Plugged

[Autodidact]

Hi,

I'm new here. I know a bit about bikes (clean out and rebuild carburetors, mechanical, etc).

I have a1994 Suzuki Katana 600. Bike runs well, but after riding for a while at a speed above 40mph and slowing down or coming to a stop, the bike wants to stall. It's as if it's running on two cylinders and has very little power. Aside from this, the bike gets very ad gas mileage i the neighborhood of 15mpg.

Since I've done carb rebuilds and adjustments on bikes, I thought I'd do it to this bike to get those things out of the way. Upon removal, I realized the carburetors' overflows were simply plugged. I know bikes have charcoal canister emission systems and in the old days these overflows were simply dumped to the ground. But this bike should have the emission evap system (it has the sticker too), but it seems someone removed this system and simply plugged the overflows.

Can this be what's causing this rough running condition as well as very bad gas mileage (15-20mpg)?

I'm not a smog expert equipment expert, but even these smog systems allow some level of ventilation (through the charcoal canister) and simply plugging these overflows seems like a bad idea. If the the carbs have too much fuel they will simply feed it to the engine.


Would it be possible to simply rig up some hoses and route them towards the ground old school style?

Your thoughts are welcome.


Jon

[Kfreak]

Depends on how much of the emissions crap was removed and was everything plugged the way it should have been after all was removed. The overflow tubes should be open. On non-emission bikes, it is a vent tube that goes up about 4". Your idle issue could be caused by clogged pilot jets and you don't know what size main jets are in the carb's.

[Kreylyn]

Woah, hold on...

1. No overflows, so they can't be plugged.

2. The Pair system (emissions stuff that was removed) only utilized vents... not overflows. On or off, should have NO effect on how your bike runs. It captures fumes before being vented to recycle/clean them instead of release to the atmosphere.

3. Your floats/float needles are what control fuel flow and level in the carbs. If that is not working correctly, this is what you need to address to resolve your issue.

You can test the floats by filling the bowls and blowing on the fuel lines. If you can blow throgh them, then you have an issue as the floats should be sealing off when the bowls are full.

Krey

[Autodidact]

The carbs have bee cleaned.

I think I may have found the problem. carb #4's vacuum reference for the petcock valves was routed to carb #3's overflow. Carb numbers #'s 1,2, and 4 had there overflow completely plugged.

So, I'm guessing that after cruising at a high speed, downshifting created a high vacuum condition and cylinder number 4 was drinking all of the fuel that it was syphoning from carb # 3's overflow.

Aside from that, the other carbs' overflows were not under the correct conditions to function properly.

2. The Pair system (emissions stuff that was removed) only utilized vents... not overflows. On or off, should have NO effect on how your bike runs. It captures fumes before being vented to recycle/clean them instead of release to the atmosphere.

I understand what you mean by vents, but what if the bike is tipped over on its side? Wouldn't fuel pass through this "vent" system? That is what I mean by overflows: fuel could pass through there. Probably not the proper terminology on my part though.

Also, I would think that having those "vents" OFF (as in completely capped off) would have an effect on their proper functioning since, if they were originally designed to be "vented", this would imply at least on "on/off" relationship within the system. If they could function properly by being simply capped off, why would there even be a tube built into the carbs?


Interesting stuff!

[Kfreak]

Ummmmm..... if the vac line from the petcock was hooked up to a vent, you got one messed up system. The petcock vac line goes to carb #4 vac tap. Someone didn't know what they were doing. Even if you had the emissions system hooked up right, there should only be two vent tubes that are tied into the emission system. If you don't have an emission system, the vent tubes become vent/overflow tubes. What does everyone do when they compromise an emission system??? They plug everything because they don't understand what it does.

[Autodidact]

Ummmmm..... if the vac line from the petcock was hooked up to a vent, you got one messed up system. The petcock vac line goes to carb #4 vac tap. Someone didn't know what they were doing. Even if you had the emissions system hooked up right, there should only be two vent tubes that are tied into the emission system. If you don't have an emission system, the vent tubes become vent/overflow tubes. What does everyone do when they compromise an emission system??? They plug everything because they don't understand what it does.

Indeed, Kfreak! This is exactly what someone did to this bike; now I'm rectifying it. And what you say is exactly what I'm trying to do.

I will finish it up tomorrow and post the results.

Thanks everyone.

Jon

[Kreylyn]

The carbs have bee cleaned.

I think I may have found the problem. carb #4's vacuum reference for the petcock valves was routed to carb #3's overflow. Carb numbers #'s 1,2, and 4 had there overflow completely plugged.

So, I'm guessing that after cruising at a high speed, downshifting created a high vacuum condition and cylinder number 4 was drinking all of the fuel that it was syphoning from carb # 3's overflow.

Aside from that, the other carbs' overflows were not under the correct conditions to function properly.



I understand what you mean by vents, but what if the bike is tipped over on its side? Wouldn't fuel pass through this "vent" system? That is what I mean by overflows: fuel could pass through there. Probably not the proper terminology on my part though.

Also, I would think that having those "vents" OFF (as in completely capped off) would have an effect on their proper functioning since, if they were originally designed to be "vented", this would imply at least on "on/off" relationship within the system. If they could function properly by being simply capped off, why would there even be a tube built into the carbs?


Interesting stuff!

I'll discuss all the ports. I'll probably go over simplified to ensure clarity, not because I have any pre-concieved ideas about what you do or don't know.

You should have 2 fuel inlet ports, between each set of carbs 1-2, and 3-4 towards the bottom near the bowls.

Mirrored above them will be 2 more T inlets that are "vents". OEM would have a line attatched to them and routed over the airbox to under the bike. IN case of tip over, this would be the most likely point of fuel to drain out of the bowls. If those lines are missing, it will have little/no effect on how the bike runs.

On the top of each carb, the black cap should have a "sync" port, capped off with a rubber top. All 4 must be capped to prevent running issues.

On the side of #4 carb (far right as sitting on the bike) is the vacuum port for the petcock (and pair system if you have CA model). This port is right beside the top of the long brass tubes top insert point for reference. This must either be used for the petcock, or plugged off to prevent running issues.

Now, on the CA emmisions stuff...

4 brass tubes, 1 on each carb body... comeing from the bottom of the carbs. There are hooked all together and routed to the charcoal canister, that is routed towards the diaphrams above the coils, where a T connector ties in with the petcock vacuum line. Additional hoses run from there to the front of the engine connecting to the engine block on each cylinder just directly under the exhaust ports.

To remove the pair system... take the canister, and all lines attatched to it, remove the diaphrams and all lines from above the coils, remove the 4 lines that plug to the engine, and discard it all. Use block off plates to cover the holes in the engine (simple to make and seal with a paper gasket), cap the 4 tubes on the bottom of the carbs to seal them, and run a single long line from the vacuum port on the side of #4 carb to the petcock. Cap all 4 sync ports on the top as should be normal, and run the vent lines off the two top Ts just above the fuel inlets, and your bike should run normally with no issues.

The CA has slightly leaner jetting than the OEM... that means you might need to do minor tuning on the carbs to richen it very slightly more so than non CA model carbs. Like 1/8 turn on the A/F screws, maybe a 1/2 mark move on the needles to shim them slightly. The pair system removed would should have no effect at all on this though... it's the size of the jets internally that causes this.

That said... I have a CA model bike. It's a 93 750 I bought new off the showroom floor. After 6 months, I disconnected the pair system and jetted the bike. I've ridden the bike since then with out that system hooked up. It's not been an issue at all if properly blocked off.

Krey

[Kfreak]

There you go. Pull off all the CA crap from your bike and sell it to him.

[Autodidact]

Kreylyn,

Thanks for that elaboration. Makes more sense now and I see that the vents and overflows are actually two different things. Haven't worked too much on bikes with smog equipment.

So, I've put everything back together now and am having some problems with the idle. It wouldn't idle with the choke so I upped the idle adjustment screw just to get it to stay on. Now the idle floats up to 3000rpm and then comes down slowly and eventually the engine dies.

Does this seem like a vacuum leak or perhaps idle mixture issue? I never messed with the carb sync (carbs were pretty well synced before and had excellent throttle response), I only cleaned out the fuels bowls and passage ways and the pilot idle mixture screws which I screwed back in all the way and 1.5 turns out on all 4.


Also, with the engine running at a somewhat of an idle and the airbox removed, I can see fuel trickling out of each of the needles and being sucked in to the engine. Obviously the engine requires fuel to idle, but it seems like that would be too much fuel even for an idle. Is a visible amount of fuel trickling out the needles during idle normal?



Thanks,
Jon

[Kreylyn]

Kreylyn,

Thanks for that elaboration. Makes more sense now and I see that the vents and overflows are actually two different things. Haven't worked too much on bikes with smog equipment.

So, I've put everything back together now and am having some problems with the idle. It wouldn't idle with the choke so I upped the idle adjustment screw just to get it to stay on. Now the idle floats up to 3000rpm and then comes down slowly and eventually the engine dies.

Does this seem like a vacuum leak or perhaps idle mixture issue? I never messed with the carb sync (carbs were pretty well synced before and had excellent throttle response), I only cleaned out the fuels bowls and passage ways and the pilot idle mixture screws which I screwed back in all the way and 1.5 turns out on all 4.


Also, with the engine running at a somewhat of an idle and the airbox removed, I can see fuel trickling out of each of the needles and being sucked in to the engine. Obviously the engine requires fuel to idle, but it seems like that would be too much fuel even for an idle. Is a visible amount of fuel trickling out the needles during idle normal?



Thanks,
Jon

A/F screws should be at 2.5 turns for a mostly stock setup. High idle, or hanging idle is generally associated with lean, and the more turns on the A/F screws the richer the setting. Go out at least a full turn on each of them, and that should help with your current issue.

Krey

[Autodidact]

Thanks Kreylyn; I will give that a try in a few.

[Autodidact]

Well,

Just got done test driving the bike. Prior to, I wa adjusting the mixture and tried even 2 turns out without success. Turned it two and a half and now it's running almost perfect. I rev it and it drops back down to idle nicely and I drive it around and it comes back down to idle. Sometimes it wants to stay almost close to 2ooorpm, but this is probably 10 percent of the whole test drive I did.

I'm guessing backing the screws out to where they are a full 3 turns out would have it running nicely?

Thanks,
Jon

[Wild-Bill]

3 turns out is probably going to be too much. You might benefit from 2.75 but if you're close then try syncing the carbs first.

[Kreylyn]

3 turns out is probably going to be too much. You might benefit from 2.75 but if you're close then try syncing the carbs first.

I agree... slightly out of sync will can cause it to hang a bit.

Krey

[Jpaul26]

Where are they??

Totally noob question, but where are these A/F adjustment screws? From the way you guys describe it, sounds as if they're accessible without disassembling the airbox, or no?