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750F Slow Return To Idle


CBVFRbikeboy

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Check your valve clearances. If they are too far off, you could have intake valves opening too long, introducing blowby and a lean mixture. If that goes on too long, it may end up burning up the valve itself.

I had a similar adventure on a Suzuki a few years back - I just couldn't get the carbs tuned properly. I finally said screw it and checked the valves and sure enough, the intake on one cylinder was WAY WAY off. Got the right shim, and the bike ran better (not perfect because I believe that valve was partially burned by then, but good enough to be rideable and run well)

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As OUR MAN Dutchy mentioned in post #5, try turning IN each idle mixture screw until that cylinder stops firing and the idle speed drops, and then return it to the initial setting. If any cylinder doesn't quit firing that means it's getting fuel from somewhere else, most likely a slightly leaky float needle that's raising the fuel level in the bowl.

well I think we're zeroing in on something. I can turn all 4 screws in at the same time, and the bike still runs pretty well. I did check the float levels, but I'll go over the needles again.

cheers

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Think we have to go back a step and learn how these carbs work..

These carbs are a sort of three stage carbs:

From 1000 to 1500 rpm - The butterfly valve in each carb is (nearly) closed. Air is going to the intake of the cilinder throughout tiny bypass holes underneath the butterfly valve. (Three or four tiny holes at the bottom of the butterfly valve) Fuel is following a different route. At the air intake port, there is a brass air jet located. From there air is taken in to the #38 slow jet where it mixes up with fuel. This mixed air/fuel is delivered to the pilot screw and is delivered to the intake port of the carb.

So to check this cirquit: take the #38 jet out, fill the hole with some fuel or starter fluid and blow compressed air through. Fuel/air should be shooting out from the intake port air jet, but also from the pilot screw side port.

From 1500 - 5000 rpm, fuel is delevired straight throughout the #38 jet to the inlet underneath the slde.

From 5000 - redline, fuel is delivered by the mainjet.

So to cut it short, two things might occur:

1 - Your idle is set too high, engine is idling directly from the #38 jets. Carbs out on the bench: check the butterflies for closing. Do a bench synch. Check if the butterflies are set the same. I use a small steel wire underneath the butterfly to check if it closes, then revert to the others to check if they have the same setting. (Like setting the valve lash)

2 - The pilot cirquit is still clogged and not functional. Check the passages in your carb, but also check the o-rings on the pilot screw. (In order before in inserting the pilot jet, there should be a spring, a tiny washer and an o-ring installed on the pilot jet.

Hope this helps you out..

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OK. Time to bite the bullet. I've pulled the rack apart, and one by one put the carbs through the ultrasonic cleaner in some yamaha carb dip. Blasted every little orifice I could find with compressed air.

ultrasonic.jpg

Just got to put them back together now :wacko::wacko:

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Is the fuel good?

Have you tested the ignition system?

Yep. fuel is fresh.

Only as far as checking for good spark at coils by taking the leads out one at a time. Nice fat spark, and engine revs drop as plugs drop out as you'd expect. Don't see any problems there. :wacko:

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Since switching carbs assembly with no change, problem is not with the carbs then. I agree with checking the valve clearance as it appears that they are not closing as they should: cams out of synch, valves sticking, etc. Going to have to dig deeper into the engine to solve this one.

But really nice job on the carb cleaning and like those metal fuel/vent lines. Mine still has the plastic OEM lines which one of them began to leak fuel which lead to me taking the carb assembly apart to replace them. Wish I had known about the metal ones as I just replaced with OEM plastic.

Gool luck and keep us informed as to what you do and eliminate.

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Since switching carbs assembly with no change, problem is not with the carbs then. I agree with checking the valve clearance as it appears that they are not closing as they should: cams out of synch, valves sticking, etc. Going to have to dig deeper into the engine to solve this one.

But really nice job on the carb cleaning and like those metal fuel/vent lines. Mine still has the plastic OEM lines which one of them began to leak fuel which lead to me taking the carb assembly apart to replace them. Wish I had known about the metal ones as I just replaced with OEM plastic.

Gool luck and keep us informed as to what you do and eliminate.

thanks. I ended up replacing the plastic with metal after I, ah, snapped one a while ago when I first put this thing together. :wacko:

Looks like a valve clearance check is next on the list. May not get to it tomorrow, but should be good by Tuesday.

cheers

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And this is with complete airbox, filter and lid in place?

Fuck, fuck, fuck.

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And this is with complete airbox, filter and lid in place?

Fuck, fuck, fuck.

Well no, but the initial problem was happening with the bike complete. Hasn't improved or worsened without the air box. I am still running the plenum though.

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Have you correctly set the fuel metering pilot screws??? item 24 in the Ron Ayers
Micro Fiche... if the screw is located in between the slide and the
engine then out is rich in is lean... if the screw is locate in
between the slide and the air box then out is lean in is rich...

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if the screw is located in between the slide and the

engine then out is rich in is lean...

Yeah. I've tried them at various positions. One of the last adjustments saw me screw them all in (lean), and the bike still ran :wacko:

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With all four screws all the way in, you have two at full lean and two at full rich...so yeah the bike would still run.

Refer to BusyLittleShop posting above.

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With all four screws all the way in, you have two at full lean and two at full rich...so yeah the bike would still run.

Refer to BusyLittleShop posting above.

Nope. They are all fuel metering screws. Out is richer, in is leaner.

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if the screw is located in between the slide and the

engine then out is rich in is lean...

Yeah. I've tried them at various positions. One of the last adjustments saw me screw them all in (lean), and the bike still ran :wacko:

There is definitely something funky going on. The bike should not idle with all 4 screws all the way in.

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It can idle, if the butterflies are open too far, but then it would idle on 1500 or more rpm..

I don't really consider that "idling".

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As already stated, the bike should not idle with all 4 screws in. If it doesn't die when the idle circuit is closed, it's getting unmetered fuel from somewhere that it shouldn't, most likely from the main jet. Excess fuel will come from the main jet if the float needle is leaky OR if the float level is set incorrectly OR both.

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if the screw is located in between the slide and the

engine then out is rich in is lean...

Yeah. I've tried them at various positions. One of the last adjustments saw me screw them all in (lean), and the bike still ran :wacko:

Post a photo of all 4 screws all the way removed including the washer spring and O ring...

Keihin-the-carb-air-font-b-screw-b-font-

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Excess fuel will come from the main jet if the float needle is leaky OR if the float level is set incorrectly OR both.

Perhaps my float measuring methodology is wrong? This is the way I did it:

floats.jpg

It's difficult to judge the point of contact with the needle, but it is fairly repeatable. I'm pretty sure they are all the same - whether or not that has been properly measured is open for debate ;-)

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Post a photo of all 4 screws all the way removed including the washer spring and O ring...

Sure thing. Looks the same as your example, and the measurements match. I was a little worried about the pointy end until I realised it's tapered. The O-ring is still in the carb. They're back on the bike, and it's a pain to get them out like this. Rest assured all four are in place.

idleJet.jpg

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Excess fuel will come from the main jet if the float needle is leaky OR if the float level is set incorrectly OR both.

Perhaps my float measuring methodology is wrong? This is the way I did it:

floats.jpg

It's difficult to judge the point of contact with the needle, but it is fairly repeatable. I'm pretty sure they are all the same - whether or not that has been properly measured is open for debate ;-)

When you check float height, you want to turn the carbs so the float is "hanging" then rotate slightly until the valve contacts the seat but doesn't depress the little spring-loaded button. Then take your measurement. Opposite of what you're showing.

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When you check float height, you want to turn the carbs so the float is "hanging" then rotate slightly until the valve contacts the seat but doesn't depress the little spring-loaded button. Then take your measurement. Opposite of what you're showing.

OK. I'll try that then thanks. I was trying to make sure the valve was seated properly, then find the point of contact with the 'button'.

cheers

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Sure thing. Looks the same as your example, and the measurements match. I was a little worried about the pointy end until I realised it's tapered. The O-ring is still in the carb. They're back on the bike, and it's a pain to get them out like this. Rest assured all four are in place.

At the point you're eliminating the unknowns... if the condition of the O rings are unknown then you might entertain replacing all 4 to eliminate the possibility of bleeding... I've had great success getting the little buggers out by shaping a hook out of .032 safely wire or a robust strand of copper wire...

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