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750F Slow Return To Idle


CBVFRbikeboy

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This 750 is kicking my butt at the moment. Latest problem is that it is slow to return to idle or power down, which has an affect on gear changes and braking etc. The throttle cables snap shut when the bike is running, so they're not binding on anything. I thought it might be fuel-related, so I fitted a second set of carbs hoping to fix it, but they're the same. Both sets of carbs are balanced pretty well. I did a short vid. of it, and in each case, I released the throttle, but the revs just slowly returned to idle:

https://youtu.be/rpVaBpVp6Uk (sorry, can't figure out how to embed video??)

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you have a vacuum leak at the intake somewhere..

I'll have to check this out. I did put new rubbers on, but just in case...

Did you share any parts among the carbs? I do not know this carb setup, but a dirty pilot jet lets less fuel in, causes a lean condition and this symptom.

No, I had a complete set spare, so I just swapped them over as a unit.

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As said above. Check if all the pilots are working. If you turn the scres in per cilinder, that corresponding cilinder should bog down. Don't force the screws in while you close them. The mixture screws have a very thin brass point that could break. Just turn them in gently. Record the amount of turns needed to get them in.

When setting them, set them all at the same turns out. Don't go over 3.5 - 4 turns out.

Also check the boots for air leaks. Spray some deoderant in the area of each boot (engine running) and see if that affects the idle..

If the boots are tight, and the idle cirquit is working ok, and still it seems that it doesn't want to drop on the rpms to idle, you can enlarge the slow jets.

From my experience, the #38 slow jet always was a bit small for the VF750F carbs. On my VF's I went to a larger size #40 jet. This has a great response on the idle drop too..

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Another thing to check is to sych the carbs that has helped me in the past...

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Looking at your video, one cylinder drops very quickly while the other 3 drop slowly as you back off the throttle. Check for leaks again, check that the carbs are seated correctly and that all the bands on the boots are tight. Did you buy new boots or re-use old ones that may be hard.

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thanks guys. I put new boots on, and I've sprayed some 'start ya bastard' around them when running with no increase in revs, so I'm confident there's no leaks. Carbs have been balanced, so I'll have a go at the mixture screws next I think, and maybe up the jets after that?

cheers

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A lean running engine will have slow to return idle... go richer on the mixture screws or install bigger low speed jets...

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A lean running engine will have slow to return idle... go richer on the mixture screws or install bigger low speed jets...

To the poster and to all others: Could testing it with the fairings off be causing this, only until it is reassembled? Are there modifications to the intake?

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Are you running / diagnosing with the air box lid off?

And / or no air filter?

yes to both. Is that a bad idea? I might pop it back on, but it was initially behaving badly while riding.

Willing to try anything.

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I thought I might have been on to something. Reading the manual it says 1 and 3 have shorter diaphragm springs and thinner jet needles. Maybe I've mixed them up at some point. After comparing springs they were in the right places, but I couldn't detect any difference in the needles?

no dice :-(


A lean running engine will have slow to return idle... go richer on the mixture screws or install bigger low speed jets...

Thanks. I just wound the mixture screws out to 4 turns, but no change :-(

Bigger jets might take a bit of time to organise. I could drill out a set of spares. What size would you suggest? I think Dutchy suggested #40?

cheers


I initially thought one or more of the diaphragms may have been holed? Would this result in my poor return? Not sure of the consequences, but I think I've eliminated this anyway by swapping carb banks. I checked them visually and didn't see any holes.

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A lean running engine will have slow to return idle... go richer on the mixture screws or install bigger low speed jets...

Thanks. I just wound the mixture screws out to 4 turns, but no change :-(

Bigger jets might take a bit of time to organise. I could drill out a set of spares. What size would you suggest? I think Dutchy suggested #40?

You're welcome... I just checked the parts manual and it already comes equipped with #40... if you swap carbs and saw no change then its possible its not the carbs...

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You're welcome... I just checked the parts manual and it already comes equipped with #40...

My model had #38s, and I've just drilled them out to #40. No change :-(

if you swap carbs and saw no change then its possible its not the carbs...

I'm starting to think that it might be something mechanical, but the throttle snaps back cleanly. Doesn't look like anything is sticking, but I'll go over it all and maybe lube some pivots?

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Make sure the floats are set at the correct height (7mm). The float height is measured with the carb held at an angle where he float tang is touching, but not pushing the centre pin on the float:


Also check the fuel emulsion tubes on the main jets.. The front and rear are different:


Last to next is make sure the intake funnels are correctly seated between the plenum and carb body. If you they are a bit shrunk, you can stretch them by using a lemonade glass and a heatsource.. (Warm water or a hair dryer) Heat the rubber funnel and push it over the bottom of a lemonade glass to stretch them out. Stretch them a bit larger than the size needed. Let them cool down on the lemonade glass, then put the funnel on the carbbody for install.. You don't have to take the whole assembly appart. Just loosen up the screws of the carb you are working on and finish one by one.

Yes, the front springs are longer.. Taking a view from the picture, you have VD61 series carbs..

Mains Front: 112

Mains rear: 110

Pilots: #38

Front slide needle: 0we

Rear slide needle: 0wc

Front slide spring: 170mm

Rear slide spring: 130mm

Float height: 7mm

Pilot 1.5 - 2.0 turns

post-23909-0-80865800-1459251073.jpg

post-23909-0-32055500-1459251156.jpg

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Thanks. I sodablasted these ones.. Also had the throttle axes and misc. bits newly galvanized..

One thing I forgot... You did check those tiny springs between the synch adjusters and the back-up springs on the the other side of them ? One loose butterfly because of a missing spring would cause weird symptoms as well..


And as a last thing: Always test the carbs with the airbox and filter installed.. Without the airbox and filter, the bike is running way leaner..

post-23909-0-02854600-1459254774.jpg

post-23909-0-78112400-1459254774.jpg

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When cleaning carbs use a bit of green scotchbrite on a finger to clean off the black carbon ring in the bore around the throttle plates or they might not close cleanly and consistently. :comp13:

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Thanks. I sodablasted these ones.. Also had the throttle axes and misc. bits newly galvanized..

Now that's just cruel :-)

Yeah, I check the springs each time I pull the bank, because I've had one shoot across the garage before ;-) Slippery little suckers!

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Last to next is make sure the intake funnels are correctly seated between the plenum and carb body. If you they are a bit shrunk, you can stretch them by using a lemonade glass and a heatsource.. (Warm water or a hair dryer) Heat the rubber funnel and push it over the bottom of a lemonade glass to stretch them out. Stretch them a bit larger than the size needed. Let them cool down on the lemonade glass, then put the funnel on the carbbody for install.. You don't have to take the whole assembly appart. Just loosen up the screws of the carb you are working on and finish one by one.

Nice tip. Just tried this on mine, and they were indeed in need of 'stretching'. Works nicely, and now they fit really well.

Unfortunately the problem persists. I've even taken to not fitting the throttle cables and operating it manually with a zip-tie rig. Still nothing :-(

time to step away and tackle something else if think.

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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.

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