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:
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.
I'll have to check this out. I did put new rubbers on, but just in case...
Knight
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.
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..
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.
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?
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?
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 :-(
BusyLittleShop
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.
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...
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 :-(
BusyLittleShop
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?
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..
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..
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:
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.
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.
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)
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.
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.
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.
[image: ultrasonic.jpg]
Just got to put them back together now :wacko: :wacko:
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:
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.
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.
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.
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...
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.
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:
[image: 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 ;-)
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.
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:
[image: 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.
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'.
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...
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...
sure thing. I have had them out when I put the carbs through the ultrasonic bath, and did exactly as you suggest with the wire hook. They looked OK, so I put them back in. Just awkward to get them out on the bike :wacko:
Still searching frog. I did check the valves and most were pretty good. #4 intake were both a little loose, which gave me cause to thank that that could well be the problem. I buttoned it all up and fired her up, but the problem persists. I'm about to pull the carbs off again and adjust the float levels down a bit. As far as I can tell, they're at 7mm, but in case I'm measuring wrong I'll set them all to 8mm and see what happens. Nothing to loose. :-(
My problem with the pilot screws is that 2 of them, (carb #1 and carb #2) seem to be stripped: they don't come out and they don't move in to seat. The other two are fine. I can't understand how the bike even ran at all, but it was strong up to about 8000 rpm. I'm cleaning and rebuilding the carbs because the P.O. left gas in the tank for years. Any tips on removing these would be a big help. I want to get this right the first time because of the "fun" of getting at the carbs. Thanks in advance.
If you do find the solution I would be glad to hear it. When I watched your youtube vid, I didn't think it was too bad. It does return to idle and I don't think it's that slow.
My VF500F would be similar. However if I screw in the pilot circuit adjusters, my bike will stop, unlike yours. I expect this is what you need to determine, how the bike will run with the adjusters screwed home.
Well, it seems I've cracked it. Turns out #3 was cutting in and out, and I could remove the plug lead without having any effect on the idle. Good, I thought, electrical. I replaced my coils, I replaced the spark units, replaced the leads, swapped the (new) plugs around, checked voltages and resistance everywhere. Even managed to squeeze my compression tester down that tunnel. 150PSI on both rear cylinders. Nothing. Spark was good at the plug through all this, so I returned to fuel. Put another set of carbs on, but before I did, I swapped #1 and #3 boots around, and reversed them.
Bingo! #3 is firing fine, and will cut the revs if I remove the lead. Idle is nice and smooth, and it now behaves properly. Too wet to test ride, but I'm confident I've nailed it :wink:
Thanks for all the suggestions gang.
cheers
idle.mov
End of thread · 57 posts recoveredWayback Machine snapshot 6 Jul 2017