vn750 rides lumpy on steady throttle... only likes open or closed position!! - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 35 (permalink) Old 05-26-2010, 03:03 PM Thread Starter
Aki
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Unhappy vn750 rides lumpy on steady throttle... only likes open or closed position!!

Hi again all. First may I say to Lance thanks so much for your suggestions for the jets and shims.

However I have yet to see the real benefit... because...

MY BIKE IS A NIGHTMARE!!!

Ok so upon receipt of the bike I posted on here about it being lumpy and lacking in the mid range. A shim and jet adjustment seemed the way forward

This cleared out the obvious difference in power throughout the rev range

However more of the problem became apparent

At first I thought it was this lack of correct jet and shim set that had been making the bike RIDE LUMPY ON A STEADY THROTTLE, but having changed the set I have realised that has only evened out the power across the range but I am now left with a inability to cruise on a steady throttle . This issue was there from the start but I had thought it all just one issue, so just to be clear this isn't new.

Its as if the bike only wants to accelerate or power down. It wont sit happy.

For a while its fine... then, once it's heated up it's much worse. I just open throttle everywhere and power down for the law

This is not doing my pocket any good lol. More importantly it isn't doing the bike any good either.

Upon a further inspection we found one of the diaphragms had a hole in it.

So we replaced the set completely.

It made minimal difference

I'm soooooooooooooo stuck!!! I'm almost at the point of just buying a whole new carb just to see if I can solve this issue!!!

Anything... anything at all here would be a great help.

PS: I have only recently had an issue with the clutch and it only seems to happen when the bike is cold ie. first thing starting it. I let out the lever and instead of meshing nicely it seems to bite and make a noise. It nearly caught me out the first time it happened and I lurched forwards a couple of times but as soon as the bike is on the road its fine and remains fine until a fresh start. What might cause this? Might this be a common vn750 issue? What are the common vn750 issues? (just so I don't add any more frown lines when they occur).


VN750 1992

42 - 147.5 - 2 shims.
Sea-level.
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post #2 of 35 (permalink) Old 05-26-2010, 04:48 PM
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as for the grabby clutch: don't sweat it. look up coffee grinder clutch in the verses. just go real easy until it warms up or you can try squeezing and releasing the clutch in neutral to get the oil moving. there is also a mod in the verses to rid you of this problem. for the surging, look for kinked fuel lines(just had that on mine after an ear shave), check to make sure the carb breather hose is not bottoming out inside the right air cleaner, try cutting it at a 45 deg angle to prevent this. I'm sure some of the other wrenches will be along soon to bring up whatever I have forgotten. Good luck

Rook
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VROC # 30581
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2009 1700 Nomad (mine)
2003 VN750 "Prozac" (now hers)
25'000 miles at purchase
Coastered
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post #3 of 35 (permalink) Old 05-26-2010, 05:27 PM
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If I read this right, you had the problem before you did the re-jet? So my question would be did you do any other mods leading up to and before this problem surfaced?

One of the rules of the game is to have a perfectly running bike before you do mods so we are pretty handicapped with the diagnosis until we understand more about what was done. If a bike starts running badly on its own, its the wrong time to be changing the intake, or doing a re-jet, adding pipes etc. You really have to find the existing problem first, fix it, then if you wanted to change the intake and re-jet, at least you know you aren't adding to an already existing problem. Can you elaborate on how the bike started running this way?

And yes, Rook nailed the clutch grab (coffee grinder) issue. It is normal on these bikes, some worse than others. Once the engine has reached operating temps, that will go away and act normal.

Fergy
Kyle, TX VN750.com member #707 VROC#19556
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post #4 of 35 (permalink) Old 05-26-2010, 05:28 PM
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I've tinkered with the carbs and have had surging/stumbling at steady mid range throttle. It feels like riding into a very gusty headwind, except there's no headwind there.

Likely suspects:
1) carb vent hoses. these lead off the carbs and allow air in. Sometimes they are joined with a T hose and there is an extra hose that leads off them and "plugs" into a holder behind the right ear. Since your ears were shaved, this hose ends up elsewhere. Exposing the ultimate open end to a cross wind will cause surging. It's like blowing across a straw in a liquid - it creates a vacuum that pulls the liquid up into the straw. Try it the next time you have a soft drink. To fix this, make sure the vent hose leads into a filter so that it can breathe without having the wind blow across the end of a naked tube. You can make your own simple filter out of foam if you need to.

2) carb boots. There is a tab on each boot which ought to be aligned with a tab on the engine. Putting on the carb boots wrong can lead to slight air leaks which can cause surging. Either the tabs are not aligned or they may be switched or they may not be tightened down properly.

Try fixing these two first. They are simple adjustments.

Scheherazade
'86 VN750
MF Battery, Iridium Plugs, RR relocated, Voltmeter, 170/80/15 Kenda Kruz rear tire, DIY Samsonite hard bags, DIY shaved seat with Beaded seat pad ('cause that's how I roll) and the dreaded STATOR CHANGE.

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post #5 of 35 (permalink) Old 05-26-2010, 06:35 PM
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Quote:
Originally Posted by Aki View Post
Hi again all. First may I say to Lance thanks so much for your suggestions for the jets and shims.

However I have yet to see the real benefit... because...

MY BIKE IS A NIGHTMARE!!!

Ok so upon receipt of the bike I posted on here about it being lumpy and lacking in the mid range. A shim and jet adjustment seemed the way forward

This cleared out the obvious difference in power throughout the rev range

However more of the problem became apparent

At first I thought it was this lack of correct jet and shim set that had been making the bike RIDE LUMPY ON A STEADY THROTTLE, but having changed the set I have realised that has only evened out the power across the range but I am now left with a inability to cruise on a steady throttle . This issue was there from the start but I had thought it all just one issue, so just to be clear this isn't new.

Its as if the bike only wants to accelerate or power down. It wont sit happy.

For a while its fine... then, once it's heated up it's much worse. I just open throttle everywhere and power down for the law

This is not doing my pocket any good lol. More importantly it isn't doing the bike any good either.

Upon a further inspection we found one of the diaphragms had a hole in it.

So we replaced the set completely.

It made minimal difference

I'm soooooooooooooo stuck!!! I'm almost at the point of just buying a whole new carb just to see if I can solve this issue!!!

Anything... anything at all here would be a great help.

PS: I have only recently had an issue with the clutch and it only seems to happen when the bike is cold ie. first thing starting it. I let out the lever and instead of meshing nicely it seems to bite and make a noise. It nearly caught me out the first time it happened and I lurched forwards a couple of times but as soon as the bike is on the road its fine and remains fine until a fresh start. What might cause this? Might this be a common vn750 issue? What are the common vn750 issues? (just so I don't add any more frown lines when they occur).


VN750 1992

42 - 147.5 - 2 shims.
Sea-level.
Is the rear carb vacuum port capped, if it is not it will cause your problem.

Set the pilot screws @ 3 turns out.


85 VN700 "Old Yella"

REBUILT ENGINE
CUSTOM PAINT
VANCE & HINES CRUZERS
EAR SHAVED AND RE-JETTED W/K&N'S
DUNLOP ELITE K591 FRONT & REAR
VOLTMETER
SYNTHETIC BRAKE FLUID
SYNTHETIC OIL & GEAR LUBE
PLEXISTAR 2 WINDSHIELD
SPLINES LUBED
ACCT'S GREASED W/TOC SPRINGS
COASTERD
LEATHER SADDLE BAGS
LEATHER TOOL AND ROLL BAG
PICKUP COILS GAPPED AT .018"
NGK CAP, WIRES, IRIDUMS
BARNETT FRICTION PLATES & SPRINGS
CUSTOM GRIPS, MIRRORS, LEVERS
ORIGINAL STATOR & R/R 14.5v
DEKA MF AGM ETX15L

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post #6 of 35 (permalink) Old 06-02-2010, 02:00 PM Thread Starter
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Heeeeeeellllllllllllllllllpppppppppppppp

SORRY ABOUT THIS... BIT OF A NOVEL REALLY BUT PLEASE STICK WITH ME IF YOU CAN.

Ok. Time to really go from the ground up here and explain up to where we have got to right now.

I bought this bike from someone who rebuilt it from the ground up. He claimed there was an issue with the transmission so it had to be stripped down so he figured why not spruce it up at the same time.
Realisticly I should have heard the alarm bells ringing right there and then but he'd done such a nice job on it i was blinded.

It later turned out that all the reciepts I have are listed to North Devon college, where he works.

I put 2 and 2 together and now firmly believe that this bike of mine was the college project.

No big deal. Engine runs great and only really a few minor issues around the rest of the bike... like an irritating buzz rattle at 5.5k revs emminating from the front of the bike but I can't tell from where. At first I believed it to be from under the speedometer and rev counter (since they are quite flimsy) but now an not so sure, it sounds more like under the tank or near the radiator. Regardless its a very flimsy sounding rattle so I feel its only something small and cheep and thus not making me immediately fear for my life. Although identifying it's source would greatly cheer me up. The front and rear shocks really are quite firm. I'm not sure if the rear springs are the correct set for the shocks. It ride's ok until you hit heavy bumps on bends and then it tends to throw about a little (keeps me on my toes, no overcooking it on bends is a good thing really lol).

HOWEVER

The carbs... have been a nightmare since I got it!

At first I just believed it to be a matter of tuning. The guy had only barely put a 100 miles on it since its rebuild (I don't think he was counting on selling it so fast. He had a 7 day listing and I offered him cash early into it). He had replaced the standard exhausts with these that you see on the bike and replaced the air filters (a'la ear shave I presume) with tapered pods. However upon closer inspection I can firmly say I don't know what make they are. My mechanic says he does not think them K&N's but more likely some cheap aftermarket variety.







The guy I bought the bike from did give me all the old air intake stuff and filters at least, though they are fairly tattered. Some of the rubber would need replacing and some of the plastic needs some cracks gluing shut.

The best way to describe this initial problem was to say it pulled nice and torquey up until about 4.5k then it would fight against the throttle and sag in power until you hit about 7k... then it would scream off like a rocket. Now that I look back I can see it also had an underlying problem of not being able to retain a steady throttle however this was heavily masked by the power issues.

I contacted the guy and he said that he'd researched his info from this very forum and that he had upped the jets to 40/140 and a 1mm washer as a make-do for 2 shim's (he was at least kinda right as upon research I found that a shim is roughly 0.5mm and so 2 of them would equate) as this was the suggested place to start.

I posted here and Lance very kindly took the info that I gave (which may have been a little incorrect as I initially believed the filters K&N's) and gave me a suggested up-jet of 42 pilot jets, 147.5 main jets, 2 shims under each needle and 3 turns out on the pilot screw. Well the shims and the jets worked well but the 3 turns out was too much (my mechanics opinion not mine0 so we settled for 1.5 turns.

This immediately sorted one very important thing out! This allowed smooth power distribution over the entire rev range. 1k-11k pulling well with a power band between 6k and 9k. Fantastic! Off like a rocket!

Only...

Could not keep a steady throttle for the life of me. The bike either wanted to just go or nothing. This is fine if I just want to accellerate or coast down everywhere (believe me its quite fun but mostly illegal lol). So back to my mechanic friend it went with more investigations.

We found:

One of the diaphragms had a split in it. We replaced both with new ones... it made no difference.

The float levels were 17mm out!!! We played with those... it made no difference.

Upon testing different shims my mech took it out and it died! He checked it over and found one of the rubber connectors to the pod filters had split!

A brand new pair of them has just arrived (because they must be replaced) meanwhile, to the old one, he has glued it back together for me so that it will run at least until he gets back from holiday and we fit the new boots.

Socks!

Before he went away on holiday he stuck a pair of socks over the air filters!!! The bike will now ride at a constant throttle! However I can really only accellerate gently. If I open the throttle quickly and full it wants to hop down the road! It coughs and pops violently and the power back's off considerably.

So here I am.

At least now if I need to I can ride my bike to work. It will even quite hapilly get to 70mph and sit there too But my fuel consumption doesn't justify the innabillity to accellerate and have fun.

So........ what's with me taking in too much air?

I'm so stuck here. Also this whole ear shave thing. Whats the comparisson to a standard system fuel wise? At the moment it seems I can just about get 90 miles out of a full tank. That's really not very efficient and given I have to ride 66 miles every day I'm wondering if I should just quit on this ear shave thing and retro-fit the standard filter system back on and jet/shim to suit? I can handle the 90 miles to a full tank if the damn ear shave is working propperly or if there isn't much difference if it were the standard air filter intake system. Am I using too much fuel or not?



I'm beginning to think maybe I should just sell it and buy a Rover Metro.

.... please help me, I'm loosing the will to RIDE!!!


1992 VN750

42/147.5 2 shims and 1.5 turns of the pilot

Pods, non standard exhaust

What you see is all I know

SEA LEVEL

GUTTED
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post #7 of 35 (permalink) Old 06-03-2010, 12:02 AM
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My stock air filter setup gives me around 48 mpg (U.S. gallons) at speeds below 55 mph. Above that, and mileage drops to as low as 35 mpg around 75-80 mph. So if most of your riding is high speed, getting 90 miles from a tank of gas is about what you should expect with the ears. Not sure what effect coastering should have on mileage, if any.

I'm keepin' all the left over parts. I'm gonna use 'em to build another bike!
_____________________________________________
"Black Beauty"
1989 VN750 acquired December, 2008, 6,711 miles
Currently 23,298 miles

Old Blue
2001 Honda CMX250 Rebel acquired July, 2008

1987 VN750 project bike, acquired August, 2009, 33,000 miles and balancer sticking out of the case, currently awaiting attention and parts
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post #8 of 35 (permalink) Old 06-03-2010, 01:44 AM Thread Starter
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Thanks for the info Flite anyone seconding his opinion would be welcome also. This is good news if it's the case. It would mean reversing my ear shave has no real economical value to me ...... now all i have to do is actually get it working!
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post #9 of 35 (permalink) Old 06-03-2010, 10:21 AM
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At 75-80 mph I get about 100 miles per tank. NV750 is really a gas saver at the 55 mph range. This makes sense because the top speed is 120mph so at 80 you're almost there and would use more gas to maintain that speed.


VROC Number: 31246
2003 Kawasaki VN750 SOLD
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post #10 of 35 (permalink) Old 06-03-2010, 10:26 AM
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Here's more about mileage. https://www.vn750.com/forum/archive/i...p/t-10336.html

I'm keepin' all the left over parts. I'm gonna use 'em to build another bike!
_____________________________________________
"Black Beauty"
1989 VN750 acquired December, 2008, 6,711 miles
Currently 23,298 miles

Old Blue
2001 Honda CMX250 Rebel acquired July, 2008

1987 VN750 project bike, acquired August, 2009, 33,000 miles and balancer sticking out of the case, currently awaiting attention and parts
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