Hey guys, I'm back with a new question. I noticed my bike is starting to act a little weird. In the past, I used to accelerate from first, shift at 4,000 which drops the rpms to around 3,000, and would often cruise in the city around 3,000 - 3,500 rpms. Now let me tell you what is happening.

I accellerate in first gear. Sudden or gradual, no problems. Upon reaching around 4,000 rpms, I shift to second. RPMs drop to around 3,000 for a moment but rise as I accelerate. 4,000 RPMs is reached again with no trouble and I shift again. Engine drops to 3,000 RPMs. I continue to accelerate. The bike reaches 40 mph at 4,000 RPMs or so in 3rd gear. I shift to 4rth gear. Engine drops to 3,000 rpms. At this poing, if I try to accelerate, the bike starts to "jerK" or "surge" or something, and you can hear the engine bog down a bit. If you try to go up even the slightest hill, or have a bit of a wind, you not only won't accelerate, but you might even slow down. You can roll on full throttle but it only feels worse and starts to bog down more.

If you downshift back to 3rd, the bike will continue to accelerate. If once you reach about 5,000 RPMs, you shift back to fourth, the rpms drop to 4,000, and the hesitancy is mostly gone, and it goes away as engine speed increases. Whats the problem? Like I said, I have no trouble with this in gears 1, 2 & 3. Idle is good and strong, with no problems there. I have not really looked to see what happens when I try to shift to 5th at 3,000 RPMs. This problem has come in sort of recently. Nothing was done to the bike, e.i. I didn't modfy anything, do anything, no accidents etc since before the problem showed itself to after it came up. I checked fluids and it all seems good. Can anyone help me?

Ben

Things you can check:

1. Spark plugs "new or old"?
2. Add SeaFoam to the gas "get it at Napa Auto Parts"
3. Fuel/air mixture screw "has it been back out to 2 1/2 turns on the carbs?
4. Check carb boots and make sure they are tight.

Those are some things you can check for now.

Carb boots are a good idea. I'll try that. I'll also get the sea foam. Figure those things are cheap and easily done. Thank you so far.

Ben

sounds like a carb issue to me.....try the sea foam...

Yeah, I just got some today. I poured it in the gas tank. I'll let you know how it works out. Thanks for your help.

Ben

pretty much this same thing is happening to me this weekend with my 86 vulcan.. all is good until i am in 5th gear then it starts surging and lurching while i am holding a contstant throttle. if i really give it some throttle it usually does fine but when i am just cruising along its surging and lurching.
yesterday before this problem was happening i had just changed the oil and the gear oil and put some seafoam in the tank and fille dup. it was after that when this started happening. you think the seafoam knocked some crud loose or something? why would it only act weird in 5th gear?

thanks.

Ben - Only 4th gear? That DOESN'T sound like a carb problem. Carbs don't care what gear you're in. That sounds like a transmission problem.

When mine was doing this...turned out to be the vaccum line that goes to the RH air cleaner. I had replaced it during the rebuild and effectively plugged it by pushing it in too far. How to fix? Pull off RH air cleaner...cut end of tube at a diagonal...reassemble and test.

I wish I understood why this particular vacuum line has this effect...I don't understand, but it does. Can't hurt to try, right?

Could this be the clutch slipping a little bit?

Could this be the clutch slipping a little bit?

is there a fix for this or just a replacement?

The tube you mention is a vent tube for both carbs. Not a vacuume tube. A tube from both carbs that run into a T with a single line going to back of the right side air filter.

There is actually no "fix" for a bad clutch. It is not a very difficult job to replace the clutch. I did mine on my Suzuki Intruder and it was a piece of cake, and I am not the world's greatest mechanic. I believe most bike shops will replace a clutch for about $200-$300. Try going up a steep hill and opening the throttle wide open in fifth gear and see if you detect the bike revving a little too much, that's how I found out my Suzuki clutch was worn out. Good luck; maybe the problem isn't your clutch at all.

someone on another board said it could be sparkplug related? what do ya think?

You said the bike only "stumbles" at around 4000 rpm, right? It's ok below and above that?

If that's the case, I wouldn't guess it's the plugs or ignition system. But since it only takes 30 minutes to pull the plugs and check them, it can't hurt, right?

You said the bike only "stumbles" at around 4000 rpm, right? It's ok below and above that?

If that's the case, I wouldn't guess it's the plugs or ignition system. But since it only takes 30 minutes to pull the plugs and check them, it can't hurt, right?

yeah thats what i was thinking. its when i am crusing at about 40-55mph in teh higher gears when it lose spower and surges at a constant throttle. the rpms don't seem to change but the bike seems to ride as if riding on nice pavement and then a section of glue and then good pavement, then glue. haha..

thanks everyone. if you have other ideas please express them!

Oh, I forgot that I had posted this problem on this board. Thanks for all of your replies.

I did find the problem, and since I bet you all want to know what it is, here goes.

One of the carb boots had come loose, and I think it had disrupted the airflow causing mixture problems. When I think I got into fourth gear, the bike needed more throttle to accelerate than at say second gear, and this was when the problem became noticable.


Once again, thanks for all of your help, and hopefully if you ever see a problem like this again, you'll know another place to look.

Ben

K.I.S.S. check the simplest first.

Hi, same problem it seems but we've had the carbs off and cleaned twice, coils checked, plugs and leads replaced twice, tank cleaned out twice and still it starts to hesitate at 4000 revs. Apart from a box of matches has anyone got any other ideas? Ian

I suppose you have checked your carb boot connections.

Yes checked that and been out with vacuum gauges on. Any knowledge on cdi unit?

That's always a possibility. Looking at the schematics, maybe pickup coils, ignition switch, side stand switch, stop switch, or lockout switch.

Can you give us some more info about the bike? Year model, mileage, recent repairs, when the problem started, etc.

It's a 1996 vn 750, 30,000 miles, no mods done and it just started a couple of months ago without warning.

Can you tell which cylinder is giving you the problem? Another possibility, although just slight, is the oil pathways somewhere are clogged and keeping the hydraulic lifters open too long. But in that case you'll usually have backfiring thru the carbs or exhaust. Another version of that would be the cam chain being worn and too much slack throwing the valve timing off. But it would probably do that at other rpms as well. Check your charging system. May not be enough voltage at the battery to keep the coils charged at that frequency.

You might consider replacing the pickup coils. Other than that, it would be nice if you could find someone else in your area with a 750 that you could swap the IC Igniter and see if that's it. They are a bit pricey, from RonAyers for about $400.