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Rough Running under hard acceleration

Old 06-27-2016, 08:26 PM
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Rough Running under hard acceleration

Forgive me if this has been discussed but I couldn't find any thing exactly like this using search.

With that out of the way I have a 98 VTR with 45k miles it has always run great but lately it has started to run rough under hard, open throttle, riding. It will start to pull then run rough almost like it is only on one cylinder then it sputters then it cleans up and has full power. Needles to say it sounds awful and is rough ride as the engine pulls hard then sputters to probably 20 or 40 hp then slams on with 80 or 90 hp. Happens in all gears and seems to be in the 5500-7000 range?

Has anyone seen this before and what could it be? Vacuum leak or carb issue?

It idles and runs really smooth under normal riding and slow to moderate acceleration. Starts right up. It has me scratching my head,thanks in advance.
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Old 06-28-2016, 05:03 AM
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Sounds to me like you need to remove the carbs and go through them thoroughly. Make sure to check the diaphragms for any abnormality.

Does/did this bike sit for any amount of time without being ran? Left sit, the ethanol can cause a deposit build up that can become very problematic. With your description, may not be this but it's usually the first thing to question.
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Old 06-28-2016, 07:04 AM
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Ditto .
Maybe you can pull the tank and air cleaner and move the slides out with your finger, release them and watch/listen for the "quiwhoosh" sound to be the same on both sides.
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Old 06-28-2016, 04:54 PM
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You might be suffering from a failing vacuum petcock, which could interrupt steady fuel flow to the carbs. Easy enough to disassemble and check anyway.
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Old 06-28-2016, 05:34 PM
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Yup I agree with the three above. Check the diaphrams on the petcock, check the slide sounds then the carb diaphrams.

Don't just look. Use a light to check the diaphrams. If any feel bad, like sticky rubber, replace. If all are good then report back.

I've had simular situations and it only got worse to wherr the bike shut down. Started as you described and quickly escalated. Also make sure the vac line going to the petcock is hooked up and in the right spot or that hasn't fallen off.

Mine was the petcock diaphrams. I took it appart and didn't notice a pin hole. With in a few days the bike died while I was in the fast lane on a major highway during rush hour.

Last edited by Meier Link; 06-28-2016 at 05:37 PM.
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Old 06-28-2016, 06:02 PM
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It sat over winter and I didn't ride it much last year. I put stable in it over the winter however I don't know however if that protects the carbs that well. I pulled out my shop manual and It looks fairly involved to pull the carbs with all the stuff that needs to come off. Is it possible to do the checks the diaphragm with out pulling the carbs and draining the coolest and pulling off all the hoses? How hard is it to pull apart the pet clock? Thanks for all the great advice. This is the first real time the bike has given me any problems, I guess I was due as it is 18 years old by now.
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Old 06-28-2016, 07:06 PM
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No need to drain the coolant, and if you remove the carb heater hoses you won't lose much liquid. I would start with the petcock diaphragm first, you just need to pull the tank off for that one, the tank cap is sealed so you can flip it upside down without draining the tank. Petcock removal/disassembly is pretty obvious from that point.

IMO the worst part of carb removal is threading the choke cable nicely out from under all the hoses, but the rest is pretty straightforward. Easiest to remove the throttle cable bracket once the carbs are loose, makes the cable ends easy to unhook.
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Old 06-28-2016, 07:21 PM
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Originally Posted by vtrguy View Post
It sat over winter and I didn't ride it much last year. I put stable in it over the winter however I don't know however if that protects the carbs that well. I pulled out my shop manual and It looks fairly involved to pull the carbs with all the stuff that needs to come off. Is it possible to do the checks the diaphragm with out pulling the carbs and draining the coolest and pulling off all the hoses? How hard is it to pull apart the pet clock? Thanks for all the great advice. This is the first real time the bike has given me any problems, I guess I was due as it is 18 years old by now.
If you get Stable, make sure it is for ethanol fuel, if that's what you fill up with. And, Stable will only protect the carbs if you put Stable in the tank and then run the engine for several minutes to get the Stable fuel into the carbs. I have run into so very many people who miss this step and don't understand why thier carbs are gummed up.

You do not need to drain the coolant to check the carbs. Once the airbox is off, you will see the carbs clearly and can check for that the carb slide movement is equal with both carbs. Slide both of the "black things" up with your fingers, then let go at the same time. They should slide down at the exact same time. If not, this could mean you have a rip in which ever of the carb diaphrams falls fastest.

About the petcock, just to clarify, it only has one diaphram. Since you will have the tank lifted to check the carbs, it is very easy to check if the petcock is working. It is vacuum operated. Speaking of which, the vacuum line must be on the back of the petcock. It does not attach underneath the petcock.

EDIT: Cadbury egg got to it before me!
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Old 06-29-2016, 07:21 PM
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Thanks everyone I'll check those things out. The stable I use is just the standard stuff I guess I will have to get the marine stuff that I use in the wave runner. I do put it in and ride it for a few miles, usually back from the gas station.
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Old 06-30-2016, 06:56 PM
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I have been using my witches brew for some years, I have never had to pull a set of carbs down due to fuel related issues when storing
1. StarTron it is much moere effective than Sta Bil, over treat by 50%
2. Lucas Upper Cylinder Lubricant, 2oz per 5 gal
3. Marvel Mystery Oil, 2oz per 5 gal
When starting for the first time in spring, add 4oz of STP Fuel Injection cleaner to the gas.
Add Lucas to the next 3 fill ups
Add Marvel to every other tankful during the riding season
Ethanol SUCKS!
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Old 06-30-2016, 07:35 PM
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My dad was a big fan of the Marvel. I remember it from wayyyy back. My boss @ the bike shop swears buy this stuff.
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