High revving on its own.

[shagadoodle]

Hey guys, since im a new member i'd like to introduce myself. My name is Rob im ASE certified in non structural and structural collision repair. Bikes and old cars are my hobby as i have a 68 mustang, 2001 yzf-r1, and i have a 79 xs400 2LO. I am also a student at TNtech University studying Mechanical Engineering.

So my dad bought this bike off of craigslist about 4 years ago for i have no idea why but it only cost him $600 The bike only has 3123 miles on it which i have only put probobly that 123 on it driving it in high school. Initially when i first started working on the bike it had carb problems. The guy before me or well whoever had the bike the logest screwed the mixture screw all the way down and broke the head off in the carb so i went to my local motorcycle salvage yard and picked up an identical Mikuni bs 34 carburetor cleaned it put it on and ran good. unfortunately i had problems where it would just quit on the side of the road on me figured it was the old points system so i went back up to the salvage yard and picked up all the components for the Hall effect ignition system off of a newer xs400. put it in all wired up correctly and it ran worse had no power or acceleration. Put the points system back on and fired right up and runs a whole lot better Unfortunately i still have a problem.

The problem i am having now is the bike wants to rev up on its own. I can start it but as soon as it starts it jumps to about 4k+ rpms but then after about a 30 second it will drop down and act like its going to die. i get it to idle at about 1500 and then when i touch the gas it revvs back up and does the same thing over again. I think maybe its running too lean but adjusting the mixture screws doesn't seem to help it any.

Things ive done to the bike since putting the points system back on
1. static timed it based on the youtube video i watched
2. recleaned the carb (complete dissasembly soaking parts in berrymans)
3. removed the mufflers.
4. Gapped points to .014

I know that removing mufflers would require me to up the jets correct? where can i get bigger jets for it at and would it be the main cause of this problem? if so ill put them back on but they had holes in them anyways.

thanks for the help its much appreciated i hope to drive this thing as much as i do my r1

 

[dburrows6735]

How are you carb boots looking? Check your fuel valve vacuum line as well. Welcome btw! Try MikesXS for new jets, not sure on the size but you will need to go up a step or two.

 

[DanMach]

Carb boots is my guess as well. really common on these bikes.

 

[JARichmond]

For new jets, you can visit a local motorcycle dealership. They will likely have jets in stock at the parts department. Be sure to call ahead to save yourself the trip if they don't have any.

 

[shagadoodle]

ok and do i need to get main jets and pilot jets or just main jets? and do i need to get a new needle while im at it? and im about to test for a vacuum leak if it revvs up after spraying starting fluid on the boot then i definately have a leak if it doesn't what else could it be?

 

[JARichmond]

This should answer your jetting question: http://www.motorcyclecarbs.com/Motorcycle_Carburetor_Jetting__W7C240.cfm
You may have to adjust your needle clip setting, but replacement isn't necessary unless there's evidence of damage.
If the boots aren't cracked, you may have air coming in through a space in your intake ducting where the collars aren't tightened down enough or have come loose. Of course, if your jets aren't right then you will get a false positive on a lean condition.

 

[shagadoodle]

Woo good news. New carb boots, new 150 main jet and a 47.5 pilot jet (they don't have a 1 step up so i stepped 2 up. new air filters and she runs like a champ! i might try bumping the pilot jet back down to a 42.5 but idk yet, i also might try a 147.5 main jet and see if it runs better with that (i order +3 and +2 jet sizes) gonna play with some combos but all is looking great! plenty of power. although i did get some complaints from the tourists about my bike being loud lol.

 

[njcafe84]

I am having the same problem with the revving to 4k on its own after I got the bike started for the first time this week but my carb boots are brand new. Carbs cleaned and running stock air boxes. I don't know where to start looking to fix this

 

[JARichmond]

You should be getting a high idle on choke if your engine is warm. If it isn't warm when this happens or your choke isn't on, look for evidence of a sticking throttle cable or your idle adjustment screw (in between your carbs) being too tight. If your throttle cable isn't snapping back appropriately, you can lube the cable with engine oil.

 

[Drewcifer]

thanks Jar, I was going to give the same advice.

The sliding nylon beneath the grip and the cable need to be lubed. Drylube for the nylon, oil for the cable. Once sorted the issue will go away.