1982 Seca XJ750 Revs to high INSTANTLY!

Hello,

I purchased a 1982 Yamaha Seca xj750, knowing that the bike needed work.
When I purchased the bike, it had already been cut down to become a bobber.
The owner before me cut the frame, exhaust, and did poor carb modifications.
He tried to clean the carbs but they still had gunk in them.
Also the o'rings for the gas-line on the carbs where pretty much non-existent.
So, I thoroughly cleaned the carbs and put new o'rings on "THINKING" that would stop
the bike from instantly revving up when starting it. "It didn't"
Then I decided to adjust the mixture screws, and play around with them, and I found a point
where the bike will stay at normal idle after ignition instead of instantly revving, BUT
then when I give it throttle, it starts max revving again.

I have read around the internet for the last few days, and I am back and forth with it either needing a specific carb adjustment or a brand new exhaust set.

 

You have a vacuum leak somewhere. First step should be to do a throough rebuild of the carbs (I suspect throttle shaft seals are to blame, and you didn't likely change those).
IN THE CHURCH OF CLEAN
The local parts man is Chacal. Start a conversation with him, as he has the correct parts in stock (not many other places do).

The second thing to look at is the jetting and intake. Does it still have the factory airbox?

The stock exhaust is very free-flowing and the lack of it would not be the cause of your problem.

More homework for you.
The Information Overload Hour
Read it all. Do it all.

 

K moe is da man....if you replaced fuel rail o ring ...then you already "broke" the rack ..you really need to do it again and replace throttle shaft seals ...soak carbs in carb cleaner and install new seals and do not forget to replace mixture screw o ring . Do a bench synch of carbs. Check intake boots fot condition.

 

Sounds like a combination of things--

1. Probably vacuum leaks at the carb to manifold boots.....most likely the seal at the manifold
2. Likely the throttle shaft seals are shot.....agreed with the others
3. Carbs are probably any out of sync

Fuel rail o-rings wouldn't normally cause this......they usually cause fuel to run all over

Other things that can be factors:

We're the jets put in wrong/ wrong jets/missing
Wrong cables
Missing vacuum cap missing
Sticky slides
Etc....

Anyway you look at it, it pretty muchcomes back to carbs needing a total overhaul. If you don't do it ALL, you're gonna chase your tail, and by the time you have done it all, you're gonna start all over again. Do it right the first time and you won't have to do it again for many many years. When ai rebuild carbs for people, it is shipped back as close to new as it can get: anything bad is gone, new seals, rings, gaskets, fuel valves, anything missing is replaced, everything set back to factory initial settings, bench-synced, ready for the owner to do the final wet-setting, running sync, and color-tuning.
If you're interested in rebuild services, contact me directly.

Dave f

 

1. I am going to go and check that now.
2. I requested my local 'Yamaha-street and trail' to look up the throttle shaft seals, and they said it wasn't in their system. ("Could someone link me a way to purchase please")
3. To my knowledge they are not out of sync. Am I supposed to adjust the Mixture screws on all 4 carbs 2 1/2 to 3 turns out, because that's what I did.

 

1. Good
2. Of course not-- they're clueless, like most dealers---these bikes are too old for them. BUT we have them right here. Go to the top right corner and click on the xj4ever logo. OR email Chacal directly at: info@xj4ever.com, and he'll hook you right up.
3. Then they probably are out of sync.
4.you are supposed to adjust each one To its own optimum setting. 2.5 turns out is just the initial factory setting. You may end up turning it back in, or farther out, or leaving it alone.

 

...and the idle mixture screws have nothing (much) to do with carb synchronization. There are separate screws for that, and it does need to be done occasionally (every few years depending on how much you ride, among other factors).

Instructions for the baseline synch HOW TO: Bench-synch your carbs a running synch is done with a manometer after setting the idle mixture.

Slow down. Read that second link I provided, along with all the links embedded therin. Educate yourself before starting any of the work; that'll make it all less frustrating for you.

 

Thanks K-moe,

I took just one seal off the throttle shaft and it's braking apart.

The family FRIEND/local mechanic told me those didn't need to be replaced. He even got pissed about asking that.
Well I am going to show him a picture of it tomorrow, and see if he is so sure now.

 

They usually don't need to be replaced...unless they are several decades old and falling apart.
If you value the friendship, keep the mechanic/friend away from the machine and do the work yourself. Easier to avoid arguments that way.

 

I'm doing a rack right now for someone the " the PO said he cleaned and overhauled them" welllll.... O rings are shot, t-shaft seals were shrunken, Loose, etc..., bowl gaskets were brittle and cracked, beanie screens covered in crud, emulsion tube clogs, missing screws, brackets in wrong places.... AND--- mixture screw caps still in place.

NOW they're being CLEANED.

 

Please read all the links and xj4ever overload hour. The only way to replace the throttle shaft seals is to break the rack ...meaning taking the carb bodies off the rails ...you have to back off synch screws there are 3 . You also have to remove 2 butter fly screws , remove plate,slide shaft out. Each carb have seals on both sides ALL need replaced. This sounds hard but it is not will take you a few hours. Ideally what you do is tear down carbs all gaskets, seals, O rings removed ...get a gallon of Berryman carb cleaner (Autozone) soak eack carb for 30 minutes , wash them in water and Dawn and dry them out. Then put new seals in with silicone grease ( ace hardware plumbing ) put shafts back in . Take pictures and keep each carb parts with the carb you work on. There is much more this just gives you a highlight . There are some you tube videos on going carbs to ....or send carbs to Hogfiddles and have him do them .

 

When you break your rack to clean it, it will look pretty much like this IF you do it the correct way: note--not all parts are shown

 

Polish the slide bores just a little too, if the slides don't "clunk" whn you st them into the bores.
Hype on prepping the carb piston/slide/cylinder

Progressively finer gits of wet-and-dry finishing paper can be subbed for th scothcbrite pad and rouge. I usually start with a 600 and work up to 2000.

 

Dave - that is like motorcycle porn, stripped down and all the bits and pieces are glistening!

 

The quote is for Ollie.

Gary H.

 

No, It was replaced with air filter pods.

I would show you a image of my carbs and everything that I have taken apart, but a error message pops up
every time I try to post a image. Posting a image "Premium Member" only permission?

 

Premium membership is not required. You need one more post to prove that you are not a spambot.

 

The good...
It'll run if the mill has in spec compression. Get the valves clearance in spec and do a compression test.
The bad...
The carbs must be (as others have stated) "Church" clean.
The ugly...
CV carbs and pods don't play well together. Xj's perform best with the stock air box. If you're keeping the pods you need to get a set of air box to carb boots, cut them down and add them to the pods.Then you can start the re-jetting process. Like k-moe stated...Xj's are free flowing so the exhaust pipes shouldn't be a problem. The air coming in, dirty and out of tune carbs is the problem. We're here to help.

Gary H.

 

The screws on one of my butterflies are stripped worse then a broad street hooker.

I guess the guy that tried to do this before I purchased the bike used the wrong size phillips head screw driver.
I am thinking about heating them up, then taking them out. Is that a good way?

 

Get some valve lapping compound put some on the head of the screw and a little on tip of screw driver the grit will keep it from slipping.

 

+1, or superglue.

 

All the screws in our bikes are called JIS (Japanese Industrial Standard) screws which are similar but different than phillips.

Gary H.

 

And the differences make for buggered-up screw heads.

 

Well still having problems.

I changed the throttle shaft seals, throttle shaft shim washers, and 2 head screws but the bike still goes max throttle.
I think for some reason the bike might be getting TO much fuel. That is the only thing I can think of.

By the way, XJFOREVER - 10/10 for the parts I purchased and reply emails!!
Parts were perfect and fast shipped!!

 

Too much fuel would cause the engine to bog or flood. You either have too much air for the amount of fuel, or throttle plates that are not closing.

Did you perform a bench synch after putting the rack back togther?

 

Gaurentee your butterflys are not centered right and your synch is off...ask me how I know this...happened on mine had to loosen screws on plates the plate moves slightly and I use a buisness card cut in half under butterfly adjusted synch screw to point where there is a slight drag on card..this will get you real close. After carb install I use a 4 vacuum gauge set to synch carbs all 4 of my carbs are all reading same on gauge only had to tweek it a slight turn ..and my idle is 1100 rpm ...I have stock airbox and since carbs have been to church have not had to touch carbs.

 

Am I supposed to leave a very slight opening like he did?

 

you are trying to make them all the same you can use a narrow strip of a card or a small drill bit. carb three is controled by the Idle knob set that. then match the other three to it
set them so there is a slight drag on what ever you use to gap them

when done you will set them all (they will move together) by using the idle knob
small gap is where you set them. then when running you set the idle with the knob

 

okay, thankyou.
I will adjust that now.

 

See the text msg I sent you---

 

Yea I got it. Thankyou. That was info that was needed.

 

I want to thank all of you for the amazing help that was given to me!
I now have the bike running, and it drives great!!!
The bike has WAY more power then I even thought it would. I can't even explain how happy I am that I found great people like you guys
that helped me to get this fixed. Thanks!!

I will be sure to come back around the forum every-once and a while to keep updates.

Thanks, XJFOREVER, for the amazing and reasonable parts!
Thanks, hogfiddles, for help over the phone!
And thanks to EVERYONE else that helped me with this thread!

 

YAAAAY!!!!!

 

I see this is a bit of an old thread I read your post sounds like your pretty knowledgeable about the carbs and I would presume the bike in whole, what I have is not a motorcycle I have a yamaha G2 golf cart I was playing with and one day I lucked upon two xj 750's for the sum of 300 bucks, this started out well as the cart came from a farm pretty well beat to he'll but for 100 I had to buy it even though a live in the middle of the city with no legal possibility of running this anywhere but hey I ain't exactly a rule follower so here I am 6 months later with countless hours days weeks making this come together and I'm near the end and had the life scared out of me thinking I burnt a hole in a piston running to lean motor just stopped then sounded weird .... Anyway got passed that hump but can't get this thing to idle down and may need any good advice that will save me any further headache as I've already been through a fire bent valve head gasket etc I wish it were as,simple as just throwing another motor in cause that would be easy except I had to make this thing work with the direct drive which in the end turned out to be a ring and pinion off some cv axle at the local recyclers I spotted one day sandwiched in between the mostly hollowed out halves of the direct drive hub that was my biggest obstacle but I maintained limited slip and I think it was like 12.5 to one ratio so yeah if ya got a minute I have a couple things I'd like to chat with you about