Runs great, but lacks power bigtime when accelerating

My 1983 Yamaha SECA XJ900RK is running very well and starting well also, she just lacks power durring acceleration. I checked the timing on the bike and it is a little off, I dont even know where to start as far as what could be the problem as I understand the timing is fixed and can't be adjusted. It has a dynojet stage 3 rejet kit, Jardine 4 into 1 exhaust, and Accel high energy coils. I rode this bike for 5 years without issue before, so this is a new development. The carbs have just been dipped and cleaned to hopefully rule out a fuel /air issue. Any suggestions would be greatly appreciated. Hopefully these videos work.

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when you "dipped" the carbs, were they FULLY disassembled; as in, throttle shafts/butterflies and shaft seals removed? If not, you may have "killed" the throttle shaft seals.

When was the last time you checked valve clearances?

Were the carbs vacuum sync'ed (YICS blocked) afterward?

 

I did not do the carb dipping, I sent to a shop to have it done, I have not checked the clearance, this happened quite suddenly and was an issue before I had the carbs done. I have yet to synced the carbs. I didn't figure the timing would be off because of a bad carb synchronization.

 

Bad move by sending them to a shop. Nevertheless, it is done, so you may have to 'correct' things yourself. If they totally dis-assembled them to the point where there was not a SINGLE piece of ANY sort attached to ANYTHING, then OK. BUT if ANYTHING was still connected to ANYTHING, then you're going to have to start backtracking.

That all said, assuming all the throttle shaft seals and fuel rail seals and enrichment rings are still good:

You must first do a bench sync to get your baseline starting point. Then you sync from there.

You must also make sure that the enrichment screws are out 2.5 turns as a starting point.

You must check your float settings.

Now, before putting the carbs back on the bike, make sure you do your valve shim checks and adjustments before anything else.

Double-check compression numbers, too.

Once shims are back in spec, go ahead and put the carbs back on. Once they are on, you'll have to go through the sync process.

YICS tool/no YICS tool......on-going debate. Personally, I don't use one. (BUT I HAVE one!!!!! LOL) I like to look at it.....it's pretty. If you don't have one, you can still do it. If you do have one and do use it, you'll be about 1 or two degrees closer to perfect than mine.

Once you have it dialed in, it'll run like a scalded dog. NO.....I haven't scalded my dogs. I just saw one once, and it was running pretty fast.

Keep us posted each step of the way.

Just because 'they dipped them' doesn't mean you're all set. You still have a bunch of things that MUST be done.

Dave Fox

 

How's your air filter? When mine's dirty it makes quite a difference. You don't really notice it a low rpms and small throttle openings but under heavy load it does affect things.

 

i sent my carbs to http://carbking.weebly.com came bacclean as a whistle that made the biggest differance

 

Could be the Diaphragm Pistons hesitating to rise.
Search "Clunk Test"

Main Nozzles might be clogged.
Clean Emulsion Tubes.

Clunk Test might be in my signature.

 

My 550 runs like a scalded dog who's still being chased by the heat! When I wind it out it scares people, me included!

But seriously unless you know the guys at the shop and can trust them then there is no way to know (w/o watching them do the work) just how good of a job they did-or what they didn't do for that matter.

+1 to Rick's idea because polishing and smoothing out the piston bore is tedious and probably got skipped if anything did-and if they took you for a ride then your emulsion tubes could still look cheesy. Could also just be badly out of synch but if you're saying it idles smoothly then that's probably out.

 

That return to idle would concern me, seems like it takes a bit too long to drop.

 

mine did the same id let off the gas and would take a while to come down i turned the idle screw about a 1/4 turn to the left and it stoped best is wait till its hot then rev and turn screw mm at a time and repeat till itcomes down right away if this helps

 

When you are in a fight to get IDLE close to specs and the bike still gallops o n you, ... you need to suspect an AIR Leak.

Some AIR Leaks are hard to find.

One Occult-type AIR Leak is:

Manifold -to- Head Joint Leak.
The O-ring Seal dries-up and the Manifold Leaks between the Manifold and Head.
The Fasteners SEEM Tight, ... but, are actually seized.

Seal the Edge of the Manifold all-around.

 

Dave
I think I know what that scalded dog was running from!!

I see a Bad Moon arising! :lol: :lol: :lol:

Once these bikes are dialed in they have seriously scarry acceleration. Don't let the back end get in front of you!

Ghost