HELP!! ENGINE WONT REV HIGHER THAN 5,OOO RPM's

Hello. I" dont know if this is a easy question to answer, but here it is: I have a 81' XJ650 maxim and it runs pretty good for being what it is but for some reason the engine wont rev higher that 5,000 rpm's. It only does this when the engine is under load... The engine revs up just fine all the way to redline, BUT, only when the bike is in NEUTRAL. I just did a valve adjustment, carb rebuild, and synchronized the carbs and ITS STILL doing the exact same thing. Could it be the jets in the carbs that are not letting me rev higher than 5,000 rpm's???? If not that, What causes a problem like this?? ANY help on this will be GREATLY APPRECIATED!!! Thanks in advance!

 

This isn't really related but I'm curious.

How fast are you going when the RPMs cap? On the way to work today I was going 65mph and the tach was hovering around 4800-5000. It doesn't go above 5k until I hit 70mph

 

It does it in every single gear.... Except 5th.
I take off in 1st gear and right as im aproaching 5,000 rpm's the engine just stops pulling and just sits there struggling to rev up any higher...
So, i go on to the next gear and it does the same thing up to 5,000 rpm.
BUT, once im in 5th. gear, it seams like i could go all the way to the 80 M.P.H. top speed without much hesitation.

 

Hmmm. Maybe it's a difference in the engine but I tend to up upshift around 3500, I've never even tried brining up to 5000+ in any gear but 5th.

 

Have you checked the fuel lines? Fuel filter? Vacuum to the petcock? Sounds to me like it might be running out of fuel at that point.

 

Really? Never? You're missin out..

I hit 9K all the time! 8O

 

3500 just "feels" right.

I'll try bringing her up around 5 on my way home, see how it behaves

 

Yeah. I have checked alll that, even replaced the fuel filter. But it hasn't fixed the problem.
Any other suggestions?

 

Sounds like fuel/air delivery problem to me. How do your fuel hoses look? New ones never hurt. Clean petcock? Inline filter isn't backwards is it? Negative pressure in your gas tank? Clean air box? Limp wrist? Ha just kidding.

It's interesting that it only happens under load... What happens if you pull the clutch in and rev?

PS - Yeah Speedwagon! I feel you.

 

Definately sounds like fuel/air problem.

Checklist

Emulsion tube blockage
Air filter
Float levels
Diaphragm leak
Sticky sliders (what was that term rick used? Stiction?)
Petcock

Each or any one of these could be the problem. Also check main jet sizes against standard. PO could have replaced with smaller.

Failing that, could be coil/ignition breakdown at higher revs. Does it backfire at all when you hit 5k?

 

Failing that, could be coil/ignition breakdown at higher revs. Does it backfire at all when you hit 5k?[/quote]
No, it doesn't backfire at 5k, but it just REALLY struggles to rev up any higher than that.... If i pull the clutch in, it revs up just fine though.
Well, i've replaced everything but the ignition coils.. I think im going to try and replace those to see if that fixes my problem.
Do you think a Dyno jet stage 1 kit would fix it? Would i see a good performance improvement?
Thanks again for the help!!!

 

Have you looked at the diaphragms in the carbs -

mine was doing the exact same thing a few weeks ago - revs to redline fine when standing/testing, but fails to go over 5K under load.

Turned out that two of the diaphragms had a tiny split in - temporarily fixed by a drop of superglue over each split - now revs all the way past redline.

 

Dyno Kit is not the answer. Need to get it fixed with stock gear first before adding unknowns.

AdeXJ has bought up an interesting point. Diaphragms are a pain and easy to miss a leak. On the weekend I was playing with two sets of carbs on my XS's and discovered leaks in them both.

Two ways to test this.

1. If they are clean then stick your mouth over the large oval hole on the filter side of the carb and GENTLY blow into them. If they are good the slide will rise. If they are leaking they may or may not rise but you will hear the air blowing straght through.

2. Lift the slide up and block the same hole with your thumb. The slide should stay up and slowly fall. If it falls quickly the remove that one and check for leaks.


How did you go with checking the other bits in the list.

 

what happened ya fix it?

 

------
I have not checked that yet. But i will most definitely check that as soon as possible. I will try and check the diaphrams tomorrow, tuesday, but if i cant do this tomorrow, i will do it for sure this weekend! Thank you very much for your help!

 

Well.. I have replaced the air filter, replace the float needles, petcock seems fine but i might still buy a rebuild kit anyway. I will try and check the diaphrams either tomorrow or this weekend. Also, where is the Emulsion tube? And im not quite sure about the sliders? Can you, please, explain them a bit more to me? Thank you very much for all your help!! Very much appreciated to all!!!

 

Have not yet checked them but what can i do to replace them if they are leaking? Do they sell replacement diaphrams or would using your glue technique be an ok fix or do you not recommend that? Thanks again!

 

An extra bonus is that the diaphragms can be checked while carbs are still in the bike - just be careful not to lose anything or damage the rubbers!!!

 

Diaphragms can be bought but have found that they are expensive here (£30 each, times four carbs = £120 [worst case]) - would be cheaper to buy another set of carbs (£50-£80 second hand).

On the plus side, if the splits are very small, the superglue method will work (had a KN250 with a diaphragm that had been superglued - lasted 6 years!!).

It just depends how comfortable you are, knowing that a blob of glue is what is helping your bike run properly.

Just be careful taking the diaphragms out - if they have never been out before, they could be sticking to the carb tops - remember they may be over twenty years old!!

 

Thanks for all your help, AdeXJ, you have been very helpful! I will surely try the superglue technique too see if that solves my problem. If you say your bike lasted for 6 years with this fix, pretty impressive! Anyway i will try that because, like you said, replacement diphragms are pretty expensive here too! The cheapest i've been able to find was $120 Dollars EACH, through bikebandit.com---- Thats just a bit too much for me right now. But as soon as i check the diaphragms i will let you know the outcome! THANKS AGAIN TO ALL WHO HAVE BEEN SUPER HELPFUL!!!!!!

 

Sliders are the big brass looking thing in the middle of the carbs attached to the diaphragm.

Emulsion tube is the bras tube that the needle on the slider goes down into. If you are going to remove the diaphragms then it's worth removing the emulsion tube for checking/cleaning as well. To remove take the float bowls off and remove the main jet((after the sliders are removed) and carefully push out the emulsion tube.


Cheers
HG

 

Well guys, I have checked all my diaphragms and none of them seem to have any holes or cracks in them. I did notice, on a couple of diaphragms, that there was some sort of residue on the tops of the diaphragms. Must have been fuel, right? This is bad correct? Also, on my way to work today the bike stalled out and for some reason didnt want to start up again. But i think it was cause i was kinda low on fuel. Anyway, i pushed it to a gas station and filled the tank up. On the way to work it felt REAL sluggish. I would have the throttle wide open and the engine wouldnt rev past 4,000K. It was doing the same thing in all gears. Well, after work i started going home and i didnt even get half way there. I HATE TO SAY THIS BUT THE POOR XJ650 FEKT LIKE A SCOOTER. I COULDNT GO ANY FASTER THAN 40m.p.h.!!! NO LIE!!! This time the bike was struggling to reach 4k. It eventually stalled out on me like a block from my work. Oh, sorry, i forgot one thing... As i would try to accelerate, i would have the throttle wide open and the engine would be strugging to pass 2k and 3k rpm's. At around 4k the bike accelerated alot faster but it would throw alot of white smoke. Then it would just bog out and die. I could start the bike and, kinda, keep it running... but as soon as i would take of it would just bog out and, again, die on me.I took it home and pulled the spark plugs off and they were covered in what seemed to be like a very heavy soot build up on the spark plugs. Well i changed the plugs and it runs alot better but what do you think is causing my problem? I still cant rev past 5k. well, 5,500 rpm's, with the new plugs. lol. This weekend, im going to give my carbs a VERY good cleaning. How would i fix a sticky slider? Sorry about the long post, and i appreciate you taking the time to read this. Thanks in advance to all!

 

Have a look at this link. It might give you some answers Hope this link works. xjbikes.com/Forums/viewtopic/t=1747.html

 

Black sooty plugs = rich = too much fuel
White smoke = unburnt fuel(lots of it) = too much fuel.

Without going back and rereading if you have done checked this already but you definately need to to a complete carb clean paying extra attention to needle seats and float levels.

From your latest description I'm surprised it runs at all as it sounds like it is flooding badly.

One thing that should also be checked is your exhaust. Does it flow freely? possible blockage?

Also try rmoving your air filter and going for a quick ride.(not too long) Does it make a difference.

Man, Where's that damn teleporter when I need it.

 

Sounds like when you put your diaphrams back together you might not have the seal lip right. There is a groove and the tab has a groove it fits into also and the spring has to be sitting on top of the diaphram and under the lid properly.
You need to test each slide to be sure that the diaphram seal is right.
To do this you simply lift the slide up and let it go, the slide should drop fairly slowly, if it drops fast then you have a sealing problem on the top or a hole in your diaphram.
also note the slot in the housing on the air cleaner side of the carb. when you lift that slide valve and cover that slot the slide should not drop very fast (slower than before you covered it)
Your engine is definatlly running rich!!
I'd like to have known what your plugs looked like before you tried any fixing but couldnt get over 5K


Good Luck
Mike

 

Doesn't sound healthy does it!!

Residue on top of diaphragms = fuel getting through somewhere - I would be tempted to take them out again and check - hold them up to a light and GENTLY stretch them - if you can see light through it, that's not good.

Black sooty dry plugs = rich = too much fuel OR not enough air - try without air filter/clean the filter. What are the jet sizes on your carbs?

Is there fuel in the gearbox (does your oil level look full when on the sidestand?) - possibly not related, but if engine is flooding - fuels gotta go somewhere!!!

 

Not healthy at all!
Also try the snif test on your oil fill hole, open and smell in the hole (when the engine is off) if it smells like gas then bingo you do have carbs flooding.
Also that goo on top of your diaphrams isn't good!!!... if you cleaned them off before, then they should still be clean when you go back in to them now.
Sounds like another carb clinic!!

 

HERE IS A LINK TO HELP YOU OUT
Maybe it'll give you some more info on those brass slides.
It did me!

 

There's also this thread

http://xjbikes.com/Forums/viewtopic/t=1772.html

which brings up the possibility of the TCI unit (but I would still make very certain it isn't the carbs first)

 

Actually, this all happened BEFORE i took the top cover off the carbs to inspect the diaphragms. I really didnt notice any holes in the diaphragms but i dont think i checked them as well as i should have. All i did was get a new set of spark plugs, and the bike ran, "ok", i guess, for a couple of days. Hasnt been more than 3 days since i got the new plugs and its back to acting like a 50cc scooter!! Well, this weekend those carbs are comming out and they're getting a REAL GOOD CLEANING!!!
But, how do i know what the proper air/fuel mixture should be? Is there a way to check it, or adjust it?
Another thing, it seems that when i leave my petcock in the "on" position the bike always stalls out on me and i cannot get it started again until i put it in the "pri" position. But if i leave it in the "pri" position, when parked, SOMETIMES AND ONLY SOMETIMES, i see that there is a small fuel leak from the #1 carb. Is this simply because i left the petcock in prime?
Well, thats it, for now... I really appreciate all the help and knowledge that everyone has given me. Thank you all!! And keep up the good work!!!

 

Everything in your post indicates that you need to yank the stack, shave your head bald and put-on an earring, there, Mr. Clean!! It's time!

Your going to find most of your problems in the bottom half. But, that don't mean you should take a shortcut and skip doing the uppers, too.
Overhaul your 4-pack. Pop-out the emulsion tubes (nozzles) - clean! Pull float valves - clean screens. Clean-tune the inside diameters of the slide cylinders. Once you have the carbs all cleaned and synced ... THEN you can play with the Pilot screws.

Too bad you are in Arizona. The guys I know on Yahoo Group "XJ-Planet" have very frequent Carb Clinic's.

I'm having a Carb Clinic - Tune & Tweak weekend on the 19th and 20th!
Pull the carbs off and hop a flight to Boston ... I'll pick you up at the airport, you camp-out at my place, YOU clean the carbs on my bench with me providing you with all the tools and "The Play by Play."

 

Please check your Private Messages - Mail - I have arranged a Private ... One-on-one -- Carb Clinic for you with a fine fellow in Phoenix.

Please acknowledge.

 

Well.... This weekend, I gave my carbs a COMPLETE CLEANING!!! I wish i would have taken pictures of the build-up that my emulsion tubes had. It was REALLY BAD. Almost every hole was either completely plugged or had some sort of nasty build up accumulated on them. I also checked the diaphragms, and none of them had any holes or cracks in them. Now.. The bike is running better than ever, at least for me. It is very smooth and the throttle is really responsive.. But it still backfires from time to time, and the fact that i cant rev higher than 5k RPM's is still there. I mean, the engine feels very smooth and all, but once i get to 5k it just sits there and struugles too much to reach a higher engine speed. Also, i checked the plugs recently, and they are now kinda turning WHITE. And now the right side exhaust seems to be smoking alot. What do you make of that?
Someone mentioned to me the possibility of it being my TCI unit... What is that? Thanks alot to everyone!!!

 

If it backfires at low speeds, just off idle, that means the mixture is too lean because the pilot jet is too small or still partly clogged. White plugs also mean it's too lean since they should be light brown. If you only cleaned the carbs once you probably need to do it a few more times. I must have cleaned mine about 19 times by now but they were really gunked up to start with.
If it won't go past 5k rpms the mixture is either too rich or too lean or it isn't getting enough air due to the vacuum slides not opening all the way or the intake or exhaust is clogged or possibly the ign. is failing because of bad sparkplug caps or wires or cracked coils. Could be the TCI but since they don't fail very often I would suspect the coils or carbs.

 

Bring the bike and yourself to the XJ-Planet Member I arranged the Private Carb Clinic for you.

You be all tuned-up and running in very short order.

People travel across three States to get tuned by him. How far is Tuscon from Phoenix ... cant be too far ... same State!!! Do it!!!!!!!

 

Hey guys! I just wanted to give thanks to all that helped me with my carb problems. As you might know, Last weekend i took the carbs apart, im suprisesd the bike even ran at all with the type of build-up it had accumulated. Anyway, the bike is running better than ever, at least for me. I have been riding all around Tucson with no problems. The smoking from the right side seems to have gone away. The ONLY issue now is that it still doesnt want to rev past 5k. I mean, the bike is incredibly smooth, i can cruise in it all day, as long as i dont try reving past 5k. What do you make of that? Well, for now, im am very pleased with the bike. No more stalling, sputtering, or hard starts in the morning. It's a NICE cruiser!! Thanks again everyone!!!!!!!!

 

If no fuel gets to the engine in the RUN position, and it leaks out of the #1 carb in the PRIME position, it sounds to me like the vacuum petcock diaphragm is shot. This would dump raw fuel into whichever carb pulls open the petcock with vacuum (#1??) causing it to run very rich. It would also prevent the petcock from opening in the RUN position, starving your bike of fuel, causing it to bog and stall. It would also cause a fuel leak when the bike is shut off.

 

Engine vacuum falls off with wide throttle openings as well, making any vaccum petcock issues worse at higher RPM. If the diaphragm hasn't failed, it at least sounds unhealthy.

 

Clogged air filter or fuel filter?

 

Screens on the float valves.

Wrong type of vacuum line from manifold to petcock collapsing on itself at higher revs.

Pozz-ib-ee da woik uh da Laud ... who doan wan cho-azz rydin no modder-sickle much fassah den da speed limit!!!

Kin ah gits huh hey-men hon dat!

 

Thanks for the useful info, MacMcMacmac. I will purchase a petcock rebuild kit and let you know how it went.

 

Air filter is fine. I, actually, removed the fuel filter, so i just have a hose coming straight from the gas tank and going from there to the carbs. Is this BAD?

 

Well, the float valves are brand new. The vacuum line seems to be ok.
Its still kinda back firing, sometimes at low RPM'S. But im hoping to fix that with the ColorTune this weekend. Just wanted to let you know, I love that last comment you posted about the LAW. HA, HA. Thanks for the laughs!! That was great!! Hey, you never know, could be true. :wink:
Thanks again for all your help!

 

Yes. It will work fine until you buy gas with some trash in it. Then you get to take your carbs back off and clean them again. That' is basicly the only problem. To be fair the fuel filter can become clogged with trash and put the bike down also. Knowing this I keep a spare filter in my bike.
I had mentioned the TCI unit and I still believe this may be the problem with the bike not running over 5k RPM. The TCI is wired apparantly so that another circuit kicks in at around 5k. My bike would run fine until about 46 to 4700 RPM and then it missed badly. Sounded like it only had two cylinders.
The tip off to me that it was the TCI was that I noticed the tach would become erratic at the very time the engine ran rough. Back off and the tach smoothed out with the engine. This is because the electric tach gts it's info from the TCI via the reluctors. This doesn't apply to the cable driven tachs. Best way to know is to put your TCI in another XJ and see if it has the same symptom. They come up on ebay all the time and sell from $20 to $50.