can i fix this ??

ok so my 81 xj750 runs good or kinda.. runs great after a couple minutes of full choke i cna drive around at normal speeds but when i try to open it up and it gets to 5k rpm in any gear it starts to bog like not enough fuel or air
first bike please help
and yes choke i turn choke off onces it warm.

 

First try a can of gumout in your tank, if there is no improvement over a hundred miles then it's time to clean those carbs. While it's a pain to do it, do it the right way and you shouldn't have any trouble. I think you may have partially clogged main jets, or emulsion tubes, but the float valves may be out of adjustment or their screens may be clogged.

 

It's time to clean the carbs.

And properly set the float levels.

And check and adjust the valve clearances,

So the carbs can be sync'ed.

Gumout won't fix anything.

 

yea the tanks fine and its getting fuel other wise itd bog like that at any speed but i just rebuilt the carbs or cleaned them at least bike wouldnt even idle with choke off when i first got it now it runs smooth until she hits 5k.. would that be because of the float levels ? and if so how do you set them properly

 

Float levels: http://www.xj4ever.com/setting%20fuel%20levels.pdf

Valve clearances: http://xjbikes.com/Forums/viewtopic/t=14827.html
Part Deux: http://xjbikes.com/Forums/viewtopic/t=29209.html

Float levels need to be accurately WET set; then you'll need to do a running vacuum sync on the carbs with YICS blocked.

But you can't do an accurate vac sync if your valves aren't in spec.

I suggest a service manual be your next investment; you're going to need it if you plan to ride and maintain a 30-year old bike.

 

ok so ima attempt this but i noticed that the lids for diaphrams have a dent in them does it matter which way they face.. towards the nose or tward the back of the bike ??

 

The bevel, or dent, is cosmetic. They can face anyway you would like them to.

 

thanks biggs. and guys some more info on my problem the bike popps sometimes when i down shift or i let off the gas by pops i mean at the exhaust

 

Get those valves checked! as bikes with bucket under shim design will experience ever tightening valve clearance as the valves wear. This is very important for the exhaust valves, which may explain the popping. It may be a pain to check, but if you catch it in time you will save yourself a ton of cash.

 

Popping can indicate a lean condition as well as ignition troubles. Tight exhaust valves will also do it and with tight valves you can get valve float at higher rpms so I would check that clearance before I messed with the carbs.

 

can tight valves be casing the weird spuddering at 5k?
and i how do i check them ?
sorry for all the question just wanna try to locate problem before ig o tearing into it

 

The first step in getting the carbs properly synced is to make sure the valve clearances are correct.

Could dirty carbs, and valves out of adjustment cause the problems you are experiencing. Most deffinately!

Look in the FAQ suggestion section for Fitz's 2 write ups on checking, and adjusting the valve clearances.

Ghost

 

Here's what happens to tight exhaust valves; first they overheat, as they are cooled by good contact with the seat, next the valve burns, cracks, fails to hold compression. If you have done a compression check and found your values are somewhat low, say 50 to 100 psi then it's probably too late. You may start crying now. If a shop has to do the job for you, your lookin' at a minimum of 5 big ones. Better to buy one from Ebay.

 

well i did a comp test today one cold and one hot when cold this is how they read
1) 145 2) 140 3) 135 4) 135
hot 135 all across so they're good and now you guys have me scared and ima stop running the bike till i check those valves
im 20 and this is my first bike so i don't have much experience but tomorrow im picking up a gauge and measuring them and i have something to go off thanks to bigfitz

 

Those numbers do not look bad, but the #3 & #4 cylinders could be low because of tight valves.

Here are the write ups for the valve adjustment.
AIRHEAD VALVE ADJUSTMENT with Pics
AIRHEAD VALVE ADJUSTMENT Part 2: The Tool vs. ZipTie -w/pics

Ghost

 

just as a matter of interest Random, have you fed the bike a diet of new spark plugs ?
Old plugs will break down like a biatch under load, like at 5k as you described....

 

You might want to actually read the answers you've already been provided:

MiGhost is correct; your compression numbers look OK. Your symptoms indicate that you need to check your valve clearances immediately; and properly service the carbs.

GET A SERVICE MANUAL. The "how-tos" are intended to be supplemental to a service manual; not a replacement.

 

Good point. Also if the plug caps are old, cracked, etc. they will leak voltage. Not a bad idea to replace these as well. Contact Len (chacal) for the correct ones as they are resistor caps and off hand I don't know the correct resistance.

 

Another thought, while cleaning the carbs be sure you have the proper size main jets in the proper carb. The PO had reversed mine ( easier catch as I only have 2) but caused a dead spot around 5 K, wouldn't not rev hiogher

 

checking valve clearance today #3 was alittle loose which was wierd but 4 was tight on both exhaust and intake bigfits i didnt see the valve clearance link but when i did it gave me the guts to go for it. i check the clearance on all of them and replaced shim with a home made tool but i checked them again still off so ima remeasure carefully this time.. also learned kawi shims are .5 wider haha lucky it was a straight trade.. once i have valves good ima check my carbs out again

 

ps if i remember correctly main jet is 120 or something like that right ? and what part of throttle will the diaphram run on the carb 2/3 to full ?

 

plugs are new got recommended maybe coils no good gotta check that too

 

setting the float valves makes since as far as the measurements go but how the hell do you get there i could use a 10 foot hose and the fuel level wouldnt measure 3mm from the top of thr bowl it even reach the bowl im sure the special hose he uses in the setting of the floats is a certain length right or im i missing something here ?

 

Nope and nope.

The hose can be as long or as short as necessary, the fuel level should end up "mimicking" the level in the bowl. It just takes more fuel in a longer hose.

 

i dont get it are you measuring how much fuel is in the line ? why cant we just do it this way say let the carb fill up and empty it into say a graduated cylindee and check how many millliter of fuel are in it and set it till it hold a certain amount

 

You're measuring the level.

By having the clear tube outside of the carbs, bent up along side of it, you will see the exact same level of fuel that is IN the carbs.
Compare if it's too low or high in relation to where the bowl meets the body, that's where it should be +-1mm.

 

Like this.

 

You could. If you first calculate the volume of the bowl, minus the float and any carb body and jet parts that reach down in and then subtract the spec height from the top of the bowl.

The hose method is far easier.

 

i get that but if the tube is too long wouldnt the level be low cause the fuel wouldnt reach up to a high enough level in the tube ?

 

No.

 

Fluids find their own level regardless of how long the tube you are using is. The wet set method works exactly the same way as using a water level in the building trades. It's a method that has been in use for several thousand years, and is extremely accurate.

Here's a link that explains the principal.

http://www.deckmagazine.com/tools-and-e ... level.aspx

 

IF you have an adequate supply of fuel in the tank and hold any length of tube up along side the float bowl and open the drain screw it will come up to the level inside the bowl where the float closes and stops the flow. The tube could be miles long, you'd need a lot of fuel but it would work.

 

finally got around to doind it feel kinda dumb but you guys were right no matter what the level always went to certain point.. thanks for the help

 

The only time a person should feel dumb is when doing something that they already know not to do. :wink: