sputtering/bogging out on initial take off.

1983 xj750mk
6668 miles

the original owner of the motorcycle used it for 2300 miles and had it stored until i obtained it in july of 2012. before moving the bike across the country i had it serviced at local automotive shop. the carbs were rebuilt. both front and rear brakes inspected and adjusted. new tires. and a few small odds and ends.

i shipped the bike to portland oregon where it is used as my only source of transportation. ive been riding daily since july of last year. up until two months ago the motorcycle had been running tip top. at that time the motorcycle was experiencing similar issues to the problem occurring now. on initial acceleration from stop, the bike sputters and bogs down and seems to want to stall. i noticed the #4 carb looked to be leaking so i replaced the gasket and reinstalled. that was not the problem and the leak persisted. eventually upon further careful investigation and hours on the forum the leak was spotted between #3 and #4 carbs. one of the 0 rings between the two carbs had a small tear in it. i replaced the 0 rings and the bike no longer leaked. the bike still had some hesitation, but was running better. i then changed the plugs as they were the original plugs from 30 years ago thinking it would help. it did.

the bike was running great up until now. so two months no problems.

the day before the problem occurred i washed the bike. and after scrubbing it down i put it in the garage where it sat until 4am the next morning. at 4 i drove to work and parked it untill 10am. at 10 i fired it up and the problem began.

i'll start the bike with the choke on full and level it out as the bike runs. i'll usually take off riding around half choke and adjust the choke as i go so that the bike isnt screaming and isnt stalling out.

at a comfortable ide just below 1500 with the choke still on. the bike can sit, but once i try and take off it boggs down and wont pick up until i get the rpms up higher. the problem is less evident once i get the bike up to running temperature.

could this be as simple as my carbs are not in snyc ?

when i last opened the carbs they were very clean and at that time i installed a fuel filter. so i tend to think its not something from the tank stuck in one of the jets.

air filter looks clean. spark plugs are new. no found vacuum leaks.

hope someone can lead me into the right direction. thank you

 

I'll help you out the best I can. Start by looking at your plugs and read them see it they are rich with fuel or lean. I under stand stuff was rebuilt. Etc. But anything can still happen. You can be running to rich on idle and lower rpms that the fuel don't burn good till it hits higher rpms. Also check gap you could always knock it out.
Then move on to your spark and see what its like make sure its strong and fires. After that make sure your linkage for you choke ain't hanging up and leavening them choked a little bit.

So in all read your plugs and make sure its not flooding out at some point in time or lean.

And as stupid it sounds maybe your buying bad gas its unlikely but who knows.

This should help till a vet jumps in to help your a bit more

 

hey, thanks a lot.

i'll recheck the plugs in the morning. i checked the plug gaps earlier today.

are you suggesting i take out a plug while the bike is running and examine the actual spark?

i did fill the tank the day i washed the bike, and have yet to add new gas.

 

sounds to me like a possible cable problem as well...can you confirm "linkage lever" at carbs is fully home when lever is fully home at the bars?

 

Try a Carb friendly dry gas additives the gas caps on the tanks does not make a good seal so you could of gotten water in it as well. Sorry forgot you washed the bike. Also wires maybe got water so its miss sparking. How ever to check spark on plugs they got a tool for it because a open plug( a plug just out side of bike ) will not have a load on it so you can miss judge it. So what I would do and have done is take a boot off a plug and put a old one in it that will check each coil for good spark. Or use the plugs in bike if you don't mind doing all the extra work.( p.s. please use a charge after test plug and as you test I don't know why but a not so par battery will not start the bike a full battery is the only way to test spark on these bikes)


Blue spark is good
red bad
Orange weak
Those are spark color reads

Also check wires on bike for water because atleast mine dose not have water tight wire connect on it so water could make it miss read. Just like you plug wires. Water and electric does not mix.

 

thanks to you both. i'll check and make sure the linkage for the choke is responding properly. also investigate the plugs properly.

 

Wish you luck bud and welcome to xj bikeing they nice bike when they run right. But there's a lot of younger and old guys willing to help here but get a user manual for your bike or find a link on here and down load it to your computer or smartphone like I did. It helps a lot if u done have one get on and free download You should have it.

 

It needs maintenance.

I'm seeing that we're now at over 6K miles and no mention of valve adjustment. The initial check was due at 3000 miles; then they need to be checked every 5K thereafter. If neglected until now, you're looking at some tight valves.

Your carbs do need to be sync'ed; and for that you need to have your valves in spec.

But your first step should be to wet-verify your float levels.

 

doughnuts and valve gasket ordered. i'll look into what it takes to "wet-verify".. thanks for taking a look. i appreciate all your time.

 

shouldn't the float levels be fine as the bike was operating good for some time?

 

i guess that is why the word "verify" was used. i'll try and figure it out.

 

Things it could or might be:

Fuel Filter "Air-locked"
Moisture in Spark Plug canyons.

"Runs on Choke"

Clogged Air Orifices surrounding Main Nozzle - (Emulsion Tubes)
Sticking Diaphragms.
Kinked Gas Line.

 

thanks very much rick.

im about to head out and check all the accessible parts, and i'll pop that filter off and on and look for a kink. im going to throw some new gas in as well.

as for clogged air orifices around main nozzle. i'll have to search for forum and once i pop off the carbs we'll see about the diaphragms.

again. thanks so much.

 

You are correct. Verify, adjust if necessary.

This might help: http://www.xj4ever.com/setting%20fuel%20levels.pdf

 

Like I said thevets will take over hope I help you get a start on things atleast and still welcome have a good rider when she's fixed up.

 

checked for kinks. looked somewhat pinched between filter and the clip that holds it so i just took it out from there and drove to the gas station.

choke lever on handle bars communicates with choke plungers things perfectly.


still feel weird about pulling the plug and grounding it to look at the spark.

any good way to check the spark other than hitting the starter while the plug is out?

a video below of the bike in the garage. with the choke almost off. just filled it up with new premium gasoline.



xj750 sound

 

still hesitating and bogging down from stop. to start. doesnt act nearly as bad once its warmed up.

 

Nope. Just be sure the plug is grounded to the engine and you (and the TCI) will be fine........unless the insulation on the plug wire is cracked and you're working in the rain :lol: (yep....did that)

 

Leave the Plug IN

Use a Spare Plug. Place the Spare Plug into the Spark Plug Cap.
Hold the Ceramic part of the Plug Cap.
Touch the Business-end of the Spare Plug to the Block.

Perform the Test.
Look for Spark at the Spare Plug's Gap.

 

spark looks nice on all plugs. lets see how the ride to work goes tomorrow and then some more tests..


also the plugs ceramic parts look like nice toasty marshmallows. so from what ive read. that is nice.

again thanks so much. i appreciate all the help.

 

Could we see pictures of plugs please?

Bike sounded OK in the video except the revs seemed to take a while to drop (couldn't hear any bogging though :? )

If the revs take a while to drop it could be indicative of an air leak/idle screw set too high, have you tried the brake cleaner/ether/wd40 spray test round all the carb joints & throttle shafts? Just spray a tiny amount directly at possible leak sites (wd40 straw is ideal to get at inner carbs) and if the engine note changes you found a leak

 

Okay. You replaced the fuel rail o rings. Which requires you to break the rack. Which would request a bench sync and running sync. That's the cause of your hanging idle, or a vacation leak. Or a combo of the two Check the valves adjust if needed. Then perform a bench sync then a running sync n then check for vac leaks. That's what id do.

 

hey everyone. thanks again for all the help trying to diagnose this issue. the problem seems to have gotten worse. the bike will preform well on the road basically 2000rpms and after. when slowing down and dropping below that is when the bike become sluggish. and if i dont raise the rpms above 2000 when i take off it will jerk and sputter but eventually pick up after i get the rpms up.


i ordered a the gaskets to do the valve shim swap. and after that plan to get someone to tune and sync or find a tool and tutorial to do it myself.

until then i would like to figure out why all of a sudden the bike is acting this way.

the last time the bike felt this way a change of spark plugs was all that was needed. and the bike preformed tip top again.

as for breaking the rack and having to re sync. if i did not adjust the mixture screws when replacing the fuel rail o ring. what would justify a re sync? i pulled it apart and put it back together only using the screws on the rack.

thanks everyone for all your comments. i'll have to shoot another video and show you the plugs after todays ride.

again. thanks.

 

so weird update. i took out all the plugs to look at them. put them back in after photographing and started the bike up and it started and ran like before but eventually started to even out. took it for a ride and it isnt acting up at the moment. i tested all the plug wires and they were producing spark. i dont see what would cause this "temporary" fix to happen.

as for the plugs. all are pretty white except the number 4 plug. yesterday they seemed nice, but maybe the dark garage threw me off. perhaps im just losing my mind.

can they be white from the small amount of additive i put in to combat the moisture i may have had?

looks like im running lean. anything i can adjust while i ride and wait to adjust the valves?





also saw some oil on the fins of the motor on the kickstand side.. is this coming from the head gasket? cant see any oil above this point, or on the right side.





i'm thinking of spraying it off with engine degreaser and monitering it and see if i can spot where its coming from. there is just enough there to see. when i go to wipe it off with my hand i dont get much up at all.

what do you all think?

going to spray later today. thanks again..

 

The plugs look very lean from the pics, although not too clear on this screen so they could just be Clean instead of Lean. (hmm, screen clean lean... :roll: poetry in motion) I think there must be an air leak somehow and I'd be wary of riding anywhere in this state as you can overheat the engine (head gasket leakage may be a sign of this starting to happen).

Also, You do need to re sync after splitting the rack (bench sync first then running sync on warm engine, and the valves NEED to be in spec before you can get a good running sync too) so that all needs done ASAP.

Sync refers to the screws on the throttle shafts that synchronise the movement of the butterflies, not the pilot mixture screws. Have you read church of clean/9 yards at all? It's long but VERY important & useful info there...

That said, looking at the plugs I would say probably lean, possibly due to air leak. Perhaps it would be worth checking/resetting the pilot screws to standard as well in case they were played with... I don't recall if you're running stock airbox and filter? stock pipes? If you put your bike details/mods into your signature it's easier for people to remember

As far as the running problem that improved with the plug check, have you checked that your plug caps are screwed tightly onto the leads? (sometimes need a bit cut off lead to get them to "bite" again) and also while the caps are off you need to check that the resistors are not loose & that the resistance is in spec (US models are 5k outer & 10k inner, caps only)

 

thanks a bunch. i'll be looking for more air leaks today. no leaks as of yet. everything is stock. i'll update my profile later when i have time. and i'll inspect those plug wires. thanks for the info regarding the resistance.

 

i sprayed and i sprayed and every time i thought it was picking up, the engine would just stay and hang at that rate. only had a little bit left in the can. so it looks like more carb cleaner for me to pick up today.

if everything is stock. and plugs are still above mixture screws. would one need to adjust the mixture screws? i dont see why they are capped off with tamper proof plugs anyway?

 

After 30 years or so, mixture screws, their springs and o-rings have gotten covered with mild corrosion and need to be cleaned to work properly. If they are still hidden under their caps, it is pretty hard to clean them thoroughly.

 

thanks for the clear answer. thanks again. looks like ill be popping them off.

 

got the valve gasket business and doughnuts. going to replace this weekend

 

As I see it, ... 1, 2, @ 3 are "Critically Lean".
The Bike loves it like this. It charms you to leave it on the Lean-side for that big pull out-of-the-hole.
That mixture is explosively hot. Generates big heat. Hot-Rod.

Air-cooled Aluminum can't handle that for too long.
Be careful.
Critical-lean, dialed-in or air-leak related, can be heartbreaking.
You can go from Show-Bike to Parts-Bike during a pleasant afternoon joy-ride.

I have my 750Max dialed-in "On the Lean-side".
I pretend the Oil Cooler added helps-out a great deal with the cooling.
I look for some brown stain on the Insulator.
I use Marvel Mystery Oil in the gas if I'm going to run long.

If you have a ColorTune Plug you can dial-in quick.
With the Pilot closed; run-it. Use fans for coolng.
Slowly open the Pilot Screw until the Cylinder begins firing without missing.
Check the ColorTune Plug. It might be Yellow or Blue.

If it's Blue, ... continue turning-out slowly until you arrive at where the Blue just fades-out rich.
When you arrive at that mix you become a full-fledged "Tweaker".

You'll probably even fabricate some-kind of "Tweaking Tool", ... so you can reach-in to twitch two or three, ... and not hardly move the
Pilot Screw > || < that much, ... and be all-like, ready for a Merit Badge and a good cigar.

 

thank you so much..

how well does the pretending thing work? maybe i'll give it a try?


im still very concerned about drilling those little plugs out. something just doesn't seem right about it. after i or someone convince myself to drill those puppies out i'll let you know how it goes and if im celebrating or not.


thank again.. so much help.

 

If you can find a 900 Oil Cooler, ... with the whole sha-bang, including Adapter, (PN 90401-20089-00 BOLT, UNION) a hollow, Hex-fitting which secures the Adapter to the Oil Filter Platform, ... it's an easy fit.



The Cooler really helps on all-day jaunts down the Interstate.
When I put the Bags on and hit-the-road, ... I bring a Plastic Bottle of Marvel Mystery Oil along.

The Marvel smoothes-out the Burn and provides Top Cylinder Lube.

 

thanks a lot. i'll be looking around. you are quite a lot of help. thanks again.

 

I'm kinda coming into this late in the game, but are you POSITIVE that the battery is charged? It sounds like the way my bike was running when I had the alternator go out. It just wouldn't run under 2000 rpms...

 

you know what.. i should really check that. it fires right up in the morning. but i'll have to put the multimeter on it.

thanks alot

checked the valves yesterday.. thanks fitz. your tutorial really helped

intake

.08 .15 .10 . 08


exhaust

.15 .15 .16 .15

looks like i'll be getting some shims.

 

well. found some shims im going to pick up tomorrow and tested the battery at idle and at 2200 rpms.. looks like its working.. road it after i fully trickle charges the battery and still shuddered at take off. after the riding for about 20 minutes the bike evened out a bit and was more responsive at take off.. i dont know what im getting into.

shims in tomorrow though.

thanks everyone for looking and helping.

 

i have a quick question. when im cleaning out the pilot jet and paths with in.

when i spray some gum out and air in i should get gum out/ air coming from two places.. the pilot air jet and and pilot outlet. or the by pass hole?

anyone have any insight on this?

 

My 81 xj650 'stumbled on tip-in' when I got it. I too tried everything I could think of in the fuel system dept. I changed the IC ignition box and guess what? It ran fine! never count those ignition boxes out!

 

Is it possible that you have an intake manifold leak like a cracked boot or bad seal to the head?

I had my carbs dialed in pretty good and experienced same "stutter" off idle and found I had a crack in the intake manifold rubber. I also discovered that a PO had removed the intake manifold gaskets in favor of some yellow goop.

As soon as I installed four hand-cut gaskets and fixed the crack with Super Black RTV and silicone tape, the problem disappeared.

 

i'll look into this. thanks so much guys. you are all so much help. this coming weekend will be the wet test and idle jet cleaning. had a yamaha mechanic ride it the other day when i was looking at shims and the service tech recommended i investigate the pilot circuit and jet.

 

well.. checked levels. looking good. cleaned out pilot jet and passages. had the carbs all apart. reassembled. problem still exists. think its gotta be a vacuum leak.. keep looking i guess

 

Yes, because intake valves #1, #3 and #4 are too tight as weel as all exhaust valves (well, exhaust #3 is not, but will be soon). YOu will need shims one size thinner for all of them. Depending on the numbers on the bottom side of the shims, you may be able to swap some of them.

 

shims are changed. had the carbs off again and cleaned them again.. they look quite clean. but maybe im wrong.

 

okay.. so. pretty sure something is either wrong with the petcock or with the fuel filter. fuel filter is laying horizontal to the ground. sometimes it stays less than half full. (this is when the bike seems to operate poorly). i noticed today while it was riding nice that the filter was pretty much full. noting this i arranged the filter so it sits vertical. took it for a ride to get gas. seems to be running a lot better now and filter seems to be mostly full except a small bubble near the top of the filter cone.

theres the update. thanks everyone for all your help. i'll keep you all updated to the success and findings to come.

 

What was wrong is the orientation of the filter. You have to make sure that it doesn't get a bubble trapped in it; a bubble will reduce or eliminate fuel flow.

 

is there a simple way to eliminate all the air in the filter?

 

well. the fuel filter rearrangement seemed to have fixed my problem.

thanks everyone.