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.