yamaha xj 750 82

Hii some time ago i bought a yamaha xj 750 from 82 in pieces.
I've put everything together and made a caferacer from it.

Changed the stock air filter box to pods to give it a cleaner look.
Upped the jets.
Cleaned carb's very well and synced them myself with lots of youtube video's.
Did a valve clearance and changed shims.

Start up is with choke, idle is fine, revs are a bit hanging i think but while driving not really noticeable.
Runs really great, feels like a rocket altough its from 82

I just drove for 20mins and it was amazing. ( full trotthle accelerations etc )
Went to 160 ish in 4th gear , all smooth.
But then it started sputtering again. ( probably when engine was 'hot' )
I had this also in the summer while i tested it previous times ( before valve check ) , then it had the same but it had it earlier ( probably because temps where 10°C higher )

Here is a sound in normal operation (cold start up)
https://youtube.com/shorts/WQAS1FoZYpo?si=uf6N0kETpQmkgfc_
And when i came home
https://youtube.com/shorts/hklmitO_nKA?si=rguhmFaFmIOUVQgB
(needed to use choque to start altough engine hot )
Also picture of the spark plugs

https://ibb.co/nC00Vk0
https://ibb.co/wLDkx7G

So the problem:

It sputters and start to die , also start up not easy unless i let it cool down a bit.
Engine feels hot,ignition cover feels hot ..
The sound sounds dampened and it won't rev properly


Guess i start again with valve clearance checks.
I think at moment of sputtering its running poor , because when its start it start in the under range of the revs and when i pop the throttle and revs go up it suddenly starts to take traction again and shoots of. However after a while that's not possible anymore and it only starts sputtering until it dies.



I don't have the original tank so might want to address it here :
It doesn't have a vacuum line thing on my petcock so i've closed the air inlet at the carbs that normally go to that. That's right i guess?

 

Good place to start, definitely not running well according to your videos. If the valves are out of spec and tight when cold, they might not be closing completely when the engine is warmed up.

Yup.

 

If it's running good when cold and needs to be choked when warmed up to get it started then you probably have a problem with carburetion. Using pods as opposed to the stock air box always opens up a can of worms!

Even in the stock configuration the carbs need to be "hospital clean" Buying a can of Berrymans or Gunk carb dip is a good start on that end. Then it's a matter of getting the jetting right, a task better left to someone that knows what they're doing or you'll nickel and dime yourself to death running down and trying different sizes.

 

Ding ding ...that was I was going to reply PODS suck on CV carbs , the stock airbox is the way to go period . I worked on a guys Maxim over the summer , rebuilt his carbs the butterfly seals were RTV , replaced those the carbs were taken to the church of clean and put bigger jets had a real joy getting it to run right. He tried to build an airbox with PVC pipes and it actually worked sort of it idled good and got rid of the hesitation, but ran out of wind at wide open throttle . We think since he put a single pod filter it needed one on the other side . He later got tired of working on it and sold it on . This is where alot of folks call the bike a POS when it won't run right, when if it had the stock airbox they could be out riding , have had three Secas all with stock airbox and no carb issues. Oh I know folks claim their bike runs great with pods ...but does it ?

 

Thanks for the replies
Yep the thing is i wanted a hole under my seat to look trough , so i needed to get rid of that airbox. (And i wasn't aware of the problems that could come when removing haha)
I also put the original boots to the airbox in my pods. ( they were a bit longer then the orignal ones in my pods ), should also help.
The thing is it runs really really great and fast when cold until its really hot hot running temperature. ( also starting when cold is easy peasy )
I've had a new kawasaki Z750 some years ago and yes its slower , but that's expected given the less hp. But it's hell a lot faster then my friends cb500. And i feel no hesitation in the rpm range, nowhere.(until its hot after 30mins of driving in high rpm's)
That's why i don't really think its the carburation.
Unless fuel suddenly starts to evaporate quickly when engine is hot or so. ( but if its gets to less fuel that means it would run way to rich the first 30minutes if its a carbu thing)
Also my spark plug readings are in an optimal condition ..

My guess is valves ( less likely ) , coils or something else in the ignition.

 

Well actually, I think I do recall you did have carb issues... Just goes to prove that the carbs on these are a right pain to get right, even without removing the airbox, which then gets blamed for the issues that would have been there with the original airbox anyway.
I've run my 650 maxim on Mikunis from a 600, my 650 Seca on original Hitachis both without filters to prove engine condition, they ran quite well, but not right.
So strip and clean again, go back to original jets, then change one thing at a time, if you really must bin the airbox.
The airbox does much more than house a filter, it ssmooths out induction pulses, acts as a resonator, and creates a restriction upstream of the carbs - this last bit alone is enough to change the fuelling, but then it changes the differential pressure or vacuum that thee piston/slide sees, which changes when the main circuit comes on.

 

Yes true i am very aware of that.
I just don't understand why my problems only come after a bit of riding perfectly until it comes hot.

Yesterday i did some testings :
measured the voltage on : frame & sparkplug caps( ceramic part)
When off : 0V
When idling it goes up from 2V to 8V when its hot. When cold it was way less.

I don't really understand why and how this is possible but atleast it doesn't seem right. My ktm doesn't have this.
So i will buy new spark plug caps to start with.

The thing is its on al 4 spark plugcaps , that seems a bit weird to me. ( one would mean okay its a bad one , but al 4 .. )

 

The proper way to check the spark plug caps is to remove the spark plug wire and check the resistance through the cap with an ohm meter.

 

Okay i removed my coils and seems i have 2 different ones :
CM12-09(like manual says) & CM12-20
Al readings seem within spec

But then for the spark plug caps
The big angled caps ohm out at 1k ohm -> should be 5kOhm says manual ( but there is 1k ohm written on the caps ..)
The small 90degrees ones give very strange readings -> 3M ohms and 130k Ohms ( tried with 3different multimeters and all give weird readings)

So all caps should be replaced imo

Also saw the spark plugs inside were not the original ones ( Resistance type) -> also bought new ones.

Just wonder if i should replace the CM12-20 coil also

 

With a cap reading 130k ohm my guess is you'll have a much better running bike once you replace them.

 

yamaha caps were 5k. you should not use 5k caps with resistor plugs. You should also not be trying to measure volts on the HT side of the coils - here lies madness...
Regarding your hot missfire - you can buy some neon plug inserts that light when pulsed to indicate HT issues. Also, a hot missfire on a non -standard carb setup would indicate a rich mixture to me, so much so that I would clean the plugs, take bike for a spin until it missfires, then pull the plugs to asses colour. But the neon plug tops would be a good start, I suggest.