Switch Vaccum to Gravity Petcock (83 Maxim 750)

Hey guys, the title says it alot,
I have a 1983 XJ750 Maxim with a bad Vaccum Petcock.
The petcock let gas reach the oil pan when stored for a while.
I live in canada and can't find a rebuild kit for the vaccum petcock so i ordered a gravity fed one.
Bike will run as good?

 

Yes the bike will run as good, but the gas will still get into the oil.
The thing that stops that from happening are the float needles in the carbs. You have one or more that are not sealing, or a stuck float.
Pull the carbs for inspection, and plan on replacing the float needles and resetting the fuel heights.
Have you ever gone thorugh the carbs since you've had the bike?

 

Just did it, it's weird because it look like everything is new in the carb probably already been restored. Float Needle and where it sit look brand new, the black rubbery tip look great on every of them.
One of the valve float was stuck on the little hook of one of the needle but i solve the problem by bending it a little bit by the outside.

Only thing i haven't done is resetting the fuel heights, any tip to do it?

 

The after market peacocks work fine except when you forget to shut fuel off , xj4ever has the rebuild kits for the stock petcock .

 

Yea this is what i saw on a post

 

Setting the fuel levels

 

Spell check on phone ....I regress petcock

 

gets

same with me when I try to text

 

Guess I'm "old" ...

 

my phone folds im old too

 

Hey guys! I have a problem with my left turn signal. I put the turn signal on and its not blinking at all, it does turn on in the front( not in the back) but its not blinking. In the back i don't have any light, weird thing is that it was working all properly 30min ago. The light in the dash turn on too. Right side there is no problem except if i leave it on too long it would stop blinking.

My bike is a Yamaha Xj750 Maxim

 

I'd suspect the rear light is burnt out

 

^ or the socket needs to be cleaned ^

 

I have been thinking of putting a 12V fuel solenoid in at some point. Would just have to find find the right one and a place for it on the bike of choice. It would work only when the key is on and it would cut off fuel when the key is off. I know if is cheating but for some it might give them a different option.
https://www.amazon.com/HFS-Electric...ocphy=9019293&hvtargid=pla-351281160128&psc=1

 

Thx for the answers! Just realised that the rear light was burn, so not pulling enough Watts fo the blinker to go on!

 

Yamaha Maxim XJ750 1983

Hey guys, my rebuild is getting together.
But I have a small problem, the Bike refuse to Idle with the choke off.
I don't think i have a Jet problem.
It's weird because i can make the Bike Idle at 1k rpm with the choke open at like not even 5%.
So basically I open the choke not even 1mm and the bike idle great and i can rev it to redline and it will go back to idle like it was new! I could basically drive the bike like that, but when i shut the 1mm of the choke it will go down to 600-700rpm and slowly die, if i give gas sometime it will go back, rev up, fall down to 1k and then 600 and die, other time i will try to rev and it would just die immediately.

Is it bad choke adjustement? Bad Idle Ajustement?
I really don't think it's the carb, because it was running great before i remove the head to do the head gasket

 

Start by bumping up the idle speed adjustment once the engine comes to operating temperature.

Also make sure that you keep the enrichment on until the engine is fully warm.
As the engine warms the idle speed will climb, just ease off the enrichment lever gradually to bring the engine speed back down so it's not idling at 2,500 RPM with the enrichment on.

 

Hey Guys, here is video, you will understand my problem.
So basically i can Rev the bike and go back to idle with the choke mid on.
But as soon as i clode the choke the bike can't idle.
I can rev it with the choke off, past 2.5k rpm bike run great but as soon that it go back to idle it slowly die if i don't rev it again.

here is the video

 

It's weird because the carb were running great 1 week ago and the bike could idle.
I didn't touch anything since i was waiting for some parts and now it do that

 

I hear a misfire.
We need to start ruling out things that are not carb related before we go back to the carbs.

Pull one spark plug at a time. Hook it up to the plug wire and lay it on the head so the plug is grounded. Start the bike and confirm if there is spark at the plug.
Repeat until you have checked all four plugs.
I need to emphasze tht you have to do this one plug at a time in order to prevent damage to the TCI.

 

And to be sure. the video was shot after the bike was fully hot (at least 10 minutes of riding)?

 

Alright, i will try that

 

Yes it was hot

 

If you do find a lack of spark, unscrew the plug gap from the wire and look for corrosion. If you find any just cut the wire back about 1/8" and reinstall the cap. That's one common cause.

 

Since i just changed the spark plugs 2 days ago before i record this video i will directly unscew the plug cap see if its the problem. But at the hand i still think i will have to pull out the carb again, the bike just old rev the ast time i tried to start, it rev up to 6-7k and stuck there. I think there is probably a float valve stuck or something

 

Hey, could it be a bad carb sync? Never sync them..

 

I rebuild them and cleaned them 1 month ago but never sync them

 

did you bench sync them?

 

Well all the valve were opening at the same time

 

if you use the idle knob for carb 3 to adjust the throttle plate to where you can see 1/2 of the first little hole in the top of carb throat (there is a set of 2) then use the sync screws to match them all to that position you have bench synced

 

They will do that even if they are out of synch.
Synchronization isn't just about them opening at the same time, it's about them resting at the correct spot to properly meter the airflow at (and above) idle, and to do so in a way that allows all the cylinders to recieve the appropriate amount of fuel mixture to produce the same amount of horsepower in each individual cylinder.

You can have the throttle plates open at exactly the same time, but not have them adjusted to allow for individual differences for each cylinder.

A bench synch gives you a baseline that will allow the motorcycle to idle so that you can then perform a running synch to get the engine to perform properly.
You can think of throttle synchronization as being like the conductor for an orchestra. Each individual can probably play very, very well, but without the conductor to keep everyone in synch the music will not be the best.

The bench synch provides the baseline. You set each throttle plate to the same location relative to the middle hole of the idle (pilot) circut outlet.
Cover that middle hole halfway (it is the rearmost hole in this pic).



Alternatively you can use something to register the throttle plates against (a business card is preferable to a paperclip IMO).

how-to-bench-synch-your-carbs.6366

You will use a manometer to perform the running synch by adjusting the synch screws until all 4 cylinders are pulling an equal amount of vacuum.

 

Alright, so tonight i will pull out the carb (f***ing again) and reclean everything, jet etc. After that i will bench synch them. After that I should be able to make the Bike Idle. If not, i will recheck everything that is not the carb ( Spark Plug, Timing, etc)

 

Alright Update, just finish pulling the carb out, rewash everything. I bench sync them too. Put back the carb into place, link the gaz tank and bam suprise, the bike would not start unless i hold the throttle full open, and then it would run at like 200rpm. Since i switch for a gravity petcock, I think i just didnt let time for gas to reach the carb. I will retry to start it again friday

 

If still not working idc i will bring the whole carb to a mecanic and get it done there

 

fair enough, we all understand how frustrating this can be, but just bear in mind that only you care about your bike. The mechanic cares about your money more than your bike. That doesn't mean he won't do right by you, but mechanics that are good with 35 year old stuff are becoming rare

I wish you all the success with your bike

 

If it is about not having time I get it, but I still think you can do it. Have you looked over the "Church of clean" link? https://www.xjbikes.com/forums/threads/in-the-church-of-clean.14692/
You will have to brake the carbs down to individual units and make sure the little jet at the bottom of the float bowl is open. That bread tie trick really does work with that.
Don't feel bad, carbs can be stubborn to clean and get them dialed in. What ever you do, just keep have patients with the bike, she is old and needs love.

 

I guess it doesnt start because of the Idle screw, i will try to adjust it because i know for a fact now that the carb is completely clean and bench sync. I didn't change anything to float valve adjustment too so it should be working properly. Ill keep you guys updated

 

Hey guys, im a dumbass who don't know anything about mecanic ( I really tought i did tho ) So i want to know something. PILOT SCREW, so you know on the top of the Carb you can remove the cap and this give access to the a round rubber diaphragm where the air/fuel mixture pilot screw is. There is a plastic bolt with an hexagon on it to unscrew the pilot screw and under that plastic bolt there is a spring, so the bolt is spring loaded ( I think i know why ). How much do you need to tight this screw when you put back the air/fuel mixture pilot screw

 

are you supposed to fully tighten it? And it's the slide needle*

 

You removed the top hat and pulled out the slide diaphram and slide. The needle you took out is for the mains (Jet needle/slide needle), not the pilot.
The plascic cap gets tightnened until it seats. It should be snug enough to not back out. If you overtighten it the cap will crack.

I think that you might have an aftermarket needle. The 750 jet needle has a plastic cap on the needle, and does not have a moveable clip. Is there a number engraved onto the needle? The stock one should read Y-13.

 

Yes they are original, i just took a random picture out of google to show what I was talking about. So just to resume what happened yesterday. I pulled out the carb out and clean everything. I started from the bottom and made my way up. In the bowl, i removed the float valve and float needle, clean the needle again with carb cleaner and where it sit. After that i pulled out both jet, the 43 and the 120 and clean them with carb cleaner (pressurized ) Directly in the hole and after that with pressured air. I did that for every bowl and was looking for any gummed up part. Nothing, it looked like it had just been rebuild since all the part look super clean , when i mean clean i mean MINT.

After i did every bowl i remove every cap on the top of the carb and removed slide, two slide wasnt going all the way down so i polish them with 2000grit sand paper and did a wet sand. After that they were getting back in their position ( Remember, they were working before even without the wet sand) After that i cleaned every port from the top. They are two other ''jet?'' on the top of every carburator, so i sprayed carb cleaner in all of them. After that i reassembled everything and did the bench sync, that was pretty simple and very straight foward. The only thing was that and the end of the ''how to'' that you linked me it said that you need to see a slive of light ( Wtf does that mean ) I mean i was seeing the same amount of light from every carb put hm i was supposed to see? The picture was not that great.

Reinstalled the carb on the bike to see if atleast it would idle.
Bad news, it doesnt even start. Choke at 100% or 0% no difference.
The only way i could start is by holding throttle at 100% and only there it would only run at like 300rpm? Not even? Wtf

I snug every bolt without over torquing them

I think they are 3 reason of why it's doing this :

1 - Idling screw badly adjusted ( Make no sens since if i would go WOT the bike would have start and rev to redline?)
2 - There is carb cleaner in the bowl and now gas is mix with carb cleaner so it can't ignite?
3 - When I re-assemble all the bowl i put the washer under the 120Jet backward (it had a littlel curve), so now the 120jet is sitting 1-2mm lower?

 

4 - And in every scenario, im a complete idiot?

Maybe you guys can see something im missing

 

Many of us have pulled the carb numerous times before we finally got the bike to run right.

If you have doubts about your reassembly you should pull the carbs again and examine each one to ensure identical assembly. Reread the carb clean and tuning section on this site; a few times to make sure you get it right. Consider purchasing a manual with pictures so you have another resource.

Carb cleaner is very flammable and can be used as starter fluid.

Bench syncing is important. Many of us also sync using the vacuum meters. Has to be running before you can sync using that tool. A YICS blocker may help. I've done it with and without with very similar results.

Seating the carburetor on the bike properly is another factor. Bike is very sensitive to air leaks and pressure. Ensure all four carbs are properly seated in their intake boots and air box boots. Ensure vacuum caps are not cracked and are all seated properly.

You mentioned carb parts looking new. Hopefully previous owner purchased the correct carb parts for your bike. Many cheap carb kits out there and I have experienced fit and performance issues with some of that junk.

Ensure your float needles move up and down freely and don't bind. Floats move easily up and down and don't bind.

Patience and trial and error are key to the new owner who doesn't want to pay a mechanic. Careful using the screw drivers and other tools to. Stripping and breaking screws and bolts in these old bikes happens more than you'd like to know.

 

Update :
Try to understand the problem i had (very hard)

Float valve #1 : 17.80mm
Float valve #2 : 17.55mm
Float valve #3 : 21.60mm
Float valve #4 : 18.50mm

Basically thats why the bike was running fine at high rpm but not at low.

 

That'll do it. Glad you sorted it out.
Did you wet-set to get them all even?

 

1 yes 2 no 3no 4no
just turn the idle up a bit and try again, i think you flooded it. when you open the throttle it defeats the choke, when the butterflys are completely closed it sucks gas like a old ford but no air. that's what this is for

 

Hey guys! We did it!
Of course the bike still need few thing
- Exhaust gasket
- Muffler
- A good Carb Sync
And bit of love but we got a idling bike again and working carb without choke!

 

She sounds wonderful and healthy.
Good work.