carb tuning

i have a 83 yamaha seca 750 yics model am tuning the bike for some time now(not using the YICS tool) has been rejetted to i think 126jet it has 4 to 1 header straight pipe and 4 pod filters mixture screw is opened to 2 1/2 turns or maybe 3turns the bike wont stay on at idle has a slight backfire seems out of one carb. so i disconnected one sparkplug off(the carb that seems to backfire) it stays at idle and will rev up a bit cause thats all i tried but once i connect the sparkplug back it back fires or shuts off. Any ideas what could be? toomuch gas on that on carb or too little

 

Im thinken its the valve adjustment

 

In this order:
Check compression
Inspect valves clearance. Adjust any out of spec.
Check compression again (in case any out of spec cyinders were due to a tight valve)

Assuming you have FULLY rebuilt/cleaned your carbs (if you haven't, now's a good time to do so)

Bench sync the carbs
Set the float levels to spec.
Reinstall and adjust sync/idle/pilot mixture screws

DO NOT UNPLUG A SPARK PLUG WHILE THE ENGINE IS RUNNING! Four cylinder XJs utilize a wasted spark system, so pulling one plug actually prevents TWO cylinders from firing (1=4, 2=3). But doing so means that the high voltage signal has no where to go and can end up travelling back up the TCI ground and arc in it, frying those electronics.

Testing for spark REQUIRES you to completely remove the spark plug from the head, plug it into the socket, and ground the spark plug on the engine.

Also, pods and free flowing exhaust might complicate your tuning efforts off-idle under load, but you should be able to get the idle and sync down even with those mods.

 

The stock jet was 120s and 125s I rejected to 134 and 136 due to straight header 4to1 and pod filters ,i also have smaller jets 130 &132 maybe i have to go smaller?

 

Your carbs shouldn't have different size jets. Please elaborate on your meaning or 120's and 125's. Having different size jets across all four carbs will certainly give you tuning nightmares.

Also, even with your mods, and if you are under 2000' below sea level, the biggest jet size you should need is 126 main fuel and 41 pilot fuel. If you pull your plugs, they are likely black.

 

Stock came carb #1 120mainjet#2 125#3 120#4 125 ,The jet kit came with those for rejetting my bike with those specs,the kit came with 2 main jets 130-msd ,2main jets 132-msd ,2main jets 134-msd ,2main jets 136-msd ,shims

 

Says put 130 or 134 outer two cylinders#1&#4 and put 132 or 136 inner two cylinders #2&#3, thats what I did also yes they are a Black

 

Stock was not different jets in center carbs. When you say stock, I assume you mean that that was the way the jets were in the carbs when you acquired the bike. I also assume it had the pods as well. Now, I do know that some kits try to compensate with different size jets from the air imbalance pods are suspected to cause. But, with the size of jets you are running, the difference in size isn't going to matter, because flooding is flooding, whether it is a little or a lot.

With what you have right now, they are just running way to rich. You really need to get closer to the 126 mains (or below depending on where you live). By the way, the mains aren't really kicking in good until about 4 grand, so you will need to do plug chops above 4k to tell if the jet size is right for your mods. The pilot jets (the color the plugs show if you just roll up in the yard. shut the bike off and pull the plugs) are what will affect the idle to 3500'ish rpm range. This you use a color tune plug for or tune by ear and spark plug check.

I really do not envy the position you are in, as my bike came with pods and 4-1 exhaust, and I chucked them both due to the tuning difficulties. You are going to incurr a whole low of trial and error and frustration. But, there are plenty of members on this forum that have pods, and have tuned them to their satisfaction, so I won't say it can't be done, only that it can't be tuned to replicate the factory smooth, worry free performance from idle thru the whole useable rpm range without an incredible amount or time, energy, money, and if your up for it go for it. Get the jets closer to what you need and test.

 

Check out this link to the changes I made.....NOBODY ever talks about changes to the air jets. Its a must!!!!

http://www.xjbikes.com/Forums/viewtopic/t=36590.html

 

I changed my jets to 130 ,132 and still nothing would it be possible to run my stock jets with those mods ,I am only rejetting haven't change the pilots or air jet

 

You're not paying attention. Larger MAIN jets are not the only changes required to compensate for pod filters and a 4-1 pipe. All you're changing are the mains, and going way too big in the process.

No, you probably won't be able to run stock jetting. But you WILL need to change more than just the mains.

And in order for any of your efforts to make sense at all, everything else has to be 100% first. As in, valves in spec, carbs religiously clean, float levels accurately wet-set; etc.

 

Tips -
* go back to your 126 Mains and try to get 41 Pilots while you are "in there"

* Double-check the cleanliness of your Pilot passages, and your float wet-settings.

* Cram oily rags in your YICS passage to plug it (cheap-O tool) then sync & tune

* Spend $50.00 on a ColorTune sparkplug and amaze your friends.