Broken pilot jets gs750

[Guest]

Hi I am new to the forum so please be gentle

My bike is a 78 gs750 I have fitted two different sets of after market pilot jets and each time i have found at least one jet to ground and break. Today i have tried to fit a set of 17.5 pilots and the very first yet simply broke off, i gingerly tried a second jet in a different carb and could almost instantly tell the same fate awaited it should i continue to install it. The ends of these jets have a slightly larger diameter and the threaded part of the jet seems to be slight longer, apart from that i can see no difference between these jets and the originals. Has anybody else had this problem? I do not believe this is simply down to poor installation or over tightening, the originals fit nicely and you can feel that they are seated right without being over tightened.

On a side note, i have the dreaded pods and a 4into1 of unknown origin, even with the needles at the top position and a 120 main the bike is still flat with any throttle and simply seems very lean. Having read many other threads on this, i was given the impression that a 115 main would solve the leaness. The pilots are still 15, i had to leave them standard after breaking one. Is this a familiar story to anyone and how large can i expect to go before the sparkle comes back?

Thanks for any help offered, i hope this hasnt been covered too many times, i could not find anything about it after a good search.

[chuck hahn]

Well post up pics of the ones you took out and the new ones..this will help us see whats what.

But on the flip side, why are you putting new ones in in the first place??? Is the broken one still stuck in the cab body or have you gotten it out???

[chuck hahn]

The carbs should be VM26, so heres a link to the pilot jets you should have. Note these are for the VM28s but they are the same that the 26s use.

http://www.partsreloaded.com/ATV-UTV/Air-Fuel/Carburetor/Jets/Mikuni/Pilot-Jets/Mikuni-Pilot-Jets-VM28-486-OEM/Mikuni-Pilot-Jet-VM28-486-17.5?cPath=314_1490_1514_1626_1627_1667_1674&

[Keithatron]

The first set came with a carb rebuild kit, which where standard size. Since the airbox refused to be refitted after to days of fighting with various hair dryer and different contraptions i submitted and replaced it with a set of pod filters, which is where the second set of larger jets come in. Thankfully i managed to remove the broken part quite simply so the carb is still usablepilot jets.jpg
The centre jet which is dull in colour is the original, the left on is the standard size aftermarket and the right one is the 17.5 jet with its broken brother

[Keithatron]

That link shows exactly what i have thanks. But it doesnt explain my problem

[chuck hahn]

The pilots only effect from idle to about just under 1/4 throttle and "assist" in the transition over to the main circuit. Seeing you have pods and a 4 into 1, the priority adjustment would be to go up to the 17.5 pilots and set the bottom pilot screws at about 7/8 out. The side mixture screws at about 2 out to start off.

Then you make adjustments for the rest of the throttle range on the main jets and the needles if need be. The stock setting for the circlip on the needles is the center groove so start there. See how she responds with the clips on the center groove first and adjust from there. by moving the clip toward the TOP of the needles you allow the tip to drop further into the air jets thus making it leaner. Placing the clip on the ones BELOW the center will raise the tip and make it richer.

Theres a balance between the clip, position and the size of the main jets thats gonna have to be found specifically for your engine and its gonna be frustrating.

Be sure the valve adjustments are within specs AND that you vacuum sync the carbs and check the timing. If these arent done and your trying to tune a carb your just wasting time and effort. Get the maintenance right so your positive a bad valve adjustment or improper timing isnt the real culprits rather than whats in the jetting. If after doing the maintenance and its still fishy..THEN your better off knowing its in the jetting and not in something else.

[chuck hahn]

Why wouldnt the airbox mount up??? Hard rubber i suspect. Take the boots out of the box housing and get some wintergreen oil and soak them for 24 hrs and they will be supple as new again. Honestly your bike is gonna run 100% better with the stock box on it.

[Keithatron]

Ok, thanks for the advice, i will check all of the maintenance things as you suggest before i continue trying to tune the bike. i have a couple more photos of the jets to share, as you said a 17.5 is a priority and currently i am struggling to fit them. The photos show how much larger the end of the 17.5 jet is to the two original size jets.
pilot comparison 2.jpg

[Keithatron]

you can clearly see how much bigger the jet is here
pilot compaission 1.jpg

[chuck hahn]

Well most "after market" kits are known to be full of crap parts and you have gotten took. Wrong jets obviously. look like its time for the original Mikuni jets so they fit right if you ask my opinion.

[Keithatron]

Ill be honest i have been thinking of having another go at the airbox, but after two days of fighting exactly what you say, very hard rubber i simply could do no more, i will try your advice with the oil, i guess i just don't want to believe right now that i wont be able to get it to run right with the pods.

[Keithatron]

ok thanks i will try to find some originals

[chuck hahn]

The ones on the link I posted are the right ones. I just got a set for my VMs on my 78 1000..same carbs as your 750 is.