79 carb jet size

[bruss]

so i bought a 1979 xs400 thats been "messed" with already, the PO put on a 2 into 1 exhaust and told me he had jetted it, it was running so rich that the plugs fouled black within 50 miles and it would not start on choke and bogged down at around 5k rpms, just for the heck of it i put on pod air filters and it will now start with a lil choke..smokes pretty bad until warm and gets to around 5.5k rpms but is a slight improvement, i just took the carbs off and the main jets are 140's..should i put 150's on it to see if it improves ?

 

[Scorpio1963]

Like everyone told me take the carbs and clean and reclean.Replace any warn parts inside then while you have it apart you can rejet up to the next size.Also you say it smokes pretty bad,is it blue oil smoke or black?
My bike smokes also when Ist start it till it warms then it goes away.

 

[16VGTIDave]

Increasing the jets to 150's will make it run richer. Assuming it is running rich now, that won't help.

I'd start with a standard tune-up and carb cleaning / adjustment. Probably start with a compression check. Then change the plugs, replace the points (if so equipped), change the oil and filter, adjust the valves, clean the carbs, replace the rubber parts in the carbs, check and adjust the floats, etc. Then get the carbs synced and see how it runs.

You have to cover the basics, and establish a baseline to work from, before you can start tuning.

Dave

 

[bruss]

ok..so wait a min here..imcreasing the carb jets will make it run richer but yet its already running at a higher size than stock at 140 ?

 

[bcware]

My '78 uses 137.5 stock mains. If the ONLY modification is the exhaust the 140 main is probably fine. What is likely fouling your plugs is an emulsion needle that has been raised or shimmed or a larger pilot jet or an excessive number of turns out on the pilot mixture screws. Also, check the float height for giggles while you have the carbs open. Finally, check the diaphragms for cracks/leaks. Check your air filters for excessive dirt and grime. Also make sure you're running the right spark plugs (why not check, right?). If for some reason someone stuck a colder plug in there you could have this problem.

This should cover it I think.

 

[xschris]

Stock for a 79 is 132.5

 

[bcware]

Then a 140 main may simply be too large. That's 3 sizes up.

Thanks Chris

 

[bruss]

thanks guys.. i'll try to adjust the needles first..see if it makes a difference

 

[bruss]

didnt get into it yet..but if i do decied to go a jet size down, which ones will fit my bike ??

thanks

 

[sly409]

check out the jets from mikesxs.net thats the only place I know of that sells jets for our bikes.

 

[bcware]

Yes, the "large round main" type will work for your carbs found on Mikesxs.net

There is a minimum shipping charge, so you may want to pick up a few things at the same time rather than spending more on shipping than the actual cost of the jets.

 

[bcware]

thanks guys.. i'll try to adjust the needles first..see if it makes a difference

Fyi, I was originally jetted too large and tried to fix the problem by adjusting the needle. My '78 carbs have adjustable notches with an e-clip. Moving from one notch to the next never fixed the issues due to my large mains. No matter how you adjust the mid range you're still going to be too rich at WOT when the needle clip position has no affect.

 

[Scorpio1963]

check out the jets from mikesxs.net thats the only place I know of that sells jets for our bikes.

Theres a couple of other places also,Old bike barn and Bike bandit.Either way they dont cost very much.

 

[bruss]

i ordered 137.5's i'll report back what happens...hopefully this weekend as its going to be in the 50's here

 

[bruss]

HUGE improvement, i couldnt ride it much but what a difference, thanks guys

 

[bcware]

Winner winner chicken dinner!

 

[XSLeo]

Most any place around you that works on dirt bikes, atvs and such should have plenty of real Mikuni jets in stock or can get them. And at about the same pride as Mike's cheap nockoffs.
Many of these places have a large assortment of gently used jets. Might save you some cash.
Leo

 

[xschris]

You can tell the bike runs better with them