"Quick" question about jets.

Line · Mar 23, 2016

hello!
i'm just about to order new jets for my carbs ( they're bs34's)
now; i'm running megaphone mufflers and a couple of cheap pod airfilters.
i've read somewhere that a good starting point is 2 up on the main jet and 1 on the pilot jet.
does this make sense?
also, i dont need to change the little air jet in the inlet right?
hoping some of you can help me out.
thanks!

rshutchinson · Mar 23, 2016

Yes, that is a good start and no, you don't need to change the air jets.

I went with 145/45 jetting and didn't change the needle clip setting (our Canadian models had adjustable needles). I am using Emgo shorty mufflers and UNI pods on the carbs.

MrStubb · Mar 23, 2016

If you need to rebuild as well as rejet your carbs 6sigma seems to have a good system. I just ordered a kit for a set of carbs from them. You give them some details like mufflers you use along with air filters average elevation you'll be driving in and how you plan to use your bike. They will use their calculators to send you the proper jet kits. I haven't gotten my kit yet but I've seen a number of positive reviews for their service.

16VGTIDave · Mar 23, 2016

I wouldn't use a "guesswork" jetting kit for more than a starting point. Actually, I wouldn't use one at all, never mind pay for it. Nothing beats actual testing. The plus 2 main, plus 1 pilot size theory is a safe starting point, but it is highly unlikely that it will be correct. There are so many factors involved, not the least of which are the air temp, barometric pressure, and humidity.

The air jets have a couple of functions, one of which is controlling when a jet circuit becomes active. The larger the air jet, the higher the RPM before a circuit becomes active. Fine tuning like this requires monitoring the air/fuel ratio and is best done on a dyno. Very few people are willing to spend the money to do this, even on expensive vehicles...

Line · Mar 24, 2016

okay, so i've decided to go with the 2 up on the main and 1 up on the pilot. also, leave the air jet alone, for now.
then i came across a new thing that puzzled me.
the jets that are already installed doesn't add up to what it says in the book. (the picture someone took of his book and uploaded in this forum)

my jets are:
main: 140 (should be 142,5)
pilot: 42,5 (same)
air: 170 (should be 45.) -this makes absolutely no sence. though these jets are hard to read properly.

has the previous owner done something weird, to save gas or something, or does this make sence to anyone of you?

ekstra quick question: how would you set the pilot screws? (and why are they different in sizes)

thanks alot for your help so far!

xschris · Mar 24, 2016

Pre 80 bikes have anywhere from 142.5 to 132.5 mains. What year is your bike. Post a pic of the carbs you have. They all have 42.5 pilot jets. They all have 170 air jets which are the ones in the mouth of the carbs. Some manuals are a bit confusing on what jets are what.

Line · Mar 27, 2016

xschris said:
Pre 80 bikes have anywhere from 142.5 to 132.5 mains. What year is your bike. Post a pic of the carbs you have. They all have 42.5 pilot jets. They all have 170 air jets which are the ones in the mouth of the carbs. Some manuals are a bit confusing on what jets are what.

the bike is from '77

xschris · Mar 27, 2016

They look like 77-79 xs400 carbs. The biggest issue with those years carbs are the idle mix screws breaking off in the bodies. Make sure the points on the ones you have are not worn. If they are get new ones. When you put them in make sure to GENTLY seat them. Then turn them out to the amount needed for a correct idle - 1/4 throttle. If you need more than 4 turns then you need to go up one size on the pilot jet. There are threads on this in the forum. And always sync the carbs with a manometer.

Line · Apr 29, 2016

xschris said:
They look like 77-79 xs400 carbs. The biggest issue with those years carbs are the idle mix screws breaking off in the bodies. Make sure the points on the ones you have are not worn. If they are get new ones. When you put them in make sure to GENTLY seat them. Then turn them out to the amount needed for a correct idle - 1/4 throttle. If you need more than 4 turns then you need to go up one size on the pilot jet. There are threads on this in the forum. And always sync the carbs with a manometer.

okay. i looked it over and i think that the PO has broken one of them. so i'll have to get new ones.
another thing is the hole in one of the carbs, where the tip of the mixture screw seats are bigger than the other. :/ could this have been caused by the PO simply tightning the screw in too hard and thereby breaking of some of the body? (i don't have a picture of this yet)

xschris · Apr 29, 2016

The po must have drilled out the broken tip. If it is bigger than the other side he did it wrong and destroyed the carb. The hole is very small and tappered. Just drilling it with a larger drill is bad. I would look for a replacement carb for that side and use new screws and for the one you have also.

"Quick" question about jets.

Line

XS400 Member

rshutchinson

XS400 Addict

MrStubb

XS400 Addict

16VGTIDave

XS400 Guru

Line

XS400 Member

xschris

XS400 Guru

Line

XS400 Member

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xschris

XS400 Guru

Line

XS400 Member

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xschris

XS400 Guru

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