Rejetting, how to determine what size new jets?

Vigo The Carpathian

XS400 Enthusiast
Messages
65
Reaction score
0
Points
6
Location
Philadelphia
Hey everyone. Once my bike is back together, I'll be using pod filters and also running the stock headers with new mufflers with no more h-pipe. I know this will mean I'll have to re-jet in order for the bike to run correctly. Is there a tried and true way to figure out what size jets I'll need, or is it more a matter of trial and error?
 
Typically its a matter of trial and error.

What you need to think of is the air fuel mixture, the more air you are pulling in with these pod filters means you need more fuel to mix, meaning larger jets. If you then put on new muffler that means you are exhausting more/less, meaning a change in back pressure.

Generally if you are using pods, and opening up the mufflers to something that is more free flowing (like shorties) you should step up the pilot jets (1, maybe 2 sizes) and the main jets ( maybe 2 sizes to start).

After installation you will then be able to see if you have a lean mixture, or a rich one, by watching if the RPM needle hangs anywhere and by checking the plugs, and by using your eyes and ears.

If its good on ideal, but hangs when you rev it, your are lean, and will need to go up. If it seems to be okay, but just not right and the plugs are really dark then its rich and you may need to go down.

That is just the basics, as it gets much intense when you start playing with your needle clip position haha

Somewhere on this site is a post or a guide that details how to go about re-jetting. Its not TOO difficult and to be honest when i did mine i just went 3 sizes up on the main, 1 on the pilot and called it a day lol It wasn't ideal, but when I could get it running- it ran haha
 
I am working on a xs400 with pod filters and straight pipes. I can not make out what number is on the main or pilot jets. Which ones would be a good starting place.
 
Have you taken the jets out and cleaned them? jet sizes differ a lot through the years so it's hard to tell you where to start
 
I picked up a rejet kit on EBay for my 81 XS400 SH. Still running stock air filters but previous owner cut out "H" pipe in the exhaust and welded up Honda mufflers. It is still 2 into 2. I received 2 sets of Mains (122.5 & 127.5) and new Pilots (45) The kit recommends installing 122.5's and going up to the 127.5's if still running too lean. From what I've read the stock Mains are 142.5's so it sounds like the kit is leaning out the fuel since the "H" pipe has been removed. So I'm guessing that removing the "H" pipe created more restriction in the exhaust reducing the air flow coming in from the intake and therefore using less fuel? I'm hoping the veterans on here can hopefully give me a thumbs up or down on the Jets I recieved and help me understand what is going on with the fuel mixture. Thanks everyone! This site and everyone here has been a great help to me in working on my Yamaha! :-D
 
I have Pod filters and open headers with no baffles and had loads of issues getting mine to run properly. Mines a 1981 4G5 model and that had a 125 main jet, 42.5 pilot and washer on 4th slot on the needle. I ended up changing to a 45 pilot which was a vast improvement and then up to a 140 main to stop it leaning out at high revs. I then had an issue with fuel bleeding through the needle from the large main jet on 1/4 throttle making it too rich, so had to lean the needle setting two slots. I did lots of runs and plug chops as I went and it runs perfect now. I had to buy about 6 sets of main jets and kept going up in size until the flat spot at high revs went. Your spark plugs give a very good indication of where your at!!
It is very much trial and error!! Good luck...
 
That is actually good to hear ^^ my bike is a 4g5, though the engine says 2A2... It had 125 mains and 42,5 pilot jets. Was going to start off at 45 pilots and 140 mains, so i'm probably in the right area... Also raised the needle 1 step iirc (that one was a while ago)
 
You definately gave me something to think about Requiem! Since the bike is 33 years old there is a pretty darn good chance those carbs have been into.....a lot! Thinking about OEM and original was probably many years ago for this ride. Probably the best thing to do is start simple and get complicated Hahaa!! This is a good thread!! :-D
 
There are many good tuning guides online. Find one. Test what you have, the guide will tell you what to change from the results of the test, retest, retest, until you arte satisfied it runs as it should. You may get lucky and only make a few small changes, or it may take many changes. Only testing will tell.
Leo
 
I am also beginning to get the idea it is a very good idea to wait awhile on gettting a new exhaust till I get this machine dialed in. No use turning a new set of header pipes blue from running too lean.....Hahaa!! :p
 
One other question I have. I noticed in the manuals they recommend replacing the float valves and seats as a set, but I've seen on EBay the valves are sold special - made for unleaded gas. Wouldn't the xs400's already be set up for unleaded fuel since it was phased out after the mid 70's? If not, would it be better to get those special valves even if not with the seats as a set?
 
Please disregard above . Was trying to find an example on EBay and didn't see one. At any rate what I more than likely read was about ethanol added fuels. I did see plenty of sets on there so no problem.
 
Kinda like to see what everyone thinks on this one....

I know everyone prefers their own poison but I've decided to soak my carbs for awhile instead of boiling. Since parts are ever more scarce and never getting any cheaper I'm going to take the conservative route. Being a 52 year old I now have patience I wished I had when I was younger....Hahaa.

So what is a decent length of time to do a Vinegar soaking? 1 week ? 2 weeks?

Should I use Distilled Vinegar or Dark?

I plan of picking up a nice plastic container at WalMart and simply fill it up and place the Carbs in there and put a lid on. I can go back periodically and clean on them a bit and set them back in there for some more soaking.

Though about picking up a tester while I have them off and giving the engine a compression check. The bike has under 15k on it so I'm hoping for the best.

Thanks for any thoughts. I've really enjoyed reading the posts. Best education on these machines right here - and gotten some real good laughs out of some of the comments along the way too!! :-D
 
as a 52 year old patient man, i would take the time to fully disassemble the carbs and boil away! i think its the convection action and the heat of the water that helps break up the gunk.

If your carbs are not totally filthy you should be able to get away with just a bottle of carb cleaner (Gummout is buy one get one at Auto Zone 7$) and an air gun. make sure you can get steady streams of cleaner through all passages and then blast it with compressed air. there are a few guides on which passages should come out where on here.

be mindful of the ball bearings behind the choke lever!
 
Make sure you take out the butterfly shaft seals before dipping them!!
 
Are these the ones to remove xschris? Thanks for the heads up - as I surly would have left them on thinking they would have been ok off the fire Hahaa......:yikes:
.
DSC02169.jpg
 
I used that mikuni carburetor rebuild tutorial pdf when cleaning mine, and that didn't mention anything about the butterfly shaft seals... I kept the entire butterfly valves etc in the carbs when i boiled them... Out of curiosity (and a bit of fear ;) ) what effect would it have to keep those in?
 
could cause the rubber seals to deteriorate. Which would cause air leaks. Dont forget the Choke plungers to!

if your not having idle issues you might have escaped A-OK, next time you tear them down would inspect them
 
Back
Top