The Journey of a wrench and his first motorcycle


XS400 Enthusiast
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Long Time lurker, first time poster.
The details.
Bike - 1978 SOHC xs400.
Me - Handy with a wrench and reader of instructions.

New Parts:
Carburetor to cylinder boots
Brass Floats
Exhaust gaskets
Emulsion tubes
42.5 pilots
142.5 mains
Float pin valve and seat
Emulsion washers
Uni foam pods
Countless spark plugs
Oil filter
Light bulbs all around
Petcock to vacuum tubing
Vacuum barbs
Couple new lengths of wiring
Spark plug wires and boots
NOS 1979 ish H pipe exhaust
in-line filter
Rigged up a new seat
H-Pipe for carburetors!!! Found a great condition one on ebay
Throttle cable
and the smiles i have riding it.

Hi folks thought i would post a proper introduction. The names Dillon and i've been scooting around on a xs400 for about 10 months now. My obsession with 2 wheels goes back rather far and I always new i would be sitting on top of one that weighed 4-6 times heavier then me. What kind of 2 wheel'ers? Bicycles! everyone's got one and there is nothing easier right? Any who i literally get paid to work on bicycles and fallow instructions to the letter. My College days brought me a few courses of Automotive technology and at one point after about 5 years of regular practice i thought i was going to do something with my welding experience, but who would've thought around that same time i would land the "dream" job. Pedal bikes baby, lots of them and some of the latest technology. These days though I could not want to pedal a bike less, suit me up for a day on the motorcycle and i'll see you in 5 hours. So here i am, as i find the time to update this thread i'll be certain to include in depth trial and error, reasoning behind plans of attack and ultimately what the fix was. The engine is finally running very well and as i compile my notes i'll be sure to post pictures soon. Thanks for any future insight!


Post 9-23-2017
What I've learned.
Petcock rebuild kits are about 8-11 dollars less then a manual petcock. There is little reason to attempt the rebuild. The time spent ordering, waiting, cleaning, and reassembling, for what i consider a 50% chance at success and a 50% chance of a gas filled crankcase make the decision for a manual petcock a no brain-er. I have to wait another month till i get mine and guess what, i carry a plastic bag around with a small rubber stopper that reeks of gas, get gas on my hands undoing my fuel delivery tubing and plug it up. My rebuild didn't take.

Update. 09/27/2017.

So i had to jump back into the petcock, i was refusing to believe that it all went well. All looked good till i took off my petcock selector plate and switch to get into the large circular gasket with the 5 holes in it. 4 in a circle and 1 in the center. So i had installed everything dry since i thought grease and gas wouldn't be ideal. The problem was that i installed dry and let it sit for about a month. I assume the 1st time i turned the selector the dry gasket attempted to pull with it and lodged part of it into places it was not supposed to go. Tried to clean the gasket up, greased it and reinstalled with the least amount of damage connecting the reserve to my outlet tube. This almost worked, sometimes it drips about 1 drip every 2 minutes and other times it doesn't drip at all. Replacement ordered as the circular gasket was 6 bucks with shipping. Long story short, a rebuild done slowly in a clean environment and greased may very well be a sure bet. That's your call though as the kit is really just a couple bucks less then a manual replacement. I will probably keep the vacuum petcock once zero leaking is observed, or if it seems mine is 100% operational.

Zero leaking has been observed once my large circular gasket was replaced after ruining the first one to a dry install and letting it sit for a few weeks. BUT, vacuum petcocks connect to many places and require all those connections to be in good condition. Carb boot to petcock vacuum inlet must be in good shape as well as the gas tank flip cap O-ring.

Don't jet yet unless you a vet.

You've got the new to you xs400, your convinced you can make it go faster by cracking open your carbs and screwing in 4 brass thingies with little holes. Enjoy your bike for a few months double checking the easy things, or you can do it my way and attempt jetting on a 99% operational fuel and air system and a 45% or less ignition system. Get your ignition in order. Inlet and exhaust valve clearances, gap points and time. Then once the expected base operations are solidified, you can try and jet. To make it more confusing i would suspect that the majority of us have yet to develop the subtle detection skills to really dial this aspect of our bikes but what i'm starting to pick up is, that these bikes will run a multitude of pilot and main combinations as long as there relatively similar to what you took out. Fine tuning requires detailed notation and observation. There is a website i'm having trouble finding again that if i can remember correctly had a pink background? or pink writing, i fallowed a link from another thread on this forum there and it was hands down the best jetting guide. I'm still trying to nail down my main jet and the guide says that is absolutely step number 1, there is no pilot and needle tuning without figuring out your main jet. If you know this site please link it again in this thread. Finally freaking found it!!!!!!!

Here it is,CV,high_rpm_engines.html

You Might need new points and or a condenser.
A new condenser would only be appropriate if you observe or notice evidence of the points sparking excessively. Double check point gap before getting too far in. I had also noticed a light coating of dust on and around the effected area, it would easily wipe away and was brown greenish yellow dust, you know, the weird "thats an electrical fire" type stuff. A bad condenser will ruin points until replaced. Pitted or poor condition points can be dressed flat again with good results. I suspect though, as you dress your points multiple times the removal of material will eventually effect the point at which the lobe would normally contact the points lever. Potentially making timing the bike gradually harder.

Gap Tip.

Rotate the engine and test the corresponding point you are gaping at all the spots it appears to be the "most" open with your feeler gauge. You will find at least 2 positions that will be more open or closed then other positions. The trick is to find the largest gap throughout. Identify the deviations in gap width and make sure the "most" open is set according to the manual. Keeping the lower point plate bolt slightly tighter then the top will allow you to utilize the small raised dots on the timing plates to help slowly and accurately move the points plate with an appropriately sized flat head screwdriver. I set any feeler gauges to a loose target size, tight next size up. Keep in mind that gaping the left and right cylinders must be done together starting with the left, immediately fallowed by the right. There is no one or the other and frankly in my mind there is no gaping without timing although the manual dosn't quite say this and the inspection intervals pertaining to each adjustment are different but I've already got my crankcase cover off i might as well triple check timing.

Time it.

If your going to statically time your machine I would recommend making the light the manual suggests. It is also of the utmost importance to triple check your gap before attempting timing. As of writing this i don't know how to pull off a dynamic time and i would have to watch someone do it, the manual says oil will issue from case, no thanks. sounds scary. I made a light out of a turn signal i had lying around and some alligator clips. Connect the positive and negative leads to the alligator clips and i'll put a clip on the point spring ribbon and the other on a cooling fin on the engine. Key set to on, with the spark plugs removed, and as your points separate the circuit will open illuminating the turn signal or other small light. Set this to the exact moment your LF/RF line crosses the timing point on your engine and you've statically timed your engine. Triple check and button up.

Helpful corner -
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It's running rich, doing half turn adjustments. Is quarter turns better? Getting diaphragms and slides first for possible fix's.

I'll post pics and more details as I can
Post some pics of the bike. I would go smaller on the mains. Did your sync the carbs with manometer?
Excited to try out the smaller mains. I made a snapple bottle, trans fluid and 1/4 inch vinyl tube homemade manometer and i think i did a good job on it. Its the type that wont suck fluid into the carbs with the middle connecting piece between the snapple bottles. I don't think it could be dialed in better but after swapping mains i'll make sure to hook it up again.
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update :
Brass floats
Carb to cylinder boots
and added what appears to not have been there before : exhaust gaskets.

Bike is running very well, the sluggishness and lack of power around 6-7k rpm was a diaphragm that had come undone up in the sealing surface and groove of the top of carb. Just waiting on a paycheck before i checkout on my online cart to replace them. I plan to hook it up to the manometer again tonight after i have a chance to warm it up and a brief plug color inspection. Why? The bike came to me with bp8es plugs and i swapped to bp7es once i started really working on the thing and cause of some strange obsession with stock setup. I had a fresh set of the bp8's lying around and decided to plug them in after i noticed my bp7's had gotten kinda fouled and reading that these things get real finicky with a fouled plug. My fuel air mix had to be turned down due to the cooler plug and im close to around 2 turns out on each. This is what i suspect to be the root of my current issue. Carb backfires at kick over for a few mins till warmish and sustained 4-5k rpm. At 4-5k rpm it could be a few times and sometimes quite a bit but it does come an go. From the digging i've done most of ya'll would say its a too lean condition. Plug color as of 2 days ago and 100 miles between color inspection lead me to believe i need to lean it a bit but wisdom might recommend enriching. I put in the squeeky clean main jets that i had, the 132.5's and im not sure what to look for. Now that the bikes running rather well i might toss the 147's back in and see if i notice a difference higher up in the revolutions.
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Rode home and let the beast cool.
Plugs are still surprisingly rich looking.
After yesterdays adjustment I believe im down to about 1.5 turns on each fuel/air regulation screw.
Plan of attack.
I ordered the next size up from stock on pilot jet and main as well as a rebuild kit that included float pin and seat. Some digging on the forum lead me to the conclusion that rule of thumb might be 1 size up on each for each modification to air flow. Air pods? Next size up. Shorter exhaust? Next size up. Since I plan on keep the NOS exhaust i practically just got, whole for a while longer I'm prepping to tune according to the addition of free'r flowing Uni pods. Riding the bike lightly and at speed limit till parts arrive and i'll be trying the pilot jet first after picking up some fresh plugs both the "cooler" bp8's and bp7's. Since um ehemm* cough* mumble one of my first major repairs was a left cylinder thread sleeve install, and some research has uncovered a gentleman's testing into the thermodynamics of the steel sleeve in the aluminum head. I forget his forum name but he seemed to be around on this board a lot till about 2015 or so. FYI The compression test i did was about 200 miles after the thread repair. Some day i'll shower ya'll with sexy pics of this machine.
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Contrary to conventional wisdom....with pods you might have to go down in jet sizes...might help with the "rich" condition.
Contrary to conventional wisdom....with pods you might have to go down in jet sizes...might help with the "rich" condition.
Huh great piece of information, I'll definitely be snagging the size down on the pilot. How about this idea. Bike backfires mildly after a brief high speed run then goes away after a short "not so fast section" with the smaller main installed could backfire be fuel starvation at sustained high speed? Suggesting a larger main? Can't wait for the parts to come in, I'll be testing with hotter plugs but will also be snagging up the size down in jets since I plan on owning this bike for a while.
Heard somewhere that backfire could be unturned fuel igniting in the exhaust from a rich condition...just reporting...
Tossed in 1 size up on mains and pilot jets, bike runs very smoothly. Take off from stop has smoothed out and getting up to 45ish feels very smooth and sounds good. I did some digging on plug color and i may have been misreading my plugs. Where in i thought i was rich i might have been a tad lean. Using Houghmades method of fuel air mixture based on the hard engine brake deceleration to listen for popping and i think its only been getting better. I started with the 3.5 turns out but i believe I'm around 4.25 on each. probably a little more on the left cylinder. Doing 1/4 turn adjustments. I still plan on getting new diaphragms and slides as the sealing surfaces seem like they have been around for 40 years.
Right cylinder spark plug color is getting really good, took a quarter turn off this morning to put me around 3.75 turns out. Left cylinder is still a tad lean, i ran it out a whole turn to put me up to around 5. It was easier to start and didn't pop and backfire through the carbs at all. Current gremlins i'm chasing is it being difficult start and backfire for a few seconds that goes away. I tried to inspect the choke assembly last tare down and ever since the choke has been a little goofy. I'll have to see if i can get new parts for it which puts my current obsession into butterfly valves, diaphragms and choke bits. first things first though is an oil change. new float pins and seats lead me to think if i had any issues with the floats not sealing i gatta get some new possibly un gassed oil in there.
4 turns out is as far as you should go. If that won't get you a good idle you need to go up on the pilot jets. I would put the stock H-pipe for the air filters back on as this helps balance the carbs and smooths out the air flow. Stick with stock type plug numbers. They where designed for the motors temp range.
I want the H-pipe soooo bad. I want to be able to vent crankcase fume/pressure/whatever back into the carb for another chance at ignition. ebay keeps coming up with stuff id rather not pay for. 4 turns max huh, i like this information. If it appears that my left cylinder is still a tad lean at 5 turns out, would the next size up potentially let me get the same flow using less turns out? Allowing me to stay around or under the 3.5 recommended starting point.

this helped me a bit as well
I have seen them for under $20 off ebay. 77-82 sohc xs400 will fit.
I'll most likely be fabricating my own h pipe for the carbs this weekend. Potentially tearing the bike down again tonight to prep for the weekend as well. Ordered a new throttle cable 47.5 pilot and stock mains. Still getting some popping on very heavy deceleration but as im at around 5 turns on the left cylinder it seems that parts need to change. I'll up size both pilots because - left cylinder still has some white on the spark plug and the right is becoming a beautiful light brown. Both are above the recommended 3.5 turns out so hopefully the larger pilot will allow me to achieve the same idle flow with less turns. Downsizing the mains to see if i get better rev and pull around 1/2-3/4-full throttle. Since i drive the machine at 1/4 or less throttle i suspect plug color will be affected nicely by the bigger pilot. Will also be taking the exhaust gaskets out i put in. I know i was getting better sealing with a good bead of high temp gasket maker then the rings i put on. I have a mild amount of escaping hot air around the exhaust outlets. Will also be triple checking the inlet and exhaust valve clearances as they may be contributing to the backfire.
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Thanks for the suggestion. Just put in a mikexs order this morning but i'll have to put in another for the condensor. I found a thread for testing but i also kinda like getting shiney new parts. I've reviewed the manual several times but haven't done the points and timing procedure. I need to make the recommended buzzer or light mechanism the manual suggests for the timing. I'm sure there's a thread on here somewhere with the hot tips one figures out after doing it like i'll be doing, the hard way
Make sure the ATU is clean and lubed. If it sticks it will cause erratic timing.
Update :
Re gapped points, exhaust and intake. Bike fires on first kick and none of the backfiring. Woooot! Plugs are very rich after about 10 mins of idle. Turned the screws down to the 3.5 turns and dialed in by blipping throttle and watching for idle drop or idle hang. Checking color tomorrow. Had a box fan in front of it to help with air flow but did not drive it tonight, potential test jets arrive tomorrow. Now that the engine seems to running correctly *I get kinda excited and jump right into things but if memory serves I made a massive adjustment to the left cylinder gap and a mild to the right as well as what I think was a beautiful slip fit on the valve clearences* I get to start jetting! 45 pilots and 145 mains currently. I'll will be downsizing main to stock 142.5 and testing the current pilot with fresh plugs.