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78 xs 400 E rebuild and modify

Discussion in 'The Garage' started by Tyler Shiells, Mar 30, 2017.

  1. After tinkering with the carbs here's were I'm at....
    I have 47.5 pilots and 137.5 jets installed.

    I can get the bike idling after warmed up. The left exhaust is very weak pressure coming out. The right feels steady and strong. When I twist the throttle the left exhaust does become stronger and more steady.
    When I adjust the pilot screws I get no change in anything.
    But when I start adjusting the sync screw, clockwise, the left intake pops a bit and exhaust pressure becomes a bit strong and then that's when it revs upto 3500-4000 RPM. Turn it back counter clockwise and the idle comes back down and so does the left exhaut pressure.
    If anyone has any idea what that means, the wisdom will be appreciated! Thanks
  2. xschris

    xschris a lifestyle not a trend Top Contributor

    Sync the carbs with a manometer. Do this after you have set the valves. Once you do this you may find the pilot jets are way too rich. Make sure also the pilot mix screws have not broken off in the carb bodies. 77-79 year carbs are very prone to this.
  3. I built a homemade manometer.

    I set the valves and timing. Benched synced the carbs with a feeler gauge. I then warmed the bike up. I hooked the manometer up and it immediately wants to pull the fluid into the left. I try to make minor adjustments but I can't get it stop pulling left unless I turn it to the point where it starts revving high again.

    The pilot mix screws are intact.
  4. xschris

    xschris a lifestyle not a trend Top Contributor

    Do a compression test. If it is pulling left the right cylinder has low compression
  5. dgoodsy

    dgoodsy XS400 Enthusiast

    Did the carb boots get replaced? They look cracked and can cause a vacuum leak like that.
  6. Well this bike was never the main priority in my life. It took a back seat to a full restoration of a 2006 Tw200 for my wife. Then a full 2nd floor renovation on our home.

    Have begun tinkering again. But need to pull the motor on our Subaru to do head gaskets and timing kit before I commit to buying new parts.

    But this is where I'm at currently. Stripped the bike down. 20190914_225953.jpg
    Going to rewire a new electrical system.

    Took the forks apart to find out one spring cap is completely seized in.
    I'll have to drill and tap it to back it out..

    Also took the shocks apart, using the ratchet straps, to test the dampener rods. A big rusty spot on one is causing some grinding sounds, so maybe replace but they feel ok for resistance.
    20190919_111920.jpg 20190919_113540.jpg 20190919_113553.jpg

    But going back to how the motor runs. I'm only getting 90 psi on the left cylinder. I've replaced rings but I'm going to take the head to a machine shop and have them going over the valves and what not.

    Any way stay tuned...
  7. So with a valve adjustment and a compression test... again. Still low and lower left cylinder.


    Ran a leak down test next. At TDC, with both cylinders, the leak seams to be thru the crank case. So it appears I need a more accurate measurement of the cylinder bores. I do have proper bore gauges now, I didnt few years back, so that's next.

    If I need to go with one size up on the pistons, hopefully those are not too hard to find...
  8. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    Check Ebay for short blocks, or completes.

    A lot of those sellers break down good bikes. Its almost cringe worthy for me to think of sellers breaking down perfectly good engines to make maximum profit. Picking up a short block would be convenient for you if that were the case and Ive seen them before.

    Have a look at the bores, they will tell you whats going on. It would be worth it to take the head off and examine IMO. Head gaskets are readily available.
  9. Thanks NH Mike

    I did check and I'll keep looking because
    20191013_193709.jpg 20191013_194203.jpg

    I took about 8 measurements across both cylinders and 69.15 was the highest 69.03 was the lowest. So she's sitting pretty fat for standard pistons Going to take a few more measurements at varying depths.

    So on to some research as to my next options..probably going to follow NHMikes advice and hope to find a set of jugs off a low mileage bike.

    Do these bikes use the same cylinder jugs across years and models? Sure hope so!

    But in the meantime going to measure everything down to the crank bearing now.
    Last edited: Oct 13, 2019
  10. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    Early 400 cylinders were square like the 360s were. The 400s got rounded cylinder fins in 1978 I believe. All 400 sohc jugs are interchangeable.

    What you have listed would be perfect for you. Make sure you read the entire description in the ads. A lot of sellers will list bore sizes, milage, and even compression numbers.

    How do the cylinder walls look on your engine? Are they glazed or scuffed?

    Having youre cylinders machined to the next oversize is an option. You will need to find pistons, rings and new circlips. This would be a good option if you had a machine shop near by.

    Buying the used cylinder would be much quicker and probably cheaper at this point if you get a good cylinder. All youd have to do is bolt it in and go.

    At this point, you should lap the valves and replace seals. Removing the valve springs is VERY hard if you dont have the right tool. You need a heavy duty spring compressor that has enough clearance for the head.
  11. I had this motor apart few years back. Valves seals were replaced and valves lapped. They arent leaking.

    Cylinder walls were honed by me as well. But I didnt have the proper tool to measure the cylinders so I though I snuck a win in there by just replacing the rings but appears not.

    I did read thru that extended thread about options regarding pistons and rings. Those Spanish ones or the ones from that Germansite are very expensive for a Canadian. Over $350 plus a re bore. Makes that a $500 job.

    Low milage cylinder block in spec is by far the cheapest and easiest options..

    Gonna hope for that.
  12. One other question. My bike has 34000 plus kms on it. Are similar milage bikes out there still within spec on the cylinder jugs and running fine with standard size?
    How did this one get worn out?
    Lack of proper oil changes?
    Just curious thanks
  13. https://m.ebay.ie/itm/Yamaha-XS400-...273111?hash=item3ab3a89557:g:Y8QAAOSwxA9aAsQt

    Damn is that pricey. But it may be the only option I can guarantee fitment.

    I've messaged 4 sellers of cylinder jugs and cant get a guarantee on cylinder measurements.

    The one link in my other post the seller has it listed as 68.9mm but after messaging the person they said its prbly a small measurement error and its prbly still with in spec. I belive the motor it came off had 23k miles so who knows.

    Dont want to get burned on used jugs just to be where I am now, in need of bore and over size.

    So I need to ask myself if 450 plus bucks for the piston and ring kit from Germany and maybe another $150 for the machining is even worth it?!?!
  14. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    My engine has around as much milage as your engine and thats not a lot of milage compared to others on here. Getting an engine to make the best power and last forever all depends on one thing, was it properly broken in? Obviously after that, theres the upkeep which is crucial.

    The old school fix would be to get new rings, deglaze the bores, and knurl the pistons. Another last ditch effort would be to dump Ajax powder in the cylinders and supposedly that will deglaze the bores and reseat the rings. I probably wouldnt try the Ajax...

    You are at a fork in the road here and its a tough call, all of which would require a significant commitment. Youre roughly at the $500 point.. The only sure way Id have confidence in the bike is going with a rebore on your current parts. Thats basically a brand new engine for you that you will be able to depend on for a very long time..

    Or, put the $500 toward another bike. ‘Tis the season when good deals can be had for every type of motorcycle. I just picked up a mint 1971 Honda CB350 for $400. Needs a full tune up and only has 5000 miles on it. Bought it as a non runner but its completely original.
  15. After about another half dozen measurements on the cylinder, all over spec and without a guarantee on one off ebay, Ive decided to rebore my cylinder block. Im going to get the next size piston/ring up. Im going to keep my eye open for some new old stock on this side of the world off ebay.

    Gonna rebuild this grumpy old motor.

    Anyway, the motor is completely disassembled and going to replace anything that needs to be.

    barely got the screws out for the shift guide plate. 30 mins later got em.
    20191014_153109[1].jpg 20191014_153100[1].jpg
    will replace em

    I didnt have a fly puller. The only bolt i had large enough to screw into it was the rear axle off a tw200
    20191014_160411[1].jpg 20191014_160403[1].jpg
    worked perfect!

    Next came the short block bolts. #12 was rust stuck.
    20191014_165325[1].jpg 20191014_165223[1].jpg
    and #1 is quite stripped so will replace.

    Also the oil plug is completely stripped and cant be removed. So that to figure it out.

    Pulled the oil seals with a drywall screw.
    20191014_171954[1].jpg 20191014_172209[1].jpg

    The sludge at the bottom was quite bad. and the engine just filthy.
    20191014_172232[1].jpg 20191014_182118[1].jpg 20191014_182125[1].jpg

    The one large bolt with a flathead on it and spring in the case? Whats its function?
  16. I found 2 pistons part# 2L0-11635-00-00 from 2 different sellers $200 shipped for em.

    Now to keep my eyes open for the over sized rings
  17. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    I paid about $125 US for each piston kit that includes the rings, circlips, pistons, and wrist pins. Another $60 was for each rebore.

    Do not remove the connecting rods from the crank. I presonally wouldnt even have touched the bottom end at all.

    You should bring the cylinder to a machinist and see if the bores are good for the 1st oversize. They may say that you need to skip the 1st size and go to the 2nd if the bores are that much out of round. This happened to me after I had already purchased everything. My machinist told me that I shouldve gone to the next size because there was a slight imperfection on the cylinder wall. We couldnt feel it with our finger nails and he said it should be ok and I decided to run it anyways and it ran fine.
  18. Ive measured each cylinder a dozen times in varying depths and degrees and I feel confident Im still within usable material for .25 over.

    I will need some wrist pins as there is a bit of freeplay within the crank rod when tested.

    I took the bottom end apart so I can properly clean the engine cases, prbly use a soda blaster. Then paint em but most likely polish them.
    Last edited: Oct 16, 2019
  19. 20191016_113122.jpg

    One more battle won with this grumpy ol motor.

    When I drained the oil last time I just dropped the sump cover because the oil drain plug had been rounded off.

    I tried lock pliers, wouldn't grip. So I ended up filing the edges down till I could get a 16mm socket on there, and it came off.

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