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XJ750 Midnight Maxim (Sounds like engine will blow up)

Discussion in 'XJ Technical Chat' started by RussianBiasGuy, Nov 5, 2022.

  1. Timbox

    Timbox Well-Known Member

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    Reading through all the treads I am wondering about the gas tank. You say that the carbs were cleaned by the shop. As other above on this post have stated, unless you get in there and take a look at them you will really ever trust the cleaning.

    The engine is only running on a few cylinders, so it is either from the spark or the gas. Being able to run a sync or other tests will be hard until you get the engine running on all 4.

    This test will be a Pin in the A.. to do, but if you removed the boots from behind the carbs you can then either use your hand to block off each carb entrance to see if that will help make that cylinder run. If the floats were not set right adding extra vacuum will force the fuel up.

    Once the bike has been running a while check the pipes for heat. Be careful as a lean engine will make them supper hot and blister your fingers in moments. I have a IR temp gun that really works well from HT.

    Stick with the basics, air, fuel, spark to get them all running.

    Good luck.
     
  2. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    Ok—it sounds like it’s not running on all cyls. I’d check to see which pipes in front don't warm up... The replace plugs/trim leads/check caps/check fuel delivery/Etc....

    Have you done compression tests, or leak-down tests?

    the goal being to get fire on all four...
    Then...... get the the cyls synced.

    THEN if it is still sounding bad, we can try further diagnoses.
     
  3. RussianBiasGuy

    RussianBiasGuy Member

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    To those questions above I dont know at all, it doesn't look like the valves were touched as someone used a gasket sealant type stuff and thats been on the bike since I've got it and it hasn't been changed out so I doubt the valves have been touched.

    I had the bike running today as well and I can tell you Cylinder 3 was less than luke warm and Cylinder 4 was stone cold, meanwhile Cylinder 1 and 2 made for lovely handwarmers (I'm going from left had side to right). It decided to act slightly better today when running and I was able to get it revving well over 5,000 RPM and that was without choke with some backfire/misfire from those two dead cylinders firing only once or twice and it felt like Cylinder 3 tried to keep going for a while whilst I was giving it the beans but it just couldn't keep up and well Cylinder 4 was still asleep. Still only idles with choke though under 3,000 RPM.

    I have to say it looks like a mixture of oily fuel was ever so slightly dribbling from cylinder 1 via the gasket it was a brown/blackish mixture leaking out. To be fair there's quite a lot of whatever that mixture is coated around the front of Cylinder 1 and 2 but not the other side.

    I will contact the garage and see if they can generate an entire list of parts they replaced/repaired as well as what they performed to the bike to "fix" it. I will try the suggestions that have been provided but I'm busy most of the time and the weather in the UK well its not great il say that.
     
  4. Timbox

    Timbox Well-Known Member

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    Good trouble shooting, now you have a start. You know that 3 and 4 are having issues so now go back to the carbs. I don't know how mechanically inclined you are, but for the cost effective for these bikes, you will have to get in there. Read through the "Church of clean" follow what the link shows you. This way you will know what was done, will not have to relay "Trust" a shop and be confident on what those carbs look like.

    I would also check the fuel in the tank. Remove the line, put a clean catch container under the petcock and turn it to "Prime" that will let the fuel flow. Then you can see if have brown, water, clear or bits in your gas. Your carbs could have been nice and clean and then the tank fuel got in there and blocked jets and passage ways.

    Weather sucks here in USA Wisconsin too, highs in the 34 F range this week. Sudden weather snaps are not fun.
     
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  5. RussianBiasGuy

    RussianBiasGuy Member

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    Right now I'm not the most mechanically inclined but I'm learning I've liked anything with an engine most of my life and I've decided well if I like them and I want such vehicles I need to know how to fix them which is why I got this bike, yes I know its not the greatest starter project at all but I liked the look of this bike and thought well I want that.

    Here's some more information when I got the bike back me not thinking instantly ran it off of the tank and I did that one more time before I realised that I've just been extremely stupid and I ordered myself a portable mini fuel tank that you can hook over the handle thats what I run the bike on now. When I drained the tank of its leftover 3 month old fuel it was rather orange with flakes of rust that washed out with it (the flakes of rust only came out because I removed the petcock all together to drain it) so the fuel has actually cleaned the tank but I ran the bike twice on that rust infused fuel so I potentially re clogged my carbs.

    I've added below the video of my bike running when I came to view the work the garage had done to it, now it sounds quite good there but I have another video which I'm talking in and you can hear the mechanic rev it and the knocking sound appears when he revs it. They also used a portable fuel tank.

    https://youtube.com/shorts/3-ZWaJ4MJyU?feature=share - Link to the bike running at the garage.
     
  6. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    That’s not running correctly there,.either.

    you CAN Do a complete tear down and rebuild on the carbs yourself… It is not impossible. We are right here to help… If you get into trouble.

    also I am right here to talk you through doing a complete rebuild, if you wanna go that route… And as a last resort if you throw in the towel and need rebuild services, I’m right here and offer rebuild services. Plenty of people here know my work, you can feel free to send me a private message and we can discuss that, as well.

    I highly recommend that you read up on adjusting the valves, and check your valve clearances before proceeding any further. If they are out of spec by now, anything you do will be negated when you finally DO get the valves back into spec.

    That also it’s not a hard procedure, it is just time consuming the first time or two that you do it. We can talk you through that, as well. And, as far as replacement shims… I run the shim pool, too… Or, you can order new ones either through Meet or through Len Chacal at xj4ever.

    dave fox
     
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  7. RussianBiasGuy

    RussianBiasGuy Member

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    Sorry for not responding straight away I've been rather busy with work. I think you are correct that I shall have to do a carb rebuild myself but I do not know a single bit on rebuilding carbs its completely new to me the only issue is I have no where to actually work properly as in a good workplace, I've been planning on building a workshop type shed I just haven't gotten around to building it yet and I need to as its becoming colder and colder in the UK. So as of now the carb rebuild isn't possible until that shed is built.
     
  8. Huntchuks

    Huntchuks Well-Known Member

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    Do you have a basement?
     
  9. RussianBiasGuy

    RussianBiasGuy Member

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    In the UK 90% of houses dont have basements and we dont use the attics either
     
  10. Huntchuks

    Huntchuks Well-Known Member

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    Well, it would be a chore getting the bike into the attic if you had one!
     
  11. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    I have a Honda 305 dream in the attic......
     
  12. Timbox

    Timbox Well-Known Member

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    hogfiddler, you probably have a deluxe elevator to the attic too..... Just saying you have lots of bike and will find a way to keep them in hidden little places.

    :)
     
  13. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    Just a pull downstairs… For what it’s worth, the bike is completely disassembled at the moment lol
     
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  14. RussianBiasGuy

    RussianBiasGuy Member

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    Nov 23 the last message.... Sorry for being inactive and just vanishing away I've been rather busy and have had no time to work on my bike as well as the weather still not helping. I'm no further forward with finding the issue of the knocking but I do have pictures of the spark plugs (I accidentally dropped one of them) but it does show the bike has been running really rich but potentially its been running on all 4 and the knocking is something else like someone I spoke to suggest tight valves. I will soon start work on the bike again which also includes taking apart those carbs and seeing how good they did it and resetting each one and then going from there as well as replacing gaskets on the engine as the head gasket has been leaking oil and fuel mixture which means its leaking from the valves. Here's the photos of the spark plugs: Spark Plug Image 1.jpg Spark Plug Image 2.jpg Spark Plug Image 3.jpg Spark Plug Image 4.jpg These photos should give some more idea of how bad this engine as been running and the spark plugs go from left to right so Cylinder 1 is far left and Cylinder 4 is far right (Yes I dropped Cylinder one spark plug but I haven't used it since and will get new ones once I've fixed the engine properly then il run it). Sorry for being inactive as well as I should of kept the updates but I didn't. Cheers.
     
  15. RussianBiasGuy

    RussianBiasGuy Member

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    hello people just did a compression check on my bike finally and i can say that it was rather good Cylinder 1, 3, and 4 being at 125 psi and cylinder 2 being at 120.
     
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  16. RussianBiasGuy

    RussianBiasGuy Member

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    Jan 17th last message, yes I'm very much alive I just post updates often I should but it seems I dont lol. Don't ask me what's gone and happened but my carbs have reblocked themselves from sitting over winter oh how fun that is for sure but I'm soon going to strip the bike of most of its parts and respray it (gloss black) and start replacing and greasing up parts as I build it back together and the first thing thats coming out is the damn engine. From it sitting over winter its show its leaky side of things and that being the Head Gasket leaking fuel and oil mixture and the Valve cover leaking oil. The spark plugs all fire really well as the engine still turns so I took them out and tested each one and they have plenty of spark so there all good (new-ish spark plugs) but it does now seem I'm going to have to rebuild the carbs as there blocked and find the engine knocking noise hmm how fun (I find it quite fun actually).

    Il put this here as well as I'm unsure of where to go to ask and I know someone will be able to point me in the right direction, my fuel tank is missing its "Yamaha" gold logo badges and it seems the tanks with the badges have an indent of where they are held in place, mine seems to be missing those indents unless they have been filled lol. I was hoping to put the badges back on but I'm stuck on what to do as I have a really clean fuel tank but no where to mount the badges if I can get them. Any suggestions?
     

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