XJ750 Midnight Maxim (Sounds like engine will blow up)

Hello I'm back again I seem to never stop asking for help but this time it worries me and its currently beyond my information of diagnosing this issue. Obviously I've had my bike back from the garage and had the carbs fixed as they were completely blocked up and I could of done it myself but I've been too busy to do it and i had a fear of doing it incorrectly and causing more damage.

Since then I've had my bike running, now when it ran at the garage they said the rattle was caused by my exhaust system which had a loose bolt but I dont believe that because this rattle has gotten worse id say recently and its coming from my engine. It sounds really bad to be fair like extreme rod knock or quite bad misfire on one cylinder. It also will only run on choke and if I try to rev it will just stall out and when it does want to rev it will hit 3000 rpm and it sounds like its wanting to fire a piston out the top. I've got myself a small camera to look down into the cylinders and I've had suggestions of what the issue could be but I dont have the facilities right now to take it apart and find the cause.

Below I've added a link to YouTube of the bike running which shows when I try to rev it stalls then you can hear the serious knocking (YouTube has changed the sound slightly so best to turn it up to hear the knocking). Any suggestions of what that extreme knocking could be caused by are very much welcomed as I dont know what it is. Oh and it was leaking a small amount of white smoke from cylinder 1 via the exhaust manifold even though its not got a radiator as its air cooled.

Cheers.

https://youtube.com/shorts/y3hyNHnfJfI?feature=share

 

I’m not hearing fire in all 4 pots.
You said you paid to have the carbs rebuilt,
Without knowing what they did I would suggest inspecting all the carbs yourself.
You need good JIS screwdriver mostly
You have it running now,make sure all pipes are hot. If any are cold they might be the only carb needing inspection. Do this before pulling the carbs.

 

If a cylinder is cold you should rule out a bad plug. Make sure it has spark.
1st thing I suppose is inspect all 4 plugs
Your bike sounds close to running properly

 

about 3 months ago when i got the bike it put new spark plugs in the bike proper NKG ones for this engine so they should be all good

 

Just to add more to this, i requested the motorbike company to clean, rebuild, replace any parts that are needed and to tune the carbs so they all are synced. They said they had done that. Ive yet to check if they have but since the carbs have just been fixed i dont see the reason for them to be causing the fault at hand. I may be totally wrong and they could be. Il check when ive more time.

 

you sure its not the starter chain hitting the oil nozzle cuz its old and no chain guide?

 

Well yes that could be another reason why its making such a noise but it wouldnt explain for its lack of being able to rev and the fact in the video you can hear is a more dominant part of the engine which is making the noise e.g. cylinders.

 

If the starter/alternator chain guide has failed it will cause gear selection problems.

 

The gears select quite well thats the one thing that actually works on this bike i know theres an issue with the clutch handle being lose because of the pin that holds it is barely there anymore but im already onto fixing that.

I spoke to some mechanics i know of and they are pointing towards a really bad misfire.

 

That's good your gear selection is working. You would need to split the cases to change the guide.

 

My two cents or pence if you prefer, agree with Simmy . First pull your plug caps unthread them snip off a couple of MM of the plug wire , rethread caps back onto wire see if any difference. The other thing with bike idling put your hand close to pipe see if each feels hot or is cold . Just because the shop days the carbs are synched , you stated it revs this idicates a vacuum leak , it is possible if your intake boots are old this could be source of leak and this can throw off the synch . To test let bike idle spray some carb cleaner spray around the boots see if idle goes up or changes pitch if so there is issue. Cheers

 

Il agree on what you said and il give this a go just some more context my bike will only run on choke if i turn the choke off it just konks out as well as it konks out if i dare try to rev it, sometimes with choke on a certain level it will allow the throttle to be used and it rather rapidly accelerates to 3000 rpm and then drops back down. Its also still got the original boots on it on both sides from the airbox side and the engine side so yes it could be causing a leak. Is there any chance that the rod bearings are potentially gone which i know can cause such a sound but still wouldn't explain the lack of throttle and it being only able to run on choke. Cheers for your suggestion I'm going to give it a try along with Simmy's suggestion

 

Very doubtful about rod bearings . Since carbs were removed and reinstalled, I would look carefully at the intake boots !! Unless it is someone you know personally that did your carbs question was the rack broke apart and butterfly seals replaced ? Or was this a spray can overhaul? Was a bench synch done and how were carbs synched ? ( vacuum gauges or Morgan tune ?) Was idle mixture screws seated and backed out 2 1/2 turns ? Were the floats wet set they could be off as well .

 

Uhhh ummm uhhhh, the company i got it done at are quite recommended by a lot of people i asked for them to clean and rebuild my carbs all 4 of them were cleaned and rebuild and synced but everything else you stated i do not know if they did or not

 

You said it will run with choke and rev with the throttle to 3k and drop back. No choke = won't run. Have you adjusted the main idle screw since getting the carbs back? With choke on, the engine should run at 3k or above initially anyway, with no throttle. Without choke it should be set to idle at around 1100 RPMs. Check your idle screw.

 

new plugs installed in a motor without the carbs dialed in can foul in 60 seconds.
Once a plug is wet it has trouble sparking. They can be dried out.
Very easy to pull all 4 and look at them, this is how you diagnose.

I had another listen and it sure doesn't sound like all 4 cylinders are running.

 

Sounds like it's only running on two cylinders. Check your coil connections, HT leads, etc.

 

To be honest with ya ive taken my spark plugs out many times in the process of trying to get my bike to run and they are coated in a medium coating of soot if that helps with anything. And i think one or two of them are quite clean most of the time out of the 4.

 

Then aint right at all this bike idles at about 1200rpm on choke unless i turn it up to the max which it then tries to idle at 3000rpm. And without choke well it wont even run let alone idle. Since getting the bike back i have not adjusted the carbs what so ever they are still as they are when the garage worked on them.

 

1. I wonder if a running sync was performed.
2. I wonder if valve clearances we adjusted— did you get a writeup if what is in there now....
3. I’m curious to know if the air jets under the diaphragms are reversed ......

I’ll check the video tomorrow when I get some time.

 

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.

 

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.

 

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.

 

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.

 

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.

 

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

 

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.

 

Do you have a basement?

 

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

 

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

 

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

 

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.

 

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

 

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: 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.

 

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.

 

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?