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.