Not all silent bikes want to be ridden...

Now, my understanding of what k-moe and fitz said is that doing a compression test first would be simply a waste of time. Checking and adjusting valve clearance and then testing the compression will be more efficient than testing the compression before and after.

 

this is why somebody invented leak-down testers. it tests the ability of the cylinder to hold compression and if it doesn't, it's easy to hear where the problem is.
but it doesn't make it any easier to fix

 

The valves are a regular maintainance item. They'll have to be adjusted no matter what. In my experience, valves are the second most common cause for poor running (after gummed up carbs), because the vast majority of owners never bother to do anything with them (this is true across all makes of motorcycles).

 

Whatever you do, don't feed it belladonna...

The Mangler

 

Hard to say. I have been asked to not touch anything until the bike can be brought to my expert, who setup the carbs, as it is possible that more than maladjusted valves could be at fault. As an auto technician, I can appreciate someone with far more experience than I asking me to stand down until he can get a good look. I trust his judgement more than I do mine. I can wait until spring to get the bike back on the road. I love this bike, enough to have me walk it home several miles in the dark, to the point the battery finally went dead as I made the final push across the road to the house. My favorite colors are black and burnt orange, and thats exactly what the 81 XJ is painted as.

So, this thread here, it might die. We have to prepare his garage for this winter project. We have to move some stuff out and prepare a suitable spot for my XJ, and get into it proper. If I dont update this thread, Ill make a new one. Maybe Ill have some nice machine porn of bent valves, and crazyness. Or maybe not. We will see.

 

Whoa, wait a minute.
Do NOT sell yourself short. The items that need attention and are being recommended are not that difficult especially for an auto technician.
You do NOT need an "expert" - that expert is YOU! (With some conditioning of course).

There are wizards here who know more about these bikes than the original Japanese Engineers that designed them. These people are here to help and to keep you from digging a hole. No one, not even your 'expert' will pay attention to detail as closely as you will.

When I got my XJ all I pretty much knew mechanically was "righty tighty; lefty loosey".
Some of the maintenance tasks may sound daunting, but going at things slowly and having meticulous attention to detail (take digital pics, lots of notes, keep work area organized, ask questions here) and you will find that you can become your own expert.

Honestly, I did my carbs this spring, shaking with fear and intimidation . . . . I practiced what I preached above and when I put the rack on and the bike ACTUALLY STARTED! I was overjoyed, proud, and giddy like a schoolgirl.
If I can do this stuff anyone can.

 

Well, Ill be there the entire time. I'm not dropping it off at the shop and picking it up when it's done, just taking advantage of a valuable resource (and an actual garage, something I don't currently have myself.)

 

I did check my valve clearances this spring and found it is an easy job, compared to anything related to my old Goldwing water pump...

 

Once it's running well you should name the bike Bob.

 

Why Bob?

 

Because it's a silent bike......

 

I am resurrecting this thread.

Todays compression check resulted in 165 PSI on 1, 175 on 2, and 180pst on 3 and 4. So I am going to adjust the valves and sinc the carbs.

 

Pretty good readings, let's see if you find tight valves on cyl #1.

 

Good to read you are back with it and there appears to not be any significant engine damage.

Compression spec for the XJ750 is 156 PSI with a maximum deviation of 14 PSI per the FSM. It will be interesting to see what the numbers are after the valve adjustment.

I just re-read through the post and there doesn't seem to be much mentioned on why #4 carb bowl was dry. You mentioned you had to tap #4 with a hammer, as if the float valve was stuck in the closed position. Sticking float valves in the shut position I would think is a rare occurrence, especially since new float valves were just installed. Maybe some more experienced members can weigh in on this as the fuel starvation issue seemed to be the root of the problem.

Since #4 carb is last in line to receive fuel perhaps there is a fuel flow issue and a couple of thoughts:

Petcock not working properly
Restrictive fuel filter installed
Vapor lock in tank caused by cap vent not functioning

 

Thats what I am getting at now. After the float on #4 was unstuck, it ran fine. I rode it about 5 miles, and it was fine, then suddenly misfire everywhere and I was left stranded and pushed it several miles home in the dark.

My hypothesis is that once the bike is at temp, the valves tighten up (as they should) and that meets carb imbalance (as it should) and it becomes a muddled mess that wont run.

So, I am looking at Sunday to be the day I can pull the valve cover and check lash. I am also suspect that I will find a tight exhaust valve on #1 or #2. Of course, Ill need to get the valves in spec before syncing the carbs. They have to be out of sync... Ive only had them out like 3 times now without syncing them.

 

This is pretty much a motorcycle version of Wangan Midnight. Likely more interesting too.