Compression Test Questions

[86FJNJ]

I found this cheap compression tester, it has a 14mm thread diameter adapter but I know the spark plugs on the FJ have a thread reach of about 19mm and this does not appear to be that long. Do I risk destroying the spark plug threads in my head by using this kit to do a compression test? Does the adapter for the compression tester need to have a 19mm reach? Does anyone have any other recommendations for a kit to use?
https://www.amazon.com/gp/product/B0BDCN8NKZ/ref=ox_sc_act_title_2?smid=A1Q7LK027197Q0&th=1

I've see conflicting answers if compression test should be done on cold or hot engine, does it matter?

When doing compression test do I leave the spark plugs in the other 3 cylinders or take them out?

[Pat Conlon]

You will be fine with that kit. The threaded hose adaptors are used briefly. I've never blown any out. Don't forget to first warm the engine then hold the throttle plates wide open.

[86FJNJ]

ok thanks, so the compression test must be done when engine is warm and running at full throttle, will do.

[RPM - Robert]

ok thanks, so the compression test must be done when engine is warm and running at full throttle, will do.

Not running, after running and warm. Take all the plugs out, install the compression gauge on a cylinder and crank with the throttle open until the gauge no longer rises. That is your compression number for that cylinder. Make sure the battery is charged and/or on a charger. A weak crank can give faulty numbers.

[86FJNJ]

ok thanks for clarifying that, I just got a new battery and the crank does seem weak, seems like it cranks slower than I am used to, is that normal for FJ's or is the starter possibly on its way out?

[RPM - Robert]

The 2 brush starter was weak from the factory they went to a 4 brush later which increased cranking power.

That said the 2 brush symptoms usually only show up when the bike is warmed up or hot. Or if you have a built motor with higher compression.

[86FJNJ]

Thanks, are the 4 brush starters from later years compatible for my 1986 without modification? If so do you know what year they started using the 4 brush starters?

[RPM - Robert]

Someone here probably can confirm but I don't know if they changed in the North American models.

I believe all the XJR models had them but they were not sold in NA.

There are "chinese" 4 brush starters on eBay. I would buy at least 2. I have seen them come apart internally but you can buy 8 for the price of one Yamaha one.

[86FJNJ]

I was able to get the bike running! First time in 15 years, smoked a lot. Would only run on almost full choke the moment I tried to give it throttle or lessen the choke it died. So I'd appreciate any advice about what's happening here since I'm not familiar with carbs.

I did the compression test and numbers are as follows:

Cylinder 1: 150 psi
Cylinder 2: 135 psi
Cylinder 3: 140 psi
Cylinder 4: 140 psi

Are these numbers in the ok range that the bike is worth continuing to restore?

Within a 60 seconds of it starting up I put my hand on the headers and the headers for Cylinders 2,3 & 4 were extremely hot while cylinder 1 was just warm. Is that normal? I thought maybe it wasn't firing on cylinder 1 but not sure how to check that. I pulled the plugs and can see spark on each one although it looks a little weak IMO but these are the original plugs not sure if that would be the plug or the coil?

Also immediately upon starting the bike I blew the 15A fuse for the blinkers and horn so I replaced it with the spare. Is this something I should dig into more could there be a reason for that which needs fixing?

As always this group is amazing and I would not be able to get the bike to this point without your help.

[Pat Conlon]

Congratulations on the start up!
Only being able to run on choke means that your idle circuit passages in the carbs are blocked. Suggest a complete disassembly and ultrasound bath with fresh o rings on reassembly as a starting point.
Compression numbers look ok. As you get more hours of running time on the engine, those numbers may change for the better.
Electrical gremlins are a bitch. They take time, logic and patience to track down. A common area for failure in the blinker/horn circuit are the 2 horn wires going down to the horn by the steering neck. The back and forth movement of the handlebars cause these wires to rub against the frame causing the wire insulation to break down. The wires going down to the horn are hot wires (w/ignition on)  meaning the horn works by grounding at the horn switch. These hot wires make it easy to blow a fuse on this circuit even if you don't activate the horn or blinkers. As soon as the ignition key is turned on...pop!

Again congratulations! Keep at it!    Let us know how you're doing...

Happy New Year!

Pat

[86FJNJ]

Thanks Pat, will be a while before I ride, no clutch or brakes and tires are 20 years old.

I should mention I had just the carb hooked up o an UV tube with no filter element, not sure if that matters.

Is there any reason to think it was not running on cylinder 1 with that head pipe being significantly less hot that the other 3?

Also for what it's worth I've been putting some assembly lube on the carbs and intake boots and that makes it a lot easier to install and remove.

[Pat Conlon]

I was modifying my message when you replied.

Get those carbs cleaned. Your cold cylinder can be the result of little or no fuel getting thru the carb.

[Millietant]

I'm with Pat, your symptoms read very much like carb issues, with blocked idle circuit passages at least.

Number 1 cylinder exhaust being a lot cooler than the rest is a tell tale sign of weaker combustion. From your description this really can only be from the carbs, or the ignition. There are plenty of potential causes on the inlet side (as you have a spark for the number 1 cylinder and decent compression), but as the bike has been laid up for a long time, blocked jets/circuit passageways are the ones I'd go for first, but while you're looking at the carbs, just make sure the inlet rubbers between the carbs and the head are not seriously cracked (especially no 1 cylinder) and also make sure the vacuum take-off plugs are in place properly, again, especially on number 1.

Good luck with the troubleshooting and keep us posted on your process/progress, with more questions as neccessary.

Cheers.

[86FJNJ]

Is this hole on the top of the carb for Cylinder 1 supposed to be capped or vented to something? I'm wondering if this (being open) might be why my Cylinder 1 header was not hot and likely not running properly during my initial test. The Carb for #2 has a long vent hose which I'm assuming goes to the gas tank and #3 and #4 have caps on them.

FYI I got the bike in already partially disassembled state so I wasn't the one to take it apart to know exactly how it should go back together.

[Pat Conlon]

Yes, that port should be capped off if you are using an auxiliary fuel tank.
 That is the vacuum port for the vacuum hose that goes to your vacuum operated fuel tank petcock. When the engine starts, the vacuum opens the petcock and when the engine stops the lack of vacuum closes the petcock.
 By not capping this port off you are letting unmetered air into your fuel/air mix on your #1 cylinder causing a lean misfire condition.

The vacuum hose you see on #2 goes down to the ignition box and is the vacuum advance.
https://fjowners.com/index.php?topic=11690.0