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.