64x different cylinder compressions

[jhall752]

A ski i bought the guy said it was a 760 and that all the info he gave me. I honed the cylinders to 84.5 no porting (stupid me) and a new ADA Head with 38cc domes. I found the motor was a 64x and I still have different compression in each cylinder. The front seems to be at the correct compression, but the back is pretty low. what is the best way to bring the compression up in the back? I am also assuming the electronics are all 64X however it has an aftermarket flywheel cover so not 100% sure.

 

[fastgtfairlane]

Unless you have a leaking o ring on the lower compression cylinder, the only way to really fix it is a fresh bore with new pistons and rings. Maybe fresh rings might do it but need to measure clearance on the rings. Did you do a leakdown test as well?

 

[jhall752]

Unless you have a leaking o ring on the lower compression cylinder, the only way to really fix it is a fresh bore with new pistons and rings. Maybe fresh rings might do it but need to measure clearance on the rings. Did you do a leakdown test as well?

it was bored and honed, and I have new pistons and rings I even swapped domes on the cylinders thinking they were miss labeled and same outcome.

 

[fastgtfairlane]

I'd say do a leak test and see if you have a an air leak somewhere

 

[bkspeedwerks]

crank out of phase possibly?

 

[jhall752]

how do I do a leak test?

I also read somewhere that the 64x motors have a weird timing or something is there any truth behind that?

 

[WFO Speedracer]

Timing is staggered ignition timing, not crankshaft phasing

 

[Quinc]

Ring might have come out of the groove during install. Or maybe the rings are upside down? Check your ring gap and with the ada head you want to torque it a little more then stock specs.

 

[fastgtfairlane]

how do I do a leak test?

I also read somewhere that the 64x motors have a weird timing or something is there any truth behind that?

You'll need to block off the carb holes and exhaust manifold exit. I typically use expandable rubber freeze plugs then use a popoff gauge connected to the pulse fitting and pump the engine up to 8-10psi for 20-30 minutes. Also, i unbolt the carbs from the speed plate of you have one so you can check the base gasket. You can also fab up some plates to block the intake and exhaust holes as well. It should hold and not loose any pressure. Then you can use soap bubbles and spray down the engine to look for leaks. Also best performed with the front cover and flywheel/stator off so you can check the front crank seal. Also verify a fully charged battery.

 

[kevbo]

I have found that often the deck needs to be lapped before an ADA head will seal. BTW, the O-rings do little for sealing combustion. The gas will go right through them if theres any space between the cylinder and dome. I'm sure Ive said it before, Yamaha cylinders need crushable gaskets. Thats why the factory uses them.

 

[fastgtfairlane]

I use some assembly goo on the o rings on the Ada heads to help them seal up properly.

 

[bored&stroked]

With the same size domes and fresh bore/pistons, there should be no mismatch in compression. This is a engine problem, not a 760 problem.

 

[WFO Speedracer]

Just to be clear on a bone stock 760 there is a slight difference in the compression on the rear cylinde versus the front cylinder , in stock configuration the rear cylinder head dpoome is larger, you have an ADA head, that negates there being any difference front to rear .

 

[jhall752]

when doing a leak test do i seal off all the water line ports as well or just the exhaust and intake ports?

 

[DylanS]

don’t block off any cooling passages, this will allow any leaks that lead into the water jacket to show up as a loss of pressure.
You can also listen for obvious leaks at your fittings on the head when you’re doing this.
How low is the rear cyl?
Just had a new oring read 20psi low after the first ride.

 

[Quinc]

 

[Big Kahuna]

Measure from the top of the cylinder to the top of the exhaust port. See if they are equal.

 

[jhall752]

Measure from the top of the cylinder to the top of the exhaust port. See if they are equal.

this seems like the easiest option first

 

[jeepcreepcj]

Another possibility, an overlooked bent rod could cause uneven compression. If you didn’t, check squish clearance w lead before pulling the head, would be an easy check.

 

[TheBuzzard]

Troubleshooting 101 in the real world means squirting some oil in the spark plug hole and checking it again. If compression goes up, it's the rings. If not, it's the valves. Oil in there will tell you if comp is going by the rings