Rebuild 4.0 twice- 60 pounds compression all cylinders

[markaboo929]

A nother way is to line up the crank with the "0" mark on the cover and balncer and look at the intake and exhaust valves as you turn the engine over.When the ehasut valve closes and the intake valve starts to open #1 cylinder is at exhaust.Watch when you come up to "0" mark again both valves will be closed in either direction...
http://www.youtube.com/watch?v=IYWUWMmimjQ&feature=related
Thats how I check to be sure.

 

[Breezio69]

I noticed that the stock piston dish and pin height are different from the Sealed Power 677cp.

Stock 4.0 cast aluminium '87-'93 #83500251, '94-'95 #4773157, '96-'06 #4798329----1.601" 13.1cc

Sealed Power (Sterling) cast aluminium 677P/677CP----1.585" 17.5cc

Could this be why the compression is low?

OK I have a little bit of knowledge from small engine builts, nothing over 2 cylinders and maybe im way off going from small engines to a straight 6...

Wouldnt the dimensions of the pistons he has in there lower the compression significantly over the stock ones? Your running a higher pin (comes up lower from the top of the cylinder), plus almost 5cc's of extra air per cylinder in a block/ head combo that probably wasnt decked for these pistons....

Am I over thinking this? Last quad I built I had a problem similar to this, I had a piston that was lower compression than I liked and noticed a huge lack of power, so I had the cylinder milled .5 mm and run a single layer gasket instead of 3 layer, jumped from 11.3:1 to just under 14:1 with the same piston in it

I realize this is apples and oranges with a single cylinder 4 stroke and 6 cylinder but its still the same principles, is it not that significant to check into on the 4.0?

 

[markaboo929]

I also see from looking at the other replies...are you using the same pushrods? Perhaps they are wrong,it sounds like valve timing,if the head was bad--compression would be different in each cylider.The piston hieght and cc dish changes the compression Ratio-allthough it changes the pressure,not that much however.You may want to use a degree wheel to check it as well...just a thought.

 

[waynecomanche40ho]

Alright. I found two problems. Whoever put the timing chain on had the wrong marks pointing at each other, and my fuel pressure regulator was bad; 60lbs at the fuel rail. So timing is good now and new pressure regulator, now it has 120lbs of compression and 40lbs in fuel rail. Runs alot better now and it seems to be getting better gas mileage!!!

So I'm guessing with the combo of timing off and too much gas equals bad compression, loss of power and fuel mileage?


Also with the excessive fuel pressure would that wash out my cylinder walls, if so after getting fuel pressure under control will the rings seat better after driven for a while.

 

[kastein]

Sounds logical to me, glad you got it fixed! 120 is a little low still in my experience but quite passable.