Loss of Compression

[Guest]

Backstory:

I originally tested the compression and received 148 on right side and 130 on left side during the test.

The bike was running well except that it hung around 35000 RPMS.

Dipped Carbs and replaced carb O-rings and replaced air intake O-Rings and reinstalled everything.

Bike was hard to start and had to use Starter Fluid to get running. Tried tunning and working with the air screws to no avail. Bike continued to run fairly crappy (Bogging out, low power, popping nosies and so forth) until it now will not start. You can hear it attempt to catch a bit now and then but basically just turns.. Decided to test compression again as a sanity check and I now get exactly 90 psi from both cylinders. I have heard you need above 100 at least.. When I tested the compression the first time there were no carbs or airbox installed. This time the airbox and carbs are installed. Also there is no difference in compression with throttle closed or open.. Let me know your thoughts..

[Souske]

have you adjusted the valves?

[mpencegs]

The valve timing was adjusted before the original compression test but has not been done since.. The bike was running well. I have not done a valve measurement or adjustment yet.. Also I noticed that the spark plugs appear fouled now as well.. Looks like Carbon fouling not oil but not certain.. I will post some pics later tonight. I have not ridden the bike at all so when I say it was running means running at idle or some throttle while testing and tunning..

[Nessism]

I'd double check the valve timing, adjust the valve clearances, and make sure the air box is fully sealed and the filter is clean.

[mpencegs]

Will do.. I am also gonna get some new plugs today and see whats up.. I bought a .04 MM feeler guage for testing the valve clearance but sounds like I may need some other sizes as it appears the range is .03-.08

[mpencegs]

anyone out there have a how to for valve adjustment for the gs 450.. I want to make sure I do it right. I check basscliff and I think his are for a four cylinder not the twin..

[mpencegs]

Also can someone with a running 450 let me know what compression they get.. I want to get an idea of what the compression needs to be..

[pete]

Valve adjustments are exactly the same, just less to do

Get the cam lobes parallel or perpendicular to the valve cover gasket surface and measure away, clearances are the same (0.03 - 0.08mm).

Here's my compression test which I expect to improve as this was taken before I got it running properly:





Ideally your compression should be better than this up towards 150.

[mpencegs]

Thanks for the info.. I am have talking with my buddy and he mentioned that I might be bleeding compression between the cylinders due to faulty head gasket.. He believes this would explain why the cylinders are now reading the exact same crappy compression when before I had variance between the cylinders.. Anyone have any thoughts on this.. How hard is it to replace the head gasket?

[mpencegs]

Holy crap.. I just noticed due to the pictures that I might not have hooked the compression gauge up correctly. I had screwed the conpression guage directly to the engine but had a feeling like something was missing. I notice in the pics that the guage is screwed into an attachement that then screws into the engine. Maybe this is all just operator error LOL.. I will check when I get home..

[pete]

Thanks for the info.. I am have talking with my buddy and he mentioned that I might be bleeding compression between the cylinders due to faulty head gasket.. He believes this would explain why the cylinders are now reading the exact same crappy compression when before I had variance between the cylinders.. Anyone have any thoughts on this.. How hard is it to replace the head gasket?

Check your valve clearances first, you gotta do that anyway right? You haven't had the head off for any reason?

Also, to take the head off you need to take the valve cover off, so once again you need to get it where you can check the clearances regardless

[mpencegs]

Makes sense. I will check the clearances and timing first.. And no I have not removed the head.. Ever since I broke the valve cover bolts I have become very paranoid about the bolts on this bike LOL

[pete]

Haha fair enough.

At least the bolts holding the head down should be a little more robust... but hopefully you'll just find you have no clearance on your valves which is holding them open and lowering your compression figures.