Do I need to replace my base gasket?

[Guest]

Hi there, my problems are shifting, so I am starting a new thread, sorry for the overlap. I have a 1983 850g that I picked up over the winter. It was idling poorly, with a climbing/hanging idle, so with lots of support and instruction from the GSR, I have done the following:

Shimmed the valves
New valve cover gasket from Real Gaskets
Replaced the spark plugs
New air intake boots and o-rings
Full carb rebuild as per bikecliff tutorial, with o-rings from cycleorings.com. (I have 3rd party needles and seats, waiting for OEM to show up)
Full airbox rebuild also as per tutorial on bikecliff
Synced the carbs with Morgan Carbtune

The idle seems to be ok, except for a constant popping in the exhaust. After about 7-10 minutes (when the engine heats up) the idle starts to go up into the 3k-4k range, although it sometimes comes back down. Since this sounds like a vacuum leak, I tried the starter fluid trick around the boots, but no change. When I shot the starter fluid lower, at the base gasket, the revs climbed to 2500 and then back down. Repeatedly.

I ran a compression test and got only 60psi in each cylinder. Throttle open or closed had no impact on the reading.

Do I need to tear down the engine and replace the base gasket? I am willing and able (I think) to do so, my only hesitation is that everything else I have done has had no impact and I am becoming a little frustrated. Also, how does starter fluid at the base gasket affect rpm? Isn't this below the piston? Or does the piston go all the way down into the crankcase? Could this be a tuning issue? Electrical? Any help?

[JTGS850GL]

A leaking base gasket will not cause low compression. If you have to pull the head to resolve the low compression problem then you WILL need to replace the base gasket.

[Ian Donahue]

A leaking base gasket will not cause low compression. If you have to pull the head to resolve the low compression problem then you WILL need to replace the base gasket.

Thanks JT. What do you think I should do next? If it was you, what would you look for to solve the compression issue?

[chuck hahn]

Starter fluid at the intake and head joints will indicate a air leak..has nothing to do at the base gasket. As for the low compression..are you 100% certain the valves all have proper clearance when you adjusted them?? I end to get as close to the .08MM gap that i can so as to ensure the valves have ample clearance to close fully. A leak down tester will quickly tell you whats the problem. One quick way to see if the valves are tight and holding open a valve is to turn the lobe of the cam up 180 from the shim bucket and see if the bucket can be turnedwith your finger or a small screw driver. Iif it does spin at least this will verify there is "some" clearance and that valve shouldnt be hung up...in theory of course.

Take off the cam cover and the points cover. Roll the engine over and watch the number 1 intake cam. When the intake valve gets compressed and the valve begins to come back up youll want the intake cam facing the back of the bike and the exhaust facing the front..both lobes pointing AWAY from each other. Put a screw driver in the plug hole and wiggle the crank just a little at this point and feel the piston come to full top dead center.

Install the leak tester hose. If the rings are leaking past youll hear the air down the cam chain tunnel.. If the air is leaking past the intake valves youll hear it in the airbox..put your ear up there and listen with the airbox cover off.. If its not leaking past the rings or you cant hear it in the airbox then the exhaust valve is the leaker...follow me?? Repeat bring the rest of the pistons to TDC on the compression stroke and leak test them. I got my tester from Harbor Freight and it works vry nicely.

[chuck hahn]

If the air is heard down the can chain tunnel its time to figure out why the rings arent seating. has the engine been recently rebuilt or is this the original factory rings in there???

[JTGS850GL]

Compression is going to be either piston rings or valves. Drop a teaspoon of oil in each cylinder as you do a compression test and see if the number go up. If they do then it's rings. If not, then it's valves. If it's a valve problem them I would tear down the top end and do a valve job along with a re-ring. New OEM head and base gaskets would be in order. If it's rings, and the bike hasn't been run for a while, I'd first try and see if I can get the rings unstuck first. I've used Seafoam with good results in the past. I'll pour some into each cylinder and let them sit overnight (24 hours is even better). Before you put the plugs in, crank it over with a towel over the cylinders to evacuate the fluid. I then run some in the oil as well and do what's commonly called as an "Italian tune-up". Basically, run the PI$$ out of it with lots of load changes over the next few days. Check compression after about 100 miles and see what the results are. If things don't improve then go to plan A and do a complete top end rebuild.

[Ian Donahue]

Thanks JT and Chuck. I'm going to pull the airbox and carbs and recheck compression. I read a post about the CV slides not lifting without combustion, so I want to be sure. After that I will do the leak down test.

[Ian Donahue]

Starter fluid at the intake and head joints will indicate a air leak..has nothing to do at the base gasket. As for the low compression..are you 100% certain the valves all have proper clearance when you adjusted them?? I end to get as close to the .08MM gap that i can so as to ensure the valves have ample clearance to close fully.

It was when I moved the starter fluid down to the base gasket that I got the RPM increase. For the valve shims, I double checked the clearances after shimming. Some were close - .04, but that's within spec so I didn't adjust, since .09 would put it out of spec. In spec is better than out of spec, right? None were tight.

I'll head out to my local HF and pick up a leak down tester.

[Ian Donahue]

If the air is heard down the can chain tunnel its time to figure out why the rings arent seating. has the engine been recently rebuilt or is this the original factory rings in there???

Not sure what the maintenance history is. PO is deceased.

[Ian Donahue]

OK, so I took off the airbox and carbs and got 60 PSI on the compression test in #4. I put 10 ml of Marvel's Mystery Oil in the cylinder, and got 90 psi, so if I'm understanding this, it's the rings. I will try the seafoam trick, and I'm also going to pick up the leak down tester to try to isolate the trouble.

[JTGS850GL]

Are #1-3 all the same 60psi? Might want to pickup up a second compression test as well. I've seen more than one compression tester give bad readings over the years. Make absolutely sure that the O-ring seal is good at the spark plug end, assuming that it has one.

[Ian Donahue]

Are #1-3 all the same 60psi? Might want to pickup up a second compression test as well. I've seen more than one compression tester give bad readings over the years. Make absolutely sure that the O-ring seal is good at the spark plug end, assuming that it has one.

#1 - 62psi
#2 - 52psi
#3 - 60psi
#4 - 60psi

The tester is new, but it is from Harbor freight, so... Yes, it has o-rings, and I feel good about the seal. Good battery power to the starter. Will try leak down test next.

[chuck hahn]

Yeah its gotta have something to do with rings if all shims were at least gapped so a feeler guage would go in at all. Follow the directions on the leak tester as far as turning the regulator knob all the way counter clockwise before adding your line pressure to get it into the SET area of the dial. I rest the regulator at each cylinder myself just to be sure I do all 4 exactly the same way.

[Ian Donahue]

OK now I'm having trouble finding TDC on my bike ('83 gs850gd). Here's a pic:
ignition.jpg
It doesn't look like the pics on Mr.Wringer's tutorial. Does the line go even with the thing on the left for 1 + 4, and to the right for 2+3?

[chuck hahn]

TDC is the T mark.