Head Gasket Replacement

So I ended up checking the valve clearances this last weekend after ordering all of the tools I needed. They actually are all within range. There was one intake valve that might have been questionable, but not by much or enough to warrant a replacement. The timing chain felt pretty decent too.

I tried my compression tester on my friend's 800cc v-twin and his engine was reading correctly, it did, however use a different size adapter on the tester for his spark plugs. I'm thinking of renting a compression tester from somewhere to see if mine is still causing issues for some reason and to compare readings. I'm just not sure why all cylinders would be reading as low as they are especially since the last time I road it, it ran perfectly fine. It flipped like a switch the next day when it started not running well. Any thoughts on what a good next step might be?

 

Do you have access to a leak down tester? If valve clearances are with in spec, doing a cylinder leak down can identify why your cranking compression is low, be it rings or valve seats. Air escaping past rings will blow out thru the crank case, air blowing into the air box says bad intake seats/valves, air out the exhaust pipe says bad exhaust seats/valves.

 

I just looked over this thread and as your valves are now checked and in spec and assuming there is a problem with the adapter for your engine perhaps this problem is to do with ignition? I cannot be sure of course but if the bike was running great all summer and you had the problem after the headlight was off you should check what voltages you have at your coils etc. The fact that the compression readings don't have a big variance over the four cylinders suggests the head gasket is unlikley to be the culprit I think. Compression cannot drop suddenly unless the head gasket fails and it surely would not drop equally accross the four cylinders.

 

That's kind of my hypothesis too, I was wondering if it is electrical-related since it happened so sudden, but an inaccurate compression reading is throwing it off. I'll try to check the voltages. Would it also be worth it to try and turn the engine over with the air filter removed to see if that changes the reading?

 

I don't own one, but I can get one. The local auto parts store rents them out. I can try this too.

 

Well you could as the air filter will reduce the airflow a bit.

If your piston rings were not sealing right l don't know if there would be a way of checking the crankcase breather because there should be increased pressure in the crankcase. It is because the change in your machine running well to the present situation l would check the ignition system. Carburettors are the same as before, you valve clearances are fine. It is because piston rings, valve clearances and valve seats and carburettors don't change quickly l think the problem is elsewhere.

 

Go back to the headlight retracing your steps and look to see if there are any broken wires or harness plugs not connected properly. What is your battery voltage when cranking the engine? I think it needs to be a minimum of 10 volts to make the transistors work in the TCI. Is the bike on the centre stand with the sidestand up? Have you checked the ignition fuse? What about your sidestand switch? Is it working properly?

 

Makes total sense. I'll check the connections. I didn't check the battery voltage, but did try hooking it up to a booster the second time I took the test, unfortunately, it gave me the same results. I can definitely check the voltage though to see where that's at.

The bike has been on the center stand every time I did the test and the kickstand should have been up every time. I'll check the fuses!

 

Yes if the kickstand should be up as the engine wouldn't start with it down.

 

With the ignition on and the kill switch set to run check the supply voltage at the red wires with white stripes on your ignition coil connections.

 

Remember the compression figures specified by Yamaha will be with the air filter fitted.

 

Yes it will provided it is in neutral (and the neutral switch is working.) Bike will start just fine with sidestand down, then die when you drop it in gear if the stand is down.

 

Well, I think I may have figured out the solution. It turns out my compression tester was the culprit for the previous bad readings. I rented one from a local auto parts store for a piece of mind and it tested much better and within range across all cylinders. I was thrown through a loop because my tester was working just fine on my car and my friends motorcycle. It still, however, didn't solve the issue as to why the bike is running so poorly. I did a bunch of tests with electronics and found that one of my ignition coils is shot, so I was essentially running on two cylinders. I'm going to replace that along with the valve cover gasket and see what happens.

 

Good point. I installed the new filter and ran the compression test with it instead.

 

Hopefully the new coil will fix it and you know the piston rings, bores and valve seats are in good condition by your latest compression test. You can double check the coils by swapping the harness connectors going to the coils. The spark will move to the other pair of cylinders and vice versa.

On the subject of ignition coils here is and alternative setup for your Yamaha.

https://xjbikes.com/forums/threads/cb-750-coils.88468/

 

Glad to hear it, because I tried the exact same thing by swapping the connectors and still saw no results. I came across a replacement nearby so I'm going to try it out and see how it runs. Fingers crossed!