valve seat?

[Anonymous]

I am getting blowback out the carburator on cylinder #1 and #2 on the compression stroke. This is after rebuild of a bike that wasnt mine when it was running. So it may have done this before. The valve clearance is fine. I pulled the head again, compressed the valves so they were open, I used a toothbrush and carb cleaner and got them sparkling. I put everything back together, timed the cams and cranked the motor...same thing. The only thing I can think of is the valve seats are bad. Anything else I can try before I pull the head again?
thanks

[Anonymous]

Are you sure you installed the cams correctly?

[Anonymous]

yes 100%

[redliner1973]

Other than the seat being mis-cut when rebuilt ( would have noticed during lapping) all I can think of is the cams are not timed right :twisted:

[Anonymous]

If the cam timing is correct and you are absolutely sure of that, then the valves are not seating properly.

You can always run a compression test before you pull the head again to verify this.

When the head is off, place it combustion chamber up on the work bench. Fill each combustion chamber with a light oil like WD40 and see if any leaks past the valves. None should.

[Anonymous]

thanks for the suggestion swanny...I'll try that

[Anonymous]

I know you said the cams are installed correctly. This also includes the valve lash.

[Anonymous]

I tested the compression. Bad compression....actually practically no compression on # 2. What are the reasons besides valve lash would the valve not seat properly?

[redliner1973]

I tested the compression. Bad compression....actually practically no compression on # 2. What are the reasons besides valve lash would the valve not seat properly?

Other than the cam timing out, which you said you looked at, a burned valve can cause it, usually from a bad valve-job or inferior valves used during the rebuild. check the lash ( just to make sure) and also see if valve is not 'stuck' open by mau=nually pushing valve open to see if it will spring back closed and gain compression again. post back and let us know further. :twisted:

[Anonymous]

checked the lash again...still good clearance. I re-used the origional valves in the head...I mainly just cleaned the valve heads and seats with carb cleaner and a toothbrush. Did not do any machining. They are as clean as they will come...no visual obstructions. How do I check for a "burned" valve?

[Anonymous]

Time for a valve job. Have the machinist check the guides and replace the seals also.

I noticed in another post the springs were weak. Is that the same head? The springs could also be broken. Weak or broken, replace then, all of them. If you had bad springs they could have contributed to bent valves. Have everything checked before you put it back together. It wil be pricey, but worth it.

[redliner1973]

checked the lash again...still good clearance. I re-used the origional valves in the head...I mainly just cleaned the valve heads and seats with carb cleaner and a toothbrush. Did not do any machining. They are as clean as they will come...no visual obstructions. How do I check for a "burned" valve?

First, if you just cleaned the valves and seats, but did not do any valve lapping or seat cutting, remove the head again and do yourself the favour of replacing at least the exhaust valves, having the valve guides re-done and cutting/lapping the seats. A 'burned' valve results when the valve does not seat fully into the head, but remains (even slightly) open. Since the valve needs to be on the seat to cool, it soon overheats and literally starts to burn away. by the time compression is noticably lower, a good chuck of the valve is gone. when you remove it for inspection, a burned valve will look sort of like a pie with a slice removed from it. failure to at minimum lap the valve to the seat can cause the valve no to seat right and result in the 'burning'. weak springs can also contribute to this problem. :twisted:

[Anonymous]

Thanks for everyone's help. I will remove the head again, take it to the shop and see what they think needs to be done. I dont think any valves are burnt. I saw no indication of what you described. Hopefully, the seats and springs are the only issues!!! Anyone know about what they would charge for refinishing the seats and changing out springs?

[Anonymous]

It is on a per valve basis. I have no idea what bike you have. My 16 valve motor was right around $250 for a valve job.

One point on lapping. I have never done it and probably never will. I know I will get some arguments on this, but to me, it has always been a poor substitute for a valve job. I also find virtually no benefit to lapping valves that have just had a valve job performed.

A proper valve job cuts the seat width to the correct angle and width - something that lapping does neither of.

The only time I would use it is if it was a last resort. Imagine a disaster where a machine shop was not available and that would qualify.