[1907]

I am in the process of rebuilding my 686. I don't have a spark in, and have the clutch and generator apart. When I try to spin the crank by hand, there is a lot of resistance. When its at ttc, there isn't any resisitance. Is this normal or is there an issue that I am not away of?

 

[onebuilder]

Did you upgrade your valve springs? I noticed the same when I built my 686.

 

[1907]

Yes, all 5 are new springs and valves. But if the spark plug is out, there shouldn't be any cpmpression.

 

[canucklehead]

There is no compression, but you are still compressing the valve springs (upgraded, and stiffer than stock) via the timing chain, which is connected to the crank! Once it's past TDC, it will almost fly around another 1/2 cycle.

What you have going on is normal.

 

[1907]

Thanks for the help, but now have the motor shooting fire out of the carb when trying to turn over. I have the timing right, checked to see if the valve seal 100%, checked the woodruff key, and still don't even have it try to turn over. I have checked everything I can think of. Please help. I have spoken to the guys at Weller and Pro-tec, and they both are puzzled.

 

[WV Hot Rod Rhino]

i would check timing again,,thats what it sounds like

 

[1907]

It matches the manual perfectly. I also had two auto mechanics look at the timing and they also said that it looks right according to the manual. It won't even try to idle. So if I was a little off, I agree. But nothing, and I mean not even one revolution. It pops out of the exhaust randomly, but shoots a flame out of the carb just as often.

 

[WV Hot Rod Rhino]

180 out??????

 

[onebuilder]

How about the timing on the decomp inrelation to the cam gear?

 

[1907]

The timing can't be out 180 deg., due to the ratio is 2 to 1. 180 on the cam is still tdc on the crank. I am not sure I understand the question about the decompression timing. The weights open freely and the pin retracts easily.

 

[canucklehead]

The timing can't be out 180 deg., due to the ratio is 2 to 1. 180 on the cam is still tdc on the crank. I am not sure I understand the question about the decompression timing. The weights open freely and the pin retracts easily.

Yeah, but if you're at TDC on what is supposed to be the compression stroke, but your exhaust valves are wide open, it will be tough to ignite the fuel.

To be honest, I'm not sure how the CDI knows when to fire... how does it know when it's on compression, and when its on exhaust? I'm assuming there is a crank position sensor... but does the spark fire every stroke, or just every OTHER stroke?

Your problem has to be something with your timing and valves, in my opinion... just not exactly sure what it is!

 

[1907]

The plug will fire on every stroke. And the when the motor finishes its compression stroke, both lobs on the cam are down (no valves move and tdc). Then spin the crank 1 time to tdc, and the lobs are straight up (between exhast and intake). I have aligned the cam sprocket per the manual, with the two little dots sitting flush with the head. How am I screwing this up? I have tons of spark. It won't even try to idle.

 

[onebuilder]

The timing can't be out 180 deg., due to the ratio is 2 to 1. 180 on the cam is still tdc on the crank. I am not sure I understand the question about the decompression timing. The weights open freely and the pin retracts easily.

Their is a timing mark on the end of the decomp pin. It must be timed with the cam and gear. Allowing 1 valve to open releasing compression when starting. Check the manual for the correct alignment positions.

 

[Duramax]

There should be 3 marks on the sprocket. One on each side lines up with the edge of the case (3 and 9 o'clock) and the 3rd one should be at 12 o'clock. Take another look at the decompression pin and see if it is holding one of the exhaust valves open. Also, check to see if one of the ears on the decompression weights are not bent. Easy to do when installing the two bolts. I am also assuming that you lined up the timing marks on the crank before lining up the 3 marks on the cam sprocket. It must be lined up and the piston top dead center before you align the 3 marks on the sprocket. Easy to get it off one tooth.

 

[1907]

I do have the 3 marks positioned were they should be. And if thats correct then the decompression pin would be sitting up when assembled. I have opened the weights and they more very easy, and the pin drops in like it is supposed to... that still doesn't explain why it shoots fire out of my carb!

 

[onebuilder]

Ok, what mark are you using on the flywheel? Their are 3

 

[1907]

The mark that is by itself. Not the mark that looks like an "H" for the timing light.

 

[Biggjim]

try a different cdi box. I had one go bad for no reason and had similar symptoms.

 

[RuttingRhino]

There has to be an intake valve at least partially open/not fully seated when it fires...

If everything else is correct with cam timing, ignition timing, and compression release then maybe for some reason a valve is not seating properly...

 

[1907]

I have checked the clearances on my valve and they are good. I thought the same thing. I also took my head off and flipped it over and filled the center with gas to see if there is a leak. Not one drip went through. I just had the valves done and thought of the same thing. How else is the air coming out of the carb instead of in? Is there to much or not enough gas going into the cylinder and it is firing were the air to gas ratio is better?