1982 Seca 750 No compression on all four pots

Ok, so heres the story. I just picked this bike up for $200. The guy had got it from a friend after it had stopped running. After buying it he did nothing with it and he left it sitting outside for the last few years. Which is where I came in last saturday, when I bought it.

So, let me preface the next part by saying that I'm quite familiar with working on bikes and have worked on many of them.

When I got the bike home I pulled the carbs off and cleaned them out. They were pretty varnished and nasty. After getting them all nicely cleaned and functioning again I put them on the bike and tried to start it. I got nothing. Checked spark, which it has, nice and strong. Next I noticed that it seemed to be cranking over awfully fast. SO I pulled out my compression tester (one of the older pressure fit ones, but good enough for trouble shooting) and low and behold, I have zero on all four cylinders.

So I pulled off the valve cover to see what was going on. Well both cams were turning, chain was intact, and I could see the pistons coming up and down through the spark plug holes. Next I checked the cam timing. This is where it gets interesting, both cams were off by a tooth or two. So I set the timing properly and....... still no compression.

I noticed at this point ( I had the carbs off again) that the compression seemed to be blowing out the carb boots.

My next thought was valve clearance (however unlikely it was that all 4 would be tight enough to cause a total loss of compression) I wasn't sure what the spec was, but I grabbed my last remaining feeler gauge which was a 0.004in. and it slipped under all the lobes and the shims. So I knew I at least had clearance, but just to be sure I pulled the shims out of two of the valves and cranked it to see if I had compression (this turned out to be a stupid idea and it scratched the cam up, live and learn) but still none.

My current theory is bent valves. Either because the cam chain skipped a tooth (The chain seemed nice and tight), or maybe the previous owner set the cam timing wrong, or...who knows.

So, before I tear the head off, I wanted to ask if there was anything else you guys think it could be?

Edit: forgot to mention. I also thought that it could be severly gummed up and stuck rings. So I soaked the cylinders with ATF for 24hrs. This also showed no imprevent.

 

I'm thinking timing still way out. Very odd to have bent valves on all 4 cylinders I would have thought.... No way of talking to original owner?

 

There's not much you can do to fix a "No Compression" issue without removing the head. And a good place to start for issues ... (like a blown gasket or cracked/warped head)

 

Yeah I'd concur with the above, if the timing's that far out it could quite easily have bent valves on all four once it was cranked over. Also possible the valves could have rusted/stuck over time or the rings gummed up.

Either way if its bad enough to have zero on all four "decapitation" is the way forward for diagnosis & repair.

 

To check that rings may be causing the no compression ... a spoonful of a heavy grade oil in each cylinder before testing each cylinder. An increase in compression would then suggest rings being at least part of the issue.

Still needs decapitation ... but I would say more than one issue going on.

 

Well its good to see that I'm on the right path. I still find this to be very odd. I still can't figure how all 4 cylinders could be affected in the exact same way. I too think timing is a possible issue, but I watched the piston moving through the spark plug hole and it seemed to be timed correctly with the valves. Just to double check (because I've made stupider mistakes) the number one cylinder is the one on the left (ignition pickup) side right? Either way I'll keep you all posted with what I find

 

If the motor was turned over with the cams out of time, you could have 4 or even 8 bent valves. These are "interference" motors, if the cams are out of time the wrong way the pistons WILL crash into the valves.

 

Yeah that's what I was figuring. I guess the previous owner must have timed it wrong and killed the valves. Oh well, I'll pull the head off and continue the exploratory surgery.

 

Well I pulled off the head and found the problem. As both you guys and I suspected the valves were bent. You can see the clean spot on each intake valve where they kissed the pistons.

Seeing as the intake valves for this bike are $50 each (so $200) I think I'll probably see if I can track down a used motor for it. Either way here are the pictures the cylinders and head.

 

GACK. I had that happen on a 12-valve Honda car engine that broke a timing belt, it's no fun.

I shouldn't tell you this, but I have a whole DRAWER full of NOS valves (except they're for my 550s) that I paid at the most, $2 or $3 apiece for.

Off eBay. But one at a time, and over the course of about 4 or 5 years. By being patient, and being the only bidder and getting it for the starting bid.

Troll eBay for a month or two. You might get lucky.

 

Or buy a cracked/otherwise damaged head with good valves.

I know it's "wrong" to profit from other peoples misery, but you at least allow them to rescue some cash from the ordeal. Maybe special edy would sell you his head (or just the valves from it) for a good price as he's just had the misfortune to crack his head?

 

A quick question, What would you say a used engine with 50,000lm on it would be worth? I sourced one from a fellow who is parting one out. He says it ran perfectly when he pulled the bike apart, but he wants $400 for it. Which seems a hair on the steep side to me. WHat do you all think?

 

Would only consider that sort of money if I'd heard it run & maybe even ridden to confirm good gearbox. With only his word that it's sound I'd say it's worth £50 ($100-120?) tops

 

There is a complete motor minis the carbs I have those at a local wrecking yard they want two hundred for it it's a 85 700 do know it ran as they fired it for me before I took carbs off

 

Are the 750 intake valves unique or are they shared with other models?

 

All 750s, '81 ~ '83, and all 650s, '80 ~ '83.

A good Yamaha fiche: http://www.yamahapartshouse.com/oemparts/c/yamaha/parts

 

Seeing as these posts are like three almost four years old, how did it come out in the end?
Currently my xj700 x gave me some oddities as you mentioned the engine was spinning faster then normal when I was trying to start it. Oddly 2-3 weeks prior the bike ran fine. Came home form work park it and three weeks later nothing. the same problems very low compression and when I took the head off I found two exhaust valves bent and the corresponding guides broken. Also the same with one intake and the cylinders where 2 and 4. I didn't notice any kissing of the valves upon the pistons or dents which is what I was expecting.
As you mentioned spark was good and everything else for the most part until I checked the valve clearance prior to taking the head off a few zero clearance or as the excel sheet showed me that I only had one valve that was within specs.
I also found that the valves and guides are 50+ apiece so I bought a used head, jugs for less then a 100.00 bucks so I got some spare parts.
So a piston up rebuild is were I am at more accurately I am waiting for new rings and wrist pins have everything else already to go. By that I mean that I have had the jugs checked by a machinist 20.00 and honed.
Removed cleaned and lapped all valves, new valve seals and such.
Anyway hope you got your problems solved.