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Here is the story of how I was gifted a motorcycle but had to put the top end back together:

viewtopic.php?t=31441

Well the top end is back together and I'm trying to get it started. I have fuel in the bowls and spark on all four. The starter turns strong and it will cough, as in burn the fuel and make a cough or a backfire, but it will not start, will not even stumble. Just pop now and then.

The carbs were drained when I got the bike, and remarkably clean. I removed the jets and made sure each of them was clear. Did I miss something? I've rebuilt the identical carb set from my running CB750 with no problems, that identical bike runs great.

I was extremely careful to understand how to put on the cam sprockets coffectly to make sure the timing was right. I've seen what damage can happen if the timing is off. If I did it wrong, however, what would be the symptoms?

The bike is in neutral, and it *is* firing, so I don't think it's an electrical disconnect. Again, each carb bowl is full of fuel, it's getting fuel. The PO did't mess with the carbs, the bike was running when he started taking it apart, and I haven't done anything but peer through the jets to see daylight, hit each air passage with carb cleaner and blow them out, just in case.

There is only one more thing that seems off to me. When I hit the start button it doesn't sound like when my running 750 is turning over and not starting. It sounds different. It sounds like the starter motor is turning over much more quickly, almost like there isn't anything there. But the PO did not remove the starter chain, and I didn't remove anything from the starter motor or starter chain either.

Many thanks for any ideas, what else could it be?

The starter sounding like it is spinning freely could be a sign of low compression. I would test each hole with a compression tester. And if it is low, I would be looking at the head before the rings.

A mis-timed cam can either cause loud, metallic banging sounds or the symptoms you have now, depending what direction it is mis-timed in, and by how much.
It is worth re-checking if all the holes seal well.

Hope you get it figured out.

Thanks Buzz....

I did re-use the old rings (except for one set that I broke and had to put on new), but they were pronounced good to run by my mechanic buddy. Still, I can hopefully borrow the compression tester tomorrow.

It occurs to me that I have never dealt with hydraulic valves at all. You say you'd blame the head for low compression; why? I did the test where you pour kerosene into the chamber and see if it leaks through; it didn't. Is there anything else that could be wrong in the head?

I just hope I got the timing right. The Clymer's was clear, I was sure I had the right side of the right sprocket on the right camshaft at the right place in the crank rotation....

Right now the silicone sealant is drying on the cracked boot seams of the air box, in case maybe it could have been an intake leak. That's all I could find still wrong.

I agree with Buzzz on the timing. Even a tooth off could cause those symptoms.

Didn't you say you honed the cylinders? Did you bore them too? That'd definitely cause low compression.

Also did you line the rings up like shown in the manual? Usually they have to be about 110 degrees from each others' openings to not let stuff leak through.

Valve guide seals good?

Gaskets new and good?

Probably stuff you've already made sure of, just going through and making sure of everything.

The reason I would look at the head first is because leaky rings still give pretty good compression. For rings to leak as bad as a cracked head/cracked cylinder/buggered valve, they would almost have to not be installed.

If (and all this is still very much in the realm of 'if') you get readings of less than about 50 psi (way too low) the cause is probably a leak. That leak could be a mis-aligned head gasket, or a crack in the head or cylinder walls (unlikely if all 4 read bad, or you have 3-4 cracked jugs - rare) non-sealing valves (for whatever reason, cam timed wrong maybe), warped head, warped block...

Important to note that all 4 jugs reading low is a sure sign, something is amuck. 1 or 2 holes reading fine points to something more specific in those cylinders.

Maybe you just have a really good starter on that bike and it's not a compression issue. Have you primed the cylinders with raw fuel?

OK you folks put out good information, helps me think around what I've done.
No boring, just touched up with the cylinder hone. Lined up the ring gaps like they are supposed to go. Didn't put on new valve seals (they were good by all appearances) but I put on new gaskets and the gasketing surfaces were cleaned right.

Did not prime the cylinders with fuel (how?) but sprayed some carb cleaner in the intake and got some backfires loud enough that the neighbors hit the deck. Oops. I hate their stupid dog anyway.

Mechanic buddy has one more suggestion: he says swap the primary leads on the coils. But I matched them up to my running bike. I'll switch 'em over before I do the compression check, anyway.

Thanks again for the information, this is what I need so I can think through what's going on....

I doubt this will help at all but...I put the cams on at exactly 180 degrees from where they should be on a Polaris once. I couldn't figure out for the life of me what was going wrong. I realized that the carb was puffing out air when I tried to start it. I knew what I had done then. Sure enough the exhaust was sucking in air.

Just a long shot, but check to make sure its sucking and blowing from the correct orifices. At least you will know if the cam is close to being right.

Thanks everyone, I appreciate every idea....


SUCCESS! Switched the leads to the coils and it fired on up!

It's tapping a lot, I think it's valves. Doesn't sound like cam chain or anything, just sounds like super-tappy valves in #1. However, when I tigtened up the exhaust stud nuts the noise got quieter, so once I run it some more maybe new exhaust gaskets will be in order.

It's also smoking a lot, I'm hoping that's the assembly lube, it was a black moly paste.

Otherwise it went on a shake-down run, shifted fine, got the idle where it belongs, no problem. A little hard to start warm, though.

I welcome more ideas on the bugs. I can't believe it didn't blow up yet.

Yay! Congrats on getting her running Coffee-Brake!

The old coils wired backwards trick. +1 for your buddy. Did he use to work on Triumphs?