T500 Won't Start -- Bent Crank?

[Stan]

Evening, folks. I have a '72 T500 that I am rebuilding after my son took it apart a couple of years ago. I took the crank to a well-known codger who said he could replace the seals on the crank. Well, I get the crank back and rebuild the engine, but it won't start. Good spark, but it won't even cough, no matter what I try. A repair mechanic came over and tried all of the same tricks with no success. Then he noticed that when I kicked the bike over, the bolt at the left (timing) end of the crank wobbled (not the timing cam, the bolt). He said that my crank was bent, keeping my engine from sealing or starting.

Can this indeed be so? If so, what to do? Can I send the entire crank off to be redone by a reputable shop? If so, any suggestions? I am not inclined to take it back to the codger - once bitten, twice shy.

I appreciate any suggestions. Thanks.

STAN

[rngdng]

I guess it's possible that the crank was not pressed back together correctly, but I don't think it would keep it from cranking. Are you sure the spark plug wires aren't crossed? That happens a lot on T500s.


Lane

[Stan]

Excellent question but no, the plug wires are not crossed. I checked this early on. Even so, I appreciate your help thus far.

STAN

[jabcb]

On some of the twins (T250/T350/T500) the HT leads should be crossed & some shouldn't.
It depends on the coils installed with the HT leads facing forward or backwards.

In my little collection I have coils installed in both directions.
Was curious about this a while back & I found official period pics (brochures, ads, etc.) that are in both directions.

[Stan]

Thanks for the thought, but the plug wires are correctly attached.

Any advice or thoughts on the bent crank?

STAN

[titan performance]

So you have a healthy spark Stan, and fuel reaching the cylinders...ie wet plugs?

[rngdng]

Take out the plugs, squirt gas into the cylinders, put the plugs back in,,,,kick it over.....it should fire. If not, something else is wrong.


Lane

[Stan]

I agree - checked new plugs and get nice spark. Also checked timing and cylinder placement so no crossed wires. Squirted fluid into cylinders - nothing, not even a cough. Also tried spraying through the carbs and everything else I can think of. Nothing.

Again, the mechanic is convinced it's the bent crank. I am open to suggestions.

STAN

[ConnerVT]

I'll start by saying that there are people here much more experienced in the workings of these engines, but I have completely torn down and rebuilt the T500 motor myself (crank sent off and rebuilt by Bill Bune). So I know a bit.

The T500 motor, when assembled correctly, is usually one of the easiest motors to start. Mine starts on the second kick when cold, and first when warm. I have even been able to "kick" it started using my hand on the kick starter when warm.

I'm not even sure how one can bend this crank. It is a pressed together design. I could see it being assembled not being true, but I would think that if it was severely out of true, you would have had issues reassembling the the motor - the bearings would not sit flush/square in the crankcase halves, and it would of been impossible to reassemble the top/bottom crankcase halves.

Even if the crank was just a little out of true, I would suspect that the motor would at least try to fire. Might run like hell, as it would be all out of balance, but would at least spit and fire.

You have checked that the right spark plug wires are going to the correct cylinders (done that wrong before). Be sure battery is good, with a good voltage. Correct point gap/timing. Condensors not shot. Poor wiring connections. Bad spark plug wires/caps. Weak coils.

Fresh gas, in a rust free tank. Gas getting to the carbs. Carbs cleaned (not just sprayed with Gumout, but disassembled and cleaned thoroughly). Carbs reassembled correctly (correct float height, slides reinstalled with cutout facing the air box).

No air leaks (Intake boots in good condition, vacuum line attached to petcock with no air leaks). If the crank seals were not installed correctly, you could have a major lean condition. Another foolish question - Pistons installed correctly? There is an arrow on the crown, which points toward the front (exhaust ports).

So, a laundry list to get you started. Good luck with it.

[akendall1966]

How much compression, do you have a gauge?

[Stan]

I'll answer the easier question first: solid compression with new rings and freshly honed cylinder walls.

Now for the laundry list:
I too have worked on this engine a couple of times. Not my first rebuild on this engine, and did a couple of others. I agree it should be relatively easy to start, which is why this is so puzzling and difficult.

I too wonder how one bends the crank. Like you, I suspect it was assembled out of true, causing the wobble and lack of seal. Would this keep the engine from starting, or at least coughing? If so, I am prepared to tear the engine down, send it to Bill Bune to be redone, and reassemble.

As noted above, I've checked the plug/cylinder firing combination a number of ways including by gauge.

Got fresh gas, fresh battery, good blue spark. I may find I need the carbs rebuilt once I getting it to fire, but the thing should still at least cough with fluid. This one does not.

Pistons installed correctly, but cannot speak for the crank seals. If they are not installed in the crank correctly (a good possibility with the wobble?), will that keep the engine from firing?

Again, if the solution is to tear the engine down, get the crank redone by a highly reputable shop (like Bill Bune), I am open to this. I simply want to have some level of likelihood I am addressing the correct issue. Any help is welcome.

Thanks again for your thoughts and questions. I hope my answers are clear.

STAN

[ConnerVT]

Glad to hear that you've at least thought of the things in my laundry list. Easier (for me) to just shotgun out the ideas of what would be keeping you from running, and let you sort through what you have, or haven't, thought of.

Pistons installed correctly, but cannot speak for the crank seals. If they are not installed in the crank correctly (a good possibility with the wobble?), will that keep the engine from firing?

Bikes with poor oil seals still typically run. They may suck up transmission oil (inner) or be have a dangerously lean running condition (either inner or outer). I would imagine if they were *really* bad (as in non-existent), that the cycles to transfer a fuel charge from carb to crankcase to cylinder could be compromised, and prevent the engine from running.

But you are getting nothing, even when introducing fuel at the top of the cylinder. It gets compressed, then the spark should ignite it. But you have nothing. I don't feel that even a crank less than true (or with shot oil seals) would do that.

While writing this, had a really stupid thought. Since the crank rebuild is suspect, are you sure that the the pistons are at TDC when the spark fires (not just that the left spark plug fires when the left points open)? I would check that the left piston is actually near the top of its stroke when the "L" indicator shows in the timing window. I don't know if it is even possible, but perhaps that old codger you hired got things swapped, and the pistons are actually closer to BDC than TDC when it fires?

[Vintageman]

I agree with VT make sure piston really near TDC when it fires ... both side. If crank assembled wrong things could be misaligned

Even though you have done a couple engines you relied on a mechanic for an opinion. Then you are even more desperate and come here

Are you sure you put pistons in correctly? There is a left and right of course and sometimes I almost mess up ...easy to do , been done many times I am sure

Put you hand over the carb mouth. You should feel it suck distinctively when you kick it. check both sides.

If you take plugs out and kick it over by hand do you feel like the crank is binding?

I once had a bent rod (bad) and it ran but made horrible noise. But, if the crank is so bad you see it wobbles that isn’t good.
I would think the whole rotor would wobble not just the bolt?

Post a video

[titan performance]

I have experienced bikes that had fuel at the plugs, and a healthy spark, that would not fire on 2 occasions. Firstly, the pistons were in backwards, and secondly, the carb slides were in backwards.

[Vintageman]

to titans point I'll add one or two more more.

If there is something mechanically wrong with engine or even too much fuel or not enough oil you can wear rings out very quickly (rings installed upside down?) ... in less than a hundred miles.


What happens is you loose compression (not obvious). If you had a reference point like OK is 140 PSI and now 110 PSI that may "feel" OK but, that is a large drop. To much blow by bike may not start with kicker... it may if you could bump start it and get it spinning fast enough.