750 twin won't start

I have a 76 750 twin that was having trouble starting, so I did an oil change since it was that time anyway. Kicked it over after the oil change, started perfectly, ran like a champ.
Then I went to bed and forgot to turn off the fuel. Now it won't start no matter how much i kick it over.
For right now the starter is not working, so it is only kick start for now.
I am at my wits end with this problem. The plugs look normal, oil level is good, has fuel. The basics seem fine. The only other thing is the shifting seems a little "off" but it straightens out after I kick it over a couple times.
Please help me:S

Left the petcock in the "on" position eh

Don't do it again, more than likely the cylinders are flooded with fuel, and it's going to drain into the crankcase. Remove the oil cap and sniff the opening, bet you will smell gasoline, change the oil then take the plugs out and kick it over a few times to clear the cylinders.

A simple check is to kick it over a couple of times and see if the plugs are wet with fuel.

My Nephew rode my 750 twin to work and had to do an oil change before he could ride it home again because he forgot to turn off the petcock

Having the fuel valve open might have drained the gas tanks contents into the crankcase. If this did happen you could have what is known as "hydraulic lock" where a fluid is trying to be compressed and has been known to bend/break parts.

What I would do is remove the sparkplugs and kick the engine over several times and see if gas is comming out of the spark plug holes. You might have to change the oil as it may well be diluted from the gas.

genesis wrote:

I have a 76 750 twin that was having trouble starting, so I did an oil change since it was that time anyway. Kicked it over after the oil change, started perfectly, ran like a champ.
Then I went to bed and forgot to turn off the fuel. Now it won't start no matter how much i kick it over.
For right now the starter is not working, so it is only kick start for now.
I am at my wits end with this problem. The plugs look normal, oil level is good, has fuel. The basics seem fine. The only other thing is the shifting seems a little "off" but it straightens out after I kick it over a couple times.
Please help me:S

Might have "flooded" overnight if float needles weren't sealing completely, and overflows failing.

As a precaution, would sniff-test crankcase oil to assure absence of any fuel contamination into the crankcase oil.

Could remove sparkplugs and allow combustion chamber to dry out. Replace existing plugs with brand new plugs, and try to crank. Even new plugs can be fuel-fouled quickly and beyond recovery, no matter how "perfect" they may appear.

With cold engine and choke on, remember not to touch the throttle while attempting to crank. The throttle slides are supposed to remain in the fully down position in order for the choke (enrichener) circuit to properly function.

Once running, get off the choke asap, using the throttle to maintain sufficient rpm until warm enough to idle without choke. Leaving the choke on too long for warmup purposes or otherwise often fuel-fouls the sparkplugs.

Good Luck!

This was so much easier with my 650. Never had to worry about the petcock.:blush:
Oh well, off to the shop i go to get another gallon of oil and 2 more brand new plugs.:silly:
thanks guys

MFolks wrote:

Having the fuel valve open might have drained the gas tanks contents into the crankcase. If this did happen you could have what is known as "hydraulic lock" where a fluid is trying to be compressed and has been known to bend/break parts.

I've been thinking about this statement for awhile now, and even assuming you are talking about filling the cylinders and not the crankcase, I don't think it would happen.

My reasoning for no cylinder hydraulic lock:

(1) There are two float needle valves, and the leakage past them is a very small amount (volume).

(2) The 750 twin has a rather large cylinder diameter, and it would take a lot of fuel to fill it to the point of hydraulic lock.

(3) At the same time fuel is draining into the cylinders, it's also draining past the rings.

(4) so there can not be sufficient flow to fill the cylinders to the point where hydraulic lock can occur.

As to why crankcase hydraulic lock is extremely unlikely:

(1) The crankcase is vented to the air box, and the point where the vent hose attaches to the air box is really close to the level of the bottom of the cylinder block, so fuel will start filling the air box when it reaches that level.

(2) If the fuel were to fill the crankcase to the bottom of the cylinders, and at a rate that exceeded the amount going into the air box, it would trap the air between the bottom of the cylinder and the bottom of the piston, preventing it from rising above the bottom of the cylinders.



I just don't think hydraulic lock due to leaking float needle valves on a 750 twin is going to happen.

Been wrong before though.

The other thing is there is an overflow pipe in each bowl. Unless the gas is coming out like the great flood and those tubes are blocked, very little gas is going into the engine.

My original petcock used to leak like an SOB and put a 3 foot puddle on the floor overnight, but I never got gas in the oil.

I have gone totally through my carbs, cleaned everything in them, made sure all passages are clean, and it never spills a drop of gas on the floor. It sure does get it in the crankcase though.

And it just occurred to me that there is one thing I didn't think of checking, the hoses themselves. I'm going to take a look and see if by some coincidence both hoses are plugged.

Or maybe it's just one carb leaking and that hose is plugged. I'll find out.

Thanks for mentioning the hoses

If the overflow circuits are functioning properly, they are supposed to allow any excess fuel in the float bowls to drain out the overflow tubes before attaining a level high enough to reach the combustion chamber.

A common problem is where the float needle/seat interface is leaking and thereby allowing fuel to continue flowing into the floatbowl, which fuel level rises to the point of flowing out the overflow circuits onto the garage floor. At least that's what's supposed to happen.

If the overflow circuit is partially blocked by crud or what-have-you, it may be unable to handle the excess fuel being allowed into the floatbowl. Some of the tiny orifices in the overflow circiuit are difficult to access and clean (some being difficult to even see without lighted magnification). And, yes, a clogged drain hose from the floatbowl overflow drain nipple, would qualify as an overflow circuit "partially blocked by crud or what-have-you."

That said, perhaps even a perfect overflow circuit might be over-whelmed by a humongous failure at the float needle/seat interface. But in such event, some of the excess fuel should be escaping from the overflow nipples to the garage floor (provided the overflow circuit and overflow hoses are clear and unobstructed).

Good Luck!

Patton wrote:

If the overflow circuits are functioning properly, they are supposed to allow any excess fuel in the float bowls to drain out the overflow tubes before attaining a level high enough to reach the combustion chamber.

Correct, assuming the overflow tubes (or the hoses attached to them) are not blocked. The stock overflow hoses on my bike were low quality rubber like vacuum line and would "melt" inside when exposed to gas a lot. Then they would block up. I griped about this to the dealer and he said the solution was to use FUEL LINE on the overflow tubes down to the drain point. But, it's very thin tubing so I had to special order fuel line that small. But, he was right: about 25 years later, those tubes are still soft and pliable and never plug up anymore. And I have yet to get gas in my oil.

Patton wrote:

If the overflow circuit is partially blocked by crud or what-have-you, it may be unable to handle the excess fuel being allowed into the floatbowl. Some of the tiny orifices in the overflow circiuit are difficult to access and clean (some being difficult to even see without lighted magnification). And, yes, a clogged drain hose from the floatbowl overflow drain nipple, would qualify as an overflow circuit "partially blocked by crud or what-have-you."

Yep. take the hoses off and use a can of carb cleaner to hose the heck out of them (wear glasses). Take the bowl off and locate the overflow tube, it should have a tiny hole at the top. Take the drain screw out and use carb cleaner to hose that tube from the top down. You should see the spray coming out at the bottom.