Topic: Rebuild or keep troubleshooting?

[Lumbee]

I have a 77 750F I recently rose from the dead.  The symptoms are these.  Seems to run OK when first started.  Runs with good power and good crusing speed when its cold.   I ride for about 5 miles (about 10min on the interstate) and I start losing power, feels like I'm dropping a cylinder.  When I get on city streets and keep it under 30mph or so it runs like a champ.  Get back up to 55 and it runs ragged again.  I've adjusted points, valves, and did a compression check, and have a solid 120psi across all cylinders.

The question for the house is should I keep troubleshooting (not sure where to go next) or should I go ahead and pull the motor and check out the top end.  When it starts to cut out at highway speeds it almost feels like I have a valve floating (bad spring ?)  I've got a few leaks around the top end, so I was gonna pull the motor and replace all the gaskets after the riding season, but now I'm thinking I should go ahead and do it now.

...any and all comments welcome...

[Magpie]

Do your carbs have the same drain tubes coming from the bottom of the float bowls as the earlier 750's? This may sound stupid but I had a similar problem years ago and it was the way the hoses from the bottom of my K1's carbs were routed out the back of the motor. They were being pinched between the swingarm and the motor so it affected the flow of fuel onto the float bowls somehow. It was like running out of gas at higher speeds and ok at lower speeds. Changed where the hoses went and it was fixed. Stupid- but it worked.
Cliff.

[Bob Wessner]

Does sound like it could be a fuel starvation problem at higher speeds. How about the fuel tank vent in the cap, is it clear? What about float height, have you checked it? How about the advance unit, is it free and working properly? You could check that with a timing light.

[Chris Liston]

Sounds like a tank "vapor lock".  My cousins KZ750 had the same symtoms.  He replaced the vent hose and it runs great now.  He would stop, open the gas cap and could here it de pressurise like when you open a gas can thats been sitting for a while.  He'd close the lid and wham, bike ran great. 

[Lumbee]

Do your carbs have the same drain tubes coming from the bottom of the float bowls as the earlier 750's?

Not sure about the earlier carbs, but I have the drain tubes coming from the bowls, but there not routed back to the motor in any way, they just allow the fuel to drain out (onto the concrete) if they overflow.

Does sound like it could be a fuel starvation problem at higher speeds. How about the fuel tank vent in the cap, is it clear? What about float height, have you checked it? How about the advance unit, is it free and working properly? You could check that with a timing ligh

Checked and adjusted float height, and statically adjusted the points.  I've got a different gas tank I could try just to rule out tank/vent problems.

[Bob Wessner]

and statically adjusted the points.

This wouldn't remedy, or rule out an advance unit problem.

[Lumbee]

...someone suggested trimming the plug wires back...gonna give that a try this weekend...

[Bob Wessner]

If in fact it has anything to do with fuel starvation, which who knows ?, you needn't switch tanks for a venting problem. The vent is on the underside of the filler cap. Just make sure it's clear and not rusted over, etc. One other possibility along these lines, have you checked the petcock screen? Maybe there is an accumulation particles, not enough to pass enough fuel at low speeds, but at higher sustained speeds perhaps it can't pass enough fuel. Just some thoughts that are easy to check.