P700 [RESOLVED] Won't start unless pressing gas

[fmjnax]

I've got a Big Red 700. I rebuilt it just over a year ago and everything has been working fine. However, it has started presenting an issue where it won't start unless you pump the gas pedal. It will crank all day long and not even sputter without opening the throttle. Doesn't matter if it's hot or cold, it requires throttle to start. Once it fires up, though, everything runs perfectly fine.

I am just now starting to teardown process so I haven't checked anything yet. Figured I'd get this post out there and hopefully have some suggestions by the time I have the engine exposed.

Ideas?

 

[osobad]

Dirty air, fuel, filter. air fuel mixture not right. Does the fuel pump run when key is turned? Valves out of adjustment, Faulty fuel pump

 

[Jerryg]

Start with the easy stuff first, like the gas cap.

 

[fmjnax]

Sorry for the delay on this. Had some other issues that I was diagnosing (that I THOUGHT I knew the cause of, which is why I went straight to a tear-down). Still haven't been able to get back to this issue (needing to give it gas to start), but will be posting another thread for the other issue. Once I get that one resolved, I'll be back to this one.

 

[fmjnax]

Resolution found in the other thread. Turns out my exhaust valve clearances had been loosening up over time, causing the camshaft decompression system to lose effectiveness up to the point that the starter couldn't turn the engine past the compression stroke.

My theory is that when it started loosening up, it was just enough to allow some of the compression to remain, putting extra load on the starter. I was having to give it extra fuel to get it to fire under the extra load. As it continued to loosen up over time, it would let even less compression out and add more load to the starter and get harder to start. By the end, it had loosened so much that the starter was working as hard as it possibly could [and then finally had so much compression it couldn't turn the engine over and stripped the teeth of the starter; which was the topic of the other thread].