Topic: problem starting when hot on a '78 750f

[my750f]

I searched the archives but did not find an answer to this problem.

I just bought a 1978 honda cb750f  and it starts right up when cold. After riding the bike for almost any length of time and turning the bike off, the bike will not restart with the electric starter (it will however start by push starting).

If the bike has been running for an hour or so it starts to want to die at low rpms (like the battery is dead).
Unfortunately I have no way to test the battery, but I did recharge it and put it back in the bike and it does not appear that it's running out of juice (the lights still work, etc.)

Any ideas where to start? Is this because the coil is getting hot?

Thanks

[TwoTired]

When was it's last complete tuneup?
Air filter clean, clear?
Are your plugs sooty?

Define won't start.
Engine doesn't turn over?
Engine turns but no fire?

How long will it crank?

Describe your choke operation when cold?

[my750f]

Just purchased the bike, so I'm not sure when the last tune up was. However, when the engine is cold it fires right up with a simple touch of the starter and purrs like a kitten. The engine sounds in-tune and is very responsive when riding.

The filter is clean and plugs do not appear sooty (they look brand new, so maybe a tune-up was done recently?)

Once the engine gets hot, it won't turn over with the electric starter. It will however start just fine by push starting, but will stall at lower RPM (like when at a stop light etc)

It will crank for as long as I hold the starter button, it just seems like there is not enough juice to turn the engine over.

Not sure what you maen by choke operation, I use the choke to start the bike cold, it appears to be functioning properly.

Thanks for your help, it's much appreciated.

(P.S.- I used to drive air-cooled vws and experienced a problem known as "hot-coil" I'm wondering if this is the same thing here?)

[Lumbee]

...couple of things...

#1-Put in new plugs...its cheap, and u'd be amazed at the diff. a new set of plugs can make...

#2-Are you gearing down into true neutral?  You can get to neutral straigh from 2nd without going to first when u'r gearing down.  Mine wont start with the electric start if it is geared down and shut off this way...just something to try...

[oldbiker]

Refusal to start when hot usually means the mixture is a bit rich. Does it help it to start if you hold full throttle when cranking?

[Mark M]

I'd agree on a rich running problem,

The main causes are either 1) not enough air getting through the carbs,
Which is most likly a cloged filter, but could be choke sticking on or carb adjustment problems.
2) Or it could be too much fuel.
This is a bit more trickey as it indicates a problem with the carbs,
Maladjusted, crud, stale fuel or oversize jets.

A good service will be the first thing to do, get a haynes manual and just follow the instructions in there one step at a time. It's amazing how much better these things work when they are set up with a little care to detail.