Topic: Need help getting a CB750 running right.

[neetones]

Hi folks.

A friend of mine picked up a 1975 CB750F a few months ago and when I went down with him to pick it up it ran beautifully.

It looks really well taken care of, and has just over 40k on it. The problems started when we got it home.

It idles fairly well, if low, but as soon as you give it gas it bogs and the engine wants to quit.  We've tried playing with the fuel screws and got a new battery, but we're stuck.

Could it be the charging system? Could we have to rejet the damn thing? It was running fine and now, terribly/not at all.

We've taken it into the Honda shop and he had it running fine.  He cleaned the carbs and said the plugs look fine.  But now we can't even get it on the road.

Any ideas?

[joehardy]

Where did you pick it up and where is it now? Where has it been for the last few months?

[neetones]

Where did you pick it up and where is it now? Where has it been for the last few months?

The bike's in Santa Barbara and was picked up outside of L.A. If you're thinking altitude change, it's all pretty much sea level.

One of the big problems is that it's so inconsistent. We had it running fine up here as well, but only for a few miles and then nothing. Now it bogs immediately and you can't really get above 4k rpm.

[Don R]

Sounds like a plugged fuel screen, maybe a tank full of crud

[Duke McDukiedook]

Yeah, sounds like something fuel related. Look at your fuel delivery first.
If that all checks out look at your spark or lack of it.

[the technological J]

i was kinda thinking choke or slow fuel delivery also whether clogged fuel screen( pull the gas line and see how well the gas runs) stick your finger in the gas tank an fee the inside of the topside of the tank and see how much rust is there

[TwoTired]

Check head pipes for even heating at idle.  Are all cylinders working?  ...at all throttle positions?

Pull spark plugs and "read" the combustion deposits on all four. 

State if the bike has had mods from stock, like exhaust and air filter, etc.

When was the last 3K mile tuneup check list completed?

Have the carbs been vacuum balanced?

Are the carbs receiving clean fuel?  Is there clean fuel in the carbs now?

You can't snap the throttle full open at idle, it will wheeze and stumble.  It doesn't have fuel injection or and accelerator pump.
It should take up to one half total throttle travel and pick up predictably.

[ekpent]

I will go with Don on that. Sounds like it runs long enouph to empty the bowls and then they can't keep up.Look for rust in tank or crud,see if there are deposits in the petcock bowl,check that your tank vent is not clogged. After you run it shut it down,turn gas off quickly and drop or drain a bowl to see how much gas it contains.