Topic: CB750K2 Will NOT idle cold

[Artfrombama]

Idles great when the engine is warm but won't idle cold. Choke has no effect. Full choke, won't run at all.

Any ideas?

[mrbreeze]

Time for a tune??...carb synch??...carb clean/rebuild??
These bikes are cold blooded for sure but can be made to idle when cold.They all run better when warmed up.

[WHALEMAN]

The rubber carb manifolds have a lot to do with this if everything else is good. If yours are original they need to be replaced. Dan

[TwoTired]

Idles great when the engine is warm but won't idle cold. Choke has no effect. Full choke, won't run at all.

Any ideas?

That's kinda normal for an engine with carbs.    Many chokes have a fast idle cam coupled to the choke to bump the idle speed with choke operation.  Some SOHC4s don't have that feature and the operator is expected to hold the throttle open while the bike is warming up.

It's not fuel injected and there is no computer or exhaust sensor for these bikes.  Fuel atomization is poor with a cold engine, leading to poor combustion efficiency.  Lack of low idle speed is expected.

For a stock Honda, you start with full choke, remove some and hold the throttle open for 2000-3000 RPM.  As the engine warms, remove the choke incrementally and let eh engine rev less until the choke is off and it will idle at 1050 RPM (about)  Depends on which SOHC4.  Some models have a fast idle cam link to the choke.
If yours, doesn't behave as described, there is something wrong that needs to be repaired, or there have been modifications from stock that alter standard behavior.

Cheers,

[Terry in Australia]

Don't forget the idle screw (knurled knob) on your carbs mate, it was designed just for that function, just fire the bike up, wind the screw in so it's idling at 2000 RPM, and have a cup of coffee or don your riding gear while it's warming up. Cheers, Terry.

[Artfrombama]

Recent tune, yes. Carb sync, no. Waiting on my vacuum gauges.
Original carb boots? I suppose, but wouldn't a vacuum leak cause a fast idle? Runs great warm.
No fast idle cam on these carbs. Familiar with Honda carbs, have ridden Hondas since I was 12. Modifications? All I can detect is 41 idle jets.
Cranking in the idle screw isn't a problem, really haven't ridden this bike long enough to get a routine of starting a cold engine without going back to the starter button 4-5 times.

20W50 oil too thick?

I really like to hit the road with a little warmth in the engine.

[TwoTired]

On the 550's, I don the riding gear, full choke, start, and drive off.  As I'm riding, I take off the choke incrementally while the engine warms.  I keep the R's above idle speed with my hand on the throttle.  But, I don't ask for high power levels during warm up.   After a few blocks, the engine has warmed to where it will idle at stop lights at normal operating temperature.

The 550 uses D7EA plugs which are a hotter heat range and therefore run better when the engine is cold.  I also use 10-40W oil, as we have no hot deserts here to run flat out with max power for long periods of time (which would tax the cooling fin capacity).
The 550 has less mass to warm up than a 750, so it probably warms a bit faster to operating temp.

If your outside air temp isn't above 60 F, you don't need a heavier weight oil, certainly nothing above 40W.  And, you'll have less wear when the engine is cold by using a thinner weight oil, that can move through the oil galleys easier.  You can also use D7EA heat range plugs for better operation during warm up.

Clutch disengages better when cold, using a thinner oil, too.  A bit less clunk when putting it into gear and on down shifts.

Cheers,

Edited to remove some stupidity

[Artfrombama]

I don't know what bike you have. 

In the subject line, don't recall the serial number, CRS

But, on the 550's, I don the riding gear, full choke, start, and drive off.  As I'm riding, I take off the choke incrementally while the engine warms.  I keep the R's above idle speed with my hand on the throttle.  But, I don't ask for high power levels during warm up.   After a few blocks, the engine has warmed to where it will idle at stop lights at normal operating temperature.

The 550 uses D7EA plugs which are a hotter heat range and therefore run better when the engine is cold.  I also use 10-40W oil, as we have no hot deserts here to run flat out with max power for long periods of time (which would tax the cooling fin capacity).
The 550 has less mass to warm up than a 750, so it probably warms a bit faster to operating temp.

If your outside air temp isn't above 60 F, you don't need a heavier weight oil, certainly nothing above 40W.  And, you'll have less wear when the engine is cold by using a thinner weight oil, that can move through the oil galleys easier.  You can also use D7EA heat range plugs for better operation during warm up.

Clutch disengages better when cold, using a thinner oil, too.  A bit less clunk when putting it into gear and on down shifts.

Cheers,

Thanks T/T, I think I'll try 10W30. My WOT (wide open throttle) days are few and far between.

[TwoTired]

I don't know what bike you have.

In the subject line, don't recall the serial number, CRS

What the saying?  "Looking for data in all the wrong places"?
Musta been the missing serial number that threw me. 
Oh well...

Thanks T/T, I think I'll try 10W30. My WOT (wide open throttle) days are few and far between.

Why not do what Honda recommends and use 10-40W?

Cheers,

[Bob Wessner]

Thanks T/T, I think I'll try 10W30. My WOT (wide open throttle) days are few and far between.

I'm not sure I would go to 10w30 weight. I wouldn't even use it here in Michigan. It seems awfully light for these bikes.

[dusterdude]

i know 10w40 is recommended for the 750`s

[Artfrombama]

10w40 it'll be

[Terry in Australia]

Yeah, don't go too thin on the oil, I sold a CB750K4 to an English tourist here in early 2001, and before leaving melbourne on his "Around Australia" trip he had a bike shop do a full service, including an oil change. They were used to servicing modern bikes so used Penzoil 10W30 and it blew smoke and rattled like a bastard.

He rang me when he got to Sydney (500 miles from here) complaining about the oil consumption so I suggested that he dump it out and try some Aussie Penrite HPR40 (the engine had done around 45,000 miles) and guess what? No smoke, and the engine ran much quieter.