Topic: Enlighten Me

[michaelkad]

    Why is it that when my 1974 cb750 is first started from cold, it runs perfect and shifts perfect with a nice click. But after it warms up the shifting gets clunky, the idle goes from a nice 900rpm idle to 1600 or so and the motor starts making more noise? I know that with heat tolerances get looser and such but is this the case with all bikes? It has been with almost everyone i have owned. They all run better when slightly on the cold side. Is there anything to do about this or is it just the status quo for air cooled engines in general?

[cd811]

sounds like a little mess(no big problem)
I start with the usual tune-up stuff firt
points, timing,valve clearance,air filter,plugs and so own...
if you still got problem...carbs are next

[TwoTired]

What oil are you using in the bike?  Thinner oil = clunkier shifts.  Your clutch may be dragging, too.

The idle changes because there is no automatic adjustment for fuel mixture vs temperature.  When cold, the fuel does not atomize as readily as when it is hot.  And, the heat makes the mixture burn more efficiently.
Cheers,

[michaelkad]

   In this case i have done all the things you mentioned, valves, carbs, complete tuneup and all. It does seem to be running a bit rich (i.e. the plugs seem abit on the sooty side after a couple hundred miles). But my question is, every air cooled bike i have owned even brand new ones have done this to a greater or lesser degree, a decline in overall performance after extended riding. I just wondered why and if something could be done. Especially the rise in idle rpm after warming up as this adds to clunky shifting. I bought a brand new yamaha 650 back in 1975 that did the same thing. I have thought about buying a colortune and a carbtune from morgan to check things out but the expense for something i would rarely use seems excessive.  Also I am using Castrol Actevo 20/50 oil .Thanks for all replies.

[Bodi]

Balancing the carbs (carbtune or similar) will help a bit with the idle change, but usually this is just an airscrew adjustment.
Go for a long enough ride to warm up the engine, then let it idle for about 5 minuts and shut off. Check the plugs, the inside tip insulator colour will indicate if the mixture is right. All 4 plugs should be a tan brown colour. Dark means too rich, light means too lean. Once you get all 4 reading the same you can make small equal adjustments to the airscrews and the bike should idle within 200 RPM or so from just warm to hot.
This can also be from a weak spark or bad plugs, the low pressure mixture in the cylinder at idle can be hard to fire.
The transmission shifting "clunky" is more likely to be a clutch adjustment issue. Make sure the engine clutch adjuster is set to spec.

[oldbiker]

As an engine warms up, the temperature of the oil rises and the oil gets thinner. This means less friction inside the engine and combustion gets more efficient so naturally, idle speed rises. The idle speed should ONLY be adjusted with the engine at full working temperature. Now when you start from cold, the choke richens the mixture and also opens the throttle slightly so if you close the choke fully too soon, the engine may well die. Close the choke slowly over 15 to 30 seconds and you should have no more problems.

[TwoTired]

Now when you start from cold, the choke richens the mixture and also opens the throttle slightly so if you close the choke fully too soon, the engine may well die.

I could be wrong.  But, I don't think the 74 CB750 carbs have a fast idle cam coupled to choke operation.  But, wasn't there a friction adjust on the twist grip that could be use to hold the throttle open during warm up?  I know that's how the 74 Cb550 was set up.
Cheers,

[L.A. Nomad........]

My 76 550K was doing that too, I switched to 20-50w oil and it went away!

It would only happen coming home after work, after a 22 mile/ 85 mph ride back home off the freeway in 90F+ heat. I would jump off at Union Station near downtown L.A. and have to stop in go with crappy slow traffic!  I think the 10-40w was too thin, I know my motor was really hot! I switched to 20-50w  and the problem went away! Evertime I would stop the red oil light would slowly come on! The oil change solved it all........

Eddie A

[oldbiker]

Now when you start from cold, the choke richens the mixture and also opens the throttle slightly so if you close the choke fully too soon, the engine may well die.

I could be wrong.  But, I don't think the 74 CB750 carbs have a fast idle cam coupled to choke operation.  But, wasn't there a friction adjust on the twist grip that could be use to hold the throttle open during warm up?  I know that's how the 74 Cb550 was set up.
Cheers,

Hi Two Tired, I am a cb400f 0wner, so excuse me if I'm wrong but according to the data I have collected over the years, page 4-16 of the cb750 manual shows an adjustment for idle speed controlled by the action of the choke. I assume this has the same action as on the cb400f.