Topic: CB750 Typical Warm-up?

[joehardy]

Just wondering if this sounds like a typical warm-up for a CB750 K model. In my case, warm-up takes about 10 minutes. She fires off quickly but you have to fiddle with the choke to keep the rpms up. Within a minute I can hold her steady at about 3000 rpm choked about half-way (no more fiddling). Within another minute or so she will hold idle around 1000 rpm- though she's lopes a bit. At this point I start driving her but she is not happy between 3000 and 5000 rpm. She goes but it's almost like she's running a bit lean- even with the choke about half. Sometime in the next few minutes she gets fully warmed up. I can shut off the choke and she idles right at 1200 and revs to 8000 eagerly- making really good power from 4000 rpm. Lot's of fun from then on. She's got 1mm over pistons, a number 41 Webcam cam, velocity stacks and Dyna ignition.

[medic09]

It varies.  Some of us (me included) seem to have cold-blooded bikes.  My ride is a '78 bone-stock motor; and it behaves pretty much like yours.  The slow-to-warm-up bit is the only frustrating thing about my bike.  Even when it is in good tune, it is slow to get going in the morning.  I though maybe with an electronic ignition it would be different; but your experience doesn't sound that way.

[madmtnmotors]

Which Dyna ignition? My Dyna III ignition (uses a "Black Box" while the "S" is self contained pick-ups only) started acting up in a similar fashion. Seems after the "Black Box" warmed up it would fire more readily. Eventually my black box quit firing on 2-3 altogether. Found a used black box on e-bay for $35.00. Oddest thing, electronics usually get worse as they warm up, while mine seemed to behave the opposite. Next time you go to warm it up from a totally cold start, hook up an inductive timing light and check for spark on all cyliders while it's still cold. My only experience has been with the Dyna III and not the Dyna S.

I see a Hondaman ignition in my near future...

And these do take a rather lengthy warm-up, especially if they haven't run for a couple of weeks.

[BobbyR]

It varies.  Some of us (me included) seem to have cold-blooded bikes.  My ride is a '78 bone-stock motor; and it behaves pretty much like yours.  The slow-to-warm-up bit is the only frustrating thing about my bike.  Even when it is in good tune, it is slow to get going in the morning.  I though maybe with an electronic ignition it would be different; but your experience doesn't sound that way.

I have the same issue. I have just started to let it run long enough to get my helmet and gloves on. Once I get out onto the main road, I just open the choke and ride her till she warms up. It is a bit unresponsive for the first mile but then smooths right out. 

[joehardy]

Which Dyna ignition? My Dyna III ignition (uses a "Black Box" while the "S" is self contained pick-ups only) started acting up in a similar fashion. Seems after the "Black Box" warmed up it would fire more readily. Eventually my black box quit firing on 2-3 altogether. Found a used black box on e-bay for $35.00. Oddest thing, electronics usually get worse as they warm up, while mine seemed to behave the opposite. Next time you go to warm it up from a totally cold start, hook up an inductive timing light and check for spark on all cyliders while it's still cold. My only experience has been with the Dyna III and not the Dyna S.

I see a Hondaman ignition in my near future...

And these do take a rather lengthy warm-up, especially if they haven't run for a couple of weeks.

Yeah, in short it seems to take a long time to thoroughly warm up, runs kind of crappy during that time but once it's warmed up it runs great. It's kind of amazing to me that it's that sensitive- but it's consistent. I've done the same warm-up dozens of times. I am running Dyna S with the green coils- I've been really happy with them so far. I had a CB360- with points and it seemed like I was always messing with them. It's been a long time since I owned a vehicle older than 1995 vintage- though I do remember a similar warm-up procedure with my dads 66' Dodge.

[joehardy]

It varies.  Some of us (me included) seem to have cold-blooded bikes.  My ride is a '78 bone-stock motor; and it behaves pretty much like yours.  The slow-to-warm-up bit is the only frustrating thing about my bike.  Even when it is in good tune, it is slow to get going in the morning.  I though maybe with an electronic ignition it would be different; but your experience doesn't sound that way.

I have the same issue. I have just started to let it run long enough to get my helmet and gloves on. Once I get out onto the main road, I just open the choke and ride her till she warms up. It is a bit unresponsive for the first mile but then smooths right out. 

Exactly.

[nancy]

Hmmm,...mine fires up with choke not fully closed..about 2/3, better than if I have it fully closed. Have to hold the throttle else it stalls,..no idle. Shove the choke fully open after 15secs or so and hold it at 2,000rpm for about another 15secs and ride off. Will idle OK'ish now showing about 1,110 rpm. No issues on the road..seems to run fine. I don't need to fiddle with the choke lever much at all. Just for that initial squit on the starter button. My plugs indicate I run close to normal range of mixture..brownish/tan plugs. Do you maybe have an issue with the idle circuit in your carbs? Maybe the idle jets need attention?
Regards
Mark

[754]

 Joe, wqhich carbs,,??

 77/78 carbs are definitly leaner running..

[HondaMan]

Just wondering if this sounds like a typical warm-up for a CB750 K model. In my case, warm-up takes about 10 minutes. She fires off quickly but you have to fiddle with the choke to keep the rpms up. Within a minute I can hold her steady at about 3000 rpm choked about half-way (no more fiddling). Within another minute or so she will hold idle around 1000 rpm- though she's lopes a bit. At this point I start driving her but she is not happy between 3000 and 5000 rpm. She goes but it's almost like she's running a bit lean- even with the choke about half. Sometime in the next few minutes she gets fully warmed up. I can shut off the choke and she idles right at 1200 and revs to 8000 eagerly- making really good power from 4000 rpm. Lot's of fun from then on. She's got 1mm over pistons, a number 41 Webcam cam, velocity stacks and Dyna ignition.

Which K model?
The K7/8 had PD carbs, legendary for their cold-bloodedness. The Dyna actually makes it worse... 

[Anti-Johnny]

Sounds typical of my 75. Usually have to kick start her in the morning. Rest of the day she starts right away with electrical about  70% of the time. I think the electrical might be going out? maybe time to clean the carbs? eh i dunno.

[joehardy]

Just wondering if this sounds like a typical warm-up for a CB750 K model. In my case, warm-up takes about 10 minutes. She fires off quickly but you have to fiddle with the choke to keep the rpms up. Within a minute I can hold her steady at about 3000 rpm choked about half-way (no more fiddling). Within another minute or so she will hold idle around 1000 rpm- though she's lopes a bit. At this point I start driving her but she is not happy between 3000 and 5000 rpm. She goes but it's almost like she's running a bit lean- even with the choke about half. Sometime in the next few minutes she gets fully warmed up. I can shut off the choke and she idles right at 1200 and revs to 8000 eagerly- making really good power from 4000 rpm. Lot's of fun from then on. She's got 1mm over pistons, a number 41 Webcam cam, velocity stacks and Dyna ignition.

Which K model?
The K7/8 had PD carbs, legendary for their cold-bloodedness. The Dyna actually makes it worse... 

K3

[BobbyR]

The advantage I have with a K8 is the fast idle cam on the choke. I don;t have to hold the throttle. it is cold blooded.

[Popwood]

My '75 is warm blooded. Even after sitting in winter at freezing temp she'll fire up on full choke in 20 seconds (not continuous but a couple on/offs). Then run with throttle twisting and after another 20 sec will have to start backing the choke off-- 3/4, 1/2 then off before a block or two away. In general, runs on the rich side all the time. My 550, also '75 takes a bit more start/ choke but but not a lot more. Both bikes are all stock.

[andy750]

My K2 (810 cc) and K4 are both very warm blooded. Minimal choke and good to go within 30 secs. Sounds like you have carb or timing issues.

[SKTP]

My K8 took forever to warm up (its all apart right now)...I'd wedge the Choke full open every morning start...If I was at a normal idle after 5 minutes, that was a really good day...once it was warm, it was amazing.

[HondaMan]

Just wondering if this sounds like a typical warm-up for a CB750 K model. In my case, warm-up takes about 10 minutes. She fires off quickly but you have to fiddle with the choke to keep the rpms up. Within a minute I can hold her steady at about 3000 rpm choked about half-way (no more fiddling). Within another minute or so she will hold idle around 1000 rpm- though she's lopes a bit. At this point I start driving her but she is not happy between 3000 and 5000 rpm. She goes but it's almost like she's running a bit lean- even with the choke about half. Sometime in the next few minutes she gets fully warmed up. I can shut off the choke and she idles right at 1200 and revs to 8000 eagerly- making really good power from 4000 rpm. Lot's of fun from then on. She's got 1mm over pistons, a number 41 Webcam cam, velocity stacks and Dyna ignition.

Which K model?
The K7/8 had PD carbs, legendary for their cold-bloodedness. The Dyna actually makes it worse... 

K3

Some things to check, then, presuming it has good sparkplugs:
1. Vacuum leaks (minor) around the carb boot hoses between the carbs and head. Start by tightening the screws when the engine is warmed up. In conjunction with this, make sure the air screws are set at about 1 turn out (Range 7/8 to 1-1/8 turn from their stops). Out is richer, but beyond 1-1/8 turns it will quickly foul the plugs.
2. You'll find the ND X24ES-U plugs work better than the NGK D8EA, especially when cold. Measure the sparkplug cap resistance: they are in pairs (1-4 cylinders on one coil, 2-3 on the other). Those plug caps must be within 500 ohms of each other and less than 8500 ohms apiece or there will be cold-engine imbalance and less-than-ideal idling.
3. Check the valve lash, particularly the intake valves. Too tight on even just one valve, and warmup time becomes very long.
4. Make sure the gap on the Dyna is near the middle of the spec. Dyna gaps vary, depending on which year they were made: old ones use gaps of about 0.015", newer ones use a little more. Your instruction sheet will tell you which yours is: make sure they are both the same gap, or very close to the same, at least.

Also, try a compression check on all 4 cylinders, both cold and warmed up. If there is more than 10% difference in the PSI value cold-to-hot, the valve(s) may be leaking in the low cylinder(s), making for uneven warmup.

[joehardy]

Some things to check, then, presuming it has good sparkplugs:
1. Vacuum leaks (minor) around the carb boot hoses between the carbs and head. Start by tightening the screws when the engine is warmed up. In conjunction with this, make sure the air screws are set at about 1 turn out (Range 7/8 to 1-1/8 turn from their stops). Out is richer, but beyond 1-1/8 turns it will quickly foul the plugs.
2. You'll find the ND X24ES-U plugs work better than the NGK D8EA, especially when cold. Measure the sparkplug cap resistance: they are in pairs (1-4 cylinders on one coil, 2-3 on the other). Those plug caps must be within 500 ohms of each other and less than 8500 ohms apiece or there will be cold-engine imbalance and less-than-ideal idling.
3. Check the valve lash, particularly the intake valves. Too tight on even just one valve, and warmup time becomes very long.
4. Make sure the gap on the Dyna is near the middle of the spec. Dyna gaps vary, depending on which year they were made: old ones use gaps of about 0.015", newer ones use a little more. Your instruction sheet will tell you which yours is: make sure they are both the same gap, or very close to the same, at least.

Also, try a compression check on all 4 cylinders, both cold and warmed up. If there is more than 10% difference in the PSI value cold-to-hot, the valve(s) may be leaking in the low cylinder(s), making for uneven warmup.
[/quote]

Thanks. Checked number 1- looks good. I'll check on the others.

[kpier883]

My '74 750 is not cold natured at all.  I turn the idle screw adjuster to slightly open the throttle, flip the choke all the way on and (if it has been ridden in the previous week or so) it will usually fire off on the second or third kick even on cold mornings.  The choke has to be backed off some almost immediately.  Within 15 second or so the choke is unnecessary and I can ride away.  After a mile or so at a stop I re-adjust the idle speed down.  Sometimes it takes a few stop light adjustments to get the idle where u want it.  

I am one step leaner on idle jet than most because it used to foul plugs and I went leaner.  I would have to look through old posts to find the idle jet size I chose.  My mains are also kind of lean - 105 I think...

Dyna S ignition fwiw.

If it has been weeks or months since riding it and it is cold out I generally just bump start it on the hill by my house.  It would probably crank by the kick or starter but I don't like to use the starter on a potentially weak battery and kicking gets old fast.