Topic: Summer tune

[Markcb750]

After fiddling with my carbs for hours this spring; including having them balanced by an "expert" with limited improvement I had come to the conclusion this was just the way it was going to run.  Really cranky when cold, never idling well, bumbling when cranked hard until it had run half an hour.


Now that it is warm this old motor screams.  Pops to life with one kick even after sitting over night. choke off before I go a mile, and man does this thing scream to 9K, I get moving position my wrist up so I fully open the throttles full when I twist,,,, freeway speeds are just 5 seconds away. 

Idles at 1000 with little nursing.

Anyone else find their SOHC motors just don't run when the temp drops to the 50's?



I think the rubber isolators keep engine heat from the carburetors exacerbating the problem. I do not remember my '71 500 four being like this in upstate NY. Memory does not suffice to but I believe the 500's carburetors where mounted in a similar way to the 750's.

So my question is; let her be, or look for rebuild kits to install this fall?

[cycleman]

All these year bikes be they the cb750f or my gl1000 or others are very cold blooded and don't like the cold.  They where built just as the EPA was starting to get into setting limits so the bikes are jetted fairly lean at idle, and need the choke when they are cold. If you ever look at a gl1000 the carb runners are about 5 times longer than anything the CB750's have.

This spring I did a lot of driving when it was around 3-5 degrees C, I guess that would be around 40 F and the gl1000 was an absolute dog when cold.  It took forever to warm up. Never really did run right until the weather warmed up.

My two cents is to just get used to the choke & if its running right ( plugs good colour) then leave it alone. My 78 cb750f is currently undergoing a restoration process as it sat since 89 ( mostly outside ). Have it running, and yes it does like the choke until it warms up. Being that its air cooled I don't think it will be as bad as the wing ( water cooled ) is when it gets cold. 

[Markcb750]

Thanks cycleman, great to have input.

Plugs look good, a little black but very light. I change to D7's this spring because of fouling. Probably not necessary now that it is warm. 

[detdrbuzzard]

never had a problem with the 750's in cool weather ( 40 to 50 f ) but like cycleman my 1200 wing is stillrunning on partial  choak after the engine has warmed up

[HondaMan]

Starting with the 1975 series (K5 for the 750), the carbs got real lean, for emissions reasons. This makes for cold-bloodedness in cool weather for two reasons: 1. lean idle mix and 2. weak spark while the points and condensors warm up.

Many have reported on these bikes that my Ignition box greatly improves the situation (no, this is not an ad...), due to the elimination of the "cold ignition" effect. The Dyna 'S' will not improve the situation, because it causes a shorter-than-normal spark discharge from the coils, and many have testified to this performance.

One thing that I have always done, before I got into doing it electronically, is to run hotter plugs in the cooler weather. I would guess that you are running D8ES plugs now: these are too cold in any case. The D8ES-L plug is normal for the bike, and in winter I run the D7ES, or the X22ES-U from ND.

The best all-around plug heatrange is the X24ES-U from ND for these 750s. This is a drop-in replacement for the now-missing D8ES-L plug.

Also, if you have old plug caps with resistance of more than 10,000 ohms, they will make the coils act very cold-blooded.

[kpier883]

Mine is a 74 CB750.  When I got it, I rarely had to use the choke.  Even in cold weather I had to turn the choke to "off" immediately after the bike fired to keep it from "loading up".  The plugs were always black and sooty and I fouled a plug or two every few months.  Upon starting off from stationary, if I gave it lots of throttle, it would lay down a black cloud behind me.  At night if a vehicle was behind me at speed and I went for a handful of throttle, I could see wispy smoke along the ground by the light from the vehicle behind me.  The only redeeming quality of this was that it would crank first or second kick most of the time without any choke. 

Here is a post from another thread where I documented my changes:

I put in a new air filter and changed the idle jets (#40 to #35) to leaner and picked up maybe two or three mpg.  I was getting 29 - 32 mpg before and maybe 31 - 35 mpg now.

I don't like to change too many things at once because then I wouldn't know which change caused a potential problem.

I have some #105 mains that will be replacing the #110 currently in it soon, and hope to improve my mileage some more.  I have noticed that when I really give it some throttle, it will billow black smoke.  Already lowered the needles and it helped but didn't eliminate the smoke.  Also notice that at wot I get less acceleration than at 7/8 throttle and I think it is due to being rich.

I did put the 105 mains in later.  I now get about 40 mpg and don't foul out plugs any more.  But, it is definitely harder to start and always requires the choke.  I often roll it off since I live on a hill just to avoid excessive cranking.  Generally speaking, it will not run without letting it warm for 30 seconds or so on the choke.  I still think it is less cold-natured than many others have reported for their bikes. 

Maybe you could check your idle jet size.  If they are very small, maybe go up one size and see if it helps.





 

[Markcb750]

I am running a Dyna point replacement with original coils and wires, I would not use resistive plugs with out changing the coils & wires. 

The D7EA's helped in cold weather, particularly on #2.  Book has D8EAs as standard.  I would not want to run any hotter then the 7's for fear of piston damage. 

Carbs have been changed from 105's to 110's by PO, I lowered the main needles one notch prior to changing to the D7EAs.

Minor smoke in cold weather, again makes me think I have the tuning set pretty well. I think I am just dealing with cold carburetors wetting out the vaporized fuel before it can reach the CC, then pouring raw gas into the CC. 

[kpier883]

I think the main difference in my start up is due to the idle jets - #40 was not cold-natured, but #35 is.  Have you checked your idle jets for the size? 

[Markcb750]

I had the carbs apart several times until I learned to clean all the pathways. I am sure they are 35s, thanks for the suggestion.

Mark

[HondaMan]

I am running a Dyna point replacement with original coils and wires, I would not use resistive plugs with out changing the coils & wires. 

The D7EA's helped in cold weather, particularly on #2.  Book has D8EAs as standard.  I would not want to run any hotter then the 7's for fear of piston damage. 

Carbs have been changed from 105's to 110's by PO, I lowered the main needles one notch prior to changing to the D7EAs.

Minor smoke in cold weather, again makes me think I have the tuning set pretty well. I think I am just dealing with cold carburetors wetting out the vaporized fuel before it can reach the CC, then pouring raw gas into the CC. 

You might wish to check out this post and method: modifying the emulsifier tubes (unfortunately, typoed "emlusigier", so no one can ever find it...  ).

My two cents: go to the 105 jets and put the needle back up, modify the emulsifier holes. Drill the upper 8 holes to .039". If that is not quite enough on the later bike (do you have -081 carbs?) then also drill, one at a time, the lower 4 holes. Try it after each additional hole.

When mixing air with the fuel, the RPM range follows the holes: the lower holes affect the lower 1/8-1/4 throttle range and the upper holes the 1/2-3/4 range. Most 750s do better with the upper holes drilled: it reduces the wetness that makes the smoke you are talking about. 

Update: Steve D. just fixed it for us. It's added here, now:
http://forums.sohc4.net/index.php?topic=7401.30

Thanks, Steve!

[vames]

I've had a 76 750F and now have a 75 400F. Never had any trouble with temperatures down to freezing. Both bikes totally stock -- the 750F with a Dyna S and the 400F with stock points. Even at freezing (or sometimes below) I turn the choke off within 5 seconds of starting and I'm idling at 1100 within a few blocks. Never any bucking or stuttering, no puffs of smoke and no fouled plugs.

I'm not sure what the problem is or what I've done right, but it should be known that these bikes are capable of running well right from startup at virtually all range of temps you'd want to ride in.

[fmctm1sw]

You might wish to check out this post and method: modifying the emulsifier tubes (unfortunately, typoed "emlusigier", so no one can ever find it...  ).

My two cents: go to the 105 jets and put the needle back up, modify the emulsifier holes. Drill the upper 8 holes to .039". If that is not quite enough on the later bike (do you have -081 carbs?) then also drill, one at a time, the lower 4 holes. Try it after each additional hole.

When mixing air with the fuel, the RPM range follows the holes: the lower holes affect the lower 1/8-1/4 throttle range and the upper holes the 1/2-3/4 range. Most 750s do better with the upper holes drilled: it reduces the wetness that makes the smoke you are talking about. 

Can someone provide me a link to this thread?  I can't find it.  I tried to theorize in another post about those holes in the emulsifier, I'd be very interested to see discussion on it.  Thanks!

[HondaMan]

You might wish to check out this post and method: modifying the emulsifier tubes (unfortunately, typoed "emlusigier", so no one can ever find it...  ).

My two cents: go to the 105 jets and put the needle back up, modify the emulsifier holes. Drill the upper 8 holes to .039". If that is not quite enough on the later bike (do you have -081 carbs?) then also drill, one at a time, the lower 4 holes. Try it after each additional hole.

When mixing air with the fuel, the RPM range follows the holes: the lower holes affect the lower 1/8-1/4 throttle range and the upper holes the 1/2-3/4 range. Most 750s do better with the upper holes drilled: it reduces the wetness that makes the smoke you are talking about. 

Can someone provide me a link to this thread?  I can't find it.  I tried to theorize in another post about those holes in the emulsifier, I'd be very interested to see discussion on it.  Thanks!

Here is my "short post" on the topic:
http://forums.sohc4.net/index.php?topic=7401.30

Steve D. just added it to the FAQ.

There's another, much longer, thread about these holes somewhere else, where I was talking with several others. The 750 bikes, until late in the K5 series, were tuned in Japan. Most American sites are higher than that. I have seen some differences in hole sizes on K5 and K6 carbs: more like .035" holes from Keihin, stock.

Basically, the top holes (8 of them, OEM size .031") should be drilled progressively , starting at .033", then .035", .037", to about .039" maximum (above that there is no additional benefit), and tried each time to see if you like the results. The upper holes improve fuel atomization in the 1/2-thottle and higher ranges. The midrange really snaps to life with this change.

The lower 4 holes bubble the fuel at lower throttle settings more than the upper holes do. I usually drill one of these at a time, until the "blubbering" in the low RPM range of 1500-2500 gets better. This problem is worse on bikes with pod air filters that don't have the velocity stacks of the stock airbox, and the drilling of these lower jet hole helps with the transition from the idle circuit to the low midrange. It will not, however, completely cure that problem: only velocity stacks will.

But, before drilling....always pull out the tubes and look very closely at them under a magnifier: the fuels we all have contain quite a few minerals that make a calcium-like crust inside these holes over time. So, start by removing that stuff. Then, when you are finished drilling to the desired hole size, take a countersink (a real one, not the next-size drill bit) to the outside of the holes, to remove the burr that will collect this crust in the future. This will make your work "stay", like a good dog... 

[Markcb750]

Thanks HM.

I may try that next time I take off the carbs, I suppose I could try to remove the jets w/o removing the carbs, but it is running well right now and I just don/t want to lose riding time...


Interesting how every little thing in the air/fuel system has  an effect on operating characteristics.  The engineers who set the production specifications must have spent many hours trying to specify what they thought was the "best" tune.

[HondaMan]

Thanks HM.

I may try that next time I take off the carbs, I suppose I could try to remove the jets w/o removing the carbs, but it is running well right now and I just don/t want to lose riding time...


Interesting how every little thing in the air/fuel system has  an effect on operating characteristics.  The engineers who set the production specifications must have spent many hours trying to specify what they thought was the "best" tune.

I've often felt bad for those poor engineers: they were on another continent, using a different type of gasoline refinement process, at nothing but sea level, trying to make a good guess at settings that would run all over the world. Personally, I think they did pretty well! Only the American magazines (Cycle, RoadRider, Cycle World) of the 1970s ever really bashed them for it. In those days, you'd think some of those journalists had invented motorcycling, from their writings!

[bucky katt]

mine has always been a cold blooded b*$ch, i would gladly ride all year round if the k4 would let me but anything under 40 degrees and it just will NOT warm up, below 50 or so and it takes forever to warm up.

[Caminokid]

My 73 is cold blooded. But the same thing. After it is warmed up it runs great. My old 77 SS had no problems in the cold. It always fired right up.

[HondaMan]

mine has always been a cold blooded b*$ch, i would gladly ride all year round if the k4 would let me but anything under 40 degrees and it just will NOT warm up, below 50 or so and it takes forever to warm up.

Bucky:
This is usually caused by 2 things:
1. Leaks in the rubber boots between the carbs & heads: maybe try some wider hose clamps, or new boots? The Honda hose clamps only get tight enough for new, soft rubber boots. After that, you need some more clamping force. I like the 5/8" wide hose clamp bands, myself: I got an extra 10 years out of the last set of boots by using those.
2. Years of calcium deposits in the emulsifier tubes and fuel jets, including the idle jets. The way to test this theory on your particular bike: turn the air screws out about 1/8 more turn from where they are now when riding in cool weather and see if it gets better. If it does, the holes need to be cleaned out, and probably the jets, too.