Topic: Runnin' HOT!!!

[mr.brandon]

Hey gang, so it has been a few years since the bike has been back on the road. I just recently took my bike on about an hour ride which, was its first time running at any length more than half hour or so. From what I remember from a few years back,.. it seemed to be running pretty hot. I do have the air screws on 2 or 3vof the carbs 1/3 to .5 over 1 turn out or (from lightly seated) which is pretty much the spec/standard.  I did this when syncing the carbs as, it made them run more smoothly.  My question (s) 3 of them... 1) what temp should be a good engine/oil temp (20/50 conventional oil)? 2) would or could possibly,  that little bit of additional turn on the air screws (at most .5 more or 1.5 out on one of the carbs, the others, 1 or 2, have maybe 1/3 more) have that much of an impact as far as the running temperature? To me, it didn't seem likely as, there really is not much more air getting in to make it crazy lean.. and question 3) if I go back and make all the air screws 1 turn even, would I have to resync the 4 carbs? Probably,  eggh? Well, bunch stupid, stupid questions from a very lost and slow-witted man.. thank you for any & all help!!!

[TwoTired]

If the engine is running too hot, it will most likely show up on the spark plugs showing lean conditions.

The oil needs to be kept below ~230-250 degrees F or it will quickly break down and lose lubricity, requiring more frequent oil changes.  Check your oil tank temp.

But really, "runnin hot" is subjective.  The exhaust head pipes get 1300-1500F temps dumped into them right where the engine first meets oncoming air.

As it is an air cooled engine it requires ongoing air replacement as it operates.  Less airflow makes the engine run hotter too, as there is less air to carry that heat away.
If you are just riding about low speed in town with hot weather, and waiting at lots of stop lights, you could be abusing the engine and engine oil.

Finally hot to you may not really be too hot without some standardized temp measurement device being employed to give us an accurate reference.  They aren't very expensive tools.

[HondaMan]

The 750 isn't considered overheated until the oil tank is over 180 degrees (as measured in the little dimples on the side of it, K0-K6 versions). In roadrace situations it could get up to 155-160 degrees, while in-town situations with full fairing and lowers enclosing the engine can get the oil cap hot enough to permanently brand your [ex] wife's inside thigh (yep, really happened, and boy was she mad - it ruined her [self image of a] perfect body when in a bikini!).

Do the 'touch test' to the oil cap (on those 750s with the chrome oil tank cap) when you suspect the engine is hot. If the touch is so hot that it threatens to burn the skin, it's probably too hot.

Something to remember: if the engine runs rich in low-speed, in-town riding, it will also run very hot because the fuel is then still burning as it exits the head. This is how the chrome pipes can get 'yellowed' right at their first bend. The fix for this is to adjust the airscrews a little bit leaner (i.e., inward a little bit).

Something else to realize: unlike the CB500/550/400F/350F/250F bikes, the 750 does not idle well. Period. It never did, and cannot be made to idle endlessly like its later siblings. If idled on a hot day it will simply boil the fuel in the carbs, causing slugs of overly-rich fuel to hit the intakes now and then, making the engine stumble. Raising the idle speed to 1500 RPM was how some shops dealt with this issue, but this also causes even more heat to build up, and very noisy shifting out of N into a gear. The earliest carbs (model 657a series, found on K1 and early K2) can be adjusted to a lower idle (950-1000 RPM) because they have the air screws with the tiny holes in their pointy tips: if yours are solid tips then you have 657b series carbs, and the idle speed should be in the range of 1050-1150 RPM. While some have tried to just swap those screws for a lower idle speed control, it isn't the best idea as the inner cone of the carb's airscrew hole is now formed to match the screws it already has. For that reason, swapping airscrews, or changing them just because they came in the carb kit, is nearly always a bad idea.

[mr.brandon]

Good/great info, thank you!!! Yes, I was gauging by the oil cap and oil, primarily.  However,  I have a cb750k (1974) bobber build and have a horseshoe oil tank that is underneath a pan-type seat. By this I mean that, it is a very lite and allows for a whole lot of air flow to the oil tank. Hondaman, thank you. I actually have changed out those screws and am not certain where the originals are as, I have maybe 10 of them both with and without the tip holes. I believe that, the solid may be in them now accompanied by both a spring and very small o-ring in each. As you know (Hondaman) I have both the 267 a and business on my rack... I believe I did mention in my thread that there was not a lot of stop and go and it was a fairly cool day but if not, those were the conditions. After sitting for maybe 10 min. At most, I pulled a thermometer oil dipstick from my other bike, and the oil read to about only 150, as I recall. I just feel as though it may not have been extremely accurate because the initial solid metal dipstick was not hot enough to brand a thigh but pretty close. Mainly, I think that I got the info I was needing to aquire aside from the one question,  after making adjustments to the aircrew, will a carb resync be necessary? Or, if only adjusting 1/3-.5 a turn in/out,.. is that miniscule enough that a resync isn't really necessary? Or, is that something that never really effects the syncing of the carbs at all? Thanks again for the help, gang

[tofan]

Something to remember: if the engine runs rich in low-speed, in-town riding, it will also run very hot because the fuel is then still burning as it exits the head. This is how the chrome pipes can get 'yellowed' right at their first bend. The fix for this is to adjust the airscrews a little bit leaner (i.e., inward a little bit).

Hey Hondaman,

I just wanted to verify that turning IN (clockwise) the airscrews makes it run leaner. I ask only because I've been working under the assumption that turning them out brings in more air thus more lean. This is for a 1976 750F with 064A carbs.

[HondaMan]

Something to remember: if the engine runs rich in low-speed, in-town riding, it will also run very hot because the fuel is then still burning as it exits the head. This is how the chrome pipes can get 'yellowed' right at their first bend. The fix for this is to adjust the airscrews a little bit leaner (i.e., inward a little bit).

Hey Hondaman,

I just wanted to verify that turning IN (clockwise) the airscrews makes it run leaner. I ask only because I've been working under the assumption that turning them out brings in more air thus more lean. This is for a 1976 750F with 064A carbs.

Yes.
The idle-air screw in those carbs (all of the 750 roundtop carbs) controls the amount of fuel-air mixture that gets pulled up for each intake stroke. It is already mixed as fuel & air inside the tiny tube on the top of the idle jet, where those holes are, when air from the little brass orifice at the back of the carb's bell met the rising fuel in the jet's metering hole. Then, the position of the screw admits more (or less) of this mixture during the intake suction.

I think the internet's description of these as "air screws" or "idle fuel screws" may be adding much to the confusion about these gizmos. It is the size of the jet (#40) that determines the RATIO of air-to-fuel, and the screw then controls HOW MUCH of this mixture gets added for each stroke. If you add too much, it runs rich, and vice-versa.

When the screw is turned more inward, it increases the amount of time required for the fuel-air mixture to rise up into the carb's throat. If turned more outward it lets this happen sooner during the intake stroke. In between each intake stroke the fuel falls back down to the little emulsifier holes in the top of those idle jets, and if the idle adjuster screw is turned inward more, it pinches off the flow, making things happen slower. This in turn makes the amount of [fuel + air] entering the engine less for that stroke, so ti runs leaner.

Years ago (notably in Honda's Shop Manuals for twins) Honda used the ambiguous translation phrase for the use of this screw that said, "For richness, turn screw in". It was this poor translation that seems to have started the confusion: in English this means, "If the engine is running rich, turn the screw inward". That adjustment reduces the amount of fuel/air mix being admitted, making it run leaner. This was an issue with the Honda Twins, as the vibration often made the screws slowly back out, causing richness and fouled sparkplugs that brought the bikes in for a tuneup. The CB350 twin was notorious for this.

[willbird]

Something to remember: if the engine runs rich in low-speed, in-town riding, it will also run very hot because the fuel is then still burning as it exits the head. This is how the chrome pipes can get 'yellowed' right at their first bend. The fix for this is to adjust the airscrews a little bit leaner (i.e., inward a little bit).

Hey Hondaman,

I just wanted to verify that turning IN (clockwise) the airscrews makes it run leaner. I ask only because I've been working under the assumption that turning them out brings in more air thus more lean. This is for a 1976 750F with 064A carbs.

Yes.
The idle-air screw in those carbs (all of the 750 roundtop carbs) controls the amount of fuel-air mixture that gets pulled up for each intake stroke. It is already mixed as fuel & air inside the tiny tube on the top of the idle jet, where those holes are, when air from the little brass orifice at the back of the carb's bell met the rising fuel in the jet's metering hole. Then, the position of the screw admits more (or less) of this mixture during the intake suction.

I think the internet's description of these as "air screws" or "idle fuel screws" may be adding much to the confusion about these gizmos. It is the size of the jet (#40) that determines the RATIO of air-to-fuel, and the screw then controls HOW MUCH of this mixture gets added for each stroke. If you add too much, it runs rich, and vice-versa.

When the screw is turned more inward, it increases the amount of time required for the fuel-air mixture to rise up into the carb's throat. If turned more outward it lets this happen sooner during the intake stroke. In between each intake stroke the fuel falls back down to the little emulsifier holes in the top of those idle jets, and if the idle adjuster screw is turned inward more, it pinches off the flow, making things happen slower. This in turn makes the amount of [fuel + air] entering the engine less for that stroke, so ti runs leaner.

Years ago (notably in Honda's Shop Manuals for twins) Honda used the ambiguous translation phrase for the use of this screw that said, "For richness, turn screw in". It was this poor translation that seems to have started the confusion: in English this means, "If the engine is running rich, turn the screw inward". That adjustment reduces the amount of fuel/air mix being admitted, making it run leaner. This was an issue with the Honda Twins, as the vibration often made the screws slowly back out, causing richness and fouled sparkplugs that brought the bikes in for a tuneup. The CB350 twin was notorious for this.

It would be cool to have access to a dyno and a bike with preferably four widebands and EGT sensors. Then run through every circuit on the carburator and make changes and adjustments and document the results.

One could play around with the "reproduction" parts too, and I suppose even the $29 knockoff mikuni carbs.