I've never heard of "round top" being applied to anything other than CV carbs (that have the big round top).
Apparently CB750 folk lore has it's own meaning for terms, that do not appear in the shop manual.
Since there are only three styles of carbs for the CB750, I've only known them to be identified by either their set up number (stamped on them), or PD for the late model, and "early", with a separate designator for the ones with 4 cable actuators (really early) rather than the bellcrank/rod/lifter arm types found on 71 through 76 models.
I understand that the roundtops like I have do not have an accelerator pump.They will die if you whack the throttle open too fast.Nature of the beast.
It doesn't matter what you call them, you should be able to snap the throttle to one half of remaining travel while at idle in any gear including 5th, and the engine will pick up reliably and smoothly. No you can't snap them full open. But, you can snap them half way, and that is the true nature of the beast when properly tuned. At higher engine revs where there is more air velocity through the carbs, the venturi effect produces such a reliable vacuum source, that throttle position changes don't produce any stumble.
So I ask again,if I am doing it wrong what is the correct way?
If it is stock, you set the air screws to book values. See page 195 of supplement chapter 20 of the Honda CB750 Shop manual. The relevant exerpt is:
Carburetor air screw adjustment... The standard adjustment that gives the best performance is 3/4 to 1 1/4 turns open from the full close position.
And restricting fuel AWAY from an engine will not make it run faster.More throttle = more fuel=more rpm=go faster
I disagree with this statement. The fuel doesn't make it run. The fuel won't do anything without the oxygen to combine with and combust. With fuel ratios greater than 5:1, the engine won't even run no matter how much throttle you give it. So, "more fuel=more rpm=go faster" is only wishful thinking. Over rich fuel mixtures do not burn completely and don't reach maximum expansion pressure onto the piston crown. Clearly more fuel is NOT the answer, unless more oxygen is available for that extra fuel.
These carbs have air bleed screws which adjust the amount of air mixed with the fuel at the pilot jet emulsion tube.
This becomes a premix froth, which is lighter than the raw gas alone, and is far easier to be pulled into the carb throat with engine vacuum.
Turning the air screw outward allows more air to be mixed with the fuel and a leaner ratio fuel air mix burns more efficiently and the engine increases in speed as all (or more of) the fuel is burnt rather than just a portion of it. To compensate, the slides are lowered to shut off more air flow. If you were able to add enough air to the premix, you could shut the slide completely, but there is an air jet to limit that and so the slide can only be lowered just so far.
The engine will run with an air fuel mix between 5:1 and 22:1. Stoichiometric is 14.7 ish :1 and best power is about 12 or 13 :1.
Let's say you were actually able to adjust the idle fuel mixture to a perfect 14.7 to one. The engine purrs, producing only enough power to overcome internal friction. Opening a mechanical slide does two things. 1) it suddenly makes more air available inside the carb throat leaning the mixture well away from any ideal power mix. 2), the engine vacuum in the carb throat is suddenly polluted with the pressure from the intake tract upstream the carb slides, from about 16 inches Hg less than atmospheric pressure, to 5 or less. The pressure differential is what drives fuel through ALL the fuel metering orifices, and reducing that differential severely reduces the fuel available for that sudden inrush of air when the slide is opened.
The result is engine stumble (I call it wheeze) when the throttle is opened suddenly and too far, due to extremely lean A/F fuel mixture.
To combat this, the idle mixture is purposely set rich. Not rich enough to carbon foul plugs during prolonged idle. But, rich enough that the A/F ratio remains in the combustible range when the slide is moved to the half way position.
I've told about this experience before in this forum. But, here it is again to prove my point. The carbs on the CB550 (022a) are internally designed the same way that the 76 CB750 carbs are designed. About 20 years ago, I intended to put the extreme fine tune on my 74 CB550. I obtained an exhaust gas analyzer. I did the standard tune up to the bike renewing plugs, air filter, etc., synced the carbs, and then set about to optimize the air screws for least hydrocarbons at each of the four exhaust pipes. As I opened up the air screws, I watched hydrocarbon levels fall, and the idle RPMs went up, so I repeatedly kept backing off the big idle knob to keep the 1000 RPM idle. It took about 20-30 minutes behind the engine heat from the fan in front. But, finally I had the lowest HC reading on the analyzer and a perfect 1000 rpm idle and the engine purred pleasingly. I was quite surprised that the 5-5.5 turns out from seated was so different from book value setting of the air screws. Still, I eagerly set about to road test my latest toil expecting the best experience to date.
First observation was that even while on the center stand, throttle twist was pretty touchy. Still, I geared up and headed out the garage toward the street. The only way to get the bike to move was to rev it up way high and ever so slowly ease the clutch out. The bike had no power on throttle twist at all, and almost total wheeze. I didn't get 50 feet from the garage, before realizing there was no way to ride the bike this way. So, I nursed it/ struggled back up the very shallow grade to the garage. I realized I had done tune ups before and never had this happen. The only difference was the "super-tune" of the air screws, so I cranked in a turn on each, and noticed the throttle response on the center stand (no engine load) improved greatly. Now off for the second try road test. It still sucked, with no acceleration power, only inertia from RPM and slipping the clutch got me any forward advancement. Back to the garage, for another 1 full turn inward on the each air screw, and off again for another road test, this time bringing the screwdriver with me.
Better, but still not good. So, I'd give the screws another turn and test, stop and turn the screws in some more and test, in repetition, until I could get predictable and reliable power from the engine in any gear from 1000 RPM, simply by turning the throttle grip, up to half way of remaining throttle travel.
Back to the garage. Shed gear. And, now to find out the final air screw setting. I count 1 1/4 turns out from seated...which is the book value for the CB550 and 022A carbs.
So, it took me about two hours to learn that book value works the best.
For your bike, either set the air screws for book value, or mark your throttle and go test drive the bike. Open the air screws to the max open setting while still maintaining smooth power from idle under load in any gear (including 5th). It won't be fast in fifth gear from idle, but it will be reliable. If you open the screws too far, you will get stumble from low RPM, and that tells you to turn those air screws back in.
You can't find this correct setting in the garage, hunting for max engine RPM with slides closed at idle. I've tried that.
The most accurate way to adjust the idle mixture is to slowly and evenly open all four screws at the same time by the same amount. Keep opening them as long as the engine responds with increased rpms. If it increases in engine speed that is your indicator that the engine needed that extra fuel.Once it stops responding to the extra fuel you know that you have found the correct amount of idle mixture for your particular bike.
You actually have this backwards. Your carbs have air bleed screws placed in the path between air jet and pilot emulsion tube. Turning them outward increases air flow from the air jet to the pilot emulsion tubes, and thus leans the mixture. You aren't giving it ANY extra fuel with a more open screw setting, just the opposite, in fact.
The engine is increasing RPMs because the standard over rich idle mixture is getting a more efficient burn with the added air so more fuel is actually being burnt. So, more air = less fuel in the mix which increases RPM in this situation with the slides nearly closed.
Cheers,
