Thanks guys. Just debating on jet size. The Uni NU-4055 is supposed to match the flow of the paper oem filter, and if it's not letting more air through then putting on the 4-into-1 shouldn't have mattered much as far as changing jet size, right?
It is not just about air volume, it is about air pressure. The carb throat pressure (negative with respect to outside the carb) is what draws fuel from the jets.
The filter membrane style/type changes the differential pressure drop across that membrane with the selection. Paper has finer holes but a lot of them. The foam has fewer holes but they are larger. And so, the pressure drop across the Uni is slightly less, leading to slightly less fuel volume from the carb's jets, as the pressure differential seen by the jet orifices is slightly less.
It is only at or around red line RPM that air volume restriction actually occurs. Still if ONLY an air filter style change, the carbs shouldn't become too lean for the engine. Remember, the carb has no way to automatically adapt to components that Honda did no intend to be installed on the bike. The Honda carbs operate purely on physics principles, and the filter membrane style alters the physics baseline. But, you didn't JUST change the filter, did you? More about exhaust later.
I mean it's probably letting less air through than the pods so it's not like I'm adding a ton or air, I'm assuming. That's the only thing that has me going back and forth on the jet size.
PODs change/shorten the inlet duct length (again a major carb throat pressure alteration) as well as changing the membrane/media pressure drop. Neither of which the carbs can automatically adjust for as it does not sense engine needs, only the physics (pressures) applied to them.
As I mentioned before, based solely on going from the paper to foam I must be letting a little more air through, regardless of whether it claims to match the oem airbox flow. I just don't know if I should jump from 90's to 105's out of the gate.
If you have it running now, a direct pointer to severity can come from reading the spark plug deposits under load conditions. (A "plug chop"). Or, a dyno fuel map print out.
What you have is custom and not like anything else that been assembled before. Did you follow some known and well understood "recipe" for assemblage of components?
The stock 78 came under direct import scrutiny from the EPA, which required a hydrocarbon sniff test on the model style. To achieve the goal, the exhaust system was redesigned as a high pressure type. The effect is that cylinder scavenging is incomplete. This meant the some hydrocarbons and less oxygen was available for the next firing cycle, so the hydrocarbons could be put through another combustion cycle with additional oxygen. More oxygen is needed to be delivered to the cylinder along with less fuel for the engine for efficiency. The result was less Hydrocarbons found in the sniff test and the EPA was happy to allow importation.
Note, the carbs were adjusted to reduce fuel and increase air being delivered to the cylinder to meet the needs of the engine with the high pressure pipes. I know of NO aftermarket exhaust system that mimics the 77-78 4 into 4 stock pipe, and almost all of the them tout "less restriction" and "free flowing" as sales attributes.
For this model in particular (77-78 CB550K), this also demands that the carb jetting be altered to accommodate the exhaust characteristic change. However, if the sales/marketing departments of the aftermarket supplier determine this knowledge may deter sales, they won't tell the prospective buyer about this at time of purchase. My, projection, (for what it is worth), tells me that the jetting orifice for the PD carbs will have to be enlarged just to restore original A/F mixtures delivered to the engine. I can't determine the magnitude precisely. But, I would start with #95 (see earlier post), and then test for effect, adjust and repeat, until the engine ran well, showed good plug deposits, and exhibited best 1/4 mile acceleration times (assuming no access to a dyno).
For this model, and to add to the confusion, the venturi shape of the PD carbs is different than early model carbs. So, jetting choices for early model carbs associated with air filter and exhaust style changes, will not directly apply to the PD style carbs.
I will add that, reports from third or fourth parties about jet selection is highly subjective, and completely unsubstantiated with reproducible facts in nearly all cases.
"Butt dyno" reports are essentially meaningless, as it can mean they were able to drive around the block and made it home before the plugs fouled, to "the person they sold the bike to never complained about awful gas mileage and nasty stumble with throttle twist". Many have never ridden a stock bike in proper tune and have no basis for any performance comparison besides, "it runs ok for me, and I don't want to work on it anymore".
Add to the above that no two 4 into 1 systems are the same among brands (any many between examples from the same manufacturer).
For many, its a style choice/description rather than having any performance parameters/data about that choice. (The cool racers have 4 into 1s, so they must ALL be good for my bike, right?)
PODs is another "style choice" with no engineering parameters appearing with most brand selections.
So, when the folklore "database" list "pods and 4 into 1 exhaust", it tells you nearly nothing about how your selection of pods/filter, exhaust system choices that YOU made will operate with the carbs and engine YOU have assembled. Particularly when the database only lists "runs good" as a qualifying parameter for selection, with no quantification of what "runs good" actually means in performance, or fuel metering accuracy.
In short, if you are going to make design changes, you become the designer, and assume the design responsibilities associated with that. If it is only about style when parked, you can avoid a big part of the learning curve and test/adjust cycles. Don't underestimate the man hours that Honda paid for that bike in stock form.
Also, TwoTired - Where did you get that nice looking back rest in your pic?
The back rest is an era survivor I got with my 76 CB550F rescue project. It sat in the Arizona sun which kept the metal bits from rusting, but ruined any plastic the sun's rays could punish. So, the cover was awful and really regraded. But, my wife liked it so much, I made a new cover, using the old one as a pattern. I primarily wanted the luggage rack, so I could strap lunch boxes, brief cases, computer luggage, and other sundries to the bike in my travels about town. The backrest is actually removable, and can be moved forward for functional use by the driver. It also has a zippered pocket on backside of it, that I mostly use to store the bungee cords when not in use. I think I've seen only one other like it...ever.
I'm sure many would label it ugly. I try not to live by other's "sensibilities".
Cheers,
