i reset the floats to 14mm, put a bit of carb cleaner in the end of the idle circuit, covered up the air screw hole, and blew air through it, just to make sure they were clear. each produced a nice spray from it's corresponding slow jet...
There are at least 4 holes from the atmosphere into the slow circuits in the bare body of these carbs; Air jet, slow jet, carb bore exit, and when the idle mixture screw is removed, the screw hole, too. When you pressurize that passageway, each one of these exits should be verified flowing.
The slow jet has a small tube as part of it inside the carb body with rather small cross drilled holes. This forms a small emulsion tube where the fuel is premixed with air from the air jet. Between, the pilot jet and the carb bore exit is the I.M.S. (Idle Mixture Screw) which meters the fuel emulsion from the slow jet.
These small emusion tube holes are very difficult to clear effectively without removing the slow jets from the carb bodies. They are pressed in, but can be removed (yank them straight out), cleaned, and tapped back into place. If you are considering abandoning these carbs in favor of others, what have you got to lose by yanking the jets?
but the tip of one of the idle srews broke off in the throttle body when i was resetting them. now i'm stuck either finding a replacement idle adjustment screw (hard for these years) or buying a rebuild kit for a '76 set of carbs i have, and changing the intake manifold out to match them. dammit!!! anyway, that other set should be easier to rejet, as the slow jet is removable and therefore replaceable.
A broken IMS is indeed bad news. I fear the valve seat was also damaged when these were way over-tightened. Distorted seats make equal settings across the carb bank difficult to achieve. If you think the seats are salvageable, I can send you a used IMS from my junker set.
thanks for the advice, two tired. i have a pretty decent grasp on the workings of a carberator, but have had little experience with the slide carbs in the sohc 4's. (i'm more familiar with the CV carbs on a lot of the twins) rejetting is another story.
This is why I alerted you to the throttle position aspect wrt jetting.
another question, however...
do i have to separate the carbs to pull the slides out and change the notch setting on the needles? the '77 carbs look like it may be possible without doing so, whereas the '76 set looks like a doozy to deal with. even with rejetting, i'll likely have to raise the needle up a bit.
No, I think it is unwise to separate the carbs from the bank. But, you will likely have to remove the set from the bike. The needles exit the carbs with the slides through the top of the carbs. Remove two screws from the carb tops. Loosen the screw on the arm at the actuating bar. Then a No. 1 phillips to remove the screws deep in the slides (slides lowered). The actuating arm then will swing up out of the way to gain access to the slides and needles. You will have to synchronize the carbs after putting it back together.
also, two tired,
i read a previous post, where you discussed the engineering of honda vs. hacks trying to improve something that was pretty damn good to begin with. i agree with you, really, but from what i understand, essentially the same carbs were "detuned" through the years to satisfy emissions standards. for example, the '77-'78 carbs using a 90 main instead of a 100. you seem to know a lot about these carbs, so i suppose i'm telling you something you know very well. in this day and age, i don't really want to decrease my MPG, but i wouldn't mind a slight power increase. either way, i'm breathing life into a really trashed bike, by doing a complete tear-down and rebuild from a freshly powder coated frame up.
Actually, I think the 77-78 carbs were highly tuned for the bike in the configuration as sold to the public. The polution control laws for motorcycles lagged behind those for the auto industry. They weren't really on the books yet in 77-78. However, the lean burn carbs WERE common in the auto industry. Honda began creeping up on what was to become polution laws applied to motorcycles by "evolving" the slide carbs. This was abandoned in favor of the CV carbs by the time polution laws actually took effect for motorcycles. In parrallel with the anti air polution frenzy was also the anti noise polution effort, though to a lesser extent. The stock exhaust for 77-78 CB550 was quieter than previous models. And, a bit more restrictive, for a bit less power at the top end.
I believe the 90 main was selected for these carbs to:
Lean up the high RPM range over previous years.
To match up the reduced main air jet size from #150 to #130.
To compensate the emulsion tube hole size and placement over that of previous years.
And, the restrictive exhuast made a bit less power at the high RPM where the main jet is dominant in the fuel equation.
The earlier carbs were actually sloppy in tuning by comparision. Which makes them easier for the home mechanic to deal with when other bits of bike are modified. However, both models had 22mm throat bores. And, with proper adjustments can be made to run equally well, regardless of what's been done to exhaust and intake changes. I've no doubt the newer carbs can be made just as sloppy and over rich as the older ones, though, just by tweaking the right (wrong?) bits internally.
This link and the attatched chart may help you tweak the appropriate bits in the carbs to achieve your goals. (However, the pics don't work for me anymore. I have old computer equipment/software.)
http://www.motorcyclecarbs.com/jetting_1_your_keihin_W46.cfmCheers,
