Topic: 78 750 Carb Tuning

[BobbyR]

See I was right. Now i know why. 

[brewsky]

What I meant to say is that after 5 years the levels had changed. I surmise that the tabs slowly bent over the years.

The change (like the Devil) is in the details: over the years, the soft brass tab on the floats dimples slightly, plus the face of the soft brass valve seat settles slightly, all from the vibration and shocks of riding. If you look REAL close at the little tabs where they touch the float valves, you'll see a tiny dent, and on the brass valve seat, a small, flattened ring where the point wears it (except on those with rubber-tipped float valves, where instead the rubber shrinks from today's ethanol).

When I refurbish these floats, I file off the tiny dimple because it can make the floats stick in the fully-open position when the bowls go from fully empty to petcock ON mode. Polish well afterward.

Wow!....I continue to learn something new every day....

I've been polishing the inside walls and seat of the float valve seat, along with the sides of the float valve.

Never noticed the tab contact being dished, just shiny, but guess I need to look closer.

Do you hammer the dimple flat from the back side first, or just file the dished side?

[HondaMan]

What I meant to say is that after 5 years the levels had changed. I surmise that the tabs slowly bent over the years.

The change (like the Devil) is in the details: over the years, the soft brass tab on the floats dimples slightly, plus the face of the soft brass valve seat settles slightly, all from the vibration and shocks of riding. If you look REAL close at the little tabs where they touch the float valves, you'll see a tiny dent, and on the brass valve seat, a small, flattened ring where the point wears it (except on those with rubber-tipped float valves, where instead the rubber shrinks from today's ethanol).

When I refurbish these floats, I file off the tiny dimple because it can make the floats stick in the fully-open position when the bowls go from fully empty to petcock ON mode. Polish well afterward.

Wow!....I continue to learn something new every day....

I've been polishing the inside walls and seat of the float valve seat, along with the sides of the float valve.

Never noticed the tab contact being dished, just shiny, but guess I need to look closer.

Do you hammer the dimple flat from the back side first, or just file the dished side?

It's a tiny "dish", a little bump the size of the tip of the float valve pin. You'll see it if it is there (under good light!). I file them flat with a jeweler's file, then polish with 400-600 grit paper.

[brewsky]

What I meant to say is that after 5 years the levels had changed. I surmise that the tabs slowly bent over the years.

The change (like the Devil) is in the details: over the years, the soft brass tab on the floats dimples slightly, plus the face of the soft brass valve seat settles slightly, all from the vibration and shocks of riding. If you look REAL close at the little tabs where they touch the float valves, you'll see a tiny dent, and on the brass valve seat, a small, flattened ring where the point wears it (except on those with rubber-tipped float valves, where instead the rubber shrinks from today's ethanol).

When I refurbish these floats, I file off the tiny dimple because it can make the floats stick in the fully-open position when the bowls go from fully empty to petcock ON mode. Polish well afterward.

Wow!....I continue to learn something new every day....

I've been polishing the inside walls and seat of the float valve seat, along with the sides of the float valve.

Never noticed the tab contact being dished, just shiny, but guess I need to look closer.

Do you hammer the dimple flat from the back side first, or just file the dished side?

It's a tiny "dish", a little bump the size of the tip of the float valve pin. You'll see it if it is there (under good light!). I file them flat with a jeweler's file, then polish with 400-600 grit paper.

Thanks, HM......I'll look a little closer.

Haven't had any issues with my 750 yet, but my 305 has.......maybe because the carb is more angled?

[savage]

Thanks for all of the knowledge and images, I am learning alot!

I know have my float bowl levels consistent and at the appropriate height. Unfortunately the bike is still running crappy. It wants to die if you dont give it more 20% or more throttle and wont idle.  See for your self below.

1978 Honda Cb750 K running poor

• I have tried the mixture screw at 1, 1.5, and 2 turns out -  it didnt seem to help much
• The idle adjustment didnt seem to help much. 
• I checked the spark plugs - they seemed black and one was a touch wet.
• I checked the valve clearances - they all were correct.

Does any one have a suggestion of what I should check or adjust next?

[HondaMan]

What I meant to say is that after 5 years the levels had changed. I surmise that the tabs slowly bent over the years.

The change (like the Devil) is in the details: over the years, the soft brass tab on the floats dimples slightly, plus the face of the soft brass valve seat settles slightly, all from the vibration and shocks of riding. If you look REAL close at the little tabs where they touch the float valves, you'll see a tiny dent, and on the brass valve seat, a small, flattened ring where the point wears it (except on those with rubber-tipped float valves, where instead the rubber shrinks from today's ethanol).

When I refurbish these floats, I file off the tiny dimple because it can make the floats stick in the fully-open position when the bowls go from fully empty to petcock ON mode. Polish well afterward.

Wow!....I continue to learn something new every day....

I've been polishing the inside walls and seat of the float valve seat, along with the sides of the float valve.

Never noticed the tab contact being dished, just shiny, but guess I need to look closer.

Do you hammer the dimple flat from the back side first, or just file the dished side?

It's a tiny "dish", a little bump the size of the tip of the float valve pin. You'll see it if it is there (under good light!). I file them flat with a jeweler's file, then polish with 400-600 grit paper.

Thanks, HM......I'll look a little closer.

Haven't had any issues with my 750 yet, but my 305 has.......maybe because the carb is more angled?

Funny you would bring that up: I learned about it from MY Superhawk!

[brewsky]

Savage,
Sounds like your slow jets or passages are still clogged up.

Did you remove the pressed in jets and clean them and blow thru the small holes in the inlet end of the carb?

[savage]

I did not remove any pressed in Jets, do you need a machine to that? or a press to put them back in?

[bollingball]

savage all you need is a pliers set and something to wrap around the jet to prevent damage twist and pull. once clean tap gently back in place this has to be done or it will never idle.

[brewsky]

Ditto....It's a little scary at first, but they will come out without damage.

Cut a small piece of shoe leather to wrap around the jet and grab it with vice grips.

Kinda like pulling a tooth.....twist,  rock, and pull ....but start out gentle.

Soak in PB Blaster first.

[savage]

I have removed my slow jets, cleaned them out and re-installed them. Fired the bike up and it still wont idle very well. Adjusting the mixture screws didnt seem to help and the float levels are at the recommended height.

I checked my points they were a bit off so I adjusted them to recommended .12-.16. Once i did that I had to spin the base plate all the way to one side to line up the F mark. The F mark is still about 3mm from the line and I have no adjustment left. My dwell angle is about 140 degrees, much larger than the recommended 92-98 degrees.

Does the point adjustment have anything to do with lack of idling?

If so, do I need a new point assembly to get it back in adjustment?

Or is there something else I should be doing with the carbs?

[HondaMan]

I have removed my slow jets, cleaned them out and re-installed them. Fired the bike up and it still wont idle very well. Adjusting the mixture screws didnt seem to help and the float levels are at the recommended height.

I checked my points they were a bit off so I adjusted them to recommended .12-.16. Once i did that I had to spin the base plate all the way to one side to line up the F mark. The F mark is still about 3mm from the line and I have no adjustment left. My dwell angle is about 140 degrees, much larger than the recommended 92-98 degrees.

Does the point adjustment have anything to do with lack of idling?

If so, do I need a new point assembly to get it back in adjustment?

Or is there something else I should be doing with the carbs?

Ah, the plot darkens....
The timing symptoms you are referencing here sound a LOT like "Daiichi points syndrome". Explanation: For the last several years, the Chinese arm of Daiichi has been making points and points plates (real cheap!) that essentially are made wrong. Their geometry between the foot and the pivot is off by about 1mm distance, so they cannot be correctly timed with the nominal .012" to 0.016" gap, period. Nothing can make them time up correctly, except trial-and-error rebending of the arm (not recommended unless you're REAL familiar with points). To make matters worse, some Honda shops have been selling them as [pirated] Honda components, where real Honda parts are made by TEC (the best) or FEW (not bad...).

If the timing, with gaps of .012" to 0.016", is not on the "F" mark at 1250 RPM or less, and does not reach in between the 2 hash marks of advance at 2500 RPM or more, the engine will feel sluggish on acceleration and will not idle for beans. This is not a carb issue, but a timing one.

There are now (or at least, WERE) Japanese Daiichi points (from the tsunami area of Daiichi province) that work better, and lately we have seen these slowly replacing the Chinese ones. Still, the best thing you can do is to get a set of TEC points and plate from Honda (pricey, but effective). If you combine that with one of my Ignitions, it will likely last for the next 5-7 years with no further adjustment (maybe more: that's the furthest testing we've had so far with great results).

[savage]

Looks like I have the TEC points. I assume they are the original points. And I assume since they are old, they could be wore out and I should get new ones?

Looks like I can get a points assembly (plate and points 623594) from BikeBandit for $77.83, is that what I would need? Not sure if its TEC.

[savage]

I have also found these

http://www.oldbikebarn.com/CB500-CB550-CB750-Ignition-Tune-Up-kit?sc=12&category=52302

http://www.cyclexchange.net/index%20pics/WEB%20points%20cover.JPG

Not sure what country they are from

[bollingball]

IMHO I would go to the Honda shop mine carries the TEC it will be stamped on the points those plates you are looking at are thin and crap. If your plate is the original keep it and get the points and condenser from Honda. Some other stores carry the (TEC) brand you may save money that way but I would verify first. I got a set from DSS a while back they were TEC but with shipping I should have gone to the Honda shop.

[savage]

I just contacted my local Honda shop. They are saying the points they can get from Honda are TOYO. Is this any relation to TEC? Are they as good?