Topic: 78 750 Carb Tuning

[savage]

I just put on my freshly rebuilt Carbs on my 78 750K. The bike fired up relatively quickly but was very bogged down. I needed to give it gas to keep it running, but if I gave it alot of gas it became bogged down.   

What should I adjust first? The air screws?

I have an old Motion Pro mercury manometer to sync with, but i think the bike needs to be running more reliably before I sync, correct?

[bender01]

Choke cable hitched up properly? Slides all set as best you can to #2 slide? (bench synced?) Idle knob set to low? Was it running well before?  Do you have enough gas in the tank for it to get fuel? Just trying to help. Happy wrenching.

[Spanner 1]

Did you verify all 4 accelerator pump nozzles are squirting when you twist the throttle ?? If they are not squirting = total bog down above idle when you give it throttle .

[HondaMan]

Do you have the air screws set between 1.5 and 2.5 turns out from stock, and did you replace the teeny-tiny little O-rings up inside those air screw holes? They often crack, leaving very poor idle mixture control.

Here's a common one: if you replaced the float valves in the carbs with Keyster valves, the float bowls are now almost 3mm shallower than with the OEM Keihin valves. This is because the springs inside the Keyster valves are too stiff. If so, try raising the float level from 14.5mm to 12.5mm, even 11.5mm if it doesn't leak around the bowls too much when the bike is on the sidestand for an hour with the fuel tap ON. This will deepen the fuel in the bowls and let the idle jets stop sucking air and start sipping fuel instead.

[brewsky]

This is because the springs inside the Keyster valves are too stiff.

I have notice this also, but thought adjusting the tab so the floats have the correct setting would take care of the slight difference in overall length of the valves.

The Keysters seem to have strong enough springs to stop fuel at the instant the floats make contact with the plunger. Are you saying the weaker spring plungers must be depressed to do so?

I always wondered why there was so much difference in spring pressure.

[bollingball]

Savage: Don't you have PD42b's You can check what the springs are doing with the clear tube method. Easy with these carbs.

                            Ken

[BobbyR]

One important point. The 78 carbs do not have air screws, they have actual mixture screws. When the bikes rolled out of the factory that 1 1/2 turns was probably accurate. 32 years later they may need more tweaking.

I also adjusted the tab that pushes down the actuating rod so I had to twist the throttle less to operate the pump. Improved the throttle response at most speeds. 

[savage]

Thanks for the help guys:

Bender01 - choke is hooked up, carbs are bench synced to #2, I am not sure what the idle knob is set too. Where is that located?  and I have enough gas.

Spanner1 - I did not verify that the accelerator pumps are firing. How would you go about doing that?

HondaMan - I have the air screws(or mixture screws) set to 1.5, I believe I replaced the tiny O-rings but i will double check. I will also check the levels of my float, what is the best way to adjust that?

bollingball - What is a PD45b? and what is the clear tube method, could you be more descriptive?


If I have to take the carbs off again (which I am sure I will), does anyone have a good method for putting them back on? It seems to so tight that you barely slide the carbs between the engine and frame.

[Spanner 1]

Take off the airbox to see into the carb throats and with the carb bowls full of fuel ( bike does not need to be running ) simply twist the throttle. This action as well as lifting the slides also depressed the acc. pump plunger and forces a spray of fuel out of each 'squirter', which is the little brass post in each carb throat. Thesa posts have a teeny hole facing into the carb and must squirt a goodly stream of gas everytime you twist the throttle from idle position......... Hope their working...  If not squirting, then the bike , especially when cold has a very diffucult time ' catching 'the slow jet's output ( best way I can say it ! ).
The acc. pump works across the rpm and throttle range and is very important to the bikes performance.

[BobbyR]

T

bollingball - What is a PD45b? and what is the clear tube method, could you be more descriptive?

The clear tube method is simple and you do it on the bike.. You buy some clear plastic tubing the same diameter as the overflow tubes, nothing special is needed since they will not have gas in them very long.

You pull the overflow tubes off the bottom of the float bowls. Cut a length of clear tubing about long enough to reach the top of the carb if you loop it around. You put the tube onto the float bowl drain hold the tube up next to the carb and open the drain screw. Fluids seek their own level. When you hold the tube next to the carb the level in the tube and the level in the float bowl will be the same. If the fuel level is just below the top lip of the float bowl, you are good. If too high or low you can turn off the petcock, drain the bowl into a bottle. The float bowl screws can be removed, tricky but possible. Adjust the tab on the float and repeat the process.

[savage]

I performed the clear tube test and my levels were very low. I think the stock measurement is 15mm. But where is the measurement taken from? I attached a pic if someone would like to mark it up.

 I need to make the levels higher, so should i bend the tab towards the bottom of the float or the gas tank?

[bender01]

Towards the bottom of the float bowl. That will raise the float allowing more gas in. I measure and bend a big paper clip in an L shape to use as a guide to (15mm) or whatever it is. its taken from the outside rim of the carb body to the bottom or lowest point on the float.
 Hope thats a help.

[Spanner 1]

Please don't attempt to set your float levels with the carbs upside down like that ( pic. ) will be wrong.!
Carbs must be on their sides with the float pivot uppermost and horizontal with the float tab barely resting against the little valve ' plunger', but not depressing it any. Holding that position measure from the bottom front ( away from the pivot ) edge to the gasket surface, should be 14.5mm for PD 42B carbs ...

[BobbyR]

Please don't attempt to set your float levels with the carbs upside down like that ( pic. ) will be wrong.!
Carbs must be on their sides with the float pivot uppermost and horizontal with the float tab barely resting against the little valve ' plunger', but not depressing it any. Holding that position measure from the bottom front ( away from the pivot ) edge to the gasket surface, should be 14.5mm for PD 42B carbs ...

+1
Download the manual if you don't have one.

[brewsky]

I performed the clear tube test and my levels were very low. I think the stock measurement is 15mm. But where is the measurement taken from? I attached a pic if someone would like to mark it up.

 I need to make the levels higher, so should i bend the tab towards the bottom of the float or the gas tank?

I performed the clear tube test and my levels were very low. I think the stock measurement is 15mm. But where is the measurement taken from? I attached a pic if someone would like to mark it up.

 I need to make the levels higher, so should i bend the tab towards the bottom of the float or the gas tank?

Where did you get the thumbscrew version of the mixture screws?
My 750K8's are slotted and recessed.

[savage]

Thanks I will try it out tonight, I have a Clymer manual but it had an illegible diagram. I will check out the Honda manual.

The mixture screws came with my Keyster rebuild kit. They are slotted as well.

[BobbyR]

I bought a parts carb rack cheap off ebay and there was a set on them on it. I will probably put that in the rack i am using. I am tired of burning my hand.

[brewsky]

+1 on the burned hands!
Idle speed knob is bad enough, but the mix screws need asbestos gloves.
Here's my clear tubes.....after sitting a while with petcock on, and immediately after running the motor a while.....some difference. But #1 & 2 definitely needs checking....must have bumped it changing jets.

[BobbyR]

The far left one seems balls on. I had mine perfect a few years ago when I did my 3rd and most complete cleaning. I had a popping on deceleration. I found one that was quite low. I think after a while the bouncing bends the tabs slowly but surely.

[brewsky]

I think you are right on the correct levels, Bobby, ........I took some measurements to see how they translated to the outside of the carb, and came up with the same thing.
"Disclaimer"....The 5mm dimension and scribed line on the float are only as close as I could measure the fuel level of the submerged float and transcribe it. It could well be a mm or 2 off due to surface tension, light bending thru a liquid or just old eyeballs.

[BobbyR]

The bikes seem to like the fuel level just below the gasket. What I meant to say is that after 5 years the levels had changed. I surmise that the tabs slowly bent over the years. I use the clear tube method as it is real world and the Honda method is subjective in that you have to tilt the carbs the right amount to take a measurement. With the clear tube you are reading the levels as mounted on the bike. 

[bollingball]

Sorry it took so long. +1 on what HM & bobby said The one on the left looks good should be 2-3 mm below gasket I like to build a little holder to work on the bench. You can rotate the carbs any angle you want


           Ken

[the technological J]

also do a 3000 mile tune up before the next sync

[brewsky]

Sorry it took so long. +1 on what HM & bobby said The one on the left looks good should be 2-3 mm bellow gasket I like to build a little holder to work on the bench. You can rotate the carbs any angle you want


           Ken

Good idea!

[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.