Topic: Lack of power on the top

[HondaMan]

Maybe also try this: get some NGK DPR8EA9 sparkplugs. The tips are extended and the resistor will stretch out the spark a little bit more. The Pamco ignition, like the Dyna S, shortens the spark duration above 6000 RPM. Adding some resistance to the sparkplug's circuit will extend the spark a little.

I swapped advancer with one from the K0, it helped a little bit.  Installing DPR8EA9 helped little bit too, but still no cigar.   

I am afraid I will have to find a set of PD42A.

Should I go to HondaMan ignition?  What do you think, Mark? 

Yeah, but then you'd have to go back to points! 
(I still like 'em, myself...)

I'm kicking myself for not thinking of this earlier: you have the F2/3 head with roundtop carbs: what hoses did you use to connect them up? If they by chance happen to be the new ones (from either Tourmaster or Honda OEM) and you don't also have the Oetiker hose clamps (#050-9, Aircraft Spruce) on the head side of those hoses, you likely have a vacuum leak situation going on... The modern hoses are about 1.5mm smaller diameter when on the head (thinner rubber) and the OEM hose clamps will NOT seal them very well, or at all. I have also had to use either brand-new OEM clamps at the carb's end of the hoses, or else re-flatten the worn clamp stops and add a trimmed flat washer to the screw heads of the clamps, on the carb side, to get them to seal 100%.

[70CB750]

I would go back to points to get the power

I replaced carb hoses not that long ago, I think I had early K before and went to reproduction Fs.  I used Euro hose clamps, but I will check for leaks when I get home.

Thank you.

[HondaMan]

I would go back to points to get the power

I replaced carb hoses not that long ago, I think I had early K before and went to reproduction Fs.  I used Euro hose clamps, but I will check for leaks when I get home.

Thank you.

While there are some clamps like these that fit in the hose's grooves, the ones that do not will also not seal at the head. I discovered that on an F2 that came here about 7-8 years ago with burned exhaust valves, caused by lean-burn mixtures at American West interstate speeds.

[70CB750]

I checked and there are no leaks - checked with starting fluid

But I improved it little bit by advancing the timing plate.  I can now rev up to 5800 in 5th gear on flat road.  I know, nothing to write home about, but in 4th gear it gives me passing power. 

[PeWe]

My K2 with K6 carbs, stock airbox, Yamiya no numbers 4-4 needed needles at 4:th notch from top. I tried middle setting to start with at 3:rd notch.
Huge difference going 1 step richer.
Mains 115, pilots 40.

[70CB750]

Thank you, already tried it. One notch richer made it worse.  Now there is like a little valley to ease over and after that the power  comes back somewhat.

[70CB750]

On my jeep after going TBI fuel injection and HEI ignition I would advance the timing till it started to knock and back it till it stopped knocking.

Is it advisable/safe to do it on CB750?

[PeWe]

I did that back in the 80's on my CB750 K6. Knocking heard in 5:th gear when twisting at 4000 rpm...knocking heard around 5000 rpm. I have found that stock marks work best, F at idle and hit full advance not before 2500 rpm.

[70CB750]

Maybe also try this: get some NGK DPR8EA9 sparkplugs. The tips are extended and the resistor will stretch out the spark a little bit more. The Pamco ignition, like the Dyna S, shortens the spark duration above 6000 RPM. Adding some resistance to the sparkplug's circuit will extend the spark a little.

I am still working on PD carbs to try but  there is another thing,  I advanced the cam shaft 4.5 degrees, to move the power peak to lower rpms.  Could it be the reason for lack of power on top?

[bryanj]

Simple andwer Yes, Honda spent a fortune getting it as good all round as posible so to make one bit better another usually gets worse

[70CB750]

It was the carburetor rack after all.

I finally installed PD carburetors and she rocks.  Revs easily to 8 000 in third and fourth, did not take her all the way, there was not enough road.   

I replaced 35 idle jets with 42s and kept 110 mains.   The choke is great, I can set it up and she will idle and warm up, with 657s I had to stay with her to keep her running.   

Overall I wish I done it sooner, she was not meant to run with 657s.

[dusterdude]

70,pm sent

Sent from my LM-X420 using Tapatalk

[HondaMan]

It was the carburetor rack after all.

I finally installed PD carburetors and she rocks.  Revs easily to 8 000 in third and fourth, did not take her all the way, there was not enough road.   

I replaced 35 idle jets with 42s and kept 110 mains.   The choke is great, I can set it up and she will idle and warm up, with 657s I had to stay with her to keep her running.   

Overall I wish I done it sooner, she was not meant to run with 657s.

You're correct in that, given the hi-RPM cam of the F2/3 engines. The roundtop carbs run out of mixing ability above 3/4 throttle or so: they tend to get lean. The PD carbs have a thick restriction right across their middle (throttle plates) that split the incoming airflow and lower the pressure more in the bottom half than in the top, which then lets the bowl push more fuel upward into the throats.

The late airbox is better at making still air for those throttle plates than was the earlier airbox, but they won't fit the roundtop carbs. The closest we can get there is to use the K4/5/6 airboxes with those carbs. That improves top end, too.

[seanbarney41]

My F3 always ran great with the roundtops...I still have not managed to get a set of pd's to even work right.

[PeWe]

The problem here was the needles in a VERY lean position right?
4th clip from top should have fixed it.
If that will cause rich after 1/2 throttle or more, reduce main jets.

[70CB750]

My F3 always ran great with the roundtops.

And good for you 

[70CB750]

The problem here was the needles in a VERY lean position right?
4th clip from top should have fixed it.
If that will cause rich after 1/2 throttle or more, reduce main jets.

I done many combinations of jets and needles and nothing worked, I will stick with PDs.

[70CB750]

It was the carburetor rack after all.

I finally installed PD carburetors and she rocks. 

You're correct in that, given the hi-RPM cam of the F2/3 engines. The roundtop carbs run out of mixing ability above 3/4 throttle or so: they tend to get lean. The PD carbs have a thick restriction right across their middle (throttle plates) that split the incoming airflow and lower the pressure more in the bottom half than in the top, which then lets the bowl push more fuel upward into the throats.

The late airbox is better at making still air for those throttle plates than was the earlier airbox, but they won't fit the roundtop carbs. The closest we can get there is to use the K4/5/6 airboxes with those carbs. That improves top end, too.

The latest round top jet/needle combination  felt like it was  too rich above 5 000 rpms.   That and also the mileage went down if I kept higher speed for longer time.  I felt the round tops would deliver the fuel but not enough air due to size of the intake horn.

Also the cam is advanced 4.5 degrees from the factory position, it might made a difference.

[PeWe]

The problem here was the needles in a VERY lean position right?
4th clip from top should have fixed it.
If that will cause rich after 1/2 throttle or more, reduce main jets.

I done many combinations of jets and needles and nothing worked, I will stick with PDs.

Needles at leanest or  ext to leanest position will not work with any other jets except for very rich needle jets (emulsifier tubes).
I write this to clear that your earlier carbs are working, not selling them cheap in the believe they are defective.

[70CB750]

The problem here was the needles in a VERY lean position right?
4th clip from top should have fixed it.
If that will cause rich after 1/2 throttle or more, reduce main jets.

I done many combinations of jets and needles and nothing worked, I will stick with PDs.

Needles at leanest or  ext to leanest position will not work with any other jets except for very rich needle jets (emulsifier tubes).
I write this to clear that your earlier carbs are working, not selling them cheap in the believe they are defective.

I know they are not defective, they just dont work with my engine and I am ok with that.