Author Topic: Lack of power on the top  (Read 6379 times)

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Offline 70CB750

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Lack of power on the top
« on: October 31, 2018, 04:52:48 PM »
Red has 78SS engine (750)and 17/48 sprockets.

She pulls like a freight train from bottom up, i can rev her up to 7-8000 no problem.

But on the express way, going - say - 85 in 5th if i try to accelerate, there is a hole and thats that.  I twist and she boggs down.

I was going to raise needles a notch and also check the air filter, any other ideas?
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Offline 70CB750

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Re: Lack of power on the top
« Reply #1 on: October 31, 2018, 04:58:14 PM »
I forgot to mention, she is running old style carburetors - like 1973 K or so - not 78SS carbs. I believe the original carbs had an accelerator pump, maybe thats the problem?

Edit: she has 657 carbs while the engine originally used PD42A
« Last Edit: October 31, 2018, 05:01:03 PM by 70CB750 »
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Offline innovativems

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Re: Lack of power on the top
« Reply #2 on: October 31, 2018, 06:57:50 PM »
If you lightly apply the choke is it better?  What mains are you running?


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Offline 70CB750

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Re: Lack of power on the top
« Reply #3 on: November 02, 2018, 02:27:16 AM »
Seems there was a rollback. Flybox's comment is missing :)

Jets are 40 and 125, needles at first notch.   I started with 120s and worked my way up notch by notch and when the 5th was inadequate, I installed 125s.


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Offline innovativems

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Re: Lack of power on the top
« Reply #4 on: November 02, 2018, 07:51:46 AM »
Does the hole feel better if you slightly close the choke?


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Offline 70CB750

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Re: Lack of power on the top
« Reply #5 on: November 02, 2018, 08:51:31 AM »
Does the hole feel better if you slightly close the choke?


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Didnt get to try it yet, but definitely will.
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Offline PeWe

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Re: Lack of power on the top
« Reply #6 on: November 02, 2018, 09:42:43 AM »
Still riding season?
"needles at first notch"  - from top?
If you use pod filters, main jet need to be bigger.
My K6 with stock carbs, pods, 836cc, ported head and 4-1.
Ran fine with 40 pilots,
needles one step richer than stock, 4:th notch from top. Stock is 3:5 (middle notch)
Main jets really huge 140 something
« Last Edit: November 02, 2018, 09:44:47 AM by PeWe »
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline 70CB750

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Re: Lack of power on the top
« Reply #7 on: November 02, 2018, 09:54:43 AM »
Yes, the most limiting position of the needle.
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Offline PeWe

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Re: Lack of power on the top
« Reply #8 on: November 02, 2018, 10:15:42 AM »
That must be too lean.
Stock airbox with stock style filter: middle notch
Pods: 1 step richer
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline flybox1

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Re: Lack of power on the top
« Reply #9 on: November 02, 2018, 10:44:09 AM »
Does the hole feel better if you slightly close the choke?


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Didnt get to try it yet, but definitely will.
This, but you'll have to do something before hand, to determine if needles need to be raised, or main jet increased.
With white electrical tape, make a baseline mark on your throttle housing.  twist your throttle 1/2 its full travel(needle), and make a corresponding tape line on your grip.  do the same for 3/4 throttle(mainjet).
If you ride, and get in that not-so-sweet bog action, put on a little choke.  If the bog goes away... check your throttle grip and see which line you are closest to.
That's what you need to adjust richer.
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Offline TwoTired

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Re: Lack of power on the top
« Reply #10 on: November 02, 2018, 11:57:13 AM »
Do a plug chop when the motor hits that "hole".  If you can, hold operation in that "hole" for as long as you can.  The deposits will tell you if too lean or too rich.   Mark the throttle so you know which fuel metering system is dominant in the throttle position selected.

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Re: Lack of power on the top
« Reply #11 on: November 02, 2018, 06:07:57 PM »
The round-top carbs that you are using now are not ideally suited to the cam you have in the K7/8 engines.

The round-top carbs go slightly lean between 45% and 55% of throttle: you can see this zone by looking down their throat - you will see the wasp-waist in the middle, which was intended to increase the lift on the needle jet at that range to bump up the torque a little in midrange before the cam comes on at 5500 RPM.

The K7/8 has a cam that comes on around 6200 RPM instead of the earlier 5000 RPM of the earlier engine. The PD carbs, while leaner overall, were made richer in the 4000-6000 RPM range to boost its torque there.

When the roundtop carbs are used with the later cams, there is a lean spot between 6000-7000 RPM because of this difference, and the roundtops essentially do not work well above 7500 RPM, in order to protect Honda's warranty department. ;)
This stacks up to a re-tune-needed situation for use in your arrangement. The floats need to be set deeper so as to increase the lifted fuel above 6000 RPM, and the holes in the emulsifiers need to be opened up somewhat to accommodate the increase. I'd recommend drilling out the upper 8 emulsifier holes to about 0.039" (they are about 0.035" or 0.0375" now, depending on which ones you have), and increase the mainjet from the stock #105 (K3) to about #120 or #125. Make the float bowls 24mm deep. If there is a stubborn flat spot at about 7000 RPM, then open up the lower 4 smaller emulsifier holes from their stock 0.0245" to about 0.028" or even 0.030" as the last resort.

When these carbs are run at full speed like this, they MUST have a steady air flow into their float bowls. This is why the expensive bowl vent parts were installed: on the K0 bikes there were originally 4 individual (expensive) vent elbows and 4 long vent tubes with clear instructions to route them '...well up under the seat for quiet air..." to pressurize the float bowls above the moving air around them. This will push more fuel up in the jets by pressurizing the float bowls more. On the later bikes the top end was sacrificed for less expense and the 2-hose (and later, 1-hose) vent systems were developed. After a while, even those hoses got shorter, mostly because in the USA, Honda's biggest CB750 market, the 55 MPH speed limit caused no noticeable issues for the shorter hoses.

Roadracers knew this, and went to a lot of trouble to solve the float bowl pressure problems at speeds over 100 MPH. The full-fairing of the Vetter Phantom and the [later] Vetter Windjammer with Lowers re-proved the concept on the street, too. On roadracers they made small containers (some with beer cans!) with one or two large holes in locations where only still air could reach them (like way up under the seat) and the vent hoses were fed from there. On the street, I drilled 2 holes in the lower half of the [stock] airbox, as shown in my book, away from the inlet vent(s) and poked the vent hoses in them. At (illegal for 1980s) speeds, this much improved the throttle response, especially in the upper gears. More recently, after my 2013 rebuild that has generated much more hi-end torque, I have found this really helps when I use very tall gears, like 19T countersprocket and 120/90x18 rear tire combo, pushing my Vetter. While this makes 5th gear feel like overdrive that should not be used below 62 MPH, it really improves the freeway passing time in the upper gears. :D
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Offline 70CB750

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Re: Lack of power on the top
« Reply #12 on: November 03, 2018, 06:08:52 AM »
Thank you, Mark!
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Offline 70CB750

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Re: Lack of power on the top
« Reply #13 on: March 01, 2019, 05:54:00 AM »
I did drill  emulsifier holes as recommended and added tubes to bowl vents.  Also set floaters to 22mm.

Yesterday, I was finally able to take her for a spin.  She seems to be more responsive but some of the power drop is still there.

I will try to raise the needle one notch.
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Offline scottly

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Re: Lack of power on the top
« Reply #14 on: March 01, 2019, 08:43:50 PM »
Red has 78SS engine (750)and 17/48 sprockets.

She pulls like a freight train from bottom up, i can rev her up to 7-8000 no problem.

But on the express way, going - say - 85 in 5th if i try to accelerate, there is a hole and thats that.  I twist and she boggs down.

So does the problem only show up after maintaining 85 MPH for a distance? What is the RPM when it happens? My first thought is that your gas cap vent is partially blocked; it will allow enough fuel to flow for short bursts to 8000, but not enough for a steady 85 MPH?
Don't fix it if it ain't broke!
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Offline 70CB750

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Re: Lack of power on the top
« Reply #15 on: March 02, 2019, 03:54:46 AM »
That would be funny, if it was just a tank cap, wouldnt it?  I will check.

Local little roads did not let me keep it at 80,  more testing needs to be done. But generally you can build the speed up slowly, if you open too fast it seems to be starving for fuel.

Before the emulisifiers it was happening under 6000, now it is definitelly higher.

Mark thinks it is because of my old K carburetor set on 78SS engine.
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Offline seanbarney41

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Re: Lack of power on the top
« Reply #16 on: March 02, 2019, 05:51:28 AM »
I think you may be compensating for an incorrect needle position with too much main.  I have F3 engine with roundtop carbs (7a).  It ran best with 40 slow 110 main needle in middle slot air screw 1 turn out stock air box with K&N, and a fairly wide open 4-1 megaphone...no lack of power at any throttle position, had it to 105mph with more left.
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Offline 70CB750

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Re: Lack of power on the top
« Reply #17 on: March 02, 2019, 06:20:41 AM »
I started with 120 mains and went up notch by notch till I ran out of notches.  Even in the highest position it would fall on the face with the twist of the throttle.

Than I got 125 mains and in top needle position it felt much, much better except for the upper hole of course.  I lived with it for a while but business trips to Baltimore make me ride few miles on US 70 and I need more power for that.
Prokop
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Offline seanbarney41

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Re: Lack of power on the top
« Reply #18 on: March 02, 2019, 12:06:52 PM »
Have you checked float bowl fuel level with clear tube?  Double checked ignition timing and advancer operation?
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Offline 70CB750

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Re: Lack of power on the top
« Reply #19 on: March 02, 2019, 04:13:21 PM »
Have you checked float bowl fuel level with clear tube?  Double checked ignition timing and advancer operation?

Changed flow level to 22mm to Mark's suggestion.  As far as ignition, I run Pamco, there is no need to change anything.
« Last Edit: March 02, 2019, 04:15:55 PM by 70CB750 »
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Offline seanbarney41

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Re: Lack of power on the top
« Reply #20 on: March 02, 2019, 04:58:17 PM »
Have you checked float bowl fuel level with clear tube?  Double checked ignition timing and advancer operation?

Changed flow level to 22mm to Mark's suggestion.  As far as ignition, I run Pamco, there is no need to change anything.
that is all fine and dandy but you need to see where the fuel level is in relation to the jets.  Measuring the floats does not necessarily get you there.  Do a clear tube test.  And pamco certainly does not guarantee to be free of problems, especially if the advanced is not working.  You can mess with carbs for years, and it will never run right, if you have spark problems.
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Offline 70CB750

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Re: Lack of power on the top
« Reply #21 on: March 03, 2019, 05:00:37 AM »
I do not have spark problems, but thank you for your ideas.
Prokop
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Re: Lack of power on the top
« Reply #22 on: March 03, 2019, 02:26:32 PM »
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.
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline 70CB750

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Re: Lack of power on the top
« Reply #23 on: July 10, 2019, 05:35:43 AM »
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?  ;D ;D
« Last Edit: July 10, 2019, 05:38:09 AM by 70CB750 »
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Offline PeWe

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Re: Lack of power on the top
« Reply #24 on: July 10, 2019, 11:06:57 AM »
Advancer work as it should?

 I noticed that mine had really sloppy springs when starting it 2014 after the first restore since the 80's.
That time with Pamco ignition. Full advance kicked in way too early. Found the cure on this forum on a thread where Hondaman recommended to cut the springs 1/2 wound and reshape that end.
That fixed it. F line up nice at idle +1200 rpm, full advance at 2600-2700 rpm. That engine has bigger bore, hotter cam, ported head

My last build, CB750 K2 (stock) had  issues to get F to line up at idle. A little bit advanced despite largest point gap and latest  plate position. I did the springs on that advancer too that sorted it.

Both my bikes have TEC points, plate and Hondaman ignition. First 250 km stock only but with Dyna 5 ohm coils. Now Denso X24ES- U plugs that make them to run smoother than NGK D8EA, NGK 5kohm caps, copper leads. No suppressed graphite leads.
The K2 runs without condensers since it had some bubbling sound as the carbs were lean.

My K6 has condensers, it could get some occasional misfires after 8000 rpm without them.

My K2 runs with stock K6 carbs that got 42 pilots that improved the behaviour. EDIT: Went back to stock Keihin 40 pilots.

It has K1 head without ex guide seals I have started to think leak air into the chambers causing the bubbling sound.
I'll see later this year when it will get a refurbished very fresh K6 head that match its year better, 1975.
« Last Edit: August 11, 2019, 03:41:44 AM by PeWe »
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967