1978 kz750 b3 engine bog at full throttle

[onkelheri]

Maybe the fuel hose is sqeezed?

All that Filterstuff above is ok if you just open the bottom of the floatchamber and the fuel "runs"(!) out there. 

The jetsize should growup apprx. 10 to 15% without an Airbox. The Airjet also had an influence of the Gasbuilding in the Brassflute, if they too less you'l drain the fire.

Heri

[Michael]

Hi Jon,

sorry, haven't watched the linked videos yet.

With slip-ons you mean the exhaust pipes, correct ?
If they are fully open you need to use bigger main jets.

Heri suspected you have mounted funnels (Trichter) or something like K+N-filters.
These also need bigger main jets to run properly.

Both combined together you need even bigger main jets ;-)

As far as I've understood everything in the low and mid rpm-range seems to work, just the top end revving is missing.
Go for bigger main jets !

The Bs have standard main jet #125.
Often plus 10% (area, not diameter!) is working good for mildly tuned engines.
(either K+Ns or open exhaust)

Let me calculate:
standard main jet:
125 / 2 = r = 62,5
a = r² x Pi (3,1415926...) = 12272

10% bigger main jet:
a = 12272 + 10% = 13500
13500 / Pi  => square root = r = 65,5
=> new main jet (+10%) is #131

To be on the safe side choose the next bigger one:
#132,5 available ?
Otherwise #135

Not the best fuel economy, maybe not best performance, but it should give the ability to run the engine even at highest rpms (red line at 7.800 rpm !)

PS: tank ventilation is tested automatically as long as the "fuel run-out test" was done with closed tank cap.
Or did you open it for that test ?

Good luck,
Michael

[onkelheri]

Did you ever adjust the idlemixscrew? Or they US-typical sealed by brasscap? Open them by taping a 4mm threat...

Greetz heri

[jon707]

Hi Jon,

sorry, haven't watched the linked videos yet.

With slip-ons you mean the exhaust pipes, correct ?
If they are fully open you need to use bigger main jets.

Heri suspected you have mounted funnels (Trichter) or something like K+N-filters.
These also need bigger main jets to run properly.

Both combined together you need even bigger main jets ;-)

As far as I've understood everything in the low and mid rpm-range seems to work, just the top end revving is missing.
Go for bigger main jets !

The Bs have standard main jet #125.
Often plus 10% (area, not diameter!) is working good for mildly tuned engines.
(either K+Ns or open exhaust)

Let me calculate:
standard main jet:
125 / 2 = r = 62,5
a = r² x Pi (3,1415926...) = 12272

10% bigger main jet:
a = 12272 + 10% = 13500
13500 / Pi  => square root = r = 65,5
=> new main jet (+10%) is #131

To be on the safe side choose the next bigger one:
#132,5 available ?
Otherwise #135

Not the best fuel economy, maybe not best performance, but it should give the ability to run the engine even at highest rpms (red line at 7.800 rpm !)

PS: tank ventilation is tested automatically as long as the "fuel run-out test" was done with closed tank cap.
Or did you open it for that test ?

Good luck,
Michael

Hi Michael, Thank you very much for the detailed information. I much appreciate the time you took to do that!

Yes the mufflers are what I meant by slip ons....I did not fool with the stock airbox. So I will take your advice and go with the 132.5 if I can find one.

I will hunt for the jets and order them. When I am done with the job I will let you know the results.

Thank you again Michael  : )

[jon707]

Did you ever adjust the idlemixscrew?  Or they US-typical sealed by brasscap? Open them by taping a 4mm threat...

Greetz heri

Hi onkelheri, Yes they are sealed with the brass cap. If the jet change does not work I may try your advice with the 4mm threads
Thank you
jon

[onkelheri]

It would'nd work without this adjustment ...

You'll need a "flatend" Tappingdrill for Blindendholes...

BSsealsonicht.jpg

this shows what happen if you do it wrong...


Drill carefull with a 2mm then 3,2mm, be prpared that you reach the "cave" above the Idlescrew, ... but then take care that the Tapper hat a straigth, flat front(!) because if it reached the Idlescrew it will start to lift the brasscap. In other, topend,  case the idlescrewslot will destroyed.

Screenshot_2017-07-10-01-19-57kl.jpg

Screenshot_2017-07-10-01-19-26kl.jpg

Screenshot_2017-07-10-01-19-50kl.jpg

Screenshot_2017-07-10-01-19-35kl.jpg

Greetz Heri