Flat spot 65-70mph

[leonardo269]

Hi,
    I have noticed that my K7 has a flat spot at about 65-70mph. If held at a steady 65 it goes into a cycle of surges followed by loss of power. At all other times it runs perfect.

I have got electronic ignition, K&N re-useable air filter and Motad 4 into one exhaust. Everything else is bog standard.
I have a feeling that the carbs are running the original jets.

Has anyone else got a similar set up to this?  

Any advice appeciated.

Cheers,

Ray

[uk_kev]

could this be something to do with the auto advance unit?

[JonnyB]

Does sound like ignition timing at high revs, What rpm are you doing 65 at, and if you slightly over rev 4th do you get similar fades in power.

Got to admit My F is doing something similar at about the same speed, and I have the old points and condensor setup. I've settled for just doing the speed limit

[leonardo269]

Hi,
    The electronic ignition is by Newtronic and it replaces both the points and advance assembly, so there shouldn't be a problem.

I will check the timing again to make sure that it is all set up correctly.

Cheers,

Ray

[uk_kev]

If it is all set correctly then I would think it is the jetting due to the filter change more than the exhaust? I wonder do the sport version with the single pipe use the same jetting as the four pipe models? if not then that may be a starting point.

[JonnyB]

uk_kev wrote:

If it is all set correctly then I would think it is the jetting due to the filter change more than the exhaust? I wonder do the sport version with the single pipe use the same jetting as the four pipe models? if not then that may be a starting point.

That's a good point, I remember before my re build I didn't have this problem, but I did have to change the carbs, from an unknown model, and now I too have a flat spot. Any one know the 750 F1 jet sizes?

[Spitfire]

Hi Ray,
            Does it drive through the flat spot ? It does sound like carbs to me but I am no expert, check the simple things first:
Make sure that the choke is operating correctly, the K7 has a choke cable rather than the simpler mechanical choke of the earlier carbs.
Give it a quick run with the air filter element removed, just leave the airbox empty.
Check your float bowls for dirt, check the main jets are clean, K7 main jet should be 115, slow 35.

The K7 series was similar to the CB750F2 and had the same later carbs with the accelerator pump.

Buy a good manual, David Silver spares sold me a good one for the CB750F2/K7.

Jonny the jetting for the F1 is 105 main and 40 slow.

Cheers

Den

[uk_kev]

Spitfire wrote:

 jetting for the F1 is 105 main and 40 slow.


Den

There you go, this could be all you need to sort out the flat spots, just change the jetting.

[uk_kev]

Someone has mailed me saying that if you change from the 4/4 pipes to the 4/2 pipes then you need 120 jets?

[uk_kev]

After reading the \"satanic\" CB750 site they recomend, For carbs with velocity stacks or individual pod filters, 125 mains seem to be a good starting point.

So it seems you pays your money, and takes your choice.

[leonardo269]

Hi,
    I checked my timing yesterday and found out it was quite a bit out.

it needed to be set at full advance (3000rpm) and I think I must of set it up at tickover. It just shows that you can never be too experienced to make a mess of things. :blush:

If it ever stops peeing down I must get a chance to test it out.:S

[uk_kev]

so is it tested, and if so how is it now?

[leonardo269]

Hi there,

I think I have now hit on the problem (or problems).

I stripped the carbs last night and recovered a slither of red material (looks like a bit of a fibrewasher (approx 1.5mm x 0.5mmx 0.5mm) in alongside carb2 float valve.

Also the push-rod of the accelleration pump (on carb 2 as well) was feeling decidedly siezed, so I stripped & cleaned it.

All the float heights seem OK

The main jets are all clear, they are 120

The float bowl gasket seals look a bit flat so I am replacing them. - Does anyone know if these can be obtained separatley (or am I going to be mugged for a \"carb kit\" at

[leonardo269]

OK - Time for a bit of an update on the K7 carb issues.

The carbs have been rebuilt with 4 new float valves and all the float heights where set to 12.5mm. my Haynes manual is a bit scarce on where the measurement actually takes place, as the floats taper down the further away from the fulcrum they get . I have assumed it is in the middle of the carb, in-line with the main jet. If anybody knows any different then please let me know.

I also noted that the spring on the accelleration pump was not strong enough to move the pushrod when the throttle was turned so I have replaced it with a stronger spring that I had lying around.

Better!

I then fitted the original Honda air filter.

Holy Sh*t - Much better! :ohmy:

I still get a slight hesitancy aorund 70mph (it basically feels like I am starting to run out of petrol), but overall the power is way up on what it was.

I am going to set the slow running mixtures with a clourtune tomorow and re-check with the vacuum guages to see if this gets rid of the problem.

I also have some 122.5 main jets that may get a test if the above doesn't work (and if I feel like poisoning myself with the smell of petrol again).

The only other option is to try standard 115 jets (Can anyone report sucessfully using them with a motad 4-1 exhaust?)

Cheers,

Ray

[leonardo269]

Hi again,

I Set up the carb mixture and synced the carbs today.

It goes like a rocket now. :woohoo:

I hope my meddling exploits can be of assistance to others who encounter similar problems in the future.

Thanks to eveyone who chipped in with ideas.

Cheers,

Ray