First ride misery.

[Graeme77]

First ride of the 750 today, took it for MOT and it passed.

However, there were 'issues' LOL.

Bike stutters, power comes in fits and starts. It did smooth out a little after a while, but not great.

Then, it cut out and wouldnt re-start. It did this a couple of times, cutting out dead when pulling up to a junction.
It would re-start, but only with a bit of choke, then off choke imediately as it doesnt like running on choke.

First thing felt like it was lean, or maybe ignition.

Then, stopped for petrol and it wouldnt re-start. Eventualy drained the battery. There is no way this bike will start on the kickstart, but normaly starts straight away on the starter.

Got it trailored home, pulled a plug, and its properly sooted up. Didnt get a good run on the bike so hard to tell, but my guess is its fouling up the plugs to the point where there not firing.

Air filter is suspect, ill remove it for now, see if that helps, but ill look into the carbs again.

Do you think the dodgy power thing could be running rich?, plugs fouling? It did feel a bit like that.


When i went over the carbs i noticed i have 2 different types of emulsion tubes. One set where the row of 4 holes are all the same, and another set where the bottom 2 holes are a fair bit smaller than the top two.
Any idea which ones my 750F1 should have?

As the chains are a bit noisy, and the starter clutch chatters, the AA man thought it had slipped the timing chain. I find this highly unlikely though. I think he just didnt want to know.

Ill look into the starter clutch though.

[Graeme77]

Oh yeah, and the clutch bite point changes as you turn the bars

Cable routing around the fairing bracket must be suspect. You can feel it in the lever. Makes pulling out of junctions 'interesting' 

[K2-K6]

Good stuff getting your test done.

Sounds from the plugs as if it's rich. It would be sensible to have the emulsion tubes the same as a good start point.

Rich usually gives the feel of abruptness and sort of chang, chang, chang as it tries to tick over then eventually dies if badly rich, also you get a much stronger unburnt fuel smell from exhaust.

Lean usually feels much less precise and sort of wafty with delayed response and usually gets very hot.

Don't let the clutch cable compromise your safety especially moving out of T junctions, that's gonna hurt you if you get caught out.

At least you can now ride it to get it sorted.

[Graeme77]

I posted on the other site, but ill add my new findings here.

I have been trying to sort the running issue.

Its hard to start, have to play with the choke then get the choke off quick. It never has liked running on choke.

If i turn the idle up to 2500+ it runs fine, but try and turn the idle down and as soon as it gets to around 2000rpm it just dies.

Ive checked timing, tappet, and had the carbs apart agian. All the passageways are clear, all jets are fine, float heights are all spot on.
116 main jets and 40 idle jets.

It did idle ok at 1200rpm before this first ride. I dont get what could have changed.

Clutch cable will be sorted, but need to get it running properly first.

I have no air filter in it right now as i thought maybe its clogged.

Maybe i should mess up the float heights again, stick the mix of 132 and 152 mains back in and unbalance the carbs. It at least ran when it was like that (but badly)

Runs great, on all 4, above 2000 ish.

[Spitfire]

If it is a CB750F1 then the main jets should be #105's, my F1 hates running on choke, it needs it to start but then I push it down as fast as possible and get moving.
Got this from the US site I think.

                                                                                        Float Main Slow JN     Screw
75   CB750F   B750A & 7A   2-Jet Mechanical Slide Pre-EPA   26   105   40   3   1
        657A & 657B   2-Jet Mechanical Slide Pre-EPA   26   105   40   4   1
76   CB750F   069A   2-Jet Mechanical Slide Pre-EPA           26     105   38   2      1


Cheers

Den   

[Graeme77]

Yeah, the 116 are a bit rich, but the smallest i have. It did run ok with these though.

Im not bothered about it being a bit rich at the moment, im more concerned why it used to idle and now it wont even think about running under 2000rpm.

[Graeme77]

I think it might be a float issue.

Ive just checked everything agian 

All is good, but it just dumped some fuel out of the carb drain unexpectedly. There is also fuel in the carb mouths when i take the filter off.

The float heights are set to 26mm and ive checked them with a bit of clear tube on an old drain screw. Levels are all the same, fuel level is around 6mm below the top of the bowl.

I think ill drop the fuel height and see what happens.

[Graeme77]

Im begining to think this must be an ignition issue.

A quick look on the boyer site shows a fault finding page.

This is what mines doing...

The Engine Runs Fast At Idle, Kicks Back on Starting
Poor fuse connection or wiring running low or variable voltage to the ignition.  As the alternator charges into the system with increasing speed the problem can clear.

So, not really sure how i go about diagnosing this, i guess i just need to check over the whole electric system.

[Graeme77]

Checked everything over, all is fine. I did make the kill switch work again though, it had been badly bypassed.

Still the same though, i think my ignition box may be fubard.

Im sure this is the problem, its not carbs, its displaying symptoms boyer themselves say are possible, but all the electrics are fine.
Seems like the only conclusion is a fried ignition box.


I have a couple of points ignition sets, but im missing some of the advancer mech. I have the advancer plate thing, and the cam, i just dont have whatever springs and bob weights are in there.

Anyone have any spares?

[Bryanj]

Check it with a FULLY charged battery, see if the charging circuit is working properly

[Graeme77]

I think its cahrging properly, but ive had the battery fully charged since this issue started and its still the same.

As far as checking the charging system, i think maybe i should explain what ive checked already.

I checked it out of curiosity a few weeks back. Voltages rose with revs but not quite high enough. I tweeked the voltage screw to get them up a little. I also re-did the core gap.

When i checked it today it was showing 15.5v at 3000rpm, so i tweeked it back down a little.

Im wondering if this voltage was high enough to kill the ignition unit. I guess i have no way of knowing if it may have gone even higher.

If the regulator was bad, what symptoms would show?

Battery is back on charge now, but it reads over 12.5v at the moment anyway.

Im sure the ignition box is holding the idle up, but nothing seems wrong, so i think its broken.
It did once idle at 1500, it then gradualy rose up to 2500, stayed there for a while, then died.
Im sure the idle stop on the carbs is right, the box is making it idle high, then when i try and tweek the idle down im just shutting the carbs off.

[Graeme77]

I recharged the battery and tried again.
No different.

It was idling at 2500-3000rpm. I got a timing light on it was half way between F and the full advance marks.
However, it doesnt change with revs.
I got the engine to briefly run at 1500rpm but the advance was the same.

The box is stuck somehow and not advancing/retarding.

This took 5-10 mins, at which point i noticed the headers for 2/4 were glowing cherry red!
I shut it down and walked away

[Graeme77]

Could i take the whole bikes electrical system out of the equation by running the electronic ignition from a seperate, fully charged battery?

See how it runs then.

That would remove all possibility of any problems with connections etc.

[BigAl (Alan)]

I would look at replacing the main jets No 105 as previous posts, and check for leaks in the inlet rubbers.
I think the early CB 750 No 120 jets have been fitted

Good luck

Alan

[K2-K6]

If the advance was not going back down to base level you would get those sort of effects i.e.fast idle and too much heat (from advance with no increase in fuel flow, making it leaner than ideal).

You should be able to rig up a power supply from a good car battery maybe just to the ignition pack to take that variable out of the equation if the pickups are not earthed back to the engine, I'm unfamiliar with the circuit used on these.

If no advance function, can you swing the timimg plate back to retard it and see what response this has at tickover?