Honda CB750K5 Restoration by Royhall

[K2-K6]

"So all is good then. Well no not really. A couple of problems have shown up for next weekends project. It's not charging the battery. Probably a cable connection I missed somewhere hopefully. And it has a slight misfire on cylinder 1. Good thing about the mini coils is you can unplug the HT lead from the coil, so I shall swap 1 and 4 at the coil and see if the misfire moves to cylinder 4. If not, then I will change the spark plug and possibly HT lead and see what happens. I think it's most likely to be electrical but you never know. I may also check that the tappets haven't closed up. After that I will be stumped. Got the battery back on the optimiser ready for the weekends mission."

From working on one the other day, my understanding is that you can just swap #1 lead with  #2 as they all four fire at the same time, all of the time. It shouldn't matter which plug they are connected to. That's with full Boyer and micro coils system. It may help with diagnostic investigation.

It was Bills 400, which although it seemed to fire on all four with the pipes getting warm, #3 was, it turned out not running on idle jet, so it got warm from chiming in as you lifted throttle onto mains , but nowhere near the others for overall heat, giving some confusion as to exactly what was what. Inspection found a restriction in that #3 idle jet when taken out.

[royhall]

All four pots are running and warming up the same, it's just a slight misfire when ticking over. Thinking about it, this Boyer was bought about 7 years ago for my 350F. But I struggled to get it running correctly so out of desperation changed back to standard points and coils. I later switched to a simpler electronic on the 350 that keeps the standard coils. It may well have been a faulty Boyer coil that's now reared it's ugly head again. Will try and figure it out at weekend.

[K2-K6]

Flipping the 1&2 plug leads should at least give you diagnosis in that case then Roy as it will put #1 onto the other coil to give comparison.

Interesting you had difficulty with setting it on 350 as we felt that the instructions for fitment are slightly odd, a little difficult to fully understand what they are trying to do in set up.

It says set crankshaft to TDC firstly, then set all the trigger and pickup in a line with the backplate fitted fully clockwise ( in their picture, schematic) but then to set the backplate in the middle of slots ? From words in instructions. This seems to retard the ignition start point, which then has to be advanced when you strobe it, that seemed to then run out of slot length without reaching maximum advance when tried. Couldn't figure why that instruction line was there. 

Went with leaving plate fully clockwise, set static point, then strobe to check maximum advance at above 4000 rpm, went correct straight away with no shift but to rotate the plate in accurately get it bang on the full advance marks.

Something it does seem to do though, looks like a condition of the "fire all" strategy, is give odd miss or popping effects if its out of correct timing as it appears to light off cylinders that maybe not on compression, blowing off the synchronization vac tube of a cylinder that did this.

Looks a good system though, but with a distinct different character to setting points up, which you need to get your head around.

[philward]

Boyer offer a great service Roy (for diagnosis) - either by email or send back for negligible few. Used them to check a running issue on the CR750 - they confirmed that there was no issue with the Boyer

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[royhall]

I had a go at the misfire this morning without any luck unfortunately. I first checked the tappets on cylinder 1 had not tightened up. They were slightly loose so readjusted them (it was only a thou or two). Swapped the HT cables around between ! &4 but the misfire stayed at 1. I made up a new HT lead, fitted a new cap, and changed the plug but still a misfire. Looks to be a good bright spark at the plug.

I did notice that the engine got hotter on the number 4 side than the number 1 so had a play around with the air screws again without luck. I'm not too sure about these carbs that Gerben built as the choke doesn't appear to do anything other than kill the engine if lifted right up. Could it be a partially blocked idle jet, the engine starts (without choke) okay and once warm will tick over okay, just with a slight popping from number 1. Or could it possibly be a set of piston rings that are yet to seal and causing low compression, as there's a fair bit of vapour coming from the vent tube.

Any ideas would be welcome. Cheers.

PS. Sill got to tackle the not charging issue as well.

[K2-K6]

I'd bet my Ulez money / fine / scam funds on it being tge idke jet Roy.

The Boyer does a funny thing sometimes if there's a missed fire point, and may give that pop back through the inlet valve as it can fire it off at prior tge bdc with some charge in the cylinder and inlet open. Not a fault of the ignition, but shows other compromise in place.

That #1 pipe cooler than #4 is a giveaway I believe. If you run them down the road it heats them all from running on mains, which kinda masks the original cause. 

Could try just blowing into #1 breather while its running to see if forcing a little fuel, via pressure in float bowl, in there will stop it popping temporarily.

[royhall]

Now the pipes have cooled but the engine still warm, I started it up and exhaust 1 is warming slower than the others. My money is on the blocked idle jet as well. As there's also something weird going on with the choke (like no fast running) I am thinking of taking the carbs off again and sending them to my usual carb guy Matt Harper. I don't think Gerben has done a very good job with these, wont be the first time he has sent me carbs with blocked jets. It will cost more money but at least they will come back bob on.

Before then I would like to solve the charging issue, where's the best place to start.

[K2-K6]

There's no throttle lift on these for choke use at all Roy.

Unlike the F2 and some of the smaller fours, you're on your own for manual throttle modulation when using choke flaps.

[K2-K6]

Charging, first stop is the plugs coming out of generating pack in around the front sprocket cover to make sure there's nothing amiss there.

[royhall]

If I disconnect the three yellows in the chain case, is it safe to run the engine and meter the output on each yellow.

I was wondering about the throttle control on the choke, thanks for verifying that, am used to the 350F and the 750F2.

[K2-K6]

Think I'd firstly check with probe in each while connected and running to see what that gives you, then go from there.

[royhall]

I will leave it all connected and try it at the rec/reg  plug to start with. What voltage should I be looking for, and I assume it will be AC on the yellow cables. Should one be dead until the lights are on?.

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[K2-K6]

All three yellow are live and ac to measure. Can't find voltage spec to check this against.

Regulator controls output by switching a resistance into the white wire going to field coil if voltage is above target, to then drop output of all three yellow simultaneously.

There's really good description in original 750 workshop manual, section 8 if that's available on Ash's library roster. Worth a read through to get your head around layout etc.

[AshimotoK0]

No night time running .that's on single phase Honda alternators.its 3 phase .. so same AC between any two yellow wires 

[AshimotoK0]

This is Honda Bulletin on 3phase alternator on SOHC Fours
https://www.dropbox.com/s/2oqrx5cvv7byqao/3-phase%20Honda%20Charging.pdf?dl=0