750 K4, poor running

[ianmck]

My 750K4 was running fine apart from a slight "stumble" when accelerating gently from say 30 up to 40 mph, also showing signs of being rich i.e. sooty deposits on plugs - hardly any choke required to start.  After some investigation I found that there were 120 main jets in carbs.  Searching through both SOHC sites, speaking to David Silver and looking at CSMNL website it was decided that 105 mains should be in carbs so duly ordered 4 of these and fitted at the same time I checked tappets, points, timing, carb synch, adjusting timing chain etc. New plugs (D8EA).  Bike now doesn't have a flat spot at low revs, but "holds back" "surges" on harder acceleration, doesn't want to go past 3500 to 4000 RPM.  Re-checked all settings, re-fitted 120 jets, still the same.  Checked fuel supply - cleaned petrol tap, still the same, any help/advice would be gratefully received. Thanks

Ianmck

[Bryanj]

What exhaust and air filter is fitted?

Is mechanical advance free?

Is battery good and charging?

Plug caps new?

[BigAl (Alan)]

Did you also replace the needle jets? sounds as if the machine is running too lean. Had a similar problem with my K4, re-built the carbs with Keyster kits, but had to order 105 jets and needdle jets from Dave Silver to solve the problem.
I have assummed every thing on your bike is standard spec.

Alan

[ianmck]

hello bryanj, standard exhaust, (new a couple of months ago), standard air filter (new), battery is new, havn't checked charging, plug caps have been swapped from my 400/4, which runs like a sewing machine makes no difference. have advanced the timing slightly, I can then clear 3500/4000 rpm, but starts surging again around 6000 revs, if I try to cruise at a steady 30, or 40 mph engine is rough,light acceleration is fine, hard acceleration is fine up to around 6000 rpm, if I set timing back to spec, then problem happens back down at 3500/4000 rpm, have checked advance, is free, also checked with timing light and the ign advances to spec.
hello Bigal, hav'nt changed needle jets, they are as they were, but have cleaned and checked the carb seems OK. regards ianmck

[Bryanj]

It would suggest to me that a PO has "rebuilt" the carbs with the wrong bits or cheap aftermarket kits. It seems that Keyster and aothers dont get the needle profile right.

I have also heard of, but not actualy seen, that its is possible to mix the slides up by putting 1&2 in 3&4 which puts the slide cuttout on the wrong sideas the location pegs are "handed".

Sorry to say i think you are in for several carb removals before you cure it

[ianmck]

thanks bryan, I will check the slides, I just whistle now and the carbs take themselves off, I'll keep you posted as to any progress or not, thanks again, take care.

[ianmck]

Problem was still there up until yesterday, when the engine cut out, no electrics at all, charged battery overnight, checked batt voltage, 12.5-13 volts, connected battery, still no electrics, no lights-nothing, checked battery voltage, 2/3 volts, disconnected battery 12.5/13 volts. Is it possible this is a rect-reg or even alternator fault, could this be the root cause of my initial problem. Please help, i need to get out on the bike.
regards ianmck

[Bryanj]

Sounds like a bad battery cell to me mate that cant give anything on load. If there was that much of a short on the bike the special smoke would be issuing profusly from somewhere

[steff750]

  just a few lines too confirm what bryanj as already said,
i went for ride today on one of my 1970  cb750's today stopped for a waz as you do after one or two tea breaks ,went to fire the bike up and bang no electrics,right change the fuse i thought ,nice try but no cigar the fuse was ok ,still no power started chaseing wires from battery to ignition and then the headlight still no joy dead as a dodo after about an hour of messing about with every connection i could find,decided too try my battery on my mates bike yep you guessed it ,flat as a witch's t*t
right time too get the van my mate said,but i told him these bikes have never let me down always got home one way or another so because we were in the middle nowhere in wild wales ,we decided let it run down the big hill we had just came up and park near some houses , as i freewheeled down the hill i tried too bump start it and hey presto it fired up, so drove home keeping the revs up on a completely flat battery got home ,swapped batteries and every thing working great
question ? is the battery duff or did it short out ? the flat battery had a bulge on the side of it so i asume it has blown a cell, it is a gel battery that bought about four or five months ago (probably byranj can answer this) are the gel batteries ok on the older bikes ,or was it a bad battery ,by the way i still run another cb750 with a gel battery with no problem so far?

[Bryanj]

Never used gel batteries, dont think they are worth the extra cost. My bikes do sod all mileage every year so i buy the cheapest battery i can get as its only gonna last 3 yrs max anyway

[ianmck]

firstly, yes it did turn out to be the battery, got a new one and all is well on the elec problem.
secondly, my poor running problem is now cured, after trying all suggestions from this and the American sister site, it was still the same, (would'nt rev over 3/3500). in desperation, I removed the new base plate points and condensors, (genuine Honda from DS), fitted the set from my 400/4, (after checking compatibility), checked the gaps, set timing, took it down the road, perfect.
So now I can get it out on the road for what is left of our super summer. Many thanks to all who tryed to help, going to get onto DS see if they can sort anything out regarding the point etc.
Regards ianmck.

[steff750]

glad you got the bike sorted, i had the same battery problem ,so here is a quick update on my bike, the battery shows 12.28 volts but as soon as i put any drain on it it goes flat wont even power the oil and neutral light,it then creeps back up to 12.28 volts over time, it has a shorted or dead cell.
      hi bryanj i agree with you 100% with your last post  about the gel batteries i have seen them for sale up to

[Bryanj]

No reason you shouldnt name the seller if you have good service----same if you get a bad one!!!! Its always better to buy stuff on recomendation rather than guestimation.


As to the bad points plate----could it have been preservative on the contacts?

[steff750]

  thanks bryanj
         the ebay seller is LMspares online they have a small shop in hereford ,but i think they do all thier selling online they do lots of other stuff aswell as gel batteries nice people too deal with and really friendly