Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Oggers on April 24, 2022, 04:00:10 PM
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Not a Honda - but I feel you gents may be able to point me in right direction as the solution may be similar for CBs . I have another bike - 78 Kawasaki KZ650 - which has performed wonderfully since restoration a year or so ago. I took it out on Friday and it ran beautifully. Saturday took it out on another spin, but this time filled it up with super unleaded as normal. After that, bike rans fine until you give it some throttle at @ 4500 rpm and it hesitates - somewhat alarmingly lurching as it does so. It tries to go, but hestitates again. Very erratic, very hesistant, will not go above 4500. Below 4500 it seems OK - ish....
Clearly I have though about rubbish in the tank being disturbed when filling up, and then finding its way to the carbs. I took the tank filter off - very clean, and the extra in-line filter between petcock and carbs looks fine and clean. Cleaned plugs - all OK, checked gap - OK, cleaned air screw needles - all OK, fuel line blown through, vacuum take off to petcock is OK, Auto advance lubed up, all seems free moving.
Taking the carbs off and cleaning jets etc. is a royal pain. Has anyone got any other suggestions that I could easily do, and do you feel I am on the right track with the root cause? - rubbish in the carbs bascially. Deeply annoying after it was running so well!
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Can you give it just a little choke as it hesitates at 4500 rpm ? It's just to see if being a little richer can alllow it to increase revs and give a pointer as to what it's doing.
If it doesn't help, it's possibly not fuel.
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Another thought about this, someone here had a pattern, I think, fuel tap on a Kawasaki that was restricting the fuel flow such that it wouldn't rev as carb float bowls depleted level during a run.
Worthwhile checking for straight flow from the tank into a pot to see what you have there.
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I’d have thought (significant) carbs issues would appear gradually over time given your filters are clean. And given the problem has appeared in an instant, surely more likely something obvious…. Plugs, coils, condenser, faulty power supply to the coils etc (if not fuel supply.)
Certainly I wouldn’t be taking the bloody carbs off until all the obvious ruled out.
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Have you checked the fuel cap vent is unblocked?
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Six months ago I bought 5 litres of super unleaded from my local BP in a nearly new jerry can. When I poured it into the empty fuel tank of the bike it wouldn't run. The fuel was off when I checked it, full of water and bits floating about. The petrol station was selling dodgy fuel, the jerry can was nearly new and absolutely clean as was the fuel tank, no rust or debris inside as it had been resprayed and rinsed prior to fitting the fuel taps.
I wrote to BP who said they would check the fuel at the independent petrol station even though it was fully branded as BP. I called the petrol station and they put the phone down on me. When I checked Google reviews others had suffered the same.
It's odd that your bike started playing up as soon as you put new fuel in, I would say that is the culprit.
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Dave
I am beginning to think the same way. No idea why it should be so as I used super unleaded from the same spot beforehand and all was well. Mind that was some time ago. I'll drain the tank and fill afresh - from another petrol station!
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Dave
I am beginning to think the same way. No idea why it should be so as I used super unleaded from the same spot beforehand and all was well. Mind that was some time ago. I'll drain the tank and fill afresh - from another petrol station!
I deleted my post about the fuel as I did not read the part where it will not rev above 4,500 rpm I read it as a hesitation then it would go on revving.
I agree too much of a coincidence that it goes wrong after fuelling - out of interest what brand fuel was it?
You might get away with draining the float chambers if you have such a capability unless dirt has got through to the carbs. I use an inline filter as well as the strainer on the tap.
Esso is probably the safest bet this link is a little old but explains the E5 paradox - in the South of the UK Shell & BP SUL is said to be Ethanol free but up North a different story due to the supplying refinery I guess.
https://www.windrushcarstorage.co.uk/blog/the-windrush-guide-to-e5-e10-and-ethanol-free-fuel.php
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It's proving not to be an easy fix. That or I'm going dotty! Filled a clear empty brandy bottle with petrol to test presence of water in fuel - so much more sophisticated than a jam jar I feel! - Nothing settled out. Replaced all fuel with fresh from another petrol station - no joy. Removed the in line filter between petcock and carbs - no joy. Rode it with filler cap open - no joy - Fuel tap all clean...Proving to be very irritating! Any more clues would be greatly appreciated......
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ordered condensors......I have a horrible feeling it isn't the fuel....
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Have you checked the timing dynamically, maybe the bob weights are sticking or one of the springs has come off and the weights are not being thrown out to advance the timing. It's a free and easy check.
Also check nothing has come adrift under the tank where the coils are, could be a loose connection. Seems odd it started after you filled up.
I was on an IAM observed ride last Friday when we pulled into a petrol station for a feedback chat. When I restarted the bike the indicators didn't work! The fuse was ok as was the wiring and bulbs. I checked the realay and heard it click but still I couldn't find the problem. When I got home I stripped it down and changed out the realay and found it was the culprit. Although I heard it click it had failed inside. A £9 part ended my ride! Strange how things fail when the engine is switched off and then back on again a few minutes later!
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Dave
Yes - I too thought about the ATU not working properly so I sprayed it - behind the contact plate - with WD40, and also oiled the pivot points - although I must admit I haven't checked it properly as the contact plate fixing screws are a swine to remove! ATU's don't often go wrong though do they?. I also visually checked the connections around the coils/earth/plus leads - nothing obvious. I did though check the condensors. According to Haynes, if points spark on breaking with the ignition on, then possibly one of the condensors is goosed. One did spark, and reading up, what I am seeing when riding the bike could be symptomatic of a failed condensor. Condensors now on order.....
I agree that all these things are very odd. One minute it is running fine, very well in fact - the next hopeless, for no obvious reason. Britsh bikes are even worse for this!
Thanks for all the info - all useful stuff. Honda RS progressing - slowly. Bottom end done, barrel back on.
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New condensers, if not Honda mega expensive ones, have a high failure rate and short life. You may be better using an old one from another points plate to check
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Bryan
Understood. It will eliminate condensors as the root cause - or not - at least.
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Can you ascertain whether it’s all cylinders misbehaving…. or 2… or just the one as this might better point you to the root cause.
If poss can you ride it for ‘an extended period’ in the rogue rpm band, hit the kill switch and bring it to a stop. Then have a look at each plug, plus of course tap each exhaust header to compare temps.
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It was me who had fuel starvation on my Z1-A due to a dodgy pattern fuel tap, but this isn’t similar, your bike ran ok and then it didn’t.
I did have issues with my CB750 K1 running rough, though, after I’d rebuilt the engine, which turned out to be rock hard ancient HT leads into which the plug caps had been screwed so many times that there were only a few threads of copper left. It all looked ok and the caps screwed on tight, but I had intermittent plug fouling and cylinders cutting out and coming back for no apparent reason. CB750 leads come permanently attached to the coils, you can’t change them without clever surgery, there is a thread on how to do this somewhere. I wanted the bike sorted properly, and was dubious about 50 year old coils anyway, so I made a large contribution to the David Silver retirement fund, and put new genuine coils, with leads, on. The bike ran perfect after that.
This may not be your problem, but worth checking out.
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Lobo
Difficult to assess which cylinders are misbehaving. It seems to run OK-ish below 4500 or so, then just runs out of power after that. I can wind the throttle fully open on the road, and then nothing - it remains at @4500 and no more. At a standstill it will rev all the way - which is why I was thinking fuel related. I will try replacing the condensors as one seems to be misbehaving. Ignition on, open points, then sparks across one set of points - which there should not be. Other condensor looks fine.....
Davo
HT leads are pretty new and decent ones. I will check all HT connections though. Simple push fit...Coils could be an issue for sure, but keep that for later....
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It may sound stupid but......Are you sure there is nothing blocking the air intake below the seat ( I am assuming the Z650 has the air intake under the seat as most SOHC do and as my Z400J has as well). Theres nothing attached to the underside of the seat pan such as a document wallet that may be flapping open when the seat is closed and blocking the air intake? Likewise, is there anything that may be blocking the air inlet between the air filter and the air box to the carbs? It could well be the condensors though, just a bit odd that it happened directly after filling up but then again this is what happened to my indicator relay last week.
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Dave
Pretty sure nothing blocking the air intakes - which is indeed under the seat, but I'll check it out. I've known stranger things to happen! Condensors - hopefully that will be the issue. Perhaps after turning the ignition off/on at the petrol station, one condensor just gave up...who knows. If it is not condensors, then I'm stumped :'(
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Oggers - my previous post was suggesting a method to determine which cylinders are misbehaving. By chopping the engine at problem RPM, rolling to a stop and then pulling the plugs, you may find one / some are wet / cooler than others, which would indicate the problem cylinder(s). Armed with that info will help you massively in determining the cause.
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Lobo
If new condensors don't sort it, I'll give it a try....
Cheers
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I have had condensors go on me on 2 separate occasions and in both cases, the symtoms where the same. The engine misfired as I accelerated - surging then holding back as I rolled the throttle on, This was across the whole rev range and not just at higher revs.
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Oggers, have reread your initial post, and see you have an additional in-line filter.
If no joy with other routes, consider taking this out for a short trial. I too used one in a small Yamaha, which would loose power at WOT (on hotter days), but run fine at small throttle settings. The in-line filter was hard to see as buried in the crankcase along side the carb, but upon inspection it appeared only half full, and somewhat airated. Removing it solved the issue.
I’m guessing it was fuel vaporisation - a fairly well documented phenomenon. Typically it rears it’s head when an engine has been switched off for a while, allowing heat to soak into the fuel lines. This fits with your petrol station fill up…
Worth a try I’d guess if al, else fails.
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Have you got one of those bottles that hold fuel while you sync carbs? Just thinking, try fitting that if you have and go for a quick run to see if it's a fault inside the petrol tap.
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Chaps
I am very pleased to report I have just ridden the bike very quickly up my very convenient, quiet test hill just outside the village, and I can report all seems well. In fact far better than before! It pulls like a train. The problem did indeed appear to be condensors, and in all probability, one of them was on the way out creating less than adequate sparking before it finally gave up.
Couple of points - pun intended....
Intially I must have inserted the fork connectors of the condensors to the small bolts incorrectly. I do realize there are insulating washers and that the contact fork terminal sits in there also. Initially the bike would not start after replacing the condensors. I then fiddled about further, and inserted both fork terminals under the washer which sits under the head of the bolt. The insulating washer follows this, which would seem to insulate the bolts/forks from a bracket and thus the contact breaker plate? When it would not start, I feel the fork connector(s) may have been in contact with bracket/contact breaker plate. I am no sparkie so lesson learned perhaps....which is carefully note how things are connected/attached before removing them!
Everything pointed to fuel as the problem. I guess the condensor died when I turned the ignition back on at the petrol station. One for the brain's hard drive - not to be wiped. Thinking about it, the bike now performs so well that it may be worth replacing condensors on other bikes as a matter of course. I have no idea how these things behave and if condensors degrade over the years, but for £10 from Fleabay, it may be a worthwhile thing to do on the 400/4 and the GS1000.
Very many thanks for all your suggestions and help. I find these forums are a truly beacon of kindness in this increasingly disagreeable world.
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Let’s all bid on it then FHO at the end?