Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: PaulS on August 20, 2021, 01:09:31 PM
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Hi.
My first post, so I hope you can help.
I bought my bike 7yrs ago in UK, recently imported from USA. It's done around 25k miles in total to date but has been sluggish since I bought it. Whilst I don't want to thrash the bike, I'm disappointed that it struggles to get much beyond 80mph and wheezes to get there. Also it sometimes only starts on 3 (sometimes 2) cylinders - but rarely the same ones! That starting issue seems to cure itself when warmed up.
So far I have dismantled, cleaned and rebuilt the carbs (using a refurb jets, needles, etc kit from David Silver Spares) and checked the rubber gaiters. I balanced the carbs with a vacuum set. The fuel lines and tap seem to be OK. I have replaced the battery, all the plugs, leads and coils, installed electronic ignition and checked the compression (OK).
And still it's sluggish.
I am probably at the extent of my mechanic skills now, so unless there is some magic answer (??) I probably need to get a specialist to breathe on it.
I had Rising Sun Restorations of Tockwith, York, carry out an initial checkover/service when I first bought the bike, but their telephone number seems to no longer be working. Are they still around?
So,
A. Any magic answers to get the bike free revving again? and
B. Where do I find the best mechanic to diagnose the problems and sort this out?
I am based near Bristol, but happy to travel to find the best person/people.
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As a US import what size rear wheel and tyre and what size sprokets are on there, they do love pissing about with Mr Hondas design specs over there
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Thanks Bryanj.
Ill check against Spec.
However, the engine is really wheezing, so I suspect its the power plant at fault.
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Thanks Bryanj.
Ill check against Spec.
However, the engine is really wheezing, so I suspect its the power plant at fault.
If it was a two stroke I would ask are the exhuasts blocked up ?
Obviously you might have checked the ignition timing - but is the advance & retard unit working as it should - can make a big difference to any engine if it's not advancing as it should..
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Are you running standard air filter box and exhaust? What colour are the plugs after a longish run?
Welcome to the forum by the way!
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I understand that John Wyatt (who owned Rising Sun Restorations) has died recently, so that's probably why the telephone is out of action.
I am not aware (others might know better) of anyone taking on the business.
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I'm sorry to ask but..
What bikes do you normally ride?
If you usually ride modern bikes, then the k1 will appear to be a slug.
50 yrs ago they was comparably fast. But now vastly over weight & woefully Under powered. US spec bars make things even worse.
Sean.
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Although comparisons to modern bikes, should still breeze past 80 mph so there is an issue we need to sort. Do agree though that if you jump off a relatively new bike to our 70's 'superbikes' will show up their deficiencies. When I step off my 'modern' 1995 ZZR1100' my 750k2 seems very sedate! But enjoyable!
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A rolling road would tell you what sort of power it is making all through the rev range.
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Let me ride it, I'll soon tell you if it's slow or not 😂😂😂😂😂😂 If I can't blow the engine up whilst flat out, you'll know its just fine 🤣🤣🤣
Joking aside, it can only really be carb or timing set up that's giving you the issue.
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A few years ago, a 750 K6 that I owned exhibited similar symptoms. It would ride normally until reaching some 4000 rpm and then just flatten out.
After lots of time and trouble, I tried easing the choke on one day whilst in the problem rev band, and immediately it ran normally.
I changed the main jets for new items and all was cured. My friend subsequently bought the bike from me and for the years that he owned it, things were also fine.
I later found that the bike had stood for over a year at the selling dealer before I bought it, so perhaps stale fuel caused a restriction within the jets?
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Two things that might be a problem you mentioned rebuilding the carbs with a kit, a lot of people have problems with the carbs going via this route, it is better to re use the original brass.
The second one is plugs, what plugs are you using, I went through several types of plugs on mine when I first got it back on the road and had problems, as the old faithful D8ES-L is no longer made I tried several different NGK plugs all with no improvement so I switched to ND plugs and have not had a problem since.
Without going down to the garage I don't know which plugs I have in at the moment but they are either Nippon Denso ND X24ES-U or X22ES-U as recommended by Hondaman on the US SOHC site.
Cheers
Dennis
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As per Dennis's comments, retaining the original brassware is the way to go. I bought an aftermarket kit for the 500k2 and mic'ed up the needles at various points on its length of the needle and they were miles out compared to the standard needles. Never fitted them and cleaned and put the originals back
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Thanks all!
In summary it sounds as though the issue is either timing, carburration or perhaps plugs.
Timing; I replaced the points with an electronic system a few years ago, so assume that won't be the problem. Or am I fooling myself?
Carburration; I checked the plugs this afternoon (last run was around 40miles) and they are looking sooty, so the mix is too rich. I'll start by lowering the needle and check again. BTW, the carb refurb kits were genuine Kehin, so I'm hoping they are OK... before doing so I might try the 'choke trick' suggested by 'flatfour' to see what happens.
The plugs I am using are NGK DR8ES. Spitfire: should I change to ND plugs? If so, which ones?
As for the bikes I am used to riding, it's a real mix. My newest is a 1 year old BMW R1250RS that I use for touring. I previously had a Ducati 1200 MultiStrada Pikes Peak. However, I have been riding a '58 Triton for 10yrs (just sold it), a Ducati 916 for just over 20yrs and regularly ride my '56 BSA DBD34 Gold Star. In the past I have owned many old and new Triumphs and an old Norton Dominator. I am really convinced that the CB750 should have more 'urgency' than it displays. My very first bike, a '62 James Captain, had more 'get up and go'!
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Definately sounds like mixture from your plug description (stating the obvious). Is the needle set on the middle notch? As per my previous post, are you running standard intake and exhaust?
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It sounds like the plug type may not be helping. You are using resisted spark plugs (the R denotes that) and if you have resisted plug caps fitted, which by your avatar looks like you are, you will be getting too much resistance. Change the plugs to NGK D8EA'S or the DENSO equivalent as Dennis suggested. You only need one element resisted, either the plug caps or the plugs, not both.
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I'm with Julie, I made the same mistake with my bike with resisted plugs and caps.
Here's a discussion form the US site
http://forums.sohc4.net/index.php?topic=77832.msg869100#msg869100
Cheers
Dennis
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Thanks philward, yes, standard (original) intake, the gaiters and clips are in good condition, and the exhaust is standard HM340 except one replacement pipe to pattern. I'll adjust the needle to get the mix a little leaner.
In the meantime I will take the medicine offered by Nurse Julie and confirmed by spitfire - I'll replace the plugs!
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Thanks philward, yes, standard (original) intake, the gaiters and clips are in good condition, and the exhaust is standard HM340 except one replacement pipe to pattern. I'll adjust the needle to get the mix a little leaner.
In the meantime I will take the medicine offered by Nurse Julie and confirmed by spitfire - I'll replace the plugs!
Original K1 exhausts are HM300, the K2 was fitted with HM340 with extra baffling so maybe they are restricting the carbs/gas flow - not sure if you have to change the main jet when fitting HM340s but someone will know?
Cheers,
Andrew
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Apologies Andrew-S; I have just checked and the pipes are HM300 (apart from one pattern replacement), all with O/E baffles.
Will let you all know how the 'tinkering' goes. I have ordered up the Denso X24ES-U plugs...
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Managed to finally track down a firm that has the Nippon Denso plugs in stock (buycarparts.co.uk) and they arrived today: from Germany!
Will get those into the bike in the next few days and let you know if I am on the way to solving my problems...
Fingers crossed.
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If you aren't able to find significant progress with above suggestions, try running it with air filter removed but airbox in tact. It should be leaner in that config and may serve to show you a direction without initially changing jetting.
In other words a quick and dirty "check see" to give low-no cost assessment and help with decision about mixture.
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Thanks K2-K; as a born and bred Yorkshireman* I like "low-no cost" suggestions.
And thanks to Oddjob for your comments. I checked the compression last year and that was really good. I also replaced the plug caps, leads and coils some time ago. I'm banking on the plugs being the issue, if not, then carburation, but will let you all know....
(*Yorkshire, where "tin tin tin" is a proper sentence)
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"WoooooHoooo!!!" :) :) :)
Just tried the bike after installing the (non-resistor) Nippon Denso plugs recommended by Nurse Julie and....
- the bike started first time AND on all 4-cylinders (that's a first!) - I tried it a few times just in case it was a fluke - and
- it rides with the free flowing urgency I was hoping for.
So far I have only taken it on a 20 mile test ride, but all is well. The spark plugs are showing a tiny bit of soot but I'll give the bike a much longer run before making any fine tweaks to the carburation.
Looks like 7 years of miserable performance has been cured by 4 spark plugs.
Thanks each!
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Looks like your problem is sorted.
Thank you for letting us know the result. So many times people ask advice on forums. But then don't come back on, to tell others what worked.
Thanks again.
Sean.
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Glad to hear that it worked, nice when a plan comes together relatively cheaply.
Cheers
Dennis
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"WoooooHoooo!!!" :) :) :)
Just tried the bike after installing the (non-resistor) Nippon Denso plugs recommended by Nurse Julie and....
- the bike started first time AND on all 4-cylinders (that's a first!) - I tried it a few times just in case it was a fluke - and
- it rides with the free flowing urgency I was hoping for.
So far I have only taken it on a 20 mile test ride, but all is well. The spark plugs are showing a tiny bit of soot but I'll give the bike a much longer run before making any fine tweaks to the carburation.
Looks like 7 years of miserable performance has been cured by 4 spark plugs.
Thanks each!
Yes it's easy to forget what a difference a change of plugs can make a few year back we had our SL serviced at the main dealer where I worked part time as I drove it away I thought to myself this seems slow and restrained. I took it staight back - they found the old plugs & the parts department had given them the wrong ones - same reach but different plug.