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SOHC.co.uk Forums => CB750 => Topic started by: Craizeehair on August 19, 2023, 11:31:12 PM

Title: What are these plugs telling me?… and other questions!
Post by: Craizeehair on August 19, 2023, 11:31:12 PM
I’m hoping some of you clever kids might be able to help a novice spannerer out?

I bought this bike a couple of months ago, with a view to get more hands on and really get to grips with my own maintenance and repairs, I’ve ridden on and off for most of the last 36 years since my first Honda XL125 as a teenager, what I never did was any meaningful maintenance or repairs, usually paying someone else to do it for me. I am happy with oil changes, brakes, cables etc, but engine work, carbs and the like are a mystery to me.

The bike hadn’t been a regular rider for a number of years since a restoration but had a wee run out every year to keep the juices flowing. So it was always going to need some TLC, only I was hoping to do that through the winter in my (hopefully) new workshop / shed I’m panning to build. The bike had a pronounced flat spot from take off and low revs, from throttle off to throttle on. I’m pretty convinced that it has got worse  but that could just be my imagination?

I have only managed to put a couple of hundred miles on the clock so far and today I decided to go for a bit of a blast, about 50 miles in total, with a good proportion of dual carriageway to blow away any cobwebs.

All was going well, when about 3 miles from home and now on smaller country roads, I noticed a noise. It was a bit faint to start with, almost like a wheel rubbing or something catching but only when the bike was moving and not stationary, however I convinced myself that it was more likely coming from inside the engine rather than from something outside. I nursed the bike back home, turned the engine off immediately and had a little panic to myself! I checked both wheels and they seem to be turning freely with no noise when raised off the ground with no play.

I have read plenty on here about how everything needs to be set up right or these bikes can sound very ropey. So I have adjusted the cam chain tensioner (not sure if that did anything as there was no discernible happening to indicate any change) and I also set the valve clearances, some of which were very tight. Tomorrow I will checks the points and static timing. I have no doubt the carbs will need attention too.

So, to the question in the title… after another question. Would it be possible that a combination of these things being incorrect could produce such a noise?

These are the plugs that I removed tonight, 1-4 from left to right. Any advice as to what they are telling me? Too rich? Big Trouble? Carb issues? Run away?

Worth noting that they aren’t all the same plug either, I presume a new matching set of the correct type would be in order.

I have a feeling that I will need to tackle the carbs, which is something that strikes the fear into me, I will need to purchase the appropriate necessities for that, as it is something WAY beyond my comfort zone! Would you recommend vacuum gauges or something like the Morgan carbtune pro?

Apologies for the waffle and the Tommy Tippee beginner questions but I guess I have to start somewhere and you kind souls seem just the type to help me on my way. Thanks in advance
(https://uploads.tapatalk-cdn.com/20230819/410b556d73196ff18296a78911f85f48.jpg)
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Title: Re: What are these plugs telling me?… and other questions!
Post by: Trigger on August 19, 2023, 11:42:51 PM
Why do you have a mixture of plugs and are they resistance plugs ? They should not be resistance  ;)
Title: Re: What are these plugs telling me?… and other questions!
Post by: Craizeehair on August 19, 2023, 11:47:29 PM
Why do you have a mixture of plugs and are they resistance plugs ? They should not be resistance  ;)
No idea? They were in the bike already, I took them out tonight to check them and they are what I found in there. I know I need a new set but that’s a good point about the resistance plugs, which leads me to my next question, how do I know if I have resistance caps or doesn’t that matter and any suggestions for best plugs for these bikes?
Title: Re: What are these plugs telling me?… and other questions!
Post by: Trigger on August 19, 2023, 11:55:57 PM
Most caps are resistance these days. The plugs you need for a 750 are X24ES-U  ;)
Title: Re: What are these plugs telling me?… and other questions!
Post by: Trigger on August 20, 2023, 12:00:38 AM
Sound like the bike needs a good service and i would change the plug caps .
Spark plugs >>> https://www.gsparkplug.com/1x-denso-standard-spark-plugs-x24es-u-x24esu-067800-1700-0678004130-4099.html
Title: Re: What are these plugs telling me?… and other questions!
Post by: Craizeehair on August 20, 2023, 12:02:57 AM
Sound like the bike needs a good service and i would change the plug caps .
Spark plugs >>> https://www.gsparkplug.com/1x-denso-standard-spark-plugs-x24es-u-x24esu-067800-1700-0678004130-4099.html
Thank you, I shall get some ordered


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Title: Re: What are these plugs telling me?… and other questions!
Post by: Trigger on August 20, 2023, 12:04:52 AM
Sound like the bike needs a good service and i would change the plug caps .
Spark plugs >>> https://www.gsparkplug.com/1x-denso-standard-spark-plugs-x24es-u-x24esu-067800-1700-0678004130-4099.html
Thank you, I shall get some ordered


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You can get the plug caps from the same company  ;)
Title: Re: What are these plugs telling me?… and other questions!
Post by: Craizeehair on August 20, 2023, 12:07:38 AM
Sound like the bike needs a good service and i would change the plug caps .
Spark plugs >>> https://www.gsparkplug.com/1x-denso-standard-spark-plugs-x24es-u-x24esu-067800-1700-0678004130-4099.html
Thank you, I shall get some ordered


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You can get the plug caps from the same company  ;)
Caps look out of stock


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Title: Re: What are these plugs telling me?… and other questions!
Post by: Trigger on August 20, 2023, 12:24:51 AM
Sound like the bike needs a good service and i would change the plug caps .
Spark plugs >>> https://www.gsparkplug.com/1x-denso-standard-spark-plugs-x24es-u-x24esu-067800-1700-0678004130-4099.html
Thank you, I shall get some ordered


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You can get the plug caps from the same company  ;)
Caps look out of stock


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Give them a call and ask when will new stock be in  ;)
I will have a look at what i have in stock tomorrow. Will have the spark plugs but, not sure on the caps .
Title: Re: What are these plugs telling me?… and other questions!
Post by: Craizeehair on August 20, 2023, 12:34:31 AM
Sound like the bike needs a good service and i would change the plug caps .
Spark plugs >>> https://www.gsparkplug.com/1x-denso-standard-spark-plugs-x24es-u-x24esu-067800-1700-0678004130-4099.html
Thank you, I shall get some ordered


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You can get the plug caps from the same company  ;)
Caps look out of stock


Sent from my iPad using Tapatalk

Give them a call and ask when will new stock be in  ;)
I will have a look at what i have in stock tomorrow. Will have the spark plugs but, not sure on the caps .
Cheers
Title: Re: What are these plugs telling me?… and other questions!
Post by: Lobo on August 20, 2023, 01:12:39 AM
As an aside, not sure why you should be ‘stricken with fear’ wrt disassembling the carbs - your basic abilities seem sound!
Certainly understand the comfort zone stuff, but there are 2 questions you need to ask yourself before tacking… (1) will the world stop turning, and (2) will I die? 😂. If no, have a go… just be slow, methodical, use 4 tubs (ie one for each carb), do it by the book, and use this forum. Best of all, no special tools are needed - not even vacuum gauges or a carb tune - assuming you keep them stock and bench set them.
These carbs are not especially high tech, you’ll come away enlightened and somewhat more confident.
Good luck.
Title: Re: What are these plugs telling me?… and other questions!
Post by: Trigger on August 20, 2023, 08:34:14 AM
Sound like the bike needs a good service and i would change the plug caps .
Spark plugs >>> https://www.gsparkplug.com/1x-denso-standard-spark-plugs-x24es-u-x24esu-067800-1700-0678004130-4099.html
Thank you, I shall get some ordered


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You can get the plug caps from the same company  ;)
Caps look out of stock


Sent from my iPad using Tapatalk

Give them a call and ask when will new stock be in  ;)
I will have a look at what i have in stock tomorrow. Will have the spark plugs but, not sure on the caps .
Cheers


Just checked my stock on plug caps and only have one set left which i would keep as, this happened a few years ago with NGK plug caps and you could not get hold of any for a couple of years.

You can test the plug caps that you have on the bike and they should be showing 5 ohms  ;)
Title: Re: What are these plugs telling me?… and other questions!
Post by: Craizeehair on August 20, 2023, 08:44:15 AM



Just checked my stock on plug caps and only have one set left which i would keep as, this happened a few years ago with NGK plug caps and you could not get hold of any for a couple of years.

You can test the plug caps that you have on the bike and they should be showing 5 ohms  ;)
[/quote]

Thanks for checking, I just removed the number four cap and it basically fell off in my hand, the connection looked pretty poor, I think that I will look elsewhere and find a new set, I think you’re advice to replace them is the best course of action.


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Title: Re: What are these plugs telling me?… and other questions!
Post by: Trigger on August 20, 2023, 08:57:54 AM



Just checked my stock on plug caps and only have one set left which i would keep as, this happened a few years ago with NGK plug caps and you could not get hold of any for a couple of years.

You can test the plug caps that you have on the bike and they should be showing 5 ohms  ;)

Thanks for checking, I just removed the number four cap and it basically fell off in my hand, the connection looked pretty poor, I think that I will look elsewhere and find a new set, I think you’re advice to replace them is the best course of action.


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[/quote]


You can also use NGK SB05E instead of SB05F for number 2 and 3 cylinders . All you need for the SB05E's is a spark plug nipple  ;)
Title: Re: What are these plugs telling me?… and other questions!
Post by: Craizeehair on August 29, 2023, 10:22:07 PM
New plugs and caps fitted now, thanks for the pointers.

The strangest thing happened though, I used the spark plug socket from my set to remove all four plugs with no problem. When I went to fit the new ones a few days later, the spark plug socket would not fit into number three to tighten the spark plug, it just wouldn’t fit, it was too big and absolutely would not fit in the hole!

Luckily, the bike came with the original tool kit and the thinner walled, supplied Honda spark plug tool slipped straight in and worked a treat??

To say I was baffled is an understatement.
Title: Re: What are these plugs telling me?… and other questions!
Post by: Oddjob on August 29, 2023, 10:29:08 PM
Maybe number 3 wasn't fully wound in?
Title: Re: What are these plugs telling me?… and other questions!
Post by: Craizeehair on August 29, 2023, 10:41:37 PM
Maybe number 3 wasn't fully wound in?
I’m sure it was, they all needed loosening to get them out, when I put the new ones in the original socket just wouldn’t get anywhere the spark plug, it was just too thick / wide to go into the cavity. Really strange, I even got a friend to check as I thought I was being really dumb?

In all honesty, I freaked out a bit and thought I must have done some damage somehow when removing it but the old Honda socket worked perfectly, I just don’t understand how I managed to use it to take it out?

I managed to get the phone and take a picture as I was convinced I’d done something wrong and buggered it!
(https://uploads.tapatalk-cdn.com/20230829/acbc2d50da6516d912a7979fe64a8362.jpg)
Title: What are these plugs telling me?… and other questions!
Post by: Craizeehair on August 29, 2023, 10:41:54 PM
Double post
Title: Re: What are these plugs telling me?… and other questions!
Post by: McCabe-Thiele (Ted) on August 29, 2023, 11:26:48 PM
I am paranoid about crossing a thread on alloy cylinder heads so to start them off I try to fit them in a few turns by hand.

When access is more difficult like on bikes I have a short bit of rubber tubing that fits easily on the plug insulation.
I am then able to dangle the plug into position and rotate the plug with the rubber tubing sounds a lot of fiddling but it works easily.
I then nip it up the last few threads with my plug spanner proper.

PS: Could it be that the new plugs have a slightly different height section to the previous ones so that the hex head is slightly raised from the boss even though the steel section is overall the same height?
Title: Re: What are these plugs telling me?… and other questions!
Post by: Trigger on August 29, 2023, 11:43:41 PM
On a Cb750 it is very easy to cross thread the plus on number 2&3. I away have an old straight plug cap, pop the new plug in it and turn it in a couple of threads with the cap on before using a socket to tighten up  ;)
Title: Re: What are these plugs telling me?… and other questions!
Post by: Pauarc on August 30, 2023, 08:01:52 AM
Great tip with the old plug cap👏👍
Title: Re: What are these plugs telling me?… and other questions!
Post by: Spitfire on August 30, 2023, 09:52:53 AM
Everytime I change the plugs on mine I am extra carefully of the two inner plugs, I tend to use the Ted method with some rubber tubing but an old straight plug cap sounds really good as well. Sods law also says that it will be one of those that soots up, though Denso plugs solved that one for me ages ago thanks to Hondaman over on the US site.

Cheers

Dennis
Title: Re: What are these plugs telling me?… and other questions!
Post by: Trigger on August 30, 2023, 11:27:47 AM
Anything is better than dropping the plug in and putting a socket on  ;) I had to make up some tooling years ago to tap number 2&3, as it was always the engine out to do a thread repair .
Title: Re: What are these plugs telling me?… and other questions!
Post by: Skoti on August 30, 2023, 06:47:40 PM
Maybe number 3 wasn't fully wound in?
I’m sure it was, they all needed loosening to get them out, when I put the new ones in the original socket just wouldn’t get anywhere the spark plug, it was just too thick / wide to go into the cavity. Really strange, I even got a friend to check as I thought I was being really dumb?

In all honesty, I freaked out a bit and thought I must have done some damage somehow when removing it but the old Honda socket worked perfectly, I just don’t understand how I managed to use it to take it out?

I managed to get the phone and take a picture as I was convinced I’d done something wrong and buggered it!
(https://uploads.tapatalk-cdn.com/20230829/acbc2d50da6516d912a7979fe64a8362.jpg)

Aye,

same here, I can take the all plugs out with a 3/8" drive plug socket, but need to use the Honda tool kit plug socket to put cylinder 2 and 3 plugs back in.
I've got a bit of 10mm hex bar attached loosely to the top of the Honda socket so I can use a 10mm spanner to nip the plugs up.

I also use the rubber pipe trick Ted mentions, I believe it's called a "plug runner" in the trade.
Title: Re: What are these plugs telling me?… and other questions!
Post by: Oddjob on August 30, 2023, 08:22:12 PM
Pity the picture didn't show both plugs out as Number 3 looks to be a little worse for wear on the seat. If both were the same it would mean they were machined like that but if the other is pristine then maybe it wasn't tight and that corrosion is the result.
Title: Re: What are these plugs telling me?… and other questions!
Post by: McCabe-Thiele (Ted) on August 30, 2023, 11:51:38 PM
IIRC most old workshop manuals advise to fit new washer if you remove a spark plug to ensure good seating & avoid overtightening when re-fitting.

Can't say I have ever seen replacement plug washers for sale - then again I have not really looked.
Title: Re: What are these plugs telling me?… and other questions!
Post by: Athame57 on August 31, 2023, 12:40:32 PM
I had a problem like this a couple of years ago, a thread problem in the spark plug hole. I took it to a mechanic early on and I really expected to be told that the engine would have to come out, but he said we can sometimes get away with it and he managed to repair it in situ with a quick tap. I now approach plug changes gingerly but no more trouble so far. But it begs the question when the engine finally comes out for rebuilding is it a good idea to get inserts put in if it can provide a more secure thread? This was common practice with CB250RS owners in the London dispatch industry, the bolts retaining the rocker box sunk straight into ally and stripped threads there were one of this bikes curses, I went pre-emptive on couple of my four RS250s. 
Title: Re: What are these plugs telling me?… and other questions!
Post by: Matt_Harrington on August 31, 2023, 01:19:16 PM
Not to digress too much, what material is ideal for an insert if repairing a plug thread - also the type i.e. Helicoil or Timesert etc
Title: Re: What are these plugs telling me?… and other questions!
Post by: Oddjob on August 31, 2023, 01:21:26 PM
Timesert are stronger.
Title: Re: What are these plugs telling me?… and other questions!
Post by: Matt_Harrington on August 31, 2023, 02:07:56 PM
Thanks Ken. Brass, stainless or steel? ( a purely academic question....)
Title: Re: What are these plugs telling me?… and other questions!
Post by: Trigger on August 31, 2023, 02:25:55 PM
Steel  ;) stainless and brass is too soft for the job  ;)
Title: Re: What are these plugs telling me?… and other questions!
Post by: Matt_Harrington on September 01, 2023, 06:47:14 AM
Er, do you mean stainless steel? or mild steel?  ???
Title: Re: What are these plugs telling me?… and other questions!
Post by: Trigger on September 01, 2023, 07:56:20 AM
They are carbon steel. On my packet the grade is 12L14.  ;)
Title: Re: What are these plugs telling me?… and other questions!
Post by: Matt_Harrington on September 01, 2023, 08:18:24 AM
Ah, low carbon steel - thanks
Title: Re: What are these plugs telling me?… and other questions!
Post by: Trigger on September 01, 2023, 08:31:24 AM
The ones you use for spark plugs are copper plated to seal better on the spark plug. If you need a taper seat insert it comes with a sliver plate  ;)
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