Rich smelling exhaust nc700x

[Xactly 5,398]

As no compression test was carried out we don’t actually know whether or not the valves were being held open, reducing compression. I don’t know whether a setting of roughly half what it should be is sufficient when the engine is hot to prevent proper sealing of the valves. Some old Hondas had clearances as small as 0.002 inch. The difference in clearance here is getting on for three times that amount, so maybe the valves were not sealing when hot. Certainly I wouldn’t want them that far out of spec. Empirically, like the bumblebee can fly, it appears to have worked….

I don’t know tbh, but I’m not as surprised as you are.

[SteveThackery 3,090]

11 minutes ago, Xactly said:

I don’t know tbh, but I’m not as surprised as you are.

 

I'm surprised because I know of no mechanism by which tight exhaust valve clearances can reduce the combustion temperature sufficient to cause blackening of the plug.  I'm also puzzled by the petrol smell in the exhaust - that normally comes with a rich mixture (i.e. not enough oxygen for the amount of petrol), which reduces the temperature (blackening the plug) and ejects unburnt fuel into the exhaust (making the smell).

Obviously I am NOT saying tight exhaust valves CANNOT cause this.  I'm just saying that having studied motorcycle powertrains at uni for three years I can't see any possible way it can happen.  That's why I think the exhaust valve clearances are a coincidence and it was actually something else.

I would be DELIGHTED if someone could explain why I'm wrong - every day that I learn something is a good day.  

[SteveThackery 3,090]

So, I've done some Googling, and can't find anything that links exhaust valve clearance with mixture richness/weakness.

 

Can anyone who recommended checking the valve clearances provide me with a link or some kind of reference?  I don't like having gaping holes in my knowledge!  

[Xactly 5,398]

I think it was a mate of the OP that suggested checking the valve clearances; it certainly wouldn't have been my first suggestion. I was simply trying to suggest how it could have been the culprit, since empirically it appears to have worked. I can't see what else could have been responsible as apart from the spark plugs and caps, already discounted unless the cap was not properly seated. It seems to me that either too much fuel was being delivered into one cylinder, or the fuel being delivered was not being burned properly. As there is only one O2 sensor and the engine ran with no fault showing I would surmise that the mixture being delivered was correct, or at least within tolerance, but that it was not being fully burnt in one cylinder. A leaking injector could have explained it and was suggested but not tried. So, with the fuelling deemed to be sufficiently accurate, the plug renewed and leads swapped over, what is left is some other cause for incomplete combustion. If the exhaust valve is not fulling seating then the mixture won't be full compressed and it is this that seems to me to present a plausible, though quite probably incorrect, reason for the failure fully to burn the mixture in one cylinder. I doubt we'll ever know unless the fault recurs. 

[SteveThackery 3,090]

4 hours ago, Xactly said:

As there is only one O2 sensor and the engine ran with no fault showing I would surmise that the mixture being delivered was correct, or at least within tolerance, but that it was not being fully burnt in one cylinder.

 

Indeed.  What I find interesting is that you could smell fuel in the exhaust, and the lambda sensor was stuck at 0.98V output (indicating rich mixture).  The ECU should certainly have picked up on that (i.e. the mixture not responding as the fuel injection period was varied) and reported a fault.

 

As you say, without further data we'll have to log it in the "mysterious" section of our collective knowledge bank.  

[Tony Roche 17]

Morning guys! I've just noticed the new posts since my last one from Steve and Xactly..... thanks guys. I have an update for you both as it seems my celebration was a bit premature! So the valve clearances adjustment certainly improved the situation, reducing the rich smell and the exhaust cough but I was a bit hasty concluding it was back to "normal". I was ready to take the injectors out but my mechanic felt that if something was significantly out with one injector, the bike wouldn't be running as well as it is. He suggested a high quality fuel system treatment. I bought the ethanol treatment by Motorex and have run three tankfuls through on the "shock" mixture which is 10ml/10litres of fuel and I'm using high octane fuel. It's further improved things and the bike is running very well!! However, there's still a milder richness in the exhaust smell......which is subjective I know, and a slight exhaust cough - which is less subjective. I'm planning to take the r/h plug out today to have a look. I'm not sure if the ethanol treatment will clean up the sooty plug or just improve the condition that was causing it to be sooty. I'll replace the plug again if necessary but will post an update later. Cheers Tony

[Tony Roche 17]

..... I forgot to add that my mechanic used his digital temperature sensor on the cylinders at idle and there's less than 5% difference between left and right

[Tony Roche 17]

....and my engineering mate did a compression test (after I'd done the valve clearances) and concluded they were close enough.

[Tony Roche 17]

As Xactly has suggested, my mechanic said if the exhaust clearances are tight they don't close fully when hot leading to improper compression and combustion which explains the rich smell.

[ChrisCB 6,672]

1 hour ago, Tony Roche said:

....and my engineering mate did a compression test (after I'd done the valve clearances) and concluded they were close enough.

One man's "close enough" is another man's "needs a rebore"

[Tony Roche 17]

:-)

I like it Chris

[Tony Roche 17]

So I've just taken the r/h plug out, which was sooty from before the fuel treatment went in, & cleaned it with a blow torch and a soft brush. (Not sure the Iridium plug appreciated that!) I've been for a 15 minute ride and the plug is sooty again. I obviously still have an issue on the right hand side. This is clearly not affecting the performance of the bike which is going really well. It's of academic interest now. I'll get the injectors out when I get a chance and get them tested by my mechanic. I might do the compression test again next week and record the numbers for interest. 

[Xactly 5,398]

2 hours ago, Tony Roche said:

So I've just taken the r/h plug out, which was sooty from before the fuel treatment went in, & cleaned it with a blow torch and a soft brush. (Not sure the Iridium plug appreciated that!) I've been for a 15 minute ride and the plug is sooty again. I obviously still have an issue on the right hand side. This is clearly not affecting the performance of the bike which is going really well. It's of academic interest now. I'll get the injectors out when I get a chance and get them tested by my mechanic. I might do the compression test again next week and record the numbers for interest. 

If I remember I’ll have a look in the W/shop manual to see what the pressures should be and maybe what the lower limit is.

[Tony Roche 17]

Thanks Xactly. I'll ask my mate to pop round with his compression test kit this week

[Tony Roche 17]

I've just checked my manual. Cylinder compression should be:

1,775kPa (257psi, 18.1kgf/cm2)

 

[Tony Roche 17]

On 15/07/2021 at 16:50, Tony Roche said:

Hi again guys. Thought I'd post a quick update. Air filter is clean as a whistle and the intake/temperature sensor is working. Resistance when cold is about 1800 ohms and 30 seconds with the hair dryer brought it down to 700. I've fitted the "new" coil but I can't remove the old one properly as the HT leads have those little clips attached which slide over lugs on the frame. Lowest one is easy to reach, second highest I did by undoing the radiator, but I have no idea where the highest one is! I can't even see it let alone unclip it. It's somewhere underneath the air filter housing which is attached to the intake on the engine...... obviously! Has anyone replaced the right hand coil before? Cheers Tony

I've just checked the service manual guys and noticed that the IAT sensor value at 20C should be between 2.2 - 2.7kOhms. As mentioned in my previous post I assumed it was working because the resistance changed between ambient and hair dryer treatment. But although I don't remember the ambient temperature on the day I tested it, my figures are lower than the service manual says they should be. Could this be a factor after all and would it affect only one cylinder?

[Xactly 5,398]

A faulty IAT sensor would affect both cylinders if, as in this case, there is only one sensor. As you reckon the air filter is clean another possibility is that there’s something restricting the r/h inlet tract.

[Tony Roche 17]

That's a possibility...... I guess the inlet tracts are down in the same area as the injectors..... behind the battery box?

[Tony Roche 17]

[Tony Roche 17]

Hi guys. Here are my injectors. I was delighted to see even with the naked eye that the r/h one looks "dodgy" compared to the clean looking one! I'm taking them to my mechanic tomorrow for investigation. I've no idea what causes this but I've a strong suspicion this is the root of my problem. I'll post another update asap. Cheers Tony

[t5er 74]

I have been encouraged by your tenacity and perseverance regarding your problem. Your pictures do indicate a problem and the methodical process you have taken does you justice.You must be near to finding the fault.....

My hat off to you sir

[Jamesc 3,655]

I've used these guys before to clean car injectors

 

https://www.injectortune.co.uk/

[Tony Roche 17]

Thanks for your message t5er..... I was in too deep to go back!! Hope to get the injectors back later and put everything together over the weekend. I'll post an update. Cheers

[Tony Roche 17]

Hi again guys. Injectors are back in and the bike is truly purring like a kitten! No exhaust cough or rich smell! I'll put everything else back together tomorrow and go for a ride. I'm quietly confident this time........

[Tony Roche 17]