C25 Exhaust blueing

[Andy Edwards]

Hello.  This is maybe a daft question, but does anyone know if incorrect valve timing could be responsible for the exhaust header changing colour?

I have only run my newly rebuilt C25 for a few minutes whilst trying to set up the carb. Initially the bike was reluctant to start or idle once started. I noticed the exhaust header becoming golden in appearance. Presuming that this was due to weak mixture I stripped the rebuilt carb again to ensure that all was as it should be. This carb was the original Concentric 928/1 with the pilot jet at the base of the carb body. I found nothing obviously wrong and also checked the carb flange and inlet manifold for leakage with nothing found.

I then double checked the ignition timing and found this to be about right at 35 degrees BTDC. After this work I found that nothing had changed and starting and idling was still poor and the exhaust header was changing colour.

Presuming this to be a carb issue I then fitted a 928/8 concentric as fitted to later B25's, this carb has the later idle bush pressed into the carb body opposite the pilot air screw. The engine now starts and idles as it should, but the exhaust is still changing colour!

As the carb seems to be set up correctly and I cant find an induction leak and the ignition timing seems to be ok, I'm now wondering if incorrect valve timing could cause the exhaust to change colour? If not, does anyone have any suggestions for other things for me to check?

             

[idie]

Usually a rapidly blueing exhaust is caused by ignition timing being out. Most likely retarded.

[V500]

I have only run my newly rebuilt C25 for a few minutes whilst trying to set up the carb.             

Blueing usually denotes timing is out and/or carb settings wrong (usually too weak), or air leak at carb or cylinder head exhaust port.
Then again, if it's a new pipe it could be just down to the material and chrome used.

[rhyatt]

Is the exhaust pipe stainless ?
If it is, then these do change colour even on a correctly set up bike

[Bigsingle]

You can restore your chrome back to shinny , with polish , I have returned blackened chrome using those felt discs for an angle grinder , with either compound or autosol. As well as the retarded timing and lean mixture suggested, a fan for cooling may help you a little , while in the garage .

[Andy Edwards]

Thanks for the replies.

The header pipe is new but is not stainless steel. I did initially suspect ignition timing or a weak mixture as being the cause, I have now though checked the ignition timing several times.

Although it starts and idles nicely, maybe I could check the fuel mixture with an old "Colour Tune" that I might still have in the garage.

The reason for asking about valve timing is that I was wondering if the exhaust valve was opening too early maybe fuel would continue burning as it leaves the cylinder head.

[DAVE BRADY]

Hi,

Unfortunately some pipes tend to 'blue' more than others regardless of timing etc. but perhaps related to the quality of the pipe.
Coating the inside of the pipe with VHT exhaust paint can help to prevent this to some extent. 

Dave.

[Andy Edwards]

Hi,

Unfortunately some pipes tend to 'blue' more than others regardless of timing etc. but perhaps related to the quality of the pipe.
Coating the inside of the pipe with VHT exhaust paint can help to prevent this to some extent. 

Dave.

Hi Dave

Thanks for that, I hadn't considered that it could be a material quality issue. I've got some VHT paint so I think I'll give that a go.

Andy

[Steve.S]

I agree with Dave. It's possible they were manufactured overseas? I notice modern Triumphs all have blue pipes.
Blueing can be removed with "Blue Away" from FART. It's a bit expensive for a tiny bottle, but it does work. Unfortunately after a lot of use, it wears away the chrome. Stainless steel pipes turn a golden colour.
Last Century, I recall reading a 'Guinea Tip' in one of the magazines, that suggested wrapping 3 of 4 turns of stout copper wire around the pipes close to the head. Mine have never blued, but I don't know if that's the reason. Might be worth a try. Who knows? 

[scifi]

As a quick and dirty method of checking your valve timing, you can put the engine at tdc on the non compression stroke.  The valves should both be open by the same amount.

Some engines do however have asymetrical overlap of the cams, so this method may not be accurate in those cases.

[Andy Edwards]

As a quick and dirty method of checking your valve timing, you can put the engine at tdc on the non compression stroke.  The valves should both be open by the same amount.

Some engines do however have asymetrical overlap of the cams, so this method may not be accurate in those cases.

Thanks for the suggestion, this is also shown in the Rupert Ratio book.  Having done this check it appears that the valve timing is correct.

I've removed the cylinder head to deal with a head gasket leak and its clear that there has been an exhaust leak between the head and the back of the finned collar on the exhaust header. Is it possible that this could have anything to do with exhaust blueing?

[V500]

Yep could do, but it depends on the severity of the leak really.

[Andy Edwards]

Having not touched the bike since last November, I'm now tackling the exhaust blueing again.

Would anyone know if a header pipe that had previously turned blue is more susceptible to it happening again later? Although I could see nothing wrong with either of the two carbs that I tried, I have fitted a brand new 928 carb along with a new tufnol spacer and flange gasket. The carb flange on the cylinder head appears to be as it should be and the valves and valve guides were replaced as part of the rebuild. The exhaust header has been sealed into the head with high temp silicone and the static ignition timing checked again. The header pipe was polished back to its correct appearance.

The bike starts and idles nicely but after about 10 minutes of running whilst setting up the slow running the header still turned blue.  Although the header and silencer did of course get quite hot, the gas leaving the silencer was barely warm. After letting it cool down, the blue on the header pipe polished off really easily in less than a minute with no real effort.  It almost felt as though I could just have wiped it off. Previously it took a bit of effort to polish away the colour. Hence the reason for asking if the header might now be more susceptible to changing colour?

[scifi]

If you set the slow running mixture to best revs, that is correct, but if you then go a little on the rich side, that will be best for the exhaust colour.   Remember, the screw is an Air-Screw, so turning it in increases the richness.

[rhyatt]

If you gave it staring, idling and running nicely , then all the required bits are there.
The fact that it goes blue , frankly wouldn't worry.
It may be something to do with the chroming process of course.
The bikes I had new in the 60s and 70s went blue as well
The manufacturers of the pipes and silencer of course use lots of different grades of steel.
Interesting that the temperature of the exhaust is moderate compared to what is leaving the head.
Could be the nature/design of the internal baffling stifling the gas flow.