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#522256 - 01/10/14 5:45 am Bsa c15 overheating and nipping up  
Joined: Jan 2012
Posts: 109
jamie weeks Offline
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jamie weeks  Offline
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Posts: 109
sussex
has anyone had this problem,the engine has been meticulously rebuilt,the bore clearance is 4 and a half thou,the timing is not advanced,the oil is good quality,the oil pump is good,it has been over bored by 2mm and has a triumph Daytona piston( a common mod back in the day) it runs beautifully--but it nips up when on a long incline.it has an anti sumping valve,but the oil returns to the tank immediately so don't suspect that,any ideas guys.

Last edited by jamie weeks; 01/10/14 5:46 am.
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#522257 - 01/10/14 5:53 am Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
Joined: May 2013
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Allan Gill Offline
Allan Gill  Offline



Joined: May 2013
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Rotherham - S. Yorkshire
did you check the ring gaps when fitting? I was asked to look at a bantam barrel/piston with the same problem the rings were too tight in the bore, I have also known of someone else who had a Daytona with the rings too tight for the bore, unfortunately he continued to run it until it fudged up the barrel.


beerchug
#522258 - 01/10/14 5:53 am Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
Joined: Dec 2004
Posts: 8,877
kommando Online content
kommando  Online Content


Joined: Dec 2004
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Scotland
Needle is set too low on the carb plus maybe the main jet is too small (what throttle opening were you using will tell which it is) would be the first thing I would look at. 4.5 thou could be a bit tight, 5 thou would be better but its marginal and still more likely to be weak mixture. What changes did you make to the carb to take account of the bigger piston.

#522259 - 01/10/14 5:59 am Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
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jamie weeks Offline
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jamie weeks  Offline
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sussex
It has a Concentric carb,jetted as recommended in Rupert's guides,it happens when it is on probably 1/4 throttle opening,the plug is not white but darkish coloured,it it is on the middle needle groove at the moment,I will take your advice and raise it,thanks,I am going to lift the barrel today to inspect the piston etc.

#522263 - 01/10/14 7:41 am Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
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Allan Gill Offline
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Rotherham - S. Yorkshire
Kommando's notes had made me add this. If the carb is worn / you have play in the slide. You can go up about 3 more main jet sizes before it starts to come on tune. If the carb is good, the bigger vacuum signal placed on the carb ( by the bigger bore size) should correct itself to a point. Fine tuning is down to you.


beerchug
#522264 - 01/10/14 8:11 am Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
Joined: Jul 2009
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Pete R - R.I.P. Offline
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Pete R - R.I.P.  Offline
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I suspect detonation could be causing this,even though you may not have heard it "pinking".
What's your true compression ratio?Have you measured it?Have you allowed for that with your ignition timing,or did you just use the standard setting?It could be over-advanced.

The piston might tell you the true story.

#522266 - 01/10/14 9:13 am Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
Joined: Dec 2004
Posts: 8,877
kommando Online content
kommando  Online Content


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You best tell what jetting you have set the carb too and what Concentric you are using as there are no recommended C15 with Triumph piston & Concentric carb settings in my Rupert Ratio.

#522281 - 01/10/14 11:30 am Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
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Alex Offline
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Alex  Offline

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Seattle
If you are not using a stock carb setup you absolutely must jet each operating range of the carb. Recommendations are just a starting point and are not enough to jet the carb. Learn to jet your carb or find someone who knows how to do it right.


A smattering:
'53 Gold Flash
'67 Royal Star
'71 Rickman Metisse
'40 Silver Star
'37 Rudge Special
sixtyseventy Lightboltrocket road racer...and many more.
#522314 - 01/10/14 3:59 pm Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
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gunner Online content
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Farnham, Surrey, UK
When running in, the usual procedure is not to let the engine labor. It sounds like running at 1/4 throttle on a long incline would cause some stress on the engine and may cause nipping up.

The BSA C15 user guide states its usually advisable to shut the throttle every couple of minutes as the vacuum helps draw oil up the cylinder bore and maintain lubrication. Additionally, its best not to let the engine labor, so change down a gear and let the engine rev a little higher.

A good way to assist running in for the rings is to find a short steep hill and charge up it in second gear a few times, shutting the throttle at the top. The increased cylinder pressure helps bed the rings against the cylinder wall and shutting the throttle at the top should help draw oil back into the cylinder.

I would also remove the anti sumping valve, at least whilst running in. These valves may be OK for some bikes but a C15 doesnt exactly have a big oil pump and may not have enough suction at 1/4 throttle to feed the engine.

Last edited by gunner; 01/10/14 4:01 pm.

1968 A65 Firebird
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#522322 - 01/10/14 4:37 pm Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
Joined: Sep 2002
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Alex Offline
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Alex  Offline

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Seattle
Inadequate running in has nothing to do with piston seizure. Piston seizure can be caused by the following:
1. Inadequate poston clearance when built
2. Piston overheating caused by
a. Incorrect timing
b. lean mixture

Since 1. Was said to be correct let's assume it is

That leaves 2.: a. while the static timing is said to be correct, there is some chance that backlash in the distributor is throwing the timing off. Early unit singles are notorious for this. Remove your primary cover and check how your timing changes with a strobe. Make some marks on the alternator to check the aau function and for spark bounce.

2b. It still sounds to me like the most likely cause is jetting due to the non-standard carb. Also, the triumph piston will give very high compression without a compression plate (10:1+) making the motor much more sensitive to a lean mixture. Search this forum for info on jetting your AMAL Concentric and forget about the stuff you read in the clymer manual about plug color.


A smattering:
'53 Gold Flash
'67 Royal Star
'71 Rickman Metisse
'40 Silver Star
'37 Rudge Special
sixtyseventy Lightboltrocket road racer...and many more.
#522328 - 01/10/14 5:11 pm Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
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triton thrasher Online content
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What clearance do the piston manufacturers recommend?

At that bore size and a typical Hepolite or Taiwanese piston, 4.5 thou sounds tightish.

It seized after a rebore. Logically, what does that suggest?

And you absolutely must follow the carb tuning drill. Relying on some suggested jetting is suicidal.

If I remember right, stock ignition timing was 1/16" BTDC fully retarded. That's not good enough. You need timing to be correct at full advance.


Amateur Loctite enthusiast.
#522329 - 01/10/14 5:26 pm Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
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Alex Offline
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Seattle
TT, manufacturer's spec is 3-3.5 thou. The piston is only 69mm in this case (triumph piston). I run 3.5 thou on my C15 MXer and have had no problems...with piston seizure, anyway grin


A smattering:
'53 Gold Flash
'67 Royal Star
'71 Rickman Metisse
'40 Silver Star
'37 Rudge Special
sixtyseventy Lightboltrocket road racer...and many more.
#522331 - 01/10/14 5:31 pm Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
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triton thrasher Online content
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triton thrasher  Online Content
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Something over the rcommended clearance should be safe, if the rebore is accurate and you have the right piston for that clearance.


Amateur Loctite enthusiast.
#522387 - 01/10/14 11:31 pm Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
Joined: Mar 2005
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John Healy Online content
John Healy  Online Content


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Boston, Massachusetts
Alex, don't forget, If the rings haven't seated you do not have to have problems with timing or fuel mixture to have a piston seizure. The source of the abnormal combustion, which is usually created by fuel mixture or timing, can be created by an abnormally hot piston.

Piston rings remove up to 75% of the combustion heat absorbed by the piston. If the rings haven't seated they will insulate the hottest part of the piston from the essential cooling provided by the rings. A lot of inexperienced mechanics will distort the ring installing it on the piston. The high spots created keep the ring from seating properly.

-Seizures caused by rings can be identified by a four corner seizure.
There will be scoring just forward and aft of the piston pin boss.
-The area on the thrust face of the piston just under the oil ring will be polished, or show some signs of seizing.
-A lot of the ring face will look like it just came out of the box. There will be a few areas where the ring has seated.
-When checked, the piston clearance is correct.
- The wrist pin will show signs of heat distress (discolored - light straw to dark purple or black).

The pistons, wrist pin and rings hold a lot of clues when you have problems caused by rings.
HTH


#522400 - 01/11/14 3:03 am Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
Joined: Dec 2004
Posts: 8,877
kommando Online content
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Scotland
And he may be using the 10/30 oil he asked about in an earlier thread, lots of informed replies but no more posts from Jamie to see what he actually thought or did based on the advice not to use it.

#522438 - 01/11/14 10:25 am Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
Joined: Mar 2005
Posts: 9,962
John Healy Online content
John Healy  Online Content


Joined: Mar 2005
Posts: 9,962
Boston, Massachusetts
10/30 would certainly stress a C15, but not the piston. What we would be looking for is a big end seizure. The viscosity of the oil at running temperature would not allow the hydrostatic wedge to form. It is this "wave" of oil that supports the loads the bearing must endure.


#522468 - 01/11/14 1:00 pm Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
Joined: Jan 2012
Posts: 109
jamie weeks Offline
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jamie weeks  Offline
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Posts: 109
sussex
Have had the bore clearance checked,it was supposed to be 4 to 5 thou clearance but was 3, it has now been taken out to 5, the needle has been raised,it is using a good quality
20/50 mineral oil and sounds good,the proof will be in the pudding when I road test it tomorrow,thanks for all you help guys.

#522471 - 01/11/14 1:18 pm Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
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triton thrasher Online content
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triton thrasher  Online Content
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scotland
It will certainly take more heat to make the piston squeeze against the cylinder now.


Amateur Loctite enthusiast.
#522494 - 01/11/14 3:58 pm Re: Bsa c15 overheating and nipping up [Re: jamie weeks]  
Joined: Mar 2005
Posts: 9,962
John Healy Online content
John Healy  Online Content


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Posts: 9,962
Boston, Massachusetts
Jamie, you make it hard to make reasoned comments. If the bike was truly "nipping-up" as you said in the first post there would have been a lot things about the piston that I would be looking at, besides the correct piston clearance.

Even at .003" clearance, it would take some abnormal combustion to increase the piston temperature enough for it to "nip-up." Were there any signs of detonation:
a. metallic spots on the spark plug center porcelain. (we routinely raced Triumph 500's using .003" piston clearance - so clearance is not your only concern)
b. Areas on the piston crown where carbon had collected that had small areas where it was literally blown of the surface. You can see the aluminum through the carbon.
c. Polishing of the thrust face of the piston in the area just below the oil ring.
d. Signs of a four corner seizure.
e. Discoloration of the wrist pin. It should remain the same color it was when it came out of the box. (The wrist pin is retains any discoloration forever, as a witness, to the piston becoming an abnormal temperature.)
f. Also the rings retain, as a witness, any abnormal wear pattern from distressing during assembly or inability, for several reason, to wear a good contact pattern with the cylinder.

Air cooled engines have very little resiliency when it comes to operating temperatures. There is no water flowing through the engine to help maintain normal engine temperature. So when things start to go wrong, they go terribly wrong very fast.

One thing you have control of... That is dynamic cylinder pressure. The pressure inside the cylinder at any one time is of course dependent upon the compression ratio of the piston you selected. But as important, maybe more important, is understanding the role of the load on the engine and the amount of throttle the operator chooses. Both of these have a direct effect upon the dynamic cylinder pressure and if the engine will develop abnormal combustion and destructive detonation. Fail to understand this, even with .005" clearance, it is more than possible to create enough heat to seize the piston.

You reduce the load, and thus the dynamic cylinder pressure, on the engine by using a lower gear when accelerating. One of the worst things you can do is open the throttle while in a higher gear at an rpm where the bike doesn't briskly accelerate. This especially on a new engine where the rings haven't fully seated. Seating the rings toughens the engine against potential detonation and once seated there is enough safety factor built in to help protect the engine from over heating.

It is deadly to come to even a slight grade in high gear and roll open the throttle and have the engine struggle to gain rpm. Opening the throttle allows more air to enter the cylinder. The more air you allow in, the higher the dynamic cylinder pressure will become... seizure is a few revolutions away. If you open the throttle, especially on a new rebuild, the bike must accelerate smartly. If it struggles you are experiencing high dynamic cylinder pressures and you don't have a water thermostat and radiator (or computer) to come to the rescue. This engineering pre-dates the luxury of computers and You are in charge. In your modern bike or car there are engine management systems that do all this for you (the first thing a car will do is shift to a lower gear), But if you ride one of these old bikes You are in charge of the dynamic cylinder pressure...

The Triumph TT is a good example of a design that has a narrow safety factor when it comes to detonation. High compression, fairly standard cams that close quite early for an engine with such compression, but ridden with a bit of understanding of the factors involved one can get away with running 87 octane gasoline and not experience detonation. Control the throttle relative the load and you have a narrow window where the bike will not experience a seizure.
Morgan, sorry for the Band Width,







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