E: Engine CR and ignition timing on a B series single

[CoreyL]

Hi All,

My B Meteor recently started to nip up - I caught it before it well and truly seized. I pulled the engine apart to look at the piston and liner. There were signs of extreme heat being seen by the piston. I rebuilt the motor (after cleaning up the liner and verifying the correct interference fit between liner and muff). I put a CP Carrillo forged +0.020" 8:1 piston in the engine with 0.005" piston clearance.

I measured the actual compression ratio while the engine was apart (using a buret and calculator) and found it to be 7.1:1.

I'm currently running a modern BTH magneto fully advance at 35 degrees BTDC. My question: does this sound like too much advance for a 7:1 CR and modern fuels?

Thanks in advance for any insights!

 

[chankly bore]

What type of spark plug? What grade of fuel? Was it a new properly round liner? What carburettor and jets? Any sign of air leaks on the inlet side or the cylinder head joint? As far as ignition timing and piston clearance that is what I would use. There are other things to check but this is a start. More knowledgeable folk will probably give you better clues.

 

[CoreyL]

What type of spark plug? What grade of fuel? Was it a new properly round liner? What carburettor and jets? Any sign of air leaks on the inlet side or the cylinder head joint? As far as ignition timing and piston clearance that is what I would use. There are other things to check but this is a start. More knowledgeable folk will probably give you better clues.

Spark plug: NGK B7EVX, 93 (US) octane pump gas, liner was properly bored and honed. Carb is an Amal Mk1 32 mm with 0.106 needle jet and 260 main jet, needle clip in the middle groove, #3 cutaway slide. No air leaks on the intake side. K&N air filter, Supertrapp exhaust..

 

[chankly bore]

Sounds like most things are roughly what other folk succeed with. Was the muff checked for lack of contact anywhere on the liner?Usual evidence is black marking in the area where the studs go through. Next thing to question is whether there's any evidence of heat elsewhere, like the exhaust valve. At what throttle opening did it nip up?

 

[greg brillus]

35 degrees does not sound like too much advance, you should easily be able to hear if the engine was pinking, detonating if this was an issue. How much tension did you use when pulling the head down........ ?........What crank assembly are you running, this can be an issue in part due to lack of oil getting to the underside of the piston, this usually shows if the piston pin is showing signs of turning blue, and the little end of the rod too.

 

[vibrac]

You should have a volume of about 81cc in the head for a 7:1 which is a lot for a claimed 8:1 piston which should be about 15cc less
(I always use play dough when engine is apart a lot less mess and leakage) anyway I concur with the others given your ratio is correct everything sounds about right I think the problem is not in the head but is somewhere not addressed by your rebuild more examination along the lines suggested above is required

 

[Bill Thomas]

Tight small end ?, Is there enough exhaust flow ?, Never seen those silencers, But have told the noise can be altered by how many plates used ?, What sort of speed do you ride at, To be safe, I would give more clearance.
If the barrel is not sitting square, You don't have the same clearance, Top and bottom.
If the conrod is not in the middle, Can be a problem. It's a strange one !.
Cheers Bill.

 

[vibrac]

Super trap is a good silencer I raced with one until the rules made it too loud the back pressure is under your control the more discs you add the better the flow and the noise, too few and you could restrict things I had a 2" inlet pipe silencer .Its difficult to find the right one because like so many web sites nowadays (hagon etc) they start and end with the modern "make?" "model?" stuff instead of also having a set of dimensions and suggestions

 

[Bill Thomas]

Just had a look in my little book, And looked back at Corey's history.
One of our Clever members said, Omega Pistons melt and go out of shape,
Because they are too big at the pin ?. Just a thought. Cheers Bill.

 

[timetraveller]

Are you sure about those values Tim? I think that for a 7:1 it should be 83 ccs and for 8:1 it should be 71 ccs. Years ago I did a very careful job of measuring the volume of a standard Vincent cylinder head. I made it 103.5 ccs That was done with a Perspex blanking plate and a good burette. Anyone else like to chip in on this?