Topic: Cylinder Head Blues, Dentonation?

[Rocketman]

One of my winter projects is upgrading my CB550 vintage racer with higher compression pistons and a higher lift cam.  With all the parts and a freshly bored cylinder sourced I tore into my engine today.  With the engine running well and only 500 miles on it no surprises were expected.  Much to my shock I found damage that appears to be detonation.  The top of the pistons have a grainy texture as do the combustion chambers.  There is metal eroded away in the combustion chambers.  The sparkplugs after the last race look good, so this may have happened earlier in the season.  Hoping some of our more experienced wrenches can take a look and advise if my analysis is correct.

Here is one of the pistons.



The cylinder head.



My understanding is detonation can be caused by gas with too low octane or too much advance in the timing.  I appreciate any and all feedback.  So the hunt is on for a good head which will need vapor blasted, ported, valve guides & seals, & over sized valves by the first event at the end of April.  No pressure!   

[bear]

Seen a lot worse.
Did you do a leak down before you pulled it?

[grcamna2]

Where did you have your timing ?

[bwaller]

Yes, how much ignition lead and what type ignition were you using?

[Rocketman]

No leak down performed before disassembly.  I use the "F" mark for static timing  which is 5 degrees, but never checked the advance which the manual lists as 28 to 31 degrees.  The ignition is an Accent unit.  Lots of used heads on eBay.  Any concerns mixing heads and rocked assemblies as far the clearance of the camshaft?

[grcamna2]

No leak down performed before disassembly.  I use the "F" mark for static timing  which is 5 degrees, but never checked the advance which the manual lists as 28 to 31 degrees.  The ignition is an Accent unit.  Lots of used heads on eBay.  Any concerns mixing heads and rocked assemblies as far the clearance of the camshaft?

Do you have any pics of your ignition set-up and coils ?

[Old Scrambler]

Check your center studs..........pretty sure the early heads have 4 studs and last-year heads have 6 or more.

I have an early head in good shape.............yours if you need it for the shipping from Wisconsin. 

[Rocketman]

Thanks for the offer Old Scrambler.  I'll PM you.

Here are a couple pictures of the electrical system.  I am running total loss.

[bwaller]

Normally in this small combustion chamber with adequate squish there is plenty turbulence for fairly efficient combustion without much ignition advance. You might want to check timing numbers at full advance. The Accent uses the stock mechanical advance? What fuel were you using?

[scottly]

What fuel were you using?

I'm wondering the same thing; the pitting looks more like corrosion than detonation.

[johno]

I was thinking the same thing Scotty,
Although if its moisture in the air from damp weather it is usually more prevalent on some cylinders than others, which ever valves were open.   If every cylinder was the same kinda gets back to detonation, mind you as a very experienced person with catastroffic detonation in my engines the head was never marked or pitted only deposits of ally but the piston sure didnt like bang bang 


If you get bored go for a close up on the head and detailed description of what it looks and feels like ie deposits on or pitted holes in etcciao johno

[johno]

PS 

also another indicator if it was moisture , will be the ring line on the bore, if detonation the bore should be clean only deposits on,  not pitting in,  due to moisture.

Kinda wish it was mine     I'd check valve seating , give piston and chamber a polish and back to work for another season.

[bear]

Kinda wish it was mine     I'd check valve seating , give piston and chamber a polish and back to work for another season.

Sounds like a plane to me

Cheers,
Brian

[Retro Rocket]

Kinda wish it was mine     I'd check valve seating , give piston and chamber a polish and back to work for another season.

Sounds like a plane to me

Cheers,
Brian

You flying planes now Brian..?   

[Old Scrambler]

He used BABLEFISH to translate..............

[Rocketman]

I'm using the stock advance unit.  I am embarrassed to say I did not check to see what the total advance was.  For fuel I use Turbo Blue 110.  I am fortunate to have a climate controlled shop, so not likely moisture.  Could be something in the fuel though.

Looking at several replacement heads.  Anyone have a concern mixing the cam covers or are they a matching set?

[bwaller]

No problem changing valve covers, no matching issues. One experienced guy in New England found he was finding huge issues with the race fuel he used. TimV knows the whole story, I'll ask him to fill us in. I have found a leaded fuel I use is definitely corrosive and I now run unleaded through it after each race weekend before draining. It's a nuissance.

I'm not suggesting your issue is entirely corrosion related, but I now understand we may not be sure of a fuel's volatility or it's ability to vaporize. If all four chambers are identical I lean towards this being a potential cause.

I'm no bloody expert on fuel or detonation.

[MRieck]

Most race fuels are corrosive especially ones that contain ethanol. I'd by some VP Small Engine Fuel to run through the system after using the race fuel. No ethanol in it. Great in yopur lawn mower etc too.

[turboguzzi]

my experience with a similar motor:
what CR are you running? 12:1 should be safe with such a small piston
how close is your squish , should be under 1mm, more like 0.75mm
did you machine the piston to have a chamfer parallel to the band in the head?  without this you get no squish action
am running 28 degrees max advance in mine, check how much you are up to.
we are lcuky to have here 98 from th epump, with some aditives i guess i get close to 100, 110 should do

[Rocketman]

Based on all the feedback I am leaning toward corrosion.  The damage does not seem consistent with detonation.  Who knew.  Thanks to all who offered advice.

My current set up has the CR is 10.5:1 and the piston squish is under 1mm.  As good as the sparkplugs look I think my advance is OK, but will need to check with a timing light once I put the motor back together.

[turboguzzi]

if there was detonation then you'd also feel it when racing, specially accelerating, the metallic pinging is really hard to miss.

[scottly]

I took a look at the Turbo Blue 110 specs, and it contains 0.0% oxygen, meaning no alcohol (methanol or ethanol).
Did you take any pics prior to cleaning the head and piston?

[bwaller]

One more thing Martin, were the plugs in the head until you tore the top end apart? If so & it is corrosion the plugs ends would likely be badly rusted. That's what happened in my case and getting the plugs out was a real issue.

[Rocketman]

When I tore the top end off both the head and pistons had black carbon deposits.  Nothing that caused any concern.  No evidence of rust on the plugs.  They look very normal, just a little rich which struck me as odd as they had been tan/white previously and the dyno runs had the fuel air just a bit rich.  Will post some pictures tonight along with the last air/fuel from the dyno.

[Rocketman]

Some additional information on my air/fuel.  Here is the dyno run for the jetting last used.



And here is a photo of a sparkplug after the last race.



With no other evidence of corrosion I am back to thinking detonation.  A head is on its way from an eBay seller and Mr. Rieck has fit me into his schedule.  Had wanted to put off a new head until next season to spread the cost, but guess I'll have the full race porting sooner as opposed to later.  A very expensive silver lining!