'72 combat detonation

I'm going to start a new string rather than go back to my old one.

Still have the pinging problem with my combat engine. Last Sunday on the Day After Ride from the Clubman's show, it ran well in the morning, but by afternoon was pinging again. This is using 91 octane fuel with a 50:1 ratio of gas to Lucas Oil Produts octane booster. Afternoon temp around 70 F.

Here are the posibilities I see
1. really poor gas out there - but have not tried race gas yet
2. something wrong with the carb mixture - though not likely - sleeved original Amals set up properly
3. something wrong with the Lucas RITAS ignition system - also not likely, but noticed the spark brightness is dimmer kicking through than when I push the kill buttom. Strobing shows it advances with maximum at 28 degrees.

Will try
1. race gas
2. switch to a single Mikuni to see if any difference
3. put the points ignition in to see if any difference

Comments? Thanks.

I'd be inclined to retard the ignition until it disappears and see how it runs. With standard timing on my 850 with the Tri-Spark ignition mine pings. I think my timing mark is out by about 5 degrees from what I can gather. Runs fine now. I don't think the carburation is an issue with pinging.

This is my third season running Race gas and premium 50/50. No mixture issues as of yet. My setup also includes a 36mm Mikuni and 1 1/2 id exhaust. (Breath in, Breath out) I saw a 36mm on Ebay today. Manifold and cables are good at RockyPoint Cycles. I also have a Pazon unit and 5 ohm duel output coil. Dead on 31 degrees.

Start with the gas.

http://cgi.ebay.com/ebaymotors/TRIUMPH- ... 45f1fab912

http://www.rockypointcycle.com/

If you've a small airport near you try to buy some avgas 100ll lot's cheaper than race gas if you plan on using it regularly

I have a '72 combat with everything stock except running a Boyer set per the instructions at 31 degrees at 5K rpm with a Dyna twin tower coil. Can run no name regular gas (from Turkey Hill near me - similar to a Wawa) in it with no pinging. Maybe we just have better gas around here - pumps marked 10% ethanol. I have a paper cylinder base gasket and flame ring head gasket.

Maybe try some Avgas from a small airport in your area just to see if that helps. The one in our area sells it no problem to bikers or most likely anyone with a gas can! Have you tried it without the octane booster snake oil?

rgrigutis said:

If you've a small airport near you try to buy some avgas 100ll lot's cheaper than race gas if you plan on using it regularly

Click to expand...

Aviation fuel, what a great idea. If race cost 5.95 aroung here what may avigas cost?

Unfortunately, here at Washington Dulles it's (100LL Avgas) $6.70 (with HiTest Auto gas at $3.10)

But hey ! Cheap expense for one's fav toy.

Is it correct that you are having pinging at low RPM's under load? Is your Combat motor stock? High compression with the 2S cam? I've known a couple of people that swapped out the cam for something more mild, and kept the compression the same. They started experiencing pinging where they didn't before. The learned opinion at the time was the a hot cam minimizes pinging in a motor with a given C.R. because more radical cams bleed off the charge till the wonders of resonance take over, and it 'comes on the cam'. They actually give away quite a bit in volumetric efficiency, yielding a lower effective compression ratio at low RPM.

Read all you can by this guy. http://www.tioc.org/tech_article.htm
This article about tuning a Mic. But it applies well here. He has some stuff on “toughening your engine to detonation” that are very good.

Illf8ed,

I haven't searched out your previous post, but the obvious question is whether or not you have the timing correctly set. Be advised that if you are running an electronic ignition, the timing should be dynamically set at a fairly high RPM. For a Boyer that's around 5K. If you set the timing at too low of an RPM, the timing will be too advanced and pinging will result.

I have a combat engine and run regular (89 octane) without pinging. Unless you have something special about your engine, I suspect that your timing is too advanced, your mixture is too lean or both.

Norbsa thanks for the Kevin Cameron article - will go through that. He's a bit too technical for me and usually pass on this Cycle World articles.

My engine has the original double S cam, paper base gasket, flame ring head gasket, sleeved original 932 Amals with 240 mains, timing on a Lucas RITA set at 28 degrees max advance at 4,000 rpms. I've played with the timing down to 20 degrees which helps, but the engine doesn't perform well with less than 26 degrees. Main jets have gone to 280 - again runs best with smaller jets - 230 is stock. Race gas is available in my town, just inconvenient to get there as I work during the time they are open - maybe will send my son on and errand.

My thinking is it ran fine for nearly a decade after the rebuild as set up now. What happenned to change the performance - systems degraded or fuel degraded.

The last time I checked compression was at the end of last season. Using a Sun screw in gauge kicking through about 7 cycles with the throttle open I got 185psi right side and 183psi left side.

I've seen a factory note advising dealers and owners to retard the ingnition by 2 or 3 degees if 5 star petrol wasn't available. Though Avgas will work I'm sure, try a pint a gallon.

Cash

For what it's worth, do a "Plug Chop" with fresh plugs immediately after you get it pinging.

Never mind if it's not pinging immediately after swapping plugs, just run out the routine on the plug chop.

Report back with results...

Seeing that it ran for 10 years after the rebuild I would look at things that would have changed over time.

Do you know what your compression was after the rebuild because 185/14.7= 12.58 to 1 (sea level) which you are at. I thought combats were 10 to 1 CR. Someone correct me if this is wrong.

You might have carbon build up. That could cause your pinging and the high CR.

Chuck

If you do the required reading you will find that it could be as simple as a spark plug just a little too long or a head gasket edge getting too hot. Carbon build up getting super heated because of to rich down low and leaner up top. The Norton head to piston is one of the best out there if we were talking BSA's and Triumph's they are way more prone. Just read and learn it's worth the time.

ChuckW said:

Seeing that it ran for 10 years after the rebuild I would look at things that would have changed over time.

Do you know what your compression was after the rebuild because 185/14.7= 12.58 to 1 (sea level) which you are at. I thought combats were 10 to 1 CR. Someone correct me if this is wrong.

You might have carbon build up. That could cause your pinging and the high CR.

Chuck

Click to expand...

Hi Chuck,

Was advised compression ratio has nothing to do with cranking compression. Carbon build up maybe, but I de-coke every 5,000 miles and it has 2500 since the last. I got a similar response in the passed when I mentioned the cranking compression numbers so qualified with instrument and condition for obtaining those numbers. Have heard from other combat owners that get the same compression readings.

Norbsa,

Will do the reading, thanks. Spark plugs are per original spec and the flame ring head gasket doesn't have any metal mesh that might get into the combustion chamber.

Thanks,