Ignition timing advice

[Joachim]

I need some advice about ignition timing.

I rebuilt my 850 MkII engine last year. I have now following setup:

• PW3 camshaft
• Raised compression to 9.4:1
• Light porting of inlet ports; widened the ports around the valve guides by a few millimeters (not flow bench tested)
• Trispark ignition
• Amal premier 932

I first adjusted the ignition to 29 degrees. The engine ran well. I played a bit with the ignition timing and realized significant changes. When retarding the timing to around 25 degrees, there was a noticeable better performance at least low and mid range. I rarely run over 5000rpm so hard to tell if it was better with more or less advance. I don’t really know where the ignition timing is now but I know the point when the bike seems to be running well and where the point is when the engine is just not pinging.

I have a dyno appointment on Tuesday for adjusting the carburettors and check the engine performance for a before and after comparison. Now I don’t know what to adjust the timing to. Should I run a few different timing adjustments on the dyno to check for max power top end or mid range? Does ignition timing affect the air fuel ratio at all? Please help!

 

[Bernhard]

I need some advice about ignition timing.

I rebuilt my 850 MkII engine last year. I have now following setup:

• PW3 camshaft
• Raised compression to 9.4:1
• Light porting of inlet ports; widened the ports around the valve guides by a few millimeters (not flow bench tested)
• Trispark ignition
• Amal premier 932

I first adjusted the ignition to 29 degrees. The engine ran well. I played a bit with the ignition timing and realized significant changes. When retarding the timing to around 25 degrees, Does ignition timing affect the air fuel ratio at all? Please help!

First, let me say that air/fuel ratio does not usually affect ingition timing, but octane ratings of the fuel does.
Didn't you get any timings advice with the camshaft?
The correct way is to set ingition timing first before attempting to set up the jets in the carburetor, always. And wright it down so you know where you are at for future reference.
I have no experience of an 850, but 28 degrees usually worked best on a 750 at anywhere between 9:1 to 10.5:1.
25 degrees sounds too far retarded, how are you checking this, are you using a degree disc and a strobe?

 

[johnm]

Have you checked the accuracy of the timing mark in the primary case? They can be wrong by a few degrees.

25 degree does sound retarded. I raced a 500 Norton twin for years with lots of dyno runs and 28.5 degree was about right for a PW3 and 10.25 to 1. Using nz 98 octane pump gas.

A larger bore engine like a 850 would generally like a little more advance. About 29 to 31 degree I would expect. Maybe a little less at your CR.

I would set the carbs as per the book and ignition at 28 degree with standard plugs.. Do a run and if the carb mixture looks safe then test the ignition back and forth 2 degree.

And then go from there. Too much advance is the dangerous area.

On my own 850 I found the timing mark 2.5 degree out and run my engine at a true 28 degree of advance strobed at 5000 rpm. Pazon ignition. I have never put it on the dyno.

Going back to my 500 I was very pleasantly surprised on the improvement possible by tuning ignition timing, carbs, cam timing , inlet tract length and exhaust.

In my opinion based on many dyno runs and results on the track I think many people run too much advance and tune way too rich. Once I got my engine right I ran well over 100 races without a DNF and several NZ club titles.

 

[Fast Eddie]

I agree with the above, 25 degree sounds a bit uncomfortable to me.

Overly retarded ign can make an engine easier to start and have better initial pick up, tricking you into thinking it’s an improvement overal. But an engine run like this will run HOT and lose significant power.

Its also possible that your marks are out and that you’re not really at 25 degrees at all.

If you’re going on the dyno you’ll be able to play with timing and see any effect on A/F ratio and corresponding output.

Personally, I have found very little to be gained by moving the timing a few degrees either side of nominal.

Please do let us know how you get on and do not be surprised if the real dyno contradicts your arse dyno !

 

[johnm]

"Overly retarded ign can make an engine easier to start and have better initial pick up, tricking you into thinking it’s an improvement overal"

Very much agree with this

 

[Joachim]

Hey gents, thanks for your input.

'Its also possible that your marks are out and that you’re not really at 25 degrees at all.' - I did check the marks against a degree disc after I rebuilt the engine. Minor adjustment was required. I am relatively confident that the marks are ok.


'Didn't you get any timings advice with the camshaft?' - I dont think so. not so relevant ayway as compression ration and ports are non stock.


'Too much advance is the dangerous area' - Agreed; I think I know the limit just before pinging


'Overly retarded ign can make an engine easier to start and have better initial pick up, tricking you into thinking it’s an improvement overal. But an engine run like this will run HOT and lose significant power.' - Is it correct that overly retarded can make the engine run hot? The power loss will showon the dyno.

 

[Fast Eddie]

- Is it correct that overly retarded can make the engine run hot?

Definitely.

 

[concours]

"When retarding the timing to around 25 degrees, there was a noticeable better performance at least low and mid range."

This is the OPPOSITE of what would be expected.

 

[MexicoMike]

If you are going to dyno it...

Two approaches: 1. Advance the ignition to the point at which max power is made and then drops as you continue to advance. Then put it back at whatever produced max power. If you use this method, you have to be careful to use the same or higher octane fuel from now on or be very careful about gear/load choice while riding.

2. Same thing but retard the timing 2 degrees from whatever produced the max power. This will drop power a bit but give you flexibility re fuel choice without worrying about detonation under normal conditions.

Keep in mind that timing settings for optimum power will typically vary a bit from engine to engine. The factory setting is a "safe" setting to account for any variations due to factory assembly/parts/tolerances.

 

[texasSlick]

Excessively retarded or late timing will turn the header pipes blue, which many interpret to be a hotter engine. Not so, .... the blue indicates the header pipes are hotter than normal, but the cylinder and head temps are cooler than normal.

This happens because late timing results in fuel being burned in the exhaust ports and header pipes, rather than in the cylinders. Of course, fuel burned in the exhaust port does not contribute to power or economy, but will raise the temperature at the headers.

Slick

 

[ILLF8ED]

"Overly retarded ign can make an engine easier to start and have better initial pick up, tricking you into thinking it’s an improvement overal"

Very much agree with this

I don’t have this experience. Too retarded and mine is hard to start and doesn’t idle.

 

[johnm]

I guess maybe too much of a good thing ?

Regarding the ignition is normal for starting.

 

[Joachim]

I think what I am trying to understand is if the ignition is correctly set when the power is highest?

When I think about AFR, the mixture may be too lean at highest power so in that case I wouldnt go for highest power rather correct AFR on the dyno.

 

[Fast Eddie]

Excessively retarded or late timing will turn the header pipes blue, which many interpret to be a hotter engine. Not so, .... the blue indicates the header pipes are hotter than normal, but the cylinder and head temps are cooler than normal.

This happens because late timing results in fuel being burned in the exhaust ports and header pipes, rather than in the cylinders. Of course, fuel burned in the exhaust port does not contribute to power or economy, but will raise the temperature at the headers.

Slick

Seems logical, never thought about it that way, just associated blue (or glowing) pipes with running hot!

 

[Fast Eddie]

I think what I am trying to understand is if the ignition is correctly set when the power is highest?

When I think about AFR, the mixture may be too lean at highest power so in that case I wouldnt go for highest power rather correct AFR on the dyno.

I think the idea is to then run it on the road and see, if it’s pinking etc then retard a little. In principle you’re right, you’d be putting the timing in a range where pinking etc is more likely.

Lets see what you discover on the dyno first, personally I’d be surprised if you see much of a difference a few degree either side of standard as I’ve already said.

Don’t forget to do several runs at each setting. That’s where you’ll learn there’s often a small variation even between runs where nothing has changed. So, when you do change something, a few runs are required to ensure any gain you might see is real. Whenever I’ve played with ign in old bikes the changes seen have been in or around this normal variation and has therefore been inconclusive, or minimal.

 

[MexicoMike]

Retarded timing under load increases heat in the entire engine. In the old days, one of the first things that was typically checked if a car came into the shop with high coolant temps was ignition timing. If the timing is retarded, a much greater portion of the cylinder is exposed to combustion heat which is normally concentrated in the cylinder head/squish area. That excess heat is then absorbed by the cylinder wall/passed on to the cooling system. The excess heat also tends to 'scrub' the oil film from the cylinder wall, further increasing friction/temperature.

 

[texasSlick]

Retarded timing under load increases heat in the entire engine. In the old days, one of the first things that was typically checked if a car came into the shop with high coolant temps was ignition timing. If the timing is retarded, a much greater portion of the cylinder is exposed to combustion heat which is normally concentrated in the cylinder head/squish area. That excess heat is then absorbed by the cylinder wall/passed on to the cooling system. The excess heat also tends to 'scrub' the oil film from the cylinder wall, further increasing friction/temperature.

MM:
I must respectfully disagree.
Your first statement "..... load increases heat in the entire engine." is true. It is true regardless of timing. Timing will dictate where the heat will be distributed, .... mostly in the cylinder (where you want it), or more in the exhaust, where you do not.

Ideally, timing is optimized to burn all the fuel in a given cylinder charge well before the crankshaft reaches BDC. Burning fuel raises the pressure in the cylinder which pushes on the piston which gets you down the road. Burning fuel in the cylinder with crankshaft at BDC does not get you any useful push.

Now, considering it takes a finite time to burn a charge of fuel (it does not happen in a flash ... that is detonation), retarding the timing means fuel is available to be burned after the crankshaft passes the ideal cut-off angle .... that is, closer to BDC. Further reduction in timing (late) means fuel is available to be burned even after the crank passes BDC .... that means fuel is burning as it goes out the exhaust valve.

There is only a given amount of heat energy in any given cylinder charge. From the above, it should become clear how timing affects where the energy is distributed.

Radical cams intentionally cause fuel to be burned late into the power stroke. This is why race engines pop and crackle ... that sound is caused by fuel burning in the exhaust stream.

Slick

 

[MexicoMike]

As the old saying goes, we can agree to disagree!

 

[cash]

A larger bore engine like a 850 would generally like a little more advance. About 29 to 31 degree I would expect. Maybe a little less at your CR.
Perhaps not. Dished and flat top pistons generally give a faster and more efficient burn.
Dave

 

[johnm]

A larger bore engine like a 850 would generally like a little more advance. About 29 to 31 degree I would expect. Maybe a little less at your CR.
Perhaps not. Dished and flat top pistons generally give a faster and more efficient burn.
Dave

The Norton twin I was comparing it to had flat topped pistons