Can somone explain advance / retard of timimg

[canuck750]

I have never really understood the physics of timing, I can set static and dynamic timing as per the manual but what does it mean?

I took my V7 Sport out for a lomg ride this afternoon and when done I set up a strobe light to check the timing.

At 1500 rpm, smooth idle, I removed the rubber timing plug, set up the strobe light on the right cylinder and the static mark was off by about 1 tooth, then checked the advance mark at 3500 rpm, off by about the same amount. I loosened the distrisbutor and by rotating it slightly counter closkwise I aligned the timing marks at static and then confirmed the marks at advance.

So what exactly was I adjusting and how does that small adjustment effect the running? I thought the bike was running 'hot'. Can the distributor being off by so little effect the running temperature?

After getting the mixtures right, the idle and sync done the only complaint I have with the bike is it wants to bog a bit off idle when I move forward unless I give it some throttle and feather out the clutch.

I would like to better understand what the effects of timing are and what the advance and retard of timing effects.

[oldbike54]

 Advancing the timing means the spark happens earlier in the upward stroke , retarding is the opposite . The idea is to set the timing at a point where all of the combustion happens before the piston is over TDC very far , so none of the energy is lost as the piston begins its downward movement . Of course advancing the spark too far results in loss of power , it is all about "timing" . Kind of a simplification , but pretty close .

 Dusty

[rodekyll]

The time the spark takes to generate and the time it takes to burn the mix is the same regardless of engine speed.  However, as the engine speed increases, if the burn begins at the same point of piston travel, the piston has gone farther by the time the burn ends.  This discboodilates the engine.  As it turns out, it is where the piston is at the end of the burn, not the beginning that is important.  So in order to keep the end point of burn at a reasonable piston position (measured in terms of crank degrees) we want to begin the burn earlier. 

Since an earlier burn happens more in ADVANCE of the piston reaching top dead center, we call that ADVANCING the timing with respect to where the static setting is.  So if the static setting is 4ºbtdc and we move the timing to 30ºbtdc we're advancing it.  If we move in the opposite direction the burn begins at a less advanced, or RETARDED point.  So if we move from our initial point of 4ºbtdc to 0º we're retarding the timing.

There was a time when distributors had their lobes offset to advance or retard the timing for a particular plug.  Some had dual point sets and a 3rd gear switch that put them in the 'power' points for heavy work and returned them to an econo-set pair for all other gears.

[harrytief]

+1 for what RK and others have said.
Basically, the plug needs to fire within the 90 degrees of crank rotation on the compression stroke. Ideally, the burn will be most "explosive" when both the exhaust and intake valves are closed with the piston at TDC and ready to turn down into its power stroke. Since the burn will always take a fixed period of time but the duration of the ideal juxtaposition of valves and piston varies with engine speed, then the burn must be STARTED earlier (advanced timing) as engine speed increases. At lower engine speed the piston and valves will spend a little more time getting to and staying at their relative ideal positions in their cycle so the timing of the burn is DELAYED (retarded timing) to allow for maximum push on the piston top at TDC with valves closed.
Harry

[canuck750]

Thanks for the tutorial guys.

So if my engine was bogging a bit off the line but takes off better with a fist of throttle does that mean the spark was occurring too late at idle?

[oldbike54]

Thanks for the tutorial guys.

So if my engine was bogging a bit off the line but takes off better with a fist of throttle does that mean the spark was occurring too late at idle?

 That is an effect of retarded timing , yes .

 Dusty

[canuck750]

Thanks Dusty, I think that explains the take off performance. Next ride I will see if resetting the timing has cure the take off  performance.

[pete roper]

Sounds more like too lean slides to me than a timing issue per se. What carbs are you running? The original VHB's? If so try and find another set of slides and take a 1/2 mm off the bottom and see if that helps. The vast majority of carbonated Guzzis have slides that are too lean.

Pete

[canuck750]

Sounds more like too lean slides to me than a timing issue per se. What carbs are you running? The original VHB's? If so try and find another set of slides and take a 1/2 mm off the bottom and see if that helps. The vast majority of carbonated Guzzis have slides that are too lean.

Pete

Thanks Pete,

VHB 30 carbs, all stock settings, stock hets and needles. I have another set of slides, I will try milling a wee bit off the bottoms and see what the effect is.

[pete roper]

Remember that when you do this it will also lower the needle so it may be necessary to shim it up a bit to maintain the same midrange/medium throttle fuelling.

Pete

[Moto]

Sounds more like too lean slides to me than a timing issue per se. What carbs are you running? The original VHB's? If so try and find another set of slides and take a 1/2 mm off the bottom and see if that helps. The vast majority of carbonated Guzzis have slides that are too lean.

Pete

 :1: I followed this advice from Pete years ago (on my only set of VHBs) and it worked as he said!

Moto

[Muzz]

 :1: on RK's explanation. A motor will run hot with a retarded spark. On two strokes especially  it can cause the piston to hole. A 4 stroke tends to have a "flat" sound to the exhaust.

It will pink under load if it is too advanced; low octane petrol will make it pink worse. Detonation (call it really bad pinking) will cause premature engine failure.

[jacksonracingcomau]

I'll add a bit more
Engine is very hard to start running it's full advance so retarding to start is pretty much essential, can start to advance very quickly, on old bikes with manual adv/ret lever, after starting you can set straight to full advance, retarding only when you feel it pinking (up hill, pulling deep mud etc etc)

The advanced figure is really the relevant part on our bikes, both the same first but final number not cast in stone, if it pinks on available fuel, back off till it doesn't, alternatively give it more spark if it likes it.

[Rough Edge racing]

Advancing the timing means the spark happens earlier in the upward stroke , retarding is the opposite . The idea is to set the timing at a point where all of the combustion happens before the piston is over TDC very far , so none of the energy is lost as the piston begins its downward movement . Of course advancing the spark too far results in loss of power , it is all about "timing" . Kind of a simplification , but pretty close .

 Dusty

 Yes, the combustion in the chamber must be complete by about 15 degrees ATDC for best power...So the spark from the plug must occur well before that depending on combustion chamber design and how long it takes the mixture to burn.. Lean mixtures take longer to burn so they require more spark timing...Some US car engines use upwards of 55 degrees advance at part throttle cruising speeds when intake vacuum is high...Carburetor equipped later  model Harleys have a vacuum device to advance timing at part throttle. Guys racing vintage Triumphs on flat track use timing to tune the engine for track condition , advance a few degrees to increase mid range power, retard a few for top end..

[Piston]

This sounds like a momentary lean condition which occurs with throttle opening. The air gets in before the fuel does making the mixture overly lean. To compensate for this condition carburetors have an accelerator pump which dumps a little bit of gas upon throttle opening. Not sure if the 30 VHB’s have an accelerator pump? Also before adjusting jets you may be able to raise the needle to see if that makes a difference.

[pete roper]

This sounds like a momentary lean condition which occurs with throttle opening. The air gets in before the fuel does making the mixture overly lean. To compensate for this condition carburetors have an accelerator pump which dumps a little bit of gas upon throttle opening. Not sure if the 30 VHB�s have an accelerator pump? Also before adjusting jets you may be able to raise the needle to see if that makes a difference.

Sorry? Understand the bit about accelerator pumps but what's this about the 'Air' getting in before the 'Fuel' thing???

Pete

[Chuck in Indiana]

This sounds like a momentary lean condition which occurs with throttle opening. The air gets in before the fuel does making the mixture overly lean. To compensate for this condition carburetors have an accelerator pump which dumps a little bit of gas upon throttle opening. Not sure if the 30 VHB�s have an accelerator pump? Also before adjusting jets you may be able to raise the needle to see if that makes a difference.

IMHO, raising the needle will affect mid range much more than off idle. No pump on 30s. I'm in the milling the slide camp, even though Pete doesn't like carbs. "They make your hands smell bad." 

carb_jet_circuits by Charles Stottlemyer, on Flickr

[pete roper]

Later VHB's did have a pump, of sorts, it was a sort of horrid piston with holes in with a spring underneath that was depressed by the needle when the throttle was closed. At that point fuel would ooze up over the piston and when the slide lifted quickly the piston, ( Spring loaded from underneath.) would rise with the needle and any fuel that couldn't ooze back through the holes would slobber up the atomiser and into the venturi! It was the same sort of principle as throwing a ping-pong ball into the dunny to give small boys something to aim at when they were taking a piss in the vain hope they wouldn't wee all over the seat and porcelain.

That doesn't work very well either.....

[Rough Edge racing]

 Changing the advance curve to give more timing off idle is an old hot rodders trick to mask an off idle hesitation....It's the lean mixture takes longer to burn require more spark advance situation...Of course there may be a carburetor problem...

[canuck750]

As usual with this campfire a great source of knowledge! I have just enough understanding of when things need to go BOOM to be even more dangerous.

The VHB 30 (and 29) have the pumps that Pete describes, I installed a new little plastic ball and spring in the pump assembly when I rebuilt the carbs.

That is a great explanation on the pump operation Pete! It would be helpful if all technical descriptions were so 'visual' (and now I know why there is a ping pong ball in the urinal at the YMCA).

Thanks for posting the carb operation chart Chuck, again a very helpful visual.

Cheers

Jim

[Chuck in Indiana]

Well, yeah.. I guess you could call it a pump.  I'd call it more of a drool..

[Piston]

Sorry? Understand the bit about accelerator pumps but what's this about the 'Air' getting in before the 'Fuel' thing???

Pete

Fuel flows into a carburetor because of a venturi effect (https://en.wikipedia.org/wiki/Venturi_effect) at normal cruising speed airflow and fuel flow are metered at the correct combustion ratio something like 14 to 1.

Off idle the venturi pressure differential is not as strong as at for example cruise speed. Consequently more air tends to flow in initially then fuel. Therefore there is more air than fuel in the ratio, known as a lean condition. This is a common problem for carbureted vehicles whether they be cars or motorcycles. The accelerator pump is designed to compensate for this temporary lag. It squirts an excess of gasoline to compensate until the fuel air mixture can be metered correctly.


"Accelerator pump

Liquid gasoline, being denser than air, is slower than air to react to a force applied to it. When the throttle is rapidly opened, airflow through the carburetor increases immediately, faster than the fuel flow rate can increase. This transient oversupply of air causes a lean mixture, which makes the engine misfire (or "stumble")—an effect opposite to that which was demanded by opening the throttle. This is remedied by the use of a small piston or diaphragm pump which, when actuated by the throttle linkage, forces a small amount of gasoline through a jet into the carburetor throat.[13] This extra shot of fuel counteracts the transient lean condition on throttle tip-in. Most accelerator pumps are adjustable for volume and/or duration by some means. Eventually the seals around the moving parts of the pump wear such that pump output is reduced; this reduction of the accelerator pump shot causes stumbling under acceleration until the seals on the pump are renewed.

The accelerator pump is also used to prime the engine with fuel prior to a cold start. Excessive priming, like an improperly adjusted choke, can cause flooding. This is when too much fuel and not enough air are present to support combustion. For this reason, most carburetors are equipped with an unloader mechanism: The accelerator is held at wide open throttle while the engine is cranked, the unloader holds the choke open and admits extra air, and eventually the excess fuel is cleared out and the engine starts."

[Rough Edge racing]

Fuel flows into a carburetor because of a venturi effect (https://en.wikipedia.org/wiki/Venturi_effect) at normal cruising speed airflow and fuel flow are metered at the correct combustion ratio something like 14 to 1.

Off idle the venturi pressure differential is not as strong as at for example cruise speed. Consequently more air tends to flow in initially then fuel. Therefore there is more air than fuel in the ratio, known as a lean condition. This is a common problem for carbureted vehicles whether they be cars or motorcycles. The accelerator pump is designed to compensate for this temporary lag. It squirts an excess of gasoline to compensate until the fuel air mixture can be metered correctly.

 .All 4 stroke engines require a richer mixture during periods of high power loading.......The air fuel ratio of a carburetor varies greatly, about 12-1 at wide open throttle , as lean as 16-1 at light part throttle cruising, 14.7 at moderate loads...Fuel injection needs similar A/F ratios ...

[rodekyll]

Fuel flows into a carburetor because of a venturi effect (https://en.wikipedia.org/wiki/Venturi_effect) at normal cruising speed airflow and fuel flow are metered at the correct combustion ratio something like 14 to 1.

Off idle the venturi pressure differential is not as strong as at for example cruise speed. Consequently more air tends to flow in initially then fuel. Therefore there is more air than fuel in the ratio, known as a lean condition. This is a common problem for carbureted vehicles whether they be cars or motorcycles. The accelerator pump is designed to compensate for this temporary lag. It squirts an excess of gasoline to compensate until the fuel air mixture can be metered correctly.


"Accelerator pump

Liquid gasoline, being denser than air, is slower than air to react to a force applied to it. When the throttle is rapidly opened, airflow through the carburetor increases immediately, faster than the fuel flow rate can increase. This transient oversupply of air causes a lean mixture, which makes the engine misfire (or "stumble")—an effect opposite to that which was demanded by opening the throttle. This is remedied by the use of a small piston or diaphragm pump which, when actuated by the throttle linkage, forces a small amount of gasoline through a jet into the carburetor throat.[13] This extra shot of fuel counteracts the transient lean condition on throttle tip-in. Most accelerator pumps are adjustable for volume and/or duration by some means. Eventually the seals around the moving parts of the pump wear such that pump output is reduced; this reduction of the accelerator pump shot causes stumbling under acceleration until the seals on the pump are renewed.

The accelerator pump is also used to prime the engine with fuel prior to a cold start. Excessive priming, like an improperly adjusted choke, can cause flooding. This is when too much fuel and not enough air are present to support combustion. For this reason, most carburetors are equipped with an unloader mechanism: The accelerator is held at wide open throttle while the engine is cranked, the unloader holds the choke open and admits extra air, and eventually the excess fuel is cleared out and the engine starts."

Good opening paragraphs of explanation.  Clear and understandable.  Did you write those?

The second part appears to be a copy from somewhere else.  Generally when we cite someone else's writing we also cite the source, both to give credit to the actual author and to allow readers to go back and read the greater context.

Well done.

[Piston]

In the links to the first webb site go to carburetor.

Also, not to confuse the issue, but if the accelerator pump is working it’s possible that a too rich condition can be imposed wetting the spark plug momentarily. In this case a hotter plug or a more potent spark coil could be needed.

[pete roper]

Ah, OK. A rather obtuse and over complicated explanation but I understand now.

Pete

[twowings]

It's really this simple:

I am rather advanced, while Dusty's development has been somewhat retarded...