Why carb 2stroke use throttle cam, 4stroke no cam

thumbnut

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Wondered this since 1974. Probably lube requirement exceeds combustion requirements at low speeds. So let in enough fuel/oil for lube but retard timing enough for slow enough speeds? I'm asking cause my '76 15hp runs best at low speeds better if I set the pickup point quarter inch past mark..more advance timing per unit throttle opening. I am jetted and run 20/1 fuel so I hope I have this covered.

I also could be full of poop...

oh...the cam adjustment doesn't affect max plate opening or max timing.
 
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oldboat1

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nah. might just be a little full of 2-cycle oil. 20:1 is pretty rich for a '74.
 

WernerF

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In carbed 2-stroke engines at idle the spark timinig must be pretty much retarded to prevent 4-strokeing (firing only every other revoloution). Since it must be done it's an easy and accurate way to adjust idle rpm, leaving all throttle butterflies fixed/closed. Especially in multi-carb engines.

Some small Japanese engines adjust idle with the throttle plate(s) and have fixed spark timing, but they are 4-stroking badly at idle.
 

Tim Frank

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nah. might just be a little full of 2-cycle oil. 20:1 is pretty rich for a '74.

+1 ^^^

20:1 makes no sense and can be as damaging as 100:1.
A little extra oil is no problem....2-1/2 X more than the design ratio is asking for problems.
 

F_R

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A two-stroke requires more air to run at slow speeds, merely because it pumps the charge through the crankcase to the cylinders. That is why if you look at the carburetor throttle plate, you will see holes or notches in it. So, to slow it down to a low idle, the timing is retarded. To say that timing does not change on a 4-stroke is not 100% accurate. True, some small engines are fixed timing, but just about anything larger than a lawn mower will have some sort of spark advance/retard built in. May be mechanical, or computer controlled electronic ignition.
 

OptsyEagle

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Wondered this since 1974. Probably lube requirement exceeds combustion requirements at low speeds. So let in enough fuel/oil for lube but retard timing enough for slow enough speeds? I'm asking cause my '76 15hp runs best at low speeds better if I set the pickup point quarter inch past mark..more advance timing per unit throttle opening. I am jetted and run 20/1 fuel so I hope I have this covered.

I also could be full of poop...

oh...the cam adjustment doesn't affect max plate opening or max timing.

How is the motor for starting, with that pickup point? If I understand you right, the throttle would be opened LESS when it comes to the neutral safety stop and I would think that it might be a little difficult to get started on those cold mornings. Maybe I am mis-understanding the position of your cam.
 

thumbnut

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I did have to move the throttle stop to open the plate slightly for starting. I got this motor in a box all apart for $100 from a neighbor. . It sat in a basement for many years as it ate a reed 4 years into its life. Never was fixed until now.

I have always jetted and ran 20/1 in all my 2 strokes since my first snowmobile in 1975. A Herth 634 twin I grafted onto an Arctic Cat. If one hasn't read Gordon Jennings stuff it might be a good read. Especially on oil mixtures.

What other 2 strokes besides outboards to they twin the throttle with timing? Usually fixed timing or some centrifugal system not directly connected with the throttle. Must be the constant load that increases the lube requirements past combustion requirements at slow speeds. Hence the 4 stroking that WernerF was saying at low speeds on outboards. I can't think of any other 2stroke that runs that rich at slow speeds with a need to retard timing for clean running other than an outboard. And most other carbs have an adjustable throttle stop for idle except for most outboards I've owned. I still feel the minimum throttle opening is set by the designers with no lesser opening possible to keep lubed enough at slow speeds on 2 stroke carbed outboards.

Never seen a 4 stroke's throttle twined with the timing advance. I have read where some small outboards use a centrifugal clutch so that rich idle can be avoided, like on my Tanaka 3 hp kicker.

I like how the etecs separate the oiling system from the fuel . Combustion requirement can be optimized separately from oiling requirements.

Again, read Gordon Jennings paper on fuel mixtures. My experience backs up is work.
 

oldboat1

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My mistake thumb. You intend to tune a '76 15hp to run on a 20:1 mix. I'll watch progress.
 

thumbnut

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My mistake thumb. You intend to tune a '76 15hp to run on a 20:1 mix. I'll watch progress.

You sumed it up with a lot fewer words. I just was thinking of the relationship between that throttle cam and intended fuel mix, if any. My motor just runs better with less throttle per unit advance at low speed but I like lube too...

I'll get pics or it didn't happen.
 

F_R

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I guess you don't accept my explanation. That's ok. I'm just a kid with a lot to learn
 
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thumbnut

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I guess you don't accept my explanation. That's ok. I'm just a kid with a lot to learn

I don't really disagree..I'm just digging in further past what happens to the reason why it has to be. I want the engineers notes and body of work ;)

See, I got my 4 year Mechanical degree in 1983. Mom had stroke 2 weeks after grad so I worked in the family plumbing and heating business to keep Dad afloat until what ever happened to Mom happened. I never got a job. So I'm a frustrated arm chair engineer trying to compete with a real one..

ahh to be human..
 

F_R

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I understand. As I said, I'm a kid. Such a kid that I remember when cars had two levers on the steering wheel. On was the throttle, other was the spark advance/retard. Talk about sync--the driver had to sync them. And if he didn't retard the spark before starting the engine, the crank kicked back and broke his arm.
 

thumbnut

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I understand. As I said, I'm a kid. Such a kid that I remember when cars had two levers on the steering wheel. On was the throttle, other was the spark advance/retard. Talk about sync--the driver had to sync them. And if he didn't retard the spark before starting the engine, the crank kicked back and broke his arm.

Yup..heard stories from my Dad who was 40 years old when I was born in 1960. People used to run those T's on the Mohawk river here when it was frozen. Several of those T's were buried along the riverbank during the 30's and as the ice flows scoured the bank they would be released again into the river to catch my anchor and lures.
 

WernerF

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Normally it's best to adjust spark timing with an rpm dependent part (centrifugal spark advance) and a load dependent part (vacuum diaphragm unit). But in an outboard power, torque and rpm is somewhat related due to the propeller load. Unlike a motorbike a boat never climbs up or down a hill. Acceleration phases are short and a bit too early spark timing is acceptable here. So the best solution (and most reliable in wet and salty environment) is to advance spark timing simply by the throttle setting.
Modern outboards with digital engine management systems consider all inputs independently for spark timing.
 

WernerF

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The CD ignition system exhibit a small rpm dependant spark advance. The timer base voltage increases with rpm and with a fixed threshold in the powerpack spark comes sooner. It is the famous 4 degrees offset mentioned in Joe Reeves' timing adjustment method.
 

thumbnut

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Thank you for that. What you said is what I couldn't seem to convey. I'm trying to make a silk purse out of a sow's ear with this motor. I'm getting closer..

My original interest was why no extra charge with retarded timing in 4 strokes..even older point ignition ones. I can see the dynamics of 2 stroke outboards allows a simplified design of clocking timing with throttle. I have been trying to figure this out since my Dad bought a used Martin 40 for our canoe in the early seventies and I would swap carbs out..even tried a Tillotson HL diaphragm carb. I could run the Martin with the cam link removed, timing at max advance at all rpms very nicely. Then I thought of this exact question and stopped doing that, figuring starving for lube at low rpms on that plain bearing motor.

I tend to beat dead horses at times so...I still think its a fuel/lube reason why run so much extra charge with retarded timing at low rpms on 2stroke outboards.

I don't want to die before I figure this out...
 
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F_R

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Aw come on, maybe your engineering degree is in a different field, but it isn't too hard to find out that for an internal combustion engine to run, the fuel to air ratio has to be within a certain range. That's why all engines have some way of metering the fuel in relation to the amount of air going in. So if it is running,it is getting enough fuel, and if it is getting enough fuel it is getting enough oil (assuming you did put it in the gas, right?).

Back to the question of timing vs throttle, interesting that you mentioned Martin outboards. The Martin "75" had a linkage whereby you could close the carburetor's throttle plate by pushing a button on the tiller handle, without retarding the spark lever, and the RPM would come down to sort of an idle. I say sort of because the idle was rough when running like that. To get it down to a nice smooth idle, you had to retard the spark. Virtually all outboards used in racing applications used the same sort of speed control. Who cares if a racing engine idles smoothly? And the Martin wasn't the only stock engine to do it either. On an early Johnson QD, you could slow down by raising the tiller handle. Various Evinrudes and Eltos had a small knob on the tiller handle. Lots of antique outboards had no carburetor throttle at all.

And I repeat, any 4-stroke bigger than a lawn mower has some sort of variable spark advance/retard. After they took the levers off the steering wheel, they put the automatic advance mechanism in the distributor, and it stayed there forever. Well at least till they started putting it in a computer in more recent years.

I also repeat, a two-stroke has an entirely different way of getting the fuel-air charge into the cylinders than a four-stroke does. So you can't be making direct comparison. A two-stroke can't run with the air supply strangled down like a four-stroke can. A Diesel always has the air supply totally unrestricted, even at idle. I know you asked "Why?" The short story is these things evolved over many years, and much was learned as they went along. Goal being an engine that runs well, is efficient, and trouble-free. And they are still working on it.
 
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F_R

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One last comment, then I'll shut up. While you are in thinking mode, consider why the spark timing is variable in the first place. It takes time for the fuel/air charge in the cylinder to ignite and burn. And the burn has to be somewhat controlled. It doesn't just "blow up" as we might think. Since it takes time for the burn, it has to be ignited sooner if running at high RPM, and later at slow RPM (or while starting). If you ignite it too soon at low RPM, it burns before the piston reaches the top of the stroke and the piston would be pushed down while it should still be going up on the compression stroke. If it ignites too late at high RPM, the burn is not complete and power & efficiency goes out the window. Balance all that with the fact that in no case is the burn complete. Lots of wasted energy goes out the exhaust. Ponder on that awhile.

Another "last" last comment. I get into thinking mode like that too. If none of us did, we would still be eating raw meat in a cave. Meat that you don't have to think in order to catch. Like bugs. So, keep it up on the thinking. You may be the next great inventor.
 

WernerF

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Lubrication is not a problem for 2 strokes at idle. Many self-mixing engines (not just outboards) have a leaner fuel/oil ratio at idle. But scavenging and controlling the combustion is.
 
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thumbnut

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Thanx to all for sticking with me..I'm go'in to my pwc forum and ask why no thottle cam on pwc's. Not really..
 
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