Fixed timing??

[Squeakit]

Most outboards have a mechanical linkage that adjusts the timing in tandem with the opening and closing of the throttle butterfly in the carb. Some of the smaller ones don't have this, and speed is controlled by the carb only. Is this fixed timing, or do they have some other way of varying the timing (electronic, maybe?) Lots of 2 stroke motors don't have the kind of variable timing that outboards have. Why or why not? Would it be possible to "fix" the linkage on a motor and just use the carb to control speed or would this burn the motor up and/or seriously hamper performance. No one can give me an answer to this, but I know someone on this board can. Just trying to increase my knowledge a little bit. Thanks in advance.

 

[iRich]

Re: Fixed timing??

I'm no expert on this but I was out this past weekend on my boat with 85hp johnson and fiddled around with a screw i thought was for the throttle and it turns out it is a stop screw for the timing advance. After turning it out about 4 turns my overheat alarm went off. I didn't realize this is what caused it at the time because i drove the boat slow back to the dock and it didn't go off again. So to answer your question you couldn't have a fixed linkage to the carb for the timing because it only needs to advance up to a certain point. on my motor at least and it is a 1975.<br /><br />Rich

 

[Squeakit]

Re: Fixed timing??

Thanks for the reply, Rich. I'm not exactly what you did to your motor that made it overheat - maybe turning that screw advanced the timing past the maximum advance for that engine. I was wondering what would happen if you disconnected the timing linkage from the carburetor linkage and "fixed" i.e. anchored the timing linkage in place, then controlled the motor speed with just the carb opening and closing while the timing stayed the same. I was idling yesterday with the cowl off making adjustments and opened the carb while the timing was at the idle (retarded) position and the engine took right off - but I didn't hold it open very long to see how much power it would have. Are engines with fixed timing set at maximum advance all the time, or partial advance -?? Wondering if you'd get a smoother idle without loading up if timing were a little hotter at low carb setting. What would happen if you had two controls, one for ignition advance/retard and one for carb opening/closing? You wouldn't overadvance the timing, because the WOT position would never be advanced past the maximum. You could "tune" your engine for best power at any given speed. Need more info from someone with more engine tech knowledge than I have.

 

[BigFellaQld]

Re: Fixed timing??

Stop mucking around with it, or you are going to do damage. Motors need to advance their timing,in synchronisation of the throttle butterfly opening. Some small motors do not use a visible form of ignition advance, but instead use electronic advance, which is built into the CDI unit, or ECU, and is triggered by signals from the crank angle sensors and throttle position sensors. The latest from Tohatsu, in the form of the TLDI direct injected engines, cost $30 million to develop, and they already had the base motors to start from. This sort of R and D takes years, and hundreds of thousands of hours of testing, and yes, quite a few blown motors. Without trying to step on your toes too much, do you really think you will be able to improve your motors performance by screwing around with its timing system? At best, you will lose a heap of performance, more probably, you will blow the thing up. Have a look around the net on damage caused by detonation, and you will see my point<br /><br />Regards Paul

 

[Squeakit]

Re: Fixed timing??

Thanks for the reply, Paul. Figured there was some type of electronic advance system in the little motors, but wasn't sure. I would still like to know more about this, though, and would welcome any further input. I have always been curious about how things work. Sometimes, a little tinkering can be productive, sometimes not.

 

[Hooty]

Re: Fixed timing??

...just remember, the spark has to keep up with the piston.<br /><br />c/6<br /><br />Hooty

 

[petryshyn]

Re: Fixed timing??

Squeakit<br /><br />Timing is quite subjective. Ignition timing advance is a required function for engines which operate within a wide range of RPM.<br /><br /> Many engines don't require an advance mechanism, because they spend most of there lives at a predetermined speed and the base timing is adjusted for that speed. (eg. powerplants, weedeaters, some lawnmowers, pump-motors, chain saws) Sometimes the manufacturer of these units will add some crude advance mechanism, just to facilitate start up.<br /><br /> With marine, automotive, ATV and industrial engine applications, a change in timing is required to have the peak combustion pressures always occuring at a specific degrees after top dead center. This is so the engine gets the best mechanical advantage of the relationship between the rod position and the crank throw angle when the highest pressures within the cylinder occur. This gives you the greatest horsepower. Since combustion takes about the same "time" to reach peak pressures under most engine speed conditions,(except when lean) we must start the burn sooner in a fast running engine.<br /><br /> Yes, over-advancing can lead to horsepower reduction, detonation and component damage. That is why you must have the appropriate timing for any given speed of the engines operating range. What you suggested (having a means of adjust timing at will) has alot of value. That is how you would plot out an advance curve for any given engine to achieve maximum efficiency. At every RPM, from Idle to WOT (in 100rpm incruments) one could adjust the timing manually and record the advance which gave the highest RPM. Once you have this spec, you could manipulate the advance mechanism to reflect the advance curve you have created.<br /><br /> Timing advance is achieved several ways:<br />>centrifugal weights and a moveable point cam or sensor <br />>simple throttle linkage position<br />>electrically triggered magneto with multiple triggers sensitive to speed<br />>ECM (computer controlled)<br /><br />Hope this helps...<br />

 

[Squeakit]

Re: Fixed timing??

Thanks for the great info, Schematic. Your explanation helps alot. If I were to have two adjustable levers (like throttle controls) one for the carb butterfly and the other for the movable breaker plate, would it be possible to fine tune these 2 levers at any speed for greatest engine output/rpm without causing damage? I had an old Model A when I was a kid with the throttle lever on one side of the wheel and the spark advance on the other which worked independently of each other. Would this work with an outboard? If you limit the maximum advance to the manufacturer's recommended setting wouldn't that prevent detonation? If you had the carb half opened up and the advance at maximum would this cause detonation? In my old Ford I usually just left the spark advance at max once the engine was warmed up. You wouldn't be changing the fuel ratio so you wouldn't be running lean, but would a lower amount of fuel/air mixture with too much advance produce a "lean" condition and burn the motor up?

 

[petryshyn]

Re: Fixed timing??

Remember, the reason for changing advance, is the changes in engine RPM. Maximum advance is much too high to be used at half throttle. Thus detonation occurs. Timing and engine speed are directly related and must be in proper proportion.<br />

 

[Backlash]

Re: Fixed timing??

Welcome back Schematic. I, for one, have missed your insightful contributions to this board. <br />Backlash

 

[Squeakit]

Re: Fixed timing??

Thanks again, Schematic. From what you are saying, it sounds like my two hypothetical levers may be too dangerous as it would be easy to overadvance the spark at a lower carb setting,causing detonation and possible catastrophic failure which I definitely dont want. I haven't actually tried this yet, and I probably won't judging from the critical nature of synchronizing timing with throttle opening. By the way, my wife is originally from Alberta - born in Red Deer and raised in Sylvan Lake. What part of Alberta are you in?

 

[Chinewalker]

Re: Fixed timing??

Hi Squeakit,<br /> My experience with engine timing has been in the realm of 2-cylinder outboard racing where often as a matter of simplification and the fact that you're primarily running at one speed (wide open!) the timing will be fixed. The end result is an exact setting for timing, often using a depth gauges, protracters & pointers, and continuity lights. This removes any changes that may occur with a rotating or sliding of magneto parts, slop in linkages, etc. The throttle is run straight off the carburetor. The only negative effects I've noted from this fixed setting is that the motor idles terribly and can be very hard to start. The motor starts well - that's not the problem. The problem comes from when it backfires and rips the starter cord out of your hand! Ouch!!!<br />- Scott

 

[Squeakit]

Re: Fixed timing??

Thanks for the reply, Scott. Simplification was basically what I was after in the first place. My little 2 horse Yammy just has a carb throttle control and if there is advancing/retarding taking place it isn't visible. I have fooled around with small 2 stroke engines and the timing was definitely fixed - you could get a little more poop out of them by removing the Woodruff key from the flywheel and advancing the timing slightly. Offset Woodruff keys are now sold as performance accessories (Go-Ped engines) I figured that it might be possible to just advance the rotating breaker plate to max setting and pin it there - then just use the carb to control speed. However this doesn't sound so good after reading some of the information here. Kickback when starting up with the spark fully advanced could break things. I know it wouldn't hurt anything to run the carburetor opening ahead of the spark - you'd just lose power. Was surprised to hear that your fixed timing engines idled poorly. I would have thought that the idle might have speeded up a bit and maybe smoothed out. My old Model A did - but an old Ford is not a 2 stroke outboard. I'd still like to try this idea out, but I don't want to have to paddle ashore with a broken motor and a bad attitude.