Throttle position for maximum timing advance

[Matysiak]

OK, i cleaned the carbs out again today. I really made an effort to get everything spotless. <br /><br />Now a quick question about link & Sync. Now that i have my carbs back on, i checked the idle timing which was 4 degrees BTDC. However when checking WOT timing, i noticed that the timing gets advanced to full (28 degrees BTDC) when the carbs are only about 1/3 open. Is this right?<br /><br />Stephen

 

[Walker]

Re: Throttle position for maximum timing advance

I've never measure it exactly, but 1/3 is about right.

 

[Joe Reeves]

Re: Throttle position for maximum timing advance

Maty.... It would help if you quoted what engine you have. However, the full spark advance (timing) is obtained before the carburetor butterflies open fully, about 1/3 open would be a close figure.<br /><br />Tinkering with the engine to have both obtain their full setting at the same time would result in having the engine die out.<br /><br />The engines are designed in that manner so as (for one reason) to set up what is called a "Cruise Range" to access a optimum fuel economy setting. This is accomplished by moving the throttle to it fullest setting, then backing off to approximately 3/4 throttle (you can remove the hood to see what I mean here).<br /><br />At that backed off 3/4 setting (after hitting full throttle), the spark advance stays at the full spark advance setting, but the throttle butterflies close to that approximate 1/3 setting. This results in a great drop in fuel usage with as little drop in rpms as possible.<br /><br />NOTE: Simply moving the throttle from a idle position to a 3/4 setting will not accomplish this "Cruise Setting", full throttle must be obtained first.

 

[Dhadley]

Re: Throttle position for maximum timing advance

As you can see that also explains why motors sometimes fail while cruising in the mid range. Youre running on full advanced timing with little fuel. Add in a spark plug thats a bit too hot or a motor thats lugging and you can clearly see what can happen.

 

[Crossflow235]

Re: Throttle position for maximum timing advance

After you have the link and sync set properly<br />You can verify max timing quite easily. <br />After warming up motor shut motor down and disconnect the timer base linkage from the throttle linkage.<br />(Look at where timer base was approx before disconnecting from throttle lever)<br /><br />Move timer base to this position and<br />start motor up, and advance timing to max timing stop. Be carefull "flywheels eat flesh"<br /><br />Check timing with timing light. Not sure of timing for your motor but I know the 235 is between 26&28 deg BTDC. Adjust timing as necessary by adjusting the stop adjustment. When satisfied shut down motor and reconnect linkage. Simple yet very effective!!!<br /><br />Note: I only test timing this way at the lake. I would do this test this on the lake, NOT on the hose because motor rpms will run a little bit higher than high idle. Dont want motor to runaway or starve water pump.<br /><br />Another thing to check out... check position of carb butterflies at WOT after you sync and link (With motor off of course)<br /><br />Good luck

 

[Matysiak]

Re: Throttle position for maximum timing advance

Thanks guys. By the way it is a 1980 115 Johnson.