The Ford Barn

john in illinois · 10-08-2016, 04:23 PM

I am in the process of tuning 2 97's on my 286 engine with a mild cam. It is a little rich. I hooked up an a/f meter and it is 11.3-5 at idle. Is this a good range?
I pretty well hve the main jets dialed in. It just seems rich at idle.

John

rotorwrench · 10-08-2016, 05:07 PM

Ideally, the mixture should be around 15:1 so your mix is on the rich side. Much more and it might quit. The big aircraft engines tend to quit around 12:1 on the rich side. Your main jets won't do much in the idle range. You generally just stick to adjusting the idle screws but you have to sync the two carbs on that as best you can unless you are only idling on one carb with a progressive throttle control.

Ol' Ron · 10-08-2016, 06:50 PM

Running the L-100 cam with 2 97's and one 48 in the center, we couldn't really get a good idle number. Something in the mid 12's. More like mid 11's. We did howeve get a pretty good cruise number int eh mid 14's WOT is still alittle rich

JWL · 10-09-2016, 12:18 AM

The idle mix should be fully adjustable and I would not be concerned with 11 to 1 if that is where the engine likes it. I have adjusted idle mixture all the way into the 9's for testing purposes. In fact, now that I think about it, I have started and flushed, with gasoline, alky injection systems on engines. They did not run great but better than you might imagine. I did not measure the A/F mixture but it was exceptionally rich.

I do not agree with trying to tune to a specific number. There can be a significant variance between A/F meters and all engines do not like exactly the same things. A number on the A/F meter can be used to indicate a possible direction for adjustment. It cannot be used to define the best performance and efficiency for each combination.

john in illinois · 10-09-2016, 10:41 AM

Rotor Wrench synching does take some time. I used to maintain 2 r985's and an r1340 float planes when I was an a&p in Alaska. If we had a carb problem we put on an overhauled pressure carb and sent the old one in. I like the old round engines.

Ron those numbers will give me a guide.

JWL I have been tuning the idle for best running and am glad to know I won't damage engine at 11.5

This engine runs good no matter what I do,but the plugs get carboned up. Thats why I got out the F/A meter. I will have to keep at it.

Thanks all for your help

John

rotorwrench · 10-09-2016, 05:46 PM

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Idle is not really where you have to worry about the mixture so much since it isn't pulling a load yet. As was mentioned, idle is adjusted for best running & vacuum. I'm not surprised it would be that rich at that low of an exhaust gas flow level. At medium to higher throttle settings is were you want best mixture likely in the 14:1 to 15:1 range depending on the altitude it's running at and the type of fuel being used. They will still run down to 18:1 before they quit on the lean side but that could do damage. Running rich is just less efficient and might load up the plugs at power.

Terry,OH · 10-10-2016, 07:04 AM

Adjusting the idle RPM on each Carb. so they both are working the same at idle (Syncing) does not take much time with only two 97's and it should be done. If the engine is running rich (off idle) the Main Jets will have the most effect. Go about two sizes smaller, and try again. Syncing the carbs will not make the engine run leaner.

Ol' Ron · 10-10-2016, 07:29 AM

I realize the 97's look great on an engine. However, there are allot better carbs. They can be tuned to function well. The ones on the 294 do just that. I have the center carb set to open the end carbs at 3/4 throttle and they only open 1/2 way. This setup gives us the best mileage, and enough power to lite the back tires. What more can you ask for.

john in illinois · 10-10-2016, 12:18 PM

Terry I have synched the carbs for air flow with a uni-syn andd attempted to set idle mixture by turning in till it misses and then out. The idle will not adjust and stays rich. I am now blocking off one carb at a time to see if I can find the problem. I think one carb has an idle circuit problem I will find out more later.

Thanks John

Bored&Stroked · 10-10-2016, 04:35 PM

With just two 97's, you should be able to adjust either carb and see the engine start to falter when the screws are in too far. With that said, are the carbs pulling the same amount of vacuum at idle? I've seen cases where the UniSyn is in sync off idle, but was not able to make it completely in sync in both idle and off-idle. This is usually related to the throttle plates and/or throttle shafts not controlling the air in the same fashion (when closed).

10-08-2016, 05:07 PM	#2
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,426	Re: Air fuel ratio Ideally, the mixture should be around 15:1 so your mix is on the rich side. Much more and it might quit. The big aircraft engines tend to quit around 12:1 on the rich side. Your main jets won't do much in the idle range. You generally just stick to adjusting the idle screws but you have to sync the two carbs on that as best you can unless you are only idling on one carb with a progressive throttle control.

10-08-2016, 06:50 PM	#3
Ol' Ron Senior Member Join Date: May 2010 Location: Chester Vt Posts: 8,860	Re: Air fuel ratio Running the L-100 cam with 2 97's and one 48 in the center, we couldn't really get a good idle number. Something in the mid 12's. More like mid 11's. We did howeve get a pretty good cruise number int eh mid 14's WOT is still alittle rich

10-09-2016, 12:18 AM	#4
JWL Member Emeritus Join Date: May 2010 Location: Fitzgerald, Georgia Posts: 2,204	Re: Air fuel ratio The idle mix should be fully adjustable and I would not be concerned with 11 to 1 if that is where the engine likes it. I have adjusted idle mixture all the way into the 9's for testing purposes. In fact, now that I think about it, I have started and flushed, with gasoline, alky injection systems on engines. They did not run great but better than you might imagine. I did not measure the A/F mixture but it was exceptionally rich. I do not agree with trying to tune to a specific number. There can be a significant variance between A/F meters and all engines do not like exactly the same things. A number on the A/F meter can be used to indicate a possible direction for adjustment. It cannot be used to define the best performance and efficiency for each combination.

10-09-2016, 10:41 AM	#5
john in illinois Senior Member Join Date: May 2010 Location: Illinois Posts: 2,184	Re: Air fuel ratio Rotor Wrench synching does take some time. I used to maintain 2 r985's and an r1340 float planes when I was an a&p in Alaska. If we had a carb problem we put on an overhauled pressure carb and sent the old one in. I like the old round engines. Ron those numbers will give me a guide. JWL I have been tuning the idle for best running and am glad to know I won't damage engine at 11.5 This engine runs good no matter what I do,but the plugs get carboned up. Thats why I got out the F/A meter. I will have to keep at it. Thanks all for your help John

10-09-2016, 05:46 PM	#6
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,426	Re: Air fuel ratio Sponsored Links (Register now to hide all advertisements) Idle is not really where you have to worry about the mixture so much since it isn't pulling a load yet. As was mentioned, idle is adjusted for best running & vacuum. I'm not surprised it would be that rich at that low of an exhaust gas flow level. At medium to higher throttle settings is were you want best mixture likely in the 14:1 to 15:1 range depending on the altitude it's running at and the type of fuel being used. They will still run down to 18:1 before they quit on the lean side but that could do damage. Running rich is just less efficient and might load up the plugs at power.

10-10-2016, 07:04 AM	#7
Terry,OH Senior Member Join Date: May 2010 Posts: 4,750	Re: Air fuel ratio Adjusting the idle RPM on each Carb. so they both are working the same at idle (Syncing) does not take much time with only two 97's and it should be done. If the engine is running rich (off idle) the Main Jets will have the most effect. Go about two sizes smaller, and try again. Syncing the carbs will not make the engine run leaner.

10-10-2016, 07:29 AM	#8
Ol' Ron Senior Member Join Date: May 2010 Location: Chester Vt Posts: 8,860	Re: Air fuel ratio I realize the 97's look great on an engine. However, there are allot better carbs. They can be tuned to function well. The ones on the 294 do just that. I have the center carb set to open the end carbs at 3/4 throttle and they only open 1/2 way. This setup gives us the best mileage, and enough power to lite the back tires. What more can you ask for.

10-10-2016, 12:18 PM	#9
john in illinois Senior Member Join Date: May 2010 Location: Illinois Posts: 2,184	Re: Air fuel ratio Terry I have synched the carbs for air flow with a uni-syn andd attempted to set idle mixture by turning in till it misses and then out. The idle will not adjust and stays rich. I am now blocking off one carb at a time to see if I can find the problem. I think one carb has an idle circuit problem I will find out more later. Thanks John

10-10-2016, 04:35 PM	#10
Bored&Stroked Senior Member Join Date: Jan 2011 Location: Central Ohio Posts: 5,064	Re: Air fuel ratio With just two 97's, you should be able to adjust either carb and see the engine start to falter when the screws are in too far. With that said, are the carbs pulling the same amount of vacuum at idle? I've seen cases where the UniSyn is in sync off idle, but was not able to make it completely in sync in both idle and off-idle. This is usually related to the throttle plates and/or throttle shafts not controlling the air in the same fashion (when closed).