TN-27 Fuel Adjustment

[440racer]

I've finally gotten my old tn-27 running (cleaned carb & installed new impeller)

I'm still confused on the fuel adjustment though.

The instructions on the cowl say "prime the carb then open the needle 3/4 turn". Is this slow or fast metering? Seems that it's slow because it's not affected by throttle setting.

Richness knob on top. I assume this is for high speed but not sure.

Last, which should I adjust first? I've read different suggestions and am assuming once I get the slow setting dialed in that it shouldn't need further adjustment. Is this correct?

 

[Xcusme]

Re: TN-27 Fuel Adjustment

Top needle is usually Low Speed Mixture
Bottom needle is usually High Speed Mixture

I do the High Speed mixture first , then the Slow mixture

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(Carburetor Adjustments - Two Adjustable N/Vs)
(J. Reeves)

Initial settings are: Bottom high speed = seat gently, then open 1 turn out. Top slow speed = seat gently, then open 1-1/2 turns.

Setting the high and low needle valves properly:

NOTE: For engines that DO NOT have a shift selection, obviously there is no NEUTRAL position. Simply lower the rpms to the lowest setting to obtain the low speed needle valve adjustment.

(High Speed) Start engine (it will run pretty rough), shift into forward gear, take up to full throttle. In segments of 1/8 turn, waiting for the engine to respond between turns, start turning in the bottom high speed needle valve. You'll reach a point whereas the engine will either start to die out or spit back (sounds like a mild backfire). At that point, back out the needle valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest setting.

(Low Speed) Slow the engine down to where it just stays running. Shift into neutral. Again in segments of 1/8 turns, start to turn the top needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running. Eventually you'll hit the point where the engine wants to die out or it will spit back. Again, at that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

When you have finished the above adjustments, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.

 

[F_R]

Re: TN-27 Fuel Adjustment

"usually" does not count with the TN Johnsons. The wheel on the tank is the high speed mixture adjustment. Start off with about 1 1/2 turns open and adjust for best full throttle operation from there.

The slow speed mixture is the little "L" shaped lever sticking down from the port on the front of the crankcase. Start off at 5/8" turn open and adjust from there for best idle. This one is very touchy.

In reality, the idle system is a second carburetor drawing fuel from the main float bowl. Idle fuel/air mixture enters powerhead through a rotary valve. The main carburetor fuel/air enters powerhead through a 3rd port system. There is some interaction between the two. Not the usual system we are used to today, for sure.

 

[wbeaton]

Re: TN-27 Fuel Adjustment

Sounds like that motor is a serious PITA. I'll start looking for one immediately.

 

[Xcusme]

Re: TN-27 Fuel Adjustment

"usually" does not count with the TN Johnsons. The wheel on the tank is the high speed mixture adjustment. Start off with about 1 1/2 turns open and adjust for best full throttle operation from there.

The slow speed mixture is the little "L" shaped lever sticking down from the port on the front of the crankcase. Start off at 5/8" turn open and adjust from there for best idle. This one is very touchy.

In reality, the idle system is a second carburetor drawing fuel from the main float bowl. Idle fuel/air mixture enters powerhead through a rotary valve. The main carburetor fuel/air enters powerhead through a 3rd port system. There is some interaction between the two. Not the usual system we are used to today, for sure.

Yikes! You are so right! This puppy is not 'conventional' at all. I just looked up the details on the TN series carbs and they are much different. Thanks for the clarification.

 

[440racer]

Re: TN-27 Fuel Adjustment

"usually" does not count with the TN Johnsons. The wheel on the tank is the high speed mixture adjustment. Start off with about 1 1/2 turns open and adjust for best full throttle operation from there.

The slow speed mixture is the little "L" shaped lever sticking down from the port on the front of the crankcase. Start off at 5/8" turn open and adjust from there for best idle. This one is very touchy.

In reality, the idle system is a second carburetor drawing fuel from the main float bowl. Idle fuel/air mixture enters powerhead through a rotary valve. The main carburetor fuel/air enters powerhead through a 3rd port system. There is some interaction between the two. Not the usual system we are used to today, for sure.

Thanks for the feedback. What you said is what I suspected. I will try this method. I have gotten it to run very well from the start to fast setting. Trying to get the motor to run well below the start position has been difficult. I think I pay more attention to the "touchy" nature of the slow speed mixture adjustment.

I'm going to try to put it in the water this weekend.


Thanks again.

 

[Chinewalker]

Re: TN-27 Fuel Adjustment

Actually, they aren't as much of a pain in the behind as you would think. Once they're set, they run VERY well. There's a reason why there are so many of them still around. I can still remember when I graduated from the 3hp JW to the 5hp TD-20 (same as TN-27, minus the neutral) when I was about 11. Man, I thought I was going a million miles an hour...
- Scott

 

[440racer]

Re: TN-27 Fuel Adjustment

Belated update.

There is finally enough water in my river to use the boat again and I put the motor in the water this past weekend and used the adjustments listed above and it ran great. starting it is still a little touchy but once it starts it works very well.

Thanks to all above for the input!