1973 RT-3 Carb Issue

Williamldarby · 03-14-2013, 08:24 PM

Ok so i got my bike up and running but i am have a problem with the carb. The manuale says the Air screw is 1 1/2 turn out but if i follow this the engine seems to run lean and the rpm is very erratic jumps between 2000 to 2500 constantly. When i pull in the clutch the rpms take a while to drop. So i adjusted the air screw to 1/2 out and idle is perfect and as soon as i pull the clutch in the rpm drops to idle like it should. What should i do. i know for sure that everything is stock in the carb by the manual. If this helps i am riding at see level. The bike originaly came from Colorado.
Best Regards
-William

Foot dragger · 03-14-2013, 08:29 PM

1/2 turn out makes it richer then 1 and 1/2 turns out,= less air through the air screw your turning. The pilot jet could be partially plugged or you need the next size richer pilot/idle jet. It wont hurt it running it with the screw only 1/2 turn out but 1 and 1/2 turns is the preferred setting.

Williamldarby · 03-14-2013, 08:30 PM

Also i just remembered the small oring on the main jet that sits in the bowl looks a little worn. idk if that would cause this issue.
Best Regards
-William

dan-c · 03-18-2013, 08:05 AM

Don't follow the instructions as to where the manual say how far to open the airscrew.
Start the bike, turn the idle jet in to it will barely idle. Slowly turn the airscrew out until the engine gains rpm. When you turn it out to where it doesn't gain anymore rpm turn it in about 1/2 turn. Reset idle speed.
That's just for the low speed (bottom end idle to ~ 1/2 throttle) circuit, may need more adjustment and cleaning to get the 1/4 - WOT adustments correct.

Also make sure the slide/cable isn't binding

WRW9751 · 03-18-2013, 08:22 AM

It would appear you may have a air leak. A good way of checking that is with a can of Gummout. Just spray around the intake manifold area see if the RPM's raise. Do your best to keep the spray away from the venturi. Air leaks may also be present behind the flywheel or base gasket. The crank case seal behind the flywheel is forty years old, if the main bearings are worn some the seal will be oblong. This might be one reason the rpms are erratic, seal warming up ect. I might also add that if the air mixture screw has been overly tightened the seat maybe distorted. I would recommend staying very close to the factory recommendations. It's trying to tell you something, listen a little closer.

dan-c · 03-18-2013, 08:36 AM

Checking the seals is a good idea, if the one on the flywheel side is the culprit, it can be changed without splitting the cases.

As to the airscrew settings, do what you want but setting it per the manual ain't the way any two stroke tuner will do it

03-14-2013, 08:24 PM	#1
Williamldarby ^OP Adventurer Joined: Mar 2013 Oddometer: 12	1973 RT-3 Carb Issue Ok so i got my bike up and running but i am have a problem with the carb. The manuale says the Air screw is 1 1/2 turn out but if i follow this the engine seems to run lean and the rpm is very erratic jumps between 2000 to 2500 constantly. When i pull in the clutch the rpms take a while to drop. So i adjusted the air screw to 1/2 out and idle is perfect and as soon as i pull the clutch in the rpm drops to idle like it should. What should i do. i know for sure that everything is stock in the carb by the manual. If this helps i am riding at see level. The bike originaly came from Colorado. Best Regards -William

03-14-2013, 08:29 PM	#2
Foot dragger beat up ex flat tracker Joined: Dec 2006 Location: chico,just below rag dump(nor-cal) Oddometer: 6,770	1/2 turn out makes it richer then 1 and 1/2 turns out,= less air through the air screw your turning. The pilot jet could be partially plugged or you need the next size richer pilot/idle jet. It wont hurt it running it with the screw only 1/2 turn out but 1 and 1/2 turns is the preferred setting. __________________ 2003 DR 650.(1976 Montesa 250 Enduro-nice!) - - 1990 MASI TEAM-3V.- 1976 Motobecane, Super-Mirage.- Kona, HumuHumuNukuNukuApua'a. Single Speed ThRaShEr BiKe. 1968 360 Greeves challenger MXer. 1999 Triumph Trophy 1200. 2011 KTM530 EXC. 2012 KONA Hei Hei Deluxe (ongoing bike issues) -2009 KTM 200XC-W.

03-18-2013, 08:05 AM	#4
dan-c Back Joined: Aug 2005 Location: SATX Oddometer: 13,513	Don't follow the instructions as to where the manual say how far to open the airscrew. Start the bike, turn the idle jet in to it will barely idle. Slowly turn the airscrew out until the engine gains rpm. When you turn it out to where it doesn't gain anymore rpm turn it in about 1/2 turn. Reset idle speed. That's just for the low speed (bottom end idle to ~ 1/2 throttle) circuit, may need more adjustment and cleaning to get the 1/4 - WOT adustments correct. Also make sure the slide/cable isn't binding __________________ "...Carved in stone is a date he had to go on, And another from the moment of his first dawn..." - In Memory of Preston Haun - In Memory of Mike Berlein - In Memory of Amanda Cassady

03-18-2013, 08:36 AM	#6
dan-c Back Joined: Aug 2005 Location: SATX Oddometer: 13,513	Checking the seals is a good idea, if the one on the flywheel side is the culprit, it can be changed without splitting the cases. As to the airscrew settings, do what you want but setting it per the manual ain't the way any two stroke tuner will do it __________________ "...Carved in stone is a date he had to go on, And another from the moment of his first dawn..." - In Memory of Preston Haun - In Memory of Mike Berlein - In Memory of Amanda Cassady

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03-14-2013, 08:30 PM	#3
Williamldarby ^OP Adventurer Joined: Mar 2013 Oddometer: 12	Also i just remembered the small oring on the main jet that sits in the bowl looks a little worn. idk if that would cause this issue. Best Regards -William

03-18-2013, 08:22 AM	#5
WRW9751 Gnarly Adventurer Joined: Jun 2011 Location: Ankeny Iowa Oddometer: 186	It would appear you may have a air leak. A good way of checking that is with a can of Gummout. Just spray around the intake manifold area see if the RPM's raise. Do your best to keep the spray away from the venturi. Air leaks may also be present behind the flywheel or base gasket. The crank case seal behind the flywheel is forty years old, if the main bearings are worn some the seal will be oblong. This might be one reason the rpms are erratic, seal warming up ect. I might also add that if the air mixture screw has been overly tightened the seat maybe distorted. I would recommend staying very close to the factory recommendations. It's trying to tell you something, listen a little closer.