Engine Stumble LM 110 HP - when hot

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rnd5553
Posts: 61
Joined: Fri Nov 06, 2015 5:22 pm

Engine Stumble LM 110 HP - when hot

Post by rnd5553 »

Recently my LM 110 HP engine when hot, has developed an annoying stumble the majority of the time when applying more throttle. It seems this happen after it's been driven a while, parked and sat for at least a hour or longer.

When it's cold no issues.

My question is what would most likely be causing this stumbling condition? The engine has a newer coil/wires/plugs and the carbs were both rebuilt within the last 18 months. The fuel pumps age is unknown.

As always, thanks for the input.
Jerry Whitt
Posts: 887
Joined: Thu Sep 26, 2013 2:42 pm

Re: Engine Stumble LM 110 HP - when hot

Post by Jerry Whitt »

Your note indicates the problem surfaces when warm. Immediate thought, what changes when warm?

1. Choke plates close when cool, and as the engine warms, choke plates open. The rich mixture when cool, changes to a leaner mixture when warm.
If the mixture become too lean, the symptoms you described would follow.

2. After the car has cooled way down, verify the choke plates close. If not closed, repair. If the choke plates close, look at the fast idle cam and make sure it engages the linkage. Then start the engine. Fast idle should be 12 to 13 hundred rpm. The choke pull off should also open the choke plate about 3/8 of an inch.

3. Watch the engine warm up. The choke plates should open fully in a few minutes. Once the choke plates are open, hit the throttle linkage
and watch for a squirt from the accelerator pumps in both carburetors. If the squirts are good, then with the engine still running, look at the vacuum advance unit. Hit the throttle again and watch the vacuum advance move the linkage on the distributor. If it does not move, vacuum hose may be installed on wrong port of the carburetor.

Let us know what happens
Jerry Whitt
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65 Monza, purchased new
65 Corsa convertible
rnd5553
Posts: 61
Joined: Fri Nov 06, 2015 5:22 pm

Re: Engine Stumble LM 110 HP - when hot

Post by rnd5553 »

Jerry - here in is my problem - when cold my throttle level is not going to the second step on the lever, so I have no fast idle. I have the Rochester Manual for the carbs and have read how to adjust this, but what else controls the throttle lever ability to move to the second step on cold starts. If I follow the adjusts as noted in the Main Shop Manual, sec. 6M-7 fig 16, 17 should this correct the non fast idle issue? Thanks Jerry for your input.
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bbodie52
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Re: Engine Stumble LM 110 HP - when hot

Post by bbodie52 »

When the bimetallic heat-sensing choke coil is cold, it should apply spring pressure to the choke mechanism to close the choke butterfly valve in each carburetor. Starting procedure on the Corvair with a cold engine calls for pressing the throttle pedal to the floor one time and then releasing it. This procedure momentarily moves the throttle linkage on each carburetor out of the way of the fast idle cam, which allows it to move into position to mechanically hold the throttle partially open when the engine starts (fast idle position). At the same time the choke mechanism pushes the choke butterfly valve into a fully closed position. When the engine starts, engine vacuum causes each vacuum break diaphragm to pull the choke butterfly valve partially open, to admit some air into the carburetor throat to mix with gasoline to continue the engine operation. The partially closed choke butterfly continues to partially restrict airflow to cause a "rich" fuel air mixture, which helps the cold engine to continue running. At the same time the fast idle cam position prevents the throttle butterfly from fully closing in the normal idle position. As the engine warms up, the bimetallic choke coil is heated at each cylinder head. As the coil heats, it compresses and reduces spring pressure on the choke butterfly. As the engine heats to normal operating temperature the choke gradually transitions from closed to a fully opened position. At this point the fast idle cam has moved to a fully disengaged position and that vacuum break diaphragm on each carburetor no longer has any effect, because the choke butterfly has moved to a fully opened position.

Choke Mechanism
Choke Mechanism
Corvair Automatic Choke Mechanism.jpg (27.91 KiB) Viewed 354 times

Fast Idle Cam Mechanism
Fast Idle Cam Mechanism

Bimetallic Choke Coil
Bimetallic Choke Coil

Choke Adjustment
Choke Adjustment
rnd5553 wrote:Thu Jun 22, 2017 10:52 am

Recently my LM 110 HP engine when hot, has developed an annoying stumble the majority of the time when applying more throttle. It seems this happen after it's been driven a while, parked and sat for at least a hour or longer.

When it's cold no issues.
As previously described, a cold engine runs better with a richer fuel/air mixture. This is created on a cold engine with a closed choke butterfly. With the choke open the engine is configured to run with a normal fuel/air mixture. However, the design of a carburetor requires airflow through the carburetor throat to create a Venturi effect to draw gasoline from the carburetor float bowl into the carburetor throat, for mixture with the air as the air passes through the throat and is drawn into each cylinder. There is a brief delay as the air begins movement through the carburetor throat, before the Venturi effect can draw fuel through the carburetor passages to mix with the air. This brief delay can cause the engine to stumble as it gets mostly air with little gasoline mixed in. To compensate for this, each carburetor is fitted with an accelerator pump, which is a mechanical plunger that is forced into motion by the mechanical movement of the throttle linkage. The moving plunger mechanically injects gasoline through the accelerator pump passages to squirt a jet of fuel into each carburetor throat. This mechanical injection of fuel momentarily provides additional gasoline to mix with the airflow, until the Venturi effect can draw fuel from the float bowl, through the main jet and into the carburetor throat to continue engine operation with the throttle open. The accelerator pump is intended to mechanically injects additional gasoline each time the throttle position transitions from closed to open, which helps to eliminate a momentary power "stumble" until the fuel gets moving in a continuous state through the carburetor.

There might be a problem with the accelerator pumps in one or both carburetors. Each accelerator pump mechanism includes a rubber boot that forms a seal to force gasoline through the pump passage. If one or both of these rubber seals has become worn, or if one of the passages has become clogged with some form of contamination, the jet of fuel that should be squirting into the carburetor throat may be weak or not present at all, which would cause the warm engine to stumble each time the throttle is opened.

You should check the operation of your carburetor accelerator pumps. If the expected injection of fuel is weak or nonexistent, you may need to repair or replace the accelerator pump plunger mechanisms as a part of a carburetor cleaning and overhaul.



Image


In the picture below, part of the float bowl can be seen on the upper left. The passageways between the float bowl and the accelerator pump reservoir on the lower left allow fuel to pass from the float bowl reservoir into the accelerator pump reservoir. As the plunger pushes downward into the accelerator pump reservoir, gasoline is forced through the passages into the carburetor throat, so that a pair of fuel jets are squirted into the air passage in the carburetor throat. This momentarily enriches the fuel/air mixture to reduce a lag in power that might occur as the carburetor transitions from a closed throttle to an open throttle condition.
Accelerator pump details
Accelerator pump details

Accelerator pump lever
Accelerator pump lever
Brad Bodie
Lake Chatuge, North Carolina
Image 1966 Corvair Corsa Convertible
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