TR2/3/3A Linkage issue? Car stalls when braking hard

Snowkilts · Oct 21, 2016

Hi folks, I've had an issue for a long time that I've been trying to figure out.

My vehicle is a '60 TR3A. When I brake even slightly hard, the rpms drop way down, even to the point of stalling sometimes. This only happens when the car is warming up, or on a cool day when the engine doesn't get quite as warm.

I suspect a throttle linkage issue because if I lift the accelerator pedal with my foot instead of pressing it I get a very similar symptom. I've been through the entire linkage. All the ball joints are good. I saw another thread about play in the bell crank, and I have a bit of that, but not too bad.

Is it possible that the car is flexing when braking? This might have the same effect as lifting up the accelerator.

Any ideas?

TR3driver · Oct 21, 2016

The throttle plate position with your foot off the pedal should be controlled right at each carb, with the throttle stop screw for that carb. That should easily override any body/frame flex.

If you're sure the shaft also turns easily in the nylon bushings in the firewall, and the correct return spring is installed at the firewall, and the bell crank isn't dragging on the bolt head hidden underneath it; then my next guess would be the throttle plates binding in the carb bore. Common cause is the front carb body and/or throttle shaft being worn enough to allow the shaft to move up and down. Of course the lever action from the throttle lever pulls it down, so the bottom of the throttle plate drags on the bore.

Another problem that I've fought with several times now is not having the plate centered properly. It has to not only be centered on the bore, but centered with the idle stop arm firmly against the side of the carb body. Otherwise, the action of the throttle linkage pulls the shaft forward, and the edge of the plate drags against the side of the bore. Over time, it cuts a step into the bore that will keep it from sliding into the fully closed position.

Also, while warming up, you can keep the choke out just a bit, which should bring the fast idle cam into play without enriching the mixture.

Installing a modern thermostat will keep engine temperature much more constant regardless of outside temps.

Oh yeah, also worth checking that the bell crank arms are at 45 degrees with the throttle closed, as well as the arm on the throttle shaft.

Snowkilts · Oct 21, 2016

TR3driver said:
The throttle plate position with your foot off the pedal should be controlled right at each carb, with the throttle stop screw for that carb. That should easily override any body/frame flex.

If you're sure the shaft also turns easily in the nylon bushings in the firewall, and the correct return spring is installed at the firewall, and the bell crank isn't dragging on the bolt head hidden underneath it; then my next guess would be the throttle plates binding in the carb bore. Common cause is the front carb body and/or throttle shaft being worn enough to allow the shaft to move up and down. Of course the lever action from the throttle lever pulls it down, so the bottom of the throttle plate drags on the bore.

Another problem that I've fought with several times now is not having the plate centered properly. It has to not only be centered on the bore, but centered with the idle stop arm firmly against the side of the carb body. Otherwise, the action of the throttle linkage pulls the shaft forward, and the edge of the plate drags against the side of the bore. Over time, it cuts a step into the bore that will keep it from sliding into the fully closed position.

Also, while warming up, you can keep the choke out just a bit, which should bring the fast idle cam into play without enriching the mixture.

Installing a modern thermostat will keep engine temperature much more constant regardless of outside temps.

Oh yeah, also worth checking that the bell crank arms are at 45 degrees with the throttle closed, as well as the arm on the throttle shaft.

I rebuilt the carbs a few years ago and I remember carefully centering the plates. Of course, this doesn't necessarily mean I did it correctly.

So, would it be accurate to say that if everything is set up correctly, lifting the accelerator pedal should have no effect on the idle?

The bell crank is definitely not dragging on the bolt. I actually have an extra spring on the throttle linkage to pull it back. I'll check the other items you mentioned, thanks.

Geo Hahn · Oct 22, 2016

An off-the-wall thought is to ask if having the floats set too low would cause fuel starvation on hard braking?

I have noticed that when coasting down the mountain in neutral at an enthusiastic speed (don't do that) that I get some fuel starvation on fast turns -- only one direction but cannot recall now whether it was lefts or rights -- never stalls but idle drops to 300 or so and the generator light comes on.

charleyf · Oct 22, 2016

I have a friend with a TR3 whose car would die as it rounded a corner. I don't remember which way he was turning, but it did it only the one direction. He tried everything he could think of and finally changed the carbs to another set he had and--- problem gone. So I am thinking that the problem is with the carbs not the linkage.
Charley

Geo Hahn · Oct 22, 2016

charleyf said:
I have a friend with a TR3 whose car would die as it rounded a corner. I don't remember which way he was turning, but it did it only the one direction...

I had the same issue on my E-Type - but in that case it was Strombergs. The problem was a sunk float which caused flooding on fast lefts - though it felt like fuel starvation at the time.

Yeah, it would behoove the OP to take a quick look at float level and also lift them out to give them a shake and listen for fuel inside.

Snowkilts · Oct 22, 2016

Geo Hahn said:
An off-the-wall thought is to ask if having the floats set too low would cause fuel starvation on hard braking?

Possible, but I can make the effect happen while only going 20 mph or so. Seems like that would make the carbs awfully sensitive. Any way to test for this?

I have not noticed any issues when turning, although admittedly I don't drive that hard.

Kleykamp · Oct 22, 2016

There are a lot of moving parts between the accelerator pedal and the throttle rod, most of which will have some play in them. They wear into a pattern and have a place where they want to "rest". Ideally that would be at the point where it idles correctly and the point at which you set the idle. I can pull back the throttle and get the idle to drop some on mine also. Mine typically idles at around 700 RPMs, but occasionally will go slightly higher or lower, but not by much. You say you have an extra spring on the throttle...not sure why you would need that, but it may be pulling back too much and binding the throttle linkage. I suggest removing the "extra" spring, set your idle screws to a good idle and try it out before doing a lot of disassembly.

Snowkilts · Oct 22, 2016

After some investigation, it looks like I have at least two issues.
1) The linkage is a bit sticky, particularly the piece between the bellcrank and the carb. I never put any lube on the ball and socket joints, just because I always thought that carb parts should not be lubed, but I see from other threads that these joints do need it. I got grease into them and the stalling issue got noticeably better, so lesson learned on that.
2) With the throttle disconnected, I can still get the rpms to drop some by turning the throttle shaft the wrong way. So I'm thinking my next step will be to recenter the throttle plate with the carbs in place on the vehicle. I tried pushing the shaft around while the car is running and the rpms did not change, so I think the shaft is tight.

TR3driver · Oct 22, 2016

Snowkilts said:
So I'm thinking my next step will be to recenter the throttle plate with the carbs in place on the vehicle.

Evidently, "centering the throttle plate" doesn't mean the same thing to you as it does to me. The screws that locate the plate are inside the carb, there is no way to access them without removing the carb.

And oh, BTW, they are supposed to be replaced every time you loosen them. The ends are split so you can stake them into place; but straightening and re-staking them makes them very apt to break later (from vibration).

6TTR3A · Oct 22, 2016

Randall, I was doing fine with your troubleshooting guide until "Oh yea, also worth checking that the bell crank arms are 45º with the throttle closed, as well as the arm on the throttle shaft" What is a bell crank arm and 45º to what? I found bell crank levers but no arms.????
FRank

TR3driver · Oct 22, 2016

Sorry if I'm not using quite the right terms, Frank. Maybe levers would be a better term? Anyway, the bell crank has a central pivot point with what I call lever arms sticking out at 90 degrees to each other. At mid-throttle, the arms should be at roughly 90 degrees to the connecting links (one to the lever arm at the firewall, the other to the lever arm on the front carb throttle shaft). But that angle changes with throttle position, so with the throttle closed, the arm with the link to the firewall should be at roughly 45 degrees to the link; while the other arm should be at roughly 135 (45 + 90) degrees to the link to the carb. The lever arm at the carb should also be at roughly 45 degrees to the link (so it hits 90 at mid-throttle).

Both links are adjustable for length so you can make this so; plus the lever arm at the carb can be rotated on the shaft. I don't know of anyplace this is mentioned in the manuals, but I do believe that is what the adjustments are for. Normally, they wouldn't need to be changed, but sometimes things happen.

Another important relationship is the pedal should hit the stop just as the throttle plates reach full open.

Snowkilts · Oct 23, 2016

TR3driver said:
Evidently, "centering the throttle plate" doesn't mean the same thing to you as it does to me. The screws that locate the plate are inside the carb, there is no way to access them without removing the carb.

Fair enough. It's been a while since I had the carbs off. So I'll revise my #2 item to "try to figure out why the throttle plates are not closing all the way by themselves", and go from there.

6TTR3A · Oct 23, 2016

Thanks Randall, I'll check it out today.. Perhaps now I can get rid of the heavy-duty spring I had installed. I'm sure it's not doing the linkage any good.
Frank

TR3driver · Oct 23, 2016

FWIW, a 1/4" drive swivel socket also makes it easier to get the carb nuts off

Snowkilts · Oct 30, 2016

Thanks for all your help on this guys.

I did noticeably free up the linkage and removed the extra spring I had put in. Now when I lift the accelerator pedal with my foot the rpms go down a little, but not much.

The car still stalls even under moderate braking, and with the choke on enough to bring in the fast idle. I have not yet looked at the floats, but there seems to be a significant temperature component to this issue, as if the problem goes away when the carbs get good and warm. Having the engine warm does not seem to be enough.

Here's an example: The other day I drove someplace that was about 4 miles away (about 50 deg F outside). Even though the engine was fully warmed up (based on the gage) by the time I got there, I still managed to stall the engine twice even with the choke out one notch.

I stayed at the place about 1-1/2 hours, then went home. On the way home I put the choke all the way in and tried to stall several times by hitting the brakes, but couldn't. I seems like letting the car sit and allowing the heat to soak in prevents the stalling.

Is there anything in the SUs that I could look at the might make them more likely to stall when cold and braking? Anything I could test to see where the problem is? Again I have no problems whatsoever when cornering.

Thanks,

Jon

charleyf · Oct 31, 2016

Jon,
This is the time that I start going back to some routine checks. One of the first ones is to check that the float bowl valves are both clear and operating properly. That means to take them out of the lids and make sure there is nothing in the lines above the valves. Check both floats to be sure they they do not have any fuel in them ( not a likely thing--but easy to shake it and find out). Adjust the level of the floats.
Make sure that the pistons in each carb are free moving and hopefully they both move about equally in their rise and fall. Take off the air filters and get someone to rev the engine so that you can give your full attention to how they move. The piston drop test is an alternative to the last one but does not tell as much. The pistons should be equal in their travel times both up and down.
The one difference that I can think of where the warmer engine would matter would be if one carb is functioning poorly and the other carb is feeding fuel through the equalizer tube to keep the car running. So when the car is warm the fuel could better vaporize and travel through the tube to the other two cylinders.
Just lots of guessing. Also have you checked the condition of the spark plugs? I am not good at this but is there a difference in color of the front two plugs from the rear two plugs? This could mean the carbs are not equally providing fuel.
Charley

Geo Hahn · Oct 31, 2016

charleyf said:
...is there a difference in color of the front two plugs from the rear two plugs? This could mean the carbs are not equally providing fuel...

Another quick test for this is to point your IR temp gun at the exhaust manifold immediately after start-up from cold. A misfire or fuel problem can appear as a big difference between the temperature of each side of the manifold as the problem cylinders may be slow to warm up.

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TR2/3/3A Linkage issue? Car stalls when braking hard

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