What does TPS on a carburetor do

Thread Starter

Autobike

Joined Feb 23, 2018
87
hello :) i need a little explanation regarding the Throttle Position Sensor.
i've seen some carbureted motorcycles where they use a TPS sensor with the carb.



googled it and got to know that it uses a varying voltage to send some signals to the TCI which advances the ignition according to those signals.

also found below explanation;

"Without the TPS in circuit you have a 2 dimensional ignition advance curve that advances the spark depending on the RPM, no matter what the load, similar to points and centrifugal weights of yesterday.
With the TPS in circuit you have the 3rd dimension. Now the igniter knows if the engine is under hard acceleration, cruising, slowly building speed or decelerating when it cross references with RPM. This is important as the speed of the flame front in the combustion chamber changes as the load on the engine changes and in turn the timing of the spark needs to change in line with the flame front to produce good ignition at the point of maximum chamber pressure".

so i have a few questions;

1) it says "With the TPS in circuit you have the 3rd dimension. Now the igniter knows if the engine is under hard acceleration, cruising, slowly building speed or decelerating when it cross references with RPM". so can't these conditions be recognized without a TPS? i mean only by the RPM values. for example if the RPM is constant, bike is cruising. if the RPM is increased rapidly, it's hard acceleration.

2) what is the main purpose of putting a TPS on a carbureted vehicle ? does it improve the fuel consumption or does it improve the engine power ?

3) sorry not sure this is a mechanical thing rather than electronic. but when people talk about TPS they talk about the "engine load". is that a similar word for the "hard acceleration" or a different thing ? i think it plays a main role in this topic. still didn't fully unerstand what it means.

thank you very much.
 
Last edited:

nsaspook

Joined Aug 27, 2009
6,704
TPS gives engine condition delta (desired rate of change) information from the current engine condition to the desired engine condition. This allows for the prediction of future changes in engine condition that might optimize (torque) the time needed from X -> Y->Z instead of just the current X->Y as we approach Z. It's much like the GPS destination position, without the destination the device can't optimize the current path, it can only supply information about current conditions.

https://www.allpar.com/fix/codes/sensors/TPS.html
 

SamR

Joined Mar 19, 2019
1,621
TPS has been somewhat replaced or at least added to by Mass Air Flow Sensors. Instead of measuring where the throttle plate is it tells the computer how much air is actually going into the fuel system in particular on fuel-injected systems. Airflow vs. throttle plate position can take out problems such as dirty air filters.
 

Chris65536

Joined Nov 11, 2019
154
This function use to be provided by a "vacuum advance" mechanism on the distributor. Higher load gives less ignition advance, to avoid detonation. I'm guessing a potentiometer and a table in the computer is cheaper than the mechanical solution.
 

Chris65536

Joined Nov 11, 2019
154
they talk about the "engine load". is that a similar word for the "hard acceleration" or a different thing ? i think it plays a main role in this topic. still didn't fully unerstand what it means.
The engine load is the amount of torque the engine is producing. It is approximately proportional to the absolute pressure in the intake manifold. Imagine these three scenarios, all with the engine running at a steady 2,000RPM:

1) Transmission in neutral - throttle is slightly cracked to rev the engine to 2000RPM. The engine load is close to zero.

2) Driving along at a steady 30mph. Engine load is low - just enough to cover wind resistance and rolling friction.

3) Climbing a steep mountain road with the throttle wide open. Engine load is maximum - doing real work raising the vehicle against gravity.

The RPM is the same in all three cases. But the engine load, manifold pressure, and throttle position are not.
 

shortbus

Joined Sep 30, 2009
7,334
TPS has been somewhat replaced or at least added to by Mass Air Flow Sensors. Instead of measuring where the throttle plate is it tells the computer how much air is actually going into the fuel system in particular on fuel-injected systems. Airflow vs. throttle plate position can take out problems such as dirty air filters.
While they did use a TPS with a carb, can't say I ever heard of one with a MAF. Many of the newer improved EFI systems don't even use a MAF today, they've changed to a MAP (mass air pressure) sensor, less problems with them.
 

Thread Starter

Autobike

Joined Feb 23, 2018
87
TPS gives engine condition delta (desired rate of change) information from the current engine condition to the desired engine condition. This allows for the prediction of future changes in engine condition that might optimize (torque) the time needed from X -> Y->Z instead of just the current X->Y as we approach Z. It's much like the GPS destination position, without the destination the device can't optimize the current path, it can only supply information about current conditions.
https://www.allpar.com/fix/codes/sensors/TPS.html
thank you. read the whole article. very informative :)

TPS has been somewhat replaced or at least added to by Mass Air Flow Sensors. Instead of measuring where the throttle plate is it tells the computer how much air is actually going into the fuel system in particular on fuel-injected systems. Airflow vs. throttle plate position can take out problems such as dirty air filters.
got it. thank you :)

This function use to be provided by a "vacuum advance" mechanism on the distributor. Higher load gives less ignition advance, to avoid detonation. I'm guessing a potentiometer and a table in the computer is cheaper than the mechanical solution.
thank you :)

The engine load is the amount of torque the engine is producing. It is approximately proportional to the absolute pressure in the intake manifold. Imagine these three scenarios, all with the engine running at a steady 2,000RPM:

1) Transmission in neutral - throttle is slightly cracked to rev the engine to 2000RPM. The engine load is close to zero.

2) Driving along at a steady 30mph. Engine load is low - just enough to cover wind resistance and rolling friction.

3) Climbing a steep mountain road with the throttle wide open. Engine load is maximum - doing real work raising the vehicle against gravity.

The RPM is the same in all three cases. But the engine load, manifold pressure, and throttle position are not.
thx a lot. had a hard time understanding and this simple example cleared it :)
have a small question though. in the first example you have mentioned that "load is close to zero". does it mean that the engine has less torque than when it's in a gear ( considering RPM is the same in both situations )? thx again :)

While they did use a TPS with a carb, can't say I ever heard of one with a MAF. Many of the newer improved EFI systems don't even use a MAF today, they've changed to a MAP (mass air pressure) sensor, less problems with them.
thx :)
 
You do need info on whether the engine is:
A) idle
B) Fuel economy
C) Wide open throttle

That's three distinct conditions determined by throttle position.


Fuel economy mode may not be position only: e,g between idle and WOT, but the lack of change.
WOT at start is a way of clearing flooding.
 

Thread Starter

Autobike

Joined Feb 23, 2018
87
You do need info on whether the engine is:
A) idle
B) Fuel economy
C) Wide open throttle

That's three distinct conditions determined by throttle position.


Fuel economy mode may not be position only: e,g between idle and WOT, but the lack of change.
WOT at start is a way of clearing flooding.
thank you :)
 
Just a note: In a traditional carburated engine, the idle system is based on throttle plate position. In this position, the carb is concerned about keeping the idle constant. It's tough to find that position, but cake if you use propane enrichment.

I won't describe the procedure unless your interested, but when you inject propane, there is only a small change in idle which is what you want. You require a propane cylinder, metering valve, on/off valve and a connection to a vacuum port and a tachometer. And, of course, a screwdriver for the adjustments.
 
More or less your first two dimensions are:

1) An engine needs a certain amount of spark advance for a given RPM for the air and fuel to be properly ignited when the piston reaches top dead center. For example: 10 degrees at idle to 30+ degrees at around 3000 rpm and levels off. All engines are different, but there becomes a point where the curve levels off and too much advance leads to melted pistons. More or less since everything moves faster at higher rpms your air and fuel need lighted earlier to burn properly. This is accomplished by your distributor weights.

2) The amount of vacuum and engine produces indicates to some extent how much air and fuel is getting drawn into the cylinders. An engine at idle (or high vacuum) doesn't fill the cylinders the same as wide open (or low vacuum). A certain amount of additional advance is added based on the amount of vacuum to allow the mixture to burn properly.

Your third dimension would be since the ECU is now doing the job of the old distributor weights and vacuum canister it can now also adjust the ignition timing based on the actual throttle position. It could either lead to better economy or better power, but not both at the same time. At that point it's all in the programming.
 
It could either lead to better economy or better power, but not both at the same time.
There's a third variable that can be optimized too. Polution. Remember the VW issues. Polution vs economy.

The major players on an ECM engine are: engine temperature, throttle position, manifold air pressure/barometric pressure, ambiant air temperature, oxygen sensors, mass air flow sensor.

So, we can do economy, polution and power optimization.

We also have the "open loop" and "limp mode". Sometime limp mode doesn't work. I replaced a MAF sensor and a day or so later, the MAF failed. A family member was driving it and it stopped and would not continue running when re-stared. With the accelerator pressed, idle was worse than rough.

I met the family member and since that was the only thing messed with, I decided to pull the connector. I got limp mode. Drove the family member home and went to the store where the sensor was purchased and replaced it in the parking lot.
 
When starting, you have choke based on engine temperature and you have the "choke unloader" . When there is vacuum, just after the engine starts and cold, the choke is opened a little bit. Gas pedal to the floor and little RPM (starting) means open the choke to clear flooding.

Then you have vacuum advance control. I had a carburated engine, but no TPS that had two vacuum advance diaphrams

It also had a solenoid enabling the EGR valve. It opened the EGR valve fully when the engine was stopped to prevent dieseling.
How do i know? I had issues with that solenoid sticking. Rough idle if the car was previosly warmed up and off for about 20 minutes before restarting. Really tough to find.
 
There's a third variable that can be optimized too. Polution. Remember the VW issues. Polution vs economy.
I didn't think of that one. The original question lead me to believe maybe the thread starter was a little hazy on some details of spark advance. I have to admit I really didn't understand much of it until I started researching to attempt to build my own timing controller for my old truck. the concept worked on the bench, but there seems to be transients fighting me in the truck and since it's falling apart and only leaves the property a couple times a month I never put much effort to getting it right. Just put in a different distributor and dialed it all in.

I was also lucky enough that when I first started driving most everything had been converted to fuel injection and the few carbureted vehicles I did own didn't give me any trouble other than normal maintenance.
 

shortbus

Joined Sep 30, 2009
7,334
2) The amount of vacuum and engine produces indicates to some extent how much air and fuel is getting drawn into the cylinders. An engine at idle (or high vacuum) doesn't fill the cylinders the same as wide open (or low vacuum). A certain amount of additional advance is added based on the amount of vacuum to allow the mixture to burn properly.
That is so wrong. Manifold vacuum is controlled by the opening of the throttle blades, has nothing to do with how the cylinders are filling. The vacuum at wide open throttle is low because there is no (or less) restriction. If you case was true, why does a diesel engine have no or very little real vacuum, whether idling or full power? It's because there are no throttle blades in the air intake. A diesel takes in the same amount of air idling as at full power, it chose which is being done by the amount of fuel added to the cylinder.
 
That is so wrong. Manifold vacuum is controlled by the opening of the throttle blades, has nothing to do with how the cylinders are filling. The vacuum at wide open throttle is low because there is no (or less) restriction. If you case was true, why does a diesel engine have no or very little real vacuum, whether idling or full power? It's because there are no throttle blades in the air intake. A diesel takes in the same amount of air idling as at full power, it chose which is being done by the amount of fuel added to the cylinder.
All I was saying is the amount of vacuum indicates how much is getting drawn in. The mixture will burn differently based on how much the cylinder actually draws in. That is where part of the old vacuum advance came in to play. I did not say how the vacuum was produced. With the throttle closed the cylinder will not fill as much as it will at wide open. Obviously the reason there is vacuum is because there is a restriction in the air flow.

A diesel on the other had actually prefers more air than it can draw in on it's own... which is why most have turbos on them and instead of vacuum you have boost. But now you're comparing apples and pumpkins.
 

shortbus

Joined Sep 30, 2009
7,334
That is where part of the old vacuum advance came in to play. I did not say how the vacuum was produced. With the throttle closed the cylinder will not fill as much as it will at wide open. Obviously the reason there is vacuum is because there is a restriction in the air flow.
Again your not thinking like a engine guy. The vacuum advance pulls in(advances) when the engine is idling and there is high vacuum. That is why you disconnected the vacuum advance back in the old days when you set the timing on the distributor, base timing. As the throttle was opened there was/is less vacuum, so the timing get advanced to burn more of the fuel.

Explain to me how a cylinder of "x" number of cubic inches fills with less than that number of cubic inches of air when idling? It doesn't get as much fuel, but it still gets that amount of air in it. As the throttle blades open the vacuum is higher up in the throat of the carb, the venturi, where the high speed jets are and more fuel is added to that volume of air and more power is made. But the cubic inches of air per cylinder says the same. More fuel means more power and speed from the engine.


A diesel on the other had actually prefers more air than it can draw in on it's own... which is why most have turbos on them and instead of vacuum you have boost.
And when you turbo or super charge you add more fuel to take advantage of that extra air. Just adding air on it's own just makes it burn lean and melt a piston top. Even non-boosted diesels have no vacuum, that is why they have either a vacuum pump, hydro-boost, air brakes or electric slave for their power brakes.
 

SamR

Joined Mar 19, 2019
1,621
On a 4 barrel carb, to save fuel, the secondary plate was typically vacuum controlled. To increase power the accelerator gets pushed opening the primary plate increasing air/fuel flow to the engine. As power comes on RPMs increase causing manifold vacuum to increase. As manifold vacuum increases the vacuum control on the distributor advances the timing and the vacuum control on the carbs opens the secondary plate to increase air/fuel flow. What we used to do on a Holly carb was to insert a screw into the outside primary plate linkage that forces the secondary plate to close along with the primary plate. Inserting the screw locked the primary and secondary plates together and eliminated the vacuum control. There was a bit of lash so I could cruise on the primary plate up to ~80 mph but if I floored the pedal all hell broke loose as both plates immediately opened creating very fast power and acceleration. I also used a dual points Mallory mechanical distributor with no vacuum advance. Ignition advance was done by a couple of flywheel cam plates that mechanically advanced timing with RPMs and had to be set up on a Sun Distributor machine using different weight flywheel cam parts and different spring tensions to tune it based on the engine performance. Let's just say it was not tuned for economy and yielded more than 1HP/Cu In along with a few other intake/exhaust flow modifications. The downside of all that was the engine was hard to restart if it was warm and I would have to open the hood and manually retard the spark to get it to start and then reposition the distributor back to it's normal running position.
 

Thread Starter

Autobike

Joined Feb 23, 2018
87
@KeepItSimpleStupid @geekoftheweek @shortbus thank you all :)
as @geekoftheweek said "It could either lead to better economy or better power, but not both at the same time". that's the other thing i needed to know. thx

i got a practical example from a local guy. he's been riding a carb bike with a TPS. he said it's lot easier to ride up a steep slope with a TPS connected than not connected. so i think as we discussed above, it advances or retards the igntion to give it a perfect combustion. so it produces a fair amount of power than the TPS disconnected ( at the same rpm )

read the @geekoftheweek and @shortbus conversation.
let's assume the bike is a carbureted bike. so if the entire volume of the cylinder is not filling 100% with the mixture ( air+fuel ) what's the rest is filled with ? is there a vacuum or something ?

thank you very much :)
 
Last edited:

Thread Starter

Autobike

Joined Feb 23, 2018
87
by the way our example says "This is important as the speed of the flame front in the combustion chamber changes as the load"

so if we consider these two situations;
1) bike is in neutral. rider revs and maintains a steady 5000rpm
2) bike is cruising at 5000rpm

it's true that those two conditions have different throttle positions. but since it's the same rpm ( 6000rpm ), does the piston reaches the TDC in a same speed ? as i understood, apparently it doesn't. that's the reason behind putting a TPS to detect the throttle position and advance the ignition. but since the rpm is same ( 5000rpm ) shouldn't the speed of the piston have to be the same? thank you :)
 
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