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Setting Tillotson Carb Popoff

Published By James McMahon     October 9, 2014    


By E.C. Birt


Your carburetor is just like your heart. When it gets sick you have to go find you a Doctor real quick to get a fix. Here your carburetor is the heart beat of the engine and you’re the Doctor… Time to go to school. 

You know 40 years ago I didn’t know what pop off pressure was either. And as the years went by and with all the tech articles I have wrote I guess I just assumed that all knew the above answers. 

A plate of ribs washed down with two glasses of red wine supper is over with, looks like it’s time to go to work. 

Any Racer that is using a diaphragm type of carburetor needs to own a pop-off gauge to service their carburetor and trouble shoot any carburetor related problems during Race time. The pop-of gauge that we make and sell to our dealers and racers comes with a machined adaptor in the bag. This adaptor has an o-ring on it for sealing purposes and the only one of it’s kind that I know of. More about this part later. 

What you need to know now and understand is how pop-off pressure works, and what its purpose is which leads to how the carburetor works. 

The two snake skins (tan colored) are called (1) a pump diaphragm and (1) check diaphragm in a double pump stack on one style of carburetor like Tillotson HL Carburetors. This is the pump stack used for alky carbs and high performance Carburetors in the HR line up. Now ever one wants to think these Carburetors are double pumper carbs. This is wrong their not two pumps, it’s a double volume carb and that is why it’s used on alky and HR Carburetors for high performance usage and bigger engines. 

Another style of HL Tillotson carburetor only has two plates on it and with this carb it only has one snake skin and serves as a pump diaphragm and check diaphragm in one. This type of pump stack is used mostly for gas carbs. The single Tillotson pump stack and clone style of the Tillotson pump design is also used on a lot of European style carburetors used on their 100 cc two cycles on gas as fuel for the day. 

Ok the above information is how fuel gets to the carburetor in any diaphragm carburetor. 

Now we get to the demand diaphragms. This is the VIP part in all carburetors and for sure I recommend that you use nothing here but original part because the copies leave a lot to be desired in performance level most of the time. This diaphragm is made out of some trick stuff. Which is a rubber and fiberglass compound type of material and a molded part with a ½ round circle resolution on one side. The higher the resolution is the better the action is in movement up and down. This is the down fall of the copy part. In short this is the black one with the alloy flat washer looking part in the center with one side having a small round knob sticking down. This pin on the diaphragm is inline with the fulcrum arm and can come down and reach one end of the fulcrum arm pushing it down. This action releases the needle off the fuel inlet seat to let fuel from the pump stack enter the wet side of the carburetor. 

How quick this action takes place is controlled by the strength of the spring that is under the fulcrum arm to push it down by the pin on the demand diaphragm to release the needle off the seat to let fuel into the wet side or return back so the needle can seal the fuel flow off. The vacuum of the engine on the intake side controls the action of the demand diaphragm. In short vacuum on the intake stroke will pull fuel out of the carburetor discharge ports which in turn makes vacuum that forms on the back side of the fuel as it leaves the carburetor to the engine, and this vacuum pulls the demand diaphragm down to let the needle come off the seat to keep filling the wet side of the carburetor as needed. Here your lucky if the pump side of the carburetor makes 1½ lbs. of pump pressure. That is why we use a fuel pump in front of the carburetor to assure there is ample supply of fuel to it all the time for a race engine most of the time. And at the best the extra fuel pump will put out maybe 3 lbs. plus a tick. 

By this time everything should be about as clear as mud to you. So take a potty stop raid the icebox, come back with a clearer bunch of brain cells and we will go from here. 

Now lets get to pop-off pressure and what it is. 

In simple terms the pop off pressure is controlled by the strength of the pop off spring in a pound pressure reading that it takes to let the needle off the seat. So came the term pop of because that is where the lb. reading was at max on your gauge as you pump it up and then all of a sudden it drops a bit. In short won’t go up any more. Pop off pressure can be anywhere from 6 lbs. up. Any thing under 6 lbs. the inlet needle never really shuts off the fuel flow to the wet side of the carburetor. Some carburetors like up to 20 lbs. of pop off before it will pop depending on the carburetor, the engine size, type of fuel, race track size, heat, and usage to just name a few of the variables of race day needs. 

I know now you say wait a minute, didn’t you say above that the pump side of the carburetor maybe makes 1½ lbs of pressure and the extra accessory fuel pump is good for 2 lbs of pressure. If that is true how in the World can it ever release the needle off the seat with 6 lbs. of pop off let alone 20 lbs.? 

Good question. A ways back up some I said the fuel when it left the carburetor out of the fuel discharge ports on the low and high side (holes) a vacuum was caused on the diaphragm that pulled it down. When you lift the throttle. The shutter goes closed and the high speed of air flow suction is for the most part from the intake port is closed off from the carburetor. So now the vent side of the demand diaphragm cover plate comes into reverse action and lets the diaphragm return back up and the inlet needle drops back onto the seat due to the spring pressure of what the pop off spring that has been used and set at. 

Ok now the way you use your pop off gauge with the pump stack off is to use the machined fitting and line that has the o ring on it that come with the gauge. Now flood the whole carb cavity around the inlet needle and seat with WD 40 because all readings have to be wet. Ok now with your finger push the fulcrum arm down to release the needle pressure off the seat so some WD 40 can run down under the needle and wet the seat. If low on fluid refill, I don’t like spray can stuff. Ok now take the machined end and push it down into the fuel inlet track to the seat and keep enough pressure on the fitting with one hand and with the other hand to pump up the gauge till it pops off. When the needle gets to the point where it won’t go any more and you here a pop it will drop just a tick. This number in front of the needle in the gauge is what the pop off pressure is in lbs. 

If your checking with the pump stack on and the carburetor is all together off or on the engine replace the short piece of fuel line on the gauge with one about 3 ft. long. Fill this line up with fuel and then plug it onto the fuel inlet cap on the carb. Also be sure and remove the high side needle from the carburetor because we don’t want to be blowing up a balloon at his point. Now pump up the gauge till it pops and then do one more time because the first pop was just to wet everything up. Now take 2 more readings they both should be very close to each other. When it pops it must pop and hold with no less then a ½ lb. Of drop then hold hard here. A very very slow seep off is ok and shouldn’t hurt you much. Most good carb builders wants it to lock solid and hold at that little bit of drop from the number they want. 

If it won’t hold pressure and drops like a rock you can sometimes clean the seat and needle tip to cure problem. If this don’t work time to go to a kart shop some times and get some parts. 

Pop off pressure you want for most carburetors, 10 to 12 lbs. for gas carbs on 100 cc and up engines. Small gas carbs and engines will like 15 to 20 lbs. Alky engines 7 to 8 ½ lbs. I know these are round numbers and from here it will take some time to find the perfect happy spot with track testing. 

Now you can wear the white coat. 

I’ve talked to Bob and this has been put in the General forum and just maybe he will make it a sticky for awhile. This way if I left something out you can post your question and I can find and answer to the best of my capabilities. E-mail won’t work on this subject any more to me and plus I just don’t have time to go look for your questions anymore. 

Maybe some day as health gets better I just may write a book with pictures and all the tricks for you to read. 

Some where Bob will put this piece in the tech article section and that is where you can go to find it after any needed up dates. 

If you need any parts just call any of our Dealers or E.C. Distributing 615/446-6807 and they will take care of you. 

E.C has left the building…

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