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Hi everyone ,can you check the pop off on a assembled tilly carb attached to motor? or do you have to take it apart? Also if you can check it on the motor is it the same reading as a dissambled one? thankyou
Here's a good read... http://www.eccarburetors.com/pdf/Tuning-Info.pdf

Short answer yes you can check it all together. Just take the high side needle out.
What is the purpose of removing the HS needle?? That makes no sense to me, and I've never seen anyone do that.

I followed the first link and read EC Birt's write-up, and I saw that he mentioned the purpose was to avoid "blowing up a balloon." Um, I still don't get it.

Some of his other commentary seems to cloud the issue a bit, IMHO. The whole point is, the force countering the "pop-off" spring is venturi vacuum working on the wet side of the metering diaphragm. With a venturi that's on the small side, there will be plenty of vacuum to pull the diaphragm in, and that little spring will be no match for it. The only reason anyone ever has to monkey with the pop-off spring is when they're running large/multiple carbs, making the venturi vacuum much less.

BTW, in order to get a crisp pop-off reading, I've always had to slather the needle area with WD-40 and then jiggle the needle around in it before EACH measurement. In the video, he mentioned that, but didn't seem to have to do it all the time.

The thumb-pump in my pop-off gauge never sealed right from day one (bought new). Highly annoying, although it has helped to build the muscles in my opposable thumbs... my near ancestors would be jealous.
Doug,

I think you have over simplified the delicate ba1ance that is maintained by the spring pressure under the fulcrum arm.
It is true that the drop in pressure in the venturi is a significant factor but your remarks left out the other major factor which is the output pressure of the fuel pump. Pressure drop in the venturi will result in movement of the metering diaphragm due to the difference between atmospheric pressure on the dry side and the below atmospheric pressure condition at the venturi.  As the delta between the venturi and atmosphere pressure levels grows the force of the diaphragm against the fulcrum arm will grow. Simultaneously, the residual pressure from the pump will be pressing on the tip of the needle that is exposed in the seat to the incoming fuel. The combination of the force on the fulcrum arm and the tip of the needle are what ultimately cause the needle to move off the seat and allow fuel to flow. I use the term "residual" when speaking of the pump output pressure as the pump is out of phase with the intake stroke of the engine. The typical built in pump as seen in the Tilly carbs produces pressure as the piston is moving down the bore and compressing the charge in the case. The pressure drop at the venturi is generated by air flow through the venturi as the piston is moving up the bore and expanding the volume of the case.
"Pop off pressure" is a simple reference point we use to quantify the spring pressure we have chosen to use at any given time. Depending on the carb design and the engine package it is used on the "correct" pop off pressure can range anywhere from as little as 5psi to as high as 30psi. Generally speaking, kart racers tend to gravitate toward setting the pop off pressure as low as possible while still maintaining enough seal to prevent drooling and flooding. However, doing so usually means the resulting mixture adjusting screw settings will be such that the openings for the low pressure signal coming from the pressure drop in the venturi will be muted. In many cases, the overall performance of the carb can be improved by increasing the spring pressure and opening the mixture screws to allow a stronger signal to influence the movement of the metering diaphragm. A very extreme example of the balancing act would be the way I ran my 125cc McCulloch enduro setup back in the late 70s. Due primarily to a very effective slippy pipe design, the BDC 16 double pumper carb pump output pressure on my engines was so high I used 30 psi of pop off pressure to keep the pump from blowing the needle off the seat at certain engine speeds. To compensate for the high spring pressure I would end up opening the mixture needles nearly twice as far as if I ran the pop off down around 15 psi. One interesting aspect of the different combinations of carb tuning setups was that I did not burn any more fuel in an hour race with the jets open twice as far.
To summarize... pop off pressure measured the way we do it in the karting world is simply a reference point to allow is to quantify the changes we make when using different spring pressure under the fulcrum arm. There is no absolute "correct" value for any particular carb. Venturi pressure is an important factor that influences the spring pressure that will work best but it must be considered along with pump output pressure and jetting to gain a better understanding of how and why changing spring pressure can help or hurt carb performance.
BTW... on a closing note.... just to complicate things a bit.... when we discuss the BDC line of Mac carbs there is another major design component that other carbs do not have which is the duck bill circuit that creates an artificial bump in pressure on the dry side of the metering diaphragm under certain conditions. Properly managed the duckbill circuit can overcome the weak signal at the venturi that occurs at low engine speeds when the carb is basically too big for the engine it is installed on. That is a subject for another day.
Steve O'Hara
Hi Mel,
Steve O is definitely the authority on kart carburetors.

However, I just want to make sure your original questions were answered...
Q.  can you check the pop off on a assembled tilly carb attached to motor?
A.  Yes.  No need to take it apart.

Q.  Also if you can check it on the motor is it the same reading as a disassembled one?
A. Same, but the readings will not be as crisp after the first "pop" or two, once air enters.  Just don't blink when you pump it up the first time.




ALSO, I think I figured out EC Birt's balloon comment (referring to the link Duke provided):
I never thought about it before, but when you test "pop-off" through the fuel inlet fitting, the pump diaphragm (aka, skin) also has to withstand that pressure, which it never sees when the engine is running.  I've never had one rip as a result, but I'm told that serious racers using Tillotson carbs buy the extra-thin pump skins and replace them every race day because they get a "belly" in them (i.e., they stretch a bit).  As long as we're still talking carburetors, I'm not sure why a slight belly is bad... at very high speeds, it must affect the skin's response to the pressure pulses.

But I'm still confused by E.C.'s instruction to remove the HS needle...

Doug
Thanks everyone for your  imput, man you guys know your stuff. Can you get different pop off springs and if you want the tilly to pump less fuel, would you increase the pop off pressure?
Yes, and yes.

E.C. Distributing is the North American importer for Tillotson. He sells a kit with an assortment of spings for the fulcrum arm. They're smaller and lighter than anything McMaster-Carr has.
Again, I defer to Steve O for sure.. but pop-off isn't usually the first thing most people mess with when tuning a carb. Many karters go their whole career without messing with it.
First help us understand what you have going on, and what you've tried.
(10-18-2017, 12:08 PM)doug tenney Wrote: [ -> ]Yes, and yes.

E.C. Distributing is the North American importer for Tillotson.  He sells a kit with an assortment of spings for the fulcrum arm.  They're smaller and lighter than anything McMaster-Carr has.
Again, I defer to Steve O for sure..  but pop-off isn't usually the first thing most people mess with when tuning a carb.  Many karters go their whole career without messing with it.
First help us understand what you have going on, and what you've tried.

Hi  Doug. When I start my Mc5 It spits fuel out of the exhaust and smokes  alot. I have the idle adjustment not even  touchin the stop, and hard to make it run clean. At the big one ,Dave pinched the fuel line and it still ran so it must of had alot of fuel in the crankcase. Thats why I was wondering if maybe I need to work on the pop off.
doug,
a bellied diaphram will mess up your track day.
the motor will start but it'll make you nuts.
carb will be all over the place in performance as the pulse signal gets too weak to allow enough vacumn from just the wasted diaphram.
because of the weak signal and insufficient fuel the driver opens the needles up to compensate.
in my mind that ads just enough of a vacumn pulse through the adjusting needle inlets to add a touch more pop off.
pop off gets more consistant - then you flood - then you thin the needles vacumn drops and start process all over again.
i noticed this after lots of practice with mis installed carbs covering the pulse hole.
i was still was able to run the motor for short times fiddling with the needles and choking but they'd never smooth out.
finding the covered pulse hole would fix the problem.
when i went to thinner diaphrams and they got belied i noticed a similar problem.
i thought about it and realized you can provide alternative vacumn path if you open the adjusting needles enough.

anyway that's my experience.

mel,
absolutely learn how to keep your pop off consistent.
that will require constant monitoring but it tells you alot about carb condition without dis-assembly.
as long as the carb reads the same as when you last did pop off you've eliminated a possible carb issue.
bad pop off can mimic worn out diaphrams and worn out fuel doors in you second plate gasket set.
if you get a bad reading assembled my next step is pull plates and see what it reads directly.
if it's back to a good reading the seat, fulcrum arm and spring are fine and you next should check the diaphram for excessive stiffness and or wrinkles.
if the diaphram seems good inspect the second plate wet/dry diaphram for a belly and/or the little doors standing off the plate thus not sealing well.
often if a tilly carb is mis behaving and the interior pop off is good i just chuck the internal gasket set and install new.
sometimes they look fine but are junk anyway.
they aren't a huge amount of money compared to a wasted week end.

if you haven't yet do take the flat side of the tilly plates and sand to flatness.
they almost always are not up to par as found especially the older carbs.
i also campher the pulse holes on the manifolds and the carbs.
check ever new gasket for pulse hole alignment.
i have a whole batch that are terrible and cover 1/3 of the pulse if you don't recut.

another issue you may find is the fuel inlet holes to be mis-cut and covering part of the passage.
that is also an issue with the gaskets sold today from some sources.

the biggest tilly gasket issue is in the fuel inlet gasket.
they were mis cut long ago and now the bad ones seem to be the standard.
one of the two fuel doors openings is too small so the door can't open completely.
this can be seen when you put the gasket directly on the plate where it covers metal.
on the fat one the extra material becomes apparent.
if it sounds like i figured something out i didn't there was a bulletin i read long ago about the issue but it's useful to know.

finally if you have a multiple part manifold that can possibly assemble wrong and cover pulse holes it's a good idea to idiot proof them.
it's extremely easy to do with gem manifolds as i can attest.
i've now  put little match arrows on the exterior of mine so i can check for mis assembley externally at a glance. Big Grin 

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