Vintage Karting Forum

well it's been several years and i'm finally running out of usable carbs.
these were build way back when i was just learning how to build a tilly thats fast.
i learned a bit since then and have decided to try and share what i've recorded as data so far. this thread like any i write is subject to life interfering so who knows if or when it'll be done. on the happy side i just rebuilt six carbs yesterday and gathered data from about twenty.
anyone who has solid real data is invited to contribute to this post.
questions are also welcome.

The carbs i chose to work on are the old cresent alky carbs i've been gathering for many years .
Cresent had their ideas pretty solid and ones that were not messed with are extremely reliable.

first i started with measuring the two holes that the fuel adjustment needles insert into.
it's the first place i look for modifications and a deal breaker if they have been over drilled to the extent that the needles no longer can seat closed.
step one
get a complete drill index #1-60 the smallest sizes are the ones you'll be using.
get a good micrometer and mike the smallest sizes in the zone you expect to use.

for ease i w folded a work tag and marked the sizes on the tag.the correct drill was placed in each fold.
see pix below for what i did
[attachment=2152]

 i then removed the idle and high needles from the carbs that had them installed.
a large amount of these varbs were nos and were never fully assembled. this is excellent as it's unlike they were modified.

as i removed the adjusting needles i miked the top edge of the barrel to size them and piled them into three main sizes
.the needles varied in size a tiny bit but that may mean something later when i track test the carbs.
so i miked them all , about fifty needles all told, brass and steel , some new some used.

the results were as follows.
1/4 of them were at .0915 +/- most from the high needle size
1/2 of them mixed between high and low needles were at .0920 +/-
1/4 of them were at .0925 most from the low needle side.
i also miked every spare adjustment needle i had. a couple miked higher .0930 and a couple thin about .0875 those are put aside for experimenting later.
i also noticed some of the needle tapers were shorter than average.
they looked like they may have been filed to increase flow?
those i also put aside for future testing.
the idea is to remove variable from the carbs so blueprint everything.

next i moved to sizing the two adjusting holes.
these carb bodies are from cr1a 194 and 264 cresent carbs.
so they span the whole production era of cresent carbs i think.
it became clear early on that the holes were all the same on the unmodified ones. out of 17 carbs the sizing was as follows
low needle hole was .0655
high needle hole was .0715

there was one carb that had an undersized high hole i put it aside.
a couple of randomly bought carbs had larger holes those too were put aside for later.
but even older worn/used carbs i measured had above sizing.
i'm guessing that was the cresent standard for adjusting needles but i could be wrong. further data from others would be nice.

after that a large amount of time was consumed so i moved into project mode and built the six carbs that were closest to complete.
i used the .0925 needle in the idle hole the .0915 needle in the high hole i left the needles miking middle range for next batch.

i DID NOT PULL THE WELTCH PLUGS AND MEASURE.
 this may bite me in the butt but they all blew clear in the three holes and visually they looked untouched. should i have issues the wetch plug will be my go to inspection.
i replaced the shutter valves and spent some time carefully fitting them to the bore.
it's tedious time consuming work carefully filing them but no light means sharp response at throttle.
i'll try to get pix at some point to explain what is needed.
the holes for the throttle shafts were all mint no inserts needed and i used new throttle shafts as most of the carbs had none.

i replaced all the adjustment needle springs as needed added washers as needed and replaced all the needle o-rings.
i added a new seat and viton needle to the mix 
for this batch i made one uncaptured needle set and the rest captured needles.
all fulcrum arms are new.
i set the fulcrum arm at .075 (ec birt alky gauge used)
pop off is varied three at my preffered 8.5-9# one at 7# and two at 6#

the covers have all been flatted in a past rehab mass project all new gaskets have been installed.
i used the red high flow cover (ec birt) all screws were replaced to.

with any luck i'll get a couple fitted up before avon and proceed with gathering more data.
heres a later pix of same carbs after avon test now ready to be retested
[attachment=2153]


for now that's all i got.

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[attachment=1937]
i'm gonna roll back this thread a tiny bit. as i'm using new old stock parts alot of what normally needs to be done was skipped.
this would not be an effective article for building carbs that are over fifty years old and often modified if i don't go over the basics before we get into doing a clean rebuild.
there is no reason to rebuild a carb if it has been drilled so far out of spec as to be useless for your application. the best method to determine this is to break down the carb ,clean inspect and mike it to create a blueprint of what you have in hand.

 my current plan is to go over a break down and inspection of carb, what to measure and how so you can determine if the carb you are using is worth rebuilding.

after that in further posts we'll cover rebuilding the carb and then i get into hot rodding the carb both mild bolt on sort of carbs and a little more intense modifying the internals both for gas carbs to alky and some alleged improvements on the carb bodies.

you absolutely need to understand how a carby works so  please read this instructional i found on the net.


okay now you know how they work so we need to inspect and measure to see what was done to your carby.


the first step in any successful build is to blueprint what you have so you know what you are dealing with.
typically any carb you purchase that isn't brand new should be blueprinted .
without an exact history you have no idea what if any modifications have been made.

please do not be intimidated by the following instructions the steps outlined are easy though tedious. the results you'll get should be way better than average and your time at the track more enjoyable.
for general purposes i'm assuming you know nothing about carbs please excuse any informational overlap.

first step is assemble a small group of tools you'll be needing to measure your carb inlet holes.

• you'll need a decent micrometer a cheap one will work but keep checking the closed setting and reset as needed.
  

• you need a small set of drill blanks or actual drill bits will be fine.
  

sizes

• get something to record date a 2x3" tag on a string works well for notes.
  

• have a strong set of cheater glasses to magnify parts i prefer jewelers headset with magnifier at 5x or better.
  

• a small screw driver
  
• a pair of needle nose plyer  smooth work face
  
• a small needle nose vise grip
  
•  a 3/8" socket 1/4" drive which you'll probably need to grind down on the sides to clear the sides of theinlet seat cavity
  

to save wasted time measuring the first thing i like to do is disassemble and clean them completely.
often after cleaning and dis assembly and inspection some used/abused carbs end up being for parts .
(TIP  when you disassemble the carb the best method if you're new is to place each part in order of removal onto your bench.reversing the order on re assembly is helpful in getting them back together .taking pictures also helps.)

All of the inspection should be done under magnification a set of cheap magnifier "cheater glasses" is useful here.

any part that is rusted or corroded needs to be cleaned and if pitted replaced.

the carb body needs to be completely cleaned and every passage cleared carefully.
have a old wire guitar string is useful as a tool here.

so now you'll need to take everything off the carb body.

• first completely wash the outside of the carb,carb cleaner soap and a toothbrush work well here clean everything.
  

• unscrew the adjustment needles. remove from carb body , take the rubber o-ring on the needles and discard these are ALWAYS replaced. under magnification inspect both idle and high tapers they should be smooth without flaws and ring looking wear on the taper or rust divots any flaws = discard and replace. you need to save the needles if good ,the tiny brass washer and spring clean any crud out of the spring.
  

•  remove the throttle plate (shutter valve - carefully they are a tight fit and oval) when you remove the screw/screws keep the shutter valve in the same position as you take it out inspect throttle plate for wear discard if sides show damage plate is twisted or anything looks wrong
  

• take off the throttle shaft retainer screw and clip. inspect clip under magnification, any sign of wear will mean replacement.
  

• remove throttle shaft ,inspect throttle shaft for wear at points it passes through carb body, if there is excessive wear allowing movement side to side discard and replace. inspect carb body holes for wear also. if the carb body holes are ovaled you'll need to add a bushing or junk the body. also inspect throttle shaft clip retainer slot if there is excessive wear allowing movement side to side discard and replace.
  
• remove plates from carb clean off anything that isn't aluminum,inspect for corrosion. when clean paint with dycum blue or sharpie for now put aside to be flat sanded later .
  
• remove the fuel inlet needle fulcrum arm retainer screw.put your finger on the fulcrum arm so the spring doesn't pop out and get lost.remove fulcrum arm pivot rod and spring. inspect screw and rod for damage or corrosion retain if perfect.inspect fulcrum arm for twist or unusual wear if it's a captured arm check and confirm arms are parallel to each other.
  
• now carefully remove needle and inlet seat.after removing seat remove tiny brass seal at bottom this needs to be replaced every time it's disturbed.a tiny paper clip sharpened to a needle tip and bent is helpful to pull this seal. carefully clean this area and make sure the passage is clear .
  
• remove weltch plug be careful drilling. the weltch plug is very thin and drilling the carb body will ruin it so set a stop slightly longer than the replacement plug you'll be needing.
  

heres a pix with advise on welch plug removal install
 [attachment=1936]

• under the welch plug in tillies theres are three holes needing checking.
  

two are in front of the shutter valve when the valve is closed.one is behind the valve when closed.
the front hole will be drilled at an angle aimed at the outside world.
 this hole is for air mixing only and is one of two air inlets your carb uses at idle.
(TIP the shutter valve of course should completely block off any other air when closed. careful fitting of the shutter valve is very important on performance carbs and in the rebuild section i'll go over how i set up shutter valves to seal as completely as possible if you don't you've modified air flow at idle which will make tuning extremely hard.)
the middle hole when the shutter is closed is your second air inlet at idle.
this changes when the shutter opens and then the second hole becomes a fuel passage.
the third hole is a fuel passage hole only.
typically but not always the holes you'll find modified badly are the two front ones.
often they'll be cut so big as to almost merge or are merged into one hole.
if the two holes are one the carb sadly is damaged beyond use unless you want to get into attempting repairs a waste of time in my experience.
if they still remain separated you'll need to check the sizing with the drill blanks.
size all three holes and note the results on the tag you've prepared for the carburator.

• now you'll need to measure the throttle screw mixture holes . using your pre=measured drill blanks insert smallest to largest on the NON-DRILL SIDE till the next drill blank won't fit.record the last drill size number that fit on the tag.
  
• finally you'll need to flat the plates. you do this by fixing a piece of sandpaper to a steel flat a granite flat or if you are tool poor a piece of glass. (FLAT ONLY ON THE NATURALLY FLAT SIDE DO NOT FLAT ANY SIDE WITH NUBS ). flatting is done by using a figure eight pattern on the sand paper holding the part lightly.usually 320 to 400 is a good grade to start with on sandpaper.sometimes 220 or even 120 is needed to get things started depending on how bad the plates are as found.after ten turns rotate the part 90 degrees and sand fugue eights again.check results the ink should only be showing in low spots.if there is no ink showing reink go to next grade paper and do a few turns check again.if the ink has again completely disappeared and you've hit the sandpaper grade desired you are done. the plates may be very nice and need little work or you may be sanding a while regardless doing this on operation will improve the carbs possible leak proof performance alot.
  

this article is incomplete but right now i gotta blast. at this point you have most of the data needed to say your carb is blueprinted with as found info.the next step is determining whether the numbers found are acceptable to continue with the rebuild or trash the carb.
ANYONE WHO HAS CARB DATA ON HOLE SIZING IS WELCOME TO POST IT HERE AS WE WILL BE NEEDING ANY WE CAN GET.
thanx










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Tillotson Operation Theory

Color key:
[attachment=1982]

Idle Operation:
[attachment=1983]

Intermediate Operation:
[attachment=1984]

High Speed Operation:
[attachment=1985]

China HL-324 Copy for Stihl 070 and 090 Saws Spec Sheet
[attachment=1986]

HL-334A/B Carb Specs:
[attachment=1991]

Factory T-Slot (Right Hand Side in Preview, Bottom When Opened):
[attachment=1992]

2nd Intermediate Jet Hole Angle (Drilled at Aprox. 38 Degrees):
[attachment=1993]

im not gonna even pretend to be a carb guru ..but ..!!!! ... how can you open up primary idle fuel orifice without taking out welch plug ..
I didn't see where  you are opening up the pump discharge hole in the center plate ..
and I could be waaay wrong ...but I don't think you can make one  work good on alky unless you gut the dump tube and open the inlet hole ..
anybody can prove me wrong ..im here to learn also ..
[attachment=2004]
far right stock .they suck ..middle has lock ring removed .,left is ball removed opened up ..
[attachment=2005]
i polished butter fly and bore ..ill try it sometime ..lol.. 
[attachment=2006]

Good info on page 18 of this http://cdn.shopify.com/s/files/1/0151/78...eb.pdf?257

Steve M. I solder the dump tube shut and then re drill them. I had a piece on here about doing the clone carbs in the carb section.

steve m,
it's late but first when you are properly blueprinting an unknown carb you always remove the weltch plug to size the inlet holes you find in there. on the carbs i'm working on they were already modified to run alky by cresent so pulling the welch plug on a nos carb was not needed.
but any mods in the low idle inlet area require removal of weltch plug.
simple job as illustrated in the pix in earlier post.

most dump tube are undersized inlets but easily fixed. i thought i mentioned that earlier. on the carb i added one i just used a hand operated drill bit and opened up the hole to the same size as the high speed needle inlet hole.
i hadn't removed the little ball in the tube but they are easily removed i'll cover all that maybe this week.

been busy got called in to a theatre job been working the deck all this week. was a long week first day went 11pm monday to 10 pm tuesday then showcall all week double on sat sun with 7 hour out immeadiately after show closed booked well over 60 hours not including 3 hours travel every day so i'm still pretty tired today.

thanx brian and steve w. for adding to thread also.
nitey nite

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steve,
i'm no carb guru either but i figure to post all the stuff i learn good and bad so others can avoid my mistakes.
i'm pretty good at screwing up simple stuff so all anyone has to do is the exact opposite of me when i vear inmto the woods with a rebuild!!

anyway,
finally got some time today.
took carbs back into shop.
disassembled and replaced all the stupid mis installed wrong parts.
popped off every carb disassembled recorded info then reassembled and popped off again.
recorded data. some worked better assembled pop off wise all are at 8-9.5 # pop off.

while i was at it i finished rebuilding the cresent that has the e.c. birt t slot modification. this one is way deeper than my version which is my best carb. am curious how it will work. tomorrow i have to assemble a few clutches before i can test carb.

i shelved the motor we busted the starter nut on at avon when it hydro locked from the badly assembled carb. it feels ok but i'm tearing it done to check the rod and piston bearing area for damage.
the clutch that was on the motor was a major woman to get off the busted starter nut was torqued in beyound belief. i got lucky as there was a ledge left when the outer part shattered.
i banged it for about 45 minutes with a cold chisel with the motor locked up trying to spin it loose.
never came loose but it finally cracked and i was able to save everything but the nut.
the clutch needs to be readjusted and while i'm on clutches i'll set up a few extras for the next project i'm trying for before rigp.
then i can test some carbs .