We removed, cleaned, and disassembled the carbs in Part I. We also talked a little about carb theory. We talk a little more theory and do the overhaul and reassembly in this part. We reinstall, tune and troubleshoot the carbs in Parts III.
Chamber Overhaul & reassembly
you look at the parts you quickly realize that all the parts attach to the main
body and can be reassembled in about any order.
I choose to start with the float chamber because it is nearly always the
source of the fuel leaks that got me interested in working on the carbs in the
floats were pried off the steel clip when the carb was disassembled. The only thing left to remove is the needle valve.
This should be unscrewed using the correct size wrench or socket.
The valve I removed was 17/32 and I just happened to have a socket.
A 14mm wrench or socket is the next closest fit and would probably have
If the valve has been acting up it should be discarded at once lest it end up back in another carb. If the valve is to be reinstalled or kept as an emergency spare, then it should be soaked for an hour or so in the lacquer thinner.
is a thick aluminum washer under the needle valve.
A new value is usually supplied with several different size washers. The
correct thickness is 1/16 inch. The
valve is installed (or reinstalled) first and then the float hinge pin with
floats is inserted into the steel clip as shown below.
Note the position of the float. Check
that the clip grasps the hinge pin securely.
(If not, remove the pin and squeeze the clip together with a small pair
of pliers.) Make sure the float
the carb inverted, make sure the tab on the float is against the valve holding
the valve closed and then check that the highest point on each float is 16 to 17
mm above the face of the body as shown in the next photo. (Note: every reference I have has the float setting as 16 to
17 mm but all have different settings in inches.
The correct conversion is 0.630 to 0.670 inches.
The Haynes manual lists 16 to 17 mm or 0.725 to 0.787 inches. (Is it any
wonder the carbs get screwed up?) Bend
the tab as required to meet this specification.
When done make sure that the tab contacts the valve at a 90 degree angle.
float chamber is then reinstalled on the body using a new gasket. (Never use an old gasket; itís too much work
replacing a leaking one. Also,
gasket sealers wonít seal gasoline so trying to repair an old gasket with
sealer is futile. Itís wise to always to have a spare gasket on hand.)
Make sure the two shorter screws are in the correct holes and then
tighten the screws uniformly. Finally,
lubricate a new O-ring and slip it on the sealing plug and then inset the plug
into the hole in the chamber. This
completes the work on the float chamber.
Throttle Shaft & Bushes
Replacing Throttle Shaft & Bushes
Inspect the throttle shaft to see if there is wear where the shaft rotates on the bushes in the carb body. If there is wear and the shaft is loose in the bushes then both the shaft and the bushes must be replaced. I understand bushes and shafts are available from TRF and Moss. I understand that the bushes must be reamed after installation using an expensive (~$100) reamer. I've not done this job yet as the bushes on the dozen carbs I have all seem to be in good shape. This may be because TRs in Ohio tend to rust through before they wear out. I'll update this should I do a bush replacement in the future.
Throttle Shaft (Spindle) Seals
The throttle shaft seals prevent air from leaking into the intake manifold. Small leaks around the throttle shaft are of no consequence when the throttle is at least partially open. However, the leaks can cause a rough idle. It is doubtful that the seals wear out because the typical Triumph gets very little use, especially here in the mid west. However, the seals are subject to deterioration due to age. If the seals have not been changed for 10 or 15 years itís probably a good idea to do so when the carb is off.
The seals are located in the outside ends of the holes that hold the throttle shaft. The seals are removed by inserting an awl or scribe along the edge of the seal and then lifting the seal out as shown in the photo.
í70 and later TR6 carbs are arranged to dispose of fuel vapors from the fuel
tank collected when the engine is idle and crankcase by-pass gasses when the
engine is running. In the í73 and
later TR6s, vapors from the carb float chamber are also collected when the
engine is off and disposed in the same way as vapors from the fuel tank.
This later addition is also used together with an added electric valve
to eliminate ďrunning-onĒ or dieseling as us yanks call it.
breathing apparatus is shown in the next sketch from a Triumph Maintenance
Manual. The fuel tank cap seals the
tank when closed. The tank is
vented though a vapor separator connected to the top of the tank and via hose #5
to the top of absorption canister #4. As
the vapor laden air passes through the canister active charcoal granules absorb the vapors
and clean air exits through tube #9 at the bottom of the canister.
Tubes #2 vent the float carb chambers in the same way on Ď73 and later
connect the valve cover (#10) and the canister to ports (C) on the carbs.
These hoses are much larger than the ones that connect to the fuel tank
and the float chambers. The ports at (C), located above the idle trimming screws, feed
directly into the mixing chambers on both carbs.
the engine is running there is a depression in the carb mixing chamber that
draws fresh air into the bottom of the canister through tube #9
out of the top of the canister port (B) and via tube #3 into the carbs.
When the fresh air moves through the canister it purges the carbon
granules of the vapors and carries them into the carbs where they and sent into
the engine and burned. Bypass gasses from the crankcase rise to the valve cover
(#10) where they exit though the port #1 to the same large hoses to the carb
ports. The by-pass gases are also
sucked into the carbs and burned in the engine.
the big opening in the side of the mixing chamber upset the depression, airflow,
etc discussed previously? The
change isnít as much as one might expect since the airflow is limited though
the canister and the carbs are designed to handle this air.
However, cracked or broken hoses can cause leaks that will upset system
operation that is most noticeable at idle.
As long as all the hoses are in good shape and connected, the system
seems to have no effect on performance.
last part of the breathing apparatus is the electrically operated anti run-on
valve introduced on the í73 TR6. Power
from the ignition switch in the OFF position is feed through the ON position of
the oil pressure switch to the valve, #6 in the sketch; the valve operates when
the ignition is turned off and there is still oil pressure. The operated
valve closes the external air input to the bottom of the canister via tube #6
and instead connects tube #6 to the intake manifold via tube #7. The depression in the intake manifold sucks air out of
the canister and via tube #2 to the float chamber creating a depression in the
float chamber. This depression in
the float chamber prevents fuel entering the carb via the jet thus preventing run-on
or dieseling. The valve releases a
few second after the ignition is turned off when the oil pressure drops.
Compensator, Idle Trimming Screw & Bypass valve Overhaul.
components are on the side of the carb body that faces the front of the car as
shown in the diagram below. There is an air channel from the air cleaner to the throttle plate side
of the air valve used by both the temperature compensator and the idle trimming
screw. The temperature compensator
consists of a temperature sensitive bi-metallic blade that moves a plug attached
to the end in and out of an orifice from the air channel to the main
chamber. The plug controls the flow
of air that goes around the air valve. Adding
air from this channel leans the fuel mixture.
When it is very cold the plug is in all the way in cutting off the extra
air path thus giving a richer mixture. When
the engine and surrounding air is hot, the plug is all the way out giving
maximum additional air and a leaner mixture.
Idle Trim Screw: The idle trim screw uses the same chamber as the temperature compensator to provide a minor idle mixture adjustment. The richest setting is with the screw all the way in blocking any additional air. The screw is cleaned and then reinstalled and initially set to the fully closed position.
Start Valve (Choke) Overhaul
The components discussed above (temperature compensator, idle trim screw and bypass valve) are positioned on the forward side of the carb and alter the carb behavior by changing the air flowing through side passages. This section deals with the cold start valve (choke) that is positioned on the rear side of the carb and alters the carb behavior by changing the fuel flowing through special passages. The job of the cold start valve is to provide a much richer that normal fuel air mixture whaen starting and operating a cold engine.
Late Cold Start Valves
TR250 and TR6s through í73 use a flexible choke cable.
These cold start valves have springs that return the valve to the off
position when the cable is slackened by pushing the choke knob in.
later cold start valves donít have a spring.
In fact, there is a spring-loaded ball that rubs against the cam on the
valve that tends to restrict movement. There
is a detent is the cam that the ball fits that tends to lock the choke in the
partial on position. The
choke cable is a stiff wire that is supposed to pull the valve open and push it
Iíve had a lot of trouble with the stiff cable. The problem is that when pushing the choke knob in, one or both of the outer covers are forced out of the fixture at the knob end of the cable. Both í76 TR6s I bought had cables that failed in this way. I bought a new cable for the one that was restored. It lasted less than a year. Bought another one and it only lasted about 6 months.
$50 a throw, I decided to repair the two broken cables.
First tried squeezing the crimp fitting of the fixture on the outer
cover. Lasted a few days.
Epoxy next, lasted a few weeks. Finally
designed a clamp to really squeeze the fixture on the cover.
Worked for a few days then one day when the knob was pushed in, the stiff
wire from one carb bent, folded and broke where it connects at the knob end.
Rats! Must have
squeezed the outer cover so hard that it squeezed down on the inner cable.
Moved the clamp to the other broken cable but didnít tighten it so
much. Outer cover came out. Made
it tighter, a wire broke the next day. What a pile of crap!
The area that
fails is shown in the following photo.
This cable has been repaired with epoxy.
The masking tape was to hold the two cables in position while the glue
dried. Another advantage of the early cable is that it can be locked in
position by rotating the knob 1/4 turn clockwise.
Another advantage of the early cable is that it can be locked in position by rotating the knob 1/4 turn clockwise.
I think this was about the time I bought that new set of carbs that were on the í76 TR6. These carbs have spring loaded cold start valves. I bought the earlier design cable with the flexible inner cable. That was in í84 or í85. It is still working. Moral of the story --- donít try to push a wire.
The later valves can be modified to work with a spring. The valve is disassembled, the frame placed in a vise and the cylinder housing the spring loaded ball is driven out with a punch as shown in the photo below. Springs are purchased (from Moss) and installed as showed in the right photo below. Note that the steel ball is gone. With the spring installed, either the older or the newer design cable can be used. Even a cable that has failed by the outer cable coming loose at the knob end will probably still work with the spring loaded valve. (The cable shown earlier had been repaired with epoxy and used several years on a í73 that had spring loaded cold start valves.)
the Top of Carburettor
The parts accessed via the top of the carb include the damper assembly, the air valve, air valve return spring, the diaphragm and the metering needle and associated parts attached to the bottom of the air valve. The four screws securing the washer and diaphragm ring to the top of the air valve should be removed, the diaphragm lifted off and then all the parts thoroughly cleaned and inspected.
following sketch taken from the Haynes manual shows a cross section of the
components associated with the adjustable metering needle on the í70 and later
carbs. A long Allen wrench (taken
from the mixture adjusting tool discussed later) is used to turn the needle
adjusting screw. Turing the screw
clockwise raises the needle making for a richer mixture.
The opposite direction yields a leaner mixture.
The needle adjusting screw has an O-ring seal that prevents the oil from
draining from the air valve guide rod.
Remember that note about the carb leaking oil that fell out of the carb
when the top was removed? Well,
itís that O-ring that is at fault. The
only reason to remove the needle is to replace this O-ring or a broken needle. (How does one break a needle????? Iíve seen several --- maybe someone was so upset with
the carb that they ran a blunt object like a hammer through the main chamber.)
needle is held in a carrier by a spring and is free to move side to side.
This allows the needle to adjust position as required when the air valve
moves within the design clearances. The
needle and carrier are removed together and no attempt is made to
separate the needle from the carrier. To remove the carrier, carefully turn the
adjusting screw counterclockwise. The retaining screw is left in so that the
needle carrier doesnít turn with the adjusting screw.
If the adjusting screw becomes hard to turn, remove the retaining screw.
The needle carrier should be extended far enough at this point so that it
can be stopped from turning with your fingers.
needle and carrier can be pulled free once the retaining screw is removed and
the adjusting screw is completely unscrewed from the carrier.
To get at the O-ring, a steel rod is inserted in the bottom side of the
air valve guide rod and gently tapped with a small hammer driving the adjusting
screw and the retaining clip out the top of the air guide rod.
These parts are shown in the next photo.
Left to right at the bottom are the retaining screw, the adjusting screw,
the retaining clip and an old O-ring.
The new O-ring is lightly lubricated before it is installed on the adjusting screw. The adjusting screw can then be inserted in the top of the air valve guide followed by the retaining clip and tapped into place with the steel rod and hammer. Be sure that the retaining clip is firmly against the needle adjusting screw. The needle valve and carrier can be reinserted and the adjusting screw tightened to lift the needle into position. The retaining screw is installed next; making sure the spigot end of the screw enters the slot in the side of the needle carrier.
Next, test the the needle adjusting screw. Turn it as far as it will go clockwise. This pulls the needle up into the air valve to the highest position corresponding to the richest mixture. Now turn the needle counterclockwise and the needle should move lower. Count the turns of the screw. You should find that after about three and one half turns, the needle stops dropping --- the screw is out of the carrier. The screw must be engaged in the carrier when in use to prevent the needle from dropping further. About three turns is the maximum the needle should be set off from full tight. This corresponds to the leanest setting. If, after the air valve is installed, the screw is turned too far so that the screw is out of the carrier, it may be necessary reach into the air input with your finger and push the needle carrier into the air valve to get the screw started again.
The correct starting adjustment for the needle is one turn counterclockwise from the upper most (screw tight) position.
the diaphragm is installed on the air valve. There is a slot on the top of the
air valve that mates with a tab on the diaphragm.
It is difficult to position the diaphragm correctly and then hold it in
position while installing the sealing ring, washer and four screws.
It's easier to install all the parts with the screws loose, then
position the diaphragm tab in the slot and tighten the screws.
The air valve is lubricated with light oil (SAE 20) and then carefully inserted in the carb body, making sure that the needle slides into the jet. There is a tab on the diaphragm that mates with a slot in the carb body. If every thing is positioned correctly, the two vent holes in the bottom of the air guide are on the manifold side of the carb body. The air valve return spring is then inserted over the air guide rod and the cover attached. The cast boss on the side of the upper part on the cover is positioned to the air filter side of the carb. The four screws secure the cover making sure that the identification tag is secured under one of the screws. A new gasket is inserted over the threaded part of the damper assembly knob and the damper assembly is screwed into the top.
We install and tune the carbs in Part III