Modifying a US TR7 to UK Ignition and Carburetor

Modifying a US TR7 to process of failure. The failures in the carburation

system are now actually worsening the emissions.

UK Ignition and Carburetor What we need to do is simplify the carburation and

Specifications rely as little as possible on electrical systems, and

By Tony Rhodes, Delaware Valley Triumphs use pure mechanical systems instead.

You can see the advantages of the conversions

listed above. The conversions give you simplicity,

The Philosophy of the Conversion reliability and easy maintenance. But what are the

I recently completed the conversion of a 1980 disadvantages? Well, you can hardly can go wrong

TR7 to UK-type carburetors and ignition. In the US with simplicity, reliability and easy maintenance. I

this car originally came equipped with Stromberg think that the only issue is the loss of electronic

carburetors and an AC-Delco electronic ignition ignition. However, the points-type distributor

using a vacuum retard to reduce idle emissions. The opens up a wide variety of aftermarket electronic

UK vehicles were equipped with SU carburetors ignition systems that are probably better (and more

and a points-type vacuum advance Delco or easily replaced in case of failure) than the original

Lucas distributor. The US ignition system worked electronic units. The choices range from the

well, but parts are no longer available. The dealers simple Pertronics points replacement to Capacitive

exchanged them to the Lucas Opus system when Discharge and Multiple Spark Discharge units. Any

they failed. The UK points-type system requires of these is simple to add and maintenance free

maintenance, but the reliability is very good. If a (do not forget to oil the distributor). But, as stated

failure were to occur it would be possible to repair previously, electronic ignitions are “fail-hard”

on the road. If the electronic system goes bad, systems. When they stop working, no amount of

then you are dead until (and IF) you get new parts fiddling at the road side will get you limping home.

shipped to you. Electronic systems were fitted in You can not fix a transistor with a screwdriver,

the US not because they are functionally better, but wrench, and matchbook cover.

because they require no maintenance (until they fail The down-side of the SU carburetor exchange

entirely). Needing no maintenance, it is less likely is the work required to modify heater hose routings

that an engine will be running poorly and creating and fabrication of a new accelerator cable bracket.

high emissions, despite the “fail-hard” outcome I suppose that a manual choke is a pain in the

with an electronics failure. By comparison, the neck compared to an automatic choke, but it is

points system is a “fail-soft” system. The engine simple and reliable. I do not believe there is any

may run poorly, but it usually gets you home. Basic liability associated with the carburetors themselves

routine maintenance will keep a point-type ignition compared to the Strombergs. If you planned

system running beautifully and give you the benefit on making any improvements to the engine for

of a fail-soft, and an easily and inexpensively increased performance, then the stock carburetor

repairable system. needles will probably not give a good mixture.

The same emissions philosophy applied Different needles will be necessary and the SU

to the carburation. The emissions regulations carburetors have many more needle options than

required a carburetor which was idiot-proof. It the Strombergs.

was designed to have no easy way to change the

mixture so an idiot would not put it out of tune. It Ignition System Technical Facts

had to have an automatic choke because people The US ignition system uses mechanical

are not smart enough to turn off the choke when advance only. The maximum advance is 18

it was no longer needed. It needed all sorts of degrees. There is also a vacuum retard unit that is

temperature compensation to prevent that last active only at idle to reduce idle emissions slightly.

fractional percentage of emissions. The price to be There is no vacuum action off idle. The UK ignition

paid for this – complexity, is difficult maintenance, uses mechanical advance has a maximum advance

tendency to failure, and literally expense. Now of 18 degrees, and the vacuum advance also has a

that these vehicles, in 2003, are between 23 and maximum of 18 degrees. [These numbers are out

28 years old, the complex systems are all in the of the Repair Operations Manual. My UK distributor

Preparations. Read the

manual on removing

the distributor. It is

important to look at

the position of the rotor

prior to removal so that

the new one can be

replaced so that the

rotor is at the same

location. Do not rotate

the engine before

the new distributor is

attached. Removing

the distributor clamp

bolts is not easy

because it is nearly

impossible to get to

them, particularly the

front one. It would

be very useful to

has markings of “6” on the mechanical advance do the job while the

unit and a “7” on the vacuum unit. This suggests carburetors are removed. You also will have some

12 degrees and 14 degrees maximum advance frustration starting the bolts in the holes after

respectively.] The advance diagram is as follows: inserting the new distributor. Take your time and

The UK-Total curve gives an example of the total have some good beer on hand.

advance curve. The actual curve you would have The white/slate wire from the distributor goes

would vary greatly with throttle position, rapidity of to the Negative terminal of the coil. There is also

throttle opening, and engine speed. Nevertheless, a white/slate wire on all models which goes to

you can see that the vacuum advance can allow the tachometer. This also goes to the Negative

considerably more total advance than the US terminal. Depending on your model year, there

distributor. Within limits, advance of the ignition may be one or two wires that provide power to

is useful to improve combustion efficiency, and the coil. Most have two modes of operation. When

therefore fuel efficiency. Timing which is more starting there is power directly from the starter

retarded than ideal reduces efficiency and also relay to the coil, providing about 12 volts. When

reduces the percentage of certain emissions. the starter is not engaged, then the power goes

This reduction is the rationale for the lack of the through a separate circuit which includes a resistor

vacuum advance. The increased emissions due to wire (or sometimes a ballast resistor) so that the

increased fuel consumption is offset by a greater coil only gets 6 volts. The coil is designed to run

reduction of some components in the exhaust. with 6 volts, but gets a bump in power for a short

In reality, the change in emissions is miniscule time to improve cold-start characteristics. These

compared to that smoke belching wreck driving just wires are white/yellow or just white. You can tell

in front of you, and you can gain possibly 10% in the difference between the 12-volt-only and the 6

fuel efficiency with the improved ignition advance. volt systems by measuring the voltage between

the Positive and Negative terminals of the coil (or

Replacing the Distributor electronic coil unit) while the engine is running. If

I have found no US source for UK specification you read something a little less than 6 volts then

distributors. It is possible, but not really worthwhile, it is a 6 volt system. If you read comfortably more

to retrofit the US Delco distributor to the UK than 6 volts, then it is a 12 volt system. If you have

specification. You can get reconditioned UK a 12 volt system, you might want to upgrade to a

distributors from Rimmer Brothers and S&S “high output” coil. I believe there are performance

blocks positioning of the rear carburetor. The fuel

coils for 6 volt systems as well, and I think they are

compatible with the 12 starting voltage, but I am bowl strikes the steel pipes that are mounted to

not sure. the engine block. The pipes must be moved, and

My 1980 Spider had a Delco distributor with since it may not be a good idea to drill a new

a Delco electronic coil unit mounted on the left mounting hole in the crankcase, the steel pipes

inner fender. I clipped the leads from the wiring may be eliminated and rubber heater hoses must

harness just behind the plug for the electronic be fabricated

unit. One of the wires was white/slate colored and Triumph used a bewildering variety of hose

the other was white. I installed insulated female sizes to get from the intake manifold to the heater

spade connectors. I removed the electronic coil hoses. It has been a year since I made the

carburetor conversion, so I do not recall all the

unit. The mounting for the coil is narrower than the

Delco unit. One end of the coil is held by one of sizes of the hoses and pipes, so you will certainly

the bracket mounting bolts, but the other requires have to assess your specific needs based on these

a separate (new) bolt. Fortunately the hole for theinstructions. Coming off the manifold are two pipes.

The more forward one is a 1 inch pipe angled

other coil mounting bolt is present, but the bracket

does not stand off the fender sufficiently to give backward and outward at a 45 degree angle. The

clearance for the bolt. I fabricated two 1/8 inch other more rearweard is 3/4 inch coming out more-

aluminum spacers to lift the mounting bracket. I or-less straight to the right . These must taper to 5/

8 to hook to the heater hoses. I found a NAPA hose

then added the extra bolt to hold the coil and it fit

perfectly. The wires were connected to the coil andthat was perfect for the 1 inch pipe. The NAPA part

I had to fabricate an extra length of white wire totapered to 3/4 inch AND had a 45 degree bend in

reach the wire from the distributor. the taper section. The part also had a second 45

degree bend in the 3/4 inch section, but I cut the

Carburetor Replacement pipe off just before the second bend. This allowed

Replacement SU carburetors are available me to position the 45 degree bend to take the pipe

from TSI Automotive here in the States, and from somewhat downward. I used a straight 3/4 inch

Rimmer and S&S in

England. In addition,

you can get a set of

TR7 carburetors from a

variety of private sellers

and junk yards. You

just need to be careful

of the condition of the

as-found carburetors.

The reconditioned units

are more likely to be

functioning properly,

and may not need

immediate rebuilding!

The exchange of the

carburetors is more

complicated than the

distributor because

the heater hoses need

to be moved and the

accelerator linkage is

not compatible. The

stock configuration of

heater hoses and pipes Figure 2: The arrangement of the new heater hoses.

hose on the other outlet from the manifold. (See the the new distributor. It is now time to attach the

photo of the hoses, and note how accessible the carburetors. I re-used the metal plate which spans

between the carburetors. I think its purpose is

distributor is!) To the front outlet pipe I attached the

45 degree hose mentioned above, then a straight to reduce the independent motion between the

adapter and allowed the hose to gently curve back carburetors. I turned the plate so the angled upper

upward, then attached to the original heater hose. end is tipped toward the carburetors, but is might

To the rear outlet I used sufficient length of hose tobe nice to flatten out that angle if possible.

drop comfortably away from the carburetor, then In Figure 3 you can see the tipped edge of

inserted a 90 elbow, and then ran the hose back, the plate edge on. The choke cable is coming in

and it then adapts to the original heater hose. from the top and the accelerator is coming in from

In Figure 2 you can also see the bracket bolted below. There is the UK fuel rail attaching to the

to the center of the intake manifold. This is used tofuel bowls. It was trial fitted with air filters in place

hold the accelerator cable which then directly pulls so the rail could be attached to the air cleaners

in a manner similar to the TR4A (I also used the

on the throttle linkage. Also, there is a black rubber

cap on a port on the intake manifold directly in the oval TR4A air filters.) I later added a throttle return

middle. Later modification to the throttle cable spring because the resistance was too little.

attachment eliminated this pictured bracket. I was The final arrangement is shown in Figures 4

able to use the UK throttle linkage which bolts to and 5. I fabricated a custom PCV hose system

the manifold. The mounting point is in a different because the original UK pipes did not fit well in

on my manifold (but I found that earlier manifolds the space available (the UK intake manifold is

are the same as the UK version). I had to fabricate somewhat different from the US). It is important

an adapter to move the linkage over to the original to keep the length of the PCV pipes the same

location, and then rotate it around slightly to clearbetween the front and rear carburetors so the air

the heater hoses. flow will be identical. For simplicity, I capped off the

At this point the heater hoses are moved down PCV ports on the carburetors for the initial tuning

out of the way, and hopefully you have installed and fault checking.

After installation of

the carburetors, fuel

rail and air cleaners,

screw the carburetor

jets up fully and

then down two full

turns. This is the

recommended starting

position for initial tuning

of SU carburetors. The

engine ought to start

now. As you turn the

engine over (have a

helper do this) use a

timing light to check

the ignition timing and

rotate the distributor

left or right to get 10

degrees of advance

at idle speed (or less).

When the fuel bowls

fill, the engine ought

to start if the ignition

Figure 3. The Carburettors are now fitted is adjusted anywhere

and synchronize the

idle air flow between

the carburetors. By

this time the engine is

probably fully warmed

up. Install the air filters.

(they need to be in

place because they

enrich the mixture

slightly.) Pull upward

on the lifting pin of

one carburetor. Lift it

upward until it contacts

the piston, then lift it

slightly further (1/16

to 1/8 inch). If the idle

does not drop then

it is too rich. Screw

the jet adjusting nut

upward a flat or two,

and re-try. After about

30 seconds you need

Figures 4 and 5. Showing the final installation. to clear the engine by

revving it up to 3000

for five seconds or so,

then let it drop back

to idle. Re-test the

mixture and re-adjust.

Once it seems close,

then do the same for

the other carburetor.

You need to go back

and forth closing-in

on the perfect mixture

until it is just right on

both sides. By this

time the carburetors

are probably no

longer synchronized.

Re-synchronize the

carburetors and

recheck the mixture.

When the mixture,

idle speed and

synchronization are

nearly correctly. correct then you are

Once the engine is running on its own check DONE! Drive the car a few weeks, then install the

the timing again and set it to 10 degrees advance. PCV hoses, and re-tune the carburetors. It ought

Shut off the engine and tighten the distributor not to be necessary to re-synchronize them. Drive

clamp bolts the last turn or so. Restart the engine the car some more and then take out the sparkplug

from #1 and #4 cylinders. They should have either! I was not so fortunate.

medium tan deposits after a good run (hopefully In my case, even with the heater hoses moved

not in stop-and-go traffic). Dark deposits suggest so the carburetors will have sufficient clearance,

a rich mixture, and white deposits suggest a lean the throttle linkage does not have enough

condition. clearance. The linkage needs to be moved to the

That is all there is to the conversion. The most left to get it to the UK-spec location, then it needs

difficult part was the fabrication of the new hoses. I to be rotated upward to clear the hoses. About 15

found that the new carburetors easily gave 28 mpg degrees of rotation is sufficient.

on the highway (before I added the UK distributor). I fabricated an adapter out of two pieces of

The UK distributor provides a subtle change. 1.75” wide angle iron (from Home Depot). I held

The part-throttle responsiveness seems to be the linkage in the proper location and guestimated

improved. The full throttle performance does not the measurement of the offset from the US

seem to be affected by the distributor. If you have position. About half of that offset would come from

any questions feel free to contact me by email at each of the angle pieces. This gave me the offset

spamian AT comcast DOT net. for the mounting holes I had to drill holes in the

first piece of angle iron. I then bolted that piece

to the intake manifold. I then vise-grip clamped

ADDENDUM the other piece of angle iron to the first and then

After driving the car with this conversion I find clamped the UK linkage in precisely the correct

that the mixture may be lean at full throttle, and I offset to line up to the bell crank on the carburetor

need to do some work on this. Most annoyingly, throttle connecting bar. I then drilled mounting

the throttle “tip-in” is very aggressive. By this I holes in the second angle iron piece.

mean that when you step on the accelerator, it is

hard to control the engine speed. It wants to rev-

up very quickly. This means that it is very hard to

feather the throttle when pulling away from a stop.

The engine wants to stall or rev at 3000.









As a result of this, I obtained a nice newly

cleaned and cadmium plated UK-spec throttle I felt it was important to keep the original

linkage. Of course, my 1980 US intake manifold geometry of the linkage and connecting bar, so

does not have the same mounting location as I took a small piece of aluminum angle iron and

the UK manifold, therefore the linkage can not be bolted it to my UK intake manifold test mule. I

simply bolted in place. Earlier US intake manifolds then took a piece of aluminum bar and made a

are the same as all the UK manifolds, therefore “U” cut in one end. I then mounted my carbs on

the mounting location is identical. For these older the test mule manifold and placed the “U” against

manifolds, the linkage can probably be bolted in the connecting bar, and then rotated it until I

place without any modifications AND probably the could clamp it to the angle iron mounted to the

heater pipes and hoses will not need to be moved manifold. I then pop-riveted the angle iron and bar

together. I now had a jig which would place the

adapter in the exactly correct location (distance

and relative angle) no matter what position it was

rotated into. I bolted the jig to the second piece of

angle iron, then put it against the first piece which

was mounted to my US manifold. I then rotated

the second piece until it would clear the hoses. It

was clamped in place. The jig was removed and

the linkage was bolted in its place, and the entire

thing was reinstalled on the manifold to check that

everything had good clearance. I found that it was

impossible to have clearance for the hoses and

also have clearance for the original air filter box.

(I will have to modify the lower-inner edge of the

box to clear the linkage.) I then pop-riveted the

two angle iron pieces together, then got a friend

to weld them. I am currently in the process of

installing the new linkage as I write this. Here are

pictures of the alignment jig, the finished adapter,

and the UK-spec throttle linkage.









Note: the throttle linkage is already slightly

modified. The bottom edge originally projected

downward about 1/2”. I ground it down until the

edge was relatively straight. The top edge has

a projection to support a choke stop, but in the

rotated position, it will not work and serves no

useful purpose. It is not in the way, so I have

not yet removed it. I do not want to wreck the

cadmium plating, so I probably will leave it as-is

unless it becomes a problem.