Update: Follow the link to a great
carb-tuning checklist submitted by Wade Lester of Daytona Beach, FL.
Thanks, Wade! Some troubleshooting hints are
at the very end.
Some troubleshooting hints are at the very end.
(Tuning) the Carbs:
carb adjustments are:
Before starting, the following should be done:
Next, adjust the idle screws for an engine speed of about 900 RPM. If it is impossible to reduce the engine idle speed to this level, then something is wrong that must be corrected before proceeding. Clearly, fuel is getting to the engine when the throttle plates should be closed preventing fuel from reaching the engine. First, examine the linkage to make sure nothing is holding the throttle partially open. Another possibility is that one or both the bypass valves are misadjusted or defective. Remove the bypass valves one at a time (slotted head screws) and cover the two holes in the body with duct/masking tape. If this fixes the problem, then examine, repair and adjust the valve for minimum sensitivity (see Part II). If covering the passages to the bypass valves doesn't fix the problem, then there is likely something preventing the throttle plates from closing. Pull the carbs and examine the shaft and throttle plate to make sure that the springs are holding the plates closed. Also, make sure the plate is aligned on the shaft properly such that it completely closes the carb opening. If the plate is misaligned, loosen the screws and work the throttle back and forth until the plate comes into alignment and then retighten the screws.
On '73 and later carbs it is a good idea to temporarily disconnect the the hoses connected to the float chamber vent ports while the engine is running. The engine speed should not change. An increase in engine speed when the hoses are disconnected indicates that the anti run-on valve is either stuck operated or the there is incorrect wiring or a fault in the electrical circuit causing the valve to be operated. This should be fixed or the hoses left disconnected.
Manifold Vacuum: If
one has a vacuum gauge, it's a good idea to measure the vacuum before
proceeding. I connect the gauge to the port to the anti run-on
valve on the later cars and the port to the brake servo on the earlier
models. The hose to the brake servo is harder to get off and
requires a reducer to match the gauge hose which is why I use the anti
run-on valve port, if provided. The vacuum should read between 17 and
20 inches of mercury (Hg) and be pretty steady. If it is slightly
lower, the timing may be too retarded. If it's much lower, like 10 or
less something is likely wrong with the valves. If the reading
is about correct but not steady, then one can go ahead and tune the carbs to
see if that smoothes the operation. If the reading is much below 17,
the problem should be fixed before proceeding because tuning the carbs will
have little effect on the engine performance.
Crossover: While fooling with the synchronization one observes that the engine can be made to idle properly with one carb completely closed. How is that? First, recall that little cutout section of the bypass valve gasket lets a little fuel through even if the throttle plate is completely closed. However, the major reason this happens is the bridge between the front and back half of the manifold that contains the manifold vacuum ports. The cross sectional area of the bridge is small compared to the carb throat so there is little effect if the carbs are running full open. However, only a few percent of the carb capacity is used at idle and the bridge has adequate capacity for a carb on one side of the bridge to supply the three cylinders on the other side of the bridge.
Bypass Valve Adjustment: Disconnect and plug the vacuum connection to the distributor vacuum retard unit. The idle speed should increase to about 1300 RPM. Turn the bypass valve screw clockwise until the engine speed increases to 2000 - 2500 RPM indicating that the valve is floating. Then turn the bypass screw back (counterclockwise) until the speed returns to 1300 RPM. Use the throttle to rev the engine to about 2500 RPM and then release the throttle and verify that the engine returns to 1300 RPM. If it doesn't, the valve is floating requiring that the adjustment screw be be turned further counterclockwise until the speed drops back to about 1300 RPM. Once the valve is adjusted such that the speed drops back to 1300 after the engine is revved, turn the adjustment an additional half turn counterclockwise to seat the valve. Repeat this adjustment on the second bypass valve.
The idle trimming screw is set as desired and then the mixture is checked and adjusted for each carb. I make identical changes to both carbs and then test both. Because of the crossover, it is impossible to isolate the effects on the individual carbs at low rpm. The tests are made with the damper assemblies removed. The mixture-adjusting tool must be removed before testing the mixture. If the carbs test too rich, the screws should be adjusted ½ turn counter clockwise; if it tests too lean the screws should be adjusted ½ turn clockwise. After the adjustments the carbs are tested again and adjusted as required. Remember that the range of the screw adjustment is 3 turns counterclockwise beyond which the screw is may come out of the needle carrier. In practice, one should not go beyond 3 turns counterclockwise from the tight position. As mentioned earlier the carb adjustments are kept identical. These are tweaked later after the road test.
open the throttle enough for the engine to run at about 1500 RPM and use the
Unisyn again verify that the carbs are balanced. If not, the previous step should be repeated until the carbs
are balanced at both idle and at about 1500 RPM.
(See why this procedure is called tuning?)
Fill the air valve guide rod to within ¼ inch of the top with oil. I prefer SAE 20 3-IN-ONE electric motor oil sold in 8 oz cans. Motor oil is also satisfactory. Install the damper assemblies in the top cover and then raise the air valve with a finger to the maximum up position to force the thimble on the damper assembly (later carbs) into position in the end of the air valve guide rod. Next, clean up the oil expelled from the vent in the top of the carb because you over filled the valve guide.
final step is to install new filters in the air filter housing and mount the
housing to the carb after placing gaskets
between the cleaner housing and the carb. I usually affix the gaskets
to the air cleaner housing using a small amount of gasket cement to prevent them
from falling out when the air filter housing is mounted.
The test drive
The best check of carb tuning is the test drive. The test drive should be such that the engine is at normal temperature for at least the last 10 minutes. During the test drive observe the following:
After the test run, pull all the spark plugs. Note that the front three are associated with the front carb and the back three with the rear carb. If the end of one of the plugs in the front or rear group looks significantly different than the end the other two, something else is wrong (consider a compression test or an examination the ignition system). If the end of all the plugs in a group of three have a brownish color, then the associated carb is probably adjusted properly. If the plugs are black, the associated carb is probably set too rich. If the plugs have a whitish color, the associated carb may be set too lean. Evaluate all these data and adjust the mixture as appropriate, maybe a half turn at a time. Take another test run of 10 minutes or so. Check for misfires as above. It is necessary to check only one plug in each group after this test run (assuming all the plugs in each group of three looked the same after the first test). Continue to repeat adjustments & test runs as required until good performance is achieved and the plugs look right.
Anti run-on valve test
If the valve operates with a click, the engine speed should increase slightly (as the mixture leans) and then stop. If the engine doesn't stop, then check the hoses from the carb vents to the valve and from the valve to the intake manifold. If the hoses are in order, disconnect the hoses from the carb vent and put a finger over the end of each hose and see if there is very slight suction when the valve is open. If there is no suction, then the valve is not functioning properly and probably needs to be replaced.
If there is suction at the end of the hoses that connect to the carbs and the engine continues to run after the valve is operated (with hoses connected to the carbs), then the vent valves may be defective or misadjusted. See the discussion on adjusting these valves near the end of Part II.
After the valve is verified to operate properly with clip leads, normal operation should be verified by listening for the valve operation when the ignition of the running engine is turned off. There should be a click when the ignition is turned off as the valve operates and a second click a few seconds later when the oil pressure drops. If the valve clicks are not heard, the electrical circuit probably has a fault. The white with pink stripe wire at the valve connects to the oil pressure switch which should ground the wire when the engine has oil pressure. The brown with red stripe wire should have 12 volts on it when the ignition is off.
The 12 volt side can be checked by removing the white/pink stripe wire and leaving the brown/red strip wire connected. Then, with the ignition off, use a clip lead to ground the terminal where the white/pink strip wire normally goes. The valve should operate when the terminal is grounded. If it doesn't, then the fault is in the wiring between the valve and the ignition switch.
It's natural to try to relate the graph to the needle adjustment. Dick Taylor passed on a couple calculations of the effect of the needle adjustments that I found really interesting. Some time later I ran across the specs of the B1AF needle and was able to enter the data into spread sheet and calculate the open area between the jet and the need at various needle depths. (The jet diameter is 0.10 inches.) The amount of fuel delivered is proportional to this open area. I assume (but don't know for sure) that the proportion is direct; if the area is doubled, the fuel delivered is doubled, etc. Recall that the maximum needle adjustment is a little over 3 turns or a little over 0.01 inch --- roughly one step in the chart below.
The next thought was --- where does it run at idle? The needle in my '76 is around the 3/8 inch height (+ or -) at idle. My guess is that carb runs at about 3/4 inch needle height when cruising at 70 mph on the level.
My '76 TR6 seems to be running great. It is a bit non standard. The head is from a '73 and the air pump, exhaust air injection and EGR valve have been removed. The compression measures between 150 and 160 psi on all cylinders. The carbs are original with original throttle shafts, bushes, needles and jets. My guess is that they have a total service of about 75K miles. These carbs have been completely disassembled in the last few months and powder coated. As far as I can tell, they are in like new condition. The ignition is standard points with 6 volt coil. The timing is set at TDC with retard unit connected. There is a slight ping at around 2000 RPM when under heavy load using 89 octane fuel (at ~ 1,000 ft above sea level).
I did a set of short test runs in the '76 at various mixture settings. All tests were made with the dampers removed and with the exception of the first run, with the idle screws unchanged. Each test run is seven miles over a country road. The middle part is straight and flat where I get up to ~ 70 mph and then turn around and head back. The first part is winding and down hill and of course uphill on the return where the engine can be loaded at low RPM. The mixture setting is based on the number of turns from the full up setting. For example, 0 is all the way up (screw tight), -1 is down one turn, etc. Small spacers were fabricated and placed under the adjustment screw to allow the needle to be raised above the normal maximum up (0) setting. One of the spacers is shown in the next photo. The spacer OD is .370 inches and the ID 0.313 inches. This allows the spacer to rest on the shoulder that the bottom of the adjustment screw normally rests and the ID is large enough for the needle carrier to pass. The sleeve is 0.065 inch high but when rested on the rounded top edge of the shoulder raises the maximum height about 0.070 inch or about 2 turns above the 0 position.
The test results are shown in the next chart The plug color refers to the major color of the darker half of the plug tip. About half of each plug was pretty white and the other half changed color with the mixture. Number 2 and 5 plug plugs were pulled and checked after each test run. In all cases the plugs were nearly identical.
The air valve lift test refers to what happens when the air valve is slowly raised about 1/4 inch and then raised to full up. For the richer settings, the the engine speed increases when it was raised about 1/4 inch and continued to run (rather poorly) as it was raised to full up. The likely explanation is that the other carb is supplying enough fuel to keep it running.
I ran another test of operating the anti run-on valve when it was idling. In all cases except the -2 and -3 setting, the speed increased before it stalled. It stalled with no speed increase in the leanest two tests. For the cases where the speed increases, the mixture at idle is richer than peak power on the curve. As the depression in the float chamber increases after the valve operates, the mixture leans. The speedup indicates that more power is generated at a leaner mixture.
There was a significant increase in the exhaust popping as the mixture was leaned. The exhaust is a stainless dual sport system with several leaks. I plan to remove the exhaust manifold for powder coating in the near future. An attempt will be made to fix the leaks in the pipes when the system is put back together. This will be updated after the pipes are fixed.
Apparently the carb design is such that it runs richer at idle than when the throttle is opened further ---- possibly to insure a smooth idle.
I did a ~ 125 mile test drive on highways and country roads at the +1 setting and measured a 23.2 mpg performance.
After all these tests I decided to use the -0.5 turn setting -- until I tweak it the next time. I also adjusted the timing to 4 degrees ATDC to eliminate the occasional pining with the 89 octane fuel. A subsequent test drive showed 26.7 mpg economy.
Note that it was impossible to adjust the carbs rich enough to turn the plugs black without using the little spacers. That would probably not be the case if the needle and jet were worn making the open area between the needle and jet larger. It is also worth noting that it appears the standard carbs can supply sufficient fuel to reach the maximum point on the power curve, and if one uses the little sleeves, sufficient fuel can be supplied to exceed the maximum power point. I guess that means it is a waste of money to replace the carbs on a standard engine with carbs that supply more fuel --- you loose power and economy really goes to pot.
Dick Taylor suggested several alternatives to the little spacer to achieve a richer mixture. One is to push the jet down about a tenth of an inch. Another is to carefully file a flat on one side of the needle. Dick suggests reducing the diameter measured to the flat by .002 inches from the thickest part to about the middle of the needle.
Note: after this note was written I built an air/fuel monitor and used it to check out carb mixture. This work is described in the accompanying notes. The mixture measurements are in the note Using Air/Fuel Monitor. While that note provides considerable additional information, the conclusion is that the best carb setting is between 1/2 and 1 turn below the maximum up position, the same as found above.
The main thing leaned from these tests is that no matter how badly the carb mixture settings are screwed up, the car runs pretty well. A corollary is that no matter how the carbs are adjusted, a screwed up engine probably can't be made to run well.
The following are some trouble shooting hints:
Possible causes of poor idle quality:
Possible causes of poor engine braking:
Possible cause of run-on (dieseling):
Possible cause of exhaust popping:
Possible cause of poor performance at high temperature: