Additional Tuning Information for the Following Symptoms:
1. Worst Case: Engine will idle, but when the throttle is applied the engine will not rev. If the Carburetor Damper Piston is lifted manually the engine will rev correctly.
2. Minor Case: Under hard acceleration, the engine will totally bog when up shifting. The engine will pickup again after a moment if the throttle is released.
Cause:
The Carburetor Damper Piston is not lifting quickly enough under load to offset the suction of the supercharger. The Piston is literally sucked down to the bridge and will not move.
During assembly and set up of the Carburetor, jets were installed to slow the Piston movement under wide open throttle conditions. This was required to prevent lean conditions when the vacuum operated diverter valve closes at higher revs and there is an immediate large suction through the carburetor which would cause the Piston to over react and allow too much air through the carburetor. Typically this will happen when cruising at light load on the freeway and hard acceleration is required.
To prevent this condition, two jets were installed in the vacuum ports at the bottom of the carburetor Piston. These ports, combined with the heavier 90 wt. damper oil slow the movement of the Piston.
Due to manufacturing tolerances and variations in engine vacuum characteristics, we have run into cases where the specified jet size does not allow the Piston to move quickly enough to offset the suction of the supercharger. When this happens, the Piston is sucked down to the bridge and will not lift until the airspeed past the bridge increases. This problem is aggravated if the Piston and Damper clearances are too tight, or if the Piston is binding in the Suction Chamber.
If you have either of the two symptoms mentioned, please follow the following steps to re-size the jets. Removing the carburetor Piston and Suction Chamber assembly and re-sizing the jets requires the following tools and approximately 30 minutes.
Flat Bladed Screwdriver
Numbered Drill Set with sizes from # 49 – 36
A caliper is recommended for verifying dimensions, but is not required
Wire Wool
Procedure:
1. Unscrew the black knob (Damper Assembly) on the top of the carburetor by hand.
2. If you have access to a caliper, measure the Damper Piston diameter at the base of the assembly. It should measure .325 +/- .002 in. This is to ensure that an oversized damper did not get through our checking procedure.
3. Again, if you have access to a caliper, measure the ID of the Damper tube that the piston was removed from. It should be .343 in.
Assuming that both of these items are within tolerance, proceed to the next step.
4. Remove the Suction Chamber and Piston Assembly from the Carburetor body by removing the 3 securing screws using a flat bladed screwdriver. Lift the assembly straight off the body, taking care not to bend the needle which protrudes from the Piston.
5. Remove the circlip from the top of the Piston Assembly. Push the Piston into the Suction Chamber against the spring to allow access to the ring groove. The image below is for illustration purposes only. Allow the Piston Assembly to slide out of the Suction Chamber with the Piston Spring.
6. Check the dye on the spring to ensure that it is red for the MGB application or Green for the MGA application.
7. We recommend that the needle be removed to ensure it is not damaged for the following steps. Loosen the screw on the side of the Piston and remove the needle.
8. Check that the needle is labeled BCA. The lettering should be on the collar of the needle.
9. Reassemble the Piston, Spring and Suction Chamber. Carefully slide the piston in the chamber and ensure that it has smooth movement. If there is any binding, the Piston should be buffed gently with wire wool to remove any imperfections. Make sure that the piston moves smoothly before removing it for the next step.
7. Looking at the bottom of the Piston, you will see that there are two ports which have drilled brass plugs inserted into them. These are the jet holes which will need to be enlarged. The jets were installed to reduce the speed of piston lift when the supercharger bypass valve closes upon hard acceleration. The jet size that worked best on our test cars was .073 in. This is equivalent to a # 49 drill bit.
If the piston is not lifting from idle, then we would recommend re-drilling the jets to .082 in. with a # 45 drill as a first step.
If the piston is only sticking during 1st to 2nd gear up shifts, then re-drill the jet to .0785 in. with a #47 drill as a first step, remember you can always make the holes larger but too large of a hole will result in stumbling when accelerating hard from a cruising speed.
Make sure that all drill chips are cleaned from the Piston. Reinstall the needle. Make sure the notch is facing the set screw and the needle collar is flush with the base of the slide.
Reassemble the carburetor and see if this resolves the problem. If the problem still persists, repeat the drilling process with the next size drill bit. Again we caution against skipping drill sizes as too large of a jet size will reduce performance and could introduce detonation under load from too lean of a mixture.
We have had an extreme case where the jets have had to be drilled out to .1065 in. # 36 drill. In the event of the piston still not lifting from idle with this large of a jet, please contact our technical department.
I DiLL MINE OUT TO 3MILL