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MG MGB Technical - black carbon on plugs

First, the motor is in good shape. Went to twin HIF-4's and now have carbon on plugs. The carbs have fixed needles and some modification. I purchased them from Eric Jones as a complete unit rebilt. The car, when I get it tuned pretty close runs good! The plugs get carbon on them, not 0il, not sooty, and not black and wet! Anyone got some sound idea's on the problem? The coil and black box are original and seem to be doing OK. Thanks if you can help.
Dale
Dale & Barb Mast

Sounds as though the car is running too rich. The carbs may need to be reset. Take one of the plugs out and smell the electrode. It may smell of unburned fuel

cheers

Gary
gary n. hansen

Dale. I am not sure who Eric Jones is, but assume that he is someone whom you feel is competent to rebuild SU carbs. The HIF carbs, as the came from the factory, had biased needles, rather than the fixed needles. The biased needles are spring loaded to push against the jet and, over a period of time, both the needles and jets will become worn because of this. The HIF series also had a "poppet valve" in the throttle plate/throttle butterfly. These can cause problems and most people either solder them shut or replace the plates with the earlier style. You mention that the carbs are "modified". It would be useful to confirm what the exact modifications are in case you ever had to get parts for them. I suspect that two of them are the conversion to fixed needles and solid throttle plates--things most of us would consider to be good modifications.

When checking on exactly what modifications were made to the carbs, also check to see exactly what needles are installed. If you are getting plug fouling, it can be from three sources.

First, the carbs are running too rich. This is an adjusment and needle selection thing.

Second is spark plugs. Most of us use the NGK spark plug, the BP6ES or resistor version of the same (BP6RS ?). If your plug is a little cold running it might tend to foul up a little.

Third is the testing procedure. The best way to "read the spark plug" is to warm the engine up, install a new set of plugs, take the car out on a limited access highway where you can run fast, then run the car for at least 20 miles. Cut the power to the engine, then coast to the side of the road, let the engine cool a little and pull the spark plugs. This will give you the most accurate reading of the mixture condition at road speed and is the normal test for correct mixture and plug.
(Note, if you have a later model car with the locking steering wheel, take the car out of gear and coast to the side of the road and come to a complete stop before shutting down the engine. Turning the key completely to the fully off position may lock up the steering wheel and prevent you from steering the car.)

If you can provide some additional information, such as year of car, engine modifications from standard, carb needles, and spark plugs being used, we might be able to dial in our recommendations somewhat more closely. But for now, the use of a set of BP6ES plugs and doing a high speed plug check would be a good place to start gathering information. Les
Les Bengtson

Thanks for all the replys. If there are more ideas out there,please send them to me.I am using the NGKBP6ES, just put them in a short while ago. Do not know the needles, but they are fixed and the butterflies are solid. Thanks.
Dale
Dale & Barb Mast

Dale. It is relatively easy to check what needles you have and also check the jet adjustment while you are at it.

Simply remove air filters. Then, lay out a piece of clean newspaper or several layers of paper towels on a clean, flat surface. If there is wind, weight the paper down so it cannot fly away, taking light parts with it. I use a magic marker to label the paper F for front and R for rear when demonstrating this process. Pull the hex headed "nut" from the front carb, using a paper towel under it as you remove it to catch the oil drips. Remove the dampener rod, attached to the plastic hex head, over to the paper and set it down. Remove the three screws holding the suction chamber to the body of the carb. Put the screws next to the dampener rod on the area of paper marked "F". You will keep all of the parts to the front carb on that side of the paper and all of the parts to the rear carb on the "R" side of the paper. That way, you cannot mix up parts. Pistons and suction chambers may not be interchangeable. Next, remove the suction chamber by lifting straight up. Pull the spring off the piston and put the suction and spring on the paper. Lift the piston straight up and out of the carb body. Invert it over a small jar or a wad of paper towel to catch the oil which will come out. The base of the needle should be flush with the base of the piston. The needle is held in place by a single set screw coming in from the side of the piston. Remove the screw, carefully, then remove the needle. Lay the piston and set screw on your paper, then read the marking on the needle to see what size it is. Make a record of the size, then lay the needle on the paper.

Do the same thing to the rear carb. Needles should be the same for each. Sometimes I have found different needles in the two carbs, but that is rare.

With both of the carbs stripped, look into the carb from the top and notice the jets sitting somewhat below the level of the flat piece in the center. (This flat section is called the bridge.) The two jets should be about the same level below the bridge. I have seen significantly different heights which will affect mixture and running. If you have a set of dial calipers, you can measure the depth of the jets below the bridge. Mine seem to be about .053" to .055" below the bridge when the carbs are adjusted properly. Peter Burgess recommended an initial setting of .060" below the bridge in his book.

Then, have an assistant pull the choke out and watch the jets lower. This is your enrichment mechanism for cold starting. Have the assistant release the choke and make sure the jets rise up properly. Several of us have, over the years, had problems with the jets sticking in the downwards position. This will give you fouled plugs.

Reassemble the carbs, adding oil to the piston rods before installing the dampeners. Then, post the depth of the jets (if you could measure them) and what needles you are running and we may be able to help more. Les
Les Bengtson

I'll try to get back to you Les and let you know. I have had major surgery back in January and my recovery is very slow and I cannot work on the car for any lenght of time----------just a little here and there. I'll let you know------------THANKS.
Dale
Dale & Barb Mast

Dale. Had some less major surgery about four weeks ago. Know exactly how you feel. Good luck with your recovery. Les
Les Bengtson

Just read this thread for the first time, sounds like my problem with my 80 roadster.

Twin HIF4's with prescribed AA needles for 270 Piper camshaft, but car is ticking over terribly, lumpy and hunting at 1000rpm, whereas at say 2000, smoother yet still up and down.

All plugs are thick black, not wet just deep black.

Distributor is 45DME I think, with Lucas electronics.

Need to narrow possible cause down to as few factors as possible, before I go nuts.

Any ideas?

Jack

Well guys, I am still trying to work on the problem. As I said, I am pretty slow and can't work on the car to long at a time. Will add to this later as I find my way. You can keep sending any information here or e-mail as I sure appreciate the help.
Dale & Barb Mast

Since you have HIF-4's you have a separate enrichment circuit so you will NOT see the jets move.

I would add to Les' description the following:

Clean the exterior of the carbs very well before disassembly!

I would also do them one at a time and reassemble as I go (I stink at keeping parts separate) and wouldn't bother to remove damper before the piston cover. Rags or paper towels are still a good idea to catch drips.

I'm not entirely convinced that altering the needle to a fixed type is absolutely necessary or successful on all HIF's as there is little means to centralize them designed in. (Uhg, bad syntax)

With the HIF you need to be absolutely certain the enrichment circuit is actually going to the "off" position as that will cause problems too.

Mike!
mike!

Mike! Mea Culpa. You are perfectly correct. I pulled out an old set of HIFs and pulled the pistons, then rotated the choke lever. No jet movement. Thanks for keeping me straight and adding to my knowledge. Les
Les Bengtson

This thread was discussed between 10/07/2004 and 13/07/2004

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