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MG MGB Technical - Too Lean

Having a problem tuning my 73B 18V SU HIF 4 Carbs. Engine running too lean white smoke in exhaust, engine slows when I lift the carb piston. Have adjusted to max rich. Dont know what to do next. I have located a vacumn leak and fixed it. Sort of runs with choke but breaks up on excelleration. Changed dash pot oil to 30 from MMO.

Ken Knize
Binghamton NY
Ken Knize

White smoke should not be caused by lean mixture. If it's white and not black it sounds more like oil. Have you removed the plugs? What color/condition are they in? If the engine is too lean, the plugs will be very clean with a white hue to them. Something sounds really wrong here. Is your car consuming any oil?
I'd have a look for more vacuum leaks and make sure all emmisions stuff is connected tight. I am assuming you still have the carbon canister ect.. Rule of thumb is to set the jets level with the base of carb, you can reach them with the air cleaner removed. Then screw clockwise two turns. Make sure you have fuel to both carbs too.
J Arthurs

Ken: Sounds like a classic case of air leak at the throttle shafts. When very worn it is possible to max out the mixture as you describe (BTDT). As you adjust for the air leaks at idle you enrichen the mix at all speeds, which is not good. Gas mileage drops and too much fuel can thin the oil and in extreme cases wash lubrication from the cylinders.
Andrew Blackley

Check for air leaks around the throttle shafts. Spray some carb cleaner aarund the ends ofthe shafts while i s running. If the idle increases the shaft bushings are bad.
william fox

Thanks Guys, I'll try your suggestions tomorrow and let you know how I make out.

Ken Knize
Binghamton NY
Ken Knize

Decided I needed to do some checking of things befor I proceed. Compression test: 140 psi all but #3 135 psi. This is in a low comp. engine. Plugs dark grey to black, installed new ones. Found the timing marks @ 11:00 not 5:00 where I was looking for them. Still have to remove A/C unit that was hiding the marks. New Dizzy dual point Mallory Centrifigal Advance. New points & cond. set static timing 10 degrees. Turned the key and she started up immediately, idle very smooth. Carb cleaner sprayed at spindles and other spots no increase in RPM. Very slight side to side movement in forward carb spindle none in rear. Took her out for a spin and she still breaks up under load. I think my problem know is the float setting. Hope to find the time to remove carbs tomorrow and check this out. When I remove the carb pistons and look down into the carb I cannot see fuel. When running and I lift the carb piston slightly she slows down. Mixture screws full in. Oh, the white smoke I spoke of yesterday was so slight it was hard to determine the color but I'm sure its white not black. Seems like she is starving for fuel. With the choke out seems to pull beter with a load. I'll keep at it.
Ken Knize

Just a thought, but with the screws full in, what location are the jets at in relation to the carb? They should be about 1/2 inch lower than the carb body. If they are you might definatelly want to check the floats, fuel filter, and fuel pressure.
J Arthurs

Ken; You mention you have a new Mallory dizzy, how was the car running with the old dizzy?
I assume you have no vacuum advance. Do you have any tuning specs for the Mallory? I mention
this because your original dizzy should be a 41491. The 41491 has around 39 degrees of total
centrifugal advance, without checking the Mallory web site it seems like I remember they have
around 25 degrees total advance. You may wnat to try advancing the dizzy another 6-8 degrees
and see if it runs ok without pinging.

If you can get the timing mark on the balancer lined up you can paint some marks to line up on
the balancer and timing cover on the left side of the engine so you can see them with a timing
light.

You didn't mention any recent work on the carbs. However, if you have Grose jets in the carbs,
Grose jets for HIF's have a small shut off ball and tend to bind and shut off the fuel flow to the
carbs, or that happened to me. After sitting a few days the rear needle jet would stick, the
car would run good for a few minutes and then start running on two cylinders after the rear carb
ran dry. Clifton
Clifton Gordon

I'm new at this and am having a problem identifying parts. Is the jet the brass circle the needle enters? If so it is lower than the carb body but not a 1/2 inch. In fact I'm having a problem identifying any movement. Is there a locking screw. I just today found the HIF portion of the Complete MGB Handbook. I don't know what a grose jet is. Thanks for the comments, I don't feel quite as alone on this. I have had the car for a year and just getting to tune up after working on interior.
Ken Knize

The last distrib was stock and wabbled. PO said it was where he gave up after many upgrades. He decided he had put enough into it. A shame because she is rust free, well almost.
Ken Knize

I recall some info that came w/the Mallory that said I would have to order a kit to increase the centrifigal advance. Well that gives me another thing to try. She idles so well, starts so well and runs so well (until I get to a hill or tromp on it) I feel this must be a sound engine or will be once sorted out.

Ken Knize
Binghamton NY
Ken Knize

Ken: Look at HIF Section 4-D in your Complete MG handbook. You should be looking at a drawing
of one HIF carb with four smaller drawings around it. The small upper right drawing shows the jet
adjusting screw. Now look at the lower left drawing, it shows a cut away section of the jet and it's
retaining bearing. The jet is the brass circle you mention. If you remove the suction chamber and
and piston you can look for jet movement as the jet adjusting screw is turned. Clockwise moves the
jet down and anti-clockwise moves it up. The Grose jet is a replacement for the standard float needle
valve.

You indicate the car idles well and runs well until you go up a hill or tromp on the gas. That indicates
your carbs may not have any serious problems.

I still think the Mallory distributor may need considerably advance the stock Lucas. It's easy to
advance it 8 or 10 degrees and try it to see if any improvement is noted. You just have to be sure
it isn't pinging on heavy acceleration, otherwise it will not harm the LC engine to advance the timing.

Keep asking questions, someone on the list has an answer for your problem. Regads, Clifton

Clifton Gordon

Pulling the choke can mask ignition problems - "90% of SU problems are caused by faults in the ignition system". You may be able to check for fuel starvation by reproducing the problem then switching off and depressing the clutch at the same time. Bring the car to a halt then switch off the ignition. If the fuel pump chatters it indicates that there *was* less fuel then there should be in the float chambers, particularly if doing the same test at a speed that doesn't cause the symptom results in no chattering of the pump.
Paul Hunt

I got a chance to pull the carbs off today, my goal was to check the float level. What a mess, I couldn't find any sign of the float bowl O ring just a mess of goop. I'm only taking one carb apart at a time so I'm not sure of the forward carb but it looks like someone used form-a-gasket or something like it. Looks like wads of black bubble gum. I'm waiting for a call back from Sean @ Moss so I can order a rebuild kit.

Peter Burgess suggests a 1 1/2 in SU carb, mine are 1 1/4 in. Where can I get the larger carb are they from another car??? My method is to try to get this engine running just to see what I've got prior to doing modifications and/or rebuild, replace. I like a high mid to low end torque engine as it is doubtfull I will ever run this car at high speed.

Thanks again for the tips I'll use them all when she is running again. I'll post again at "Too Lean" to let you know how I make out.

Thanks, Ken
Ken Knize

Aren't 1.25 SUs a midget carb instead of a B Carb?
David

David, Thanks for the heads up. I went back to the garage and checked. Mine are 1 1/2 after all. The last time I measured them the outside throat was a little less than 1 1/2 so I thought they were 1 1/4. Today I've checked the manifold and outlet throat and sure enough 1 1/2. One, no Two less parts to buy.
Ken Knize

This thread was discussed between 14/07/2002 and 18/07/2002

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