MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Engine skipping at 3000 RPM

One of our club members is having a problem with his 73 B at cruising speed. At light acceleration the engine skips pretty bad starting about 3000 RPMs. Under hard acceleration it goes great. He has checked timing, changed to an electronic ignition, adjusted and readjusted the carbs (twin SU HIF), sprayed WD-40 looking for vacuum leaks, done a compression check, and nothing helps. He said he did get a slight change in engine speed when he sprayed the WD-40 near the throttle shafts, would this cause this problem? Any advise would be greatly appreciated.

Thanks,

Ken
Ken Waringa

Ken, Leaking throttle shafts wouldn't make a difference at 3000 rpm. It sounds like the carbs may have a lean surge. I would suggest turning the mixture screws in about 1/2 turn, in is richer on HIF carbs, and see if that helps.
Due to the leaking throttle shafts the idle speed fuel mixture settings may be off enough to cause a mid speed surge. The HIF jet metering needles for 73-74 were designed for emissions and are very sensitive to fuel mix settings at mid range speeds. Regards, Clifton
Clifton Gordon

If I may jump in here, I'm new to this group. Ken just "introduced" me to this particular BBS.

I agree with Clifton's suggestion, but I've enriched each carb a flat at a time until both were at full rich. No change.

Running stock HIF4's (ABD needles), however, the engine is not stock. 3/4 race cam, head has been ported and polished, header.

Timing is set at 12.5 degrees (vacumn line disconnected). Advance seems to work well when the vacumn tube is reconnected to the distributor advance and rpm's increased.

When I purchased the car it ran well. Rich at idle (bit of black smoke), but generally okay. I started a "to-do" list. One of the items was to balance and adjust the carbs. Both carbs are currently adjusted and balanced properly by the lift-pin test and measuring vacumn at the air intake.

Converted the distributor to point-less, checked intake vacumn (15-15 1/2 lbs at idle), etc.

One interesting note: There is no difference if the vacumn line from the intake to distributor is disconnected from the distributor. Still missed badly at around 3300 rpm. It just stumbles and falls flat on it's face, unless I pull the choke out. Runs great then<G>.

Thanks,

John

John Staub

Sounds like it's hitting a lean spot. You might try a different needle (perhaps AAA?), this might help. If there is anyone in the area with a chassis dyno (rolling road) and a knowledge of SU tuning, that would be the way to go. These people are very thin on the ground in the US, so good luck in your endeavors.
Paul K

There is a needle profile chart included in the Haynes book on carbs. The title is a bit misleading, as it makes you think it's only Weber, but it also covers SU and Z-S carbs. The needle chart lists the diameter of the needles at each of the 16 metering points. Smaller diameter = richer, so you can compare profiles.
Paul K

John, If it runs well when you pull out the choke, Paul K's suggestion to try AAA needles will probably solve your problems. The AAA is smaller in diameter than the ABD at the critical cruise speeds. Actually the ABD is thinner at its outer extremes than the AAA so that should explain why you miss stops on full acceleration. I had to change to AAA needles in my 74 when I switched to K & N filters and installed the Bob M. type filter caps in place of the Cooper cans. With the cam you have in the car it may need more distributor advance. As Paul says it's best to use a rolling road to sort it out but if you are like me none are close by so I have to sort things out bu trial and error. If you do decide to get some AAA needles you can get them from Joe Curto. He can also give some good carb advice so it may be a good idea to call him. His web site is: http://www.joecurtoinc.qpg.com/
Good Luck, Clifton
Clifton Gordon

One item you might check is fuel delivery, pressure and amount.Things like a poor fuel pump or even a clogged filter can cause problems such as yours.
While you are at it have you checked the spark plugs?
Ric
Ric

Hi John,
You haven't mentioned the coil. While you're changing everything else, borrow a coil from your friend's car just to exclude this as a possibility.
Rgds
Kevin
Kevin Cornford

Hi All,

First, thanks for all of the excellent suggestions!! That's one of the things I love about the LBC community!!

I may just order some AAA needles. What the hey, it's a cheap test<G>.

I've changed the plugs; the coil; Ken offered a dizzy so as soon as I can get over there to pick it up, I'll try it. I tested the pressure from the fuel pump - it's at 2.5 to 2.75 psi.

The PO, a really good guy BTW, is pretty stumped, but he also suggested checking the fuel line that runs from the tank to the pump. As soon as I have time, I'm going to check it also.

Thanks,

John
John Staub

John, I'm pretty certain it's the needles. With the mods you've made, especially the head porting and polishing, your carb needles just can't keep up with the air flow, and you hit a lean spot where the engine stumbles. The key symptom here is that it cleans up when you pull the choke, a very good indicator of lean mixture. Try the AAA's, and thanks Clifton, I should have told him where to get them in my post.
Paul K

Hi Paul,

Just got off the phone with Joe Curto...Heck of a nice guy. New needles should be here tomorrow afternoon.

Think I may be calling him more often.

Thanks,

John
John Staub

Hi All,

Put in the AAA needles late yesterday afternoon, adjusted the carbs some (man was it running rich<G>). Still stumbles.

Hopefully I'll be able to put it on a scope this weekend. I'm out of ideas.

Thanks,

John
John Staub

This thread was discussed between 23/09/2002 and 26/09/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.