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MG MGB Technical - Too Lean

Many helped me out a few weeks ago when I thought my cabs in my 73 low compression HIF4 carbs were too lean. After finding my timing marks (11:00 not 5:00) I set the timing closer than ever befor. I purchased an SU rebuild kit from Moss. When opening up the float chamber of the carbs I found black bubble gum but no gasket. The carbs have now been put straight as best as I can tell. I replaced the dizzy w/ a Malory dual point. I have the centrifigal advance kit but have not yet tried a change. I also have a new Lucus Sport Coil but have not yet installed it. Tomorrow is Saturday an under hood and road test day. After a road test today I have found as I change gears I get some backfire. Not a big deal just a few pops in fact this is the best the car has run since I'v had it. My question is where should I look next. My guess is the timing advance. Oh, I installed the KN air filters along w/ the carb needle change Peter Burges recommends, sorry can't remember spec's, package in the garage. I know air leaks in exhaust are likely but I have not be able to find any.

Ken Knize
Binghhamton NY
Car's name "Ruby"
Ken Knize

DIdn't change anything today having too much fun driving the car for the first time. But, the exhaust backfiring between gears can't be good. Where do you suggest I start looking to eliminate it?

Ken Knize
Binghamton NY
Car "Ruby"
Ken Knize

Ken,
When you refer to "Peter Burgess & his recommended needle change for K & N filters"..... Is it in his book on tuning the B series engine? If not where can I find the information?
Regards, Graham.
Graham

Graham; Try pages 68 and 103 in the Peter Burgess book. Regards, Clifton
Clifton Gordon

Thank you Clifton. I guess I'd better get a copy of the book.
Regards, Graham.
Graham

Peter says "Fitting the high flow filters----will in 90% of cases, need the std. AAU needles changed to ABD's the other 10% will need AAA's. I specified when ordering from MOSS the ABD's but as I look back at the packaging there is no mention of ABD. I compared what I received with those in the car and the new ones are a bit shorter and seem to be thicker. I don't have mics or caliper to verify.

Ken Knize
Binghamton NY
Car "RUBY"
Ken Knize

The needle number should be lightly stamped on the top part of the needle, just beneath the shroud where the spring fits on top. Hope that helps.
SteveO

Ken; The shop manual specifies ABD as the standard needle for emission controlled engines
from 73 on, so the needles in your car should be ABD's. I have two sets of HIF carbs and they
both had ABD,s in them. I switched to K & N filters and used open Bob type filters and AAA's are
what I needed. As far as I know all needles for HIF carbs are the some length. Your best bet may
be to call Joe Curto and tell him what you have and he will know what you need.
http://www.geocities.com/jcurtoinc/
Clifton
Clifton Gordon

I guess I'll have to open the carbs up again to verify what I have and do some trials. The car is running very well with max torque at approx 3000rpm. but I do have the exhaust popping at gear change. I can all but eliminate the back fire by a quick shift. OH by the way the poppet valves were soldiered shut so I cut the valves off to improve air flow. I'll try to get butterflys w/o valves. I am also going to slow down the idle, now it's @ 1000rpm. I can stand outside the car reach in and twist the key the car immediately starts no cranking. BUt I'll wait for a cooler day or until my travel trailer is out of the driveway. I can't get the car back in the garage and it's too hot to work in the sun today, close to 100 degrees.
Ken Knize

Thanks, Steve & Clifton. Went out to the garage and checked the needles with a magnifing glass and those that I removed were infact ABD's. Now that I know where to look I'll remove one from the car tomorrow to see what I have.

One of the needles I removed had a crack in the lower collar. Didn't notice untill I used the magnifyer. I can't see how this would effect performance but I wonder how it happened.
Ken Knize

Ken,

sounds like a classic case of a bad Gulp Valve. There's a simple procedure in Haynes on how to test it. The gulp valve equalizes the pressure in the inlet manifold when you let off the gas between gear changes, or else you get gas being sucked into the engine and hence unburnt gas gets into the hot exhaust pipe and explodes which is the popping your hear. New ones are about $65 form Moss but you see them on ebay all the time for less than $15.

Andy
Andy Preston

Andy, Thanks for your advice I didn't have a clear picture of what the gulp valve did but I wondered if it might be involved. I'm going to go ahead and order one.

Ken Knize
Binghamton
car "RUBY"
Ken Knize

Andy, Put the new gulp valve in and sure enough the popping stopped. But now the higher end break up is worse, or I notice it more. I received The Mallory advance curve change kit and will give it a try tomorrow. As this thred is getting beyond the original "Too Lean" I' start a new thread "Mallory dual point"

Thank you all, for the guidance.
Ken Knize & RUBY
Ken Knize

This thread was discussed between 03/08/2002 and 07/08/2002

MG MGB Technical index

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