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MG MGB Technical - Performance

My MGB 1978 roadster( SU carbs) stutters when 3000rpm is reached. I have renewed ignition from coil through to spark plugs (including new distributor) but still holds back/stutters when approx 3000 rpm reached. Does however rev freely to max when in neutral. Is this a fuel/carb issue. Any suggestions
D Meeney

It's going to be difficult to make a diagnosis on this type of problem without getting more data. You could guess and change lots of things and get lucky or maybe not..
You really need to get it on a rolling road with all the measuring points for fuel and electrics eg exhaust gases, timing, points dwell (assuming your new dizzy isn't electronic) etc.
My recommendation is take a day off work and book it in to Peter Burgess in Derby, limp it there and enjoy the blast home when it's running sweet.. (Peter, commission fee invoice is in the post :0)

Best of...
M McAndrew

Seen comments before where fuel starvation occurs > 3K Check your pump has good flow (IIRC about 1 pint per minute)
Bernie
B Anderson

You will have different manifold depressions as well so vacuum a/r may influence things. Does blanking it off make a difference?
BTW I would vote for not enough fuel though.
Stan Best

"IIRC about 1 pint per minute"

That's an absolute minimum, in practice it should be at least twice that, in a continuous series of pulses with minimal bubbling.

If it were anything to do with the ignition LT circuit (with the exception of the condenser) it would have shown up on the tach.
Paul Hunt

what air filters do you have on the car.when i got my bgt it had k n filters already fitted.it used to hold back at high revs for the 5 yrs i had it.the last 6mths i found out that when k n filters are fitted you have to put richer needles in the carbs. done this and it run superb.
might help. bob.
bob taylor

Yep, I agree with Bob.....wrong needles in the carbs. If you remove the needles and use a magnifying glass, the A** code should be on the needle. I use AAA's on my K&N equipped 78' car. AAM needles also work well.
M F L Sherrit

i pint / mi = 1 gal in 8 min
whole tank in 1-1/2 hours!!
Where does that come from?
Art Pearse

here's my usual pots and pans non-technical thought - I'm guessing it's to cover maximum demands plus an excess tolerance

bad simile - H tyres, up to 130 mph (for 10 minutes!) same tyres as when you're traveling at 30 mph and 70 mph, the rating is at maximum (for 10 minutes!)
Nigel Atkins

Thanks for the comments. Start with fuel pump and Filter I guess and then go from there
D Meeney

or set up/check/recheck in this order - tappets, points, plugs, timing, mixture
Nigel Atkins

FWIW When I replaced the distributor on my 1980 GT I had to go to richer needles AAUs
Vic
V Todman

"i pint / mi = 1 gal in 8 min
whole tank in 1-1/2 hours!!
Where does that come from?"

Er, delivery pipe disconnected from a carb and directed into a container? Hence being able to watch the pulses and bubbles.

Your calculation for a whole tank shows why it would have to be a very extreme MGB indeed to need more than the standard pump.
Paul Hunt

checked timing and all ignition ok. Definitely a fuel problem which was brought to light by once achieving >3000rpm and the stuttering started, if I pulled out choke the engine would instantly pick up and ride right through to redline. Not understanding the carbs in great detail could anyone suggest what part of system is failing or blocked etc

Thanks
D Meeney

just thoughts as usual from me

see my previous post about setting up order

plus perhaps more info (and photos)

what combination of air filters, carb needles and stub stacks do you have

new distributor is what make and model and is it new, recon, rebuilt, fully or partly electronic

does it have vacuum advance and where from

breather pipes and cap clear, cap chrome or plastic

consult your Driver's Handbook for setting up including carbs

John twist video, 35 Tuning HIF Carburetters - http://www.youtube.com/watch?v=ASeMfXfjNpw&feature=plcp
Nigel Atkins

If pulling the choke clears it it is unlikely to be a pump or delivery pipe problem.

As a 78 I presume you have HIFs. Instead of the single path from the float chamber to the inlet manifold for normal running as well as enrichment the HIF has separate paths, in which case it could be a partially blocked main jet in a carb, or a defect with a jet or needle. Could also be a problem with the rise of one piston. It shouldn't be sticking down, as that would cause a rich mixture, so not alleviated with the choke.
Paul Hunt

Checked pump and flow and all ok.Carb stripping time I think. Thanks for all the advice
D Meeney

Sounds like a good plan.
Has the car had a lot of standing time? When petrol is left to evaporate it leaves behind an emulsion which could/will restrict the main jet or more likely, the tube to the inlet manifold. An easy job to fix if you get a service kit and follow the instructions there in.

Spend a leisurely couple of hours stripping an rebuilding the carb is a good way to spend a Saturday and it's surprising what you'll learn about how it works.

best of...
MGmike
M McAndrew

Cheers Mike,

No the car hasn't been standing and was performing well for the first 3 months when I bought it. Makes me think it mat be a partially blocked jet. Like you suggested a leisurely and patient Saturday stripping the carb, new seals etc. It runs ok with gentle driving so may persevere until winter and then hole up in the garage for a few weekends. Make the most of our sunny summer!
D Meeney

stripped and cleaned carbs, 1 jet blocked so it blew clear. Took the opportunity to clean all the components with petrol and carb cleaner. Put back together tonight .......goes like a dream

Thanks for all the posts and advice
D Meeney

great

now continue to use and service it and it will get better and better but if you lay it up then you'll be starting again on some of the things that got sorted and run to smoothness
Nigel Atkins

This thread was discussed between 22/07/2013 and 19/08/2013

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