MGB & GT Forum

I've got a bit of a hybrid engine and I'm seeking advice over the needles and distributor to use. The engine is an 18G something - no cutouts in the block until I added them. It has a 40 thou overbore. The head is a 2708 big valve head with 3 angle valve seats, back angle relieved valves, 35cc chambers +/- 0.5 cc and some mild porting. When I calculated the compression ratio about 20 years ago (when I built it) I seem to remember it was about 9.5:1. I have a standard cam, standard cam timing, standard manifolds, peco exhaust, and twin HIF4 carbs (rebuilt) with K&N filters.

I currently have AAA needles and the distributor is unknown, but believed to be a brand new 1972 UK spec. Its set to a static timing of 10 deg BTDC.

When I drove it on the drive today after freeing the clutch it felt rough. When I blipped the throttle with a warm engine and no choke I got a black cloud out of the exhaust. Does the choice of needles and distributor / timing seem reasonable? I also have a set of ACD and AAU needles and the original 1979 spec dissy (thats done 93K miles). I'm aiming to take the car for an MOT (which involves an emissions test) in the next week or so. Under UK law the journey to (and back from) a booked MOT are the only time I can legally drive the car on the road without road tax or an MOT, so I've only got this change to get it almost right. First real journey will probably be to a rolling road.

Check your plugs. Check your timing. Static timing means little, should be around 32-36° total at 3kRpm(16-18° at idle). It's hard to say without hearing, feeling, seeing(plugs) how the current setup is working for you.

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"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

"Who do you think you are? I am."...Pete Weber

73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...

Has the car been standing for a while, why the stuck clutch ?

If it has it might pay to check the float levels and that everything is working properly linkage wise and make sure your vacuum advance is working.

Black smoke is an indication of a rich mixture so where are your jets set ?

Two full turns down is the base line but in my experience that's always rich and 1.75 or even 1.5 is closer to optimum.

From memory the AAA needles were recommended for use with K&N filters but in truth were not really necessary.

For the MOT they only measure the emissions at idle and at that setting all three needles you have are effectively the same so run as weak a mixture as you can to actually drive the car and you should sail through, you can wind it up again as soon as you have your pass certificate

Don't worry about the timing.

It's been standing for 18 years thus the stuck clutch. I've rebuilt the carbs and the vacuum is working OK.

I've measured the CO on a gunston CO meter and its giving about 3.5% (ish). A colourtune in all cylinders gives a good blue colour. Not sure where its set to. It requiers some choke to start and after about a minute it will run without choke. It's lumpy on tick over, but I'm use to seeing injected / super charged v8s idle at work and these don't move at all. Idle is set to about 950 rpm.

I've got a set of AAU needles. Looking at the charts the AAAs are a lot richer. I'll swap them over and see what I get.

Sounds like its real close to where you need to be.

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"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

"Who do you think you are? I am."...Pete Weber

73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...

I would only use AAA needles with the larger K&N filters, with normal filters or pancake K&N normal needles can be used. AAU are the standard ones, ACD are the leanest ever, I suggest to try AAU instead. I wonder about the relevance of your 3,5 CO value, are older cars like yours not exempt from CO level requirenents in your MoT rules?
PS I myself used ACD with paper filters, AAl with pancake K&N and now AAA with K&N in the Cooper housing. Also using Colortunes and finetuning on plug color I can reach any CO level between 3,5 and 2,0, but normally aim at 3 as the max level the engine is happy with. Max advance indeed on 32 degrees no vac at high revs with RON 98 petrol
I forgot to add that needles are very close to each other at lower stations, so on idle where CO tests -if any- are relevant you, one may find little difference in changing needle types. Most of the needle variations are aimed at middle ranges and cruising speeds, with the ACD needle leaning out further on higher stations as the main exception



Edited 1 time(s). Last edit at 2015-05-29 03:23 PM by Donthuis.

I'd forget about the colourtune and go by the seat of your pants, as I said typical optimum for the HIF is something short of 2 turns down but if you're getting 3.5% you're not far off.

Give it a good thrashing on the way to the test and if you get a sympathetic tester he'll tweak the carbs to meet the spec if it doesn't already.

I'm running AAA's with K&N cones, 16/34°'s timing and nice tan plugs. Holds a rock solid idle at 700RPM...but I prefer around 800 for everyday driving. Jets are set just on 2 turns down from the bridge and new main jet/needle. I also must use choke for the first 30 sec or so, after than high idle (1500k) for a minute or two and I'm all set.

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"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

"Who do you think you are? I am."...Pete Weber

73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...