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MG MGB GT V8 Factory Originals Technical - Factory V8 Carb question

I'm trying to set up the carbs on my 73 V8
problem is by checking spark plug colour one bank of cylinders is very lean, the other side rich, I've tried adjusting carbs assuming right carb feeds right bank, and when that made it worse tried the left carb feeds right bank method, not a lot better,
At the moment I've just screwed the mixture adjusters right in, and then wound them back 4 turns, Havn't tried it out yet, also I'm sure adjusting the mixture screw in richens the mixture right?
thanks in advance for your help
N.T Griffiths

Take the dashpots off and look at the jets. Are they level? I use a tyre depth guage to check, but you can do a good job by eye.
Once the jets are set level, you need to reassemble and check the airlflow by tube or Carbalancer.Pull on the throttle cable, do both linkages move together?
Also make sure that the choke cable activates both sides at the same time.
You can do most of this fiddling without having the engine running. Once everything is mechanically set, then start adjusting the mixture screws by exactly the same amount each time.
Dave Wellings

a while back i was talking with someone that had the same problem and it came up that the heater valve extension runs right close to one of the fuel bowls, and that the heat from it may be an issue. this gent wrapped his with some heat tape i believe, and reset them and all was fine. maybe it was the heat tape, maybe the reset. i tend to set my SU carb jets with the end of a micrometer after removing the dashpot, so i know theyre the same depth in, then adjust each mixture screw the same number of turns in or out. it seems to get the mixture right and i have a carb synchronizer for the airflow. Dave's right in what he said above too about loosening the linkage to both carbs, setting them individually and then hooking them back together.

Each carb feeds two cylinders one side and two the other. The jets may not be level and yet correctly adjusted as each is set to give the correct mixture, which takes into account a number of dynamics including carbs. Start with the jets flush with the bridge (unscrew the adjusting screw) then turn the adjusting screw in two full turns. Neither of these settings is critical, it is just a starting point for fine tuning. Balance the airflow at idle then by using the lifting pin and seeing how it affects idle speed you fine tune each mixture. Raise one pin 1/32" (approx) and if the revs die the mixture is weak so screw the mixture screw in a little more. If the revs rise and stay risen while you hold the pin the mixture is too weak so unscrew the screw a little. You are aiming for the revs to rise as you raise the pin then settle back down again with the pin still lifted. Needsa careful listening. You can do each carb independantly as there is no balance tube between the two inlet tracts as there is on the 4-cylinder. Then check the balance again but this time off-idle, i.e. making sure both butterflies open the same amount at the same time. By independantly adjusting each fast idle screw make sure operation of the choke does the same. If yours needs to meet emissions then check the tailpipe and adjust both carbs by the same amount to get the correct reading. Once they are correctly balanced for airflow and mixture it is *then* that you must only ever make the same adjustment to both carbs or you will destroy the balance and have to start all over again.
Paul Hunt

Thanks for suggestions and advice
reading the owners manual I raised jet height to level of bridge, then srewed in adjusting screw 21/2 turns, started engine and balanced carbs using a tube.Manual suggests turning mixture screw either way until fastest idle reached and then backing off slightly, engine now ticks over fine but still getting slight missing on constant speed, but ok on full cceleration, I have renewed all Ignition components over last week already, timing, dwell angle, plug gaps spot on
N.T Griffiths

Jake is referring to me above and I think the name of the thread was 'SU CARBURETTOR MIXTURE - RICH' 3 September this year.

My setup is a copy of the factory V8, but because the carburettors are HIF 6 from a P6 Rover, the fuel pipe connecting them runs from the outside of the right carburettor to the outside of the left carburettor. This pipe is underneath and sits there quite neatly.

I have now wrapped this pipe in an insulating material plus the heater hose from the tape which runs close to the right side fuel bowl.

I seem to be having less problems and have almost finished the plumbing to recirculate a little fuel.

Paul Hunt has given an excellent explanation on how to set and balance the carburettors, but I never had much success. I feel that HIFs are much harder to get right than HS carburettors. I went to a dynamometer a couple of times to try to get my car to stop fouling plugs.

As I just said, it is a lot better and I am currently experimenting with different dashpot springs. The ones I had were an unknown quantity as there was no paint left on them. I recently bought 2 sets, 1 red set and 1 yellow set. You can also buy a green set.

Red springs are 4 ounce, yellow springs are 8 ounce and green springs are 12 ounce. Apparently your mixture is leaner with the red and gets richer as you progress through yellow and then green.

I have started with the red springs and the cars seems to be more responsive in the mid rev range in the higher gears. You seems to get more instant acceleration than with whatever was in there before.

I do recommend you read the other thread, combined with this, there is some useful information.

Ian G Buckley

Ian is right, I also find the HIFs harder to judge the difference between rich and weak than the HSs, it does require concentration and careful listening and very small adjustments.

Heavier springs, along with richer mixtures, will also reduce the volume through the carb at any given throttle opening and hence power. This might well reduce the max power you can get at full throttle, but economy will be great! Ideally the springs should be chosen such that the pistons rise fully at WOT, mixture can be richened by choosing needles that are thinner towards the point. I used to get decent accelleration at WOT, but just sqeezing it open at, say, 70 or so was very flat-spotty i.e. almost seemed to make the car go slower unless I continued to press it past a certain point, or pulled out the choke which acted like an after-burner! Changing the needle profile made things much better.
Paul Hunt

Paul very correct on the small adjustment advice
I spent 15 minutes yesterday adjusting mixture till revs rose then fell, when dashpot pin lifted, same rise in revs both sides achieved. I did notice on blipping the throttle a dose of black unburnt fuel occassionaly, so I backed off a quarter turn, engine now pulls smoothly again. Strange I must have gone through this same process last time I adjusted carbs (over a year ago)seemed much easier to achieve then, I guess I was being a bit too heavy handed!
Thanks again for all advice received

Nigel Griffiths

The only point I have some difficulty with is the 'dynamics' reasoning as to why the jets might be at different heights. Although what Jake & Ian mention about the effect of heat is clearly relevant, so I suppose that's a 'dynamic'!! If the jets are unworn, or worn the same amounts, and ditto for the needles and springs, then before heat sink has it's effect, if the jets are not level, then the mixture will have a different baseline in each carb. What this thread has made me realise is that it's more accurate to set the jet height cold. Recently, I've fitted some 'heat blanket' under the carbs, but that was to reduce vapourisation which only became evident during long periods of tickover - at the test station for example. After rebuild, on the gas analyser, I found that I'd set my engine up far too weak, although it ran really well like that. I finally settled on 2.5CO and 430ppm HC. It had previously run for a year at 0.19CO without ill effect. The tester is astounded each year as to how a non-cat exhaust can be so clean. I wish I knew because I would market it!! Use a gas analyser if you can because the manual adjustment method is nowhere near as accurate.
I've also considered a fuel tank return, and the method of doing this exists in the old brain, but getting round to it is another matter. I suggested to Roger P that the Club ought to produce a kit - high pressure pump/regulator/restricted return/modded sender etc, but it's not likely. The fuel return method is the best way to regulate float chamber temp, and hence the bi-metalic effect on jet height. But so far my blanket is doing OK.
Final point, I've found the HIF much better than the HS over the years, because once set, the HIF tends to stay in tune whereas the HS seems to go all over the place quite soon after setting.

Dave Wellings

Nice one Paul - as you're in Solihull(?) I think I'll bring mine down to you!!?? What you have said is very much in line with the official advice in the handbook but what bothers me if I try to do it myself is a bit which says to the effect all tuning should be done with the engine up at running temp and within three minutes otherwise switch off, run to temp again and start from square one (haven't got it in front of me now but it is something like that). For a mere mortal like me that is virtually impossible, it would take me three minutes to read the bloody instructions.!

Dave - The dynamics issue is the same as why it's advisable to check dwell after setting the gap, and why strobe timing takes precedence over static - it just takes account of any variables between carbs. I have never bothered to see if they are different to each other in terms of jet height. 0.19CO?! You'll be pumping fuel out of the filler cap next!

Martin - the information I have is that whilst one should carry out the adjustments within a given time, if you haven't you just run the engine at a 2k or so for 30 secs to clear any pooled fuel then carry on, no need to let it cool down. Never heard that. Bring it down any time you like ...

What more can I say, other than regularly getting mid 30s mpg from the roadster, and to Le Mans and back this year in the V8 averaged 31 with 322 miles from a single tank averaging 34 mpg on the last leg home.
Paul Hunt

Might as well add a little more to my favourite thread!

Fuel return, I have almost finished it.

My car is 1963 and therefore has the 10 gallon tank held in place by the straps. I don't think it would make any difference if I had the 12 gallon flange fitting tank. If my tank expires I will install the later tank.

I am fairly sure my HIF6s and fittings came from a P6 Rover and I have used them on a Clive Wheatley plenum as per the factory set up.

Fuel enters on the left side in much the same way as it did as a 4 cylinder car. I then ran a pipe under the carburettors which I have described earlier in this thread. Apart from this the plumbing is the same as it was on the Rover.

On the right side carburettor there is a T, where fuel coming from the left side, through the pipe I have just referred to, enters the right side carburettor. The third part of this T, I had previously blanked off. I am sure it was intended for a fuel return, however I had difficulty sourcing the correct type of thread and profile to close this off.

The answer is the fuel reserve tap on a P6 Rover. The female connections to this tap use the correct size olive and thread. But there is more! I have carefully hacksawed (is that a word?) one of the male threads from the brass tap and will solder this to the fuel tank for the return fuel entry.

The Rover fuel tap gives me both ends to my fuel return pipe. I will remove the tank, solder on the male thread and drill the hole AFTERWARDS.

I have gone to this detail because I want it to look like the car was originally made this way.

I have secured some 3/16 brake pipe under the floor of the car using metal P clips as were used to hold cables and pipes originally. I mounted the P clips on the same studs that keep the cables and pipes in place in the channel under the car. The return pipe is not in the channel, but runs alongside it. The pipe is easy to bend and sits flush.

It is easier to make this metal return pipe in 2 pieces. One from the bell housing to the original fuel pump mouinting and another from above the diff to the fuel tank return entry point. Join the pipes with clamps and a short piece of flexible fuel pipe.

The female fittings from the Rover fuel tap can easily have modern fuel pipe secured to them with stainless clamps. These fittings can be shortened a little by carefully tapping the brass olive down the tube it surrounds and cutting a small piece of this tube off. This may be necesary to clear the rocker cover adjacent to the right side carburettor.

One final point, I am advised by the supplier of my fuel pump, which is an SU electronic, that I will need to put a restrictor in the pipe with a hole between
.020 and .060. I have made a restricter from a piece of aluminium which is easy to install and remove from the fuel return pipe fitting at the carburettor end.

I have done everything except solder the final fitting to the tank which will happen shortly, because I have 6 weeks holidays about to start.

Bit long, but that's me!!

Ian G Buckley

Thanks for that offer Paul - will let the holiday season (and related traffic) peter out and then contact you. She's not running too badly now since having the new plugs and leads but the tickover still drops down to about 400 (doesn't stall though even when opening the garage doors etc when the car is left for a few minutes). It's when cold that it feels as though it is going to stall until the temp needle gets to about six o'clock then it suddenly all evens out and starts running sweetly except for the dropping tickover. I haven't messed with it because I thought
if I raised the tickover speed it would be racing when the engine is cold and it's easy enough to heel and toe
even if it is a bit of a pain.

Martin - exactly the same as mine. I have never been able to get the right amount fast-idle. It is fine during the summer, but in the colder weather gradually slows, then quite suddenly starts speeding up again. The book says to adjust to 1500 rpm when hot when the arrows on the cams are under the screws, but that gives way too much. I'm increasing it bit by bit at the moment, but no doubt when the warmer weather comes back it'll be way too high again.
Paul Hunt

Paul - sounds as if it is bit of a 'common' fault then.
Trouble is that if it is taken to an ordinary tuner with a crypton or something they never seem to carry out the tuning by starting from scratch but just tweak here and there. I wonder if that is the problem i.e. that it really should be tuned by going back to square one each time. I had both carbs rebuilt about three years ago by a guy who'd done the SU course at Burlen I think it was. He had it running beautifully but is no longer in business having gone back to the F1 side of things.
Mine is definately worse now that the cold weather has returned. Got stuck in a slow moving traffic jam yesterday for about 30 minutes and it was a bloody nuisance having to heel and toe all the time, not to mention my left leg going numb - boy! that clutch can get a bit heavy in that situation.


I have same problem with Holley when temp drops to zero, tickover returning to normal when engine warm.


You should only need to go back to first principles if the tuning is in an unknown state or you know one carb has subsequently been adjusted differently to the other. I'm always very careful about adjusting both by the same amount in the same direction, and once a year or so check the air-flow and mixture balance and find them spot-on on both cars. My MOT-er used to tweak the carbs to get it through the test so I then had to redo from first principles when I got it back. I now have a gunsons analyser and make damn sure it will pass before I give it to them, adjusting them back again afterwards.
Paul Hunt

I take your point Paul. Gave mine a bit of a tweak on Saturday after reading your last. Got it nice and hot (in fact the fans were coming on and off) and just increased the throttle adjustment on both carbs slightly until tickover was about 800/900. I noticed that nothing much happened on the right carb but as soon as I touched the left one the tickover increased.
It seems to have done the trick and it is running nice and sweetly now. Low speed pick up has improved noticeably too. Will see how it goes but I'm happy for now.

This thread was discussed between 13/12/2002 and 23/12/2002

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