So I'm following Barney's "casual" tune up directions with respect to carb adjustments. I warmed the car up, loosened one linkage between the carbs and confirmed that each side moves independently. I backed off the idle on the back carb until the screw wasn't touching - that one should be closed off. I adjusted the idle on the front carb to about 1000 RPM using a shop tach. The engine was shaking violently and I began to adjust the mixture toward rich trying to get past the engine shake into smooth, then past that into galloping (too rich). I don't seem to be able to get it past lean. I have turned the mixture adjuster probably 3 whole turns (18 flats) - maybe more and the engine still shakes. Sometimes so much I can hardly get the mixture wrench on the nut, or get a screwdriver in the slot to adjust idle if it goes a little slow or fast.

The car was running well for the last 8 months with no carb adjustments. It's only been the last few weeks that I've felt it was running lean as engine shake at idle had crept in. Doesn't seem possible it slipped this far out of adjustment so I'm concluding I'm doing something wrong. I checked spark plug gaps just the other day, but didn't re-check the timing since I'd set it just a few months ago. I'm about to go check that, but thought I'd ask for any other advice. For those who have read the other thread about the stiff/loose damper - the one I'm adjusting is the easier moving one if that matters. I put some fresh 20W50 in it and it is a little closer to the stiffness of the other one so I decided to move on with the adjustments.

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Geoff Howard
http://mgaexperiment.blogspot.com
Now Playing on the Blog (1/6/2013): Sagging With Age

Wait a minute. Took the spark plug out to find TDC for re-setting timing and found it was now sooty and black so maybe I was getting to rich after all. Last week they looked more tan when I was checking the gaps. I was looking to get to "gallop" as mentioned by Barney, which I assumed meant the engine revving up and down a little by itself. Did I misunderstand how to tell when I got to a rich condition?

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Geoff Howard
http://mgaexperiment.blogspot.com
Now Playing on the Blog (1/6/2013): Sagging With Age

It is not unheard of for the throttle butterfly to not completely close off even when the idle adjustment screw is totally backed off. This will make it difficult to perform the 'adjust one carb at a time' method.

Note that the MGA SU carbs have a small 'lifting pin' under the dashpot. If you have the carbs adjusted approximately right then using that lifting pin to raise the dashpot (and thus the fuel needle) by ~ 1/32 - 1/16" you will slightly lean out the mixture. If the engine immediately stumbles you were too lean. If it runs markedly better you were too rich. If it intially runs a little better/faster then settles back to the prior speed the mixture is just right.
With a good ear (and/or an accurate tachometer) you can adjust by one 'flat' and discern the difference.

Synchronizing the carbs can be a little trickier. The old way is to listen to the inrushing air through a length of hose (1/2 heater hose about 2' long) and try to match the sounds.

I use a synchrometer - an air flow meter that measures how much air each carb is drawing in.

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Eric Russell ~ Mebane, NC
1960 MGA, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

Also, remember that the mixture adjusting nut has right hand ('normal') threads but it is hung underneath the carb. You screw the nut 'up' (as if you were tightening it from underneath) to lean the mixture & screw it down (as if you were unscrewing from under the carb) to richen the mixture.

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Eric Russell ~ Mebane, NC
1960 MGA, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

For what it's worth Geoff:

Not sure which, but I have H6 or HS6 carbs on my car - the ones with the 1 3/4 inch throat - look like from a twin cam.

When I've adjusted them, I've separated them on the throttle bar, but then syncronized them before adjusting the mixture. At first I had a real hard time with the mixture adjustment (because I didn't know where I was, I think). Then I said to hell with it and turned both nuts all the way up to lean (clockwise, but upside down feels almost counterclockwise). Then I backed both adjustment nuts off 13 flats (2 1/6 turns from lean). That worked pretty well, but I tweaked a bit and now run 14 flats from lean.

I've mostly just used the piston pin to raise the piston the slight amount and then re-syncronized as I made adjustments. When I was satisfied, I reconnected the two carbs at the throtle bar.

(I have three sets of needles, marked RU, SM and KW. The car was running with the RU needle and I decided to keep those. Comparing these needles on the needle charts showed that the RU ran richer at full throttle than at low throttle, SM ran leaner at all throttle applications, and KW ran richer at all throttle applications. Since I mostly run at wide open throttle, I kept with the RU. That needle also ran richer than the KW at wide open throttle.)


I also think you are having trouble because, by shutting one carb all the way down, you are running on only two cylinders.

Using the piston lifting pins is a good test and seems to work as advertised.

If you don't get consistent results, you might have an air leak in the gasket at the manifold.

When it heats up it might return to being rich if the opening closes up.

I had a piece missing from the thick manifold insulator - ordered new ones from Moss.

JIM

Thanks, all - I am closer to sorting it out. I decided to reset by going all the way up and backing off 2 full turns (12 flats). That got me back to a sane place without severe engine shake. Then lifting the piston made the rpm drop off quickly - too lean. I went richer a few flats testing each time with a shop tach (cheap Actron - not sure I trust it) and maybe saw a slight increase in rpm (I think less than 50 - more like 10-15 if any) and settled back. I went a little richer on one and maybe got it to rise more with more piston rise. I didn't see any effect with the pin. I will investigate that later.

I tried synchronizing with a length of fuel hose held to my ear. All I can hear is engine noise - can't quite tell any difference with idle adjust. I'll look for a tool to borrow while I learn the ropes.

This time I didn't try to isolate one carb at a time. I have no doubt that Barney knows exactly what he's talking about but I did suspect that it wasn't working right for me. I wondered if I have some air leaking somewhere. The manifold gasket is new so I don't think that's it. It was installed by me, so I could definitely believe I screwed something up, but I'm not looking there first. I sprayed some wd40 around the shaft where it enters the carb body and didn't see a difference in engine running. So, I'm wondering about butterfly sealing. Any suggestions on how to test that? I feel as if I can back the idle screws all the way out on both sides and the engine still runs. I haven't actually confirmed that or tried to kill the engine this way but it should die, right?

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Geoff Howard
http://mgaexperiment.blogspot.com
Now Playing on the Blog (1/6/2013): Sagging With Age

Oh, and the needles are GS, so I think that's correct/standard for my 1500.

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Geoff Howard
http://mgaexperiment.blogspot.com
Now Playing on the Blog (1/6/2013): Sagging With Age

Yes, if the idle speed adjusting screws were back out completely the butterflys should completely block off the flow through the carb. However, when the butterflys are installed into the throttle shaft if they are not centralized (or perhaps even installed backwards...) they would likely not close off the carb throat completely.

Click here for Barney Gaylord's description of the centering process

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Eric Russell ~ Mebane, NC
1960 MGA, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

Along these lines - story from this weekend.

Went on a 300+ mile drive with my wife over the last two days, staying up in the mountains overnight. Lots of climbing loooong hills.

Car running well, set up the carbs with the pin lifting technique, and seemed spot on.

Seemed to struggle going up the hills, no power in 4th gear, can't get above 3500 rpms - have to downshift and then can hold steady in third. Mileage about 18MPG for the trip up. Richening the mixture with the choke does nothing - thinking it's too rich actually. Kind of a "bogging down" under max load - just can't get any more out of the engine.

This morning, stop for gas, lean out the mixture nuts about one flat. Engine was hot, so this was not very precise. A bit more giddyup when flooring it, a wee bit more power at the top end. Didn't fill it up again yet to check the homebound mileage.

I think that the pin lift trick is fine, but with the standard GS needles, I apparently have a running too rich condition at the top end.

My idle is now a bit more erratic, not a smooth lope when at idle like before. Am I now into playing with needles, or something else?

JIM

Geoff,

Take your dash pots off and then lift off the pistons. You should be able to see the brass jets centered in the sleeve (don't know technical name) The jets should be at the same level on both carbs. When you go all the way up to lean, they should be more or less flush with the sleeve.

I'm having trouble with one of mine (see "engine stumbles at constant speed". One jet doesn't seem to want to move when I turn the nut. Perhaps you have the same general problem. If one carb is tuned correctly, you can probably tune the other to it by comparing the jet locations.

Jim Mail had a thread where I think he tuned the carbs by the jet locations.

Yes, for my initial set up I measured down from the bridge to set the jets to the same height rather than counting flats. Worked to somewhat smoothe out the idle.

JIM

To check that the butterflies are completely closing--take carbs off car and look thru the throat/carb body as you close the butterfly. If light passes, air passes. If needed, loosen buttterfly screws and "slap" the throttle shaft till butterflies center and close off all light. hth

The first task is to make sure the butterlies (throttle discs) are syncronised. To do this without an air flow meter slacken off the link between the two throttle spindles and turn the idle adjustment screws anti clockwise until both butterflies are closed. Make sure the thottle cable isn't holding the throttle open. Turn the screws clockwise until they just start to open the butterflies then turn them one full turn. Now as mentioned previously remove the suction chambers and pistons to adjust the jets up to the fully lean condition after having disconnected the choke lever interconnect. Check that the jets are the same height relative to the bridge ( the flat portion where the jet is central) Adjust to achieve the same height. Inspect the needles to ensure they are installed correctly with the shoulder of the needle level with the bottom of the piston and check for any wear marks and /or bent needles. Reinstall the pistons and chambers and check for freedom of movement of the pistons. If there is any sticking then the jet(s) may need to be centralised to the needle(s) or the piston may be rubbing on the suction chamber. All being well turn the jets down two turns (12 flats). There are other checks to make such as freedom of movement of the choke mechanisms etc. Assuming the timing is correct at around 7 degrees btc the engine should start perhaps needing a bit of choke. it should start with one choke lever connected. Once started the choke can be released as soon as possible.
When the engine can run without choke, adjust the butterfly idle screws equally to achieve an idle speed of approx. 1000 rpm. (If the engine speed cannot be adjusted down to this level then the carburetters will likely need to be removed and the butterflies will need to be adjusted to close correctly.) When the engine is fully warmed up now adjust the mixture by turning the jet adusting nuts both the same amount until the highest idle speed is achieved. REadust the butterfly adjustment screws equally to the desired idle speed (800rpm) and then adjust the mixture again equally to achieve the best idle mixture. There should be no necessity to adjust more than 3 flats from the initial setting Check by lifting the pistons 1/8" with the pin when the idle speed should increase slightly and then settle down again. Another check is by removing the damper and then pushing it back in when the speed should decrease. If the speed increases when pushing the damper back in then the mixture is too lean. Only adjust the carbs individually at the final point and if there is a necessity to adjust one carb more than 2or 3 flats more than the other then there is probably another problem such as worn jets and or air leaks.
I ran British Classic Cars in Chantilly VA for over 20 years

Ah, Phillip! You are partly to thank/blame for my diving into MG ownership. I used to drive by your spot in Aldie/Lenah and ogle the cars. We did a nice tea for mothers day at the attached British Pantry that I assume you owned as well. In fact, I bought Jeff's car from the Liberty station near your old place. I imagine you had your hands on my car at some point in the past.

Thanks, I'll go back through things recently. Jeff told me the carbs had been professionally rebuilt while he owned it. I don't suppose that was by you? They have been spot on all year until this recent episode.

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Geoff Howard
http://mgaexperiment.blogspot.com
Now Playing on the Blog (1/6/2013): Sagging With Age

Goeff,
Yes, I did work on Jeff's MGA in 2007 to get it running. I seem to remember it was a nice solid example. I hope the adjustments work to get the car running. If all efforts fail I can rebuild the carbs for you. Check everything else including the timing, points, fuel pressure, compression and valve clearances also change the plugs if fouled. Let me know.
Philip