MGB & GT Forum

HI Guys : As topic says I'm having problems. I have a 1972 mgb roaster I'm trying to tune the twin carbies
using a unisync , when I place it on the rear carbie I get the float to rise about quarter of the way at 1000rpm.
If I leave it on there to long motor tries to stall which I understand. MY problem is the front carbie the motor has to be doing about 2800 rpm to get any movement out of the float. If I hold the unisync on at any revs it makes no difference to motor the motor keeps revving .. The revs will come down if i close the throttle. I thought I knew about carbies but this one has me stumped. I squirted wd40 around carbie but made no difference. to me seems like an air leak some where. Any one any ideas ???
The car is a Aussie built car stock standard. ignition ,valves are all correct.
On the road car runs ok . I havn't tuned carbies for a while thats when i found this problem.

Thanks
Ray



Edited 1 time(s). Last edit at 2016-12-09 02:17 AM by mgbal.

I freely admit I'm no expert here... in fact, my car not has only one carb at the moment....

But I'm thinking your engine may be running mostly only on the rear carb. They are connected through the manifold. Experts will chime in soon enough.

If they haven't been pulled and properly cleaned in awhile...I'd do that rather than troubleshoot. One, it certainly won't hurt and two, you may find/solve the issue while they're off. I can easily pull both of mine in 10 min...probably less time than you have into it at this point. May be a flaky gasket or seal. MUCH easier to deal with them off the car and on a bench.

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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...

Hi Ray,

I have written a series of articles, on how to tune HS4 SU carbies, attached.

You will note that I don't think a unisync is the best way to go.

Before attacking the carbies, set your valves and ign.

Herb

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I also had the same issue. I bought a Uni-syn thinking that was the best tool but the motor stumbled and died every time I tried to sync the carbs. It never gave me a consistent reading and it seemed like I never had enough time to view the ball before the engine died. I switched to the bent wire method (I bought the SU kit that included them but am guessing that you could fab your own for pennies.) and found it much easier and I could make sure the sync was consistent at any RPM.

Hi Ray,
I have no problems using my unisyn balancing tool, however you must, and I assume you are doing so, ensure that the foam base is absolutory flat with the throat of the carb.
However as mentioned, when tuning, the carbs are the very last item to adjust. Set the valves, then and if the car starts, the timing ,then the carbs. Make sure that each carb is disconnected from the throttle bar.
Good luck.

Two synchronizing points:

1. When using a Unisyn you have to adjust the tool depending on how much vacuum you are dealing with. It's easy, and the motor won't die if you use the tool properly.

2. You first must synch the carbs at idle, but you need also to check that the synch is holding at higher RPMs. Place a screwdriver or feeler gauge such that you can increase RPMs where the engine speed holds, and check for synchronization again. Usually, it will be off, and you will have to do a lot of fidgeting to get it where you get synch at both idle and high RPMs.

Whilst being neither an expert on carbs, nor having an SU experience, by following all the steps carefully, I returned an acquired family, and serially badly running MG to absolutely perfection (using a Synchrometer).

My emphasis is two-fold:

1. Any shortcut won't prove to be!
2. These cars can be made to run perfectly on factory kit and settings.

I make the latter point as I do read so often about having to 'tweak' something to make it run properly - it is just not the case for a UK car, although can't speak for US versions with their added equipment.

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MG Roadster 1980 UK Factory wire wheels



Edited 1 time(s). Last edit at 2016-12-09 01:44 AM by RJKflyer.

Hi Ray,

The links below may assist you.

http://www.mgexp.com/phorum/read.php?1,3373851,3374390#msg-3374390

http://www.mgexp.com/phorum/read.php?1,3377036,3377036#msg-3377036

Ross H.

Any day above the ground is a great day, especially "Driving Miss B".

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Any day above the ground is a great day, especially "Driving Miss B".



Edited 1 time(s). Last edit at 2016-12-09 03:30 AM by Tar Trekka.

For the rear carb - Open the little disc slightly until the engine will not die no matter how long you hold it on. If the disc is too tight you are choking the engine and not getting a true reading.

For the front carb - close the disc completely - if the car does not die then you are running on the rear carb only and you must have a massive vacuum leak on the front carb and/or no fuel flow on the front carb.

When I sync the ball rises to the first (lowest) line on the tool. for both carbs.

There is at least one tuning article out there that advises to "block" the piston up on the rear carb when adjusting the front carb. If this is done the carb that is blocked up is not developing a low pressure over the jet and no fuel is flowing - i.e. the engine is running on the non blocked carb only.

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I'm pretty much in agreement with Herb Adler (and others) who don't like the Uni-Syn. I'll admit that I only tried it once, but once was enough to convince me that there are better ways. The main problem with it is that it disturbs the very thing it's trying to measure. So, why not use the carburetor itself as the measuring device and not have the measurement disturbed.

Also, like Herb, I recommend the bent wire method. My own design for those bent wires differs a bit from Herb's, but either will get the job done. Some of you may find this link useful:

https://thosbryant.wordpress.com/2015/10/04/su-carburetor-tuning/

Tom,
Just scanned your very detailed and well photographed Volvo SU page. I would make a small disagreement on one point. I agree, we see too many useless fuel filters in the wrong place. Half of them are the stupid clear mesh ones that won't stop a piece of buckshot anyway. But, I am an absolute advocate of having one before the pump. I usually run a Carter gear pump. They make it clear in their literature it is imperative and it comes with one mounted to the input. I use a physically larger one ( big Wix can) so no matter how much crud our 50 year old tanks may pass, the pump and carbs stay safe. Many of those horrible little cubes also come with one on the input. Never had a vapor lock issue in any of my old cars. Now, this may depend on what is inside the tank. Some cars have a nice large fine mesh inlet screen. Ours has a pipe. I don't like a fine inlet screen as it is inaccessible and so maintainable. I prefer a pipe with multible holes just smaller than the pipe sitting right off the low spot.

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Cogito ergo sum periculoso

Scott,

I'd say that your favorable experience with fuel filters is probably due to the large size that you use. My own experience is with smaller ones, which I think we can both agree, are bad, and the very worst place to put one is upstream of the fuel pump, where the large pressure drop it induces will surely starve the pump for fuel... eventually... and leave the driver stranded by the side of the road. Meanwhile, I personally have never observed any negative effects from having no fuel filter at all in front of SU carburetors. The original equipment on my cars, which includes a wire mesh screen in the fuel pump, are adequate for the job at hand.

Of course, I don't have enormous quantities of crud in my tanks.

BTW, the reason putting a fuel filter upstream of the fuel pump can be so detrimental is that the pressure drop across the fuel filter can be high enough to decrease the pressure in the fuel between the filter and the pump so much that it's below the vapor pressure of the fuel, causing it to essentially boil, resulting in vapor lock and preventing the fuel pump from working.

Tom, also dangerous.



Edited 1 time(s). Last edit at 2016-12-09 07:01 AM by Tom Bryant.

Chuck the Uni-syn. Awful things. Get some wire and read Herb and Tom's instructions. (Tom's final wire design is a great improvement) The only reason the Uni-syn is popular is people love gadgets... and they haven't tried the bent wire method.

Adrian

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Ray, clearly your linkages are WAY out of adjustment or your front dashpot piston is hanging up on some physical obstruction. Remove and clean them for starters, then loosen all linkages and start over from scratch on balancing.

I must be the only guy who like Uni-Syns. Balancing is more visual and its used for only the very last efforts to fine tune the air flow into the carb, and only used for a brief moment - not held over the carb face until you choke the engine, adjusted to one for the highest readings you can get from it. The pic Adrian shows is probably the best tool for someone not very familiar with tuning SUs or someone who may only do it once a year. The Uni-Syn or better yet an anemometer is a tool that you can take real comparative measurements with, although yes in the wrong hands they don't help much. And many of the new Uni-Syns are terrible, awful things that do leak and don't work repetitively.

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jeff@advanceddistributors.com