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MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Tuning Newly Rebuilt SU Carbs

I will be replacing my carbs with a set of newly rebuilt SUs. They are a thing of beauty, so I hope they run as well as they look :-) They are the earlier TC/TD type with Brass pistons and the tickler pins removed and the holes sealed. After my recent leak thru the rear carb tickler pin, "it's a good thing." I did install Grose Jets as I did in my current set after the leak. See pic. (the items under the dashpot caps are just there for shipping. New copper overflow tubes were included)

I have both a Unisyn and a Colortune, but even though I have read so much on the process, and memorized it I think, I have never done it before other than to adjust idle speed.

I will definitely use the Unisyn for airflow sync, as my hearing won't allow for use of a hose to the ear. I've tried my stethoscope and can definitely hear a higher pitch in the front carb, but have never heard of anyone using one :-)

So my question. I am trying to decide whether to use the colortune to adjust mixture, or to use the "lift the piston 1/32" method. I believe the Colortune would be much easier, but have read that they are not always reliable or easier as it would seem to read correct color.

So, I am open to advice and suggestions from this esteemed group.

TIA

Larry


L Karpman

Larry - The carbs look beautiful, who did them for you?

Regarding the color tune, I think that where most people have a problem with them is trying to use them in bright light. They work best if the ambient light is somewhat dim. I have used the color tune for many years on both the TD and the MGB and have had great success with it. Cheers - Dave
David DuBois

Thanks Dave. The carbs were done by a retired airplane mechanic and MG enthusiast in WA. I will provide his name off list to anyone that emails me at:

lkarpman at tx dot rr dot com

He is out of the country this month, but when he returns I will ask him if it is OK to post his name and contact. Can't see why he wouldn't, but I'd prefer to ask him first. I have emailed his name to you Dave.

Cheers

Larry
L Karpman

Very nice Larry. I have both but usually leave the color tune in the box. I used to be able to balance two SU's by ear with a piece of fuel hose. My hearing ain't so good any more so I use the unison for balance. The mixture I find I can set by using the lifter pins as well as using the Color Tune. If do use the Color Tune by all means do as Dve suggests and keep the ambient light low so you can see the true color of the flame.

LaVerne
LED DOWNEY

Larry,

I've recently have had trouble using a color tune because all the flames seem to be yellow now (ox. fuel?). So last August, I used the "get the fuel at .16 down from the bridge" to set the floats with the dash pots and dampers out, I set the floats that way because my fuel heights were wildly different using the 7/16 bar method. I then set the jets (using a dial caliper) .070 down from the horizontal bridge and use that as my starting point. I put the dashpots and dampers back on and then use the lift the dashpot method to get the mixture right. The easiest way to lift the dashpot is with a small screw driver and then instead of lifting, rotate the handle to change the angle of the blade. The most I've had to change my jet height after that is .010 inches (range .060 to .080)

I still like a rubber hose for balancing the carbs, but the unisyn works wonders as well.

Have fun,
dave
Dave Braun

I've never been around the 1 1/4" carbs Dave. Do they not have the lifting pins at the base of the carbs?
By the way I didn't mean to offend you about your web site. I might someday take you up on your offer.

LaVerne
LED DOWNEY

Thanks Dave. I assume you are using the term lift the "dashpot" synomously for "piston."

Cheers

Larry
L Karpman

LaVerne, the 1 1/4 SU H2 carbs do not have lifting pins.

Cheers

Larry
L Karpman

Oh Gosh, LaVerne, I wasn't offended, my quote was

"ps: I take issue with 'setting the bar so high'. There are a lot of pictures I wish I had taken. I really like your collection."

I meant that I think that collections of restoration pictures, regardless of how much verbage and how many pictures are posted are valuable, as we never seem to have enough angles and exact photos that we wish we had. If we decided not to publish because we are concerned about comparisons to other sites, then none of us would. For example, there is Chris Couper's site, Bud's T-Talk, and Dave DuBois highly technical offerings. Mine is not like their's, but it's different. I think your pictures are a great addtion to the hobby, and while my site is pretty useful and attractive, it is due to the software Gordon Lawson directed me towards, not my own talent. I would be happy to share that with you, helping or doing.

Larry, sorry for the thread jack. I'm tired (been writing FAA stuff all day) and I need to go to bed, but yes, it's the piston that is lifted.

warmly,
dave
Dave Braun

No problem Dave. If it wasn't for your site, and others, and this forum and it's archives, I'd be lost :-)

Cheers

Larry
L Karpman

Larry, I always use the step by step directions in the NEMGTR's T-series restoration guide. The unysin will help with the idle speed adjustments. Make sure you have oil in the dampers. It is very important to make sure the jets come all the way up after each adjustment (pull/push them home), and to adjust the connector bar so they are both fully up when the choke is pushed in. Lastly, the various fiber washers will require frequent snugging at first- the bowl to body at the bottom and the fuel inlets as well. I have always found the new ones compress and start to leak when new, eventuall this stops. George
George Butz

I've often wondered about the initial setting of the Unisyn. Obviously, using the center wheel the opening can be adjusted from full closed to wide open. Is there a preferred opening setting for the SUs?

Also, as my hearing prevents the use of a rubber hose, should I choose that route, I can plainly hear a difference with a stethoscope. The front carb that's on the car now has a higher pitch than the rear when held against the bottom of the dashpot covers. Is there a reason a stethoscope can't be used. Sorry if I've missed the obvious, but I'm a work in progress when it comes to SUs, and for that matter, piston engines. But I can talk turbine all day :-)

Cheers

Larry
L Karpman

Larry, quite a few years of shooting on USMC rifle and pistol teams raised havoc with my hearing. However, I can still ascertain the sound difference between out of sync carburetors. I'm not sure how you would use a stethoscope for this. You're not listening to the carburetor body. The open end of the tubing is placed so that the air entering the carburetor is going over the end of the tubing. The sound that you hear is determined by the velocity of the air across the opening.
Bud Krueger

As a crude test, you can also very quickly cover the opening with your palm. Easy to tell if you are way off- strong suction on one, minimal on the other. Idle speed should drop equal amount when each is covered. Remember it will run really lousy if you forget to tighten the connector. George
George Butz

One last question before I put my newly rebuilt crabs on tomorrow (Sunday). When "adjusting" and then "fine tuning" the mixtures, are the jet lever return springs left connected, or are they disconnected until the mixture setting is done?

TIA

Larry
L Karpman

Everybody must be out driving/working on their TD/TF. I really need an answer to this question.

Larry
L Karpman

Hello Larry

You can either leave them connected or take them off. The most important point is to ensure that the jet is fully "up" when making your final mixture adjustments.

just be sure to push up on the bottom of the jet while adjusting.

I usually leave the spring on when making the final adjustments.

J. W. Delk

Thanks! I believe I have all installed and fine tuned! When my wife returns we can more accurately check the rise and fall of RPM when the piston is lifter 1/32". Right now I'm 10 1/2 flats down on the front carb, and 14 1/2 flats down on the rear. I may need to adjust 1/4 turn here and there ( I hope :-)

For all you Grose Jet fans (like me), the latest 2 Grose Jets I got from Moss via LBCC BOTH locked in the ball down position on the very first start, causing overflow from the tubes! I have never heard of such simultaneous failures on Grose Jets, but I thought it would be interesting to note. Obviously I reinstalled the jets supplied with the carbs, and all is seemingly well with no leaks either (yet). Test drive later today.

Thanks to all for your info

Larry
L Karpman

Oops, my bad. I'm 11 1/2 and 14 1/2 flats down.

Larry
L Karpman

Hi Larry,

I'm a little late to the game so maybe this suggestion is coming too late.

I'd suggest junking the Unisyn and getting an SU toolkit as sold by Moss & others. Here's a photo:
http://mossmotors.com/Graphics/Products/Large/BAC-438.gif

(item B)

With it you can sync the carbs (not only at idle but check the sync at any rpm), center the jet, check float drop, and adjust the mixture. The wires in the kit make judging the 1/32" lift on the pistons a snap. The Unisyn is crude by comparison. My $.02 of course.... ;-)

Cheers!
Rob Edwards

Rob: I agree that the Unisyn is crude, but all I have. One thing that threw me on the SU tool was the starting point for the first carb. On the Unisyn you can get the airflow adjusted and see that the float is about in the middle of the tube, and adjust the air on the other carb to match (or close :-) What tells you you have proper airflow with the SU rod on the first carb? i could find no info on that, or I missed it.

In any case, I probably will get a set. Can you adjust the carbs with the SU tools with the air cleaner manifold on the car (removing the oil batch portion of course)?

I had the wife check the tach when I lifted the piston to confirm a rise in RPM, then drop on both carbs. I ended up 10 1/2 flats down on one and 13 1/2 flats down on the other, then set them both 1/2 flat leaner to compensate for reduced airflow and richer conditions when the air cleaner was installed. Hopefully the test drive tomorrow will convince me I'm done.

But, since BOTH Grose jets stuck open on the first filling of the float bowls, I'm buying a lottery ticket tonight, and avoiding thunderstorms for a while :-)

Cheers

Larry
L Karpman

Not quite sure I follow you re: proper airflow on first carb. You want the correct idle speed and both carbs to have the same airflow, but other than that there's no correct or incorrect. I typically do it as follows:

I slacken the linkage between the carbs and set the jet height on both carbs to about 0.050" with the depth gauge on my dial caliper. The I reinstall the damper pistons and chambers and insert the tubes & rods from the toolkit, making sure the wire pointers are perfectly aligned. I then start the engine (& let it warm up if it's cold) and begin to tune. If the idle speed is too high, I adjust it downward, starting with the carb that has it's wire higher than the other. If it's too low, I raise it, starting with the carb whose wire is lower. When the idle speed is correct and the wires are aligned, the carbs are synched. I then start working on the fuel mixture. By watching the wires, I can easily see how much I'm lifting the piston to judge the rise/fall in engine speed. I lift about one wire's thickness. I make a small adjustment and check again. I'll usually check the other carb as well -- there's a little interdependence. If the mixture is WAY out, I'll mirror any adjustments I make on the carb that I'm working on to the other carb -- if I started on the front carb and it's way out, I'll make the same adjustment to both the front and rear. As they get closer, I start making independent adjustments to them, and I alternate front and rear. Changing the mixture affects idle speed, so whilst I'm checking fuel mixture I'm also adjusting idle speed as I did at the beginning. When I'm satisfied that fuel mixture and sync are correct, I tighten the linkage between the carbs. Finally, I rev the engine up several times and ensure that both pistons rise the same amount at the same rate (do that with a Unisyn!)

You could sync the carbs with the filter manifold in place, but without the lifter pins you could not check the mixture. (Well perhaps you could by grabbing the toolkit rod and lifting, but I've never tried that). I have a Mk II, so I have H4 carbs with lifter pins....

HTH!
Rob Edwards

Rob...I'm glad someone else uses the old SU Tube and wire rod carb tuner. I've been using that set up since 1962 and find it easy, simple and accurate and you can be hard of hearing too.
colin stafford

Thanks for the info guys. I'm having trouble with fuel overflow with my front carb. I have a working Grose Jet in there. At first I could not figure out the problem. Then I discovered the float filled with fuel and sunk. Not very observant on my part.

I replaced the float with a good one from my old carb, and all was well. I took a 5 mile test drive at speeds up to 65 mph. All seemed well until I returned home. The front carb was overflowing again. I tried changing the float lever depth to 7/16, not good, 1/2 no good, 9/16 no good.

I note that the float bowl appears misshaped at the top and the cover cannot rotate freely. The position to hook up the front banjo correctly seems OK.

I have run the car at idle, and all is well. but after shutdown for 1/2 hour or so, it overflows again. Each time I check the jet and it is ok. The arm of the lever is straight and only the curved end has been adjusted.

is it possible the misshaped bowl is hanging up the lever?

TIA

Larry
L Karpman

I've seen a few brand-new Burlen needle valves that have just a tiny amount of casting flash on the three "fins" that centre them in their seat tubes. This makes the needle stick in the tube -- either the open or closed position. A little bit of scraping with a knife or a few strokes on sandpaper should be enough to fix the problem. See if your needles move freely & easily in their tubes....

HTH!
Rob Edwards

Have you checked the float lever (per David D.) to make sure it isn't dropping too far and jamming? Pretty simple system- the float has to be able to bob up and down freely, and the lever must move up and down to actuate the needle (or grosse jet). Either it is mechanically hanging up, or another bad grosse jet, or perhaps trash in the fuel? When you say after shut down- is that when it overflows when shut off, or at restart?
George Butz

George, it does it on restart. Dave's page on making a tab excepts the Grose Jet. This one is working fine. Here is a good example of what's happening:

The car was shut down just now for 1/2 hour. Was running fine. Then turning the ignition on, fuel pump goes click, click, click... very slowly then keeps it up until fuel come out of the overflow tube. Car has NOT been run. I take the float bowl cover off, check everything again, reinstall, turn on the ignition 1 click, it stops, no overflow. This is about the 10th time today I've done this. The fuel barely drops enough in my opinion in a half hour to cause the excessive float drop that Dave addresses. I'm confounded.

Larry
L Karpman

Duh -- Grose Jet. Ignore what I said.... ;-)
Rob Edwards

I think that Grose jets are great. Also I use the "lift the piston" method - I found colortune confusing - and after a short while I look at the plugs to see how the mixture is. Then I go by ear till the engine sounds revving properly at idle. I don't think it's critical - I'd say the spark advance is much, much more critical.
DLB Denis L. Baggi

I had the same issue- except it was when the car sat for a few days-I would whack the float chamber and it would stop. It was indeed a defective Grosse jet. I went through everything- cleaned and inspected it, etc., and replacing it cured it.
George Butz

Well, I'm happy to report that the problem seems to have been resolved (I hope :-). Everything was done/checked except changing out the float bowl cover. I put one of my old covers back on and have had 10 fillings, starts, runs, shutdowns, and restarts 30 minutes to hours to overnight in between, and all seems OK now. Hopefully I'm done with this issue.

While all this was going on, I kept thinking of junk in the fuel, and that old fuel filter I recently removed that was "before" the pump. The consensus seemed to be that was not a good idea, and could burn up the pump or points if it became clogged. In any case I would like add a fine particle fuel filter "after" the pump. I would be interested in any pics or advice from those of you who have done that, especially with the stainless fuel line from the pump to the rear carb.

TIA

Larry
L Karpman

Larry,

I wouldn't bother with a filter. There are already filters on the SU pump and the inlet to the float bowls that will catch anything big enough to clog a float bowl needle. I've driven my car seasonally every year for 25 years with the stock fuel line without EVER having a float needle not seal because of fuel debris. Additionally, I've been stranded twice by SU fuel pump problems, once on the TD and once on the B, and I figure I'm either due, or it just won't happen.

just my thought,
dave
Dave Braun

"I'd suggest junking the Unisyn and getting an SU toolkit as sold by Moss & others."

Interesting, because if you actually read the instructions that come with the kit (I have the one from Moss), they state that using the wires to balance the carbs is the crude way, and to use a Unisyn for better accuracy...

I just re-did my carbs today using the Unisyn along with the SU toolkit. Including engine warm-up, it took about 25 minutes. 15 of that was warm-up.

It helped that the last time I attempted this, I found that the float level was incorrectly set by a shop in Portland, and also that the float level valve was defective. Getting the ride float settings made tuning the carbs simple & easy.
Scott Linn

This thread was discussed between 25/03/2009 and 05/04/2009

MG TD TF 1500 index

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