SU Carburetor Rebuild (H-6)

Today’s post is about my recent rebuild of a pair of H-6 carburetors from a Triumph TR3B.

Again, the primary purpose of this blog is to teach people about their SUs and to show the types of problems that are often encountered during a rebuild so that others can benefit from my experiences. I try not to be terribly repetitive. Once I’ve shown something in one of my posts, I usually won’t go into much detail on that particular item in a later post.  None of these posts on SUs will stand on its own; there is information in most of these posts that pertain, not only to the specific type being discussed, but to other SU types as well. To fully understand the process, and the pitfalls, as they apply to all the various types of SUs, you really need to read all my SU posts, I’m afraid.  Sorry about that.

The flip side of that is, that each subsequent post should show new information that I haven’t covered before in my blog.  If I don’t have something new or unique to show, I’m not going to make a new post; it’s not that much fun.

SUs have gotten a bad reputation because not many people know how to fix their designed-in flaws.  I’m trying to rectify that for future generations of antique car lovers by showing how to fix those flaws so the carbs are actually better than new, that is, will work better than new and will last virtually forever without ever again needing a rebuild.  A repair or two now and again, perhaps, but never another rebuild.  When properly rebuilt, SUs are a very robust design and will perform extremely well for many, many, years without any significant attention from the owner.  On numerous occasions I have heard reports back from my customers, years after the rebuild, that they adjusted their carbs once and have never had to adjust them again.

Okay, enough of that.  On with describing this particular rebuild.

Here’s  a photo of the carbs “as received” by me:


Before sending the carbs, the customer e-mailed me this note:

“In the photo, you’ll notice that someone chrome-plated the suction chambers, which I find ugly, not to mention incorrect…. How about replacing the suction chambers?  If you do that would you also possibly replace the pistons, since I understand they are individually fitted to the chambers?”

As for the comment about the chambers and pistons being individually fitted to each other: I have heard that also.  But, you know, in all my 45+ years of repairing and rebuilding SUs, I’ve never seen a piston or a chamber that couldn’t be interchanged with any other of the same size and type. Every time I’ve ever tried it, for whatever reason, I’ve found that they would interchange perfectly.  That’s one thing that SU apparently took great pains to hold a tight tolerance on.  Of course, there have to be exceptions out there, but it’s not a great worry.

I have to agree with the customer; those chromed chambers are flaking, pitted, and ugly.  So I agreed that I could replace the chambers for him at no additional charge.  What I didn’t realize at the time was that the chrome plating was on separate, removable covers, not on the vacuum chambers themselves.  I probably should have realized this from the photo he sent me because of the funny shape of the “chambers”, which are much fatter at the top than normal.  But not having seen any Type H-6s in several years, it slipped under my radar until after I received the carbs in the mail.

Here’s a shot taken just after I removed those chromed covers:

cIMG_1327The dashpot dampers are used to hold the covers on.

Here’s another shot, showing the inside of the covers:

cIMG_1329Notice the red ATF that has leaked from the dashpot of the left (rear) carburetor.  A few comments about that:

1. ATF is a lousy product for this purpose.  It’s not thick enough.  You need a higher viscosity oil to provide greater resistance to upward movement of the piston and needle on acceleration.  That causes a bit of “choking” action on acceleration which reduces the hesitation often noticed on acceleration with cars that have SUs.

2. I highly recommend Mobil 1 15W-50 for use in dashpots.  It provides the damping needed, and it doesn’t seem to disappear from the dashpots as quickly as lighter oils (especially ATF) do.  So much less topping up is required.

3. By using Mobil 1 15W-50, you can adjust your jets a bit leaner and still avoid hesitation on sudden acceleration.  Whereas, with ATF, you will have to run a richer mixture at all times to keep the hesitation down to an acceptable level.

4. That ATF should have been all dumped out and washed away with carb cleaner before shipping the carbs to me.  The post office doesn’t like it when carbs leak oil (or gasoline) during shipment.  So, please, if you’re going to send your carbs to me for a rebuild, make sure you dump all that dashpot oil out and flush it out thoroughly with carb cleaner.  And dump out the float bowls, too.  Please.

In the photo above, notice the manifold gaskets are still attached to the carbs.  Normally, I remove such gaskets, and they often get destroyed in the process.  This time, because I had to change my drill press setup anyway to accommodate the longer carbs, I decided to leave the gaskets in place.  But, usually, if you leave your gaskets on your carbs when you send them to me, the gaskets will come back to you in pieces, so be prepared for that.

Here’s a shot of the front (right) carburetor’s fast idle mechanism, as received:

cIMG_1330Now, that is just plain wrong, and there’s no way it could have gotten into that predicament without the help of human hands.  There’s no way that carburetor could function properly in that condition.  We will see another shot of this later, after it has been corrected.

Next, a shot of the two “choke” return springs.  Sorry it’s out of focus:

cIMG_1333Notice that one has been shortened.  I don’t know why.  I replaced it with a new one.

Next, a shot of the front float bowl cover and fuel connection, as received:

cIMG_1334Notice that the tube is bent.  I made no attempt to straighten that tube.  Too much danger of its breaking.  I learned that one the hard way long ago.  If those tubes are bent, leave them alone unless you absolutely have to straighten them.  And don’t think heat will help; this ain’t steel we’re working with.  You heat brass up and it changes its crystal structure, weakening it, often to the point of disintegration.

Here’s a shot of the two throttle shafts:

cIMG_1335There’s very little wear on these shafts, about 0.002″ maximum.  At first I thought it was even less and was considering re-using the shafts, but after cleaning them up and rechecking them, with a more accurate caliper, I decided the customer might just as well have brand new shafts.  I try to save the customer a few bucks when I can, but this was just a little over the line.

Next, a shot of the two float bowl connections:

cIMG_1336Notice how these float bowl connections differ from those shown in my earlier post on Type H-4 carbs:

These are held on by “banjo studs”, rather than the “banjo bolts” used on the Type H-4s.  That results in the float bowls being about 1/4″ lower than they would otherwise be, and that reduces the tendency of gasoline to flow from the float bowls, through the jets, and down into the engine, often causing a backfire after shutting the engine off.

Next, a shot of the banjo studs:

cIMG_1337The right stud has been taken most of the way out, revealing the copper washer gasket.  The left stud is still in place, and the fiber gasket has expanded a bit, making it impossible for my wrench to get a bite on the hex head.  I had to cut the fiber gasket away with my knife before I could get my wrench on and remove the stud:


Here are the two studs and two gaskets after removal:

cIMG_1344At reassembly, I found another similar copper gasket and used that to replace the fiber gasket.  I believe these carbs left the factory with fiber gaskets.  This pair now has copper on each, which should prove superior.

Next, a shot of the two jets and piston springs:

cIMG_1346The jets are very slightly oversize, 0.1004″ and 0.1006″.  They don’t appear to be worn, rather, they appear to be perfectly round and to have been machined a bit oversize at the factory.  These jets are okay as-is, and I had planned to re-use them, but when we decided to order new shafts, we also decided to buy two new jets.  We want this thing to be right.

The springs are in perfect condition.  Far too frequently, I receive carbs in which one or the other, or both, springs have been stretched.  Not a good idea.  I usually replace a stretched spring either with a new one or with a salvaged spring that happens to match the other one.

Recently, however, I’ve started to un-stretch these piston springs.  I did that on the pair of HS-6s that I did immediately after this pair of H-6s.  It’s still an undeveloped art.  When I’ve perfected it, I’ll document that process in a future post.

Here’s a piece of linkage that the customer sent along with the carbs and asked me if I could do anything to fix: cIMG_1349

Note the wear on the ball stud.  Not much I can do for that, especially since I don’t have the mating half (socket), which is probably also worn.  The ball and socket need to be replaced with new parts.  I suggested a google search for “throttle linkage ball joints”, which turned up several possibilities.

Next shot, setting the drill press depth stop for reaming out the old throttle shaft bushings:

cIMG_1350Because the throttle bosses on a Type H-6 carburetor body are longer than those on the more typical H-4 or HS-6 types, and because my bushings are made about 5/8″ long, I decided to ream these bushings a bit deeper than my normal 0.500″.  So, I set the depth stop to 0.610″.

Here’s the resultant reamed hole:

cIMG_1351As mentioned earlier in my post on Type H-4s, the remains of the old brass bushing sometimes falls out, leaving an oversize and irregular hole.  That happened on this, the first hole on this pair of Type H-6s.  It didn’t happen on any of the other three bushings, but this particular hole is oversize, as seen in the next photo, and would not hold my normal bushing securely:

cIMG_1355That bushing was cut out with a “T” (0.358″) reamer, but because the bushing material fell out, it measures 0.370″.

So, I had to ream this first bushing out larger, as seen in the next shot:

cIMG_1357That hole was cut with a 0.384″ reamer, which is what I normally use when I have to install one of my oversize Delrin bushings.  The oversize bushings are 0.393″ OD, providing an interference fit of 0.009″.  Of course, I ream them to the same standard (0.3125″) ID.

Here’s a shot of the other bushing on this same carburetor body, after reaming:

cIMG_1358This time, the old bushing stayed in place, and I was able to use one of my standard Delrin (0.368″ OD) bushings.  The other two bushings, on the other carburetor, also stayed in place, and I used my normal 0.368″ bushings.

cIMG_1359Yes, indeed, that hole measures 0.358″, same size as the reamer.

Next is a shot of where the 45 degree flare fitting for the vacuum advance distributor attaches:

cIMG_1360As mentioned in my earlier post about Type H-4 carbs, that fitting frequently leaks vacuum around the threads.  I always seal the threads with blue Loctite to ensure that the vacuum advance distributor works properly.  Unfortunately, the customer neglected to send me the fitting.

The missing fitting looks like this one, which is from one of my Volvo B16’s with Type H-4 carbs:

cIMG_1360aThat’s the last one I have, and I don’t intend to part with it, so the customer will just have to find his own and apply the Loctite himself.

Next photo, cleaning up the carbs a bit:

cIMG_1361That is all I have for carburetor cleaning equipment, and that’s one reason why I insist the customer clean them himself first.  One of these days, I hope to outfit Le Garage Mahal with an ultrasonic carburetor cleaner.  Then maybe I won’t mind so much cleaning carburetors.  But, when I do that, you can be sure I’ll be upping my price to help pay for the extra investment.

Here’s a shot of the two needles from this pair of TR3B H-6 carbs:

cIMG_1362Notice that the needles are mis-matched.  The top one is an “RH”, and the bottom one is an “SM”.  According to my Haynes SU manual, the SM is the standard needle for TR3s and TR4s, whereas the RH is the “rich” option.  Notice that the RH needle is thinner at the tip, providing more fuel flow at wider throttle openings.

It should also be noted that, no matter what needle is used, carbs are adjusted at or near idle, and always provide whatever the mechanic adjusts them for at that point.  “Perfect”, “rich” or “lean”, you can make whatever adjustment you want.  It’s all in the eye and ear of the mechanic at the adjustment point; the specific needle profile is irrelevant until you actually drive the car and open the throttle up a bit.

Notice also that each needle shows some damage from improper handling, probably from someone trying to remove the needles from the pistons.  The RH needle is more badly damaged.  We decided to play it safe and replace both these needles with new “SM” needles.  I did have to charge my customer extra for the new needles.  Charging extra is something I almost never have to do, but new needles are expensive.

Next, the new shafts and jets, purchased from Joe Curto:

DSCF0745The new shafts are only a smidgen longer than the originals.  I only had to cut off about 1/4″ from one end of each shaft.

Here’s a view of the throttle plate and new shaft:

DSCF0746Next, a shot of the same throttle plate when rotated to full open position to properly align the throttle stop hardware:


Because the throttle shaft of the Type H-6 carb is so far forward of the manifold flange, not much of the throttle plate sticks out, so it’s hard to tell exactly whether or not that throttle plate is at the proper angle, so I to tried something new:

DSCF0748I took the throttle plate out of the carb, placed it in a vise as seen above.  Then, I placed the carb down onto the throttle plate as seen in the next photo:

DSCF0749That held the throttle shaft at the proper angle while I positioned the throttle stop arm as shown above and while I applied green Loctite (290, wicking grade) as shown next:

DSCF0751The Loctite holds the hardware in place while I remove the shaft from the carburetor, then drill it and pin it as shown in the next photo:



The shaft in the photo above is for the front carburetor, which is why it has the longer throttle arm, with both the idle and the fast idle screws in it.

Moving on to the rear carburetor and repeating the process we get this:

DSCF0754Notice how the idle screw in this throttle stop is overhanging the end of the carburetor throttle boss.  That’s because someone bent the brass throttle stop.  No idea why someone did that, but they definitely did it, and it was definitely a bad idea.

For comparison, here’s a nearly identical piece of hardware, but from a Volvo B-16 carburetor and stuck onto this Type H-6 carb shaft:

DSCF0757Notice how the screw is well away from the end of the throttle boss.  I probably would have just given my customer this piece of hardware, but unfortunately, it has the idle screw located in the wrong ear, so it wouldn’t work on this pair of carbs.

Next, we have a shot of both brass throttle stops placed on the same shaft:

DSCF0759Notice how bent the upper one (from the TR3B) is compared to the straight one from the Volvo B16.  This called for some straightening:

DSCF0765To straighten, I tapped both arms a bit with a small hammer, using the setup shown above.  I didn’t dare apply any heat because, as mentioned above, that upsets the crystal structure of brass materials, making them brittle.

Here’s the straightened throttle stop in place on that same carburetor body:

DSCF0770Now the screw is well away from the edge of the throttle boss.

Now, secure the throttle stop in place using green Loctite:

DSCF0772Give it about a minute to harden before drilling and pinning.

Here’s a shot of it all pinned in place:

DSCF0774The ear of the brass throttle stop is up against the aluminum stop.  The throttle plate is slipped out of position so that it sticks out far enough to see clearly whether or not it’s at the proper angle.  It’s okay, so slip it back into the shaft and insert the two shaft screws, then spread the screws at their slots.

Now we move on to installing the jets:

DSCF0775As explained in the H-4 post, you need to chamfer those holes in the sides of the jets.  This has been done using the round file shown above.

Next, assemble the jet into the jet bearing:

DSCF0777I’m showing one extra Teflon o-ring in the above photo.  The one at center top has some black stuff on it.  Actually, it’s more like “in” it.  There was some sort of defect in that o-ring, a groove with a lip of Teflon material over the groove.  Dirt got stuck in that groove and I couldn’t remove the dirt .  I’ve never seen that happen before.  I tossed the o-ring in the trash.  The point here is that you really do have to check everything.  Don’t assume that even a new part is good.  You have to check.

Here’s that same jet and bearing assembly, a bit further along in the assembly process:

DSCF0778Notice that the spring and adjustment nut have been left off.  That makes the jet centering process easier to do and also more accurate.

Next, that same jet after installing it in the carburetor body:

DSCF0779Notice how the jet sticks up about 1/8″ above the bridge, holding the piston up a bit.  That extra travel, provided by leaving the adjustment nut off, places the jet orifice at the very top of the needle, where the needle is thickest.  That forces the needle to be as nearly centered as possible.

Wiggle the jet around a bit, check for drag on the needle.  Make sure the piston slides smoothly, with no needle drag, then tighten the bearing nut:

DSCF0780After tightening, check the jet centering again by lifting the piston and making sure that the needle does not drag on the jet.

Next, pull the jet out of the jet bearing, and install the lock spring and jet adjustment nut:

DSCF0781Make sure that the jet bearing is tight and will not turn when the adjustment nut is turned.  Use a 5/8″ wrench on the adjustment nut, turn it all the way up, onto the jet bearing, and make sure that it can be turned only so far and that the jet bearing will not turn.  If the jet bearing turns, tighten it again, with a 13/16″ wrench, as shown above.

Now, shove the jet back into the jet bearing, as shown below:


We can get away with pulling the jet out and then re-installing it because we’re using Teflon o-rings.  If you were using the original cork seals, you would destroy those seals when you tried to shove the jet back in.

Make sure that the jet is the same side to as it was when you centered it earlier.  That’s because the jet orifice is often a bit off center, and if you rotate the jet 180 degrees, it is likely to be off-center.

There will probably be a marking on one side of the jet yoke to help you identify which side is which:

DSCF0783Check jet centering again; rotate the jet 180 degrees if required.  If that doesn’t fix any problems, you’ll have to go back and repeat some steps.

Now, install the banjo studs, using either copper or fiber washers and a 15mm wrench:

DSCF0784Next, install the choke mechanism and check for jet travel:

DSCF0785Make sure the jet opens sufficiently and that the spring returns the jet fully.  If you’ve done everything right, it should work well.  If not, you may have some rework to do.

There’s a problem to be seen in the above photo, one that I almost missed.  I should have noticed right away that the jet shown above was not opening as much as is normal.  Well, actually, I did notice that, but I missed the cause, and I nearly shipped these carbs back with the problem unfixed.

I mentioned this problem in my post on Type H-4s.  It is caused by unenlightened “improvements” that people sometimes make to Type H SUs.  If you’ve already read the Type H-4 post, you may think about what the cause may be while I proceed with a few more photos:

DSCF0801Above, we see that this jet is opening only 0.303″.

DSCF0804And the other jet is opening only about 0.359″.

Okay, here’s the cause:

DSCF0807Do you see that little bushing, lying on the “choke” arm, just to the left of the hole?  Someone made that little bushing and slipped it into the hole, then pinned it with the pin lying to the right of the hole.   They were probably most proud of themselves, too.  Well, that hole is made large and loose for a reason.  It permits the choke arms to move a bit and to operate the fast idle cam before the jets open.  That’s a good thing.  You need that.

Now, do you see the other pin, the one that the horizontal pin is pointing to?  It’s just to the left of the pin in the bottom of the jet, and it’s just above the hole that the loose (horizontal) pin goes into.  That little pin limits the travel of the jet lever arm when the choke is pulled.  There’s a little hook on the end of the lever arm, just above that superfluous bushing, that hooks onto and around that pin.  Well, with that bushing in the hole, the hook is thrown out of alignment, and the end of the hook hits the pin, rather than the bottom (“U”) of the hook. That reduces the amount the “choke” arm can travel, and that prevents the jet from opening properly, and that makes the engine hard to start when it’s cold.  It also prevents the fast idle cam from operating properly.

So, yes, that bushing is a bad idea, and it has to go.  It went back to the customer in a plastic bag.

Here’s a shot of one jet after removing the offending bushing:

DSCF0808This jet now opens 0.472″, as opposed to only 0.303″ previously.  Notice that the offending bushing is lying on the adjustment nut, where it will do no harm.

Here’s a shot of the other jet after removing the bushing:

DSCF0811This one is now opening about 0.475, instead of the previous 0.359″.  Again, notice that the bushing is lying on the bearing nut, just to show that it’s no longer taking up space where it’s neither needed nor wanted.

Remember that fast idle cam I said we’d come back to:

DSCF0812Well, here it is, now properly installed so that the “choke” can operate and so that the fast idle screw will actually produce a fast idle when needed.

Here we are, making progress:

DSCF0817But, we still need to install the floats, valves, and float bowl covers.

Here’s a shot of the front float bowl cover with the as-received (but not original) “Grose Jet” float valve and showing the float height setting:

DSCF0818Notice that that float was set way too low.  Off by a mile.

Here’s a shot of both float bowl covers:

DSCF0821The one in the foreground is the same one as shown in the previous photo.  The other one, in the background, is worse; it’s float height is even lower.  Someone didn’t know what he was doing. (I’m not pointing a finger at my customer; these were a spare pair he acquired somewhere.)

A close-up view of those Grose Jets:

DSCF0822Notice those fiber washers?  Such washers are not usually required on SU float valves.  I’m guessing that when a PO installed these Grose Jets, he removed the original float valves, which probably did not have washers, then installed the Grose Jets, with the washers, and didn’t check his float height.  The presence or absence of washers makes a significant difference in the float height.  You gotta check each time you remove or add a washer.

Below, we have the new Viton float valves:

DSCF0825One is fully installed; one is not yet installed, left with the threads visible to indicate that they were both installed without washers.

Next, we have the float bowl cover that was most out of adjustment:

DSCF0826Lo and behold, with the new Viton float valve, installed without a washer, and without having made any adjustments, the float height is now perfect.

Next, we have a shot of the other float bowl cover, the one that was low, but not as low as the one shown in the previous photo:

DSCF0827This float is now adjusted too high.

And here it is again, now properly adjusted:

DSCF0828The float lever arm is being held down, against the float valve spring, as far as it will go.  The gauge measures 7/16″ across.

Next, a view of the two carburetor “choke” arms, side by side:

DSCF0829I thought for a bit that one of the arms was bent.  But then I took a closer look:

DSCF0830And I decided that the bend in the choke arm was there for a reason, to provide proper clearance for the choke cable.

DSCF0833Here we are, all boxed up (without the newspaper packing material) and ready for return shipment.  I’m using a USPS Large Flat Rate Box.  I usually use a Medium Flat Rate Box, but these Type H-6 carbs are a bit larger than most, and I just couldn’t make them fit in the usual size box.

























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3 Responses to SU Carburetor Rebuild (H-6)

  1. Guy Pierce says:


    First off I want to say how much I appreciate you taking the time to explain & show your rebuild techniques. I wouldn’t have even considered rebuilding my SUH6’s otherwise.

    I have a 60′ Triumph TR3A & I purchased Curto SUH6 rebuild kit from TRF.
    However, I would like to replace the fibre washers( in the kit) with your Teflon jet washers & vitron o-ring housing & bolt washers. My float bowl is the variation “A” type, with the fibre washer at the carb housing junction.

    Do you supply these? Or can you tell me where & what to get?

    Best regards,

    Guy Pierce
    Chapin, South Carolina

    • Tom Bryant says:

      This is the sort of question that is best answered via telephone, or by e-mail. Given the fact that I don’t quite understand some of what Guy is saying, a phone conversation would work best here, I think. My contact information may be found on my “About” page.


  2. Pingback: SU Carburetor Rebuild (H-4) | Tom Bryant, Wiscasset, Maine

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