MG-Cars.com
MG MGB Technical Best carb conversion setup? BBS discussion at MG-Cars.net
MG-Cars.net
Welcome to our resource for MG Car Information.
Recommendations
Parts
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 MGB Technical - Best carb conversion setup?
| I have a stock 1977 MGB. It runs well, but I would like to increase it's performance. What would be the best way to go if I wanted to convert to a pair of SU's? For example there's a set up on ebay now that claims to be ready to bolt on, but it looks like it might need an exhaust, among other things: http://cgi.ebay.com/ebaymotors/aw-cgi/eBayISAPI.dll?ViewItem&item=1806165556 I'm curious to know what others like and didn't like about their conversions. Thanks, Randy Olson 1977 Tahiti Blue/Tan |
| randy olson |
| In the Burgess book (How to Power Tune ... MGB ...), Peter doesn't give the HIF6 much recommendation, unless your engine has been heavily modified. Essentially, because of the way the SU carb works, the larger carb throat creates a situation where the car has less performance, because the piston doesn't move as much to move a given volume of air, thus doesn't expose as much needle. As a result, don't bother going with the HS/HIF 6 pair unless you plan on taking your B up well beyond 5000 RPM. I converted from the ZS on my '75 to dual HIF4's, and they are wonderful. Be warned however, the brake booster gets in the way, preventing an _easy_ swap. I took some pictures of my work, they can be found at http://www.wsu.edu/~aaronpw/MG/SU/index.html Also, on the exhaust, don't bother with a header. Get the stock manifold for the carb setup. They flow as well as any header (or close enough), and don't leak. |
| Aaron Whiteman |
| Randy. I don't have a rubber bumper car and have not made a carb conversion, but if your car is stock the set on eBay would probably be too much carburetion without lots of other performance modifications. Most people seem to go with the downdraft Weber or a set of SU HS4 carbs. I'm sure you'll get lots of responses plus the archives is loaded with information on the subject. Clifton |
| Clifton Gordon |
| Randy. As both Aaron and Clifton have noted, the HS-6 swap (if that is what is being auctioned) is not the best way to go. First question is, "Do you have to meet emissions standards/testing". If that is so, what will your local inspectors allow? In my area, where we have to have our cars tested, the "waiver station" will allow us to use the Weber DGV carb since the proper Z-S carb is not, currently, available new. The Waiver Station has the final decision on "what is correct/allowable". If you do not have to meet emissions standards, the dual SU HS-4 or HIF-4 carbs are excellent. As to the exhaust manifold. There are several options. First is to modify the existing, combined, intake/exhaust to an "exhaust only" manifold. This requires someone knowledgeable about seperating the two and capable of welding up the hole in the exhaust when the two are seperated. I am currently using such a system. The second choice is headers, which allow the introduction of an "after market" cat into the system. It works and will meet emissions requirements. The last area is the original design of exhaust manifold. If you do not have to meet emissions testing, this is the way to go. You use the earlier model exhaust system with the early manifold. Be aware that you have to match the exhaust manifold to the intake manifold. They must be of equal thickness or must be shimmed to the same size. An adaquate pair of dial calipers are available from several sources (Harbor Freight, Rutland, Enco) to measure this. The HS and HIF intake manifolds and exhaust manifolds were of slightly different sizes--you need to make sure you have the same thicknesses or shim to make them the same. Bottom line--SUs are the best when no emissions standards, Weber DGV is the best when there are emissions standards, Zenith-Stromberg is a Triumph hold over and is junk. Les |
| Les Bengtson |
| Aaron, Great photos of your conversion. Questions: Are new exhaust pipes necessary, or did you just want to install them? Is a new heat shield required? Are the air filters adequate for keeping the engine clean? Did you end up moving the brake booster; it looks like it's still there in the last picture. The heater pipe...are there more "correct" ways to deal with the pipe rather than go with the copper tubing? Overall, this looks just like I was hoping. I expect that the difference in performance was very noticeable. Les, No emissions here in NY to consider. I have already removed the catalytic converter, plus the air pump, with good performance results. On the exhaust, it sounds like the best way to go is to find an earlier exhaust manifold AND matching intake manifold. This would preferably be from the same car that the carbs came from, or at least I would have to confirm that they are compatible, which seems a little more troublesome. Other questions: What about the choke? I have a standard automatic choke now; would that need to be changed to a manual? Is the stock distributor OK with new carbs? When I desmogged the car I followed the instructions of John Twist. This was a step-by-step guide that was easy for a normally-not-a-mechanic-type person. Are there similar guides for carb conversions available? Anything I missed, but should have asked about? The consensus so far seems to be that the HIF/HS4 carbs are the way to go. I would need to have a compatible exhaust and intake manifold. I would no doubt need another heat shield. I would have to reroute the heat pipe. The brake booster would have to be______________(?). And, I would probably have to install a radar detector! Thanks for the tips from all. This is fun! Randy 1977 MGB Tahiti Blue |
| randy olson |
| > Are new exhaust pipes necessary, or did you just > want to install them? No, and yes. If you keep your stock exhaust, you will have to do some work on them, as the stock setup for earlier B's is a whole lot different up front. Easiest way (and the route I eventually took) is to buy the front pipe for the appropriate year (71-74 for me), and let a competent exhaust guy do the work for you. You will likely have to put the manifold on yourself, they don't like doing that. > Is a new heat shield required? I used the parts that came with the HIF4 set. Essentially, none of the Zenith parts will work. > Are the air filters adequate for keeping the engine > clean? Yes. In a year of driving, the filters started to look a little dirty, and I live in Eastern Washington, we get dust. > Did you end up moving the brake booster; it looks > like it's still there in the last picture. Nope, but one of my filters got a bit mashed. I also had to install smaller spaces between the heat shield and the carbs. > The heater pipe...are there more "correct" ways to > deal with the pipe rather than go with the copper > tubing? Yes, and I did that too. I bought the plumbing for a 74 B, and it hooked right up. The piping for 75+ will not work, it runs right where the carbs need to be. I will try to get another picture up soon, but my digital camera is no longer compatible with my computer's OS... The ugly copper tubing was a temporary measure to get the car on the road. >Overall, this looks just like I was hoping. I expect >that the difference in performance was very noticeable. It was noticeable, but didn't blow me away. The distributor and cam for 75+ isn't really tuned for performance, and the carb change only helped so much. I did this as a "first step", the distributor is next, then I plan of buying _new_ HIF4 carbs that are correctly ported to take advantage of the dizzy (I currently am running without vacuum advance). Finally, the cam well need to be replaced, though that is sometime in the (more) distant future. > What about the choke? I have a standard automatic > choke now; would that need to be changed to a manual? > Is the stock distributor OK with new carbs? Yes, it will. My car's PO had already converted the Stromberg to manual choke, so I didn't need to worry about it. If you go with HIF carbs, they will work without any problems with your dizzy, as they were already set up for manifold vacuum advance. > Are there similar guides for carb conversions > available? I am not aware of any, but there might be. If you buy used carbs, rebuild them! Also, if you buy the K&N filters (if you leave the brake booster, you will), you would want to run a slightly richer needle to compensate for the increase airflow. > Thanks for the tips from all. This is fun! that is the important part! |
| Aaron Whiteman |
| Randy. Aaron has covered most of it. If it were me, I would change out the distributor to one that could be used with the 68 spec HS-4 carbs. I would buy the carbs new. That way you do not have to rebuild them. The needles supplied are not correct. Go to AAA needles from Joe Curto (1-718-762-7878) in New York. Also, ask his advice. He is one of the best SU people in the US. For intake/exhaust manifolds and heatshield, contact Bob Schaulin at gbmg@aol.com and mention my name. I have bought a lot of parts from him over the years and have never gotten a bad part. He has a good reputation on E-bay. You might also try to get an early specification dizzy from him and have it rebuilt by John Twist of University Motors Ltd or get an Aldon modified distributor for the 67 spec car. After that, you will be getting all of the power your engine is capable of developing. The next step would be cylinder head modification. Mike Brown in the US and Peter Burgess in the UK are probably the two best people in the world on this. Examine what both offer and decide what you want. I was running my RB, with full emissions gear, today at 80 on the interstate (speed of traffic in my lane) with no problems. I used to cruise at 85 on the German Autobahns with no problems with a stock, emissioned 79B. If you want more than this, you need to start a complete program of suspension upgrade. The RB cars tend to begin to float a little at much over 85 and the handling is not the greatest at higher speeds. While you are at it, make sure your brakes are functioning properly. The best thing about MGs is how well balanced they are. If you are going to add more power, make sure the other systems--suspension and brakes, are up to the task. Les |
| Les Bengtson |
| I found a set of carbs described as follows that appear to be correct for my car: "a set of 2 used HIF4 SU carburetors from a running 1974 MGB. I just finished rebuilding the 2 carbs with new gasket kits when I decided to switch to a weber setup. So these are in good order, with new gaskets. The setup inludes the 2 carbs, the linkage between them, the intake manifold, the 2 carbon spacers, heat shield (cracked, but repairable), 2 complete air cleaners with elements (used - need sand blasted and straightened) and the air cleaner bolts" If I got these could I use my existing exhaust manifold and simply shim things to match? I would need a manual choke, plus new air filters, probably. These carbs would probably need a rebuild. Comments? Randy Olson 1977 MGB |
| randy olson |
| The ZS manifold is nothing like the SU. While with enough work, you _might_ get it to work, you are losing half the reason to go to SU's in the first place. The ZS manifold is utter sh*te. |
| Aaron Whiteman |
| Randy- The Rubber Bumper intake/exhaust manifold is a one-piece affair. You'll need the exhaust manifold to go with the SU carburetors and their intake manifold. The aircleaner boxes will be worthless to you as your master cylinder projects too far into the engine compartment for them to fit. You have two options if you go this route: 1) Use a pair of K&N conical airfilters (easy, but restrictive and can cause funky running due to induction pulse problems at high rpm, such as above 5,000 rpm), or 2) Fabricate a custom box to put onto the carburetors and run a hose to a remote airfilter housing box. (Hard, but best for performance if you keep the OE power brakes.), or 3) Get rid of the intrusive master cylinder by mounting the earlier non-servo-boosted master cylinder and pedal box, complete with pedals, and modify your brake lines to fit, and then run a pair of 6" x 3 1/4" K&N airfilters. (Lots of work, but the best way to get more performance.) If you're interested in this idea go to http://www.theautoist.com/ and then go to "Garage". |
| Steve S. |
| I used a complete HIF-4 setup off a '74-1/2 car on my '79 for quite some time before I started my V8 conversion....only, I used a PECO header & a custom 2" aluminized exhaust made up with 24" center & 14" rear cherry bombs....then, I went to Moss & got a set of offset cone K&N filters that cleared the brake booster easily & allowed me to use the standard width carb spacer.... ...not using the heat shield or reducing the thickness of the spacers could affect the fuel as it enters the carbs/engine because of heat, etc.... ...I also changed from the electronic distributor to a points type one & used a Lucas Sports Coil... ...the whole setup was easily accomplished & made the car a sweet thing to drive... |
| Tony Barnhill |
| Guys, If the exhaust manifold on my MGB is changed out with one from, say 1974, can I maintain the muffler and tailpipe from the '77, which is newer and doesn't have the catalytic converter anymore? In other words, is there an adapter piece that would run from the '74 exhaust manifold to the '77 muffler pipe? Or do you have to also buy a new exhaust pipe, muffler, etc.? Putting all the pieces together starts to add up: HIF-4 carbs Rebuild kits Linkages Heat shield Pancake-type air cleaners Manual Choke Intake manifold Exhaust manifold Exhaust piping and muffler Anything missing? Thanks, Randy Olson 1977 MGB |
| randy olson |
| Randy, I did his conversion last year using HIF4's on my 76 B. It took me about a year before that to gather all the parts, as I went with a Peco exhaust header & pipe system, as well as an in situ rebuild to the the motor. A cracked head started it all for me. You have listed most if it, you'll also need a new throttle cable for an earlier 70's car. You may need a shorter return spring for the carbs as well. I think the spring from the ZS is longer. Keep the check valve for the servo from your existing manifold, in the event one is not included with the SU type you purchase. Mine fit nicely. Start collecting the parts for next year or whenever as you can afford it. You won't be sorry once you get it done & sorted out. Pete |
| Pete Haburt |
| You may be able to get away with only buying the front pipe, and adapting it to the rest of your exhaust. |
| Aaron Whiteman |
| Is there any preference between the HS4 and the HIF4 carbs? It seems both are readily available. Randy Olson 1977 MGB |
| randy olson |
| HIF-4 will fit in the confined area by your barke booster w/o modifications..& the K&N offset cone air filter will work with it.... HS4 will be somewhat trickier & require some mods because of its external fuel bowl....you'll have to slice the insulator in half & take a chance of the heat oiling your gas before it gets into the carb itself...plus, the K&N offset cones won't work & you'll have to go to those little constricting pancake filters (&, there'll still be a chance that you'll bust the fuel bolw on the booster if your engine mounts are bad & the engine rotates under torque) |
| Tony Barnhil |
| Randy, In past commentaries on this swap, it has been noted that the HIF-4s were superior in at least a couple of important ways. One was the integral float bowl system, alluded to in Tony's comment above. The other was the choke system. I am sure there are other reasons, but consider this, that the HIF-4 is simply an evolution of the prior HS-4 design, and SU attempted to improve the operation of their carbs, both because it was just good engineering, and because the impact of emissions standards requirements to maintain tighter control of the fuel/air mixture under all conditions possible. In short, you should be getting a better and improved designed carb with an HIF-4, and those who have spent time tuning them and using them, seem to confirm this. FWIW. |
| Bob Muenchausen |
| Next question: Are the inlet manifolds the same for the HS-4 and HIF-4 carbs? In other words, can you use an earlier HS-4 inlet manifold if you are mounting HIF-4 carbs? Randy Oson 1977 MGB |
| randy olson |
| Randy. No, the intake manifolds are not the same. But, to the best of my knowledge, the difference is in the thickness of the flange. The carbs will bolt up to either manifold, but I am told that an HS heat shield should be used with HS carbs and an HIF heatshield should be used with HIF carbs. That being said, my 68 GT had an old set of HIF carbs on it when I bought it. To pass emissions, I purchased new HS-4s and bolted them up to the heat shield, then made my own linkage out of 5/16" drill rod and some scrap 1/8" cold rolled steel. Been working fine for several years now. Les |
| Les Bengtson |
| I am also looking into converting my 77b ZS carb to SU's. Does anyone know what carbs are on a 73 1/2 mgb? Are they HIF-4? |
| Kate |
| The 73 should have HIF-4 SU's. Clifton |
| Clifton Gordon |
| This is an easy one: If you buy an aftermarket header, to go along with, say, the HIF-4 intake manifold, how do you know if the mounting flanges will be the same thickness? Or, do you have to shim the header flange to match the inlet manifold flange. And, if we assume that shimming is necessary, can the shims be held in place with a gasket sealant (if a welder is not available) until it's bolted up? Getting closer, Randy |
| randy olson |
| Randy. This is an easy one--you measure. Les P.S. To get into the somewhat less easy, you need to use a set of dial calipers or micrometer to measure the "average" thickness of the intake manifold flange. I found a slight variation in thickness from pad to pad on the intake manifolds I have had to measure. This is in the neighborhood of .003" variation (plus or minus). Then, measure the average thickness of the flanges on the exhaust manifold or header. Subtract the larger number from the smaller and that will give you the thickness of the shim material you need. As to how to hold on the shims if you cannot weld them on (too thin) or do not have a welder available--I would try Super Glue first and five minute epoxy as my second choice of adhesives. Use as little of it as possible while still holding the shim in place. With the heat involved, no standard adhesive will provide a secure, long term bond. Hence, look for the one that will be the easiest to use and create the least mess. I love simple questions. Les |
| Les Bengtson |
This thread was discussed between 24/02/2002 and 03/03/2002
MG MGB Technical index
This thread is from the archive. The Live MG MGB Technical BBS is active now.