Midget Carb Upgrade

[Loren]

I guess this warrants its own thread.

I'll be swapping the original Zenith-Stromberg 150CD with automatic water-activated choke for a 175CD with manual choke. This will give me the same carb area as the popular SU HS2 (1.25") dual carb setup, and should yield a notable improvement in power.

I've got the carb, a replacement stock manifold to modify, carb mounting spacer and gaskets, and a carb rebuild kit. Still need to get some carb mounting studs and a choke cable. But, I think I have everything else that I need.

Got started on the manifold today. This was exceedingly quick and easy since I bought the right tool for the job! I got an electric flex-shaft die grinder with an 1 x 1/2" aluminum cutting burr.

[Loren]

Here's where we started. Notice I'm moving the center of the hole down a bit. That's because on the top side, if I centered the hole, I'd have to blend in with the back wall of the manifold. Moving it down a bit, I can leave that alone. The bottom side is open down there, so just a simple straight cut.

175CD Manifold Before.jpg (146.83 KiB) Viewed 8667 times

First pass, this didn't take more than 5 minutes, if that. Man, it's cool having the proper tool for a job! I'd have tried to do this with something like a round file and/or a drum sanding attachment on the drill before. Would have taken forever, if I could have even done it, and would have looked like crap.

175CD Manifold First Pass.jpg (160.25 KiB) Viewed 8667 times

Now, I just need to cut past this lip on the back side.

175CD Manifold First Pass Close.jpg (132.41 KiB) Viewed 8667 times

And here it is, roughly cut to shape.

175CD Manifold Rough Cut.jpg (175.39 KiB) Viewed 8667 times

But, wait... is that daylight I see?

175CD Manifold Hole Closeup.jpg (126.04 KiB) Viewed 8667 times

Sure enough! There's a hole. One just like it on the other side. Not a lot of extra meat on the bottom side. But, it should be easy enough to patch over with some JB Weld.

175CD Manifold Hole Outside.jpg (148.51 KiB) Viewed 8667 times

Here's the final cut with the carb spacer that it needs to fit. Nice!

175CD Manifold Final with Spacer.jpg (172.95 KiB) Viewed 8667 times

Filling the old carb stud holes with JB, as well as the newly created ventilation. This being a carb, it's just going to have a mild vacuum on it, no pressure. I think JB Weld will work just fine for this.

175CD Manifold JB Weld.jpg (134.37 KiB) Viewed 8667 times

Once all the JB Weld has cured (I may put a second layer over my hole to add thickness), I need to drill and tap the mounting stud holes.

[Loren]

This is coming together much quicker/easier than expected. Having the right tools helps a lot. I guess I have some experience in my old age, as well.

Holes patched and smoothed. Stud holes filled and redrilled and tapped. Not perfect, but adequate.

If I can figure out how to connect the throttle cable, I might just slap this on the car tomorrow. Only have two carb studs, but I could test it with that.

New studs are on order, and I'm about to order a choke cable, and start figuring out how the throttle cable connects.

175CD Manifold Threaded.jpg (152.15 KiB) Viewed 8661 times

[Loren]

Okay, let's see if I can make sense of the carb linkage and other stuff that I'm going to need to deal with that's different.

First Hurdle: The replacement carb doesn't appear to have a means to attach a throttle cable. That's weird.

175CD Throttle Shaft.jpg (154.63 KiB) Viewed 8660 times

Further research indicates that because it's from a dual carb installation, the throttle shaft comes out the other side, connects to the throttle shaft from the other carb, and there's a linkage that connects in between them that I don't have. (I don't really need it... but, I don't have it... which aided in my confusion)

The stock carb throttle attachments looks like this.

Midget Original Throttle Shaft.jpg (125.38 KiB) Viewed 8660 times

Okay, now I'm there. I just need to remove the nut on the end of the 175's throttle shaft, remove a little spacer and replace it with this little linkage bracket. Then I can connect to the stock linkage. Cool.

This is the original throttle linkage. It translates the "pull" motion from the throttle cable to a "push" motion downward on the throttle shaft. I'm sure it also does whatever geometry translation is required to get the pedal "throw" and linearity where it's supposed to be. Probably don't want to try to reinvent that wheel.

Midget Original Throttle Linkage.jpg (182.01 KiB) Viewed 8660 times

Second Hurdle: The replacement manifold doesn't have bosses cast into it to mount the throttle linkage assembly.

Here's a shot of a clean manifold similar to the one on the car. It's got a vacuum port, an EGR port, and two bosses to mount the throttle linkage. (the one pictured doesn't have those two bosses, can't find one that does other than the one on my car!)

150CD Manifold 1976.png (282.82 KiB) Viewed 8660 times

And here's the manifold I modified. It's an earlier manifold with less "stuff" and larger ports. It's got a vacuum port (different location, but that's fine), no EGR (also fine), and no bosses to mount the throttle linkage (dang it)!

175CD Manifold with Carb.jpg (137.49 KiB) Viewed 8660 times

I looked to see how the throttle linkage was done on the earlier Spitfire that my modified manifold came from. If I could find one that was complete with all the mounting hardware, it MIGHT be a solution, though it probably also requires mounting holes on a specific manifold to work. Modding up some way to mound the existing throttle linkage is probably the way to go. I'll just drill and tap a couple more holes in the manifold and fab up an interface bracket.

Third Hurdle: This photo shows the original carb installation on my car, as it sits. Mislabeled the fuel line. That line actually goes to the evap canister, the fuel line goes in below that.

Midget Original Carb Setup.jpg (253.63 KiB) Viewed 8660 times

Things I'll need to deal with:
Water Choke bypass - no problem, just loop the hose. But, I'll have a big ol' coolant leak if I forget to do it!
Intake Manifold coolant - no problem, the new manifold is set up just like the old one. Hook it up.
Fuel - no problem, should be the same, even on the same side of the carb.
Evap line to the carb - I'll need to find a place to plumb this unless I want to delete the evap system. (I'd like to keep that)
Breather - There's no port for this on the new (old) carb. I guess it needs to go back to the airbox pre-carb.

The only other oddity is that the new (old) carb has a throttle bypass valve that has a hose inlet on it. It's a fresh air input to keep the car from going rich and backfiring on decel. I guess that'll need to go back to the airbox, as well.

I've got some homework to do on the evap system, PCV system, and the carb itself. None of it is difficult, I just need to wrap my head around how each system works before I go trying to modify it.

But, at least I got my choke cable ordered.

[Native]

Winning.

[Loren]

Digging into the carb side of things.

My new carb is a 175CD, of which there are 6-8 variants. This is an early variant, specifically, a "175CD-2", pretty simple.

It has but three ports on it:
Fuel Inlet - Okay, that's easy.
Air Bypass Valve Vacuum - It's supposed to add air under extremely high vacuum (when you slam the throttle shut) to prevent fuel from igniting in the exhaust.
Ported Vacuum - Vacuum source just before the throttle plate, not normally used. In fact, the one on my old carb has a rubber cap over it.

I just read up on the bypass valve. It's an emissions thing more than anything else. I don't mind a little exhaust burble on rapid decel, as long as it's not excessive popping. Those in the know say to block off the valve and it's vacuum source and you'll likely not even notice the difference. So, that's what I'll do.

So... carb connections, other than mechanical throttle and choke cables, are nil. Hook up the fuel and go!

It's got a temperature compensating valve gizmo on the side of it. Most people say they usually don't work, anyway. So, I'm gonna ignore it for now. It's got a bimetal strip in it that opens when warm to let the engine run slightly leaner when warm.

As far as adjustments, it's either got an adjustable jet at the bottom (that's what the needle fits into to regulate fuel), or an adjustable jet, possibly both. I need to look into which setup this particular carb has. I don't think there's anything else disregarding the temp comp and the bypass valve. Just throttle stops for idle speed and such as that.

Fun fact: The choke on this carb is not actually a choke. It progressively adds more fuel by uncovering a series of 5 holes in the choke mechanism, and couples that with a cam that trips a fast idle setting on the throttle. Pretty basic.

I need to come up with an air cleaner. Could modify the stock air box, but if I'm going to modify, I can do better. If I want to spend money, there's a K&N pancake filter available for $100 or so. Meh.

The other issues, I'm still looking into.

There are more ways to handle crankcase ventilation than you can imagine, and an equal number of opinions on which is best!

But, that's what I'm up against. Crankcase breather routing (PCV valve or no?) is the big question. The current setup has the breather hose going to the carb, and then another hose from the carb to the charcoal canister. Is there some kind of PCV valve internal to that carb? Still digging, but I'm betting that's the answer. Just stick a PCV valve in that breather > cansister line and call it good.

As far as vacuum, I should be able to hook that stuff up to the manifold, just as it is now. It's got a T, one side goes to the canister, the other side is the distributor vacuum advance.

Throttle linkage. A lot of cars seem to use a throttle linkage assembly that looks like it mounts directly to the carb. I'm not sure why they went this way on this carb... possibly due to how they had to route the throttle cable fitting the 1500 into the Midget. The original engine in the car has the carb on the other side of the engine.

If I went with the carb-mounted linkage, I'd have to route my throttle cable directly across the engine rather than around the front of the valve cover as it is. I think the Spitfire did it that way, sort of. It came around the back side of the valve cover. But, the spit doesn't have that big heater/blower box right there that the cable would have to route around. Probably why they didn't do it that way.

So, I think I'll stick with plan A for throttle linkage. Just make an adapter plate and thread some holes into the intake manifold to mount the stock linkage.

[Loren]

Carb studs arrived. I just finished going through at least 3 iterations of ideas for an air filter setup. Finally decided to just order one that's "not quite right" from Amazon for $17. All I have to do is modify the base plate to fit the carb. Simplest solution.

I guess Friday will be my day to try to fab up some kind of adapter to make the throttle linkage work.

[Loren]

Choke cable arrived. I need to check and be sure it's long enough. And, I'm annoyed... I selected one with a chrome knob with the letter C on it. I got one with a T on it. Not worth the trouble of trying to return it ($10 eBay purchase). I'll put a sticker or something over the T. Fuckin' people.

I guess I'm running out of excuses to not do a test-fit, huh?

I did drive the car once the other day. (after tightening up the loose choke mechanism) Drove it again today and got to experience the difference between a warm afternoon drive, and a cool evening drive. We've all been there. Cars always make more power with cool air. But, it just made me realize what kind of gains I might get with the bigger carb if it's tuned right. It's gonna let more air in. Should be noticeable.

[Loren]

Didn't do shit on Friday. But, I did get to doing my "test fit" today.

Of course, nothing is ever as easy as it should be. I've owned Spitfires with this same 1500 engine before, I should know. One of the notoriously difficult things to do on this engine is remove the lower two intake manifold nuts! I spent a couple hours on it, and had to fabricate a special wrench for the job. But, I got the old manifold removed.

Here's the miserable lower intake nut. Can't put a socket on it. Can't reach it from the top or the bottom. It's a pain.



Check my bitchen wrench, tho'! Man, it's cool to have some fab tools and skills.



I thought I'd be able to just set the old manifold aside, or drop it down and fit the new manifold without completely removing everything. Nope. Not enough slack in the coolant lines. So, I ended up removing the EGR plumbing, the coolant lines on the intake manifold AND the carb, and all the other breather/evap and vacuum lines. Only thing I got away with not disconnecting was the throttle cable. (which would have actually been the EASIEST thing)

Got it on the car. Yes, I tried to start it. Figured I might get lucky, and it would have been inspirational. Didn't happen. I feel like the float bowl never filled, which is very possible. Either the float is stuck, or the valve is stuck. No fuel, no fire. I'll just do what I should have done to start with: disassemble, clean and rebuild the carb.

Gratuitous "Yes, I actually drove the car last week" Photo:


Hoping I can keep the down time to a minimum!

To Do:

• Fit bypass hoses for the intake and carb coolant lines. The new carb doesn't have it, it never gets cold here, and if it's coldish, I'm probably not driving, anyway. My opinion is that I don't need to put coolant through the manifold. It doesn't need to be warmed up sooner, and it will stay plenty warm enough otherwise.
• Fabricate a bracket to attach the stock throttle cable assembly to the new manifold. Lots of other ways I could do this, I just figure this is the simplest and makes it easier to go back to the stock setup if I want to.
• Install choke cable and figure out how to connect it
• Rebuild carb and fit whatever lever I need on the throttle shaft to be able to connect throttle linkage to it. (it's set up for a dual carb, so doesn't have the lever)
• Delete the EGR
• Fit an air filter assy with a port for the breather hose, and find out if that hose needs a PCV valve in it
• Fit a vacuum port on the manifold for both the vacuum advance and the evap system
• There's another evap hose that went to the old carb from the canister... need to figure out where to put that
• I should probably fabricate a heat shield to go under the carb & manifold to block some of the exhaust heat

[Native]

Nice pic.
Long list.
Get to work!

[Loren]

Ordered some parts. I'm good at that part of the process!

Too lazy to go to the parts store. Ordered 6 feet of heater hose on Amazon. Also ordered a 3/4x16 plug that should fit the EGR port on the exhaust manifold. I don't have to plug the one on the intake because my replacement intake has no EGR port.

Also ordered some vacuum hose, hopefully it's the right diameter.

Got some info from MG forum people. Should be able to just run the evap hose and the breather hose both to the air cleaner. That's easy. And no PCV valve required. Somebody recommended keeping the intake manifold water pipe, and their reasoning is sound. It's a crappy log manifold. Keeping it warm will help ensure good fuel atomization, keeping the temperature CONSISTENT will help the engine run more consistently. Easy enough to keep the manifold in that loop of hose, no biggie.

I guess I'll find some time this week to get the carb back off of the car and get ready to rebuild it.

[Loren]

Nearly 2 weeks have passed, and I've been procrastinating.

But, today, I pulled the new manifold and carb back off of the car. Soaked the water pipe in the manifold in vinegar to dissolve some surface rust. Will wire-wheel the outside of that pipe and apply some paint. (the steel pipe is embedded in the aluminum manifold, so good to try to "preserve" it at least a little bit)

And I disassembled the "new" carb. Which is, of course, an eBay purchase from a 1968 Triumph TR250. I think it's very possible that it's never been rebuilt, and is very gunky!

I'm learning that there are LOTS of variants of these carbs.

I just ordered a float assembly just to ward off any problems with that. The rebuild kit includes the float valve. With that, I have the parts to replace almost everything except the needle and jet. The jet is normally not replaced. The needle could be, man is it gunky!

I was going to order a replacement needle, but this particular carb is one with an adjustable jet and a fixed needle. (newer ones have a fixed jet and adjustable needle... and somewhere in there they had ones with fixed jet AND needle, no adjustment!) The one needle that I found reference to working well on a Spitfire engine with a 175CD happens to be an adjustable needle, which won't fit my carb. So, I'm going to go ahead and try just cleaning the existing needle and see how it goes.

[Loren]

I didn't do a whole lot today, but I took cool photos.

Got the EGR valve out of the exhaust manifold. Big adjustable wrench wasn't getting it, kept slipping. Vice grips and a rubber mallet got it to move! Once it was moving, additional penetrating oil and a few minutes of manipulation loosened it up and it came right out!



Here are the plug threads compared to the EGR threads. Definitely 3/4-16. The threads in the cast iron manifold are decent. The threads in the EGR valve are pretty wiped.



Smeared some anti-seize on the threads and installed the plug. I didn't fully seat it (I might give it another go later if I feel like it), but it's in there good and tight, certainly won't come out on its own.



And, lastly, here's your Andy Warhol inspired op-art still life of the day. I call it "carb parts in the sink". I'll get out there and clean things this evening when it's cooler. Wife is out of town tomorrow, so I can reassemble the carb on the kitchen table. (hehe)

[Native]

Kitchen table???
A/C FTW !!

[Loren]

Pretty much didn't do shit today, though. Cleaned the water pipes on the intake manifold and applied some paint. That's a step in the right direction.

Got the carb parts moved to the kitchen table. I just can't get motivated. I guess my big hold up is that there's one part that I would like to remove, I suspect that it should have a rubber O-ring in it. But, I'm not sure how it comes apart. And, there being 4-5 different variants of this carb, it's hard to find specific info.

I actually have a carb manual open in another window... I'm slowly researching it. Once I get past that, then I'll take inventory and be sure I have all the required gaskets and seals and other bits... should have. Then, it'll probably go together in about 30 minutes!

That said, I had a delightful nap this afternoon.

[Loren]

Okay, I took a minute to start inventorying parts. They seem to be there.

Went ahead and started fidgeting with my problem child. What I was worried about was the jet adjuster that comes out of the bottom of the float bowl. It's notorious for leaking, so I wanted to be sure to replace the O-ring on it. I just wasn't sure that it didn't have some kind of snap ring or something holding it in, because it was REALLY stiff. But, I remember reading something on a forum where someone said "they fidgeted with it long enough" and it came out. So, I fidgeted with it... and it came out.

Ditto the jet assembly itself. It just needed to be wiggled and twisted a little bit, and it came right out. Good thing, too... it has a couple of really worn out O-rings on it!

That was the only part of the carb that I'd not taken apart that I WANTED to take apart. Now, just a little more cleaning of those parts, and it can go back together.

I opted to not disassemble the "temperature compensator" because one of these screws seems too stuck and I didn't want to strip it, plus the general consensus is that it doesn't really do anything, anyway. (so, I don't care if it works or not) And I didn't disassemble the throttle shaft assembly, as it all seems fine. Not loose from wear, nor sticking from crud. The throttle is good.

Everything else is apart. Word is that I should uses silicone spray on the gaskets to keep them from sticking. I'll do the same on the O-rings. Especially on that jet adjuster assembly, as it does need to be adjusted... at least in the beginning. It's the main mixture adjustment, as the needle itself is non-adjustable. On my old carb, the needle is adjustable (with a special tool), and the jet is fixed (but, it actually IS adjustable, it's just a press-fit, so not EASILY adjustable). This one is easier to adjust, just use a wrench on the jet adjuster that comes out the bottom! The model just after this one had fixed EVERYTHING (before they added the adjustable needle), as they were experimenting with emissions stuff, and they didn't want anybody to fuck with anything at all!

Anyway... sometime RealSoonNow(tm), I should have this carb back together. From there, not much keeping me from getting it back on the car. And it should run.

For the record, the reason it didn't run when I test fit it was that the float valve was/is completely stuck closed with fossilized fuel. Wasn't even getting fuel into the bowl. That part is being replaced. Other than that, most of this carb was pretty clean and functional and should have at least worked. Should kick ass now... if I can figure out how to adjust it properly.

[Loren]

Alright! Got the carb all reassembled today. Everything moves as it should. Float height is adjusted to the proper 17mm. The piston moves freely and snaps back down with the expected satisfying "clink". No reason this carb shouldn't work!

Now, I need to work out the throttle linkage. With that, I can fit it to the car, figure out some coolant and vacuum lines, and the car should run. Need to do the choke cable, too... but, I can get the car on the road without that.

For the throttle, I want to use all of the original linkage rather than trying to mix-n-match from other cars, or go completely custom. So, I need to make a bracket to install the stock linkage assembly to the manifold because the replacement manifold doesn't have bosses and screw holes for it. The only other thing I need to do for the throttle is put the little ball fitting on the throttle shaft for it to attach to.

Of course, nothing is ever as simple as it should be. I pulled the old throttle lever with the ball fitting on it from the old carb. Throttle shaft is a different diameter. So, I need to either enlarge the hole (and it's an oval, not a round hole... of course) in the throttle lever to fit the new carb, or make a new lever. I think I'd rather leave the old part on the old carb and duplicated it for the new carb. It's a little bitty bracket, should be easy to fabricate.

Maybe, just maybe, I can at least START the engine on the new carb tomorrow. Driving it would be a bonus.

[Loren]

Man, I'm a slow fabricator. But, "do it right, do it once", eh?

First up, had to duplicate this throttle lever with a larger throttle shaft hole to attach the ball stud. Easy enough.



That's all done. Ready to attach to the stock throttle linkage.


Then, I had to make the throttle linkage assembly fit the manifold. This shot was during one of my test fits, but it shows what I was trying to do. The two bolts on the old rusty bracket are the original mounting points on that bracket that attached to threaded bosses on the original manifold. On the replacement manifold, my best mounting point was an unused 1-1/8" diameter unused boss. Nice and flat, just a little lower than where it needed to be. In hindsight, I could have made this bracket a LOT simpler and just put a couple fender washers under it! But, I like fabricating fancy little brackets.

The shiny slot-thread adjuster is the top of the rod that goes down to the throttle shaft, and has the fitting to fit the ball stud.



Here's my cardboard mockup, which is almost exactly what I ended up with. On the left side, there wasn't enough room under the bracket for the head of a bolt, so I ended up threading that hole, cutting the head off of the bolt, and tack-welding the bottom side of the bolt to the bracket. The other one is just a through bolt. I didn't want to put two bolt holes in the manifold (didn't want to swiss-cheese it more than necessary), so I just have one bolt holding it, and I cut a slot into the boss on the manifold and a tab in the bracket to fit that slot. That should keep it from rotating.

To get the level of the top of the bracket down to the level of the mounting boss, I did a double-90-degree bend to drop it down 1/4". The first bend was easy. But, then, how do you bend it back the other way? I had to figure this out a while back doing something else. You cut half-way through the metal to make it easier to bend. Then, after bending it, you run a bead of weld where you cut it! My welding isn't pretty, because I didn't want to melt the thing into oblivion trying to make it pretty. (I'm just not that good!) But, it'll work.



And here's the final bracket. Just waiting for the paint to dry before I try to assemble it. (See that thumbprint in the paint? I guess I've left my mark.)



Then, I'm down to plumbing. Fuel line, cooling hoses, vacuum and evap hoses.

[Native]

Looks good, thumbprint and all. Much work, but hopefully a lot of fun, too.

[Loren]

Back to it today! The word of the day is "almost". Can you say that kids? I thought you could.

Ran a bypass hose for the non-existent water choke. Hooked up the old water hoses to the intake manifold, I'll replace them with new later. Rigged up a vacuum hose, taped over another open vacuum port on the manifold. Connected the fuel line. Bolted up the throttle bracket and got the throttle all connected.

Um... that should be just about it!

Quick inspection to be sure.

Almost.

[video][/video]

Because the carb is physically larger, the throttle shaft ball is a little further back. I knew this when I made the bracket. (I almost used the word "designed"... heh... nah, I just "made" it) I did set the bracket a little further back, but clearly not enough!

So, I'll need to fix that. Almost right!

But, it's all close enough that I should at least be able to START the car, which I've not done in about a month. That would be good.

Almost.

The ignition turned on. Fuel pump ran, and I could hear it bog down when the bowl filled and the float valve closed. But... no crank. Just a click.

I included a voltmeter on my switch panel when I built it. I didn't think to look at it until the engine wouldn't crank. 9.3 volts. Almost enough to think about doing something.

The battery is on charge while I take a break. I'll be really happy if the engine starts. Then I can get back to modifying a throttle bracket and thinking about all of the other loose ends.

Now that I think about it... I probably moved the position of the bracket THE WRONG WAY! I had the center position marked... moved it about 1/8"... I bet I moved my bracket to the rear rather than moving the actual throttle bracket to the rear.

That would do it.