Carburetors - I think I've learned to like them

[javacontour]

Amazing how having a minor crisis changes your priorities. Your focus was 110% on the car and not on meeting girls.

Now, with fuel injection, you can spend more time focused on women.

Originally Posted by meep
Carbs gave me a couple of opportunities to meet girls. Either someone else's car wasn't starting and I'd go lend a hand, or one day I was pulled over in our ‘57 Chevy (I was 17 in the late 80s) and she pulled over and offered to drive me home for tools.

I was such a dimwit. I knew I could fix it with what I had and turned her down.

M

 

[MCompact]

Originally Posted by bunnspecial
I'm a young guy(32) and grew up driving and mostly riding in FI cars. My only real exposure to carbs was on small engines, which of course even though they are the same in principle are often a totally different beast.

5 years ago, I bought my MG, and what followed over the next few months was learning the ins and outs of constant velocity SU carburetors, and also learning how to sync/balance 2 of them! A lot of people fuss about SUs, but I've "bought in" to the superiority of the design and aside from a few weakness in early models(throttle shaft wear that causes vac leaks) they really are an excellent and reliable carb. An SU metering needle is designed with 8 "stages" to reflect varying speed/load conditions. The needle is used to change the size of the jet, which means that an SU has the equivalent of 8 jets vs. your usual 2-3 on a fixed venturi carb.

Right now, there's a set of HD6 SUs sitting on my bench that I want to at least spruce up, if not rebuild, and maybe see if they will run decently on my MG. With some recent performance improvements, I MIGHT be able to take advantage of the larger 1.75" carbs(the ones on it are 1.5"), but I need to experiment.

BTW, pull the choke out, turn the key, and my car starts right up. Leave the choke all the way on for a few seconds, then slowly push it in over the next couple of miles of driving. I much prefer that to the water choke fitted to the later MGs that can go out of adjustment and has a bad habit of corroding.


My Datsun SPL311 had SU carbs that were built under license by Hitachi. The SU carbs are much like the Rochester Quadrajet and some Zenith Solex carbs- they are excellent carburetors but you have to take the time to learn how and why they work the way they do.

 

[bunnspecial]

Originally Posted by MCompact

My Datsun SPL311 had SU carbs that were built under license by Hitachi. The SU carbs are much like the Rochester Quadrajet and some Zenith Solex carbs- they are excellent carburetors but you have to take the time to learn how and why they work the way they do.


I've heard great things and horror stories about the Quadrajet, but have no first hand experience with them.

With the SUs, they really are elegantly simple, but just very different from most anything fitted to an American car. Now that I'm comfortable with them, I don't have a huge desire to learn anything else(except maybe similar designs like the Zenith-Stromberg fitted to later MGs and a lot of other US market British cars). I'm most comfortable with the HS type, but when I rebuilt the HIF on my Marina I had a few moments of things like "Just how does this choke work?" and then when I finally saw it I realized how brilliant it was. I have a set of H4s off an MGA here that I want to dig into, then of course the HD6s I mentioned earlier. The HD-type is probably one of the most maligned SU designs, but none the less I'm up for the challenge and if they end up being a dog on my MG(which they could be) I still have a valuable set of carbs that is actually specced(per the tag on it) to the Healey 3000 BN7.

 

[Silk]

The SU is my favourite carb, so simple and good performance. I used to fit them to motorcycles (some Triumph 650's had them) and fitted one to a VW I had in a sidecar. The HIF is different, but it is still an SU and operates the same. The last SU's used in the Montegro and others was electronically controlled with a stepper motor, maybe with an o2 sensor, brilliant idea. But the SU has no emulsion tube, and so is a dirty carb, not passing later emission tests. Put them on a digital exhaust gas analiser and you can see it. I have some motorc ycle carbs that work on the same no main jet, no idle circuit as the SU. Here is another of the Japanese SU's we used to get on some Toyota's and Mitsubishi's.

 

[CBR.worm]

I would never want to go back to carbs, but there was a time that you could easily get more power and drivability from a carb than the primitive EFI (or mechanical FI) systems. Today, I can't imagine going back. CV carbs were ideal - so easy to tune them to work perfectly. Keihin FCR Flatslides were great for performance - but you had to be judicious in your use of the throttle. I have never had to do an 'EFI rebuild' on a vehicle, I'm not sure what would be comparable today.

 

[RDY4WAR]

I love carburetors for recreational vehicles. They will usually make slightly more power than EFI, all else equal. EFI is more efficient and better for changing climates. Carbs have a complex learning curve for really dialing them in between the jetting, metering, power valve, etc... and it can easily be influenced by other factors. Carb spacers of different sizes and shapes have different effects on carb metering and fuel distribution pattern, for example.

They are more sensitive to fuel density though because of the floats.

 

[ctrcbob]

Well if you like the theory of a regular carb you should check out a CV carb. Talk about correct design from the beginning, rather than add-on circuits to correct poor design.

Yes. I've had vehicles with CV (Constant Velosity) carbs. Also known as Concentric carbs. Very familiar with the twin SU (brand) on my cars and the Bing (brand) on my BMW Motorcycles. Easy to work on. Easy to clean.

 

[Yah-Tah-Hey]

I wish I had my first vehicle again. 54 Ford 4dr Customline with a romping, stomping 239 cu in Y block V8 that couldn't get out of its own way.Plenty of room under the hood and everything could and was worked on.

 

[Motu]

Yes. I've had vehicles with CV (Constant Velosity) carbs. Also known as Concentric carbs. Very familiar with the twin SU (brand) on my cars and the Bing (brand) on my BMW Motorcycles. Easy to work on. Easy to clean.

Quite a bit of difference between a motorcycle CV carb and an SU. The SU has no idle circuit, no emulsifying jet and no main jet....or atleast, just one main jet to cover the range for that carb, all adjustment is done by changing needles, not main jets.

 

[WylieCoyote]

Tried my hand at carb work years ago on a Yamaha Virago. Don't have a talent for it. I still don't know how to get the bowl level right. Got it running good, but was never confident about what was going on in the bowls. Solved by trading it for a F.I. bike.

More recently, picked up an '82 Mustang 3.3 I6 (gone now) and attempted to just replace the entire carb myself. Turned out not to be tuned for that specific engine, and ran very rich. So, had the original carb professionally rebuilt by a guy who rebuilds carbs for show cars.....top quality stuff. Car ran fine for a while, but I guess somewhere along the line, I must've gotten ahold of some gas that had a higher-than-normal percentage of enthanol in it, and it began boiling in the bowls whenever the engine idled, essentially flooding the engine at stop lights. You could actually hear it boiling when you shut it down (or when it shut itself down)., Car had run fine the entire previous 2 weeks until I filled up. Failure #2 was enough to put me off carbs for life. The carb guy warned me that some engines have a hard time running on 'modern gas'. He was right.

 

[andyd]

My last go round with a carbed engine was my last '88 Grand Wag. Wouldn't pass MA emissions, dyno test just like CA. 30' of vacuum hose series-ed widgets together, new cat, working smog pump, the whole deal. Last but not least, was the Ford 2150 2 barrel carb that I built from best parts of 3 carbs. and a new EGR. It Passed with flying colors, finally. In 25 yrs of port injection EFI, maintenance was mostly hoses. and one fuel pump relay.

 

[philipp10]

I grew up with carbeureted vehicles. Rebuilt so many I was the go to guy in the neighborhood. But do I miss them? He!! no, bunch of flakey junk is what they were. Your comment that if fuel injection goes bad, you are screwed. Well I got news for you, if your carb goes bad.....your screwed just as badly. And having a carb on a recreational vehicle....why? So you can get stranded and no one around that knows how to fix?

They are still the go to for small engines and that is fine as they are low cost. But let's not pine for the days of constant issues....

 

[Zaedock]

There are pros and cons to both.

IMO, you have to grow up and/or use carbs often to appreciate the simplicity and WOT power. They will never meter or drive as well as EFI.

In the off road world, EFI rules. My Wrangler's 2.5L will idle on it's side (seriously, it has). That said, the poor man's EFI - aka Motorcraft 2100/2150 2 barrel, is going on my rock crawler's Chevy 305 (was rebuilt and then given to me!). I will likely source the parts to put the TBI back on, but for now it will be fine.

My best friend just built a 68 Vette with a crate 6.2L/6 speed. He's had my Snap-on Solus Edge scan tool for about a year and half figuring stuff out.
He's building his wife a 454 powered '69, which will be carbureted! LOL Again, pros and cons. Both systems need expertise on how to tune.

 

[bunnspecial]

Quite a bit of difference between a motorcycle CV carb and an SU. The SU has no idle circuit, no emulsifying jet and no main jet....or atleast, just one main jet to cover the range for that carb, all adjustment is done by changing needles, not main jets.

I'm sort of speaking out my rear end on this, but aren't a lot of motorcycle carbs "slide carbs" where you directly control the position of the metering jet in the needle? On SUs and similar/identical designs, the needle is indirectly controlled by airflow through the carb throat.

Moss use to(maybe still does) sell a twin Mikuni kit that is a bolt on for many dual SU applications. I've heard of a few being installed, but they're expensive. The reports I've heard are that they feel a lot "snappier" than SUs(SUs don't necessarily have the throttle punch of other carbs since they lack a true accelerator pump) but the trade off is that the gas mileage goes into the dumpster. The reports I've heard are in the 15-20mpg range, while a Weber DGV(properly jetted) can get 20-25 and a dual SU set up is probably badly out of tune if it's lower than ~25mpg around town at least on an MG.

 

[Motu]

A CV motorcycle carb has an idle circuit and a main jet, so the CV and needle part only covers mid range, between idle and main. CV carbs do have a flatter response to cable slide carbs...and that's why they are used on streetbikes, slide carbs can be a bit aggressive, CV's give a better riding experience...and balance out a bit of carb inbalance. As you probably know the damper is the accelerator pump with an SU. We used to use ATF, but as things got worn, we'd up the viscosity of damper oil to cover airleaks etc which lean out a carb over time.

SU's less economical ? I dunno....you could go back to the '50's and do a comparison between an Austin and a Morris. although both now BMC, Morris brought SU into the stable, and both used the Austin based engines, sometimes an Austin would have a Zenith, and the Morris version an SU. I used to fit SU's to motorcycles, and on my Triumph 650 would get 100mpg (UK gallon), much better than with Amals...single or twin.

If I sound familiar bunnspecial, I was Silk on the other platform.

 

[350Rocket]

I grew up with carbeureted vehicles. Rebuilt so many I was the go to guy in the neighborhood. But do I miss them? He!! no, bunch of flakey junk is what they were. Your comment that if fuel injection goes bad, you are screwed. Well I got news for you, if your carb goes bad.....your screwed just as badly. And having a carb on a recreational vehicle....why? So you can get stranded and no one around that knows how to fix?

They are still the go to for small engines and that is fine as they are low cost. But let's not pine for the days of constant issues....

I've been daily driving carbureted cars since I was 17 (I'm 37 on Tuesday). Yet to be stranded by a carburetor. I replace the mechanical fuel pump every 10 years (probably should sooner), rubber fuel hoses crack after 5-7 years with the ethanol gas). I have had a carb issue make my car run like crap (needle and seat sticking once from ethanol corrosion), idle jets get plugged up occasionally, causing a rough idle.

Maybe because all of my carburetor experience has been quadrajets from the 1980s and a motorcraft 2 barrel on a 78 f100. They are known to be reliable carbs. My parents did have an 84 Plymouth reliant and a 77 Toyota Corolla and both had carb issues at one point while we had them. They were also old by then.

 

[Dinoburner]

The 454 in the 77 in my sig has a 800 cfm Edlebrock that does not pull like the Quadrajet that was in it previously. It needed a complete rebuild that will now get done and the Edlebrock will go on the 327 rebuilt last year. Helped a neighbor tune his Holley EFI and while it is consistent and all WOT isn`t as strong as the the Holley 750 was.

 

[Cujet]

As a young man, I had a couple of Mustangs 1966-67 and would do track days, autocross and enthusiast driving events. I was enamored with Weber carbs and ended up with a homemade downdraft setup on my Mustang (with 2 oval K+N filters) . They were easy to tune, as the jets, (high speed mixture) emulsion tubes (mid range RPM/throttle mixture) and air bleed jets (low RPM mixture) were all removable from the top. Even the venturi inserts were replaceable. So it was "easy" (not cheap) to make the carbs bigger for more flow. Which, of course, I had to do! If I remember correctly, the originals were 32mm and I installed 40mm venturi's. The question is did it help? Not as much as it would seem, the top end HP was ever so slightly better.

The system ran wonderfully, with instant throttle response, no carb related cornering problems (a big deal in those days when on sticky tires) and plenty of impressive looking equipment. It did freeze up on long trips, as the ice would build up on the outside of the carbs and lock up the throttle linkage! MPG was not good.

My friend had a similar car with a single plane intake manifold and modified Holley double-pumper 750 CFM carb (with milled off choke and radiused inlet) He made more peak HP than me. But his Holley would dump fuel in the corners, and I had far better response. In the end, I was faster, as I could power out of corners instantly.

Fast forward to today. Other than for a touch of nostalgia, I hope to never play with carbs again. I much prefer modern fuel injection!

In reality, the HP produced by any of these old engines was directly related to the camshaft choice. Those who could run a big camshaft successfully could make big HP at High RPM. What happened with the Webers is that a camshaft with a lot of lift/duration/overlap would cause a good bit of airflow reversion and double-carburete the fuel (mix some fuel as air was flowing backwards) and go "over rich" in the low and mid RPM range. This limited the Webers to either milder cams or very high RPM use. The 4 barrel carb was simply an easier way to get big HP from a high strung engine.

An interesting old-school setup in a real Lola race car. Not a big HP setup, but pretty effective.

 

[RDY4WAR]

If you really want to see some really interesting ingenuity, hang around with some of the guys running pure stock or street stock oval classes where they rules force them to run a 350 or 500 cfm 2 bbl carb on 302-360ci V8s. Cam lift rules, stock smog-era iron heads, compression in the toilet, low rise dual plane intakes, restrictive 2 bbls, hydraulic flat tappets, and yet they still squeeze 350+ rwhp from those cars. They are constantly finding loopholes in the rules to increase the flow and response of those tiny carbs.

 

[supton]

I remember having a carb ice up back in '93 or so. I made it a mile from home and it just quit. Ran fine later in the day. That vehicle, and a subsequent one, both had dead chokes too.

But. My Tundra had an idle issue one day, and that was ultimately because the throttle valve was "dirty" (cleaned it myself). Ignoring dead chokes I've had the same amount of problems with both systems.