MGB & GT Forum

ZS 175 Problem

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

Threeputt Avatar
Threeputt Thom Hunter
Houston, TX, USA   USA
1978 MG MGB
Imagine that-- a ZS that won't cooperate. Desmogged my 78B while I had the head out for a valve job, also cleaned the tank to avoid more stuck valves. Put it back together and it wouldn't start. Disconnected the tappet vent from the carb and it started right up. Running rich but the choke worked fine and I had plenty of acceleration, plus no backfiring or dieseling. That didn't last long! Now the engine will only run richer by the minute and no amount of twisting the supposed mixture adjustment will do anything to correct the problem. I was lucky to get the car back into the garage.
Choke seems fine.
Disconnecting the overflow line to the canister does nothing.
Reconnecting the tappet hose just makes it lug down.
The diaphragm is fine.
The gulp valve 90 is sealed well and spraying carb cleaner around it does nothing. Can't find any obvious vacuum leaks (except to the brake servo. New hose on the way).

Spraying carb cleaner around the 1/2 port on the manifold did make a difference in idle speed-- it almost quit. Is it possible to misalign the manifold upon installation?

Anything obvious I haven't checked?

. Hide banner ads & support this website by becoming a > Supporting Member <
ohlord Avatar
ohlord Gold Member Rob C
North of Seattle, N.W., USA   USA
1957 Land Rover Series I "EYEYIYI"
1971 MG MGB
1971 MG MGB "Bedouin 2"
Intake and exhaust manifolds torqued to specs?
Did you have hardened exhaust valve seats fitted?
Valve lash adjusted properly?
Timing checked?

"I'm a long way gone down this wild road I'm on
It's gonna take me where I'm bound
It's a long way around"

"These are the days that must happen to you"

RD2 Radar/ Electronic Warfare Technician
Vietnam 1969-1972

glbishop Avatar
glbishop Gary Bishop
Spring Hill, FL, USA   USA
The slide(air-valve) is probably in backwards.

When installed properly, the little tab on the diaphragm sets into a slot on the carb body.

The two little holes on the bottom of the slide face toward the engine.

Also make sure the tappet vent line and the line to the charcoal canister are connected to the proper ports on the carb.

The EVAP canister line attaches to the port closest to the brake master cylinder. No harm leaving it disconnected while running the engine. In fact it should always be unplugged while tuning the carb. If connecting that hose slows the engine then it, or the hose feeding fresh air to the anti run-on valve, may be clogged. The anti run-on valve could also be clogged.

Edited 1 time(s). Last edit at 2014-02-09 06:42 AM by glbishop.

. Hide banner ads & support this website by becoming a > Supporting Member <
Nicecar Avatar
Nicecar Gary (ex "Harv") G
Victoria, BC, Canada   CAN
1980 MG MGB "Red On Red On Red"
On my /80 B, some years ago a mechanic removed the large hose (on left, coming from front of engine) from back of carb. The disconnected end looks like it's connected to auto choke, but it's not. When connected it idles rough.

This past summer I paid our local most respected MG mechanic, $175 to work on carb, and he decided it was best to leave this hose disconnected.

When you write: "Disconnected the tappet vent from the carb", is this the same hose as I'm referring to?

Do others have this hose disconnected too?

Engine anti pollution:hose.JPG    50.3 KB
Engine anti pollution:hose.JPG

jimb Avatar
jimb Jim Brown
San Francisco Bay Area, USA   USA
1967 MG MGB
If spraying carb cleaner caused the idle to change, then you've got a leak there. This doesn't seem like the only problem, though. (I had a manifold that warped and wouldn't seal well without resurfacing.)

The open tappet vent line is a big vacuum leak, and if you need it open for the engine to run, it sure sounds like it's running way rich, and needs the extra air to compensate.

The mixture adjustment screw only makes a small change in an air bypass channel. The main mixture adjustment is moving the needle via the hex screw at the bottom of the damper. The needle position is unlikely to change by itself, so there is probably something else going on. Maybe a stuck float valve?

The choke (enrichener, actually) has some tiny o-rings and a gasket that will pass extra fuel when leaking.

But if the car was running OK before this, I'd start by verifying the stuff that changed before messing with stuff that hasn't changed. How was the de-smogging accomplished? Maybe it's responsible? Hoses left open that need to be plugged? Hoses plugged that need to be left open? Stuff like that.

barry s Avatar
barry s Silver Member Barry Stoll
Alexandria, VA, USA   USA
1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
Luckily I don't know much about Strombergs. If its a B that I own, it has 2 SUs. That said, when those of you who disconnect the tappet cover vent hose from the Stromberg, do you then 'plug' that port on the carb? If not, you are providing a huge flow of additional air into the venturi, making tuning impossible. This is in contrast to the 'ambient' bowl vent line to the charcoal cannister which should have no effect on performance (unless of course it is plugged preventing venting of the fuel bowl).

Threeputt Avatar
Threeputt Thom Hunter
Houston, TX, USA   USA
1978 MG MGB
First--the tappet vent hose. It always went to a port on the carb which dumped oil mist into the carb just before the throttle plate. My manifold was nasty and oily so I figured I'd try venting the tappet chamber into the atmosphere instead. I had read that I was not the first to do this, by the way, but I did not read about the fate of the carb port. Intuitively I plugged that port but the car would not start. Unplugged the port and it fired right up. Put the hose back onto the port, wouldn't start.
Please note--this car has no air pump, no air rail and no gulp valve. I discovered that somebody (not me) had blocked the EGR valve off with a solid gasket made from a Busch beer can. All the stuff I removed is in a neat pile and the resultant holes are plugged and sealed. The lines to the canisters are intact and hooked up correctly, haven't tinkered with the anti-run-on valve.

Ok--with the carb port open the car starts better than it ever did before. Also, now that the gulp valve is gone the car no longer backfires when decelerating--which is what the gulp valve was supposed to prevent in the first place, right?

The carb is put together correctly but I could very well have a float problem. I'll take the carb back off, reclean, set the float a little lower and try it again. I have the $tromberg tool but so far it hasn't made a dent in the mixture problem. I have John Twist's articles (I think his idea about using a larger hammer is a great idea at this point) but I'll keep them handy this afternoon.

Thanks for your help, guys. I'll let you know how it goes.

jimb Avatar
jimb Jim Brown
San Francisco Bay Area, USA   USA
1967 MG MGB
Just so you know what they're for, the tappet vent to the carb is supposed to pull a small amount of air from the crankcase, metered by an orifice in the valve cover to carbon canister hose. The rest of the crankcase is supposed to be sealed. Any crankcase/valve cover leaks are effectively vacuum leaks.
The egr is supposed to send a little bit of exhaust back through the intake to reduce combustion temperatures (and Nox creation).

Both of these are small, metered, dilutions to the mixture. The normal carb mixture is calibrated to those. Any significant change there would require adjusting the mixture at the carb.

Your symptoms indicate a rich mixture, requiring wholesale vacuum leaks as compensation. The carb must be the source of the problem.

Nicecar Avatar
Nicecar Gary (ex "Harv") G
Victoria, BC, Canada   CAN
1980 MG MGB "Red On Red On Red"
For years I had run with hose disconnected and carb port unplugged. Mechanic this summer plugged it carb. It didn't seem to make any difference in performance.

Just today I fitted unsecured end of hose into a small plastic bottle to collect oil vapour.

A different mechanic who installed my OD this fall said that the oil vapor from hose was cause of dirty engine bay.

bobnorway Avatar
bobnorway Silver Member Robert Ligmanowski
downers grove, illinois, USA   USA
1960 MG MGA 1600
1977 MG MGB "orange"
Question-I have the same carb - runs great when it starts and runs for awhile,but if it's outside and its cold(not outside much smiling smiley it takes awhile to start ,but after it warms up it runs like a top.Any thoughts
thx bob

Threeputt Avatar
Threeputt Thom Hunter
Houston, TX, USA   USA
1978 MG MGB
You know what, guys--you're all right about the tappet vent hose. I only unplugged the carb port because it wouldn't start otherwise. Wonder why?

The air filter element looked fine on the outside but the inside got fried when the engine was spitting back through the carb--stuck intake valves. I pulled the element but left the rest of the assembly in place, never mind the fact that Twist advises against trying to run with a vanity air filter or without one at all. Now it starts and runs fine but you can certainly hear that carb port gasping for air. NAPA should have my new element tomorrow and I can start all over with trying to get this thing to idle consistently.

Oh, yeah-- the brakes have very little pedal but pumping the pedal makes the car idle at about 2000 rpm. Ya think I might have a vacuum leak there, too?

Bob-- the choke on mine has stuck a couple times, always in the "open" position and, of course, the darn thing refuses to start since the carb has no accelerator pump. Yours is probably just a bit out of calibration (Twist has some good articles and videos about the ZS choke) since it will start eventually. Sounds like you have a British Garage Ornament like mine. Not sure why a 62hp engine needs a carb built like a Swiss watch but I have no intention of changing it--- I doubt if I have driven that car 5000 miles since I bought it in 1986.

. Hide banner ads & support this website by becoming a > Supporting Member <

To add your reply, or post your own questions

Members Sign In   or   Create an Account

Registration is FREE and takes less than a minute!

Having trouble posting or changing forum settings?
Read the Forum Help (FAQ) or contact the webmaster