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MG MGB Technical - Mystery problem

Ok, this may be similar to the fuel starvation post, but I'm really clueless so here goes.
I was driving up from Santa Cruz to Oakland, blasting along at a good clip, spinning about 4500 rpm in 4th gear for about 20-30 minutes. I decided to back off the gas a bit. I re-engaged the throttle at about a quarter throttle, and the car began to spit and cough. Backfiring through the carb. Initially I could just floor the gas all the way, and the problem would fix itself, but as I kept driving it got worse and worse, to the point that when I floored it there was absolutely no power (almost like fuel starvation but not positive). After letting the car sit for a bit on the side of the road, I headed out again. The problem seemed to go away. But it reappeared the next day when I took it on the freeway. I cleaned the filter in my carb, I'm running a DCOE 45 Weber with integral filter, I also bought a new fuel pump to replace the one on the car, which still works, and I added a second fuel filter between the gas tank and the pump. The symptoms are still there on the freeway. I know the fuel pump is running b/c it is obnoxiously loud (an after market electric). It really starts when I drive above 3500 rpm. If I cruz lower than that things seem to be ok. Also there are no signs of problems on the street or in any other gears. When I cleaned the fuel filter there was a bit of crud but nothing to horrible.
I am guessing that either my carb needs a rebuild or I have a serious distributor problem. But I will appreciate any guesses or feed back.
Cheers
Bill
69 B-GT
Bill Mertz

Carb icing??

I suffer from this on damp cool days.

Leaving the engine for 5 mins thaws it out and all is normal till the ice reforms.

Constant throttle problem - doesn't happen around town.
Chris Betson

Have you tried removing the fuel filler cap to check for unwanted vacuum when the problem occurs?
George B.

Every time I run into this problem it turns out to be crap in the fuel tank that packs up around the fuel pickup - the tip is that it's O.K. except at high demand or long highway speeds . I'd drop and clean the fuel tank .

You can easily ' T ' a fuel pressure gauge just before the carby to check this whilst driving , if the fuel pressure drops when it begins to run poorly , there's a supply problem downstream of the carbys .

Too much dirt & crap in U.S. gasoline these days .

-Nate
Nate

Carb icing in Santa Cruz? You gotta be kidding, Chris! :^) Bill: check the ignition system- especially the rotor and wires.
Terry

I agree with Nate. I had the same problem back in '96. A group of us were on the road to the MG convention in Indy and my car would drive along for awhile then start sputtering out just like what you described. It did it about 10 times on the way over. We'd all pull over, wait a while, then we could take off again. To get me home, we bought a length of fuel pipe and some fuel hose from Auto Zone. We bent the fuel pipe and placed it going into the filler neck, out under the gas cap (that was taped down), under the car, and hooked directly to the fuel pump with some fuel hose. Didn't break down once on the trip home. After I got back, I pulled the tank and drained it. There was so much rust and dirt in the tank, it was awful. Long story, short, I replaced the tank and all was well again.

Robert
Robert Rushing

Bill i had a rubber fuel line at the pump that was cracked on top, wouldn't leak gas but was sucking air. You might want to give all the lines a good going over.
James Skinner

I had this happen and it was one of the venturi(spell) had come loose so blocking the air inlets. If you look at the outer bottom of the carb, there are screws that hold the venturi in the neck of the inlets, if these come loose it can cause the problems you describe, I was lucky and the screw fell onto the car, you may have one loose or missing.
Tatty


Do not rule out carburettor icing.

From my days as a pilot I remember being taught that ice is formed at or near a partly closed throttle valve. The water vapour in the induction air condenses and freezes due to the venturi effect cooling as the air passes the throttle valve.

Since the temperature drop is usually around 5°F, the best temperatures for forming throttle ice would be 32-37°F; BUT the text books state that carburettor ice will form in temperatures between 32 and 50°F, and in my experience at even higher temperatures, when the relative humidity is 50% or more.

I have suffered serious and continued carburettor icing, even in British Summers (no jokes please), in two cars, a Mini Cooper 1275 S fitted with a DCOE webber, and a Sebring Sprite with twin SUs. In each case the problem was resolved by warming the induction air.

Some cars have a pipe feeding the induction air to the air filter which can be adjust to draw air over the exhaust manifold in the winter, and cooler air in the summer.

Nigel
MGA 1600 Roadster
MGB GT V8
MG R V8
Nigel Steward

This problem fascinates me, and since my first post I have researched further.

I searched the web and found a report into an air accident in 1999 by the UK Air Accident Investigation Board

http://www.aaib.dtlr.gov.uk/bulletin/dec00/gbihe.htm

which states:-

“Carb icing is not restricted to cold weather, and will occur on warm days if the humidity is high, especially at low power settings. Flight tests have produced serious icing at descent power with the ambient (not surface) temperature over 25°C, even with relative humidity as low as 30%."

Note that 25°C equates to 77°F

The New Zealand Civil Aviation Authority in its handbook on icing:-

http://www.caa.govt.nz/fulltext/Safety_booklets/Aircraft_Icing_Handbook.pdf

states that temperature reduction can be as much as 30°C ( 86°F); suggesting that icing can take place at ambient temperatures of up to the mid 80s F.

This ties in well with my recollection of suffering power loss when racing a Mini-Cooper S in the Summer 1965 which was ultimately diagnosed as carburettor icing. The problem was that the evidence, ice in the venturi, had disappeared (melted) by the time that one started to investigated the problem.

Conclusion Bill you may well be suffering from carburettor icing - sorry Terry !

Safety Fast

Nigel
Nigel Steward

Carb icing may not be obvious but there is often a tell tale layer of condensation on the outside of the carb body near the jets on HS carbs - HIF carbs don't show this.

It doesn't have to be cold, just high humidity - or if water gets in the fuel tank.

Steady speed motorway driving is ideal for ice formation with relatively small throttle openings and high gas velocity, the vaporisation of the fuel drops the temp and the reduction in pressure through the gap between body and plate causes the temperature of the air to drop even more - only a little ice has to form round the jet to upset the mixture or round the throttle plate to stop it shutting properly.

A sure sign is that the car doesn't slow as quickly when you lift off the accelerator and if you depress the clutch the engine doesn't drop to normal idle speed.

I have had the engine race up to 4000 rpm when changing down to leave a motorway and have had to use the brakes hard to slow the engine down whilst still in gear. Other cars wonder why this MG is accelerating with the brake lights on!
Chris Betson

Yes! My '68 Cooper would regularly ice up in Freeway driving in the summer time.Yes Chris, and there were ( as you suggest) goose bumps of condsn. around the carb throats, and the top of the inlet manifold cold as . . .ice. Same story for my VW Bug in the AUS desert in January, on long straight stretches of road.
Ken R
Ken Rich

Bill, Bill, Bill...so many DCOE problems, eh?

My best guess is that a bit(s) of junk has found it's way in one or more passageways in the Weber
and/orrust has acculumulated in the fuel chamber and is choking things up

How can this be? Afterall, you've got not just one, but two, fuel filters hooked into the line. Well, fuel
filters, no matter how good they are, are not absolutely 100% and let in small amounts of rust and bits.

Also, on a DCOE, adjacent to the left throttle bore - is the fuel chamber vent port. Although it is
probably covered by your aircleaner, junk can still find it's way in there (bypassing any fuel filters) - and
get into the fuel chamber. Backfiring through the carb exhales a plethora of carbon and bits of debris
and it pressurizes the space inside the aircleaner (it's like sneezing inside of an enclosed helmet).
Any debris or particles that happen to be laying in or near the vent port will be blown in further and can
eventually find it's way into the fuel, then the passages, then the jets. Then the mixture leans out
unevenly...or your car conks out irradically - or worse - your car dies from starvation altogether.

Recommendation: Get a can of aerosol carb cleaner (with the thin, long nozzle). Pull out all of the jets
and passage covers and clean them (easy to do on a DCOE). Pay particular attention to the idle jet circuit. Remove the top of the carb and give every passageway a burst or two of compressed carb cleaner (wear safety goggles). Remove, check and clean the float needle valve. Remove the bottom
of the fuel chamber (it's the square cover, held by 4 slotted screws, underneath the carb). Clean out the cover as well as the entire fuel chamber - including the vent port.

Reassemble, making sure that everything remains surgically clean.
Daniel Wong

Above 3500 rpm, the second jet of your Weber is opening up. Also when accelerating after constant cruising. Could be a plugged passage- look here 1st. Also look for air leaks in the Weber, due to gaskets wearing. Easy to fix by tightening the carb body screws. Tell-tale signs are discolorations/stains on the carb body.

Finally, I had a very similar experience when the coil on my TD was failing. It only happened when very hot or under sudden load.

Hope this helps.

Ira
Ira Spector

Don't rule out water in the gas. I know that folks are probably sick of my suggesting this, but damn it, I often find this Sh*t in my gas here in wonderful Idaho and elsewhere I travel. Both the quality of the gas we get plus the lousy storage and contamination of it from refinery to pump is often a problem in my experience driving around the US, and not just in my humble backyard.

If there was a slug of water in your gas, it would only hasten and enhance the formation of ice as has already been rightly pointed out. It will make for crappy running (since it really doesn't burn) and it will carry some contaminants better than gasoline. And it will pass right on through your filters, fuel pump, and into your carbs with ease.

A while back I posted a query about rough running of my GT. I never could find the problem. Elusive and frustrating. However, I poured a bottle of octane enhancer into my fuel (because it is winter here, more than for "blinding acceleration") and low and behold, now the crappy running has smoothed out. Naw, it couldn't be crappy gas, could it??? Cynicism is often bred from experiences such as this. If you disagree, let me know, I would really like to get to the heart of this phenomenon.
Bob Muenchausen

Bob,

I agree with you on this aspect. I drove my car to the Chicago area in May 97 for a school. The car drove great the whole way there (about 2000 miles). Getting ready for the return trip in June, I developed a high RPM miss. I took the car to Northshore Imports, and the mechanic there couldn't figure it out either. Even ended up replacing the fuel pump, thinking it may have been failing. The problem solved itself somewhere in Iowa, after I had burned up all the crap gas. The "gasohol" sold in Iowa may have helped with the water problem.
Paul Konkle

Well guys thanks for all the suggestions. Like I said, I cleaned the jets in my carb with brake cleaner, replaced the fuel pump, cleaned the out the mesh fuel filter in the Weber, and added an additional filter betweent the gas tank and fuel pump. None of this seemed to help. However the other day I pulled the fuel line off of the carb and turned the ignition on. Only about 7 ounces came out in close to a minute. This didn't seem like much to me, so I cranked my fuel pressure regulator up 1 psi, and tried again, this time a lot more fuel came out. I took a sample of the fuel, and let it settle for a bit and it looks clean as whistle, no setiment or debris or discoloration. The problem seems to be gone (knock on a morgan). I'm thinking there may have been some debris clogging the fuel pressure regulator, and at high speed my carb may have been drinking slightly more than the fuel pressure regulator would let in. Over time float bowl would slowly empty causing the car to crap out. I'm going for a longer test drive tomorrow and will let people know.
Daniel I will pull the carb apart and clean it out with carb cleaner as all the backfiring has likely made it dirty. There also appears to be a few dirty spots on the body of the carb that look like leaks, perhaps a new gasket set is in order. Do you know where to get rebuild kits or gasket sets for this carb? Or could silicon or RTV work?
Thanks for all the replies, I'll keep my eyes open for carb freezing, bad gas, and a dirty tank (another likely culprit). But unless the problem resurfaces, I'll be left in the dark.
Cheers
Bill
69 B-GT
Bill Mertz

Well I made the 75 mile drive from Oakland to Santa Cruz without problem. So here is my new theory. The old fuel pump I had mounted was too high pressure for the Weber carb, so I put a fuel pressure regulator on it. Junk from the tank or bad gas clogged the filter causing the intial problem. After I replaced filters, I also put in a new fuel pump which was lower pressure, however I didn't adjust the fuel pressure regulator at all, so gas was still being restricted mimicing the old problem. Once I cranked the regulator open, presto problem gone. But thats just my theory. Incidentally, the purolator electric fuel pumps are now available at Kragen for 30 dollars which is a good bit cheaper than moss.
Bill
Bill Mertz

Bill, a warning on using silicone RTV or any other form of silicone sealant around "gasoline". DON'T!!!!!

Whatever "gasoline" is these days, will turn these sealants into a gelatinous mass which will cause you more heartburn than the problem you have just cured. NEVER use those sealants for anything that comes into direct contact with fuel. Oils, water, antifreeze generally are OK, but never fuel. FYI.
Bob Muenchausen

This thread was discussed between 08/01/2002 and 14/01/2002

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