MGB MGB Won't Restart When Warmed Up!

[jhayes]

I am totally baffled and really need some help!
I have a recently acquired 79 MGB. 'B' starts and runs great as long as you don't shut it off and expect it to start again within the next 45mins or so.
I'm sure that it's flooding on restart attempt, but cannot figure out why. I have two temp guages and temp while driving stays about 185 Degrees, so it's not over heating. Carb is a single Z-S 175CDSE. The SE only has a manual choke and a temperature compensator. The floats are adjusted to spec, float needle valve is fine, temp compensator seems fine, no clogged holes in the carb, no bad gaskets, metering needle valve adjusted and looks good, air piston is not binding, diaphram is good, took choke apart and it looks fine, Vacuum to distributer disconnected and plugged at carb, hose to charcoal cannister plugged at carb, no smog pump, no gulp valve, all air holes on side of head plugged & I have good hot spark to the plugs. I blow gobs of smoke out tail pipe on restart attemt(raw gas) and the vent hole in the manual choke cover starts leaking gasoline. I have good vacumm but I'm beginning to think my vacuum is so good, it's somehow (when the engine is warmed up) sucking huge amounts of extra gas from the float bowl out though the choke and into the carb even though the choke is rotated off. One other thing I found out tonight, the only other hose not mentioned that's on the carb, goes to the EGR valve. I pulled the EGR and found that it has been purposely blocked off. I'm not sure if this will cause a problem or not, but I know that vacuum line from the carb is not doing anything.
ANY HELP WOULD BE GREATLY APPRECIATED.

 

[piman]

Hello JH,
have you tried the usual trick of opening the throttle wide when starting, this will usually start a flooded engine?
The cause of the flooding may be harder to find. It is not due to a good vacuum by the way. What fuel pump is fitted, could the pressure be too high? Is the car running rich in general?

Alec

 

[jhayes]

Alec,
I can usually get it started if I really try, but you have to pump the throttle to try and keep it running and it has zero power. It will always die, but if your persistant, after 10 minutes or so of doing this it will eventually clear itself up and drive normal again.
I assume the fuel pump is the original and fuel pressure was about 3.3psi. My fuel pressure gauge is loaned out, but I will borrow it back and recheck.
The car was running very rich originally and is still running slightly rich.

 

[Guest]

SU pumps normally run around 2 lbs or less. Much more than that and the needle wont cut it off. Does it (the pump) cut off when it initially comes up to pressure before cranking the engine? If not, it may be an aftermarket. I went to the Grose-jet needle and seat on my carbs for positive cut off. Also there are adjustable fuel pressure regulators available if that turns out to be your problem.
You say it runs good after warm up so that would probably eliminate the ignition.

Also, you might take out the float and make sure it isn't leaking. This will cause it to flood the carb. Shake it and see if you hear gas inside the float.

[ 02-22-2004: Message edited by: mrbassman ]</p>

 

[jhayes]

DaveR,
Originally if I drove the vehicle around for an hour without shutting off the engine, the plugs would have heavy dark sooty carbon deposits. Since then with adj of mixture via needle and seat, I can perform the same experiment and the plugs are light grey/brown with a barely perseptible hint of some carbon deposits.
An overly rich mixture does not explain why the choke cover leaks gas out it's vent hole when it's warm or why gas puddles in the carb throat between the air cylinder and the butterfly valve when it's warm.
Experimentally however to prove or disprove your hypothesis, I could lean out the mixture to the point of pinging, come back just a touch and see if my problem persists.

 

[Guest]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by jhayes:
DaveR,
Originally if I drove the vehicle around for an hour without shutting off the engine, the plugs would have heavy dark sooty carbon deposits. Since then with adj of mixture via needle and seat, I can perform the same experiment and the plugs are light grey/brown with a barely perseptible hint of some carbon deposits.
An overly rich mixture does not explain why the choke cover leaks gas out it's vent hole when it's warm or why gas puddles in the carb throat between the air cylinder and the butterfly valve when it's warm.
Experimentally however to prove or disprove your hypothesis, I could lean out the mixture to the point of pinging, come back just a touch and see if my problem persists.<hr></blockquote>

If the gas is coming out the vent hole it has got to be the needle not stopping the gas when it should. You checked the float level and the float isn't leaking, its got to be too much pressure for the needle to stop it. My pump runs at around 1.5 psi. and never runs out of fuel and I have two carbs.

I assume you have the fuel filter on there and it is fairly fresh. Trash getting into the needle could cause the problem you are having keeping it from closing all the way.
Bob.

 

[jhayes]

Well it appears that too much fuel pressure may indeed be the problem, which makes sense given the symptoms. Got my fuel pressure gauge back and the pressure is much higher than it was when I originally checked it. Wonder how that happened or maybe it's just a glichy guage. At any rate pressure now reads 4.5psi. My carb is in pieces and I don't have a regulator so I won't be able to check things out for a couple days, but I have high hopes.
THANKS to Piman, Mrbassman, DaveR and Bret for steering me in what appears to be, the right direction.
I'm guessing that if this is the cause I'll be adjusting the mixture again, since it was probably forcing gas up through mixture needle-seat in addition to forcing it up through the choke assembly.

 

[jhayes]

Mrbassman,
I checked the floats again last night and they are not leaking(no gas in the floats). Float level for both floats are ~0.645" According to my book, float level should be 0.625" to 0.672"
When the engine is cold, the fuel pump does shut off usually after 2-3 clicks. I've never thought to listen to it when it's warm, on a restart.
Everyone could be right in that my fuel pressure may be too high, although I don't know why that would only cause problems on a restart of a warm engine. I definately will need to check it again thou. Also, I do have the carb heat shield installed. On restart attempts the carb seems cool to the touch and the interior of engine compartment is not overly hot, so I'm guessing my problem is not vapor lock.

Any ideas on a blocked off EGR valve?

 

[Dave Russell]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by jhayes:
Mrbassman,
Everyone could be right in that my fuel pressure may be too high, although I don't know why that would only cause problems on a restart of a warm engine.

Any ideas on a blocked off EGR valve?<hr></blockquote>
You DID say earlier;
"The car was running very rich originally and is still running slightly rich."

From your description it is still running very rich, more than slightly. Since the engine requires a very rich mixture for cold starting but not when warm, it will cold start easily & be hard to hot start & run poorly when hot.

The EGR valve should have no effect on your particular problem.
D

 

[Bret]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by jhayes:
<<SNIP>>
Any ideas on a blocked off EGR valve?<hr></blockquote>

The only reason I can think of is that it was blocked off for the same reason all of the other emissions equipment removed. I mean I don’t think you really gain or lose anything from the EGR valve with all of the other stuff removed.

Welcome to the forum jhayes!

[ 02-22-2004: Message edited by: Bret ]</p>

 

[Johnny]

Once again the BCF solves a members problem. Thanks for all the help. I too have a 75' MGB that has the same problem JHayes has/had. Going to check the fuel pressure, and floats.

I checked the fuel pressure and it read close to 10psi!! Is that possible? Bought a "Spectre" fuel regulator and set at 2psi. Works super, and car now starts. Thanks to Jhayes for posting. Never thought of fuel pressure as the problem I always thought it was ignition. Speaking of fuel pressure, the gauge I saw at Auto Zone for $10 is meant to be just screwed into the fuel line without a way to allow fuel to escape. Is this the correct way to measure fuel pressure. Or, should it be measured as it flows, so to speak thru the system?

[ 03-07-2004: Message edited by: Johnny ]</p>

 

[aeronca65t]

Glad your car is OK....I'm *racing* on 2.5 pounds. 4.5 pounds would almost certainly cause flooding problems.

Just a comment to all about fuel pressure regulators: A popular model is the Purilator regulator. I recieved a recall message on mine, saying that it might start leaking suddenly. I let it go for a few weeks and one day it failed, spilling fuel all over my (hot) headers. If you have one of these,and you bought it 1 or 2 years ago, you should check with your parts vendor...they'll replace them for free.

Also, regarding the plugging of the EGR valve. This valve is designed to "leak" exhaust gases into the intake manifold at crusing speeds. In essence, it's a "controlled leak" and the carb is set up slightly rich in the cruising range (to "compensate" for the "leak"). If the EGR valve is plugged, the car will run slightly rich at highway speeds (but not at idle). It's not a big deal...you may not even notice it at sealevel. But generally, you should lean the high speed fuel curcuit *slightly* after disabling an EGR.

When hot-starting is a problem, it's not a bad idea to check valve adjustment too. Although it doesn't seem to be the case with your car, "tight" valves can mimic flooding.

[ 03-05-2004: Message edited by: aeronca65t ]</p>

 

[aeronca65t]

I just thought I'd add to my previous comments to help clarify some points (and part of this is based on e-mails I've recieved from some of you).

I mentioned adjusting fuel pressure: on the the Purilator regulator, this is easy to adjust. The regulator has an adjusting dial with increments of half pounds. You wouldn't need this device if you are running a normal SU electric fuel pump. If you are running an aftermarket electric pump, you probably should run a regulator, since many aftermarket pumps are suitable for much larger engines with higher pressure demands.

If you don't have a regulator and are unsure of your pump pressure, you can always disconnect the line to the carb(s) and connect it to a pressure gauge. If it reads more than about 3.5 pounds, you need a regulator.

If you don't have a gauge handy and are curious about your fuel pump output, you can test the pumping volume for one minute. This will give you a rough idea of how the pump is performing. A standard, small SU fuel pump (as in a Sprite or MBG) should pump about a pint of fuel a minute.

Regarding my comment about EGR valvers being "like a controlled vacuum leak" that will make the car run slightly rich when the EGR is disabled : this is really only true in cars that have an air pump and no oxygen sensor (like the MGB in question).
Modern computerized cars with an oxygen sensor and other feedback controls would not run rich if the EGR valve was disabled.

 

[Dave Russell]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Johnny:
Speaking of fuel pressure, the gauge I saw at Auto Zone for $10 is meant to be just screwed into the fuel line without a way to allow fuel to escape. Is this the correct way to measure fuel pressure. Or, should it be measured as it flows, so to speak thru the system?
[ 03-07-2004: Message edited by: Johnny ]<hr></blockquote>
J,
Closed would approximate the float cutoff condition Which is important to prevent flooding. The pressure would drop to zero with the flow wide open, which it never is, & probably drop a bit at maximum engine power.(As it flows through the system)
D

 

[Johnny]

Thanks Dave, makes sense. I set the "Spectre" fuel regulator to 2 and drove the car. Started fine but upon accelerating starved the engine (coughing, stuttering etc.)Re-adjusted to 3 and car now runs okay. Did notice some leaking from the hoses, need to re-tighten them. Since I own both a MGB and a 1960 AH 3000 I'm considering purchasing a fuel pressure gauge and test the AH. Even though I have a new SU from England on the AH with the modern specs I'd better check it also.

 

[Dave Russell]

Johnny,
You are on the right track. Many pressure regulators are not so good, in that they also present flow restriction in addition to regulating pressure. In that case you need to set the regulator a bit high to give enough flow at max power. A really good regulator would not do this.

On a typical SU fuel pump (the AUF 200/AZX 1200) which is rated at, for example, 2.7 psi at a rate of 7 gallons per hour,(gph) The pressure would not drop much until the flow exceeded the 7 gph rate. Then the pressure would drop in proportion to the excess flow required from the pump.

To put this in perspective, in round numbers, a typical engine will require .08 gallons per horsepower per hour. Eg. the above 7 gph pump could supply a 88 HP engine at this sustained power level without the fuel pressure dropping.

A higher capacity SU pump (the AUF 300/AZX 1300) rated at 15 gph & 2.7 psi, could supply a 188 hp engine at this sustained power level without the pressure dropping much below 2.7 psi.

Since street engines are not driven at maximum sustained hp for very long, a smaller than maximum pump capacity can be used.

More than you wanted to know?
D

 

[Bugeye58]

For what it's worth, I had one of the Purilator regulators come apart many years ago. Luckily, with no ensuing fire, but have run the low pressure Holley regulator ever since. Infinite adjustment, cast aluminum construction, two outlet ports, allowing either a separate feed to each carb, (unless you're running triples, of course), or a pressure gauge tap on the second outlet. I have one that has been on a 1500 Spitfire race car for at least 15 years, and have never had a problem with it. Another plus is that, should it fail, rebuild kits are available. Right now, a new regulator sells for about 30 bucks.
I'm not trying to shill for Holley, but I know what works, and will make the same recommendation for any product that I have had good luck with.
Jeff

 

[Dave Russell]

Jeff,
I found that if you have the extra pump capacity, a return line from the regulator to fuel tank, through a 1/16" restrictor, works well to keep the fuel cool & reduce the chances of vapor problems.
D

 

[Bugeye58]

Dave, I concur. But, racing rulebooks tend to be of the "if it doesn't say you can, then you can't"
type.
And, a return line on a non injected engine falls under the rule about fuel coolers.
I can't.
DRAT!
No coffee cans, coils of copper, and ice, either!
jEFF

 

[Johnny]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Dave Russell:
Johnny,
You are on the right track. Many pressure regulators are not so good, in that they also present flow restriction in addition to regulating pressure. In that case you need to set the regulator a bit high to give enough flow at max power. A really good regulator would not do this.

More than you wanted to know?
D<hr></blockquote>
I am impressed with your infinite knowledge Dave. The "cheap" Spectre regulator ($20) only restricts pressure, but, on an MGB it seems to work well set on "3". However, I havn't driven it really fast yet at highway speeds. It may have to be adjusted even higher in order not to starve the engine. At least now after warm the MGB will start right up. Yeah, my wife can now drive it to work.