Weber - gas coming out of 1 throat

[promotor]

I've been working on a conversion from Strombergs to Webers. I'm at the point where the engine starts easily and idles well. When accelerating or cruising at 3k RPM, the engine feels rough.

Engine is 100 miles after a lower end rebuild. Compression is 190 +/- 5.

After a 12 mile run, about 1/2 at highway speeds, back in the garage, there is gas on top of the right side footwell, and inside the airbox, primarily under the throat 4. Spark plug 4 is significantly darker than the others.

For this run the crankcase vent hose was removed from the airbox, the airbox inlet was plugged, and a longer hose used to route the vent to under the car.

Any ideas what's causing this?

IMG_1291 3.jpg

IMG_1286 3.JPG

Could a faulty spark plug be causing unburnt fuel to be pushed back out of the trumpet? There's plenty of fake fake plugs out there. I've had a few NGK - first issue is that the gasket ring doesn't last two torqueing events. Didn't run them in the engine even though they were untested. That was enough for me.
Getting a decent spark on the faulty plug? Plug lead / igntion system etc?

If you are confident it isn't the pump delivering too much pressure and that the auxiliary venturis (ie chokes) are sealing correctly, other than fuel height and the seating of the needle valve, there isn't much else it can be except for machining tolerances on aux vents and trumpets etc. What do you think it could be?

[2cams70]

Don't dismiss fuel pressure issues quite yet. A bit more digging on the net suggests 2psi to 3psi fuel pressure is the optimum range for Weber DCOE carbs, and that they will start to flood at pressures over 4.5 psi.

The fuel pressure according to the LTC workshop manual for Weber carbs is supposed to be 1.25 to 2.5 lb2. Given the decimal places in the figure you will need a sensitive guage. Not all glass bowl fuel pumps are the same so don't just assume because it has a glass bowl that it is the correct one. The delivery pressure is determined by the diaphragm spring according to the application. Also carefully check the needle and seat - is it worn? Is it genuine? Is the sealing washer OK and has it been torqued correctly in the cover? You won't know if the float is good or not unless you throw it in some hot water and watch for bubbles indicating a leak.

With regards to spark plug thread repair I recommend locknstitch. Quality stuff that won't come loose again in that difficult application. See link below.

https://castingrepair.locknstitch.com/c ... ad-inserts

[Frogelan]

On a very good YouTube channel Mike's Restorations concerning a 67 911 (worth watching as the fellow is a first class car restorer), Mike (Las Vegas) had quite a few problems of the same type with IDFs.

I suggest you watch this.

[h20hamelan]

I've been working on a conversion from Strombergs to Webers. I'm at the point where the engine starts easily and idles well. When accelerating or cruising at 3k RPM, the engine feels rough.

Engine is 100 miles after a lower end rebuild. Compression is 190 +/- 5.

After a 12 mile run, about 1/2 at highway speeds, back in the garage, there is gas on top of the right side footwell, and inside the airbox, primarily under the throat 4. Spark plug 4 is significantly darker than the others.

For this run the crankcase vent hose was removed from the airbox, the airbox inlet was plugged, and a longer hose used to route the vent to under the car.

Any ideas what's causing this?

IMG_1291 3.jpg

IMG_1286 3.JPG

Could a faulty spark plug be causing unburnt fuel to be pushed back out of the trumpet? There's plenty of fake fake plugs out there. I've had a few NGK - first issue is that the gasket ring doesn't last two torqueing events. Didn't run them in the engine even though they were untested. That was enough for me.
Getting a decent spark on the faulty plug? Plug lead / igntion system etc?

If you are confident it isn't the pump delivering too much pressure and that the auxiliary venturis (ie chokes) are sealing correctly, other than fuel height and the seating of the needle valve, there isn't much else it can be except for machining tolerances on aux vents and trumpets etc. What do you think it could be?

You need to rule out #1, & #2, above.
Colourtune, or a laser thermometer and measure each exhaust runner @ head (less precise)
Pressure, verify

[RichardHawkins]

Steve,

Keith’s VP (Venturi Pump) conversion works on a different principle. My understanding of this is that the larger the dry jet (air jet or air corrector jet) the more fuel your engine will inhale. Keith is a physicist I am a mere engineer, but what Keith has come up with appears to work on the same principle as an ejector. I have only come across ejectors using steam, the flow of steam through the small hole (dry jet) induces a flow around that jet, the bigger the jet the more flow. In my case the purpose is to suck the air out of a chemical process vessel.

Hope this helps

Richard Hawkins

[steve lyle]

Could one of you who keep putting the fuel pump on the suspects list please explain to me why the fuel level test doesn't clear it? I would think if the pressure was too high, the fuel level would be way too high in the bowls, not 1.2 - 1.5 mm low. What am I missing here?

[steve lyle]

Steve,

Keith’s VP (Venturi Pump) conversion works on a different principle. My understanding of this is that the larger the dry jet (air jet or air corrector jet) the more fuel your engine will inhale. Keith is a physicist I am a mere engineer, but what Keith has come up with appears to work on the same principle as an ejector. I have only come across ejectors using steam, the flow of steam through the small hole (dry jet) induces a flow around that jet, the bigger the jet the more flow. In my case the purpose is to suck the air out of a chemical process vessel.

Hope this helps

Richard Hawkins

Thanks, it does. I need to ask Keith about that. I would have hoped that if switching to his tubes meant that the standard rule of thumb for air correctors doesn't apply, he would have mentioned that on the website or in the doc sent with the tubes. But maybe physicists don't think like that.

[steve lyle]

What do you think it could be?

Darn good question.

There are two problems here that may or may not be connected.
1) Fuel seeping out of the throats.
2) Rough running, most clearly felt during acceleration, but always there to some extent.

For 1, it sounds like based on several replies in this thread, and Keith's view, that it's the nature of the beast. Run a bead of EZ Turn around the base of the trumpets and call it a day. If fuel still seeps out, go back to the drawing board.

That leaves problem #2. I've got to believe that the soot on plug #4 is a clue. I have checked the temps of the header runners, and #4 is as hot as the others, but I'll recheck. The head is newly rebuilt, so maybe it's involved here, although the compression #'s seem to say it's ok. But I'll take a closer look when I pull it. Rough running indicates that 1 or more cylinders aren't pulling their weight, so what's different between the cylinders? Well, the thread insert on #2 came out, so that plug is different than the others, both in type and length of service. So get that fixed and it will remove one difference.

On to the carbs. What's different about throat #4? No idea, but one test would be to swap the carbs, front to back, and see if now #2 looks like #4. That would definitely (?) point to something in that carb being the culprit.

I'm not sure if this is significant, but when I run with my O2 sensor stuck in the exhaust pipe (bung not yet installed), the numbers vary pretty wildly, particularly at higher revs. That seems to indicate that there's significant mixture differences between throats. Since all have the same jets, the fuel levels are almost identical, etc., it's either how the jets were assembled, part variation, or I don't know (tried to follow the rule of 3, but...).

So tomorrow I'll get back to what I know, and pull the head to get it fixed. That will give me some more time to think this through.

[2cams70]

With all due respect you are not doing things in a logical manner. Sort out your fuel pressure and report back what it is. I don’t know what Keith Frank is about but it’s normally the sort of stuff I steer clear of because it sounds too much like black magic. I could be right or wrong on that but just sharing my approach for you to take it or leave it. If Weber got it to work with their standard range of jets and calibrated components there’s no reason why anybody else shouldn’t be able to do the same. Get it to the point where it isn’t a fire hazard and drive it to the nearest rolling road dyno to have the jetting and maybe other issues sorted out.

[steve lyle]

With all due respect you are not doing things in a logical manner. Sort out your fuel pressure and report back what it is. I don’t know what Keith Frank is about but it’s normally the sort of stuff I steer clear of because it sounds too much like black magic. I could be right or wrong on that but just sharing my approach for you to take it or leave it. If Weber got it to work with their standard range of jets and calibrated components there’s no reason why anybody else shouldn’t be able to do the same. Get it to the point where it isn’t a fire hazard and drive it to the nearest rolling road dyno to have the jetting and maybe other issues sorted out.

I would, if someone could explain how I can have correct, or slightly low, fuel levels and high fuel pressure. Barring that explanation, fuel pressure is a red herring and pursuing it a waste of time. Which seems logical to me.

[2cams70]

I'd suggest you need to accurately measure the fuel pressure because that determines whatever else happens in the carb. Whether you wish to take that advise is entirely up to you however. The level of fuel in the carburettor when it is running on road may not be the same as when the engine stopped especially if the carb is leaking.

[StressCraxx]

With all due respect you are not doing things in a logical manner. Sort out your fuel pressure and report back what it is. I don’t know what Keith Frank is about but it’s normally the sort of stuff I steer clear of because it sounds too much like black magic. I could be right or wrong on that but just sharing my approach for you to take it or leave it. If Weber got it to work with their standard range of jets and calibrated components there’s no reason why anybody else shouldn’t be able to do the same. Get it to the point where it isn’t a fire hazard and drive it to the nearest rolling road dyno to have the jetting and maybe other issues sorted out.

I would, if someone could explain how I can have correct, or slightly low, fuel levels and high fuel pressure. Barring that explanation, fuel pressure is a red herring and pursuing it a waste of time. Which seems logical to me.

Too much fuel pressure will force fuel past the needles causing a high level in the carb bowls especially if the engine is not running smoothly. The floats bounce around from the vibration and bounce the needles off the seats. Target fuel pressure should be 2-1/4psig. 2-1/2 max.

[promotor]

What do you think it could be?

Darn good question.

There are two problems here that may or may not be connected.
1) Fuel seeping out of the throats.
2) Rough running, most clearly felt during acceleration, but always there to some extent.

For 1, it sounds like based on several replies in this thread, and Keith's view, that it's the nature of the beast. Run a bead of EZ Turn around the base of the trumpets and call it a day. If fuel still seeps out, go back to the drawing board.

That leaves problem #2. I've got to believe that the soot on plug #4 is a clue. I have checked the temps of the header runners, and #4 is as hot as the others, but I'll recheck. The head is newly rebuilt, so maybe it's involved here, although the compression #'s seem to say it's ok. But I'll take a closer look when I pull it. Rough running indicates that 1 or more cylinders aren't pulling their weight, so what's different between the cylinders? Well, the thread insert on #2 came out, so that plug is different than the others, both in type and length of service. So get that fixed and it will remove one difference.

On to the carbs. What's different about throat #4? No idea, but one test would be to swap the carbs, front to back, and see if now #2 looks like #4. That would definitely (?) point to something in that carb being the culprit.

I'm not sure if this is significant, but when I run with my O2 sensor stuck in the exhaust pipe (bung not yet installed), the numbers vary pretty wildly, particularly at higher revs. That seems to indicate that there's significant mixture differences between throats. Since all have the same jets, the fuel levels are almost identical, etc., it's either how the jets were assembled, part variation, or I don't know (tried to follow the rule of 3, but...).

So tomorrow I'll get back to what I know, and pull the head to get it fixed. That will give me some more time to think this through.

On my DCOE Tipo 31's I have just measured how far it is from the top of the main body with the lid removed to the bottom of the fuel delivery port/hole that drops the fuel from the emulsion tube wells down into the carb throat - that measurement is averaging 22.6mm and will be around 18mm to the top of the hole (can't measure that and it's not really relevant, but provided just for the sake of it).
Whenever fuel gets to or above the bottom of this hole, the fuel spills over and runs into the carb throat - the float level being too high will cause this. I think of this port like a hand wash basin overflow port.

Add to this the fact that the hole for no.4 side sits lower in relation to the fuel than no.3 - due to fuel sitting level but the engine is higher at the front than the back - then you can get a situation where one is ok and the other isn't.

The important question is: is solid fuel leaking over (ie solid rather than proper air/fuel mixture) during proper running / driving of the car when fuel demand is higher, or is it just when idling at low demand / idle.

If I were using Weber original emulsion tubes I'd be setting the fuel level just below this hole between 0.000-0.080" or so and experiment. If it's too low the emulsion tubes won't have enough fuel for all fuel demands.
If I understand correctly you have Keith Franck custom tubes and he clearly goes against the grain to make his work differently, with a potential downside.

[Davidb]

Perhaps this is the time to mention a fuel pressure gauge that I just purchased from Amazon-
I confess I have not used it yet but it is surprisingly well made and very easy to read since it only reads from zero to five psi:
https://www.amazon.ca/dp/B08TQJPY17?ref ... tails&th=1

[TWebb]

Steve,

Don't know if you recall, but I have a Fuel Pressure regulator (with a gauge) immediately prior to the carbs. The regulator is a Holly product if I'm not mistaken.

Tim