Weber Fuel Seeps

[type26owner]

Getting around to fixing all the leaky bits on the Webers and it looks as though the majority of it is coming from out of the threads on the idle mixture screws (IMS). Always assumed the cold start units were the culprit but upon a closeup examination that looks not to be the case. To cure the leaks from the IMS I cut some lengths of 3/16" ID vacuum hose and replaced each compression spring with a hose making sure it was crushed enough so it would seal the fuel leak and be compliant enough so the IMS could adjusted for the best lean idle mixture which is 1/2 to 1 full turn from closed on my carbies. Well here's the part that took me back it responded much better to the IMS adjustment than ever before. I even had to open the throttle plates via the idle speed screw by a full half a turn to get the engine to idle back at 1000 rpms because it would stall out at 400 rpms otherwise. I'm pretty sure it's not only been leaking fuel out but also sucking in enough air to seriously impair the idle. I thought I had it idling really smoothly before this discovery but clearly that can be improved upon by a lot with the hose. I'm going to have to improve upon the hoses though and come up with a trick seal arrangement.

Anyone else have fuel residue all over the rear of the carbies besides just me?

[john.p.clegg]

Not had this problem but how about o rings under the springs-that might be an effective seal?
John

[type26owner]

Hi John,
That area of the idle mixture screws has threads so an o-ring would have little affect there.

The hose solution works okay for the time being as long as the IMS don't come loose. I need to drive the car for another month or so and confirm that the fuel leak is now gone. I can do this because I cleaned off all the fuel residue. Whatever causes the fuel leaking never happens when the car is stationary and the engine is running or I'd be able to see that happening. Also the amount of fuel leaking must be quite small otherwise I'd be able to smell the fumes wafting back at me. There haven't been any noticable fumes or this would have gotten my highest attention much sooner than this. The leaking is just enough so over time it's makes a real mess and is an eyesore to spoil the fantastic looks of the Webers.

Luckily the amount I had to open the throttle plates (TP) to re-establish the idle speed was not so much that the first progressive hole was brought into play rendering the IMS ineffective. I would have had to go back and enlarge all the air bleed holes that are in the TPs to reposition their edges back downstream again in the sweet spot.

Thankfully the racing season has started here again in the USA.

[type26owner]

My car-nut buddy at work has suggested perhaps some high viscosity grease which is not dissolved in fuel might be the trick to seal up the threads. Only downside I can anticipate is blobs of it migrating into the needle and seat area interfering with the mixture flow at idle. Maybe since that area has a pocket which ports to the progressive holes it might self-clean any chunks of the grease away. Worth a try I guess since it's easy to do.

[type26owner]

Well it's been a week's worth of driving about 300 miles and there is no sign of any fuel has been seeping back out. Also the idle mixture screws have not moved any even though the spring force from rubber hose has now relaxed a bunch.

Realized this is another case of two-phase flow impacting the performance of the idle circuit. The density difference between the petrol and air is so great that with the same vacuum force applied it's a situation analogous to the air as the hare and the petrol as the turtle. Hare's been winning up until now.

#4 cylinder has always reacted poorly to the adjustments of the screw. Not anymore. Betcha it was just leaking in more air than the other three.

[type26owner]

Hey John,
Had a conversation with Mike Ostrov this morning and he pointed out the newer 151 and 152 type Webers have an additional plastic cap under the idle mixture screws spring that captures an o-ring. Pretty good indicator maybe that does work okay to seal up the air leak there. I apologize for not considering that as practical solution. Bet it's a special oring with threads or soft enough durameter so it coldflows into the threads. My brother has those carbies on his Caterham Super7 so I can examine how they did it firsthand. This kinda confirms the fact there is sizable airleak there or they would never have placed any o-ring seals there. Be cool if I can buy the bits and they'll retrofit onto the older carbies.

Pierce Manifolds has exploded views of the 40DCOE 151 and 45DCOE 152 which shows and lists the items. They identified as #44 and #46.
<a href='http://www.piercemanifolds.com/weberexplode.htm' target='_blank'>http://www.piercemanifolds.com/weberexplode.htm</a>

[type26owner]

Rohan,
The behavior is a bit bizarre with the air leaking down the idle mixture screw threads. By turning the idle mixture screw one is still able to force a misfire by leanning out the mixture or fattening it up. It seems to work okay. Symptom that it's not okay is the persistant partial or occasional total misfire. Here's my thoughts on the process. I've not done any actual testing or observations yet.

In all the exploded pictures of the Weber the mixture is shown as mostly liquid fuel with lots of nice tiny air bubbles evenly distributed through out. Betcha that's a fantasy. The only time I've ever seen that kinda flow was in the sightglass on the dryer of an A/C system. Hey, just realized while typing I can make a transparent plexiglass threaded plug which screws in covering up the three progressive holes and possibly see the type of two-phase flow that is actually at work. Suspect since the mixture's velocity is rather slow at an idle that near the idle jet the bubbles are regular and small but as the mixture moves along that changes. The reason it changes is the air moves at a faster rate through the liquid then the liquid and the air bubbles join up probably into a slug-flow configuration. If the slugs are of the wrong size then this means the cylinder is slurping in a changing mixture. Sometimes it's getting more liquid and at other times it's getting more air. Combine that effect with the constant supply of air leaking down the threads and that mixture variation is worsened. The surface tension of the liquid fuel if it wets the threads could seal up the air leak for awhile and make this effect wander around to different cylinders at various times. This is the reason for the nasty idling misfiring of the early carbies I think.

The whys and therefores of two-phase flow is way too complicated for me to tackle in definitive terms. I could give a flying hoot but not understanding the affects will cripple your ability to tune the carbies. Why has every expert that's written about tuning the Webers been totally silent on this subject? Could it be that all of them are actually clueless? Scary to realize they possibly all don't understand the physics which controls the process. I have run across this type of systematic ignorance of supposedly experts in academia too. BTDT. Just gives one ample opportunity to blow the doors off the dumb ones on the racetrack.

[type26owner]

With my Innovative O2 sensor I can observe the mixture at like the 1 milli-second time domain if I remember correctly. What would be cool is if there were a way to feed in a timing signal into the datastream on one of the channels so you could identify the signal from each cylinder and know which one(s) are misfiring. I'll have to visit their website and see if anyone has asked how to do this and if not then pose the question to Klaus myself. This is another way to confirm the mixture varying at an idle. Haven't gotten around to doing that test yet.

BTW, there are three online videos on the Innovation website which gives a quick overview on the usage of their sensor. Suggest folks take the time to check them out. Best diagnostic tool out there for the price. No serious racer would not have one by now. I'm hoping these guys don't go out of business in the near furture.
<a href='http://www.innovatemotorsports.com/' target='_blank'>http://www.innovatemotorsports.com/</a>

[rgh0]

Keith

You just need to put a sensor fitting on each exhaust branch to pick up each cylinders mixture. But I think I remember you still run the original cast iron exhaust manifld so this is proably a little harder to do copared to steel tube headers.

As for 2 phase flow. I dont think many people know much about it at all. I did some work on it when designing steam boilers and hydrocarbon distillation boilers many years ago now. Most of the available design information was based on very limited and specific experimental setups. The theory behind it all was not much use in real world design situations. The normal design approach was to keep the liquid flow rates high and limit vaporisation to levels so you stayed in the relatively stable bubble flow regime.

However changing flow regimes was the biggest problem and ensuring you operated in a steady regime the challenge. The weber carbs face the same problem and I agree with you that changing 2 phase flow regimes are probably the cause of many of the supposedly unfixable problems.

I had a look at the 45DCOE- 54s I run on my Elan today. I had never noticed before but they have a washer and o-ring hidden under the idle screw springs. Probably not original and fitted by someone who owned them before I got them who also understood the air leakage potential.

Rohan

[type26owner]

Hey Rohan,
Remove the bloody seals and experience the dark side! Just kidding!

I'm just happy there is one other person out there that knows what I'm babbling on about.

[tdafforn]

I have Dellorto's on my Elan, and from what I can judge 'O' rings and washers under the idle jet screws are standard to the deisgn.
Mind you, mine had turned to a tar-like substance consistant with them never having been replaced in 30 years
cheers
Tim