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.
Thanks, Rohan. By choke, do you mean the cold start mechanism, aka the starting circuit? If so, yes, the return springs are engaged, and the lever(s) is in the closed position. But I’ll double check.
I’ve got the Pierce Manifold book, and the Pat Braden Weber book - Baden mentions that you can get gas coming out the throat if the fuel level in the bowl is too high - so I’ll double check the float level. That should be correct - these carbs were sent to me from the Weber warranty guys, who found a problem with the first set I got, and supposedly checked everything on these. We’ll see…
Be very careful because it could result in an inferno if you have liquid fuel leaking. I’d suggest don’t drive it further until you identify the cause. As I recall it in your previous post about this car you were having issues with carbs back then and my impression at least was that it wasn’t a case of the carb supplier telling the whole truth and nothing but the truth because things seemingly didn’t add up. Ultimately it’s really worth getting carbs properly set up on a dyno unless the engine and carbs are standard Lotus production spec and settings are reasonably well known
By “Choke” i meant the venturi assembly in the main body of the carbs. These are held in by the trumpets and the auxillary venturi that the main jet feed goes through is sealed by a sping clip on its side pushing it against the side of the carb body where the main jet feed comes out and into the auxillary venuri. If this clip is weak then fuel can leak out past the venuri and into the airbox
Thanks, 2cams. We’re of one mind on the caution. But after taking action, I’ve got to drive it to reproduce the problem - I haven’t figured out how to do that in the garage. Your impression is not correct - nobody misled me - the initial set of carbs sent did have issues, but was replaced under warranty by Weber/Redlilne. As for the dyno - in theory this engine and carbs are pretty standard Twin Cam kit. In any event, that can and has to wait until this gas issue is resolved, of course.
On to progress, or lack of. I called Weber warranty - they wanted me to check the needle valves and float levels. Which I did and tweaked the levels - 14mm at contact, 22mm at droop. It seemed like driveability improved a tad, but the roughness was still there. And back in the garage gas was now coming out of all throats, front and rear, not just the rears.
So on to the venturis. My auxiliaries are held in by set screws with locknuts that go through the body of the carb and seat in an indentation on the outside of the auxiliary. Braden says these are used in 45’s and “some 40’s”. So either I’m misunderstanding you, Rohan, or my carbs (40 DCOE 151’s) are just different. Braden also says the venturis are held in by the trumpets, but I would think that on the set screw models, those do the job sufficiently.
So, now what? Since I need to get the head repaired for the spark plug thread insert that came out, maybe I should do that, that’ll get the carbs off the car and I can go through them more completely (?).
Alan raises a good point. For whatever reason, Webers either have floats that are small, or needle valves that are big, but they are very sensitive to fuel pressure. Too high a pressure lifts the needle valves from their seats and the carbs flood.
Given that you have this problem from both carbs, either you have been sent a couple of duds, or there is a problem common to both - fuel pressure would be one possibility.
There are threads on here about how to measure fuel level in the carb float chambers, and that might be a good place to start. If you have a low pressure gauge (or buy a cheapo from eBay) you could try checking the fuel pressure, from memory I recall it should be somewhere between 2 and 3 psi.
One other thought, you haven’t overtightened the thackery washers? The carbs need to be able to move relative to the engine or the vibration causes the fuel in the float chambers to froth which messes with the fuel levels. This can lead to similar problems to the one you describe.
I prefer to set the level in situ using a rod of sort from the round mating face of the cover plates (e.g. depth caliper set at 25mm after removing a main jet assembly and running briefly the car at idle to let the level stabilize).
Steve, if the fuel pump is aftermarket (no glass bowl) I have found, and verified by others, that the
fuel pressure is too high and must be regulated down.
From your picture that Weber carburettor was not originally fitted to a Lotus Twin Cam. The picture is not very clear but from my reading it’s either a DCOE 36 or 86. From my references if it’s an 86 it was originally fitted to a BMW 1800 Alpina - this is of course assuming the cover hasn’t been changed at some stage. I don’t have any record for a 36.
If that’s the case your jetting and progression hole positioning will likely not be tuned for a Lotus Twin Cam. It should not be the cause of fuel spewing out everywhere however.
Check your floats too to see that they are correct and not leaking
They are 151’s. Main jets 120, air correctors 200, emulsion tubes are Keith Franck’s VP tubes. Idles are 55 F11. Floats are plastic, not leaking.
It’s got to be fuel level, right? So I’ll get a HF fuel pressure test kit, and figure out how to make an in situ fuel level teater with an acrylic rod.
I think it is most likely fuel level, I’d do the down the main jet depth test with engine running to confirm the level.
Other might disagree but I think its not uncommon for a very small amount of fuel to seep around the outside trumpets.It shouldn’t be much and no pooling in the air box or leaking out the air box.
I’ve fitted o ring under my trumpets to ensure any fuel stays in the throat.
Keith Fankck recommended that I seal the base of the trumpets with EZ Turn Lube, so similar idea. But I agree - there shouldn’t be fuel there at any significant quantity. The Weber warranty guys never heard of this sealing approach, nor have I seen it in any Weber book. But heh, I’ve got his recommended sealant on order…
Workshop manuel +2 fuel section “L” page 55 L22 weber carburetters Air Trumpets.
Commencing at engine n° C 22414 the Air Trumpets sealed with ‘Hylomar SQ.32M’
Never done it myself but mybe it can help.
Alan
Hi Guys,
I have always sealed the Weber trumpets on my S3 due to leaks. I have had the car 13 years now and have done it many times.
Not knowing any better I used what I had available. Sometimes silicon sealer, the last time I think I used Wellseal.
And always a blob on the threads of the trumpet clamp screws to prevent the nuts coming off and being sucked into the engine.
Eric in Burnley
1967 S3SE DHC
I have used the EZ Turn fuel valve lubricant suggested by Keith Franck. I also used it on anything with threads on the Webers such as the mixture control needles and the covers for the transfer ports and it seemed to make everything better. I got mine in US from Aircraft Spruce and Specialty, No affiliation with either company
I am only guessing here, but propane / gas ptfe type sealant is close.
Seems EZ Seal is not readily available.
I have used propane gas fitting in the past, just wondering if it is not correct?
