Engine difficult to start following basic service

Sorry Richard, but the idea of me paying $150 for a job I might be able to do myself for nothing is a bit out of my comfort zone.
Ask any of my friends.
We have heard from Keith Franck today that I was right to be cautious about the hole size. it turns out that his jig is actually for a 0.8mm drill. That was lucky, yet I am convinced that he stated 1mm some time ago, otherwise how have I come up with that when I new nothing about it before.
I did consider making a jig but when I found that the other progression were not very accurate anyway it did not seem necessary.
You may be surprised what you can do if you make your mind up and have lots of time but that is up to you.
Cheers
Eric

“ The 152 also came with just three holes. I bought some used 152 for my Lotus 41 and had to drill two new 0.81mm p-holes to cure an off-idle stumble.

The need for more p-holes really depends on how much idling manifold vacuum an engine has. The weaker the vacuum is means more holes are required to make up for the loss of adequate pumping speed. This is called the Vacuum Throughput Effect.

I mentioned this subject many times on the sidedraft forum. Though I’m not sure anyone understood what I was talking about. It is not intuitive.
-Keith“

Edit, this was a post from Keith
Double edit, posted with his knowledge

Has anybody else out there done this?. It might catch on. It does no harm and might do a lot of good.
Eric

Eric, can you post a picture of the progression holes and point out the one you drilled?

Greg,
I will try. It is accessible but vey small and my camera may not be up to it.
Eric

[/attachment]Hi Greg,
I think I have done it.[attachment=0]DSCF0824.JPG
The photo is taken from the right hand side of the car. The word WEBER on the top of the carb. is right way up.
This is the right hand barrel of the carburettor. The left one is a mirror image of this.
The new hole is the small one at the top of the photo. You can see that as the butterfly underneath expose the holes one by one that the new hole fills in the bit just before the old first progression hole.
Previously the first movement of the butterfly allowed air to pass without contributing any fuel resulting in a weak dip.
I am aware that I have caused some thread drift with this but I think it is still relevant to the original question as it allows weakening the idling mixture without causing the off idle stumble.
The source of all this is of course Keith Franck.
Cheers
Eric

Hi Eric,
I am fascinated by this progression hole mod you have done and how successful you say that it has been. Well done!
I think you were quite brave in doing this and i am not sure at this stage if I have a steady enough hand and confidence to do the same.
So, thinking further…
I think if I am correct you have 151’s fitted on your car? I am running with the older 31’s on my car but I have always had a slight off idle stumble and wonder if I too would benefit from this mod. The problem is I guess I won’t know unless I try and do it!
What was your backup plan if the mod hadn’t worked?..Or it had a negative outcome in the running of your car? Presumably you would have injection needle filled the hole with something like Araldite or similar?

Alan.

Thanks Eric! I suppose if it doesn’t work out, then a really small dab of JBweld or epoxy would put it back.

yes, from far away I would think trying that Idle circuit richness decrease may be of interest (a bit more fettling…) - then of course if one wants to finely optimize the tune the help of a lamba sensor make the process faster and more accurate imho.

Hi Alan
Thanks for your comments. Unfortunately I cannot comment on earlier carburettors as these 151s are the only carbs I have worked on since way back when I had a nearly new TR2. I must once again refer you to Keith Franck in the states who runs a website called Sidedraft Central. He is a retired scientist and has been making improvements to Weber carbs for a long time. Keith says that this is the best improvement available to the standard system. He describes it as a design weakness. Keith also makes and sells main and idle jets to his own design which are said to produce spectacular improvements to the smooth running and increased power. You cannot mix his and Webers as he requires a higher fuel level.
Yes, it was a daunting task at first. I spent a long time making drawings and measuring the position of the holes using metric drills as gauges. As I said earlier drilling the hole takes a few seconds. The last resort backup was to plug the holes and revert to the original state.
I did this in August 2020 along with fitting leaner 45F8 idle jets and BPR5ES plugs. The car accelerates smoothly from rest with no hesitation and plugs were (sort of) brown the last time I looked at them and last a couple of years or so.
If you are thinking of having a go, tell us know how it went.
Cheers
Eric

I agree that 55f8 is likely on the rich side. One easy check to get an idea is how many turns you have on the idle screws. If you only have less than two turns that MIGHT signal the jet is too big.

On progression holes, I’d be checking the position of the throttle plate relative to the first progression hole for your idle setting (remove progression cover and shine a light down the barrel throat). They should be positioned so that the progression hole is exposed to engine side just as you come off idle but not at idle.

If that isn’t the case you need to investigate why, too much air going through the bypass so more closed throttle?

Or maybe webber miss placed the progression holes on some carbs. I don’t have this issue on my 151s.

Thanks for that Eric,
Yes I will let you know if I decide to ‘have go’.
Currently I run with the floats at 29mm and Spark Plugs BPR 5ES’s to stop fouling, however 45F8 idle jets are hopeless for me in my Weber 31’s. I have been running for a while now with 50F14 idle jets which seem ok’ish but are not perfect and as always I am on that constant quest for improvement…

Thanks,

Alan.

webstore.com/item/Progressi … /116562782

Keith has them for sale, I would guess you UK friends could share one as postage from the good old US of A will be $50 / £50

Just before you do any tweaking I would go back to the air filter. You scrunched it down to get it to fit so you’ve probably partially blocked its flow capacity, so the air to fuel ratio will be rich. No amount of tweaking will sort that out so have a look first.

Reading through from the original posts, lots of good thoughtful responses. Two things come to mind.

My own experience with webers on the two Twincams I own - One stock, one full race.

Cold start with fuel present in carbs:
3 or 4 quick pumps, crack the throttle barely off idle, crank the starter and it fires almost immediately. Let it smooth out and then go.

Hot start:
No pumps. Crack the throttle barely off idle, crank the starter and it fires almost immediately.

One issue that will make for difficult starts is worn ignition points that are almost closed. Check the points to make sure the gap is within spec.
Second issue is a dying condenser. Starts fine when cold, gets hot and doesn’t want to “condense” any more.
YMMV

Good point on the condenser

If you have resistor ignition system, this will be less of an issue.

Full 12+V kills condensers, so if one accidentally leaves ignition on…