Engine difficult to start following basic service

[Niklas777]

Thanks guys, those are all adjustments i'll try for sure (if those are not too complex to put into place)

[RichardHawkins]

Eric,

How did you drill the extra progression hole?

Richard Hawkins

[ericbushby]

Hi Richard,
yes, I know, that is the question. I will say it took a long time.
First though, Keith Franck sells a jig for drilling the extra hole. It is $150 with half back if you return it.
He also states the hole has to be 1mm diameter and 1mm downstream, which was all I needed to know. I also found someone on the internet advising the same modification.
I spent about two days on and off wondering if I could do it myself, measuring the jet hole where they are situated,
making drawings and plucking up courage.
I found that the existing holes are not so accurately positioned anyway and thought that I could do it as good as that.
Careful marking out, scribing and finally a centre punch mark.
A Dremell turning a pin vice with a 0.8mm twist drill made the hole in about 10 seconds each.
I am a scaredycat so I went under size at first but I have never needed to enlarge it.
Oil on the drill collects some swarf and any I missed just gets blown through the engine. There is very little.
The stumble was immediately gone.
I don`t know of anyone else who has done it so if you do please let us all know.
Good luck
Eric in Burnley

[prezoom]

It would be interesting if one could duplicate the progression hole pattern of the 152G carbs. When I installed the Zetec in the Plus2, these carbs were recommended by the vendor, and included the the correct jetting. Sceptical, but upon inspection I found them to have 5 progression holes, and now I understand. Having 5 holes, makes a huge difference during the transition phase. The first time I started the engine, it ran incredibly clean, and when I drove it away from the garage, the engine ran as smooth as silk. No stumble or any flat spot. Can pull away in a higher gear when driving through town without having to change gears to avoid the hesitation. This is with a 3.5 diff and a close ratio gearbox. For the life of me, I cannot understand why all the 150 style carbs do not have the 5 progression holes. If they did, I would chuck all my 18 and 31's for this design.

[RichardHawkins]

Eric,

I was hoping that you had bought a jig from Keith or made your own. I don’t think my shaky old hands are up to the task.

Richard

[ericbushby]

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

[h20hamelan]

“ 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

[ericbushby]

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

[gjz30075]

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

[ericbushby]

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

[ericbushby]

[/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

[alanr]

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.

[gjz30075]

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

[nmauduit]

55F8 seems very rich, in relation to the other jets you have and engine setup, which is 'assuming'
an SE configuration. Try a 45F9.

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.

[ericbushby]

Hi Alan
Thanks for your comments. Unfortunately I cannot comment on earlier carburettors as these 151`s 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 Weber`s 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