jonbill

It sounds like it is going lean as you say. The fact that it takes 4-6 blocks to come on and another block to recover suggests its fuel supply not jetting (as ZH says). Another test would be to stop on the hill when the problem happens and see if it struggles to idle.

https://www.ebay.com/itm/221709332289

the runners look quite slim, but I couldn't see a diameter given for them. they may be an airflow limitation compared to the Jenvey ITBs.

and the idle jets too? much of the part throttle running is on the idles, not the mains.

I think they're both m8 bolts, 22 nm.

Are the jets stacks all the same then for 5 as for the other cylinders? I don't think the linkage can make #5 alone rich. It has to be something unique to #5. its jets, tubes, air bleed, cylinder rings, valves, throttle plate etc.

I think that says they share a design, not the part though. The body diameter is 2mm different, the choke couldnt fit.

sure, why not? chokes might be harder to come by than 40 or 45s, but I expect thats the only practical difference.

I'd check the numbers on the jets and tubes first, that should be a 10 min job. swapping carbs around can be a fair bit of work, depending how the linkages are done.

obvious guesses are that cylinder 5 unit is different from the others. have you taken the jet stacks out and compared them? is #5 idle screw the same as the others, and screwed out the same amount? its still a dcoe, so I dont think its different from a 40 or 45 in how it works.

I only have two idle holders, .2 and .1, so I work with them.

if you were in the UK I could lend you my box of spares. :)

I tend to compare the L28 to 2.0 Pinto engine. they've got a lot in common and the size per cylinder is close. I wouldn't at this point go for leaner holders. I'd just go for 40 jets and see what difference it makes.

if you know what size you want, these guys can make them for you. I think I paid £120 for a set a few years ago. http://www.precisionshims.com.au/

do check that the chokes are completely off. I think 34mm would be a better choke size than 32, but it's not a big difference and I don't want to go spending your money :) I'd go with 40 idle jets as the next step. idle jets are reasonably cheap so that's where I'd start. maybe check your float heights while you wait for them since it's easy. there's a very good book "how to build and power tune weber and dellorto dcoe and dhla carburettors" by Des Hammill which is full of useful info. we'll worth getting a copy.

just a comment on that choke size. I know that there's lots of advice around to use chokes that small, but I think that's mainly driven by ease of setup. if you want to use all the rev range then those chokes will be limiting it's performance. changing the choke sizes however invalidates all your jet choices so if you do want to get the most out of the engine ever, best to change the chokes first.

mtx-l I think should be accurate. How do the AFR values change as you drive around? what do you get cruising on steady throttle below 2500 rpm? and what is it at full throttle above 3000? when you're just off idle and it goes rich, it's on the progression holes and I'm not sure the float level is influential there. I'd guess its just the combination of idle jets and idle jet holder is too rich. unless the choke is on of course. have you got a choke cable connected? is it definitely all the way off?

they'd be very black I think if it was running 7-9. what kind of afr kit is it? fuel pressure is probably not relevant as long as its not overcoming the inlet valves. 2-5 psi should be fine.

I don't think it can run well at afr 7-9. my experience is it'll bog down once your down to 10 and lower. what do the spark plugs look like?

I've got an LD28 block and crank build on the go. At 84x90 (stroke/bore) It's still only 3.2. The Rebello 3.35 is an 89mm custom Crower crank.

I wouldn't really expect stumbling and backfiring at AFR 15. I'd expect more like 17 /18 for that. It could be that your carbs aren't balanced, so you've got one at 17/18 and one at 12/13 averaging out on the o2 sensor around 15. The imbalance could be throttle opening or needle lift. check and adjust so that they both flow the same air at idle and 2000 and 3000 rpm. then check that the needles are being lifted equally at all 3 points.

I have a vague recollection mine has one spring. This could be the "euro" distributor's slightly faster advance curve?

it's obviously man made, the plan was developed by the G8 a few years ago as the solution to the problems of an aging population and continuously advancing (and expensive) medical science. which countries are in the G8? which countries have high death counts? launching it in China is just a smokescreen.

yes, that should get you in the right area. As zedhead says, best to set it with a timing light, once it's running. having watched your video, I don't think that movement is right. it's normal to have some movement again as ZH says, but the mechanical advance is sprung so it should return when you release it. is there more resistance in one direction than the other?

the check points gap thing is an aside. I.e. check the points gap before you try to set the timing. it's a pre-requisite of setting the timing. to set the points gap, you rotate the distributor until the points are right on top of one of the 6 lobes, the most open point.