VM28 Carbs - 5I4 Needles Unobtainium?

[IR8D8R]

 I continued my conversation with AI...
I asked about the YL1. "The YL1 is the stock UK jet needle for the 75 RD350?"
<<No, the stock jet needle for a 1975 UK-specification RD350B is actually the 5L1.
The YL1 label is an aftermarket part code quirk. It stems from modern pattern part manufacturers (like Keyster) using the stamp "YL1" or "YL2" on the brass components inside their European/UK market rebuild kits to denote their exact replica profiles of the factory 5L1 needle.
The difference between the two specifications highlights the distinct setups used:

UK/Euro vs. US 1975 RD350 Specs

The 1975 UK market RD350B utilized a different carburetor internal setup than its American counterpart to comply with local noise, fuel, and emissions standards:
• UK/Euro Spec (1975 RD350B): Features the 5L1 jet needle paired with a significantly smaller 105 or 110 main jet because the European models used a heavily baffled, restrictive intake air filter box.
• US Spec (1975 RD350B): Retained the 5I4 jet needle paired with a much larger 140 main jet because it drew air through the standard open-element foam filter box.
If you are sourcing an aftermarket rebuild kit labeled with a YL1 needle, it is designed to mimic the leaner European/UK factory 5L1 sweep profile.>>

 I'm not claiming that this is right or wrong. Just pasting what AI said. I haven't learned to trust AI yet even though we are experimenting with it at my company in a regulated environment. Not even at "trust but verify yet". It does make creating SOP's easy without having to look up a lot of specs. ...At least until the "verify" point. I've caught it out on a few things but it usually admits to being wrong when challenged.

So what happens if you use the 5L1 with a 110 main jet in an engine without restrictive filter? <<(Paraphrasing): too Lean>>. How about 5L1 with a larger main?:
<<Running a 5L1 needle with a larger main jet in an open airbox configuration is a proven, highly effective tuning strategy. It is precisely how many vintage racers and performance tuners configure their RD350s to clear up mid-range bogging while maintaining massive top-end fuel delivery>>

IR8D8R

[m in sc]

its the same profile needle as he uk spec 75b. i know this for a fact.

that ai response sounds like a drunken conversation at deals gap on a wednesday at 1230 am. especially when it refers to a stock 75 as an unrestricted foam air filter w a 140 main jet.

maybe leave the ai comments on fb, they are usually off just enough to cause damage or lots of problems in the vintage tuning realm. 0.02

[IR8D8R]

Wow Mark. That's a strange reaction. I'm not arguing with you. I agree that the Google AI's language is too informal due to them seeking broader acceptance. Other systems I've tried are much more technical and better suited to exacting information selection. It was getting in the way of searching because it was contradictory to what I had specified. I do trust your knowledge.

 I had no idea that a UK airbox was different than U.S. spec in that way, and that affected their jetting choice. I'm still learning this stuff. I assumed that all of the RD350's were the same world-over mechanically. That was not the answers I was seeking or my focus in queries. It just came up...

 I'm mainly curious about using the UK jet needle as a basis in an unmodified carburetor pair because the 5I4 is NLA. I have a bone stock RD350 I'm working on and previously had the same issue as the OP sourcing original needles. I just thought the information was interesting and within the topic theme and I want to understand the history.
 I don't plan to use chambers and my understanding was that the Dave F mod was a compromise for midrage power loss using aftermarket exhaust. I struggled with that tuning for several years. Chuck tried to help me get it correct but we weren't able to get several sets of carbs to perform with the Dave F done. It was good enough but never good.

 It probably WILL be a drunken discussion at Deal's Gap. At 11:30. Not 12:30. I don't like to stay up that late anymore.

[m in sc]

i see ai fumble a lot, and at work a few people rely on it too much and its been a real issue.  not strange.

regardless,  if you can source a uk jet tube it should work just fine, the lines to the airbox arent really nec. or used needles. 

imho, if you can't get it tuned w dave f mod, then it would likely do no better w stock 5l1 needle, and a carb change may have been the only answer on your setup.  theres only so much tune ability you get get out of stock carbs tuning wise. the dave f mod is good because you can them change jet tubes to.. a p2 or 4 if need be. etc.

the next step is a tm or a pwk, and the tms are good because of the (over) tune ability between the needke and jet tube.  i have thrm on my lc, but i prefer pwks over all. only down side there is a 'fixed' jet tube.. which really doesn't matter 99% of the time

my 0.02