Idle jets

[webbslinger]

For awhile now I've been running 50F9 idle jets in my 145 (ish) hp stock-stroke engine, but they are too lean so I just moved up to 50F6s. I havent heard of many tc's with idle jets that rich, but I'm using an A/F guage and it needs more gas than the F9s.

The plugs look about right with the F6s, but the guage shows they are a bit rich so I tried drilling out the side air holes to .8mm (from .7mm). I was expecting that would lean them out some but instead it ran significantly richer. The F9s have 1mm air holes so I thought I was on the right track, but ...
What am I missing? Is there a way to get between a 50F9 and a 50F6? Isn't the air hole the only difference between them?
Victor

[Craven]

Correct, only difference given in book is side hole dia, there is a F12 but that has a central hole of 1.5 & one air hole of 0.9. I’ve have found playing around with F numbers can have very strange effect on drivability.

[Verve]

I wouldn`t worry about 50 F6`s ... I`ve had richer progression (idle ) jets than that on a 140bhp twink ... & when you say your A/F Gauge says they are a bit rich ( even though your plugs look about right ) ... do you mean at idle ( shouldn`t be as it`s adjusted out on the mixture screw ) , or on the important progression/off idle up to 3000RPM ?

Certainly increasing the air hole size should not make it richer ... I tend to only need to use F8`s/F9`s/ or F6`s in order of richness , to iron out any hesitancy or stumble ( leanness ) on progression ... & there is a very slight fine tune on the "ramp period" on the mixture screw itself as well if required ...

Paul .

[SENC]

There is a good discussion on sidedraft central of these idle jets and issues they have with engines with lower manifold vacuum. If you're not on that group, worth joining I think. Unfortunately the ioGroups structure is difficult to navigate until you figure out how it works, but there is some useful information. Keith makes and sells his own idle jets (and e-tubes), so realize he is also selling his product, but he shares his testing openly and you can follow the evolution of the e-tubes in particular and he is back testing his idle wets now. There are also a number of quite knowledgeable folks that participate regularly, so good discussion to learn from in the threads.

I did put in a set of his idle w-jets and found a notable difference in idle and transition.

https://vintagetechnologygarage.groups. ... essage/378

[webbslinger]

Thanks for your suggestions and comments. These are 151s btw. It is reassuring to know that others have used F6s, that was bugging me. The idle is good, the screws are out 1/2 turn, and there is no hesitation off the line, or flat spots. But on the way to 4k it is in the 11s, sometimes low 11's, something I probably wouldn't know without the guage. I expect there would be a bit more power to be had with a slightly leaner progression, but I don't know what else to try and my drilling experience was a flop and it won't go on f9s so I'm going to leave it as isfor awhile.
I used to follow Sidedraft Central for and even tried a set of Hyperjets but they were so rich the fuel milage suffered. I decided to stay in charted water until I know more.

[h20hamelan]

I cant recall exactly, I think there are two or three newer iterations of the hyperjets.

[1owner69Elan]

For awhile now I've been running 50F9 idle jets in my 145 (ish) hp stock-stroke engine, but they are too lean so I just moved up to 50F6s. I havent heard of many tc's with idle jets that rich, but I'm using an A/F guage and it needs more gas than the F9s.

The plugs look about right with the F6s, but the guage shows they are a bit rich so I tried drilling out the side air holes to .8mm (from .7mm). I was expecting that would lean them out some but instead it ran significantly richer. The F9s have 1mm air holes so I thought I was on the right track, but ...
What am I missing? Is there a way to get between a 50F9 and a 50F6? Isn't the air hole the only difference between them?
Victor

Also running 50F6 in a stroked (1692cc) 181hp engine. Started with 45F9.

[StressCraxx]

I cant recall exactly, I think there are two or three newer iterations of the hyperjets.

Keith has been working on his W jets to replace the hypojets. He is also working with the progression holes to get rid of the lean stumble. The application depends on the vacuum signal at idle and just off idle.

[webbslinger]

Unresolved threads are annoying, so two things - first, I didn't want anyone thinking I was going to leave it for long with the progression afr in the low 11s, so tried going back to F8 idle jets. Previously I'd tried F8s up to 50, which was the biggest F8s I had, and then jumped to F950s, thinking that was the next step. At first they were fine but a number of other changes rendered them too lean. Then I tried the F650s and they were too rich, which is where this thread started.
So that gets to the second thing - the answer to my search for something in-between a F950 and a F650 turned out to be a F860.
Full disclosure though, I didn't come up with that myself, I called an expert. He said that TCs liked F8 idle jets and given where I was at, F860s were the first ones he thought I should try. Bingo - that's why I refered to the guy as expert. Now the whole progression phase afr is in the 12s, the idle is easily kept in the 13s, and there's no hesitation anywhere

[Verve]

Hi Victor ,

That`s interesting ... I always wondered if there was an overlap between fuel hole sizes , depending on the F air hole size used ie: is the rich 45 F6 , richer than a weak 50 F2 ... but I have never found any definitive information on this anywhere ?

So , I have just rechecked the Des Hammill book on Webers ... it certainly "reads" as though there isn`t an overlap ... ie : you work through the same fuel hole size ( say 50 like you ) adjusting the richness up one step at a time (F2 thru to F6 ) , before doing exactly the same with the next fuel size up ( 55 in this case ) .... which sounds logical enough ?

Can anyone confirm this ?

In your case , you`ve found that a large 60 F8 is "weaker" than a 50 F6 ... but you also found by weakening ( drilling out the F6 air hole larger ) on your 50 F6 made it significantly "richer" ... so , I`m wondering if there are other dark forces at play here ( maybe your A/F gauge is playing around ) ... as I`m not sure any of this makes sense ??

I`m not suggesting your new 60 F8`s are wrong b.t.w. ( I`ve had jets as large as that ) ... but the process that has got you there is very puzzling to me ??

Paul easily confused.com .

[Craven]

I’ve considered the idle jet holder/arrangement as a, if you will, mini emulsion tube, hence explaining the effects on various throttle openings and why moving away from F8 on a twincam will not improve overall performance, be it flat spots, smoothness on gear change, etc.
This may help found on a web page somewhere.
DCOE Idle Jet Selection Arranged by Inlet Area (approximately Richest at the Top)

[mbell]

I find the good running on 60 idle jets interesting. I've gone to 50 equivalent but still trying to tune out very mild none consistent hesitation.

I don't understand the phsyic's but the information I got with my hypo jets show that the fuel supply response isn't linear across vacuum levels (aka engine rpm and throttle openings). I think the instruction state that more air holes weaken mixture at idle (highest vacuum level) but richen the mixture at throttle openings. The reverse also being true.

I suspect something similar is true with standard webber jets, i.e the jets don't give a linear amount of fuel to vacuum. So need jets that provide the right fuel curve based on the needs and vacuum curve of the engine. Which will be function of jet size, body(outlet) size and air openings. The relationship being defined by physics beyond my knowledge and understanding,