weber carb question

PostPost by: nmauduit » Thu May 14, 2020 7:45 am

mbell wrote:Standard pump jets are 35, so going quite a bit bigger. Assume you only get when increasing throttle noticeably?


indeed, I would try 35 before doing anything else, it is also possible that the hesitation when stepping down is too much fuel, all else geing perfect otherwise...
S4SE 36/8198
User avatar
nmauduit
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 2037
Joined: 02 Sep 2013

PostPost by: seniorchristo » Thu May 14, 2020 4:13 pm

I see where 40 pump jets are supplied for SE and Sprint versions.
67 Elan Super Safety
67 Elan +2
seniorchristo
Fourth Gear
Fourth Gear
 
Posts: 616
Joined: 19 Dec 2013

PostPost by: nmauduit » Thu May 14, 2020 4:48 pm

seniorchristo wrote:I see where 40 pump jets are supplied for SE and Sprint versions.


I did some jet trials when I tuned (to a point) my street S4 a while ago concluding with a dyno session, mostly other jets but looked a bit into the pump jet by means of the wideband sensor to get a quantitative notion of what was going on : one can see the enrichement when stepping down, but the initial conditions may vary between stepping down say at 1500, 2500, or 3500 ... and on my setup the corresponding enrichment could end up being too much.

nb: the pump jet acts 2 ways simultaneously, as it sprays at a given rate (more if larger) but only as long as fuel is available for it, while the bottom "leaks" at a constant rate, so with a smaller jet the enrichement is both longer in duration and of a lesser total amount of added fuel.

I had a 40 pump initially (standard bottom, did not play with it), and my goal was to tune relatively well between the idle (closed ) and main (opened) circuits so that the transition would not overly rely on the pumped enrichement (identified around 2900 on my engine), my target being a 12.3-13.3 window for the whole range, and typical use would be to drive at least at 2500, most likely stepping down at 3000 to 3500. This said, I observed on this engine a 40 pump would take the richness way below 12 (possibly 11) and the further take up was a bit lean (13.5 ish or more), while a 35 was more efficient, smoothing both the initial enrichement no further down 12.5 and helping longer, i.e. out further in the revs (granted, this depends on gear, road conditions etc.) till 3400-3600 where the main circuits takes over and is much better controlled... so this is what worked best for me.

anyway, one can spend quite a bit of time on Weber tuning, and one can keep fine tuning to the point he deems appropriate and retain the compromise he prefers - I figured it was enough for a street car, even for the occasionnal track day.
S4SE 36/8198
User avatar
nmauduit
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 2037
Joined: 02 Sep 2013

PostPost by: Craven » Thu May 14, 2020 6:40 pm

If the pump jet system is operating correctly no extra fuel will be supplied under normal throttle operation. Fuel via the pump circuit happens only on snap throttle opening. Jet size, pump stroke, and bleed jet size are all involved in quantity fuel delivered.
Craven
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1623
Joined: 14 Sep 2013
Previous

Total Online:

Users browsing this forum: No registered users and 17 guests