|Yet another Lemans IV Jetting Article
There is a very good reason that there are so many Lemans IV jetting articles: both the Euro and US stock jetting specs are UTTER CRAP. It’s amazing to me that these great bikes with the hottest engines that Guzzi had made to that point were cursed by rather dull throttle response.
The US specs…it’s been so long since I messed with them that I can’t even remember the specifics of their crappiness. The Euro specs…waaaaay too rich in the midrange and up top, and too lean near idle. As a result, my bike sputtered and stalled at idle and bogged at high RPM. My Lemans IV, in its stock trim (and many moons ago…) wouldn’t rev past 6k RPM due to an overly rich mixture. Being a Guzzi newbie at the time, I assumed that it was something (anything) besides the carbs. Heck, I even shelled out $150 for the Euro spec jet kit, so jetting should have been perfect, right? WRONG! WRONG! WRONG!
It’s my perspective that the PHM 40/LM IV combination takes a bit of fiddling to get right. In my bike, the 40’s seem very sensitive to altitude and small jetting changes. I’m not sure if this is the case on other PHM 40 applications like the Ducati Darmah. Changing the atomizer from a 268 to a 265 or a needle position one notch up or down can send the bike into fits of coughing, stalling and bogging. That makes the jetting process a bit frustrating.
Refer to the Dellorto tuning guide if you want to see which jets and components correspond to a specific throttle opening. This guide is available in the links section of guzzitech.com. Note that this guide is not necessarily definitive. For example, figure 2, section 2.2 of the online manual explains that idle speed and idle adjustment screw are dominant in the 0-1/8 throttle positions. I found that slide and idle jet selection also affect mixture at 0 throttle… and why wouldn’t they??!!! Similarly, I found that the needle selection affected mixture very close to 0 throttle. In other words, if your experience is contrary to what the Dellorto manual states, trust your experience- the manual is, after all, a translation.
The approach that I took in these jetting iterations was to make the bike work with the Euro spec K19 needle. The needle forms the basis for making adjustments to jetting, as it affects a huge midrange portion of throttle opening. The size of the idle, atomizer and main jets can then be adjusted to fine tune the mixture at “near idle” and “near WOT” openings. If you drop the needle up or down a notch in its holder, you are changing the transition point between the jetting regimes (idle, midpoint, main). Change the needle entirely, though, and you’re back to square one again.
Note: The dellorto manual states that needles affect 1/4 to 3/4 throttle opening, but I saw the effects of needle selection in this case at just off idle, too.
In this case, atomizer selection made a huge difference in the midrange throttle opening. Dropping the atomizer from a 268 to a 265 made a huge difference in throttle response at around ¼ throttle. I knew that I was on the right track then. I don’t know the mechanics of the fuel metering of the atomizer vs. the main jet- they’re simply two fuel orifices in series. For whatever reason, the atomizer is influential in the midrange, while the main jet is influential at WOT.
Boy am I glad that I didn’t whittle down the throttle slide cutaways 1mm in the way that Guzziology suggested. No disrespect to Dave R, but this is another example in which the “recipe” approach to jetting didn’t work. In that case, I would’ve been out $50 to go back to the 5mm cutaway slides that came with the US spec (stock) jetting. Getting the idle and slide dialed in made a huge difference in the idle and “gnat’s balls off idle” throttle opening.
First: stock Euro specs:
K19 Needle- 1 up from bottom
So…after a few months of iterations, I ended up with:
K19 needle- bottom clip
Note the difference in main jet sizes! Needles were raised one notch, but I went from a 145 main to a 130!!! What the hell was Guzzi thinking ?!?!?
I also richened up the idle circuits, which eliminated sputtering and stalling at idle. The bike still needs some choke to start in the 60 degree mornings here in SoCal. Once warm, it fires right up with two twists of the throttle (accelerator pump priming) and no choke. That’s another indication that I was on the right track.
My IV engine, by the way: 11:1 compression, stock bore, B10 cam, port and valve work, Mistral exhaust, etc. (i.e. NOT STOCK).
IMHO, the Lemans IV jetting is something that you’ll have to fine tune on your own, due to variances in bikes. Understand the jetting procedure, and what each component affects. Use these specs to start you off in the right neighborhood. It should be a short journey from there.
The next step is dyno work with A/F ratio measurement. Until my jetting results are confirmed by measurement, this jetting info is as proven as anyone else’s (i.e. NOT), and should be treated as such. Doug Lofgren’s quote that “until you check your work, you can assume that it’s perfect” is exceedingly germane to this discussion.
UPDATE: After the dyno lambda sensor results (see “Lemans IV Dyno Runs” report), I dropped main jet sizes to 127. I may eventually go down to 125. I also switched to a non-EPA K-4 needle, using the bottom clip. This needle has about the same taper as the K19, but was slightly richer on the bottom, and slightly leaner on the top. According to Paul Montgomery at Pro Italia, the non-EPA needles are numbered K 1-9. Anything higher is EPA.
The dyno sniffer showed that the bike was slightly lean in transition throttle, and slightly rich at WOT, so this needle change seems to have addressed these issues. We'll see the next time I dyno the bike.
The bike ran very well before the needle swap, but the K4 needle seems to be the icing on the cake.
FINAL UPDATE: *Important*
OK. This is my final jetting go around. The bike screams with this setup. If your needles have two digit numbers, they should be binned.
K4 needle, bottom clip
Yes, mains went from a 145 to 128. Amazing.
This should provide a good baseline for PHM 40 carbs near sea level. In my experience, engine work such as cam, porting , etc that affects flow doesn't affect jetting that much except at WOT. So, with this baseline, you might have to change mains and needle position for your bike and that should be it. Now go forth and carbourate.