Godfrey's LeMans 1000 Jetting

These articles pertain specifically to the LeMans 1000 big valve motor with 40mm Dell'Ortos. My 850T is bone stock when it comes to jetting and seems to run just fine, so did my 850T-3; I don't have any specific advice to offer on other setups.

Jetting is a fussy, detail-oriented business. To be successful at it, you must understand the factory setup in valve size, port size, cam timing, you must understand the carburetor instruments and how all the different circuits interact, and you must spend time and effort to get them all right by doing objective, careful analyses at each change along the way. There really aren't any short cuts. A dyno can make the job go more quickly as it provides an artificial environment and a quantitative measurement that you can only approximate otherwise, but even a dyno doesn't always give you a 100% proper jetting matrix.

I usually do jetting top down: mains first, needle/needle jet next, idle and idle transition last. The reason is that this is the order of sensitivity ... it's much easier to get the main jet circuit right before you get the needle right and it's similarly of less total importance. Very few people spend much time at full throttle with today's big bore bikes so if it's off by a half step or so it rarely impacts our use of the bike by very much. You just need to proceed this way, using the data from your observations and logic that is required from an understanding of how things work. It's not so complex and magical as it is tedious.

The rewards are definitely worth the effort and expense. Nothing is so satisfying as a motor that works as it ought to, and knowing that you made it so.

Godfrey DiGiorgi -- Kalessin@eWorld.com
January 1996

I took the LeMans out for a nice 120 mile ride today, kind of getting myself refamiliarized with it again after not riding it for several months and riding the 907IE a lot. The two bikes are worlds apart in response and feel but both have similar ultimate capabilities. The LeMans is essentially a 1970s motorcycle that has been developed to near perfection, the Ducati is a mid-'80s machines which has similarly been developed to a very high specification. They are about as fast as one another but require very different control techniques to obtain the most out of them.

As you may recall, I started fussing with the jetting on the LeMans soon after I got it. The K&Ns that were on it, the US OEM jetting and the stock throttle/carb return springs made forward progress a surging, stumbling mess and I was determined to fix this. People tell me that Dell'Orto PHF/PHM40s are not possible to tune to be smooth across the board. I don't believe it.

I started by obtaining the complete euro-spec jetting from Moto International (different slides, pilots, needles, atomizers, mains and mixture adjustment screws). I also obtained a stock airbox/plenum assembly and a K&N air filter element to go in it from Harpers Moto Guzzi. From Parts is Parts, I received a set of the standard Ducati-style slide return springs which replace the two heavy springs that Moto Guzzi fits.

I got to this point just before the Colorado trip, it was running reasonably well but idle seemed a little weak and it would occasionally spit. Throttle action was improved but not yet at the level I wanted; the trip was coming up so I was out of time. I bagged a batch of alternative jets and took off.

The small transmission leak that had been an occasional annoyance prior to the trip turned into a gusher during the trip. It was the rear transmission seal, pissing trans oil at the rate of 400cc per 700 miles onto the back of the bike. Yuck. Carburetion during the trip worked ok, a little rich at the higher elevations *except* for the idle, which was smooth and stable as could be. Hmmm. As I returned to sea level, I noted that it began to spit and stumble at idle a bit again. Also, I'd get some pinging if I tried to open the throttle too abruptly at any engine speed under 4500 rpm.

I took the bike apart in July to see what it needed. Indeed, there were a number of details to attend to. The oil leak had made a real mess of the bike, vibration and such had loosened the exhaust header bolts and the horns. I decided that while I had it apart I'd fix the annoying niggles and be done with it.

One of the things that I'd been planning to do was to put in timing gears, just like I had on the 850T. But I figured that the LeMans had the late model tensioner and I could put that off for quite a while yet, it only had 22,000 miles on it. Well, discussion with John Hoffman at Cycle Specialties revealed that I was mistaken ... same old shitty chain tensioner as the 850 had had. Grr. I ordered the gears and they arrived quickly. Last Saturday I took the timing cover off and found 3/8" of play in the timing chain. Looking at the timing marks, it was obvious that the valve timing had become retarded by several degrees. So that's a likely cause for the detonation problem.

Gears went in easily, the fancy external filter sump and lightened flywheel, etc were all assembled into the bike. The trans seal was replaced easily.

Next mod was the throttle itself. I ordered a set of LeMans I race kit throttle cables (for 40mm vs 36mm carbs) and a used Ducati 900SS start/kill switch from Cycle Specialties. From Parts is Parts, I obtained a black finish, Tomaselli Daytona-2C throttle assembly. The Ducati switch requires a little rewiring to mate with the Guzzi harness and the master cylinder needs to move up the right hand bar by 8mm to fit the new switch and throttle. Cables route easily, the inner RH fairing panel is discarded to allow space for the cable runs. Looks factory and fits beautifully. I reset the ignition timing and balanced the carbs with vacuum sticks. Also upped the idle jet to #62s, which seemed to cure much of the idle-spitting.

On my test ride today, there was still trailing throttle backfires and a little bit of spitting. Chatted with Dale about it and he felt that it was still too lean. So this evening I upped idle jets to #65. Idle mixture screws are at just barely 1/2 turn from lightly seated.

No more spitting; stable, lo rpm idle. Idle to off idle transition seems smoother too. Satori.

For the benefit of others who might want/need to get their LM 1000s running right (includes early 1000S models with 40mm carbs and big valve heads), here's my setup:

It starts easier, warms and idles almost instantly, and is smoother all the way around with a big, fat midrange now. Throttle effort, while not light and soft, is at a very good level with perfect, linear action and response. 40mm Dell'Ortos can work extremely well if you are fussy enough, I guess, and they don't have to cause you tendonitis to use them.

Godfrey

I'm at the "extremely fussy" end of jetting now. I really am getting to the very very end of it, finally. I'm determined that these 40mm Dell'Ortos will work correctly.

The last jetting setup worked very well. However, in the course of the past two weeks and a nice leisurely 400 mile bimble this weekend, I noted a small small off-idle spit/stumble and a bit of backfiring which troubled me. Analyzing throttle position/behavior carefully, I became convinced that the problem was too lean a slide cutaway. Since I had them to work with, I swapped in #50/3 slides and rebalanced the carbs when I changed the engine oil and filter. It ran well but lots of deceleration backfiring.

I looked for air leaks and thought about it carefully. No air leaks, hmm, well, this last change had me dropping the idle mixture screws to nearly closed to get a good idle. I suspected the idle jets were too rich at this point; I had likely overcompensated for a lean slide with too rich a pilot jet. So I swapped in #60 idle jets, which cured backfiring and returned the idle mixture screws to 1/2 turn out for a good idle.

Best throttle response, idle, off idle response yet. I might yet try the #57 pilot jets again as they should put the idle mixture screw out another 1/8 turn which would be ideal. And now I think I can drop the idle even a little more to a nice, smooth 900-1000 rpm tickover.

By the way, for those swapping to timing gears: I made a mistake when I first put the gears in. Without thinking, I put in my usual Torco 20w50 semi-synthetic oil. The gears had quieted down reasonably nicely over the first 800 miles and then stabilized into a light, ringing whirr. When I changed the oil yesterday, I realized my error and used plain Castrol GTX 10w40 instead. I noticed in the short 30-40 miles that I rode the bike yesterday in the course of this latest jetting trim change that the gears had quieted down even more. So if you're putting the gears in for the first time, do use non-synthetic oil to break them in.

I must also say that the G&G external filter/deep sump met my expectations handsomely too. Changing the oil and filter took all of 10 minutes and spilled nary a drop of oil anywhere, a marked improvement from the royal pain in the butt that the OEM filter arrangement offers.

Godfrey