BikeBoy.org - Ducati 750 F1 Laguna Seca (Just Rambling And Some Dellorto Tuning Info)

750 F1 Laguna Seca (Just Rambling And Some Dellorto Tuning Info)

Updated Jan ’06 – table of needle tapers/specs at the very bottom.

Yes, old stuff. We have a customer who has quite a lot of bikes, so we have to work on them all, old and new. Some of them had been sitting for a while, so I got to give them a quick go through to make them run nice again. After his ’76 900SS and MHR Mille, the 750F1 Laguna came in. It was running a bit funny and allegedly dropping a cylinder at times.

I’d never played with one of these before, but have had a few 750F1 in over the last year or so, and have always had a real thing for them. Regardless of the fact they have Dellorto carbs and don’t go that well and my knees hit the fairing, they’re still pretty cool. So the Laguna promised to be something a little special. Turns out it is a little special, but not in a good way.

The std 750 F1 runs the same cam timing as the later 750 Paso and SS, both carb and ie. Not my favourite cam, but considering how old the grind is, it’s quite acceptable. The horizontal cam is in fact the same part #, the vertical being different because it’s the old style, inlet at the rear, Pantah thing. It also has 36mm PHF Dellorto carbs, and the last one we had in amazed me with how well it carburated. Really, really nice. Couldn’t believe it. The ’85 models have the small 500 Pantah valves (37/33.5mm, same as the later 600M/SS), whereas the later ones have 750M/SS sizes of 41/35mm.

The Laguna has cams that first came out in the Montjuich I think. There were three F1 specials – the Montjuich (to celebrate victory at the track of the same name), the Laguna Seca (to celebrate Lucchinelli’s victory there) and the Santamonica (to celebrate Lucchinelli’s victory there in the ’86 F1 World Championship race). According to the Motociclismo book, they had 73 Hp at 9,250 RPM at the axle, while the works bikes had almost 90. Hmmmm. Good dyno that one.

They also have 40mm PHM carbs, the biggest production Dellortos. Given they have really big cams and big carbs I could easily have been convinced that it ran like the bucket of warm puss that it does. My job was to try to make it nicer. Not a very nice job.

I didn’t have any specs for the cams, so asked Doug Lofgren if he had some. I thought he’d said at some point that he had a set, so he sent me an email with a cam doctor graph, comparing the Laguna cams to the std F1 cams. Here it is:

750SS vs Montjuich cams

I don't have the specs anywhere, so I just centered the point of overlap. If it's in that range it should be pretty good.

At this position, the specs are

@ 1mm (0.040")
IO 51 BTDC
IC 74 ABDC
EO 76.4 BBDC
EC 48 ATDC

The inlet is off the seat 360 degrees at 0.004" clearance. Ya gotta ask yourself 'how does that work?'

The curves are: red – std F1 exhaust, green - std F1 inlet, orange - Laguna exhaust, purple - Laguna inlet.

And another email with some more info about these cams:

I have a set of those cams. I'm toying with installing them in my F1!?!

Some friends raced one, nationally, in BOTT. I ported the heads and offered to put it on the dyno but they never took me up on it.

I was told that it didn't pull at all, below 5000RPM. I wouldn't doubt that the one I did with 900SS cams made more power as well as having a wider power band.

These cams really need huge ports with a really huge seat ID. No low lift flow makes it act like a much shorter cam. Also helps kill some of the out-flow at over-lap.

The other side is that they would work with really small ports because they are so long. A small port needs time to accelerate. The fact is, the 'factory' bikes used 43 and 38 valve sizes. (Is that a familiar size?) Can you imagine how sunken the valve seats are on that head? And the dome on the pistons?

Therein lies my quandary with using them on mine. They really deserve the right size valves, but my heads are the earliest F1 heads and they have 37 and 33.5 valves with 14mm spark plug holes. I can't put big valve seats in them without cracking from the sparkplug holes to the exhaust seats. If I use the smaller valves (the seats are big enough to take 39mm inlets) it might actually work OK, but then it wouldn't be anything like the original.

Oh, and there's one more problem. The cases are pre-Paso and are prone to cracking with high RPMs.

Kind of sums it up. If you look at the cam profiles you can easily see just how much bigger the cams are. Both lobes have over 300 degrees duration at 1mm lift, whereas the std cams have 261/262 degrees. And most of the increase for both lobes is in the overlap area. The specs are actually rather similar to the 4V G cam Corsa specs, even the lift is similar. Just like the std 750 profiles are similar to the std 4v cams.

Add to that the 40mm holes that open when you crack the throttle and you’re back to that bucket of warm puss.

Initially, I tried playing with carb set up, going through everything without finding much. Everything was pretty much to original spec. Not that that meant it was any good, just that it was spec. With big carbs and big cams tipping in a whole lot of fuel when the throttles open can help stop the bog, so that was the next place I looked. I went through our selection of slides until I found the pump ramp that I thought would be about the best – in this case, a /5 slide. The ramp is all done by _ throttle, but it starts almost instantly. The std /1 slide ramp starts at about _ throttle, finishing at the same _ throttle point.

However I wasn’t overly impressed with the result. Usually you can tune Dellortos to work well without the pumps operational – most race bikes will have the pumps disconnected, and the big 41mm Malossi’s don’t even have pumps – just a steel plate riveted over where the pumps usually bolt on. The result was a bit better, and you could fuel it up with the pumps to get it moving quite well, but anything over _ throttle under 4,000 RPM still met with the same old “burrrrrrrrrrrr” and total lack of performance.

Then I started playing with the mixture. I needed it pretty rich down low to help with the getting off the line stumble, but then it hunted on cruise. So I started looking for ways to lean it off without adversely effecting the getaway. At low throttle the slide cut away, pilot jet and the needle root diameter/atomiser size (needle jet) control the mixture, as the needle is still on the parallel section before the taper begins. And you need to calculate some of the size changes too – going from a 2.45mm to 2.50mm base needle diameter in a 265 (2.65mm) atomiser is an area change of 25%! Which can really kill it. Often fixing a stumble off the line problem is as easy as going bigger on pilot jets – I’ve fixed lots of things that way in the past, including bevel 900SS, Guzzi LM1000, etc – but not here. Even my old Eldorado responded well to a couple of sizes up on the pilots.

After I’d made it somewhat better I headed for the dyno just to see what things were looking like. Just to get a better feel for what was happening, and to see what the power curve looked like. After the results from this first session I decided to see where the cam timing was. It was about the specs Doug had mentioned, so I checked the piston to valve clearance that I had and advanced them as far as I could (8 degrees), just hoping that this would help. It didn’t do much. The first graph shows before green and after in red, an 8 degree cam timing change. You can see the way the curve goes flat from 7,000 to 8,000 RPM, and then starts heading off again. If we were gunning for power from 8,000 RPM upward then they might be ok. Maybe.

There’s a fellow in Sydney who works at Gowanlochs with one of these who says his is great from 9,000 to 11,000 RPM, so maybe I wasn’t revving it enough.