1990 750 Sport - questions on custom carb setup

[donzielke]

Hey gang, it's been a while.

I finally got the Sport running. Ended up taking it to HordPower here in Ohio. Runs pretty good I guess. That's part of my problem - I have nothing to compare it to... my Monster is a totally different beast (full DP exhaust, jet kit, K&N filter, otherwise stock airbox). I'm amazed at the difference a riding position can make (bent over vs upright).

JD Hord replaced the belts, checked the valves, new plugs, pulled the carbs and cleaned them, put new rubber carb boots on, adjusted the jets, and had it on the dyno. 61 HP and 46 lb-ft torque. He said that's not bad for carbs and slip-ons. He also put some foam pod air filters on the carbs. The problem, he said, is that the carbs are 38mm... WAY too big for the stock compression. He said this carb set is much better suited for a hi-comp piston setup. Well, I can't do that right now, so he got them set up the best he could.

The bike ran GREAT when I rode it home from Bucyrus to Columbus (about 71 miles). Rode it to work on the following Wedhesday and noticed a few things.

1. Brakes suck. Can pads go bad from lack of use? There is a strange feedback through the lever, not unlike the feeling of pads going over slotted rotors... it's not the same pulsation as a warped rotor, but I haven't checked them so that's not ruled out either. I'm going to the shop today to get some new EBC pads and new brake fluid. There's NO brakes if they get wet, as the last part of my ride home was in the rain and I had very little brake power.

2. Hesitation. The bike runs better at hard throttle applications. At steady highway running, it seems to be greatly affected when it's in "clean" air vs. "dirty" air (from heavy traffic or passing trucks). Is this normal for pod filters as compared to airboxes? Does hot air off the engine make it behave like this? I didn't notice this behavior on the ride home Saturday. The hesitation is very noticable and feels like a " I'm running out of gas" condition rather than a "I have no spark" condition.

3. Clutch drag. The clutch drags when the clutch is pulled and the bike is in gear (like at a stop light), and the bike is impossible to get into neutral when running. I don't know if the clutch fluid has ever been changed, I'm going to start with that. I've read a number of articles that stated the clutch slave is problematic and a new part was issued by Ducati. Any info on that? The previous owner put on these chrome H-D style fat grips and I'm wondering if it's not letting the clutch completely disengage... I'm planning on replacing those too.

Anyone with comments and advice, please let me know. Thanks in advance!!!

Here they are together:


A better shot of the Sport:

Thanks,
Don

[nydesmo]

Don,

Does the bike have the stock 'carb' or aftermarket 'carbs'?

The original Weber is the first thing people remove from this bike and replace with (2) single carbs. The Weber was made for a 1200cc Fiat and is known to have a flat spot in the 3 - 4,000 rpm range. My guess is you still have that (1) carb.

Replace it with a set of 36mm Dellorto's and the bike will run much better.

Sounds like your bike has been sitting for a while.

Regarding the brakes ... replace the fluid and clean out the caliper and lines, an easy thing to do. Replacing the lines with braided always helps the feel, as does new pads.

The Clutch ... again if the bike has been sitting, it could have air in the system or fuild could be low. The system could be leaking and that could be from something being dryed out. This happened to me at the resevor on my 750 Sport.

Good luck,
Greg
http://www.nydesmo.com/images/cad_032507_219_good.jpg

[donzielke]

Originally Posted by nydesmo

Don,

Does the bike have the stock 'carb' or aftermarket 'carbs'?

The original Weber is the first thing people remove from this bike and replace with (2) single carbs. The Weber was made for a 1200cc Fiat and is known to have a flat spot in the 3 - 4,000 rpm range. My guess is you still have that (1) carb.

Replace it with a set of 36mm Dellorto's and the bike will run much better.

Sounds like your bike has been sitting for a while.

Regarding the brakes ... replace the fluid and clean out the caliper and lines, an easy thing to do. Replacing the lines with braided always helps the feel, as does new pads.

The Clutch ... again if the bike has been sitting, it could have air in the system or fuild could be low. The system could be leaking and that could be from something being dryed out. This happened to me at the resevor on my 750 Sport.

Good luck,
Greg

Hi Greg -

Thanks for the note! Yes, the bike sat for over a year before I bought it.

The bike has an aftermarket set of carbs. They are 38mm though, too big according to my mechanic. I did pick up some new brake pads and fluid, and I'm looking for new stainless lines.

The clutch fluid has been changed. Mechanic thinks I've either got a warped clutch disk or the big Harley grips (see pic above... came on the bike) are interfering with the lever. I bought some new grips to put on, just need to cut the old ones off.

I read somewhere that the clutch slaves were prone to leakage, did you ever have to replace yours?

Thanks again for your help,
Don

[DukeRdr]

Originally Posted by donzielke

Hi Greg -

Thanks for the note! Yes, the bike sat for over a year before I bought it.

The bike has an aftermarket set of carbs. They are 38mm though, too big according to my mechanic. I did pick up some new brake pads and fluid, and I'm looking for new stainless lines.

The clutch fluid has been changed. Mechanic thinks I've either got a warped clutch disk or the big Harley grips (see pic above... came on the bike) are interfering with the lever. I bought some new grips to put on, just need to cut the old ones off.

I read somewhere that the clutch slaves were prone to leakage, did you ever have to replace yours?

Thanks again for your help,
Don

My bud put 40mm Dellortos on his Sport back in the day. It was all Jimmy had in stock, so we took them rather than waiting.
It took a while to jet them, but he finally got it to run fine. No flat spots or surging.
Both his Sport and my F1 had the old style clutch slave cylinder on them. Like a Paso. Built into the clutch cover.
The slave cylinder wears inside and goes barrel shaped. My friend put new o-ring in his and greased the little throw out bearing and it worked fine until he sold it.
Mine had more miles on it. We had a machinist stick a stainless liner in it, and adapted it to use a small high speed bearing for throw out bearing. And a standard size SAE o-ring. Available at any tractor supply house.
My bike with race cams and stock 36mm carbs, and his with big carbs and stock cams made the same HP. 56. Yep. 56 smokin' horsepower.
They would both run neck and neck down I-77 south out of Canton. He had slip-ons like yours.
Matter of fact, that bike looks like his. He traded it in on a '93 888 in '94 in Columbus.
Oh, and don't ever put Silicone Brake Fluid in anything on an old Duke. It eats the rubber!

[TonyTuzz]

That looks like a fine machine- it takes me back to mine which I sold 7 or 8 years ago (has it been that long?)

Mine only ever made 55hp at the rear wheel with the weber carb, slip ons, and the top of the airbox cut off, so you're doing pretty well with those numbers.

1. Brakes- Mine worked very well- comparable to the newer Brembos on my other Ducatis- I had stainless lines and aggressive pads. When the bike would sit for the winter, the rotors would oxidize unevenly and give something like the strange feedback you describe.

2. Hesitation- Yeah, the weber always had a bit of that. My solution was to ride flat out all the time. Easy to do with only 55hp!

3. Clutch drag- There was a fair amount of drag, and neutral was always hard to find. Mobil 1 20/50 made it a lot better. I could always tell when the oil needed to be changed because shifting and clutch performance would always drop off around 1,500-2,000 miles after an oil change.

Hope that helps!

[nydesmo]

Dellorto's are the preferred set up for Ducati's, but I assume most any carb will work ... some might be more difficult to tune. The limited edition Pantah's (Montjuich, Laguna Seca and Santamonica) had the same 750cc motor with 40mm Dellorto's direct from the factory.

If tuned properly small motors can run big carbs ... the TT2 is a 600cc motor and ran 41mm Dellorto's and put out 80hp. So it can work.

The trick is getting someone who understands them to dial them in.

Good luck,
Greg

[DukeRdr]

I should have posted this the other day when I saw it.

I think I saw a pic of your non-stock carb set-up, are they 38mm Mikuni CVs?

If they are, they should work fine. Unless they have jetting from a 900.

All you would have to do is get some numbers off of the parts inside. Pilot jets,main jets, needles and needle jets, etc.

Compare them to what would be spec for a 750SS. It would help.....

.

[bevelsport]

Originally Posted by donzielke

I read somewhere that the clutch slaves were prone to leakage, did you ever have to replace yours?

Thanks again for your help,
Don

The slave cylinders can be an issue. Is the problem static or does it get worst as you ride? If it does not get worst, you should be able to bleed it to get it back. Does the throwout bearing ever squeal? If it does, it should be addressed. A bad bearing will accelerate the wallowing out of the cylinder bore leading to leakage.

The direct replacement cylinder and piston are no longer available. You can get a later model (Paso style) thru Ducati. I believe it is 2mm larger so you have to get the entire setup - vented cover and piston. About $400+. I have it on mine.

Not all dealers will be able to help you. I have found the guys at ProItalia are able and willing to take the time to cross reference the old Microfisch to moderm part numbers.

[donzielke]

Originally Posted by bevelsport

The slave cylinders can be an issue. Is the problem static or does it get worst as you ride? If it does not get worst, you should be able to bleed it to get it back. Does the throwout bearing ever squeal? If it does, it should be addressed. A bad bearing will accelerate the wallowing out of the cylinder bore leading to leakage.

The direct replacement cylinder and piston are no longer available. You can get a later model (Paso style) thru Ducati. I believe it is 2mm larger so you have to get the entire setup - vented cover and piston. About $400+. I have it on mine.

Not all dealers will be able to help you. I have found the guys at ProItalia are able and willing to take the time to cross reference the old Microfisch to moderm part numbers.

The clutch action and shifting gets progressively worse throughout the ride. My uncle rode it last Friday and we did about 125 miles or so. By the time we got home the clutch fluid reservoir was nearly empty and the bike was extremely hard to get out of gear. It doesn't seem to be affected by just holding in the clutch lever (doesn't bleed out as it's held in, which would indicate it's not the clutch master, right?), so we're thinking it's a leaking clutch slave.

My dealer says he doesn't have listings for the bike in his Ducati parts catalogs (they are a new shop and don't have historical fiches or anything like that) and gave me the name of a guy to contact about getting it rebuilt. BUT... our dealer does have a selection of aftermarket clutch slave pistons from Evoluzione and some seals that we might be able to make work with a little machining.

The carb hesitation is nearly gone now. My uncle thinks it just needed to have the cobwebs run out of it. As for the brakes, we're going to get some custom SS lines made for the fronts. And my Uncle thought the brakes were great compared to his old Nortons and BSAs...