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Just tore the upper end of my motor apart because of low compression only to find that everything looked like brand new. After careful evaluation I've come to the conclusion that the low compression problem was due to the rings not seating properly because the engine was being ran on synthetic oil too soon after the upper end rebuild. Je recommends using marvel mystery oil as assembly lube. Has anyone done this a the viscosity is very thin.
 

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10/40 standard oil, a few half-throttle runs from 4k to 7k or so, close throttle and coast back down to 4k. a few more using 3/4 throttle, a few more using full throttle. tootle around for a few minutes. then a few full-throttle runs to full revs. tootle around again and a couple more full-throttle/revs and you're done. change oil to whatever you normally use. most people are petrified to use any revs or throttle and end up doing exactly what your engine has suffered. and, most of them aren't even aware. if you don't load the engine, the rings will never truly/properly seat.

as for JE's recommendation, they're only saying to use MMO as an ASSEMBLY lube not to run it as the main oil. I know guys that used 3in1 as assembly lube on some engines.
 

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No I didn't have a leak down tester and I bought the engine used as the original had cracked cases when I bought the bike. I've been riding it off and on and just wouldn't run right no matter what I did quadruple checked timing did all the carb stuff did a compression test and wala it only had 70 psi comp. So I squirted some trans fluid and it only came up to 90psi. So I was really guessing because it was out of an ss it already had a big bore kit with high comps in it and they detonated it and it crushed the top comp ring groove as it never used any oil. Or possibly a bent valve. Turns out none of that. But anyway just finished everything up today did the 2deg. Timing retard and I had already bought the procom cdi boxes with the relaxed timing curve. I fattened up the jetting a scosh hand lapped the valves in set the valve clearances and I'm running it in with sunoco 100 octane and I have to say it sounds very healthy. I just heat cycled the motor a couple times. If the weather holds I'm going for a ride tomorrow night.😉
 

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Rode tonight for the first time. Still jetting in the carb for new compression. The sigma stage 1 kit I bought a while back is now coming in very close to where the spark plug color should be. It's funny how a spark plug will still turn a nice chocolate brown on race gas.
20191029_185445.jpg
 

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Discussion Starter #7
Looking to replace carbs now. The fcr's and tdmr's are to expensive and for the tdmr's the parts are hard to get.
So I'm leaning on using a set of Mikuni tm40mm with accelerator pump. The big question is this do the carbs need to be phyisically mounted to one another and why can't I make my own intakes that are shorter. I've already read that power is limited by the length of the intake runner. Anybody played with this yet?

Thanks
Gary
 

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I'm no expert by any means, but wouldn't shortening the intake runners be of greater benefit to a more high strung power curve (higher revs benefit)? I wonder if it would make the bike more pipey?

Nice Monster btw.
 

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Non stock runners are always shorter than stock, aren’t they ? Shorter , straighter runners should give better throttle response as well as better top end power. Fuel has less time to drop out of suspension or heat up. Having the correct length for peak power should improve the numbers, but only at the tuned rpm. Not usually noticeable on a street engine that doesn’t see sustained peak rpm’s . The stock runners in all likelihood are just a convenience for packaging the engine and air box in the frame. The stock cv carbs would especially benefit from shorter runners but I’ve never read about anyone doing it.
 

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Non stock runners are always shorter than stock, aren’t they ? Shorter , straighter runners should give better throttle response as well as better top end power. Fuel has less time to drop out of suspension or heat up. Having the correct length for peak power should improve the numbers, but only at the tuned rpm. Not usually noticeable on a street engine that doesn’t see sustained peak rpm’s . The stock runners in all likelihood are just a convenience for packaging the engine and air box in the frame. The stock cv carbs would especially benefit from shorter runners but I’ve never read about anyone doing it.
Like I said, I'm no expert on the subject (for sure!). But you essentially confirmed what I pointed out .... shorter runners benefit upper RPM ranges better than longer ones.
 

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i think an issue with the mikuni is that they like to be quite level, they're not like a dellorto that can be tilted quite a bit. on a short manifold split fcr set up you use a side draft on the vertical and a down draft on the horizontal. the mikuni are only side draft, so you'd need a near 90 degree bend in the manifold. i think.

power wise.


depends how much time you spend north of 7.
 

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Discussion Starter #12
I checked the mikuni tech spec and you can do up to 40 degrees. My thought is this Ducati also needed to conform to emission standards in the 90's and that's the package they came up with cv's long runners etc. So anyway I've been in touch with Steve at bevel heaven and his advise is to not use 40mm carbs they suck for street riding instead you want 36mm and change the rear sprocket to a 39 and you will be very happy. He sells the short intakes, and 36mm Delorto's that are a little pricey but sweet and trumpets with screens. It'll probably be after christmas till I get them on. Also I'm adding a hall effect programmable ignition. ;)
 

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36mm Dellortos are what came on the 650 and 750s back in the '80s. Literally the equivalent of a 39mm CV carb.

If I was putting some TMs on my 900, I would go with the 40s...but that's just me. ;)

But if the Dellortos have the accelerator pumps on them, you'll like them. They really help the bike jump off a corner. (y)
.
 

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Discussion Starter #14
They do and they're brand new. I'ts just such a pita to take the cv's on and off to do anything to them. I'm 90 percent sure after all the reading I've been doing I'll be going Dellorto's.
 

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i've had a couple of belt drive dellorto'd 900ss in recently, and they're both 40. 36 is too small.

the issues with dellortos are 1/ throttle travel and 2/ throttle heaviness. fcr are much lighter, and can have shorter travel. go short travel on phm40 and the feel is heavier. i've been told the mikuni can also have heavier throttle than fcr, but haven't tried them myself.
 

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Do you think the throttle heaviness might be caused by too stiff return springs ? If so you might be able to modify that. How about throttle leverage, is that an issue ?
 

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The reasons FCR's are special is the slide runs on rollers. all the rest are simply a slide in a bore. The engine vacuum sucks the slide against the bore causing throttle hangup , to cure this you use heavier springs. Some times you certainly can lighten springs but understand you may find your throttle staying open even when you shut it. push pull cables help but with a slide that it is stuck you may or may not close the throttle.
 

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The reasons FCR's are special is the slide runs on rollers. all the rest are simply a slide in a bore. The engine vacuum sucks the slide against the bore causing throttle hangup , to cure this you use heavier springs. Some times you certainly can lighten springs but understand you may find your throttle staying open even when you shut it. push pull cables help but with a slide that it is stuck you may or may not close the throttle.
Your experience and knowledge never cease to build my respect for you, ducvet.
 

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...advise is to not use 40mm carbs... ...you want 36mm and change the rear sprocket to a 39 and you will be very happy.
41mm FCR´s and a 39mm rear sprocket & was very happy!!! Nothing wrong with 41´s on the road - sweet as can be.
 

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the dellorto phm40 fitted to moto guzzi lemans 4 and 5 do the sticking slide thing. i've not experienced it with ducati 900ss bevel or belt, but i've worked on and ridden far less of them than the lm4/5.

guzzi have 3 available springs for the phf/m series carbs, all the same apart from wire diameter. i fitted some of the thinnest ones to a sport 1100 in search of a lighter throttle a while ago and haven't heard anything bad as yet. not sure which spring is generic or oem fitment to ducati applications. might be another dellorto spring again.

if you try to reduce the long travel you end up with the same cable load over a shorter travel and that makes it feel heavier again. we fitted a few 851 throttle housings to sport 1100 back in the day to reduce the travel, but we didn't go lighter on springs due to the sticking concern. not sure if it'd be worse from a wot and slam shut or a low throttle where there's already strong vacuum on them, then close the throttle and have them not move. you have to pull the clutch in and drop the engine rpm to fix it.

having the cables come straight out the top helps too.

with a 36mm carb you have 4mm less cable travel at wot and it helps, but it's a smaller carb.

the mikuni flat slides aren't as bad as the dellorto ime, but they're still heavier than fcr. someone who had used the mikunis was commenting on another thread here about the throttle heaviness recently i thought.
 
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