Ducati.ms - The Ultimate Ducati Forum banner

21 - 40 of 56 Posts

·
Registered
Joined
·
404 Posts
Discussion Starter #21
My stock carb bike is exactly this, so if you sort it out by all means share!

Edit: to get to this relatively satisfactory performance I went back to the stock airbox lid, put Db killer's in the exhaust on their 2nd quietest setting and fiddled excessively with the needle height and idle enrichment settings. All new needles on a zero mile NIB carb. Now after 1,200 miles on two sets of plugs with perfect looking burning I've accepted the low rpm performance although it does rush through the 2-3krpm's very quickly with a Nichols flywheel and a 41 tooth rear!

Could be a co-incidence but in the last week I've replaced the throttle cables and for a while the "surging, unevenness" at steady throttle was worse until I loosened the return throttle cable. Now while I can still hear and feel the steady throttle roughness below 5k, I can no longer see the variation in the tach. Slightly too tight throttle return cable amplified the unsteady issue.
OK I am using the stock airbox top not modified w/ K&N filter, & Carbon Tech exhaust, Factory pro stage 1 jet kit everything else is bone stock---I am using 130 main jet, Slide needles 2nd position from the top & 37.5 pilot jets that is the set up as of now( had 40 pilots-too much), I tried moving the slide needles to the top position this did not work so put them back air screws 3 turns out-if I go in it gets worse-3-4 turns out works. I am still getting a little stumble off idle under load. I am thinking of trying the 127.5 main jets that I have and see what I get-the stumble is not real bad and goes completely away over 4K rpm. It should take me about 1 hour to change the main jets and get it back together. It's a lot better but not exactly where I want it-- though it is now rideable
 

·
Premium Member
Joined
·
3,550 Posts
Nice job on a period correct early 900 carby.

Keep in mind those are fuel screws not air screws so the further in you put them you are leaning it out. Early 900's used 40 pilots but the carbs may not be original but later similar looking carbs that did come with 37.5's. Bottom line it will tell you which it prefers and if the heads were ported then it will want less fuel so if the carbs are right but has ported heads that might account for the smaller pilots working better.

Odd looking reaction rod on the rear brake I do not remember seeing that design before. back in the day they were mostly stock or carbon fiber tubing.
 

·
Registered
Joined
·
1,600 Posts
My bike had only a few upgrades on it when I bought it - carbon fiber rear hugger, clutch cover and Remus pipes, rear view mirrors all from the Corse catalog, Goodrich brake lines on the front brakes and oil lines. The original seat was included but that is a Corbin. This has the original early model black brake and mounting position on the rear, original stickers. NOS body panels with a darker windscreen added. I'm the first owner, those are manufacturers plates.

 

·
Registered
Joined
·
404 Posts
Discussion Starter #24
Rode her home tonight for the first time it is very close to perfect running, This was a European sold bike not a N.A. bike, I think the numbers on the carbs were a little different but I dont have the book here at home to look, I have the fuel/ air screws out right now 3 turns with the 37.5 pilots, the mains right now are 130 and pulls great up to redline. I had a spare airbox lid (the one that was originally on the bike was modified) with the intact original lid I could not clean the running up where I was happy, So I tried removing the snorkles this was too much air way too lean cleaned up all of the stumbling but under hard throttle when I close it it would still stay at higher RPM --then it would finally start to slow down way too lean--so I then tried the original lid that was modified-( with the original snorkles) picture a triangle cut between the snorkles,-this cleaned everything up almost-It is night & day better -tomorrow I may play with the fuel screws a bit more just to see if I can dial it in better but she is very close
 

·
Registered
Joined
·
404 Posts
Discussion Starter #25
and the reaction rod yes I have not seen one like that before -who knows with Renato being the original owner --it's not like he had a problem getting or having parts made lol
 

·
Registered
Joined
·
404 Posts
Discussion Starter #27
I'll have to do that tomorrow but I'll post a pic tomorrow
 

·
Registered
Joined
·
404 Posts
Discussion Starter #28
OK here are pictures of the modified airbox top, The triangle cutout was there when I got the bike-I drilled the 4 extra round holes--the last picture is a rubber plug that I can use to block off any of the round holes if I find it's too lean. the plug is a BMW part --it's the timing plug for any BMW twin from 1952-1995
 

Attachments

·
Registered
Joined
·
5,791 Posts
The screws will affect mixture to at least 5 turns out IME, so rather than play with restricting air flow with the air box you’ll be able to get the mixture right by just using the screws. My carby was tuned at sea level in Florida to run smoothly even at just above idle. If the idle hangs , adjust the screws out 1/2 turn at a time . Don’t bother with tiny adjustments when the idle is still hanging, you’re too far off. If you go past 5 1/2 turns out, go up a pilot jet size and start over. Remember these are vacuum slides. Your mixture and especially smooth running at low rpm’s is dependent on the slides rising smoothly and simultaneously. After confirmation that the butterflies are in synch, with the lid and filter out raise rpm’s slowly and make sure both slides are rising together with no dancing around. If they don’t, check the diaphragms, slide movement in the bores, and that the diaphragm lids aren’t warped/ leaking. The hoses on the air intakes to the diaphragms are prone to collapsing when hot, and the little “cans” get easily clogged. I’ve replaced mine with fuel line and fuel filters.
 

·
Registered
Joined
·
404 Posts
Discussion Starter #32
Well I talked to Mark @ Factory pro today, I was going to order new needle jets since they did not come in the kit, But suggested trying 1 thing first, I have the floats set @ 14mm -he suggested resetting them to 15mm to get the low end stumble out, said by lowering the fuel lvl in the bowls slightly this very well may solve the issue--I'm going to try this tomorrow if it works great--if not then I will probably order new needle jets and hope that works --I'll let you know what if anything gets this exactly where I want it to be
 

·
Registered
Joined
·
404 Posts
Discussion Starter #33
Well still screwing around with these damn carbs, on the recommendation from Mark @ Factory pro I have new set my float lvl's @ 15 mm -I have the 40 pilots in, 135 mains & slide needles set to 2nd slot from the top. Now thru all of this the bike has pulled great during acceleration --the only thing that made that worse was I tried to change needle position-then went right back to the 2nd slot --there seems to be perfect. My issue continues to be that @ steady throttle say 4K the bike seems to be too damn rich it stumbles or burbles very lightly. it is rideable but I dont like this. I have tried different combinations stock airbox & modified airbox, I guess the next thing is go back to the 37.5 pilot jetsto see if I can make this damn thing run like I want it to.--More to come
 

·
Registered
Joined
·
1,412 Posts
i never mess with the float level on these carbs. if the fuel level is right (and i've really not seen one that wasn't) i pay no further attention to the float level.

sounds like it just has worn needle jets to me. but, in saying that, i've got a 750ss here now that has a wacky miss on cruise that i can't work out.

i've also seen a jet carrier that for some reason was bad.
 

·
Registered
Joined
·
404 Posts
Discussion Starter #35
I think I am going to order a set needle jets on Tuesday and just replace the damn things if for no other reason then to rule that out as a problem
 

·
Registered
Joined
·
5,791 Posts
I didn’t reread the whole thread so this might be a repeat:
1) You can adjust the needle position less than 1 groove by using small thin washers.
2) These carbs can leak internally and if they are leaking internally you will never be able to get the tune right. If you haven’t already, replace the gasket between the needle/ seat block and the main body. Also replace the gasket between the main jet holder and the carb body.
3) If the issue is at 4000 rpm remove the filter lid, start the bike, raise the rpm to 4000 and hold a few times. If the vacuum slides are not rising together, or if they are dancing around when held at a steady rpm, then that may be the problem. Remember, the needles are attached to the vacuum slides. If the slides don’t hold steady then the needles will be fluctuating with them. This probably contributes to the needle and emulsion tube wear in that rpm range. If they don’t stay synchronized, check the diaphragms, slides move freely, replace the o rings under the caps, verify the caps are flat/ sealing, verify the breather tubes flow freely in both directions.
 

·
Registered
Joined
·
404 Posts
Discussion Starter #40
I was mistaken --like Belter -it happens at my age lol
 
21 - 40 of 56 Posts
Top