Trying to dial in some 41 FCRs on my brothers 97 750. We don't know what main jets are in them, but I am assuming they are 155s. It has an open airbox and Sil Motor high mounts. When opening the throttle it bogs and then dies. Seems like it is getting to much fuel. I am thinking we need to move down the main to around 148.
Anyone have a similar setup or any ideas?
I have 41s on my 750.
Does it bog from low revs at idle? If so, not main jet in my understanding.
Main jet circuit deals with upper rev range.
The pilots are too big for sure. Check float level. Floats effect all jet circuits.
I changed my pilots to a stage smaller. Needle in the middle and leaned out the floats by 1mm.
Granted I run straight pipes and no airbox, just aluminum velocity stacks.
in my experience the 41 run leaner than 39 with the same jetting because of the bigger bore and therefore lower venturi vacuum (pressure differential), all else being equal. so I wouldn't think the std mains would be too big.
my 750 runs 158 mains with the fcr39, and they're possibly a touch small. also, ime, you get a more "non flat" a/f curve with the fcr as you go bigger on the carb bore vs engine capacity ratio, with a leaner spot in the middle. http://bradthebikeboy.blogspot.com.au/2012/10/minnie-gets-some-fcrs-aka-day-full-of.html (http://bradthebikeboy.blogspot.com.au/2012/10/minnie-gets-some-fcrs-aka-day-full-of.html)
but, if it's not getting to wot you need to work thru the other stuff first. I find 52 or so pilots are a good place to start. can you get to wot if you roll it open slowly or if you nail it?
Thanks Gents, I'll start there. It won't get to WOT either by slowly rolling on or by whacking it quick. I'll look for pilots.
Do you think moving the needle clip to the second spot would change anything?
Quote from: YellowDuck on January 14, 2014, 10:36:03 AM
Thanks Gents, I'll start there. It won't get to WOT either by slowly rolling on or by whacking it quick. I'll look for pilots.
Do you think moving the needle clip to the second spot would change anything?
That might help.
But really, it's best to check what jets are actually in the carbs first.
Are the carbs new units from Ca-Cycleworks or.....?
Quote from: Speeddog on January 14, 2014, 11:09:41 AM
That might help.
But really, it's best to check what jets are actually in the carbs first.
Are the carbs new units from Ca-Cycleworks or.....?
They are originally from Cycleworks, but I bought them used from a DMF member.
OK, that's good.
Still need to check to see what you've got in there.
My 900's slow fuel jets were too big and I couldn't make it down the street without the plugs fouling and the bike stalling. +3 on knowing the current jetting. What color are the spark plugs?
Speeddog and Brad have both tuned their own 750s with FCR carbs, so they should have some good tuning tips for you.
They are great carbs by the way, and once tuned, give wonderful throttle response. They transform the performance of the desmodue motor.
[popcorn]
Hey YellowDuck
How's it going with the carb tuning? Interested to hear what progress you've made and what jetting you found in the carbs.
Cheers [popcorn]
Quote from: brad black on January 13, 2014, 04:49:32 PM
in my experience the 41 run leaner than 39 with the same jetting because of the bigger bore and therefore lower venturi vacuum (pressure differential), all else being equal. so I wouldn't think the std mains would be too big.
my 750 runs 158 mains with the fcr39, and they're possibly a touch small. also, ime, you get a more "non flat" a/f curve with the fcr as you go bigger on the carb bore vs engine capacity ratio, with a leaner spot in the middle...
Odd. I get what you're saying. Makes total sense and technically I'm not in the place to question your findings. However, out of the box, even with open v stacks I was getting very rich indications (reeking of petrol fumes, poorer running when at temp, and quite dark and somewhat damp plugs). The leaning out of the floats seemed to have helped me the most I think.
52 pilots are inbound from Cycleworks. I haven't pulled the carbs apart yet because they are on my brothers 750 at his house, I finally got it out of my garage, and I was trying to get some stuff done to my 900. I have to do valve adjustment, belts and new fuel pump on the 900 and get the 750 carbs dialed. Plus I am gonna switch back to my clip-ons on the 900 from the straight bar setup I put on a couple years ago.
Anyway, I'll post up something when I find where we are and results.
Cheers yall
Did some work on it yesterday. Pulled the bowls and had to do a fair amount of cleaning in there. New lesson, pull carbs apart when you get them.
Had the standard Cycleworks jetting. 60 slow and 155 main. Dropped the 52 slow jets in and it fired up. It is still bogging a little but can get up in RPMs if you work on it. And when it does go, man it goes. I am going to move the needle clip up one notch and see what that does.
Thanks for the help so far.
Which groove is the clip in?
Quote from: Speeddog on January 27, 2014, 09:27:27 AM
Which groove is the clip in?
Don't know, I didn't have time to look yesterday.
You get them tuned?
Quote from: koko64 on February 01, 2014, 08:58:43 PM
You get them tuned?
Not yet, I worked on the 900 and had other chores to do this past weekend.
I hope to this weekend, but I see two kids birthday parties on the schedule, YAY. [bang]
Hey YD, what was the outcome in the end? He happy with them (FCR's)?
Quote from: koko64 on March 01, 2014, 07:06:07 PM
Hey YD, what was the outcome in the end? He happy with them (FCR's)?
Ran into another problem we are working on: http://www.ducatimonsterforum.org/index.php?topic=66873.0 (http://www.ducatimonsterforum.org/index.php?topic=66873.0)
pregnant dog won't start now. We have both had limited time to wrench, so it's going a bit slow.
Bringing this one back from the dead.
Finally got the FCRs on the 750 running. Ended up with the 52 pilots, slow speed air screws out 1 turn. I started with the main fuel screw at 1 turn and ended up between 1-1/8 to 1-1/4 out on both carbs. It has a few small stumble between 1st and 2nd gears but it's going. Gonna run it like that for a bit then tweak some more.
Now if I can just get the carbs done on my dad's SuperChicken, I'll have time to work on my own bike.
How's it going?
Quote from: koko64 on January 13, 2015, 02:16:35 AM
How's it going?
It is running pretty strong. There is still a slight stumble at small throttle I haven't fixed yet. I am going to go in a bit on the air screw and see if that helps. Otherwise looks like I need smaller pilots, maybe down to 50s.
The idle mixture screws under the carbs or slow air screws in the bellmouths?
The slow air screws can have an effect with even a 1/8th of a turn. Handy for fine tuning the sub 1/4 throttle range and how the throttle comes on down low from a closed throttle. Turning them in will richen.
Im down to 50 slow fuel jets on my 900 after a few extra mods. At least the jets are cheap up there.
Watching with interest.