41MM FCRs on a 750

[YellowDuck]

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?

[memper]

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.

[brad black]

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

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?

[YellowDuck]

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?

[Speeddog]

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.....?

[YellowDuck]

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.

[Speeddog]

OK, that's good.

Still need to check to see what you've got in there.

[koko64]

 
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.

[koko64]

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

[memper]

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.

[YellowDuck]

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

[YellowDuck]

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.

[Speeddog]

Which groove is the clip in?

[YellowDuck]

Which groove is the clip in?

Don't know, I didn't have time to look yesterday.

[koko64]

You get them tuned?