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Help with 40mm FCR jetting and poor mpgs

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So I gave up on the CV carb and installed a 40mm FCR-MX carb off of a 2007 CRF250R and got the jetting really close, atleast it feels close. I haven't done a plug reading but the bike doesn't bog at all, doesn't blow black smoke and pulls really strong.I am only getting 29 mpgs with mostly highway and some spirited riding and I think that is really low considering what I have heard others are getting and the stock carb always got me 44 mpg.

Do you guys think I'm jetted to rich (I must be right), I pretty much copied DotComs jetting but leaned out the pilot and main jet one size. I figured out my pilot was too rich from turning the air/fuels screw all the way in and the bike would still run so now with the smaller pilot I was able to set the A/F screw to 2 turns out. My jetting is as follows;

NCVT needle 3rd clip from top

40 pilot jet

155 main jet

100 slow air jet (stock)

N/A main air jet

68 starter jet (stock)

70 leak jet (stock)

The engine and exhaust is mostly stock with stock header and muffler, Uni air filter with some small holes drilled in the air box, stock cam, and a 102mm JE piston at 9.6:1 compression (XR600R 10.8:1 piston installed)

Tell me what you guys think. I am planning on buying more jets tonight including a 35 leak jet and leaner main jets.

Thanks

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Something isnt right... I have a 41mm with the same basic jetting mix. I get between 45 and 50 mpg around town.

I use an adjustable leak jet/pump on mine. I have it set so there isn't much of a squirt.

Edited by ThumpNRed

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I wonder if the 70 leak jet is a contributed to the poor fuel economy? I think my main jet has to be too big considering it being mostly highway miles with the throttle open as much as it is. Smaller jets are ordered by the way.

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Is your odometer correct? small sumo wheel on front?

Edited by bork

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As far as I remember the smaller the number on the leak jet the more squirt, the larger leak bleeds off more fuel so it doesnt get pumped up and sprayed through the carb.

 

I settled on the smallest leak jet I could find (30 or 35 cant remember) which would put me richer than you. I suspect your 70 leak isnt causing the issues with your mpg's

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I think you are right. Heres a link i found on youtube showing 70, 35, and a blank leak jet. They have different durations too.

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Alright sorry for the late reply, I was in the middle of moving and fixing up my new home.

Anyways I swapped out the leak jet for a #35 from a #70 and also reduced my main jet to a #152. While I had the float bowl off I figured I would check the AP pump diaphragm to make sure I installed it correctly and to my surprise it was installed upside down. So I flipped it over and reassembled. I took the bike out and rode 81 miles with varying roads from highway to twisties and I only managed to get 30 mpgs. I noticed that toward the end of my ride it started to hesitate at 0-1/8 throttle and when I pulled the plug it was really black and sooty. This is why it started acting up I'm assuming.

So I think the #35 leak jet is too much so I went up to a #70 leak jet to reduce the squirt. I also went down to a #150 main jet to help the already rich condition I had before swapping the leak jet. I took it out for a couple of miles around the neighborhood and it feels like it has the most power I have felt yet especially mid to high rpms. I'm going to try and take it out tonight and put 50 miles on it to see where I'm at. I'm also going to pull the sparkplug before I leave and see what it looks like from the short ride I took it on. I'll post back after I do a mileage check and sparkplug reading.

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I suspect riding street your rarely ever on the main jet, I tend to think most of your riding is on the needle ? drop the needle down a clip position or two and see what happens.

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This bike spends 90% of the time in the dirt, right now I'm using the street for tuning purposes. I think your right, I'm going to have to mess with the needle to really dial it in. Thanks for the recommendation. I need to do a full throttle test and plug reading to find the right main jet.

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Has anyone looked into the flo commander 2000 for the 650 l manufactured by performance design.net?

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When jetting a carb you should always start up top (main jet) and work down . Do a long high rpm run ,pull in the clutch and shut the motor off before you even slow down . You want the motor to shut off at the rpms you want to read the plug . The time it takes to slow down and stop can change the plug color and not give an accurate reading. Do that which each rpm range.  

 

Even better is a wideband O2 setup , plug chops are not the ideal way to get things right .

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Its my understanding that the pilot jet is the jet to get sorted out first. This is the jet thats going to let the engine start and idle correctly and if you cant get that, then you cant find the right main jet.

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For final tuning it is from top to bottom. The size of the main effects the needle shape and where it's height finishes at. Needle shape and height effects when and how it takes over from pilot so that effects what size pilot and fuel/air mixture settings. That's a very general description , obviously the FCR has a few other things in play.

Edited by jjktmrider

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Alright, so I put roughly 50 miles and did an mpg check Im getting 32 miles per gallon. I did a plug reading and Im slightly lean but it runs really good but it still has a bog/miss at 1/8 throttle. Maybe the AP pump isnt timed? Im open to suggestions if anyone has any.

Im going to Death Valley in 2 weeks so if I cant sort this out Im going back to the CV carb pretty quick here.

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Did you ever check your mileage with your stock CV carb? (instead of hearsay from others) Maybe you should put it back on & check it, maybe there are some other issues in engine, altitude, terrain, tires, weight, windage, etc..? :excuseme: just throwing out ideas.

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I actually I threw it on last night and went for a ride to check my mileage and I'm getting 44 mpg with Daves mods and 55/158 jetting. I was getting this before the swap.

I do have full knobbies on right now, the engine is fresh with 1,500 miles, the bike is kind of stripped down (lighter), the altitude varies from sea level up to 2,200 feet, and not much wind here this time of the year.

I'm using the the CRF cables and throttle assembly which has a small amount of twist compared to the XRL throttle assembly, so I'm wondering if with that short amount of action is making the carb use more fuel. What I'm saying is there isn't as much control with the CRF throttle assembly compared the the XRL.

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There is nothing in your jetting to account for a near 30% drop in mileage. That is huge! 10% I can see.

The only thing that changes going from your CV to the FCR is really the leak jet/AP. A ride where you hold the throttle steady more or less, the AP is not doing anything. I am surprised that you do not have a bag with the 70! Most big bore engines have the leak jet closed off completely to get as much squirt as they can. You do not seem to need it.

Have you ensure the loaf is set right and that the float needle seat is not leaking?

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So I gave up on the CV carb and installed a 40mm FCR-MX carb off of a 2007 CRF250R and got the jetting really close, atleast it feels close. I haven't done a plug reading but the bike doesn't bog at all, doesn't blow black smoke and pulls really strong.I am only getting 29 mpgs with mostly highway and some spirited riding and I think that is really low considering what I have heard others are getting and the stock carb always got me 44 mpg.

Do you guys think I'm jetted to rich (I must be right), I pretty much copied DotComs jetting but leaned out the pilot and main jet one size. I figured out my pilot was too rich from turning the air/fuels screw all the way in and the bike would still run so now with the smaller pilot I was able to set the A/F screw to 2 turns out. My jetting is as follows;

NCVT needle 3rd clip from top

40 pilot jet

155 main jet

100 slow air jet (stock)

N/A main air jet

68 starter jet (stock)

70 leak jet (stock)

The engine and exhaust is mostly stock with stock header and muffler, Uni air filter with some small holes drilled in the air box, stock cam, and a 102mm JE piston at 9.6:1 compression (XR600R 10.8:1 piston installed)

Tell me what you guys think. I am planning on buying more jets tonight including a 35 leak jet and leaner main jets.

Thanks

I never saw if you stated what elevation your running at.  The needle is what controls most of your fuel economy.   I would put the 155MJ back in and raise the needle clip to the 2nd from the top. You could even try the first clip but then your going to increase your engine temp as you lean it out.  35-40mpg (using US Gallons which is 3.8L) is about what I get.  Some use imperial gallons which is 4.54L/gal which would work out to 42-47MPG, so make sure your comparing apples to apples.   Your leakjet is what controls your off-idle bog and you can calibrate it independently as it does not affect your AFR.  A leak jet #70 is way too big.  Put it a #35 or completely solder it shut.  Also if you have the old AP linkage you will need to do the o-ring mod and mod or replace your diaphragm to increase the pump pressure and duration.  The timing screw is to adjust it so that the fuel stream out of the nozzle doesn't hit the throttle slide.

Edited by D0T-C0M

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Thanks for the recommendations Dot-com, I will deffinately give it a shot. By the way I'm at sea level. I have the carb on the bench so I can set it up in one shot. I will also check the float height, which was good when I cleaned the carb.

In regards to the off idle bog, it didnt start doing that until I went with a smaller main jet (from 155). Maybe I was too lean?

Ill post back here with your recommended setup as soon as I get it put back on the bike and do a mileage test. I may wait until I get back from my trip so I dont put uneccesary miles on my tires.

Thanks for everyones input.

Edited by craftsman13

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