FCR carb requires large pilot jet ATC 350x?

[jays375]

Been having some issues with the 37mm FCR carb on my 350x.It requires a large pilot jet for some reason,72.Main is currently a 160,not done jetting.I even tired a 39mm and had the same issues.The motor is mildly worked.

[El Camexican]

You are saying you tried a different carb and it still ran best with the same large pilot jet? If so, is it running good with the jet and if so what is the problem? Also, does that carb have a choke, or does it just use an accelerator pump to get started? I know one of the Keihin carbs is like that, but I don't recall which model.

[xrider]

I haven't had the chance to get up to the garage to dig up the numbers from tuning my 350x. Give Jesse at Knobbies motorsports a call at 419-394-8776. He is who built and dyno tuned my motor. He can most likely help you out.

[Jason Hall]

Sounds like you have something adjusted wrong to me.. Have you checked to see how far open the slide is at an idle?? My guess would be you are bombarding the thing with a huge pilot because the slide is Way to hi?? If that is the case, then it will transition up to the needle badly also.. Also on a FCR slide there is a plate with a round seal. If the plate is upside down, or the seal is not sealed it will run bad also. A 72 pilot is Absolutly Monsterous in any engine with a FCR, unless you have a air leaking somewhere..

[jays375]

Well it did the same thing with a different carb and intake also.Both carbs I tried were from a Yamaha.Was told they weren't the best ones to use.There weren't any issues with the stock carb.

[Jason Hall]

182807182806
Not sure if these pics will work correctly, as I'm on my phone, but your slide should not be open more than the one in the pic, and the round seal under the throttle valve plate needs to be sealing good, and installed correctly. The parts I'm talking about are numbers 21-22. The seal is availible through Yamaha seperatly. Honda wants to sell you the entire plate and seal. There is also supposed to be a air jet in the back of the carb where the air filter goes on. If someone took that out you will chase your tail also.

[Dirtcrasher]

Jason, You educated me on the leak jet. Of course that was near a year before I got an FCR.

I believe you explained to me (although I didn't right it down like a meathead!!) was:

1) If a leak jet were too large, too much fuel from the accelerator pump was returning.

2) Too little a leak jet and too little fuel was available for the accelerator pump causing a person to run a large pilot?? (could this be his case)??

OR, Am I completely lost?? I have a package to send your way soon if that is AOK by you while I'm busted up.........lmk - I will call first.

THANK YOU!!

[Jason Hall]

No DC you are correct!! If his leak jet is way off, it could cause his problem. But if both his carbs are doing the same thing, it leads me to believe his slide is way to hi, or he is sucking air at the intake boot. I guess we just don't have enough info. Does the engine Idle? Does it have a big dead spot causing you to install a bigger jet to try to keep it from falling on its face. If that is the case, then you need to work on the accelerator pump, and open the fuel screw up some. The fuel screw on the FCR helps with throttle response up to about 1/4 throttle. The accelerator pump gives the engine a quick shot of raw fuel so as soon as the slide opens, the engine has gas so it won't stumble. I put a o-ring or small wire tie on my accelerator pump linkage to make sure to get a even smooth shot from the pump. The spring in the acc linkage keeps the pump from pushing as hard as needed. IMO this linkage spring is a bigger problem with a Thumb throttle, as compaired to a twist throttle that these carbs were designed with. A thumb throttle can be slammed open quicker than a twist throttle can be twisted. So the quick motion of the thumb makes the spring stretch instead of pushing the acc pump shaft. The smaller the leak jet, the bigger the acc shot. The way I explain that So people can understand it is by using the toilet plunger explanation lol.. The leak jet is like having a hole in the top of you toilet plunger lol. The bigger the hole in top of the plunger, the less pressure it will push down the toilet. The smaller the hole the more pressure it will push down the toilet. Lol. Hope you understand that bit of nasty info.

[jays375]

Have tried different intakes.One thing that scares me now is I have used carb cleaner.Which maybe has ruined both accellerator pumps.Maybe there are other parts ruined now.It definately acts like a lean pilot condition.Starts hard without throttle.Backfires on deceleration.Won't dle without throttle applied..No bottom end power.If I run the bigger pilot it is fine.Nothing has been disassembled besides the float bowl.I have no lid or frame connector tube to air box.The head is ported.Running aftermarket header,Supertrapp can.

[Jason Hall]

182854182855I would pull the slide out and see if the seal on the throttle plate is wrinkled and not sealing. Carb cleaner will trash that seal.. Also make sure the plate is in the correct way. If it is upside down the slide will need to be held open as no air can go under it. Notice the hole and cutout go down.

[jays375]

Well both carbs the seal was missing.

[Jason Hall]

So the next question is, was the plate upside down also??

[jays375]

No that was on correct.

[jays375]

Well thank you Jason!I didn't even notice that seal was supposed to be there.Got the pump working too.Removing old diapharm was fun,kept clogging.Little more tuning and things should be all good.Learned my lesson,never use carb cleaner on a FCR carb!

[Jason Hall]

That's great news!! Glad I could be of some assistance, and your getting it figured out.

[Dirtcrasher]

I guess the non chlorinated brake cleaner should be used? or GAS??

[jays375]

Think that is what I read.Or just disassemble it and remove seals and pump to be safe.

[Dirtcrasher]

Crap, I wonder how many get "Carb Cleaned" and ruined before we buy them!!

[jays375]

Exactly.So figure another $50 to buy pump and seal.Another problem can be a worn slide cover plate.Take your chances on a used carb for a few hundred dollars.Or blow the wad and spend $500 and up on a new one.