How-To: FCR carburetor accelerator pump timing adjustment
If you own a modern 4 stroke dirt bike, then it is equipped with a Keihin FCR carburetor. Most service manuals and even aftermarket companies have really complicated procedures for setting up the pump shot timing. I explain an easier way in the video above.
The Goal: Perfect Timing
I have found that if you adjust the timing screw until there is very little little play in the rod, then that is the perfect setting. This setting does 2 things: it allows the diaphragm to fully fill with fuel and allows the squirt to start as early as possible. You will want to double check the squirt either on your workbench or while it is installed on the bike (subframe/airbox removed).
Here is a link to the auxiliary fuel tank I used in this video. Super useful when bench testing carburetors.
Retarted Timing
If you turn the screw in, it will delay or retard the squirt, which is not good and it will most likely create a bog. You are basically adding more slack/play into the rod and it takes a good amount of throttle to take this play out and start the squirt.
Advanced Timing
If you back the screw out, you will advance or make the squirt come in earlier. If it is too far advanced, the squirt will usually hit the back of the slide, which is not desirable. Also, no play will be present in the rod and it will actually preload the rod, taking away usable pump shot volume.
Other Helpful Videos
If you have a bog when when whack the throttle hard, I would recommend that you check out my 4 stroke carb clean video first, then watch the 4 stroke jetting video to get rid of that bog and get that front wheel off the ground!
Comments
Have any comments or questions? Did the video help you out? Feel free to comment below.
Thanks for the good explanation about how it all works. And thanks for your videos! You are a good explainer.
Hope it’s ok I ask a question?
I am reasonably new in 4 strokes!
I have followed your advice to set the keihin fcr carburettor on my KTM 250 sxf. When my Fuel mix screw is 3 times out, the engine really goes smooth and throttle responding really quickly. But it is very difficult to start. When the engine is hot, it is impossible even with hot start activated. When I turn the screw a half turn in again, its starting right away even hot. But it doesn’t sound that crisp and smooth. Don’t you think I’m going to size up my pilot jet? Right now I’m running 175 main and 40 pilot, needle in 5th clip. Only change on the bike is a DOMA exhaust and o-ring mod on accelerator pump.
Thanks in advance
Leave the fuel screw at 2.5 and give it a richer acc pump shot.
Hello sir, I’m in the process of a 07 CRF450(semi)re-build and want to make this part of the process along with the QS3 and JD jet kit. Im wondering if it is possible to make this timing adjustment while still leaving the carb on the bike. I can get the O ring on there relatively easy but the screw is another deal entirely unless you have a trick you wanna share 😉
Thanks in advance for your time.
-Chase
Not sure on your model, however on most 250f’s I can adjust the screw while carb is on bike.
Squirts too long no matter which way I go. Have qs3 on the bowl 2004 wr450f
What is too long? What is the issue you are trying to resolve?
I wish I had watched this video before I installed and rebuilt my carb. You explained some things that the manual doesn’t do well. I am amazed my bike ran after sitting for over four years, but cleaned the carb the best way I could and installed all new parts. The best way I can describe is that sometimes it seems to have a slight hiccup in acceleration but only from idle. It runs and pulls extremely strong. Going to try and adjust the pump like you described. It’s for an 08kx250f.
I have jetting specs for that bike here:
https://howtomotorcyclerepair.com/how-to-kx250f-jetting-specs/
Attempting to tune a new 33mm FCR for a high compression 4 valve 190cc motor. Obviously, the carb has never been setup on this bike/motor. Considering a 32mm works for the crf150r, I can’t imagine this carb is too big. But I can’t get anything once I crack the throttle. Assuming I need a leaner needle due to needing a larger main jet for WOT with reduced vacuum, but not needing the extra fuel at low throttle openings. Any suggestions on how to make a shot in the dark guess for needle size?
So you can get it running and idling? If so, first tune the fuel screw and pilot jet. The correct pilot jet should net you about 2-2.5 turns out on fuel screw.
Watch this video: https://youtu.be/kUw41sUQbTY
Yeah, got it running and idling. Seems to rev decently, but as soon as I try to move the bike with a load on the engine, it falls flat and dies or nearly dies. Because of that, have never been able to try WOT for getting a main jet sized.
Do a quick test: turn choke on and see if it gets worse or better.
You can also block the air intake with some masking tape to see if things improve.
It ran well enough to get about a block away under it’s own power, but then started falling flat again. It still didn’t run well for the whole rpm range either, just partial to mid throttle.
What does the plug look like after that short run?
White-ish on the electrode, but dark on the outer ring.
OK.
You never answered my Q: what is the fuel screw set to for peak idle RPM? I’d start with that and then move on to needle.