Topic: CB400F: okay to run smaller pilots?

[Alaxy Galaxy]

I've installed some #35 slow jets on my CB400F, stock airbox and exhaust, because the jets that came with the carbs were drilled out and fouling up my plugs. Typically, with a stock bike, the jets should be #40, so I ordered up some used Keihins on eBay and had them shipped to a forwarding box in upper-state NY (I'm in Montreal). Well, the bike has been running really well with the #35s and with the air screws about 3/4 out, instead of 2-1/2 as prescribed.

I took the bike on the back roads to pick the jets up, racking up over 80 miles in the process and she runs like a dream. I was riding at about 90kph (55mph) at 1/4 throttle the whole time, which is where I'm going to be doing most of my riding, I guess.

My dad always told me to not fix something that ain't broken, so I'm kinda weary to install the #40s. What are the disadvantages of running smaller pilots, if the air bleed screws have been adjusted correspondingly? Less low-end pickup? Not like these smaller fours have any low-end pickup anyhow...

[540nova]

Listen to your dad. I'd leave it alone.

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[Alaxy Galaxy]

Listen to your dad. I'd leave it alone.

Unless there's a real danger of running lean at lower throttle openings, that's what I'm planning to do.

[rupaulpierce]

I have a similar question about bumping up to 80 mains. Perhaps I should also listen to your dad.


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[BRG-BIRD]

I feel your airscrew adjustment is compensating for a smaller pilot jet. I would leave it as is unless it runs lean or is tough to start cold.

[Alaxy Galaxy]

I feel your airscrew adjustment is compensating for a smaller pilot jet. I would leave it as is unless it runs lean or is tough to start cold.

It's quite tough to start cold, actually; I can't kick it over when it's cold. I've got to run the starter for a good 5-10 seconds to get it to fire. Would adjusting the choke stopper on the backing plate help, or would it just raise the fast idle speed? (It's about 2500 rpm when warm, apparently a bit under spec).

[Don R]

 If it's hard to start cold, that means it's lean. Put in the proper jets. There's no sense modifying multiple things to crutch a simple fix.

[Bankerdanny]

no, adjusting the choke won't make a difference. When cold the choke plate should be fully closed, no amount of adjusting will make it more than fully closed  (assuming it is working properly now). All the idle speed knob does is raise the slide a bit, just like turning the throttle.

The pilot jet only comes into play during idle and a very small amount of throttle movement, then the main jet takes over. On the road you are running on the main jet and the smaller pilot doesn't matter. If you don't mind the extra effort to start then leave it be. If you want to be sure you can kick it over if the battery acts up then order the correct size jets from jetsrus.

[Alaxy Galaxy]

no, adjusting the choke won't make a difference. All the idle speed knob does is raise the slide a bit, just like turning the throttle.

I figured as much. Thanks.

If you don't mind the extra effort to start then leave it be. If you want to be sure you can kick it over if the battery acts up then order the correct size jets from jetsrus.

I already have the proper jets; I'm just reluctant to pull the rack and fit them on a fully functional and great running bike.

If it's hard to start cold, that means it's lean. Put in the proper jets. There's no sense modifying multiple things to crutch a simple fix.

Thank you; you're right. I'm not going to fudge with it anymore change the jets to the correct size on a rainy weekend.

[Bodi]

The choke doesn't solely use the pilot jet. The idea is to close off the carb inlet which causes intake strokes to create a pretty high vacuum in the carb throat, that vacuum sucks fuel up through the main and slow jets. The Bernoulli suction at the venturi helps but most fuel delivery is due to the choke effect.

Yes I know suction is a myth and it's actually ambient air pressure pushing your Mountain Dew up the straw... but everyone understands suction - incorrectly, but the usage is not confusing.

[Alaxy Galaxy]

The choke doesn't solely use the pilot jet. The idea is to close off the carb inlet which causes intake strokes to create a pretty high vacuum in the carb throat, that vacuum sucks fuel up through the main and slow jets. The Bernoulli suction at the venturi helps but most fuel delivery is due to the choke effect.

My initial plan was to adjust the choke stopper screw to make the needles go up as much as they could when the choke is fully closed. That extra fuel from the main jets might be just enough to help with cold starts. This wouldn't have any effect on the setup when the choke is opened. Since it's much easier to do than to change the pilots, I might as well try it out just to see.

When the engine is warm, a limp kick followed by a crack of the throttle (no choke) is all that's needed to bring it to life.

I took it out for a 110 mile ride over the weekend, and it's great. Really screams.