Topic: '75 CB400F Bogging under accelleration

[RAFster122s]

Good morning. Scott here. I have a similar problem with my 1973 cb500. Seems to bog off idle under load but goes like a raped ape after about 3 grand. I rebuilt the carbs this past summer with aftermarket kits. I recently put all the original brass back in except the slow jets. I had 1 slow jet broken in carb #1 but I was able to get the little broken tip of the jet out of the carb. With that being said I stayed with the aftermarket slow jets. Does anyone think this bogging is ignition related? BTW  Stock carb adjustments, Stock air box but aftermarket baffled 4 into 1 exhaust.

The small SOHC4s like and need to be rev’ed to perform as intended. If you are expecting the bike to have lots of low end torque and power you are misinformed.
Get genuine Keihin brass from Jetsrus.com if you do not have original Keihin brass jets. The Keyster and K&L and other brass cause nothing but frustration and tuning problems as Herr Supersport has said.


Your bogging is caused by lack of revs. Slow jets control the low end of the rev range and that is exactly where you describe your trouble...so, likely you aren’t twisting the wrist enough and your slow jets are causing issues.
The 500 and 550 start waking up after you pass 5k rpm, So, don’t be afraid to run the revs up to 8-9k as the bike is designed to be ran well above 5k for long periods of time with no issues.

[Bodi]

The advance should be checked. Adjust it with a strobe timing light, increase revs until you see full advance, and set the timing to be at or just past the advance line. The idle/low RPM advance may not be at the F line, probably won't be... but that really does not matter at all. Full advance is where you will be doing almost all your riding: this engine loves to rev, and riding on the street below 4000 or so will be lugging the engine with just about any throttle increase.
Possibly you're just unaccustomed to the anemic acceleration possible below 3000 RPM from a 400F? You get a bit more from 3000, the engine wakes up and pulls pretty good from around 5000 to redline.

[jaytee-nz]

I had the same issue after rebuilding my 1972 CB500. It was a definite "bog" situation and you could ride through it with more revs but that was not the cure.
I fixed mine through a combination of ignition timing (just as Bodi has described) and fine adjustment of the slow jets, as in very small turns each time.

[Deltarider]

... and fine adjustment of the slow jets, as in very small turns each time.

Do you mean the slow jets or the airscrews?

[scott jones]

Good morning. Scott here again. I have a strobe timing light & I have adjusted the timing several times. Some places I have read is to adjust the timing @ 600 rpm. One guy said he does it @ 900rpm. And I think I also read somewhere it's supposed to be adjusted @ 1000rpm. I can see the advance working. Not sure about the springs though. The bike has about 20,000 miles on it. I have adjusted the air mixture screws many times. Right now I have them about 3/4 turns out. Stock air box. Honda says 1 turn out plus or minus an 1/8th. At 600rpm the timing shows a little retarded from the F mark on all for cylinders. Do I want the timing right on the F mark or in between the F & the T?

[Alaxy Galaxy]

If you can get your 400 to idle at 600 rpm, I'd say it's well sync-ed indeed! 

[Deltarider]

Good morning. Scott here again. I have a strobe timing light & I have adjusted the timing several times. Some places I have read is to adjust the timing @ 600 rpm. One guy said he does it @ 900rpm. And I think I also read somewhere it's supposed to be adjusted @ 1000rpm. I can see the advance working. Not sure about the springs though. The bike has about 20,000 miles on it. I have adjusted the air mixture screws many times. Right now I have them about 3/4 turns out. Stock air box. Honda says 1 turn out plus or minus an 1/8th. At 600rpm the timing shows a little retarded from the F mark on all for cylinders. Do I want the timing right on the F mark or in between the F & the T?

I once synced the carbs of my CB500 so well, it could idle @ 800 rpm. You seem to have managed to have it run even @ 600 which makes me wonder on what scale... Anyway, I'm told by an old mechanic that the lower the rpm, the more the cam suffers. I have mine set at an idle of 1100.
Actually the timing @ idle doesn't matter much. Anywhere between the yellow mark and the letter 'F' is fine. Fully advanced it should be at the two marks (blue and green in the pic) but 2-3 degrees more advance will not hurt and can result in a better respons on the throttle. So concentrate on having the timing right at full advanced and accept whatever it will be @ idle. Prior to setting the timing, in a conventional ignition, breakerpoints are to be checked and possibly adjusted. Before amateurs began to do maintenance, professional mecs used to only touch the breakerpoints for adjustment and forgot about the plates altogether. It is nice to have 1+4 and 2+3 set the same. That's what matters most. And listening to the engine can help. Practice and become a pro.

[maxheadflow]

Back when these bikes were new, I used to set the timing at idle on the back of the F as a marker then bend in the full advance tabs until the full advance was between the 2 full advance marks.   

Also when checking idle mixture, I'd blip throttle so the motor runs over about 4K rpm then close the throttle quickly. If the idle dropped below set idle RPM, idle mixture was too rich. If the RPMs drop back to idle too slowly, idle mixture was to lean. This only works when the adancer is working properly and carbs / manifolds are in proper condition. 

[jaytee-nz]

... and fine adjustment of the slow jets, as in very small turns each time.

Do you mean the slow jets or the airscrews?

Sorry - I meant air screws.

[Deltarider]

Back when these bikes were new, I used to set the timing at idle on the back of the F as a marker then bend in the full advance tabs until the full advance was between the 2 full advance marks.   

Hm... that procedure is a bit odd, isn't it?

[scott jones]

Yeah Deltarider. I had it idling @ about 500 the other day. I'm looking at the tach for the idle speed. Engine is noisy at that low of an idle. Cam chain noise?

[maxheadflow]

Back when these bikes were new, I used to set the timing at idle on the back of the F as a marker then bend in the full advance tabs until the full advance was between the 2 full advance marks.   

Hm... that procedure is a bit odd, isn't it?

Not really.. All it does is advance the low speed timing a little. Top End stays in spec.

[scottly]

Back when these bikes were new, I used to set the timing at idle on the back of the F as a marker then bend in the full advance tabs until the full advance was between the 2 full advance marks.   

Hm... that procedure is a bit odd, isn't it?

Not really.. All it does is advance the low speed timing a little. Top End stays in spec.

WTF?

[maxheadflow]

Back when these bikes were new, I used to set the timing at idle on the back of the F as a marker then bend in the full advance tabs until the full advance was between the 2 full advance marks.   

Hm... that procedure is a bit odd, isn't it?

Not really.. All it does is advance the low speed timing a little. Top End stays in spec.

WTF?

I guess it's FM?

[scottly]

Oh, I'm quite familiar with #$%*ing Magic. What I don't understand is why you would want to arbitrarily bend the full advance limit tabs, changing the range of mechanical advance? On a 750, the F mark is 10* before tdc, and the center of the double full advance marks is 35*, giving a range of 25*. The advancer is working properly when the F mark is aligned at idle, and the full advance marks are aligned at 2500 RPM or so, but, as been said, as long as it's at full advance above that, it's not an issue. 

[maxheadflow]

Oh, I'm quite familiar with #$%*ing Magic. What I don't understand is why you would want to arbitrarily bend the full advance limit tabs, changing the range of mechanical advance? On a 750, the F mark is 10* before tdc, and the center of the double full advance marks is 35*, giving a range of 25*. The advancer is working properly when the F mark is aligned at idle, and the full advance marks are aligned at 2500 RPM or so, but, as been said, as long as it's at full advance above that, it's not an issue.

It's not arbitrary and yes.

[Herr Supersport]

Back when these bikes were new, I used to set the timing at idle on the back of the F as a marker then bend in the full advance tabs until the full advance was between the 2 full advance marks.   

Hm... that procedure is a bit odd, isn't it?

Not really.. All it does is advance the low speed timing a little. Top End stays in spec.

BTW: that's exactly what I did and proposed: more advance at low RPM, normal advance at higher RPM.
Electronic ignitions make it easy to manage such things.
Initial question was "it really bogs down when accelerating from a stop" and that problem was solved by advancing the ignition.

[maxheadflow]

Oh, I'm quite familiar with #$%*ing Magic. What I don't understand is why you would want to arbitrarily bend the full advance limit tabs, changing the range of mechanical advance? On a 750, the F mark is 10* before tdc, and the center of the double full advance marks is 35*, giving a range of 25*. The advancer is working properly when the F mark is aligned at idle, and the full advance marks are aligned at 2500 RPM or so, but, as been said, as long as it's at full advance above that, it's not an issue.

BTW on a stock  CB500 The F mark is at 5 deg BTDC and max advance is 28-31 deg RPMs 1200 to 2500. Idle set to 1000.

[scott jones]

Ok thanks!