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Hey all!

I've been struggling to figure out a mid-range bog on my 78 CB400t II. I've set it up the same way as the set up from the thread "cb400 cv and pods - How to make them work," jets and all (great post, very helpful).

When riding, I can't get the bike to go over 6,200 rpms. It has no problem reving higher when the clutch is engaged, or when it is in neutral.

Another interesting thing about it is that if I leave the petcock in the "off" position, when the fuel is lower in the bowls, it easily surpasses 6,200 rpms (I found this out by being a dummy and forgetting to turn it to the "on" position, twice). So, my first thought was that the float levels weren't adjusted to a high enough setting, creating too rich of a mixture. Also, pulling the choke when it bogs causes it to bog even more, which also makes me think it's too rich. So, I made the float levels higher, but still no change.

I also thought it could be an issue with the clutch, but seeing that it runs well when the fuel level is lower makes me think otherwise. Another thought was electrical, but since it revs fine with the clutch engaged or when it is in neutral, and the fact that I've already replaced the coil and the CDI box, I doubt that this is the problem.

Other tid-bits: the valve clearances are within spec. No exhaust leaks. New fuel.

Any thoughts or ideas are welcome.
 

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First some terminology:

The clutch is "engaged" when you release the clutch lever and the clutch connects the engine to the transmission. I think what you mean is that it accelerates better when you pull the clutch lever and "disengage" the clutch.

Adjusting the float level higher results in a higher fuel level in the bowls and a richer mixture. I suspect what you mean is you are adjusting the float higher as you hold the carb upside down. When you change the fuel level you will change the mixture through the whole range so fixing your wide open throttle problem may well cause a lean condition at low and mid range.

The sticky can't tell anyone specifically which jets will work in their bike. Once you get it running you can do a series of plug chops to determine which jets are too big, which are too small, and which are just right. Your symptoms sound like your secondary main jets may be too big or possibly the metering rod is too small, shimmed up, or opening too early.


All this assumes that the carbs are cleaned and everything is functioning properly, sounds like you've already covered that.


The slow jet plugs are in place, right?
 

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Yes, Ryan you probably should pull it out and clean the screen. You'll be surprised how well the screen works by the amount of crud that comes out on it. Use a small screw extractor to get the screen out.
As for the problem listed the fuel level dropping and the engine performing better is indicative of too rich a mixture starting in the mid range and getting worse with higher rpm.
 

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Yes, Ryan you probably should pull it out and clean the screen. You'll be surprised how well the screen works by the amount of crud that comes out on it. Use a small screw extractor to get the screen out.
As for the problem listed the fuel level dropping and the engine performing better is indicative of too rich a mixture starting in the mid range and getting worse with higher rpm.
Oops, I just reread the OP and realized he said was comparing ON-to-OFF not ON-to-RESERVE. I thought he was saying he could only pull past 6.2k when in the RESERVE position.
 

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Hi all,

I’ve got the same issue with my ’78 400T. It revs all the way to the redline when it’s in neutral (or if I pull the clutch) but revs to about 5.000 rpm when riding.
I cleaned the carbs, got new spark plugs, adjusted valve clearances to spec and have no exhaust leaks. So I’m a bit puzzled over this… Haven’t ridden with the off position on the petcock though.

I’m really anxious if new jets will solve this issue.
 

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OK, has anyone checked their timing? No it's not adjustable but it may not be advancing or it maybe advancing very slowly and very late. Idle is at the F mark which is 15* BTDC, full advance is between the 2 lines which is 43* +/-2 (41-45) at 4500 to 5350 rpm. If the timing does not advance with rpm the engine will not develop power under load.
Timing is controlled by the stator and CDI unit, see section 16 of the FSM "Technical"
 

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OK, has anyone checked their timing? No it's not adjustable but it may not be advancing or it maybe advancing very slowly and very late. Idle is at the F mark which is 15* BTDC, full advance is between the 2 lines which is 43* +/-2 (41-45) at 4500 to 5350 rpm. If the timing does not advance with rpm the engine will not develop power under load.
Timing is controlled by the stator and CDI unit, see section 16 of the FSM "Technical"
Thanks,

Considering the rpm-range, a timing issue seems plausible.
I haven't checked the my timing since I'm simply not able to at the moment. I try to get my hands on a timing light shortly.

In the mean time I wil check the resistances on CDI & AC wiring, one of them (or both) should be faulty if the timing is incorrect, right?
 

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The issue seems to mainly be with the stator resistances. If you can make sense of this below it shows the readings I got when playing with different stators and CDI units. All of them were late on timing but one combination worked better than the others. I was doing this to try and figure out the original curve Honda used and it appears that it's roughly 1* per 100 rpm. That's what I programmed my Ignitech module to do and it's been great.


Stator A resistances: Blue/White 85, Pink/Green 135, Brown/Lt. Blue 206
Stator B resistances: Blue/White 82, Pink/Green 136, Brown/Lt. Blue 208
Stator C resistances: Blue/White 84, Pink/Green 135, Brown/Lt. Blue 211
I only tested stators A and C with 3 good CDI's
Stator A CDI A CDI B CDI C Stator C CDI A CDI B CDI C
RPM as close as I could get to this number
1200 17 17 17 17 17 17
2000 22 22 20 22 22 17
2500 27 27 27 27 28 27
3000 31 31 30 32 32 31
3500 32 32 32 35 35 34
4000 35 35 33 37 37 37
4500 37 37 35 39 39 40
5000 38 38 38 41 41 43
5500 38 42 38 43 43 43
6000
6500 43 42 42
This number was hit at 5350 and this one was 22* at 2200rpm
Reasoning behind not testing stator B was I grabbed C first at random. After seeing the timing changes between the lower resistance A and higher C stators it's apparent that the higher resistance brings the advance curve in earlier and stator B is in between those 2. Stator C combined with CDI B looks to be a good combination and decent curve to work from.
 

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Hi all,

Update:
Apparently I was so preoccupied with getting the bike to run (it didn't run when I got it) I didn't notice it was running on one cylinder! :oops: I don't know how, but the float valve on the right side carb was completely stuck so there was no fuel coming into the right side carb. Also the float height was incorrect, so I suspect someone has fiddled with the right side in the past 37 years without knowing what he/she was doing... After a short test drive I can safely say that my midrange issue's are gone :D

Thanks for all the help, it really gives confidence to someone who is new to restoring an old(er) bike :mrgreen:
 

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Interesting coincidence, but the OP hasn't been around in over 4 years... not likely to respond
I just realized that 😩, but I got some info about my CDI from another fellow on here, maybe being the fault, but I went back to my shop and tore down my carbs one more time and realized my air cut off valves have tiny holes and rips in them!! I held them up to my flashlight and realized holy crap! I didn’t think the air cut valves had anything to do with acceleration due to them richening my mixture on hard deceleration to avoid popping but could this cause my bike to have a vacuum leak through the carbs causing my top end to bog out!? Let me know what you think.
 

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That;s entirely possible. Replacing them would be a good way to find out.
 

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Look at the Blue lines in this image. Notice the path to and from the air cut valve? Now put manifold vacuum there since the diaphragms are torn.
306056
 

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This visual representation of a carb cut in half is EXACTLY what I needed to see! This is so awesome, thank you a lot for this. It connected the dots that were not connecting in my head lol.
OK so I replaced my hair cut off valve and the O-ring inside it, It did help my problem a little bit but it’s definitely not fixed. So this is what’s happening sitting in neutral the thing will rev Two red line with no problems, the only problem I’m having is under load first gear pulls pretty good. Second gear gets worse and so on by the time I’m in fifth it wants to completely die on wide-open throttle. And it’s not too lean if anything I’m pretty sure it’s too rich because my plugs are a bit fouled. It could be a timing problem I’m realizing now I don’t think it’s the petcock screen. Because when I open my reserve flow it flows extremely well. I’m about to loose my mind on these carbs 😂
 
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