Topic: No Power, No Idle, Full Throttle to Run at 2,000 RPM CB400F

[PHeller]

As the title states.

New points, carbs recently cleaned (i think), plugs are dripping with fuel, timing is static set as best as I can do it.

Normally I'd start with the carbs, but this is different because the bike WILL NOT IDLE and WILL NOT RUN without full throttle.

Additionally, it seems to like half-choke.

Order of events:
Kick start
Full throttle (engine revs to about 2,400rpm)
Let off throttle (engine drops to 2,000 rpm)
Full throttle (engine stays around 2,000rpm)
Let off throttle (engine dies and cannot be saved via throttle inputs)
Engine dies

[Triffecpa]

sounds like you probably aren't running on all four cylinders.

When you start the bike, take some carb or brake cleaner and spray it on the exhaust pipes as they exit the head.  Depending upon the temp of the pipes, the cleaner will evaporate at different speeds.  This will give you an idea of which cylinders are firing.  Of course there are other methods of measuring pipe temp, from using a digital infrared thermometer to using a gloved or bare hand. 

If you can determine which cylinders are firing and which aren't then it will narrow down your search for problems.  If 1 and 4 aren't firing or 2 and 3 aren't firing, then it's probably an ignition problem.  If a single cylinder isn't firing it's likely carbs or fuel.

TR

[kajtek]

i would make sure the take offs on the carb boots for vacuum synching are screwed in, or check for any other leaks. make sure theres no block in the air intake also.

ashley

[strynboen]

in my 400 2 floters hang..try to open bottom skrevs..and tjek float lewel

[PHeller]

Awesome idea about the pipe temp. I've got a infrared laser thermometer that I've wanted to find a good use for.

I think I'll pick up a set of new spark plugs as well.

New jets might take a week or more to get here, so no rush on the carb.

[flybox1]

....carbs recently cleaned (i think)..

 
you think you cleaned them, but cant remember, or you think the PO cleaned them?

either way, you better clean them....clean and inspect all jets are free of debris, check the float valves to their job, ensure the float heights and needles are set properly, and bench sync them.

plugs = D8EA's

[PHeller]

Previous owner did a lot of stuff to this bike...but judging by the appearance of the bike (and its current running condition), I'm not sure how much of the stuff was "quality". No offence to him.

All I've done so far is made sure to do a Static Timing check to the best of my ability.

[flybox1]

HA...PO's do a lot of stuff, and most, save the folks on this board who've sold bikes, do a crap job.   

got a clymer and shop manual?

follow the 3000mi maintenance sched.
take your time. do a thorough job.
i bet it will clear up the majority of your issues.

[PHeller]

The symptoms stated are applicable to clogged up pilot jets, though.  But, other causes are possible.

I think clogged pilots would result in a bike that didn't idle, but had power (and revved) above 3,000rpm.

My bike has neither.

[PHeller]

i would make sure the take offs on the carb boots for vacuum synching are screwed in, or check for any other leaks.

That's actually very possible. It runs like it could be a vacuum leak. I'll check it out.

got a clymer and shop manual?

I've got numerous manuals that I can download online and print out. Kinda nice.

follow the 3000mi maintenance sched.

Luckily the PO did brand new points/breaker, but I've got no idea on the valve clearance, condition inside the carbs, or spark plugs (#1 looks crappy, hence why I'm doing them).

Because it's got brand new K&N pods which look super clean, I don't think that's an issue.

I guess one way I could check if its a carb issue would be to spray some ether into it and see if it revs on that. If it revs on ether or starting fluid, then the timing is probably correct or close enough, right?

[flybox1]

I guess one way I could check if its a carb issue would be to spray some ether into it and see if it revs on that. If it revs on ether or starting fluid, then the timing is probably correct or close enough, right?

do what you want, but many here have made knowledge-based suggestions to help you fix your problems.

[PHeller]

I guess one way I could check if its a carb issue would be to spray some ether into it and see if it revs on that. If it revs on ether or starting fluid, then the timing is probably correct or close enough, right?

do what you want, but many here have made knowledge-based suggestions to help you fix your problems.

No I'm not saying that I won't try to the thermometer method, but I was just thinking of fuel vs ignition issues.

Lets say all of my pipes are different temperature...that could mean ignition, or it could mean I've got wacky fuel flow.

If the bike revved past 3000 rpm or revved normally on ether, that'd tell me that that fuel was the issue.

If it doesn't well on ether, then its either a vacuum leak or ignition.

Not saying that wisdom isn't heard, but rather there is more than one way of doing things.

However, I don't like using ether in the first place, so this would be a last ditch effort.

[flybox1]

you set the timing. good.
with new plugs, pull each one and rest it agains the head.  got a good spark? 
if yes. then you gotta get into your carbs and find out why they are drowning your plugs.

i'm guessing your idle circuit and pilot jets are clogged, emulsifier tubes are all crudded up, your float heights are off, your slides all need to be bench synched, (confirm clip position is the same on all), set your AF screws, and then your whole rack of carbs need to be vacuum synched.

[PHeller]

Apparently the whole carb set was dipped in cleaner over the winter. I would assume they were blown out with compressed air...but no idea. The PO did mention that he had them separated and all the jets out, hence why he thought the snyc might be messed up.

He swore up and down that he thought it was the timing...but carbs are easy to mess up.

[flybox1]

he could have dipped them in astroglide for all you know....
assume he knew nothing.

[jessezm]

Apparently the whole carb set was dipped in cleaner over the winter. I would assume they were blown out with compressed air...but no idea. The PO did mention that he had them separated and all the jets out, hence why he thought the snyc might be messed up.

He swore up and down that he thought it was the timing...but carbs are easy to mess up.

This is just one more idea for you.  I had a very similar issue/symptom on my XS650 after doing all kinds of crazy performance mods.  I tried and tried and tried to figure it out and finally I realized that when I bought my fancy new mikuni vm 34s, the wrong slides were sent to me, and installed backwards!

So, since your PO had the carbs apart, please do check to make sure he didn't reverse the direction of the cutout in the slides.  That's an easy thing to check for ya, and an easy thing to fix!