Topic: 1976 400/4 No Power off idle

[tmht]

Alright here's my situation...

Finally got my 400 to the point where it was ready for some test rides and the results were less than stellar. Basically I have almost no power and cannot get the bike to rev above 4K under load. I seem to have decent throttle response when it is sitting there in neutral. We have found that 2 was rich and 4 was slightly lean. But we have not really been able to get enough miles under it to really make a good judgement by the condition of the plugs.

What has been done so far to diagnose the problem:
1. Carbs completely disassembled and cleaned.
2. Tappets adjusted to .002 in on both sides.
3. Air screws are 1.5 out
4. Main and Idle jets cleaned.
5. Carbs synched.
6. Stock airbox and carb boots inspected for holes or tears... none found.
7. throttle cables properly adjusted.
8. clutch properly adjusted.
9. checked brakes for dragging... front and rear are fine.

This is getting really frustrating and I want to have the bike ready to go up to Barber on the 21st. Any help would be greatly appreciated.

[supersport_CB400F]

What size jets do you have in the carbs and what sort of exhaust system, stock?

[Jay B]

Have you checked timing? Are your flyweights free? Sounds like the first things I'd check.

[tmht]

What size jets do you have in the carbs and what sort of exhaust system, stock?

Stock jets, stock header, Emgo Shorty muffler.

[tmht]

Have you checked timing? Are your flyweights free? Sounds like the first things I'd check.

I will re-check the timing. When it was first started it was 180 out... I'll check the flyweights as well.

[supersport_CB400F]

Does it have good throttle response when in neutral with no load? and go passed the 4K revs?

[cmorgan47]

points gapped too high?  i had that problem on my 400 once and lost lots of top end

[tmht]

Does it have good throttle response when in neutral with no load? and go passed the 4K revs?

Yes.

[tmht]

points gapped too high?  i had that problem on my 400 once and lost lots of top end

I'll triple check that. The points have been checked numerous times during this process. At this point I would be happy to even get to the top end regardless of the power level there. At least that would be progress...

[supersport_CB400F]

I’d guess you may have to mess about with the mixture and possibly use bigger jets to offset the non stock exhaust system on the bike, if your points/timing checks out OK.
My friend had all these problems when he fitted a new set off non stock pipes

[dusterdude]

to determine if its too lean,pull the choke a tad and see if it will run that way,if it does you know you`re lean.

[Kevin400F]

Some things I don't see listed:

1. Is an air filter in place?  Is it clean?
2. In regards to timing....does the spark timing move to the "advance marks" when you rev to 2500rpm+?
3. Float level verified at spec?   (I have a 350F that once wouldn't rev past about 5 grand, and the only issue was insufficient fuel level due to my improper technique at adjusting the floats.  Make sure the spring pin is not being depressed when you set the 21mm float level.)
4. Coils getting full voltage?

All this being said, on a resurrected bike (especially one with a rusty fuel tank), its pretty common to have to open the carbs and re-clean the jets a time or two before the gremlins disappear.  Line the tank with POR-15 and the jets will have a fighting chance of staying clean.

[flatblack]

Are all the jugs warming up at idle?

Something else to check is whether the carb slides are in correctly: Is the cutout to the airbox side?

I had this problem early on in my 400F's resurrection. Turned out to be a combination of junked up mains and float valves. No matter how clean you think you got things, there's always some trash floating around in the system.

And, oh yeah: Make sure the petcock is flowing enough gas.

You'll get there.

fb

[tmht]

Some things I don't see listed:

1. Is an air filter in place?  Is it clean?
2. In regards to timing....does the spark timing move to the "advance marks" when you rev to 2500rpm+?
3. Float level verified at spec?   (I have a 350F that once wouldn't rev past about 5 grand, and the only issue was insufficient fuel level due to my improper technique at adjusting the floats.  Make sure the spring pin is not being depressed when you set the 21mm float level.)
4. Coils getting full voltage?

All this being said, on a resurrected bike (especially one with a rusty fuel tank), its pretty common to have to open the carbs and re-clean the jets a time or two before the gremlins disappear.  Line the tank with POR-15 and the jets will have a fighting chance of staying clean.

Thanks Kevin.
1. Yes to both the lid is even on it.
2. I need to check that this weekend. Timing was set statically so I need to beg, borrow or steal a timing light.
3. Yes the float is at 21mm, but while I still have the bowls off I will verify.
4. I will double check this as well.

I am going to be ording the POR-15 today. Hopefully I will have it for the weekend.

[Kevin400F]

I'll see you over there this weekend, and I'm confident we'll find the root cause of the carb problems.  In the mean time, I'm curious about one thing....since the carbs were completely disassembled, is there a chance some of the slides got mixed up from one body to another, and now may have the slide "cutout" toward the engine rather than toward the airbox?  Take a peek in the carb throats, and let me know.   

[aptech77]

Air screw should be set at 2 turns (+/-) 1/2.

[750goes]

With the tank off if you need to  - can you see if anything is stopping the throttle cable from moving further

or the carburrettors from opening all the way???

is the choke connected and working ??

it gets better- just takes time

[tsflstb]

2. Tappets adjusted to .002 in on both sides.

That's a little tight on the exhaust side.  I don't know if that would cause your problem, but you might want to back the tappets out to .003-.004 to keep from burning a valve.

[tmht]

2. Tappets adjusted to .002 in on both sides.

That's a little tight on the exhaust side.  I don't know if that would cause your problem, but you might want to back the tappets out to .003-.004 to keep from burning a valve.

According to Clymer and the specs on this site, .002 is spec on both sides. And if you think that is tight you should have felt them before I moved them.

[tmht]

I'll see you over there this weekend, and I'm confident we'll find the root cause of the carb problems.  In the mean time, I'm curious about one thing....since the carbs were completely disassembled, is there a chance some of the slides got mixed up from one body to another, and now may have the slide "cutout" toward the engine rather than toward the airbox?  Take a peek in the carb throats, and let me know.   

There is a chance, though slim that could have happened. I disassembled the carbs one at a time and placed them in labeled zip-lock bags. Bud reassembled them. So if they are a little goofy you will have that much more to talk smack on.

[aptech77]

2. Tappets adjusted to .002 in on both sides.

That's a little tight on the exhaust side.  I don't know if that would cause your problem, but you might want to back the tappets out to .003-.004 to keep from burning a valve.

According to Clymer and the specs on this site, .002 is spec on both sides. And if you think that is tight you should have felt them before I moved them.

That is correct unless you are run a regrind cam. It sounds like yours is stock.

[Kevin400F]

Here's an update on the "voodoo carbs":  After giving them a thorough rebuild, I installed them on my stock 1975 CB400F and took a spin around the neighborhood.  Airbox, stock air filter and airbox lid were all installed per OEM configuration.   The bike pulled cleanly and ripped up to 10 grand like any good 400 should.  Idle quality, off-idle throttle response, midrange and top end were all fine. 

Attached is a pic of the carbs being pressure tested for float seat leakage, and another of #4 plug after the neighborhood romp (sorry its a little blurry).  No fluffy carbon on that plug.....unlike what we saw riding it with the other carb set last weekend.

[tsflstb]

According to Clymer and the specs on this site, .002 is spec on both sides. And if you think that is tight you should have felt them before I moved them.

Hey you're right.  The Honda shop manual says the same thing right on the first page.  Minus 100 points for me.

I think I misread a HondaMan post a while back and got those numbers stuck in my head.  I tend to not check my clearances as often as I should so I set them loose anyway.

[tmht]

Guess I know what I'm doing when we get back from Barber then...

[wilson11]

Hey Kevin. Thats a nice burning plug picture you posted. What is the # of the plug you're using? I have NGK d8ea in my 400f. The manual says to use d8esl but I can't find those. I can find dr8esl but get the impression you are doubling resistance if you don't switch to a different non resistance cap.Does anybody know the difference between the d8ea and the d8esl? Also has anybody used a hotter plug in their 400 say a d7ea and if so was there any benefits? Mine runs a bit rich and I was hoping that might be a easy fix instead of getting into the carbs. Thanks.