Topic: Flat Spot, just off Idle.......... 400/4

[Ben]

Hiya all,

The 400 has been fine for a while, but I am sick of the massive flat spot just off idle (since fitting velocity stacks). I have been riding around it, but am sick of not being able to crack the throttle open quickly!
The flat spot is only evident under 1/4 throttle (all the time, including cruising where a contant speed cannot be held if on 1/4 throttle or less), and the throttle can be cracked open after 4000 revs with no problems.
I am thinking that it may be very lean on the pilot circut? Any thoughts on increasing the pilots to 50? (from 40).
Thanks and Cheers!
Ben.

[Gordon]

I tried for about six months to get my 400 to run right with pod filters.  Always had the same problem you're describing.  I was never able to find the right size jets because there aren't many options available.  I finally went with the stock airbox and jetting, and was very happy.

BTW, is that your bike in your avatar?  I've admired it many times on bikepics.com.  A really nice piece of work!

[Ben]

Hi Gordon,

Yeah, it is my beast, built with much advice from the old Greenspun forum. Thanks for the compliments!
Maybe this is a common problem when opening the airways on a 400/4? I am having the same problems; the mains I have been able to get easily, but the pilots are only available up to #45....... time to start drilling..................

Anyone else solved this problem (by richening up the pilot circut, or otherwise)

 

[heffay]

ok guys i might need a hand here cuz my memory is shot... too many finals at the moment, too many hard nights since mmi, you get my drift i'm sure.
but, at mmi i was riding an 89 kawi ex500 and had a similar problem after doing a bit of moding.  (yes i realize we are discussing two very different carb styles cv vs. what we have on our sohc's.  but, as we were dyno'ing the bike we made an adjustment to the slide by drilling a hole somewhere in it (sorry memory)  if i remember right it had something to do with getting the slide to be more responsive to the flick of the throttle.  It worked better than nearly all of the mods!  first off, does anyone know what i'm talking about cuz i kinda dont and if so... could this be a technique that might transfer over to our style of carbs?  btw, before mods i was crankin about 48 horsies  afterward... just shy of 70 

[Gordon]

Ben,

Now that I think about it, my problem was a little bit higher up in the throttle range (more of a main issue than pilot).  What size mains are you using?  I wan't able to find ones that worked for my setup.  I think it turned out I needed something around a 95, but they just weren't available.  Have you messed around much with the air screws?  You can make quite a change in the pilot circuit just by richening up the mixture with these.  Maybe you should try fitting some pod filters instead of the stacks.  You'll still get the performance enhancement of the extra air flow, but they might be just restrictive enough to straigten out that flat spot. 

[MRieck]

Raise your needles .020 or a notch (.040). Less slide cutaway( to richen) might help but replacement slides are not available. Larger pilot jets are needed also.

[Ben]

Thanks all. Gordon; yeah, tried many times with the mixture screws. I was running #90 mains, but found them a little rich so I have now changed back to the #75's and have lifted the needles to the botton notch.....................Also drilled the pilots out to #50 today, and have modified some unifilters I had laying around to go over the stacks.

I am putting it back together tonight and will let you know how it goes! Hopefully I will have something useful to put in the FAQ section!

Cheers.

[Gordon]

Good luck, Ben.

Hopefully you'll get it figured out.  This problem has come up a few times in the past, and it would be nice to finally be able to give a difinitive answer.

On a side note:  At this point, how fast are you at pulling/replacing the carbs?  By the time I finally gave up and went back to stock on my 400, I could pull the carbs, disassemble them, change the needle heights, check the float heights, and have the bike running again in 20 min.  At least now what was once a daunting task is something I could probably do in my sleep.

Gordon

[thirdman]

.
For what it is worth, I had a similar problem on my 350/4, and it turned out to just be the idle mix.  It would be fine at startup, but as it warmed up it would have no guts coming off of idle.  I adjusted the idle mix in half turn increments, and finally got it dialed in.

[Ben]

Man, this is driving me insane................ Still no good, flat spot still evident..............Gordon; I am getting pretty fast at tearing the carbs down! Not quite 20 mins (30 mins if only jets, but the needles get a bit fiddly!), but getting there!

Tried the following (that I can remember):

1. Standard set-up with stacks - No good, flat spot off idle
2. #90 mains, needle on 2nd notch from bottom, #40 pilots, air screw @ all settings possible - Good at WOT (a little rich), flat spot off idle - Tried with stacks.
3. #75 mains, needle on bottom notch, #50 pilots, air screw @ all settings possible - tried with pods. - Flat spot off idle, not easy to maintain smooth running under 1/2 throttle (rich and lean spots all over the place)

Next one to try -
1. 75 mains, needle 3rd from bottom notch (standard), #50 pilots, air screw @ all settings possible - with pods.

2. I will give anyone here 50 bucks (australian!!) if they can tell me exactly what jetting to use, with standard carbs to get the 400/4 running smoothly with, or without pods.

3. Go back to standard and forget about the extra few horses gained, its not worth it when the bike is off the road so much!

Maybe its just not worth it! In the few years I have been using this site (ie this and the greenspun), and the five years I have owned the bike, NO-ONE has supplied a definitive answer! I was hoping to be the one but hey........................................ it really is a pain in the A$se!

[Gordon]

It's becoming a bit of a "Holy Grail" for 400F owners.  Many men have tried, and many have failed, but in the end, isn't it really all about the quest? 

[Ben]

Indeed it is, but it would be nice if things just worked sometimes! (dont mind me, I have two projects on the go at the moment..... neither of which are going as planned.......)

[Ben]

Right, I have tried just about everything! Enlarging pilots, any conceivable idle screw settings, all needle settings, enlarging mains, bellmouths and pods..................... NOTHING is getting rid of that flat spot! I am now convinced that it is the standard slides that need playing with, but as I cant be bothered I am taking it back to standard airbox/75 mains/40 pilots and unifilter foam air filter...........I can deal with a few lost horses if the Bl@#dy thing will run smoothly!

I think the only real way to run a 400/4 on open carbs smoothly would be to replace the whole set with some mikunis/amals or similar..........................................

Maybe later!

[TwoTired]

You say there is a flat spot just off idle.  That is the domain of the throttle valve cutaways.
Operate the bike in the flat spot regime for awhile, do the plug chop test and read your plugs.  This should tell you if you need to go more rich or more lean.  I don't know how much modification is needed to achieve desired results.  But, as stated below,  increasing the height of the cutaway reduces the choking effect, producing a leaner air-fuel mixture ratio, and vice versa. Probably gonna have to be a cut and try process.  Go too far and well, next set of slides, please.  However, if you are already too lean, I'm not sure how one would reduce the cutaway height.  Ideas?  Rivet on a restrictor tab to the front of the slide?  I'd hate to add something that would get sucked into the engine...
Perhaps the following will help...


An Excerpt from a booklet-
Honda Motorcycle Carburetion (1975)  Honda Technical Series

Throttle valve cutaway:

The needle jet begins to discharge fuel at about 1/8 throttle opening and supplants the low speed system as the chief fuel supply. It is important to have a smooth transition from the low speed system to the intermediate system as the throttle is opened.  Otherwise, there would be a momentary fuel delivery failure causing a flat spot in acceleration.  The height of the throttle valve cutaway is crucial in obtaining smooth system transition and good initial acceleration.

While the position of the throttle slide determines the venturi constriction in the carburetor bore, the shape of the bottom edge of the slide determines the extent to which induction port vacuum is maintained at the needle jet.

The bottom edge of the slide, on the upstream side (air cleaner side) of the needle jet acts as a choke, but is cut away to a height the will produce the exact degree of choking needed to ensure proper delivery from the needle jet at low throttle openings.

Increasing the height of the cutaway reduces the choking effect, producing a leaner air-fuel mixture ratio, and vice versa.

The throttle valve cutaway controls the air-fuel mixture ratio primarily at 1/8 to 1/4 throttle openings and ceases to have any effect beyound 1/2 throttle opening.

[Gordon]

Thanks for the update, Ben.  Sorry it didn't work out, I was really rooting for you!

[Uncle Ernie]

Ben,
Reading all this with mixed emotions... I went back and forth,too. Tried stock and went back to pods. Anyone want a stock airbox, by the way?  I hate to say it, but I've realized while reading this- 400s don't move below 4K rpm. Period.
I'd be endlessly facinated to know how mikunis work if you try them. People are always recommending them for my other bikes even when they run fine, but this may actually be a find for the 400...

[Ben]

I got the mikuni idea on the weekend. I was at a classic race bike meet and saw a 350/4 racer with a full bank of lovely looking mikuni carbs, it ran beautifully! Got me thinking........................ If we are to mess around with carb mods, maybe replacing the whole bank of carbs is the way to go rather than messing around with many (expensive!) parts on the standard ones.