Author Topic: 400 stuttering/bogging from 6000 rpm [SOLVED] (Read 33221 times)

Gouraami · « **on:** April 13, 2015, 04:32:43 AM »

Hi Everyone

I am having a problem where the bike only wants to rev cleanly till 6000 rpm

From there the bike just bogs, feeling like it runs out of fuel.

I have searched the forum, finding two posts that relate to this. One was a 250 where the guy had mixed up the jets while cleaning it. The other, a 400, was due to a dirty carb.

I am currently running the bike from a small extra petrol tank, eliminating any petcock issues. The filter I have connected flows with a good flow rate, so I doubt it is a fuel delivery issue.

I have cleaned the carb very (Very) well, so it isn't a dirt issue in the jets/emulsion tube or needles, and the jets cannot be mixed up on the 400 carbs.

I currently have the needles set on the richest setting, otherwise the engine just bogs if you turn the throttle. On the richest setting it can be revved and driven without bogging till 6000rpm.

When revved in neutral when very light on the throttle it can be revved till 10000rpm where it starts sounding like it runs out of fuel, if more throttle is given it bogs and the revs drop till around 6500rpm.

Any assistance will be greatly appreciated, thank you

ventYl · « **Reply #1 on:** April 13, 2015, 05:50:38 AM »

it sounds to me like you have exactly opposite problem as you try to solve - your mixture can be way too rich. you mentioned that your jets are all correct and carbs are well cleaned but you are saying nothing about float heights setup.

if floats are set far too high engine can get too rich if throttle is pulled. mine had heights just 3mm above normal and I got hole under throttle when accelerating around 6k RPM anything faster than by absolutely gentle revving up.

another thing i noticed is that floats tend to bend easilly when carbs are removed from bike and put aside buttoms-up. the weight of floats itself combined with ratio of arm length (float : needle push nose) seem to be enough to bend the nose resulting in higher float height.

so my advice: check the float height; bring needle to their nominal setting, check tightness of float needles AND tightness of O-rings on float mounting cradle. if not replaced since bike was made they may leak fuel into float bowl even if needle is fully closing the inlet.

Gouraami · « **Reply #2 on:** April 13, 2015, 06:04:18 AM »

Hi ventYl

Thanks for the reply

I set all the floats at 14.6mm, according to the manual. The first thing I did when I got the bike was replace all the o-rings + choke rubbers with the Litetech viton replacement.

If I set the needles to the #3 setting it simply bogs when even slightly blipping the throttle. When being VERY gentle on the throttle it will rev up (also to about 6000 rpm before it starts stuttering) It seems to run lean at this setting as it will continue revving a while once the throttle is closed. On #5 it doesn't do that.

It does have an aftermarket exhaust on, but it should surely still run on the std carb settings?

The needles seem in relatively good condition, but I will check how tight they are tonight after work.

I think I will also make a video to help diagnose the problem

ventYl · « **Reply #3 on:** April 13, 2015, 06:33:28 AM »

check the hole of the emulsion tube where needle fits while travelling down. they may have slight wear and become much larger than original allowing too much fuel go in. i'm still thinking it is too rich. my experience is that if engine is too lean it revs good at neutral but dies when under load.

also i had similar issue with different motorcycle when it was idling very nice, doing nice job at low RPM but if revved above some limit it bogged completely. the bog was the harder the faster I revved it up. also resulting in huge black smoke from exhaust (not necessary true for you, mentioned bike was two-stroke). reason? main jet had damaged thread and unscrewed itself due to vibrations. fuel was passing into emulsion tube much faster and clogged engine when needle was pulled enough for engine to suck exceeding fuel. i am not saying that your jets are also faulty, but something similar may do that.

you did you also replace the "secret" O-ring sealing main system emulsion tube into slider guide?

Gouraami · « **Reply #4 on:** April 13, 2015, 08:33:20 AM »

Quote

you did you also replace the "secret" O-ring sealing main system emulsion tube into slider guide?

Yip, was part of the kit

I will definitely check the emulsion tubes and needles out tonight.

Thanks again for the assistance

Squishy · « **Reply #5 on:** April 13, 2015, 08:57:02 AM »

This is either an incorrect main jet, or airbox problem.
At about 6000rpm full throttle, the needles should be open or almost open. Doesn't sound like a needle problem to me.

What is your setup? Airbox?
Sounds like it's getting way too much air.

I tried to run with just the airbox cover off, but stock airfilter and it would also bog down.

Gouraami · « **Reply #6 on:** April 13, 2015, 09:01:52 AM »

Hi Squishy

For testing purposes I have been running it without the air box... I received the bike in a stripped condition, so it has never had the air box on while with me

I take it these bikes don't like running without the air box?

I will put the airbox on tonight before I do anything else,

Squishy

Squishy · « **Reply #7 on:** April 13, 2015, 10:58:02 AM »

Quote from: Gouraami1 on April 13, 2015, 09:01:52 AM

Hi Squishy

For testing purposes I have been running it without the air box... I received the bike in a stripped condition, so it has never had the air box on while with me

I take it these bikes don't like running without the air box?

I will put the airbox on tonight before I do anything else, Squishy

What.

No, it won't run even close to OK without airbox. At least mine won't.
I don't think there are many bikes that will, unless heavily tuned.
Like I said, even removing the top cover from the airbox will cause my 400 to run bad..
So u aren't even using an air filter??

Gouraami · « **Reply #8 on:** April 13, 2015, 11:09:41 AM »

No filter or anything at the moment

, wanted to get it running right before I put the extra bits on. It still needs a lot of work, spraywork etc. I will start a build thread to document what I have done so far

I have been running it as in the picture below:

Squishy · « **Reply #9 on:** April 13, 2015, 12:22:00 PM »

Well I hoped you removed the toiletpaper from the carbs before starting it

Anyway without any filters I'm surprised it even ran OK to 6000rpm. (well, it should go to 6000rpm in neutral but probably not when actually driving it)

ps. airbox and especially air filter is NOT "extra bits". It's an essential part.

Gouraami · « **Reply #10 on:** April 14, 2015, 03:38:12 AM »

WOOHOO! Thanks squishy and ventYl! She is pulling well, took her till 10500rpm last night

You are right, turns out the the airbox is an essential part

I thought it was only essential on two strokes

most my experience is from cars, so I haven't run into this air box problem before.

It ran surprisingly well till 6000rpm, as it does now with the airbox on.

Quote from: Squishy on April 13, 2015, 12:22:00 PM

Well I hoped you removed the toiletpaper from the carbs before starting it

Due to load shedding I was unable to work any further (I am sure it has made global news due to Eskom's miss management) however I will check the emulation tubes for wear.

Currently it will be running rich, so I will return the needle to #3 and go from there.

The air filter is very dirty, almost beyond washing. seems like a previous owner put grease instead of oil on it. I will try and wash it, if it doesn't work I will buy a new one

Once again, thanks so much guys! You saved me from fiddling with other things that would have made it worse!

I have many more questions, but will start new topics accordingly

Squishy · « **Reply #11 on:** April 14, 2015, 05:35:08 AM »

Yes running on highest needle settings will make it run rich, but I've found it to run way better +1 or +2 (from the middle).
On the stock needle setting, mid range is very weak and has a flat spot at around 4-5k RPM.
Unless you live high altitude you might put it on +1 rich (or keep it as it is if it runs fine).

Good luck.

Ps the airfilter is only €/$ 15, so you might just consider buying a new one

Gouraami · « **Reply #12 on:** April 14, 2015, 05:42:50 AM »

It has an aftermarket exhaust, so expecting it to run higher than stock.

At Sea level, will see which setting works best.

I must still finish drilling out a pilot screw due to it being abused in the past, so carbs must come off again. Lol, I can probably remove the carbs with my eyes closed by now

Thanks!

ventYl · « **Reply #13 on:** April 14, 2015, 05:54:42 AM »

after four or five remove - dismantle - rebuild - remount cycle in row each of last two days I can do it in time shorter than average commercial break in TV :P

Squishy · « **Reply #14 on:** April 14, 2015, 06:26:52 AM »

Quote from: ventYl on April 14, 2015, 05:54:42 AM

after four or five remove - dismantle - rebuild - remount cycle in row each of last two days I can do it in time shorter than average commercial break in TV :P

Hehe... very familiar.

I can't count the amount of times I have removed the tank, air filter or opened the carbs..
I don't even put the screws in the side panels anymore because I'll just take them off later.

I've been looking for a way to quickly connect/disconnect the fuel hoses but only found very expensive couplers.
I wish it had the same mechanism as the SV650 tank where you can just pop the tank up