HONDA BROS 400 Possible Misfire/slipped clutch

[dietryin]

Hi,

THis is my first post on here but hope someone can help. I need a new back tire on my bros so I have only once tested it to see what top speed it will do and got to 93/94 with a little left in her but I had to slow down as I was about to cross a bridge with strong winds that day.

I was on the motorway friday and sat and noticed the same thing happen twice. doing 85 and almost flat out at 71/2k revs the bike suddenly burst forward and the rev meter shot into the red.

A biker I met thought its a possible misfire, a mechanic thought maybe a slipping clutch which makes sense but does not seem right to me. The misfire explanation is that when it eventually fired I got that extra burst of speed.

Any Ideas?

I read that if any of the spark plugs causes problems its usually the front left which has burn marks so Ill change that later today and see if it fixs it.
____________________
Kevin

[P.addy]

If it shot forwards and then redlined... i'd say it downshifted

I had clutch slip on the GSXR, when I accelerated, it got to about 6k then it was like the clutch was half pulled in and the revs flew up but bike went nowhere.

[dietryin]

Yeah, I can see how it sounds logical and it might be the case.

Maybe im just being stubborn but the clutch has never given any other trouble.

Next time it happens ill try change up a gear and if it does go into fifth then it will definatly be the clutch having slipped down into fourth.

We shall see,

On the same note, im going to change that front left spark plug today as its only 6.50 for a new one and cant do any harm. There are burn marks around that plug socket but that could very well be use or an old prob, its a 93 reg bike.

Thanks for the post, ill let you know if the clutch gets worse or i figure it out.
____________________
Kevin

[Pol Anorl]

I had that issue, spark plugs changed seemed to fix it, also changing the back preasure on the bros makes a hugeeeeee difference it kills the thing.
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[dietryin]

Yeah, I changed the front right spark plug, it had burn marks on it. The left hand front, which I read usually causes problems, not sure why, is harder to get at and the mechanic I went to was busy. Quoted me 20 euro to take of the rad and change it but he couldnt do it then and there.
Anyway, for now now the change of the the front right has cut back on the exhaust back fire and the bike is a lot more responsive.
It is possible the front left still needs changing.
I wont know until later how much it really is fixed as I just had a brief run through town and traffic this morning. Ill prob know later if the weather holds.
Thanks for the posts
____________________
Kevin

[Kickstart]

[dietryin]

I never thought it was the clutch even though the mechanic I went to thought it might be and it made sense. The bike would have slowed down if it had slipped into fourth and I suspect I tried to change up again when it happened but I cant remember for sure.
If the clutch was slipping just in that spot it would have made sense it would slip at other times in other gears, the misfire possibility made more sense to me.
The weather is clear outside and I get a break at 12.45 so ill take it for a spin then to the open road and see if there is a change in the bike. Even in slow traffic this morning, with the new spark plug, the bike was more responsive.
I only have the bike a month now or maybe 6 weeks and it has been backfiring most of that time if not all. It was hard for me to say its doing this or that, because thats the way it was when I bought it and its only my second bike so I dont know very much about bikes. I do try and learn reading online about my and other bikes.
Your replies here and other posts I read on here have been very helpful.
The embarasing reality is that maybe the front two spark plugs have never been firing right but I never even considered that as I took it up to 93/94 mph and thats about what I read she will do.
SOme say it will do a ton just about. If two were misfiring the she would hardly do 93??????
I think the one change might be problem solved. FIngers crossed.
____________________
Kevin

[Kickstart]

Hi

The clutch alone will not cause it to change down a gear. The clutch disconnects drive between the engine and the gearbox. However if you were resting your foot on the gear lever then you might have it change gear if the clutch is operated.

Clutches tend to slip at the point where you have peak torque (NOT power), and also tend to slip when you are in the higher gears (in the lower gears you are at lower speeds where there is less resistance to the drive compared to higher speeds - at lower speeds the bike accelerates more easily so less resistance).

A misfire is possible, but being a twin if one cylinder was cutting out you would be a long way down on power. In this case yes the cylinder cutting in would cause it to surge forward, but I suspect it wouldn't manage much over about 60mph (so not surging forward when doing mid 80s). While it has 2 spark plugs per cylinder I doubt one of these being dead would make a massive difference to performance.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[dietryin]

Yeah, Two plugs per cylinder. So its very possible one was firing and the other was not and The two cylinders were still creating enough power for me not to notice that much.

My foot always rests below the gear lever so no, I did not change accidently.

Max power is at 6 and a half k revs not sure about torque, but I often go up to 7 or 7 and a half before I change if I want to move fast and never had any type of bother.

My last bike had a clutch problem and It slipped with fast acceleration in any gear.

Im clinging to the hope that the new spark plug has fixed the problem and Ill will find out in about 15 minutes.

Thanks for the input,
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Kevin

[Kickstart]

Hi

Twin plug systems are normally used on engines with a large bore and lousy combustion chamber shape. Idea is that the mixture starts burning from 2 different places at the same time. Wouldn't expect the little 400 to suffer in this way, and even if it did I would expect the effects of one plug out of the 2 for a cylinder not firing to be nominal (and both not firing the effects would be major - you wouldn't get anywhere near top speed).

Peak torque is probably at lower revs than peak power. Torque has probably dropped off by 7500rpm (in imperial measures, torque and power will be the same figure at 5252rpm).

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Pol Anorl]

It will still go to around 85mph on 1 cylinder, trust me i tried lol.

To get the front left and back right plugs i just used a 17mm socket and grinded the shit out of it so it would fit.

Ive had an indicated 108mph ish out the bros so ye probably around 100mph tops with shitty carb diaphragm and shitty carbs running 15 front standard 43-44 rear (not sure what standard rear is)
____________________
GOOD GAME BODYGUARD: https://i.imgur.com/8WePGgf.jpg
20:30:37 Pyro.: I don't sort of like men, I take every advantage to choke on dick.
Jewlio Iglesias: You live in Liverpool - Chances are, the front door has already been kicked off the hinges

[dietryin]

That changed spark plug has deffinatly stopped the backfire and made the engine more responsive.

I have not tried to get max speed because of bad weather conditions here in waterford. I found ice on my saddle twice yesterday after parking it up.

All the same, because the backfire has stopped and the engine is more responsive I am assuming for now the problem is solved but wont know for sure until the weather clears. Prob fixed.

I will over the next week change all the other three plugs because they were originally most likely changed together the last time and if one is burned out then the other three are prob old enough to need a change.

Thanks for all the input, everyone, and I will post again with any news.

Kevin
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Kevin