Causes of backfiring / raised idle?

[no_8wire]

Well my bike (GSX400F)has been acting up lately, not smoothly idling below 2500rpm and when warm will raise to 3000. Any attempt to lower the idle by using the idle screw results in dropping away from 3000 down to 750 then dieing.

I thought the idle jets could be blocked since it worked fine at the higher revs so stripped the carbs down to clean the jets, and when I put it together it worked fine for 45min then was back to the same.

So I stripped the carbs down again, cleaned, this time adding an extra inline fuel filter and giving the tank a good clean too (bit of rubbish came out to be honest)

I thought while it the fairings and tank were off I would do the plugs as well. Apparently the plugs that were in it were two heat ranges hotter than recommended ( NGK cr7 rather than a CR9)
After I put the bike back together it will only start if you dont use the choke at all. Further upon starting it will give a very sharp backfire, almost like a bolt breaking.


Would this be caused by the new plugs?
(also another problem is when I start the bike it will happily idle at 1800 rpm, but as soon as you blip the accel it will only deccel to 3000 and hold there for 20-30second before dropping down to 1800 again - ideas?)

[kwaka_crasher]

I would start by looking for a vacuum leak.

I presume you've balanced (synchronised) the carbs, checked valve clearances & compression etc...

[kwaka_crasher]

So this is an ongoing problem since purchase according to this post.

Hi.

I just bought a 92 GSX400 from a dealer.
I had previously identified that the bike was idling high (3000-4000rpm) and it was agreed that the bike be repair prior to purchase.

When I picked the bike up today I was assured that problem had been fixed and it started up ok so I bought it and rode it off.

However on the way home the bike would alternate between idling at 4000rpm, or simply dropping until it stalled.

I tried to control it via the choke switch however it had little effect.

The bike would sometimes die as the revs were to low, however when you restarted the bike it would jump to 4000rpm.

I am going to take it back to the dealer as it was supposed to be repaired prior to purchase, however what could cause this problem?

Stuck choke slide?

Take it back to the dealer and demand they do something about it. Quote the CGA saying you'll reject the vehicle through the Motor Vehicle Disputes Tribunal unless the issue is resolved in a timely and satisfactory manner. Do it in writing. Do everything in writing. Don't let them fob you off. You paid extra for the protection of buying from a dealer and now it's time they gave that protection.

You should really have done this much sooner.

[slofox]

About a hundred and fifty years ago I had an old banger that did something similar - turned out it was an air leak in the intake - warped carb face where it bolted on to the inlet...

[Gareth123]

Does it stutter at all while you ride it? Perhaps your timing?

[no_8wire]

Thing is it went away for a while (the issue relating to the linked post)

With regard to stuttering while riding, not at all. Infact once it is above 2500 is rev nicely, and doesnt stutter at all while just maintaining speed either.


With regards to valve clearances etc, checked them and they seemed within tolerance range according to the manual.

Only thing I havent done is synchronized the carbs as I do not yet have a vacuum gauge to do it.

I have been trying to find any airleak by spraying carb cleaner round the rubbers and connections to see if it revs up. Also tried to listen by hose to ear but nada.

Will change back plugs to see if that rids me of the backfire anyway. *edit* Changed the plugs backfires a little less but still does it.
Will tighten all rubbers again to see if makes any difference (including the ones to head)

Do the rubbers go on dry or do you use a grease/oil as to help create a seal?

[kwaka_crasher]

Take it back to the dealer. That's why you paid the extra. It sounds like they just did a stop-gap repair that has failed again.

[breakaway]

Sounds like they sold you a lemon. Take it back, NOW!

[no_8wire]

Too be honest, I dont think I can really take it back. 6 months is a long time to go back to a dealer and say fix this on this second hand bike I bought off you.

Plus, if they couldnt fix it the first time why would I trust them to try fix it again?


I will try another shop or MCD and see what happens

[kwaka_crasher]

The problem appears to have existed since you purchased it. It seems hasn't been properly resolved and has reoccured. It's not unreasonable to expect it to be as agreed at the time of sale.

Oh well, your loss if you don't follow up on your consumer rights.