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Discussion Starter · #1 ·
So after putting the bike together, started riding to work. It starts up fine, accelerates through the gears fine till I get to about 110kph. I try to accelerate but it will start to sputter and won't accelerate.

Also, the engine won't acheive top rpm. It starts to sputter around 6000 rpm.
This is fuel issue correct. Does it need to be jetted and the carbs synced?

I built my airbox. It has 2 K&N airfilters on it that tuck under the tank but are not protected from the wind stream blowing past them.

The carbs are the stock carbs for the RF900. Can I raise the needle jet by one level to supply more fuel?

Any advice would be great.

Thanks
 
G

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Yes imo its a carb issue

I would say from your pics that the airbox you built has less capacity than stock & its anyones guess how much flow it is getting from the K&N filters all you can do is mess with the jetting going up (most likely) or down (probably not) on the main jet sizes & test the results you can also come out a little on the air screws & try the result of that but do 1 thing at a time & test each time

Post the results & we may be able to help

Failing that its dyno time I'm afraid
 

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Discussion Starter · #3 ·
Thanks 4cups. Teething problems are to be expected on a build like this I guess. I will post what I do and the results.
 

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Is my bike ok?
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Not wanting to disagree with 4cups, who is a better carburetor mechanic than me, and who has been giving me advice on my own issues. But your issues are identical to same symptoms I've been battling. It's all in the last few pages of my build thread for my '80, link in my Sig, but I'll try to be concise here.

I fiddled around with fueling to the point that I swapped on completely foreign carbs. Still was flat at wot / high rpm. I ended up finding a cylinder with too-tight valve lash ( creates low compression/vacuum on that cylinder, therefore throwing any synch off) and my ignition timing was too far retarded. Jetting up and down didn't cure it, but advancing my timing more and more pushed the stutter higher and higher in the rpm range until it finally disappeared. And I'm running exposed pods on a CB750 with GSXR carbs, just for the sake of argument.

Just my $0.02. If carb work doesnt help, maybe just check the basics. A compression test you can do without removing any covers/ losing gaskets and should tell you of you've got a valve that's too tight and costing you compression. Just another avenue of thought.

Good luck and keep us posted on what you find.
 
G

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Not wanting to disagree with 4cups
Carry on mate, disagree all you want :) if I am wrong its fine as long as jitterbug ends up with his bike working as it should its all good & don't put yourself down you have already proved yourself to be a decent mechanic with a good methodical way of working that gets results.

I did consider your build when posting shiny but thought on balance that a carb issue would be the most likely culprit here, but you are right to mention that other things have to be considered, I was lax there :oops:
 

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Sick of being frozen
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Hey don't forget your fuel tank and petcock, sounds like it's starving for fuel, try running with the lid open or an auxiliary tank before chasing carb settings needlessly.
 
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Discussion Starter · #8 ·
So, I finally got the correct jets. I went from jet #112.5 to #120. No change! It still falls flat at 110 kmh.

Shinyribs, how do you go about advancing the ignition?

Thanks everyone for chiming in here. Any and all advice is appreciated.
 

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Discussion Starter · #9 ·
So I just tried a run with the fuel filler open. Same result.

Just narrowing things down. Every test result whether positive or negative is more information and a step in the right direction.
 

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Discussion Starter · #10 ·
Could this problem be caused if I installed the cams wrong? Maybe one tooth out of position?
 
G

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Could this problem be caused if I installed the cams wrong? Maybe one tooth out of position?
yes yes yes :) if the cam timing is suspect in any way you have to check it before going further, I would suggest a compression test first then cam timing & while you are there you may as well set the valves, if all is healthy its back to carbs or ignition as shiny said

Carbs....... only an educated guess but I would have thought your airbox was pulling a lot more air than stock & 116 to 120 mains isn't much of a jump, I would go a bit more extreme with 150's just as a test on the effect than at least you know the range you need to be in

Ignition ...... my first thought is an electrical fault rather than component failure, a loose wire or connection, a dodgy plug cap, a rubbed pickup or coil wire earthing out something like that.

if the coils are breaking down under load any gsxr/bandit etc coils will do just fine for replacement to test,next the pickups in the r/h case have been known to fail or breakdown when hot, CDI's rarely fail so I would try other options for now but do check the connections are good

let us know how it goes mate
 

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Discussion Starter · #12 ·
I changed the main jets as I said earlier and there was no change. You would think it would have made a small change if the problem was in the jetting. So that makes me thingk the problem is somewhere else.

If the cams are out of time, would it run fine at lower rpm but not at mid to high rpm?

I also was thinking CDI box but as with the cams, I lack the knowledge.

The spark plugs and plug wires are new. Again, if it runs at lower rpm but starts to cough around 4500 - 5000 rpm is it electrical or fuel related? It's not like a miss where I can feel and hear a misfire, it's not like that. When it happens, I feel a surge like it wants to go but it's being held back.

The symptoms seem to point to fuel . . . all input here is appreciated very much.
 
G

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Ahhh now there is a point worth looking at ..... if its stock coils you should NOT be able to change the plug leads, I know it looks like you can but you really are asking for problems if you do & this could be your problem

If its dyna's or something like that changing leads is normal but are the coils the right ohms for the bike ? common mistake on gsxr's so maybe the same on the RF ..... from memory the stock are the same as gsxr's bandits etc at 3 to 5 ohms
 

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Discussion Starter · #14 ·
The coils are stock parts I believe. There were little plastic clips that held the wires ignition wires in the coil, but they broke when I removed them . . . so the ignition wires are not positively held in at the coils.

Do the power wires to the coils need to be attached to the correct terminal or are they interchangeable?
 
G

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Yup plastic clips & the breaking of them tells me they are grey stock coils used on most of the oil cooled bikes & lots of the watercooled ones of the Rf era, you cant change them mate & its a damn good bet that is a major part if not all of your problem..... buy or borrow some stock ones that have not been messed with before doing anything else is my advice

Coils are marked + and - right by the connections ..... its the 2 orange for + & the other 2 colours (black & something else ?) are - those will swap from side to side happily on 99% of Suzuki's but not on the odd one or two
 

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Discussion Starter · #16 ·
Cool . . . I'm gonna go check my connections. Thanks
 

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Discussion Starter · #17 ·
I looked at the connections on the coils but I couldn't see anything marked + or -. I guess I'll look for coils.
 
G

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the + & - is either cast into the coil on the join right by the connections & a pain to see if they are on the bike or very faint on the side again near the connections or in the case of later ones there are coloured tabs on the connectors themselves 2 the same on a pair are live feed
 
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