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So what did I break this time?!

6K views 120 replies 10 participants last post by  IanDoohan 
#1 ·
I previously found that having a random piece of carboard taped over the air box made things run but removing it caused the engine to bog down and die. I finally got around to doing the air/fuel adjustment yesterday and appear to have cocked something up

It's a CBR900RR in a kit car and, after a bunch of reading, it looked like I could wind each mixture adjustment screw gently to it's stop then out 2.5 turns. I did this and went for a drive, all was well.

After about 10 minutes, on the way home, the engine died. The fuel filter was empty so I think it did genuinely run out of fuel. 2x5L cans later and it fired up but wouldn't allow anything more than about 1500 RPM. I limped home and tested the fuel pump, all fine and pumping well. I re-set each adjuster to 2.5 turns, no change. I tried 3 turns, no change.

I removed each adjuster completely, all looked fairly clean, popped it all back together and now it's popping and banging and over revving but still won't take any load from the throttle.

Although I thought I was gentle seating each screw, my only guess is that I damaged something doing that but was after any thoughts/suggestions

James
 
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#6 ·
Doubtful it has anything to do with your fuel screw adjustment as that will typically only affect idle to 1/8 throttle.

Check everything else ie. spark, solid contact on all your spark plug wires, loose connections and possibly dirty carbs.

There is also the chance it skipped time. Much to check James.
 
#8 ·
I previously found that having a random piece of carboard taped over the air box made things run but removing it caused the engine to bog down and die.

After about 10 minutes, on the way home, the engine died. The fuel filter was empty so I think it did genuinely run out of fuel.

James
Hello James,

If this was a bike, we'd know the sub-systems and be giving you the right answer immediately <honest :angel> .
- Do you have any photos that show your intake system with/without the cardboard?
- What elements are in your fuel supply line - from fuel tank to carbs?

Since everything was going well until you ran out of fuel, does this indicate that some rubbish has got into the carbs?
Whip one or more float bowls off and check? While you are in there make sure that the float needles are OK?

Can you check and make sure that the accelerator pedal is moving all 4 carb throttles?
Can you remove the air filter and make sure that the main jet sliders are moving freely?
And are they moving as expected with the throttles when the engine is running?
Might be worth taking the caps off and making sure that diaphragms (and jets) are OK?

I agree with Ian's comments.
 
#9 ·
:) It's exactly the same set up as the bike, they've just hacked the top of the airbox off because the bonnet wouldn't close and taped some card over it. Pic attached

Tank through flexi lines and a filter then to a 2-3 psi pump to the carbs. I have used the exact same set up before and it worked well (and has been so far)

I thought the same about pulling some muck in, I'll have a check in the carbs as soon as I can

All throttles are moving and I'm checking the jet sliders tomorrow all being well

Thanks for the comments
 

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#10 ·
:) It's exactly the same set up as the bike, they've just hacked the top of the airbox off because the bonnet wouldn't close and taped some card over it. Pic attached
Crumbs! I was not expecting that! :eek:

You've lost a lot of the intake volume and it looks like more than half of the volume above the air filter?

Which I am assuming is a Std CBR900 filter? Does this kind of set-up run rich? And, if so has it been re-jetted?

When you removed the cardboard, what else did you do ......?
 

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#12 ·
All plugs are the same, looking lean

Checked the timing this morning and it's bang on still (thankfully!)

Yes it's the stock air filter and not rejetted that I'm aware of

Just removed the cardboard, nothing else

Propshaft bolts straight onto the engine with a greased joint

Just fired it up again and still running like crap but after idling for a few minutes it cleared a bit and was revving a bit before bogging down and stalling. I left it a bit longer and could rev more and now I can get all the way through the rev range and it sounds fine.

So it's fixed itself. I guess something in the carbs?

I'll pop out for a drive and see what it does but I think I'll get a full carb rebuild kit next
 
#13 ·
I think I'll get a full carb rebuild kit next
I re-built my carbs with this kit, it contained what I wanted and, at the time it was good VFM:

https://www.ebay.co.uk/itm/4x-Honda...782562?hash=item1ccf773da2:g:VNkAAOSww4BcuKed

However, just looked to get the above details - Search "CBR 900 Carb Kit" and there is a more comprehensive kit - additional parts include the fuel enrichment slider, trumpet seals and main jet holder&needle - at £39.99!?! But, I cannot vouch for the quality.

And, there is another company but the picture is quite small and I just cannot seem to make out what is in the kit.
 

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#20 ·
Well, a picture says a thousand words! I decided I wouldn't be comfortable just hoping it was OK so started stripping the carbs today. I found:

- A missing washer on a pilot screw
- 4 very badly damaged O rings to match on the pilots
- 2 large pieces of foam melting away inside the carbs (looks like part of an air filter)
- Bits of dirt in various places
- Assortment of jets with muck in them

It wants a good clean up and a bunch of new parts but I guess the foam was the biggest culprit and would most certainly have continued to plague me had I ignored it!
 

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#28 · (Edited)
Well I did promise some follow up questions didn't I?!

The kit arrived today and generally looks pretty good with most of the bits I need. I do have some oddities though as shown below:

1. I'm missing the tiny O ring for the mixture adjustment screw (shown on the left) but I have an extra medium sized O ring I don't have a home for, any thoughts?
https://www.fireblades.org/forums/attachment.php?attachmentid=95250&thumb=1

2. The main jet is physically different in size (Edit: see next post)
https://www.fireblades.org/forums/attachment.php?attachmentid=95252&thumb=1

3. Each kit has three jets in it, all marked '122', any thoughts on why I have 3 and is 122 correct?
https://www.fireblades.org/forums/attachment.php?attachmentid=95254&thumb=1

4. The choke needle doesn't have a point, can I just pull the old one out? It spins freely and looks like it might be held by an O ring?
https://www.fireblades.org/forums/attachment.php?attachmentid=95256&thumb=1
 

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#29 ·
Well I did promise some follow up questions didn't I?!

The kit arrived today and generally looks pretty good with most of the bits I need. I do have some oddities though as shown below:

1. I have this little silver needle screw with an O ring which I wasn't expecting, any idea where that goes?!
https://www.fireblades.org/forums/attachment.php?attachmentid=95258&thumb=1
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You should have 4, they are the "drain screw" you should find one in each float bowl.

Withdrawing the screw allows the carbs to be drained of fuel, and can be performed with the carbs still on the engine.
 
#30 ·
Sorry 2 more revelations as well!

5. The long jet needle needle with the circlip and 6 adjustment points is replaced by one with a solid end and no adjustment. This is the equivalent to using the bottom circlip groove as shown in the pic
https://www.fireblades.org/forums/attachment.php?attachmentid=95260&thumb=1

6. Also, the main jets I have don't fit the jet holder!! They fit the original jet holder though so that's a problem!
 

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#41 ·
Duncan

The only ID I can see is "VP" "Kei Hin" "Japan" on the carbs

I think it's a 1993 893cc engine


Ian

The jet needle holder O ring is supplied so it's not that one I don't think

Pretty unimpressed with the Chinese set then :)

What makes the jet wrong? I can see the emulsion tube is a problem but thought I could use the original one cleaned up with the new jet (which seem to fit correctly)

Do you know if the choke needle comes out? It looks like it should but I don't want to damage it by removing incorrectly

Sorry for the confusion, it's one "kit" of 4 rebuild sets. Each one has 3 jets in

The original markings are 120 adn the new ones are 122 so I assume a bigger holes/richer mix?

The other gaskets look good for fit
 
#49 ·
OK to summarise where I think I am, I can use:

1. The gaskets
2. The O ring on the jet needle holder
3. The jet needle spring
4. The slow jet
5. The main jet
6. The float valve
7. The float chamber drain
8. The choke spring
9. The mixture adjustment screw
10. The mixture adjustment screw spring

I can't use:

1. The SE valve (choke) because it's missing the end, I'll re-use the old ones
2. The needle jet holder for the main jet - It simply doesn't fit!
3. The jet needle on the vac assembly - because it sits too far down against the old one, re-use them
4. The small O ring on the mixture screw because it's missing

I'll assemble based on the above and know it could well be coming apart again!
 
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