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Discussion Starter #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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Symptoms point to fuel starvation. Are you using the low pressure pump from e.g. an early CBR900RR or similar bike, or are you using a car fuel pump with a pressure regulator?
 

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Discussion Starter #3
Thanks - It's a low pressure 2-3 psi pump, if I pop off the feed to the carb it gushes out and if I block it then it kicks off at low pressure
 

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all the airlines ok, no leaks? no pinched pipes either? inmey rubbers all ok and carbs seated in them properly?

I had issue years back with my 98 rr.w and found it was a pinched breather to the carbs. I know you have more room in a kit car but worth starting with the basics
 

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Discussion Starter #5
Sadly I don't think so, I had a good look around and no signs

I've got a tachometer coming so I'll adjust all the set screws to the full procedure and see if that helps but it feels like something's gone wrong with the fuel screw adjustment, I just don't know what
 

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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.
 

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Discussion Starter #7
Thanks Ian, all good suggestions as usual!

I checked all plug wires and will do a spark test next

Never thought about the timing skipping again but will pop the cam cover off and check that too and report back!
 

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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.
 

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Discussion Starter #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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:) 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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Discussion Starter #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
 

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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-CBR900-RR-Fireblade-1992-1995-Carb-Repair-Kit-Overhaul-Carburettor/123739782562?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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:) 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
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 ......?

James, I think it would be cool for you to put the standard top back on with an OEM filter, then cut out the bonnet to allow for the proper top of the airbox to fit, protruding through the bonnet like the old "shaker" intakes on the 70's Trans AM :nerd
 

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Discussion Starter #16
:D I have had a couple like that in the past but they tend to get very wet!!

I've been a bit brave and gone for a Chinese carb kit after all because I liked the look of the extras, will take some pics when it arrives and see if that reveals anything. Expect lots of 'how do I rebuild this bit' questions to follow!
 

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Discussion Starter #18
Nope rigid apart from the UJ

I've had a few of this ilk and tend to (as with this one) stick a Kevlar clutch and give it death all year round without issue
 

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Discussion Starter #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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