I'm making REALLY good progress now with my bike carbs, following someone's post the other day regarding good work done by Bogg Bros.

Previously I'd started with 1.6mm jets, but experienced very bad coughing and spluttering when I'd put my foot to the floor. At first I thought my Facet fuel pump wasn't up to the job so replaced that with a decent bike pump. The engine behaved just the same. Went from 1.6mm to 1.65mm thinking that it was fuel starvation - and it was just as bad, if not worse. The plugs were badly sooted up too - only the tips of the electrodes were even approaching the right colour.

Then someone posted that Bogg Bros had put 1.5mm jets onto a 90BHP 1800 Pinto, and mine is a 1660 x-flow that gave about 90BHP with the Weber DGV carb, so I thought I might try going down in jet size. As it's quite hard to drill a hole smaller I made some new jets on my lathe - 4 at 1.5mm and 4 at 1.4mm. Today I fitted the 1.5mm ones, and the difference was amazing - no coughing and spluttering at WOT. I connected up my JAW AFR meter and it looks like I'm still running rich, as I'm seeing the following AFR figures:

tickover = ~14
low to mid throttle = ~10 - 12
WOT = ~10
On over-run = 14

It's a lot better now with the 1.5mm jets, but I reckon that I should try the 1.4mm jets.

To be absolutely sure, it would be useful to know what sort of figures I should aim for with bike carbs on a car engine.

So, what sort of WOT AFR should I aim for? Once I've got that near-enough right I can worry about the mid-throttle settings.

cheers,
David

[Edited on 23/6/08 by David Jenkins]

at WOT (wide open throttle) you want to run slightly rich....

its better to keep the valves cool...
and thats done by some extra fuel...

from memory I'd say 12.5 to 13 at WOT but I'm sure you'll hear from an expert soon enough

mmm interesting David, I drilled mine 1.6 and am now thinking about reducing mine too as mine doesn't seem to rev as cleanly as I think it should. I may try soldering them up and redrilling. Good interesting post David and I'm looking forward to getting my Jaw as well.

Cheers Rich

i had my JAW wideband temporarily connected to my car and set a fuel map in the ecu to aim for 12.5 at full throttle .I found it to be running near to 10.5 before making adjustments and think it was a bit to rich,for a normally aspirated engine 12.5 should be fine richer looses power.
I found the cruise to be near 14.7 and less on deceleration which is not to bad as i have only previously used a narrow band sensor for adjustments.
Paul.

12,5 at WOT is fine....

10:1 is eye wateringly rich (both literally and in cost these days). As has been mentioned, for a normally aspirated engine somewhere in the region of 12.5:1 at WOT is the target.

On low to mid throttle you should be able to happily run between 14 and 15:1 (bear in mind OEMs will map any car with a catalytic converter to run as close to 14.7:1 as possible for a big chunk of it's operating range). On really light throttle you can run right up to 17:1 or so if you feel so inclined, but personally I wouldn't try it with carbs.

You don't actually need any fuel at all on overrun, with injection you can turn off the injectors completely.

The consensus seems to be for 12.5 or thereabouts at WOT.

Sounds good to me!

BTW: The old Weber DGV downdraft was worse - and that was set up on a rolling road!

yep 12.5

Incidentally - somebody asked me about the home-made jets:


Carb jets

I'll try to make a diagram of the dimensions and put it on my website - they were easy to do, but did require a fair bit of lathe knowledge to get them all exactly the same size, with all the holes concentric. I probably spent as much time making the jigs and tooling as making the jets - but once they were done I made 2 sets of jets and a couple of spares. May not be a job for a novice...

Add-on: Oops - forgot to mention that they were for Honda CBR600F-X Keihin carbs.

[Edited on 21/6/08 by David Jenkins]

fancy making a 1.4 set for me and payment in beer tokens? This would save me messing about with solder, I notice we have the same carbs which is fortuanate.

You know the saying if you don't ask

Cheers

Rich

I knew someone would ask!

Unfortunately it took a few late evenings over the past week, time that normally I can't spare... I doubt I'd be able to find the time - sorry!

no problem, just thought it was worth an ask
Rich

Well, after making a range of jets and blasting up and down the local roads, I've found a jet size that works just nicely - 1.3mm!

Considerably less than the usually recommended 1.6mm, but at the bigger size the plugs got sooted up and the engine wouldn't run at WOT (wide-open throttle).

Now the engine pulls like a train, I get between 12:1 and 13:1 at WOT, with 15:1 - 16:1 on over-run. The plugs are just the right colour so I'm confident that the AFR meter's telling the truth. And before anyone says it - I know plug colour is only a crude guide.

The only thing that now needs fixing is mid-throttle - the sort of throttle you need when running at a fixed speed - as that's still around 10.5:1 - 11.5:1. Unfortunately I have non-adjustable needles so there's no easy fix for that.

Does anyone know a cheap source of adjustable needles for CBR600F-X carbs? I have no intention of paying over £100 for a dynojet kit!

Cheers for everyone's help over the past few months,
David

[Edited on 23/6/08 by David Jenkins]

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Originally posted by robocog
Cant you lathe some grooves in the needles to make em adjustable?
Or just pack em with shims to raise?

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Maybe you could face the top of the piston or deepen the locating recess for the needle then, and then make some spacers to adjust the needles?
Paul.

quote:

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Originally posted by David Jenkins

quote:

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I'm reluctant to muck around with the originals - I don't know if I could easily get replacements (and they'd cost a packet even if I could). They're pretty thin anyway, so they wouldn't stand much machining.

Anyway, it's not packing they need - it's some means of lowering them a little so they don't come out of the jets so quickly.

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