Bike Carbs Lean - strange solution

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davidimurray

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posted on 11/5/15 at 08:18 PM

Bike Carbs Lean - strange solution

I've been setting up the Duratec over the past few weeks and after various issues I found something very strange today

Having got my fuelling pretty good as per -
http://locostbuilders.co.uk/forum/9/viewthread.php?tid=197958

I refitted my airbox and cold air feed. Took the car out for a spin and while fun on cruise, WOT had gone from 12s to 17s. Disconnected the cold air feed to the box with no change.

Then removed the airbox leaving the carbs open with just the trumpets/backplate fitted - better but WOT still in the 14s/15s

As a last resort I removed the backplate and trumpets and suddenly everything was good - WOT back in the 12s!

What the heck is going on? I need to try refitting the airbox tomorrow without the trumpets and see what happens. The airbox is not 100% sealed to the carbs, the backplate just rests against the carb mouth. The carb breathers are ducted into the airbox so should be seeing the same air. Also, the air correctors in the mouth of the carb are open to the inside of the airbox as the backplate is cut out for them.

I can't see what the difference is between no airbox with the backplate/trumpets in place and the nothing on the carbs (no backplate/trumpet)

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posted on 11/5/15 at 08:44 PM

I assume that adding the trumpets have changed the way air flows through the carbs.

Just do all your tuning with the setup that you intend to run

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posted on 11/5/15 at 09:04 PM

You need to map it again with the trumpets on.

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posted on 11/5/15 at 09:04 PM

quote:
Originally posted by dave_424
I assume that adding the trumpets have changed the way air flows through the carbs.

Just do all your tuning with the setup that you intend to run

Exactly that.

Trumpet design greatly changes the flow of air. Length, tapper angle and end of trumpet profile.

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posted on 11/5/15 at 09:05 PM

We need pics. Particularly of the back plate. 'Just resting against the carb mouths' doesn't sound right.

davidimurray

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posted on 11/5/15 at 09:43 PM

Thanks everyone. I can understand the tuning 'it to the setup' but I am intrigued as to what is going on. I would have expected that if anything the airflow would be restricted by fitting the airbox - not improved! Also, assuming that the trumpets are actually making WOT lean through increased airflow then it shows how important they are!

Currently the mains are 1.7mm so it looks like I got some more drilling to do tomorrow!

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posted on 12/5/15 at 05:44 AM

Some bike carbs have air feed and jets in a ring around the trumpets, are you blocking these off with the backplate?

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posted on 12/5/15 at 12:29 PM

quote:
Originally posted by snapper
Some bike carbs have air feed and jets in a ring around the trumpets, are you blocking these off with the backplate?

That was my first thought as well. It's very hard to see in the OP's facebook gallery, but it does look like this could be the case.

davidimurray

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posted on 12/5/15 at 12:40 PM

quote:
Originally posted by MikeRJ

quote:
Originally posted by snapper
Some bike carbs have air feed and jets in a ring around the trumpets, are you blocking these off with the backplate?

That was my first thought as well. It's very hard to see in the OP's facebook gallery, but it does look like this could be the case.

I will get some pics later but the nozzles/jets in the mouth are open to the airbox. The hole in the backplate is the same size as that of a zx6r backplate.

The one test I haven't tried yet is with backplate but no trumpets - chucking it down outside at the moment!

Think I am going to try and put a set of 1.9mm mains them and see what happens then.

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posted on 12/5/15 at 03:29 PM

Trumpets

davidimurray

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posted on 12/5/15 at 04:01 PM

quote:
Originally posted by r1_pete

Trumpets

Thanks for that - I've been looking for that diagram but couldn't find it. So potentially I could b seeing 5% more airflow.

I've just been out and drilled a set of mains to 1.9mm and put it all together, complete with airbox - running around 14-15.5AFR at WOT.

Will try opening out to 2.1mm and see what happens.

Some pics of the setup with/without trumpets below -

Description

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posted on 12/5/15 at 05:05 PM

Possible theory:

Without trumpets the air drawn in to the main choke will be passing over the top of the air corrector jets. This fast moving air will create a low pressure in front of the jets which would prevent or reduce the amount of air entering. This would create a rich condition, and in fact many people using bike carbs on car engines have had to block off the air correctors to get the mixture rich enough.

With the trumpets on the air correctors will now be in still air (as they should be), allowing air to be drawn in and emulsified with the fuel, weakening the mixture.

Have you tried with only the trumpets fitted, and no back plate? If the mixture still weakens off then it would add weight to the air corrector theory.

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posted on 12/5/15 at 07:40 PM

My main jets are 1.55 which came with carbs.
I have blocked off the fixed main air correctors.
Dropped the needle 1 click down to lean it out a bit.
Changed idle jets from 12.5 which came with carbs to a 17.5 now only 2.5 turns out on mixture screw.
Idle air jet 125 which came with carbs.
Blacktop Zetec 2.0 - goes very well .. AEM AFR reads all good.

Description

If you look carefully I have matchsticks in the main air when first test.
Carbs need to be set with full load first to size main jets (block off fixed air correctors) then mid loads to set needle heights then tweak screws to sort idle mixture (more than 3 turns out go up a jet size).

[Edited on 12/5/15 by johnemms]

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posted on 12/5/15 at 08:08 PM

Mike - some good thoughts there. I didn't get chance to try just trumpets as the weather foiled ,my tinkering.

John - I did try years ago when I first put the carbs on the Pinto blocking the air correctors. It all went stupidly rich and I couldn't get it back lean. I suppose I could fill and drill them smaller as a half way house.
When you say you changed the idle jets do you mean the pilots? Do you run an airbox or a sausage filter?

I tried the car again today with 2.1mm mains and it is definitely better now in the 12-14AFR range. Looks like I need to go bigger again so will try 2.3mm. I dread to imagine what the fuel economy will be ....

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posted on 13/5/15 at 08:01 AM

This is what I have done with my Ebay R6 carbs - 2.0 Blacktop..

Important.. Buy inlet manifold & fittings from DanST Engineering..no leaky

Very nice manifold..

Description

Description

(Remove 30 year old Dellorto's - many many hours of jetting - never ever got them right - store away for Ebay).

Zetec |Inside

Fit bike carbs - set fuel pressure to 2psi - started first time - stand back amazed - jump in - drive - check AFR's

Description

Very first run - WOT was like 16's 17's - simply stuck matchsticks in the air correctors & WOT dropped to 13's on 155 main Jets (see pic).
(I would have needed massive main jets to bring the AFR down with so much air correction).

Description

Description

Filling & drilling the fixed air correctors to smaller sizes looked difficult but could be done - however blocked gave me immediate good results.
(Blocked for me seems better & predictable with or without trumpet/stacks or air boxes to complicate things).

Description

Description

Next - Drilled & tapped 5mm holes in manifold - used bike manometers - balanced all carbs up... very nice and smooth.

Description

Next - Mid and cruise (needle circlip in the middle position) - Decided to drop my needle down one slot to lean my cruise to 14.5 to 15 AFR.

Description

Pilot Jet Air Corrector (removable) - next to the fixed main jet - this air corrector is marked 125 - this seems fine & designed to give emulsion smoothing effect up to the onset of the needle main jet coming on - so left alone.

Description

Description

Finally - Fuel Pilot Jet (removable) - carbs came with 12.5 size fuel jets.
At at 950 rpm tick over mixture screws needed more than 3 turns out for 12.5 to 13 AFR.
(If more than three turns go up next jet size or risk mixture screws falling out)
Changed for 15's - still 3 and a bit turns out but seem good will leave them in..

Description

Description

On the subject of Drills.
Every drill seems to lie about their actual size - therefore measure with a micrometre or whatever - before using one.

Description

Trumpets/Stacks air flow principals are applied to all your fuel jets.
The angles of the fuel inlet jets are all machined with special conical inlet shapes - diameters - lengths - and are all tuned for correct fuel flow rates.
My Locost soldering up & drilling will get me in the ball park - but a set of machined jets although same diameter may differ in actual flow rates.

Trumpets

Final thoughts are - blocked means WOT AFR is now adjustable by main jets only - the needle when fully open at WOT has no more effect on mixture.

With no variable air emulsion - the main jets WOT can now be easily sorted.

Cheers ...

Caution - I am no expert!!

[Edited on 13/5/15 by johnemms]

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posted on 13/5/15 at 08:58 AM

quote:

I tried the car again today with 2.1mm mains and it is definitely better now in the 12-14AFR range. Looks like I need to go bigger again so will try 2.3mm. I dread to imagine what the fuel economy will be ....

I'm also no expert but surely if you keep making the mains bigger at some point you will need to get different needles as they will be too thin to close the main jet at tickover / low throttle openings?

I'm really surprised a 2.0 Duratech would need jets so much bigger than a 2.0 Zetec on the same carbs. at 1.6mm mine are on the rich side at WOT.

If you were closer I'd lend you my spare carbs to try as I think something isn't right.

BTW I have none of the air correctors etc. blocked on my carbs.

However I do have the bike air box back plate between the Pipercross backplate and the carbs This ensures the carbs are sealed as they would be on the bike.

My setup is a bit different in that I use the bike manifold rather than a fabricated one so very little chance of manifold leaks. It means re-spacing the carbs though.

Stu

[Edited on 13/5/15 by whitestu]

[Edited on 13/5/15 by whitestu]

dave_424

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posted on 13/5/15 at 10:22 AM

Why do people block the air correctors on their carbs? They were needed when they were on the bike so why change the way that the carbs work. Also it makes sense that a set of zx6 carbs will need to have the jets opened up to fuel a 2l car engine.

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posted on 13/5/15 at 12:26 PM

quote:
Originally posted by dave_424
Why do people block the air correctors on their carbs? They were needed when they were on the bike so why change the way that the carbs work. Also it makes sense that a set of zx6 carbs will need to have the jets opened up to fuel a 2l car engine.

Because the engine characteristics are different, a 600 cc bike producing say 80 or 90 bhp at 16-17000 rpm has much less of a pulsing action on the carbs as a car engine producing 140 bhp at 6500 rpm. The slower revving needs to pull more fuel per stroke but less often, hence the air correction requirement differs.

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posted on 16/5/15 at 10:09 PM

Just to update - drilled out the mains to 2.3mm today and then took the car out for a test which turned into a 2 hour long hoon!

Fuelling is much better now, see below but WOT is now in the 12s. Despite the big mains fuel economy doesn't seem to bad soa far - on my 75mile test run it averaged 27mpg.

Description

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posted on 16/5/15 at 10:13 PM

WOT wants to be around 12.5

Looks pretty spot on

[Edited on 16/5/15 by dave_424]

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posted on 17/5/15 at 08:33 AM

Is that with the trumpets and filter back on?

Looks good.

Stu

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posted on 8/9/15 at 08:09 PM

Just a thought.. but do bike carbs have a special air box? ..
At WOT the air box would have a different atmospheric pressure and almost cancel out the air corrector..
Now.. if you run the carbs without the proper box and open to atmosphere then the air corrector would be free to suck too much air..
Probably load of tosh ... lol

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posted on 5/9/21 at 10:24 AM

Sorry to bring up old thread. I am running between 140-150bhp 5800-6500rpm

My airbox was 2 litres in capacity. 1600CC 16V engine on 264 road cams.

Running rich only at WOT 11.5 to 11.18 between 3500-4500 rpm then 11.2 between 4500-6500rpm.

Could do with it at 12.2 before I increase the timing further.

Airbox for 1600CC should be 3.5 to 4 litres in size?

The main jets are the correct size.

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posted on 5/9/21 at 10:57 AM

Important.. Buy inlet manifold & fittings from DanST Engineering..no leaky

Very nice manifold..

Description

I'm sorry. That may not leak, but it's a terrible manifold for airflow. Ridiculously restrictive. Air doesn't like going around sharp corners, and you're getting nowhere near the maximum airflow through the carburettors. You should have the carbs respaced to match the inlet ports and have nothing more than a gradual curve on the inlet manifold runners. The straighter the better.

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posted on 5/9/21 at 02:47 PM

I have a inlet manifold that doesnt leak. was built by bogg brothers and was port matched.

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