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Author: Subject: Missing that last 10bhp - running rich at WOT
bikecarbfred

posted on 28/8/21 at 05:00 PM Reply With Quote
Missing that last 10bhp - running rich at WOT

1.6 AFH 16V overhead schrick cam on carburettor.

Main JET 150 80%-100% range; The the air fuel ratio at WOT at high rpm 4000rpm - 6500rpm is very rich. 11.2-11.5.

The pilot jet 20%-80% range 13.5 to 14 ish AFR . Im good with that. A nice smooth delivery of power.

So when I do WOT from 4000rpm to 6500rpm the AFR drops like a stone to 11.2 - 11.5.

I won't downsize the main jet as this is the same jet and carburettor I was running on my 1.4 AFH 16V overhead cam and somehow bogg brothers got it running around AFR 11.9 to 12.5 . Thier timing ignition was between 30 to 34 at 4000rpm through to 6500rpm. The compression on the 1.4 is 10:5:1

My timing on the bigger displacement 1.6 AFH is 26 to 30 from 4000-6500 Rpm because I was concerned it being a high compression engine so I thought 30 was my limit. The compression on the 1.6 is 11:5

It now seems I have to further advance the ignition and see if it makes the AFR leaner.

Risky. I have been using the phormula knock sense to tune it so far and it is so accurate.

I'm abit stumped lads. The mixture screws are for pilot and idle only.

Rolling roads around me not interested in megajolt tuning but I do feel confident in what I am doing so far.

I have been told by an engineer that I now have to advance it until I get knock.

[Edited on 28/8/21 by bikecarbfred]

[Edited on 28/8/21 by bikecarbfred]

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gremlin1234

posted on 28/8/21 at 05:18 PM Reply With Quote
what are you using as air filters?, could be starved of air?
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bikecarbfred

posted on 28/8/21 at 05:31 PM Reply With Quote
True;

It is the same Ramair large filter at front grill; from there the same 70mm pipe going into the air box.

The air box was made by me and I made it smaller than the one the one I had on the 1.4L because 1) I thought that was over kill and 2) I curved it at the far end for smoother air flow instead of having a sqaure box.

All spark plugs look good and are identical.

Does anyone know the formula of the airbox size in relation to displacement.

@gremlin; If it was lack of air at high rpm then shouldnt it be sucking out less fuel from the bowls also. Shouldnt it be relative.

[Edited on 28/8/21 by bikecarbfred]

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gremlin1234

posted on 28/8/21 at 06:16 PM Reply With Quote
quote:
Originally posted by bikecarbfred
@gremlin; If it was lack of air at high rpm then shouldnt it be sucking out less fuel from the bowls also. Shouldnt it be relative.

yes if there were plenty of air, but just think how the 'choke' works.

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bikecarbfred

posted on 28/8/21 at 07:02 PM Reply With Quote
I need to look at that instead of messing with ignition timing. I need to get the fueling right.
The jet sizes must be correct. I have even gone up displacement but obviusly the engine is now needing more air especially with the bigger cams.
I might just take off the air box and see how it reacts. if you are right then thats where my minimum 10bhp is missing. Could be more I hope.

70mm diameter sounds big enough. It might be the air box.
I have a pic of it below. seems big enough to me. I will have to find out that formula of air box capacity to engine cylinder capacity.


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coyoteboy

posted on 28/8/21 at 07:30 PM Reply With Quote
The afr you read from the gauge is affected by ignition timing and fuel. You they're not independant. Tweak one and you'll affect the other.
I spent ages going round and round trying to get my arf target and get the ignition dialed in, only to find my afr shifted again. I'm glad I am injected and left it to do closed loop for all but the last 10%

Going to knock and dialing it back a few degrees is the old school way, but won't get you peak power, sometimes peak power comes earlier than that.

[Edited on 28/8/21 by coyoteboy]





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bikecarbfred

posted on 28/8/21 at 08:01 PM Reply With Quote
My intake size is wrong. I ''should' be running above 150bhp therefore bigger diameter pipe required; more expenditure.

I also need to work out the air box capacity.



197CFM required

[Edited on 28/8/21 by bikecarbfred]

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Sanzomat

posted on 29/8/21 at 06:56 AM Reply With Quote
I'm a bit surprised by that inlet sizing chart. On Rover K series lots of people claimed that fitting the 52mm throttle body added power but it was proven that the standard 48mm throttle body diameter was not restrictive until above 150bhp and only then did the 52mm one make any difference and then that was good until about 180 so for that chart to say you need 75mm for up to 150 sounds wrong?
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bikecarbfred

posted on 29/8/21 at 07:17 AM Reply With Quote
It's a tricky situation;

I have now been told by someone who has done a similar setup to mine that my AFR is safe enough for me to increment slower the ignition advance and that is how I am going to make more power.

So at end of the day it is simple.

I remove the air box completley and run it like that . And if all those 4 sliders open up fully on the carbs then air flow is not the issue and cannot be the main jet because it is the same jet used on the 1.4.

Tune the advance ignition without the airbox. Then put the airbox back on and see if it runs rich. If it runs rich then the problem is the airbox. Problem solved?

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bikecarbfred

posted on 29/8/21 at 07:34 AM Reply With Quote
infact what am I mumbling on about. I should just remove the airbox and see if it runs the same AFR values lol
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