HRC Carb setup/tuning

[28hodge]

I’m needing some help to iron out a bit of a flat spot.

Was at Oulton on the 13th March for the first trackday of the year (Technically it was the second, the first was on the 15th Feb when the CDI decided enough was enough after half a lap) this was my first chance to test out my HRC Carbs.

Overall the bike ran well, it’s hard to say how much different the bike felt compared to my previous car set up as it a very long time since I last rode it (bout 11 months) thanks to breaking my foot & ankle last year, but it did feel pretty good and the stumbling I had encountered at 7-9k rpm was completely gone.

But it felt as though I was encountering a bit of a flat spot from around 11-12500 which then disappears and the bike revs cleanly from there to 15.

On my previous carb set up which was pretty much standard but with larger 122.5 and 120 mains, the bike ran outta power at 12-12500 as shown up by the dyno run, so I never revved it much past this, now I have to rev it quite a bit harder.

I’m not against revving it but I am if it’s going to hurt the engine eventually, I know it’s got to be revved but I don’t want to be over revving it all the time. Is this just a characteristic of the HRC carb set up?

I’d also like to try and get rid of the flat spot from 11 – 12500rpm, this is on main jet though so I’m unsure why it would stumble up there if it will rev clean just beyond??

Short of dyno time, anybody got any settings to try, current set up is as below.

HRC Spec carbs complete with HRC short stack (RO replica items) HRC fibreglass air tray and drilled carb plate running the following settings

Rear cylinders
Pilot 40
Emulsion 5 hole
Main 147.50
Needle 8zf
1mm washer

Front
Pilot 40
Emulsion 4 hole
Main 145
Needle 8zf
1mm washer

Full micron 4-1 exhaust.

I think I will try and get some dyno time booked as I want to make sure that it is not lean at the top end as it’ll soon lunch itself at 15000 rpm if it is running lean!

Any suggestions?

[Cammo]

But it felt as though I was encountering a bit of a flat spot from around 11-12500 which then disappears and the bike revs cleanly from there to 15.

I’d also like to try and get rid of the flat spot from 11 – 12500rpm, this is on main jet though so I’m unsure why it would stumble up there if it will rev clean just beyond??

I agree - unlikely that main jet size is the problem, more likely to be an issue with the needle type/height.

Those settings are pretty similar to what I used to run apart from needles (I used 8zk needles), I never experienced any flat spots, pulled like a train.

A dyno run would really be the best way to diagnose, make sure you get a copy of the the air/fuel ratio chart to post up here. It's not a sure fire solution (since the carbs will see more air velocity on track than the usual dyno fan provides) but it will point you in the right direction.

[lukemillar]

I had issues running washers under the needles on my setup. I ditched them in the end and switched to the 8ZG needles.

[28hodge]

Cheers so its looking like a needle issue then, got another trackday booked on the 10th and wanting to try and dial it in a bit better before then,

As a first run its pretty damn good to be honest, got Dunken Munkey to thank for that as they are exactly as I bought them from him, good work Tony!

Luke, where do you shift up, what sort of rev range are you trying to keep the bike in? I run the standard CDI where I think you run the Ignitech yeah? plus i guess your bike is a couple of hop ups from mine at least!! still it'll give me a rough guide. I would watch one of your vids to find out but im at work so can't at the minute.

Struggling to get on the dyno before the 10th as im away to Norway to see the other half's family wednesday next week and got no time this weekend, have to try and squeeze it in on the 6-7th April.

Does it sound as if it is over fueling in the 11-12500 range? if so taking the washer out should help eliminate this yeah?

Which way do the needles go, ie which is leanest and which is richest?

Thanks

[Cammo]

Does it sound as if it is over fueling in the 11-12500 range? if so taking the washer out should help eliminate this yeah?

Yes it does sound rich, lowering the needle (removing the washer) will make it leaner. Still, a dyno is best to see what's going on.

I used to shift up at 14k rpm (going off standard tacho which is probably not that accurate).

[lukemillar]

Cheers so its looking like a needle issue then, got another trackday booked on the 10th and wanting to try and dial it in a bit better before then,

As a first run its pretty damn good to be honest, got Dunken Munkey to thank for that as they are exactly as I bought them from him, good work Tony!

Luke, where do you shift up, what sort of rev range are you trying to keep the bike in? I run the standard CDI where I think you run the Ignitech yeah? plus i guess your bike is a couple of hop ups from mine at least!! still it'll give me a rough guide. I would watch one of your vids to find out but im at work so can't at the minute.

Struggling to get on the dyno before the 10th as im away to Norway to see the other half's family wednesday next week and got no time this weekend, have to try and squeeze it in on the 6-7th April.

Does it sound as if it is over fueling in the 11-12500 range? if so taking the washer out should help eliminate this yeah?

Which way do the needles go, ie which is leanest and which is richest?

Thanks

From what I remember.... higher the letter, leaner the needle.

I usually shift about 13-14k from memory, though last time out there was a mild dip right at the top of the rev range so I think I started shifting a little earlier. It was about 8 months ago now, so trying to remember.

I ran smaller mains on my standard engine, but I haven't changed the needles between that I my tune done. The motor isn't that tuned yet though.

I also only switched to the Ignitech after I HRC'rised my rear mudguard, didn't protect/waterproof the CDI and went racing in the wet! Have only run 1 round on the Ignitech, but I really didn't notice much difference to be honest.

[28hodge]

Thats great cheers luke, sound like i have a rich spot then, as i think im right in saying, you're running a leaner needle with no washer which will also lean off that area of the carb compared to me.

If nothing else this at least ensures its not running lean at the top end so is safe, although i am definately not shifting before the red starts at 14500, think i'd best stop that and get shifting a bit earlier, as per you and cammo.

its going to have to go on the dyno to get it 100% but if i can get it to 90% without makes it a bit quicker* finishing it off on the dyno with a bit of luck.

*CHEAPER!!!!!!

[blueflag]

Sounds like it's coming on Kurt!
Dyno is the way to go mate, I went in with mine producing about 58 horses which I was happy with (what do I know) Came out after cleverer people than me moved the needles and changed the mains with 60.8 horses and a nice curve. Was wheelieing out of the hairpin at 3 sisters which it didn't do before!
Are you doing the practice day in May? If so I'll come down and put a stopwatch on you.

[28hodge]

Its running well at the minute, now i need to leave it alone and stop playing with it and get some riding done.

The guy i use for my dyno is mega busy at the minute so im struggling to get a slot with him, was at oulton on the 10th though and it was a bit warmer than when i was there in March, bike seemed better in terms of carburation so pretty happy with it at the min.

Just sent my entries off for Wirral 100 at Oulton on the 27th Mike, all going well i'll get the day off for the test at 3 Sisters and come along too.

Cant tempt you outta retirement yet can we???

[blueflag]

I'll be at Oulton to cheer you on Kurt! I'm there in the capacity of Gerry Burgess to my mate's Rossi. (or summat!) Hopefully it'll be nice and dry with no frantic wheel changes.
Knee wont let me ride yet, never mind race, I'm looking forward to the prospect of being an ex racer anyway, just think of the possibilities for bullshit. "No mate, you don't want to brake there, I used to change up there and then brake 30 yards later...." You get the idea!
I'll pop over and see you in the paddock.