Mixture Settings on the Track

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I've had a friendly dispute with a well known UK enginemaker who is a great bloke but a bit stuck in about 1972. This isn't a moan about him - he is a national treasure.

He's built a lovely new top end for my 850 with larger inlet valves, and what with other engine mods including large dia. straight through pipes he thinks its capable of 80hp. It isn't. When I took it on the dyno, set up for an AFM of 13.25%, it was so rich it was off the scale on 320 main jets. I got about 59hp, but leaning out to 270s brought me 72hp and was bang on the AFM. Torque was about 20% higher and smoothed out a number of holes which I'd noticed on the track.

When I told him the news rather than being pleased with the outcome he went ape. 320s is what they ran in the 1970s, they didn't need dynos to tell them when a bike was right etc, etc. He took the bike for a run on the 320s and announced it "crisp and sharp." It doesn't feel like that to me, instead of pulling cleanly it seems to drown. He's convinced that on 270s the bike will blow, but the graphs don't lie, or do they? Is 13.25% too lean? I read in a recent old bike mag that 14.5% is ideal... I'm worried. I'd love the extra 13hp, but I'm not going to enjoy the blast unless I think the motor will hold together.

Dave Jackson
 
I would suggest very cold plugs, NGK 9 or colder. Then I would jet it for maximum horsepower. Keep an eye on the plug insulator for the salt and pepper flakes that indicate detonation. 13.25 is about right for maximum horsepower but right on the edge of being too lean for safe long wide open pulls if your on a big track. 13 is safer and is what I usually jet customers bikes at- but I would be likely to go leaner on my own bike.
Required jet sizes have changed from 15 years ago due to the change in specific gravity of the fuels. Jim
 
Interesting situation - and not very different from when I first went racing.

I agree with comnoz. The very first step in carb tuning is correct ignition timing. If you google Gordon Jennings on spark plugs this is the first thing he says.

Check your timing is exactly the same both side. Depending on your compression your full advance timing should be somewhere between about 28 and 31 BTDC. High compression less advance. Carefully look for any erosion around the electrodes. The pepper spots on the white insulator is the very first sign of detonation. (I have read that people attribute them to burnt oil getting past the rings as they vibrate under the first sign of detonation)

My experience was that many people run too much advance and then dump lots of fuel in there to try and control detonation.

I could not get my bike to run on the recomended main jets and went to the dyno to get a reasonable setting - and yes mine was off the scale too.

Depending on altitude I used to run 190 to 210 main jets on my 500 Norton twin with twin 30 mm Concentrics - NZ 98 octane pump gas. 50 bhp on a inertia dyno. Take the hp numbers with a grain of salt but the bike is fast and extremely reliable.

The only concern I have with jetting on the dyno is the old story about disturbed airflow at speed. The old wifes tale is to jet on the dyno and then go 10 more.

My suggestion is to get your timing in the range , both sides, jet a bit richer than the dyno suggests. Say 280 290. Do proper plug chops on the track using race fuel, get your igniton right as per Jennings advice and then bring the mains down until the plugs look correct. Dont go further than the dyno though. I guess you might argue the air supply on a dyno is better so you could go leaner but I sure wouldnt push my luck.

I use NGK BP9 ES plugs. P mean projected and in my engine there is enough room for them without fouling valves or pistons. I use projected plugs if I can. Also note what Mr Jennings has to say on this subject.

John
 
I definitely agree on the disturbed airflow at speed. You will want to do some plug chops from speed to see the effect. Start out slightly rich.
You will also want to be richer than 13.25 at the torque peak of the motor. More like 12.8 at the torque peak and then leaner at the horsepower peak.

John, I am surprised to hear you run projected plugs. I always went the other way with high compression motors and used a recessed gap along with an extra degree or two of timing . Not to say one or the other is wrong but I always worried about a glowing side electrode with projected tips and high compression. Jim
 
"I always worried about a glowing side electrode with projected tips and high compression. Jim"

Hi Jim,

Actually thats excactly why I do use them.

If you read Jennings http://www.strappe.com/plugs.html

He argues that having the plug right out in the centre means it is cooled by the incoming charge and therefore gives a better burn, needs less advance and a gives cool plug.

However you do need space to run them and my bike doesnt have very high compresion. Only 10.25 :1. I ran the bike like this for about eight seasons with no issues. Our races are not that long - 15 to 18 minutes max so that could differ from your situation as well.

These days its on methanol but ran for years on petrol on BP 9 ES plugs.

John
 
Hi Dave
My Maney 750 came with 38mm carbs & 320 jets.
I ended up working down until I got a really good drive everywhere.
It now runs 34mm smoothbores with 230 main jets.
Just keep running it until you find what works for you.
Didnt like the big hole the 38s made in the drive & I cant get out of corners as fast as the quick guys, so I kept finding it.
No point having all the goodies if you cant run the bike the way it suits you.
Chris
 
This is knid of a double edge sword for the masses. Too rich and foul out in the city, not rich enough and blow up on the highway.
I like to cruise around the countryside but I also like to cruise 80 to 90 down the highway.

Maybe those power arc units could be made switchcable. Or even FI for the rest of us. City/map....flip....highway/map. Just thinking out loud.
 
Fuel injection is like cheating with a modern OHC 4 valve. Put a CHT ring around a plug and a temp needle probe in the header and dial in while watching the gauges and mph speed per distance testing. Let comoz tell yA the number to aim for as who'd believe me.
 
Fuel injection doesn't need any switches for regular or performance use. It is already right there in the twist grip. Jim
 
One thing to bear in mind when using a dyno to set up the jetting is that they need very well controlled airflow through the dyno room.

I have been to some poorly ventilated dynos where the bike uses up the oxygen in the room and not enough fresh air is coming in. That ends up making the bike run rich and you have to jet down to get a correct mixture. Then you get to the track and it runs lean 'cos it's in fresh air with plenty of oxygen.

Many dyno operators consider exhaust extraction fans to be the only thing of importance, but you have to replace the volume of air that's being sucked out of the room with an equal volume of fresh air. Many don't do that.
 
john robert bould said:
comnoz said:
Fuel injection doesn't need any switches for regular or performance use. It is already right there in the twist grip. Jim

Jim ,are exhaust gas meters any good ?

If you are referring to O2 sensor they they can work pretty well. To be accurate at anything except stoic you need to use a wideband sensor and controller. I use one along with a 5 gas analyzer which has a chemical oxygen measuring unit as a backup. The most common problems with them are air contamination from taking the sample too close to the end of the pipe and interference from ignition systems.
 
Hi All,

Sorry to butt in on your conversation, but you guys seem to know what you are talking about.
I have a string on the spray tubes in Amals. Are you guys using Amals and if so do you know whether
the cut away (stepped) spray tube is better than the full tube type?

Thanks
Staytite
 
Staytite said:
Hi All,

Sorry to butt in on your conversation, but you guys seem to know what you are talking about.
I have a string on the spray tubes in Amals. Are you guys using Amals and if so do you know whether
the cut away (stepped) spray tube is better than the full tube type?

Thanks
Staytite

Better or not just depends on the application. The style of spray tube will change the fuel delivery curve. It is used just like changing jets to get the fuel mixture that works best on your motor. Jim
 
EGT.

http://thesensorconnection.com/applicat ... ance.shtml

http://www.iwt.com.au/EGT_CHT.htm

This is how I fined tuned ole Peel lines with tach, speedo and EGT and oil temp. Made note of what speed and angle made one tire or the other let go then could reproduce the max entry per turn, per the approach and exit intended. EGT just over 1375'F, CHT close to 400'F. Oil 200' F. Not very good power until Exht system dialed in, then POWIE!!
http://www.foxvalleykart.com/gauges.html

http://www.jpitech.com/press_releases/Bush_Article.pdf
 
If you apply choke , and it goes faster ; Its LEAN

Agree -. On full throttle and stablised speed - If you apply choke , and it goes faster ; Its LEAN
 
Duh dudes even I know at WOT near top speed if applying the choke helps then its sign of way too big a carb not Lean-ness. Easing back on throttle to get a brief surge implies lean-ness while actual choke on too much makes it misfire not power surge.
 
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