Safe jetting? Race bike

Hi

Monday night


Wednesday night


Loading for Donnington Thursday night

John who built my engine would like me to jet up while bedding it in? Your thoughts?
It runs 36mm Smoothbores full 960 engine.

all the best Chris

Chris said:

Your thoughts?

It runs 36mm Smoothbores full 960 engine.

Click to expand...

My thoughts is that it looks like a weapon !

On the jetting, would not have a clue.
You don't mention what cam, or fuel either, double the trouble.

Hopefully, your engine builder or carb supplier would have some ideas ?
Have you previously run these carbs on anything ?
Anyone you know used them ?

Well done Chris!

You have run this motor before haven't you?

If so, I would work from there. I personally would not jet up for bedding in reasons per se.

Running a tad on the rich side is common to help keep things cool in general, and may not be a bad bit of insurance in yer big motor.

Stoichiometric AF efficiency is 14.7:1 (if I recall correctly) whereas my Dyno man prefers closer to 13:1 on modern race bikes and is 'happy' anywhere between 12:1 and 13:1 on carb engined old bangers like ours as this gives a little safety margin for temp, altitude, fuel quality, etc.

Jog our memory again - what happened to your previous motor.
There are some awful looking pistons on your photobucket pages there, those JE 81 mm ones.
If those are related to these carbs, maybe your builder is steering you in the right direction with bigger jets...

Hi Rohan

yes those pistons are mine. Steve Maney has reduced the compression ratio & suggested the fuel I must use!!!
John who put the engine togeather thinks the ignition slipped & was retarded. Thats what melted the pistons.
The carbs were on the 750 (to rich) & the 960 (good) so just a matter of playing safe???

Chris

Thanks Nige hopefully at Donnington

Chris,
In the closed season I would strongly suggest looking into ceramic thermal barrier coating. It is not as expensive as you might think, and is great insurance.
As you may have read recently, my timing had 'slipped' and was very retarded. And this is how I rode it at Mallory and a whole half day on the Dyno! It should have melted but didn't. I reckon the coating was the reason for not having a melt down!
I did my piston crowns, combustion chambers, valve faces, exhaust ports. I used a company called Camcoat. It's a bit of a faff sending bits off to them etc. but well worth it IMHO.
The head coating:

The piston coating:

Dam danergous looking weapon! Do not quess install CHT plug rings and EGT needles in header and fit a G-meter, start rich and work lean going by better times and safe temp indicators plus V8 set of plug chops. Spark is set optimal 1st of course.

what frame is that?

Hi all

Thanks. Had some exhausts cam coated nice work. Will see about the engine one day.
Hi 84ok its a Seeley Mk2

Chris

Larger engines with increase swept volume can sometimes run smaller jets as compared to same carbs on smaller engine displacement .

Junk Yard Dawg
AHRMA 5X
F750

Chris said:

its a Seeley Mk2

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heard about those ..man, ata quick glance makes the commando frame look spindly tx

Junk yard Dawg said:

Larger engines with increase swept volume can sometimes run smaller jets as compared to same carbs on smaller engine displacement .

Junk Yard Dawg
AHRMA 5X
F750

Click to expand...

Now what do you know you old dawg????

That's the Bernoulli Principle, or part of it. The volume through a fixed size Venturi increases at the Square of the Velocity of the flow past the Venturi. Velocity of the flow past the Venturi increases in direct proportion to the increase in engine displacement, as does the need for more fuel.
So when increasing engine displacement with the same Carb and jet, the amount of gasoline getting pulled into the mainjet increases more than needed. This means the big engine will run rich on jets that were right for the small engine, all other things being equal.

Glen

When you jet carbs on a a motor, four things affect it - comp. ratio , ignition advance, the exhaust system and the fuel you are using. If the comp. ratio is high usually requires bigger jets. If the ignition advance changes it can have the same effect as changing the jets in the carbs. I would set the ignition advance to the specified value for the commando engine for the fuel you are using. Fit a main jet which is miles too big. Fit the normal sized needle jet and needle with the clip in the middle groove, and a number 3 slide, and start the bike. As you wind the throttle on, if it gasps you know the needle jet is either too small or you need to lift the needle and change the clip to the bottom groove. If the motor still gasps, you need a bigger needle jet. Getting the needle and needle jet right is critical. You should try to get the situation where lowering the needles one notch causes a cough as you change gears when riding the bike - then raise the needles one notch. The last thing to try to get right is the main jet, that takes high speed runs and plug chops.
The needles come in various tapers and getting the right taper is important if you are trying to get the most performance. Two things interact - the shape of the taper on the needle and the ignition advance curve.
Don't ride the bike for a large distance if it is jetted lean. If you take a muffler off your bike and fit a megaphone without re-jetting, you will probably burn a piston if you ride the bike any distance.
Hope this helps.
Best Regards,
Al

All in depth tuning references state to get ign timed 1st and that depends only on effective CR and octane in our hemis. All tuning below WOT can be done w/o main jet installed. Once ready for top end keep richening till power-time drop then back off some. A maxed out all out tune will cause some light-mild detonation but most hot rods are able to tolerate this over lengths of time and mileage of race engines. No its not safe nor is motorcycling but same as slightly over powering tire somewhat below 10% max traction is bad for rubber life but more lasting memories. Detonation is the buggaboo and comnoz has knock detector timed/filtered to self adjust his spark and fuel but us lessors may have to adapt a knock sensor to head phones or just a stethoscope tube attacked to barrel.

When setting the main jet size, start big and lean off gradually until the plugs have the 2mm black ring on the porcelain right down inside where the porcelain meets the metal. If you use one range hotter plugs than normal, just for tuning purposes, you will always be safe.