Weber Jetting For Altitude

[type26owner]

Hi Dave,
Yup, it sounds easy. In fact finding a jetting combination that flows with a constant mixture between an rpm range of 1800 to 6500 is a lot of work. I changed the settings at least 30 times until I was lucky enough to hit upon the present jets that I'm sucking on now. I'm hoping that average holds and I can find another jetting combination which flows as well but at a mixture of 14.7 to 1. Hitting the jetting jackpot turns the Webers into a dream otherwise they are a total pain in the butt. The typical behavoir was to lean out to 18 to 20:1 at 6500. No wonder none of the tuning books would elaborate on how to find the best jets. It's a lottery.

Tuning a racecar is much easier than doing a roadcar. On a racecar you don't care about such things as being fuel efficient. Just am I getting full power at WOT and that it doesn't foul the spark plugs right away at partial throttle. It doesn't even have to idle. On a roadcar the operating conditions are much more stringent. IMHO, tuning the Webers on a roadcar is at least 10 times harder to do it to perfection. Could not do it without my wide-band O2 air/fuel meter. Reading spark plugs is a cruel joke.

[type26owner]

The 89 octance fuel has stopped the detonation when executing a gentle takeoff. Have remapped my spare 23D dizzy to provide 34 degrees of mechanical advance so I can run it retarded enough at idle so it does not detonate constantly but still pull hard at the higher rpms.

Going to investigate if I can purchase a knock sensor to retrofit the twinkcam. My hearing has degraded to the point I can't rely on it anymore. Have to have my son ride along now to listen for trouble. Getting old sucks! Any experts out there that have done this before?

With this dizzy I should be able to advance the total timing up at altitude by about another degree per 1000 feet and not cause it to detonate at the lower rpms.

[type26owner]

Received my new tuning toy yesterday. It's a MSD Knock Alert sensor p/n8964. Have to figure out where to mount the piezo sensor on the engine though. Thinking about making an aluminum bracket that bolts onto the two protruding bosses in the head which are visible through the center opening of the camcover and are centered between cylinders 2 & 3. The only problem with that location is the throttle cable runs directly over that particular area and there's a wire which plugs into the sensor that naturally would want to stick up in the cable's way. Hmmm....

[lotus026]

Let us know how it works when you've had time to figure it out - I've also thought about getting a knock sensor for the couple of my old cars that are running Webers!
Dave
lotus026

[type26owner]

Dave,
Looks to be really simple to install this knock sensor unit on most engines. On the twinkcam it's not though. The only place which has pre-existing threads is the drain plug for the cooling water in the engine block behind the exhaust and it won't fit there anyways. Okay, that just means it's special bracket time. The bracket will jog towards the rear and project down to nearly the bottom of the well where the spark plugs reside. That way the wire in the sensor will be positioned well away from the throttle sheath. I'll post a picture of the mount here next week.

[rgh0]

Keith

There is a thread lug on the top front left hand side of the block that may be a suitable mounting point. The threaded hole appears to have been used to mount the block for some of the machining operations.

Rohan

[type26owner]

There is a thread lug on the top front left hand side of the block that may be a suitable mounting point.

Thanks Rohan, I'll look for it to see if it's suitable for others to use. After 3.5 hours of fabricating the bracket and 1 hour of installing it's done and functioning. At first when I followed the instructions and started the engine to adjust the sensitivity nothing happened. I had to rap the bracket gently with a ballpeen hammer to get the damn thing to light up at all. It did all the way up to the top red bar. Drove it home which took all of 15 minutes and it lit up the first green bar almost continuously above 4000 rmps and flicked on the second bar occasionally. Seems my son was spot on perfect with his timing done just by ear. I'll advance the timing by a few and force it to detonate to evaluate this knock sensor under a real pinging/pinking/detonation circumstance. It could turn out to be a piece of junk afterall. I've learned never to get my hopes up too much when trying out a new tool for the first time.

Just reinstalled my Air/Fuel Meter to start searching for another main/air corrector jet combination which stays within a single domain pattern of two-phase flow. The reality is I'm going to have to probably swap out several hundred sets to find one which is close to the desired 14.7 to 1 holy grail. This the darkside of Weber tuning. May the force be with me. :rolleyes:

[type26owner]

Rohan,
It's close but no cigar. The sensor has a 1/4" NPT thread. The provided adapter has a 3/8" coarse thread fitting but the protruding boss for the bolt on the timing chain cover which is used to fasten the top of the generator/alternator is in the way so it can't screw in that place in the engine block you mentioned at all. An offset steel bracket would work but you'd be so close to the exhaust manifold I'd worry it would cook the sensor. Don't think that would be a good place to mount this sensor afterall.

Had an idea on how to store more clamping energy for the internal sealing arrangement I came up with to keep the oil from weeping out the 8X 1/4"-28 studs that fasten the camcover down. Went to the hardware store and bought some extension spring stock that just fits over the fuel hose pieces when they are slipped over the studs. The spring ID is 1/2" with 1/16" wire coils. When the fuel hose is crushed it expands in diameter but now I've got a spring there to resist it doing that quite so freely. In theory this should negate the relaxation of the hose due to thermal cycling due to it being emersed in hot oil. If the spring is too long it will merely get pushed down over the 5/16 nuts on the cambearing caps. Hope this works better cause I'm trying it for the first time today!

[type26owner]

The solid extension spring made the hose go completely solid while trying to cinch down the camcover. Tried a compression spring with .05" diameter wire and a relaxed pitch of about 3/16" per turn. That seemed to work just fine by the force it took to tighten up the 1/4" nylock nuts. It'l take a day or two for any oil leaks to appear if they are going to at all. Took some pictures which I'll link to here tomorrow.

[type26owner]

[sk178ta]

Keith,
I`m still intrigued by the idea of changing carb. jets to compensate for altitude. I`m imagining mechanics on vintage aircraft sitting astride the engine cowling making adjustments in flight ! especially in WW2 when performance in fighters was critical.
Looking at our domestic barometer, in a temperate climate as in England, the difference in barometric pressure between cyclonic and anti-cyclonic weather can easily be the same as the difference between that at sea level and 3,000 feet of altitude. I respect your determination that your carburation should be optimised but surely you don`t change jets according to the weather. Or does it really "never rain in California"?
Jim

[type26owner]

Hi Jim,
I'm concerned with the jetting getting so fat it fouls out my spark plugs. I've already experienced this at sea level by applying jets which measured a 9:1 mixture. My present jets are delivering 11:1 now at sea level. If you have a copy of latest Weber Tuning Manual (p/n_95.0000.54PM) open it up to page 59 where it talks about 'Altitude Operation' and you'll see why I've got a bad feeling about this. It would be really dumb to go to a trackday totally unprepared. Having this problem go all wrong would really peeve me off. Popping in a smaller main jet like the manual suggests is just asking for it to run like total crap.

Realized I have 4000 ft mountain just 20 minutes way. Wonder if the park rangers would mind me doing WOT testing in the parking lot at the top! Just kidding...maybe

p.s. The manual also illustrates an Aneroid Bellows Mixture Compensator to automatically correct for altitude. The DCOEs don't have one installed though.

[rgh0]

Jim

Serious racers ( eg in days of carburettor formula 1 cars) they used to change their jetting depending on temperature and atmospheric pressure for each racing session to maintain optimum mixture settings.

Your observation that difference in the weather is significant and can affect mixture is absolutely correct. These day your modern fuel injection system does all the needed adjustments for you automatically

Rohan

[type26owner]

Hey Rohan,
I've been dreaming of having enough horsepower to pull up and stay in the draft of my brother's Super7. Been looking into the pros and cons of a nitrous oxide system. With my present mixture of 11 to 1 I'm actually perfectly tuned to add a 'dry' system and boost the power up a little. By dry the nozzle would inject the gas into the airbox. Would only add enough NO to get the mixture up to 15 to 1. Don't know if the twinkcam will allow this without detonating though. Leaner mixtures do burn slower so maybe the detonation is a non problem. Guessing the weak point of the engine is the head gasket. They are easy to replace though in about two hours effort.

Adding a NO system has to be far less work then me finding another main/air corrector jet combination that flows correctly. I'm intimidated by the task ahead of swapping hundreds of jets and am just looking for a reason not to have to do it at this point.

Venture a guess as to the hp increase one would expect?

[rgh0]

Keith

My engine simulation program allows for simulation of NO injection. I will have a play in the next few days with some of my twincam models and see what appears doable without destroying the engine. My guess would be at least 180hp before things such as head gaskets and bearings and combustion temperature start to give you problems due to the high BMEP's

Just a bit busy currently swapping my new engine and spare gear box into the Elan. I have a race meeting in 12 days and still need to run the eingine in !!!

Rohan