DP
I have been tuning carbs on racing engines for 50 years , similar to the time you have been active with cars. I have been on dynos 200 times . I have re-tuned too many carbs to remember .
This is where my thought process comes from

Ian

I have huge respect for the people that can tune, maintain, and rebuild carbs. I know there are some really impressive engines out there running them flawlessly, but they aren't for me. I read the various manuals, books, and watched the YouTube tutorials. I just didn't have the patience, time, and perhaps talent to manage them myself, so I went EFI, and I do not regret it for one second. Turn key, start engine, drive off. No funny starting procedures, ongoing diagnosis, or jet swaps with weather change.

I’m trying to teach Ian my secrets about carburetors but I’m not sure I’m getting through to him.

Tip in
Tapping on butterflies.
Silent Float leveling.
Vacuum porting
Pump jet bleed holes.
pump jet diaphragms and hatchets.
Eliminating Black Magic from the floats.
Float inlet valves.
Synchronized balance and air flow.
Precise adjustments of linkage to be equal through full throttle.

And much more.

Seriously, Ian has gained about 95 HP on my car from ignition, timing and carburetor adjustments over the last year. I thought I had it tuned, was I ever wrong.

PMO tuning question.

I keep hearing how carbs are difficult to tune, is the difficulty getting them to run as close to 100% as possible?

For example: If you are at 95%, is it the final 5% that's difficult? Wouldn't the car run great at 95%?

I've been wanting PMOs for a long time but I keep getting discouraged from threads like this. I've had muscle cars and dirt bikes all my life and doing some simple tuning wasn't all that difficult on a street car and dirtbikes are fairly easy with only one carb.

Sorry if I'm high jacking the thread.

I’m leaning toward a set of PMOs and if carbs are what you want you shouldn’t be discouraged. Appreciate that the cis system is 30+ years old. I personally would never try to regulate a fuel pump designed to deliver 70-80 down to 5-7. You are asking for trouble! Just because you were told it’s ok and that currently it works for you, well when you have an engine fire then that’s on you. I’ve spoke with our host about a set of PMO’s, they will spec them for you based on info you give them about your engine. I’ve looked at refreshing my current cis system and it’s not cheap. Fuel lines, regulator, fuel pump, fuel distributor etc etc.

The difficulty is knowing what perfect is. If you’ve never been there then you don’t know what can be achieved. I think that most people don’t realize how the smallest adjustments can make an incredible difference, both in smoothness and power.

I put carburetors on and ran them for years thinking I had them perfect, I wasn’t even close. But they felt better to me then the CIS I replaced. Then Ian did some small adjustments to the linkage, then the mixture and probably increased the smoothness and horsepower by 50 or so. Then I purchased an AFR gauge, and a 123 distributor and we started dialing in perfection. There are a few other things we did to increase air flow and timing. But mostly it was experimenting with jetting and finding the right plugs.

You need to use all your senses and the right tools. You could just bolt the carbs on and tune to best lean mix, and think you got the right jetting. But you won’t know until you’ve checked your AFR’s, plugs, and timing etc. it’s really the same with any intake system your using, with carburetors it’s in the jetting. There is no standard jetting, every motor is different.

One of the hardest things to do is to get the linkage dead on synchronized. But there is a way. I’ll share it later. I’ll probably have to make a video.



http://forums.pelicanparts.com/uploa...1675297451.jpghttp://forums.pelicanparts.com/uploa...1675297545.jpg

Thanks and yes, I totally understand what your saying, just because it's good to me doesn't mean there's not a lot left in the tank. I've already decided that CDI+ or 123 will be done before I would do the conversion and already plan on having O2 bungs installed on my SSIs so that I can run a AFR guage on each bank.

My Thoughts
 

I've enjoyed my PMO carb experience.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/821976-gordos-pmo-carb-tuning.html

Favorite quote from this thread:


Agree - thank goodness it's a set it and forget it process, unless you need to remove the carbs and linkage brackets for some reason.

Parting shot - installing and tuning carbs or aftermarket EFI requires considerable interest, study and patience / that or a good chunk of $$ to have someone knowledgeable do it for you.

Gordo is a great example of an owner being committed to the process of tuning his carbs .
Look at the time line , he stuck to the plan of learning , writing down the data and building a plan. ...... More than 12 months of diligent work and research.
It is a worthwhile endeavour , and takes persistence , Good on you Gordo.
I'm not trying to scare people away from the idea, just trying to help them understand the process.
Give it a shot , all you can do is learn.
Ian

Back in the stone age all we could do was make runs, read plugs and exhaust pipes

Off the wall question.

Does anyone know if a Holley Sniper EFI could be fitted to a CIS System?

I could see it being a sweet option. Cut the CIS Air box to allow attaching the unit to the base of the CIS Air box, Block the injector holes, tune and go!

Replaced the stock CIS fuel pump with a low pressure pump;

http://forums.pelicanparts.com/uploa...1662850103.jpg

This is a L shaped bracket I had on hand, just drilled a couple holes and it mounted in the stock location perfectly.

http://forums.pelicanparts.com/uploa...1662850103.jpg

Anything is possible however the cis intake runners would be a poor choice for sending atomized fuel to the heads.

OP. If you keep the CIS pump with the PMO regulator, then you can keep the accumulator. The accumulator is meant to manage the volume and pressure pulses from the CIS pump. If you go to the low pressure pump then bypass/remove the accumulator. You can keep the stock filter.

No matter what system you choose, use the PMO fuel pressure regulator and keep the return line.

The bottom line is that the high volume low pressure pump is the best system for carburetors.

Jess P,

When you say you spoke with the host, I'm assuming you mean Pelican Parts? Reason why I ask is I don't see PMO carbs available for sale on their site. Do they need to special order it from PMO?

There are authorized EMPI dealers of the artist formerly known as PMO. I think Rothsport racing, Patrick Motorsports and their ilk could be suppliers now. Expect a 20% increase in what they cost unfortunately.

My bad, PMO appears on Pelican if I search for it without entering a specific 911 model and just do a generic search. It looks like its $6200 starting
https://www.pelicanparts.com/More_Info/PMO8030.htm?pn=PM-O803-0&SVSVSI=911M

I wonder if the PMO EFI systems are going to become available.

What the heck, everyone else has an opinion on this...

Listen to Ian. He knows of what he talks. Maybe a bit of hyperbole thrown in ("at least 25 years"), but either building your own experience with tuning carbs, or finding someone with that experience, is not trivial. I've been cleaning, setting, tuning carbs for 50 years.

Did I mention cleaning? Yeah, with the introduction of ethanol-contaminated fuel, the problems with carbs getting gunked up internally has increased dramatically, ESPECIALLY with cars and equipment that are not used regularly. Any fond memories I have of tuning carbs have been displaced with unending frustration with cleaning and resetting them due to ethanol gunk. That applies to everything from chainsaws to big V8's. Full disclosure: I have a Kawasaki KLR 650 with a SINGLE carb. That I can deal with cleaning once in awhile. A single 4-bbl carb, maybe. Multiple carbs? Fuhgeddaboudit.

And the amount of ethanol contamination is already set to increase further, from 10% to 15%. This will exponentially increase problems.

So for me, there's only one real answer now--EFI. I still run CIS on my SC 3.2 engine because it works very well and I have become a CIS aficionado (I keep a 3mm long Allen wrench in the glove box), but if it craps out, I'll be replacing it with ITBs and EFI, and I will use a fixed distributor with a programmable curve, or I will just go all-in for an integrated engine management system with coil-on-plugs, etc. Why screw around with anything else?

And about the idea of grafting an EFI onto the CIS intake system--forget about that too. Carbs and intake manifolds had to be carefully designed to minimize changes in mixture to different cylinders due to the differences in "wet flow" versus "dry flow" inside the manifolds. I won't go into it, except to say that the CIS manifold was designed for air only (dry flow). It will not work well with a throttle body style EFI system. There's good reason why manufacturers replaced throttle body injection with port injection several decades ago.

The really good news is , PAUL ABBOTT is back in the biz of weber recreation

Paul Abbott
Weber service specialist
Performance Oriented

Ian

Pete , that conversation was lots of fun!!

Ian