Question about running Carbs vs CIS
 

Hi -
I have some general questions about running carbs. My only experience w/them has been on my KTM 300 (dirt bike). Please excuse my ignorance! My car is a '74 911 w/a '77 2.7S engine w/CIS - which generally runs fine at the moment.

1. How sensitive are they to altitude change? I live at 6000', often drive to 4000' and 9000' - will I be cursing them while driving up the pass to 9000'?

2. Assuming I have someone to help me set them up, how difficult are they to maintain?

3. I seem to always be reading about how carbs get better throttle response and more power - is that really true?

4. How much will my gas milage suffer?

5. Are there any other differences that I should be concerned about? If I'm spending money on carbs, would I be better off just getting into another system like EFI instead?

Thanks,
Tom

1 - no
2 - not bad if in good shape or new
3 - yes & maybe see Note
4 - a LOT; expect about an 80% reduction in mileage
5a - a top EFI system will be superior to anything else but will cost 2-3x carbs.
5b - smog regs are a concern as will be resale value (you cannot sell the car in Calif., most of the PNW or the NE) much less many other places in the US. So if you do it, you are going to be stuck with the car, which may be just fine.

Note - your cam will not allow more power with carbs and you'll also have to change the pistons. Cams + pistons = $$$

i dont think i'd bother going to a carb setup unless i was rebuilding the engine.

a rebuilt engine with more compression and say a 964 cam would be nice with weber or pmo carbs

from personal experience on other cars with big weber & EFI setups i dont think gas milage would be that different, assuming you are driving the car in the same manner. odds are you'll drive more agressively then milage will suffer

Re: Question about running Carbs vs CIS
 

Hi Tom:

I'd offer another opinion,...:)

1) Carbs do not handle big (3K) altitude changes very well and IMHO, you should rejet when going fom 4000 ft to 7000+ unless you are just transitioning.

2) These are VERY easy to maintain as long as they are in excellent condition and have been setup correctly. The devil is in the details,...:)

3) Yessir!

4) Mileage depends on both carb setup/jetting and throttle position. My motor ranges bwteen 24-25 at light freeway cruise to <12 mpg with aggressive driving.

5) EFI is neat but requires a large commitment in time for programming and adjustment. Many EFI'ed engines do not as well as they can,...:)

I recently did the switch from CIS to carbs and I've never looked back. I went with a new set of PMOs and once set up they've been great. It's true that mileage goes down, but this is also thanks in part to the fact that you will be enjoying full throttle more often than before!

It's true that in some states the car will not pass emissions, but you'll be OK in CA (if you ever decide to sell) since your 74 is now smog exempt.

"when going from 4000 ft to 7000+"

But note that he is going down _from_ 6,000 ft. -- or up from there. (so apparently he lives at 6k). So the difference on each swing is less. And he is in dry air, not wet (usually).

Also, O2 concentration is somewhat curvilinear with altitude.


Despite my careful reading of his location, I missed the part re it being a 1974! So ignore my comments re resales in Calif. (unless they change the law again....)

Thanks for the responses so far everyone. I do live at 6000' where the air is typically VERY dry. My main concern w/the altitude changes - going to the track which is at about 4600' I think - will I have to rejet when I get there? How much of a chore is that to do? Is it practicle to rejet for the weekend and then change it back a few days later? Also, one of my "fun" drives is up and over a minor pass, but the top is close to 9000'. I'm not sure I'd enjoy smoking and popping my way up! Just trying to get a general sense as most on this board seem to be psyched after making the switch.

Thanks,
Tom

Altitude changes and carbs will make you nuts. The CIS is very forgiving.

CARBS will drive you nuts! yeah, they can be nice, but for everyday use, fuel injection will make you happy

I would rejet for the track. You are going down where the air is denser (more O2) so you'd want more gas (bigger jet). You are racing, so might want a bigger jet then maybe.

You really want to get the thing setup just right to begin with... Steve is not all that far away from you, and Portland is known for its brewpubs...
If you do that, then carbs will not drive you nuts. I've had 2 cars with carbs (those are recent 911s; not counting my old Pontiac GTO, Olds..., and assorted Japanese buggies).

My guess is that you'd be 'ok' on the pass - besides are you gonna change jets 2x (up + down) on each pass? They didn't do that way back when...

Who do you know in the Silver Sage PCA group or others near you that have carbs?


The ultimate is to have a fuel and air control system that
1. knows what is going on in the combustion chamber (or, at least what is coming out the exhaust) at all times
2. can compensate for all relevant variables

ONLY EFI can do the above.

You can put a choke on a carb, put a metal diaphragm on MFI, duct hot air to either, hang 8 dinguses (or is that dingi?) off of a CIS system, but none can do what EFI can.

EFI is ~2x the cost of carbs...
and only those with a computer and expertise can set the best ones up right. That's why I have carbs...

But don't think you've solved the compression and cam issues just by strapping a pair of $1,000 bills on top of the motor.

how carbs are tuned reflects mileage.

I'm rich with 15mpg at 3.4-3.6k for hrs. Around town at sea level is around 10mpg. I'm as rich as possible and still able to keep plugs clean. I've been riding this for 2 years and want to compare what happens when it's in 110F deserts at noticably higher rpms this spring.

135 mains/ 34 vents even cools EGT about 25F over 3.6k and maybe another 15 at 3.8-4.2k.


Not recommended reg gas added at least 1mpg and maybe another 1.25-1.5 on hy's.

ronin, are you saying that you get better mpg with reg. than with high test?