75 911S Help me decide if I should swap to PMO's

[varacoustic]

I recently bought a 75s coupe, gorgeous original car, from ebay- 2.7 CIS setup.
Drove the car a few weeks, performed pretty well, despite having silicone gasket sealer gunked all around the injectors, etc , poor mans vacuum leak fix-
Car suddenly had a few large intake backfires, it does have a pop off valve installed, and I began hunting leaks myself.
I changed the intake boots, new injector sleeves, seals, new injectors and o rings...
put it all back together, took it on a drive up the Angeles crest, more backfire, particularly when the temp gets up around 210 on the gauge..
Took the car into a good Race oriented 911 shop here in LA, had some suspension work done, engine tuned a bit, mixture set, timing set, etc Mechanic said everything was good after his tuning, car was way rich before, timing off, etc and no vacuum leaks found, his pressure gauges showed everything to be fine.
I drove the car again up the crest, performance is less than stellar, kind of lumpy feeling up through the revs, but ok so far.. Then way up in the mountains where there is no cell service(!) huge backfires, enough to shut the engine down. stranded.
I let it cool down and limped it back to Burbank to the shop.
Mechanic tells me he suspects the Fuel Distributor, won't really entertain the idea that the airbox may be cracked, etc.. Noone wants to drop the motor and remove the CIS system it seems, including me.
My frustration is high at this point, can't really find a CIS expert in LA, or anyone really after using 3 shops that is willing to actually take the time to consider all the moving parts in the CIS setup and how to tweak the car to be perfect with that setup.
Most of the shops I've been to kind of frown upon a 2.7 CIS car like its the bottom of the line in a world of hotrod Backdates or Gt4 motors stuffed into $300k 964 race cars .
In short i'm a professional guitar player/ 911 enthusiast , not a rich Beverly Hills doctor with a race car budget.

I'm about ready to throw $7k plus at a set of PMO'S and later early style exhaust for this car and be done with it.
I have 3 porsches with CIS systems, and frankly I'm a little tired of the multi component troubleshoot blackhole every time something isn't right.

I've also had many american carbureted muscle cars through the years, and they all have been infinitely easier to deal with that this pesky CIS system.

Any thoughts or advice greatly appreciated!

[unclebilly]

If you want to do more mods, then carbs. Personally, having done this twice, I would switch to zeniths with 34mm throttle bodies.

The first race 2.7 I built has webers. The second has the zeniths. I sold the zenith car this summer but plan to build up another engine (I already have it and the zeniths) using zeniths, distributorless ingotion (again), modern cams and 9.8:1 or higher compression.

Carbs let you run more aggressive cams than you can with CIS. To run more aggressive cams you can't use CIS pistons so they need to be changed as well.

[unclebilly]

Another option is efi

[winders]

Carbs are so '60's technology. Go with EFI. You will get better performance and better fuel economy.

[75 911s]

There's a guy in Long Beach developing a new EFI setup. His test subject is his own 75 911s. you can see the thread here

[914efi]

Since you have other cars to drive, why not drop the engine yourself [recruit a helper for an afternoon] and just pick your way through the CIS system? People here will talk you through it. You can buy new injectors if needed, most lines, have the WUR and FD rebuilt [CIS Flowtech] for less than 1/2 the price of the carb conversion. If the airbox is cracked you can find one used.

Bonus is that you can clean, re-seal and find/fix other problems with the engine out that will make the car that much more reliable and fun. The CIS system is really pretty simple once you get the hang of how it works.

I get it if you want carbs, it is a different experience, but the CIS properly tuned is a good system and very reliable. Carbs have the appeal of simplicity, bolt them on and go.

[al lkosmal]

If the CIS is a mystery to you and your mechanics and you know your way around webers/Zeniths/PMOs, I'd go in that direction and put the CIS on the shelf to keep the resale value up. The CIS system, when working well.......works well. However, you are correct....very few people/shops will spend the time or have the experience to make it work right. That is why so many have been swapped out for carbs.

However, if you would like to get your 2.7 running with CIS....or EFI......feel free to contact me.
I can help.

https://www.youtube.com/channel/UCA2Ea55IQplBIL2F025_P9g
https://www.youtube.com/results?search_query=koz77911

regards,
Al

PS: Getting those flippin carbs to work right is not always a walk in the park either.....

[manbridge 74]

CIS is simple if you understand the principles of operation. I start at the fuel tank and work toward the rear when I don't know cars history. Many guys are not dealing with these on a day to day basis and understandably are hesitant to condemn an expensive part like FD if that doesn't fix the problem.

I think you could tackle this yourself if you're patient and methodical.

[James Brown]

i did the Zenith on a 3.0 SC w/new ventures. they worked well looked and sounded sharp but there 40+ years old. if you can swing it, go with the PMO's along with the backdate SSC, the engine bay will astound you, the sound will intoxicate you, the performance will increase a bit but butt feel will be off the chart. PMO fuel regulator (with bypass) will work well. then slippery slope stuff-duck tail, long hood, RSR interior, etc. DON'T track cost!!!!

[vintagecarlover]

I've got a perfect set of Weber 40's that came off a 2.7 with cams. Bought them from Matt at Reno Rennsport. He's the man! Might sell to replace my deck at my lake house. Carbs are literally complete to air filters and cross linkage and absolutely sexy and beautiful and plug and play. 706-392-2580. I can send pics. Don't need to spend pmo $$. Call me. Mark

[James Brown]

webers are a good choice also, properly rebuilt and set up