PMO question

[dipso]

I am kind of in a postponed position on my engine rebuild.
Currently I have a stock 2.7. Heater backdate and exhaust backdate SSIs.
I am eventually ( next year) going to build a 9.5 single or 10.5 twin plugged engine.
Most likely twin plugged either way.
Either way I will need appropriate cams. I know this.

My question now is, since I always have a little time at the end of the year I was going to get a head start on things.
I am going to put on the elephant ribbed oil lines and wide mouth oil cooler.
I am also going to do the MSD6AL ignition with MSD blaster coil.
(both of these are already bought and will be done in December)
I think both of these will work with either motor.

The area where I think I might make a mistake is with the Carbs.
I would like to pull the CIS and put the carbs on during December as well.
My question is, Can I buy carbs for a stock 2.7 put them on and not have any major problems or changes when I upgrade.
I understand their are adjustments and jets that may have to be changed. Is rejetting expensive?
Or will the carbs I buy for the stock 2.7 just plain not work when I build a better motor.
Or even better yet, Will the carbs I throw on the 2.7 be just fine for better built 2.7?

I have no idea what 40mm, 46mm and 50mm mean. I am sure those guys will tell me.
I have heard good things about (I think it is Richard) at PMO as far as setting things up properly before they ship.

I don't mind tinkering, in fact I enjoy it. I just don't want to jump the gun and buy something expensive for my engine now and have to replace it when I rebuild the motor.

What do you think?

Thanks
Lance

[Steve@Rennsport]

40mm carbs would be appropriate for a street-driven 2.7. This describes the diameter of the throttle bores,.......

Indeed, Richard (or any of his dealers), will pre-jet the carbs according to the specifications provided at the time of order and they will be pretty close.

Its imperative that you install and set them up correctly to assure a successful outcome,...

[dipso]

That sounds good Steve, but what changes on the carbs will I have to look forward to after I rebuild.
If any, and will those be simple and/or inexpensive.

Thanks

[RoninLB]

I'd install a single or double EGT meter with its exhaust probes. Order SC bungs and caps from SSI through pelicanparts before you install carbs in case you want O2 metering later. If done early you'll learn to read the meter beforehand. You'll love it when dialing in carbs.

most likely the exhaust should come down to drill the tubes anyway and you're setting up for a future O2 meter.

party hearty

[Steve@Rennsport]

Originally Posted by dipso

That sounds good Steve, but what changes on the carbs will I have to look forward to after I rebuild.
If any, and will those be simple and/or inexpensive.

Thanks

Jetting changes will depend on what changes you make during rebuild. Cams, compression ratio, displacement, and exhaust all affect jetting however, this is not hard nor expensive to do.

[sc_rufctr]

Re jetting is easy...

Back in the day when I was running an air-cooled VW with twin Webbers I used to change jets at the track just before an event. The size of jets I used depended on the event, track conditions and the weather. I had spent many hours on an empty track near my house with a stop watch and a note pad. I had it down to a science... About 10 minutes total to change all of the jets!

They used to laugh at me in the club but my trophy shelf was full so I must have been doing something right.

[moneymanager]

Yes jetting is easy but getting things right can take weeks if you lack a dyno and a good understanding of what you're dealing with. But you're lucky with PMO's. Richard can set you up for the stock motor. Then when you change he can recommend all the jet changes you need for the new set up. He'll be pretty close.

[dipso]

Sounds great. I guess i should order them.
Ronin, I already have that LM2 exhaust gas meter and the exhaust sniffer. Its a little hard to work with a Mac but luckily it came programmed. I guess if I put a little time into it i can figure it out.
I bought that in advance too and tested first on my CIS.

Excellent, so it sounds like 40mm PMOs is the way to go. I didn't want to buy 40s now and then have to buy a different set later.
I can just re jet and mess with it.
Sounds like fun!
I should probably order carbs, a book on tuning carbs, a synchronizer float? ( if that is what they are still called), I have my LM2.
Anything else I will need?

I used to have dual carbs on my Bug as a teenager, I am sure balancing two is easier than balancing 6. Only had 2 on the bug.
Definitely going to be a learning experience.

Any suggestions on the book and float?

Thanks.

[jtkkz]

It took us about a week of re-jetting, etc... before we got it dialed on my 2.7 rebuild.

[dipso]

Originally Posted by jtkkz

It took us about a week of re-jetting, etc... before we got it dialed on my 2.7 rebuild.

How come? Shouldn't there be a formula for specific builds?
How did you know when they were dialed in?
Were they PMOs?
Did you give your build specs before ordering?
Were they new?

[dipso]

I just looked in Waynes project book. There are 2 sections, one on rebuilding carbs and one on adjusting/balancing carbs. Both of these very detailed except they are for Webers, not PMOs.
Are the adjustments the same for both?
Or should I look for something PMO specific.

I am thinking the details in the book should be fine for the PMOs.
I hope.

[GaryR]

PMO's are Weber clones AFAIK, what is applicable for one should be for the other.. please someone correct me if i'm mistaken.

[RWebb]

PMOs have the float guage windows built in & have many other detail improvements.

I can't recall whether the knobs are all in the same places or not...

[Eagledriver]

Your 2.7 should use 40mm carbs. They can work on your upgraded engine if you don't get too wild with it. If you will be going above 7000 RPM with your upgraded 2.7 or going with a bigger motor then 40mm will limit your power slightly.

-Andy

[RoninLB]

carb size is matched to displacement air flow.

usually a tad lower than text is better atomization than a tad higher

all out racing gets other responses

there are a couple of formulas around for figuring all this out

[racer_X]

dipso
Forgive me for high jacking your thread. I am a newbie to the 911. I have a 1977 911S. Bone stock.

You said:

"Currently I have a stock 2.7. Heater backdate and exhaust backdate SSIs"

What is a backdated heater (I am assuming you mean heat exchangers) what back date will work with the 77 2.7? The reason I ask is that I would like to by a set of SSI (hopefully used) and am confused by the thick flange - thin flange options.

Also, I am assuming backdate exhaust is the twin pipes coming out around the middle of the back (like a stock bug : ) not like the stock set up with the single exhaust out the side.

[dipso]

Hi RacerX.
Heater backdate means I backdated to an early style heater (no heater fan, just ducts running off the alternator fan) it's a really nice way to go. I would highly recommend it.
I also have the backdated exhaust, I think it is Dansk, with heat exchangers. 2 in, 1 out.

I don't know if the backdated heater is relevant in my original question, but as you will find out around here. The more information you put on the table, the more precise of an answer you can get.

There are always different ways of doing things. If I didn't put that info in I might of gotten 2 different answers. One for headers with a heater, one for headers without.
I am sure somebody would of asked if I already backdated the exhaust.
Some carbs, I think, won't work properly at all without SSI.

I am pretty sure it is the diameter that changes, and in our cars the thermal reactors disappear with a backdate. Both very important.
Check smog regulations for NY first. Im in Cali and 75 is exempt.

[jtkkz]

Originally Posted by dipso

How come? Shouldn't there be a formula for specific builds?
How did you know when they were dialed in?
Were they PMOs?
Did you give your build specs before ordering?
Were they new?

I had brand new PMO carbs & match ported PMO manifolds, and Richard jetted them according to my engine rebuild spec. but we still had to play with the jet sizes to get it smoother and really dial it in.
Once it is dialed in it is very smooth, almost fuel injection smooth except for the bad gas mileage those PMO carbs loves to drink up the fuel in the tank.... my car is not a daily driver so it was fine with me, since I wanted to max out my HP on my single plug 2.7 engine.

[RoninLB]

Originally Posted by moneymanager

Yes jetting is easy but getting things right can take weeks if you lack a dyno and a good understanding of what you're dealing with.

I think the rich mix "seat of the paints kick" throws many people off

[RoninLB]

Originally Posted by dipso

Hi RacerX.

Check smog regulations for NY first. Im in Cali and 75 is exempt.

25 years then a safety sticker and a visual only, no smog