Jetting help needed: PMO 46's on 3.5L

[Bobboloo]

Hi guys,

I could use some help on jetting for a 3.5L engine I built with PMO 46 carburetors and Mod S cams from John Dougherty.

The engine started as a 3.0Lbuilt as follows:

3.6 crankshaft
3.6 oil pump
Mahle 98mm high compression P+C set.
10.3 to 1 CR
Patrick Motorsports Twin Distributor
Heads ported to 40mm intake 38mm out
Mod S camshafts
MSDS headers

Current jetting PMO 46's

190 air corr.
F11 emulsions
155 main jets
38mm chokes
55mm idle jet?

The car runs great but it's a bit rich.
Here's some numbers I got with my LM-2.

RPM's AFR
Idle 12.3:1
2500 11:1
3000 11:1
3500 12:1
4000 14:1

Any suggestions on leaning out the 2500 to 3500 range?

[gamin]

Richard Parr recommended opening the idle airs 1 or 2 turns. Little slotted screws with a lock nut on each barrel. I did this on my PMO's and it made a difference. Haven't had to change plugs in a long time. Bit better gas mileage too.

[snbush67]

Originally Posted by Bobboloo

Hi guys,

I could use some help on jetting for a 3.5L engine I built with PMO 46 carburetors and Mod S cams from John Dougherty.

The engine started as a 3.0Lbuilt as follows:

3.6 crankshaft
3.6 oil pump
Mahle 98mm high compression P+C set.
10.3 to 1 CR
Patrick Motorsports Twin Distributor
Heads ported to 40mm intake 38mm out
Mod S camshafts
MSDS headers

Current jetting PMO 46's

190 air corr.
F11 emulsions
155 main jets
38mm chokes
55mm idle jet?

The car runs great but it's a bit rich.
Here's some numbers I got with my LM-2.

RPM's AFR
Idle 12.3:1
2500 11:1
3000 11:1
3500 12:1
4000 14:1

Any suggestions on leaning out the 2500 to 3500 range?

I think your chokes are way too small, typically smaller chokes will mask improper set up and balance. The mod s cam is docile for your 3.5.

190 air corr. Probably good
F11 emulsions. Maybe
155 main jets. 165 maybe bigger
38mm chokes. 40’s and maybe 42’s (more air in, better Signal)
55mm idle jet? Maybe try a 56.5 or 57

Your AFR 14:1 at 4000 is bad. That lean at 4000? That means you’re likely going receive lean damage at 6500.

[icarp]

I’m sorry to say, but I think you need some serious professional help the likelihood of you really hurting your engine is reasonable I don’t think you have a concept of the relationship of the fuel Jets and the air and the exhaust and the camshaft and the compression ratio
I am glad you’re asking for help. That’s the first step.

Just my thoughts, Ian
Feel free to PM me if you’d like

[Bobboloo]

Thanks for the replies.
I had similar thoughts on the 38mm chokes. They were suggested by Richard Parr when I initially built it as a 3.5L with stock Carrera cams. It ran great but I never had a chance to check AFR's before the owner decided he wanted to go twin plug and that presented the opportunity to go with the Mod S cam since Mahle makes a P+C set for twin plug that allows for a cam with overlap. The owner loves the torque curve with the Mod S cam. It fits for him and I must say I really like it too.
I was hoping there might be an adjustment with air correction I might do before buying another set of chokes.

@ icarp Regarding the 14:1 AFR at 4000. I should have mentioned, this is a street engine. 14:1 is lamda and the owner won't be cruising at 4k or above. Only brief excursions above 4k so I don't see it as an issue.

[targa72e]

I am with icarp. If you are seeing 14 at 4000 you will be much leaner above that and with S cams peak power if going to be around 6K plus. At that point you are probably so lean that you will burn things up. Lamda or leaner is only relevant for low load and light cruise. Chart below gives general mixtures to shoot for.



Your 11 to 12 between 2500 to 3500 is not a problem, your 14 at 4K is. You should probably shoot for 12 to 1 thru the entire rev range.

From the Performance oriented site, the 3.2 is closest to your configuration.



There is some good tuning information on how to correct the A/F mixture at different points here as well, link below.

Performance Oriented

Hope this helps,

john

[snbush67]

Bobby, check the jetting that I posted, it’s Icarp approved.

[icarp]

OK Bobalooo do what you think is best. I’m just trying to save your engine.

Ian

[icarp]

Originally Posted by targa72e

I am with icarp. If you are seeing 14 at 4000 you will be much leaner above that and with S cams peak power if going to be around 6K plus. At that point you are probably so lean that you will burn things up. Lamda or leaner is only relevant for low load and light cruise. Chart below gives general mixtures to shoot for.



Your 11 to 12 between 2500 to 3500 is not a problem, your 14 at 4K is. You should probably shoot for 12 to 1 thru the entire rev range.

From the Performance oriented site, the 3.2 is closest to your configuration.



There is some good tuning information on how to correct the A/F mixture at different points here as well, link below.

Performance Oriented

Hope this helps,

john

Yes, you are right this is great information from performance oriented, but none of it crosses over into the PMO carburetor world , well most of it doesn’t

[icarp]

The Mod s cams are not really providing the good torque , the greatly undersized chokes are most likely to take the credit .
For all others reading this post , improper jetting in carbs can provide many gremlins to chase down . Coking of the back side of both intake and exhaust valves , combustion and less than optimal temps allowing for hot or cold running , bad fuel mileage , poor throttle response , catching the air filters on fire and burning up the car , premature cylinder wear, poor piston ring performance,
and many others .

Carb Jetting is an ART

just some thoughts

My car Kermit has a 3.2 ss, M1 cams , 46 PMO's , 8 to 1 dynamic compression ratio and makes
225 lb / ft of torque at 2050 rpm and 243 lb / ft of torque at 4800 rpm,
1/4 mile @13.0 sec @111mph and revs safely to 7350 rpm.
It is a TQ monster , I love it

Ian

[Bobboloo]

Ian,
I came here for suggestions because the majority of my experience is in building engines with MFI, CIS and EFI and I appreciate your input. When I took on this build I told the customer I'm not a carburetor guy. If there was someone here I could send him to I would, but that isn't the case. I know the shops in LA and who works at them. There is one guy and he's way too busy and doesn't need the business.
John,
Thanks for the link to Paul Abbots sight. That's some really great info. Next run I do in the car I will take some sustained readings at 4k thru 7k to get a more accurate AFR's.
SNBush,
Thanks for the suggestions. The engine I originally planned for this client was very similar to yours but he changed direction on me a couple of times. I should have done a better job managing the client but it's his engine and his money and I want to make him happy. I don't think we're too far off and even though I'm not a carburetor guy I think we'll get there with the LM-2 and some good info.

[Bobboloo]

Update,
I pinged by another user on this board who is well versed with PMO's and has a lot of PMO parts that can be used to tune this engine.
It'll take a bit of time to arrange getting the car to him so he can tune it and I will get back here with some numbers to post of the results as this may be helpful to have it listed here for a future build.

[icarp]

Bobby, you are in good hands with your choice of PMO artisian.
He called me and included me in his thoughts

Ian

[911 SLANT]

Keep us posted. I'm very interested in the outcome. I am also running 46 PMOs with a Mod S cam. Is your PMO guy local to Los Angeles?

[Bobboloo]

911 SLANT PM sent.

[snbush67]

I suspect that this PMO expert is nimble and quick.

[icarp]

Nope he's plumb crazy