# Afr problem



## Nz66gto (May 28, 2016)

Hi first dyno test on my 66 Gto was a bit of a disappointment but still a fun day my max rwhp was 250 at 4697 rpm max torque 305 3391rpm but my main problem was afr top reading was 13.9 the dyno guy said it was to lean and engine needed more gas he put a fuel pressure gauge on to test and it was 5-6psi at 2500rpm then dropped to 1.2-1.5 at 4000 rpm does that sound low it's a mechanical fuel pump and I have tripower set up 400 engine.cheers for any ideas help.


----------



## Nz66gto (May 28, 2016)

Dyno sheet


----------



## Nz66gto (May 28, 2016)

Dyno sheet


----------



## PontiacJim (Dec 29, 2012)

*The 5-6 psi is about right. BUT, you may have a problem somewhere along the car's fuel system and not necessarily the pump it self. Here is a thread that may provide ideas, click on the red link:*
http://www.gtoforum.com/f50/fuel-pressure-dropping-28814/

*I am no expert on air/fuel ratio's, but here is what I found on the web, but 13 to 1 seems to be the correct number. Remember, this is a carb'd engine:*

"Performance engines runs best with an air/fuel mixture in the range of 12 to 13 to 1. This isn’t the cleanest or most efficient mixture, but it typically delivers the most power.

Emissions-controlled engines, by comparison, are calibrated to run as close to the ideal or “stoichiometric” air/fuel mixture of 14.7 to 1, because this produces the lowest overall emissions. Leaner mixtures are better for fuel economy, up to a point. But beyond 18:1, there’s a risk of lean misfire, a significant loss of power and the risk of detonation/preignition and possibly burning a piston.

Richer mixtures (less than 12 to 1) just waste fuel and don’t deliver any more power because there’s not enough air to burn all of the fuel. And if the mixture is less than about 8:1 , it may not ignite at all causing the engine to misfire.

A far more accurate way to determine the air/fuel ratio is to run the engine on a dyno with wide band oxygen sensors installed in each exhaust header. A wide band O2 sensor will reveal the exact air/fuel ratio, which can then be adjusted by changing the jets in the carburetor until you find the sweet spot of about 12- to 13-to-1. The dyno horsepower and torque readings will also tell you if you are getting more or less power with each change in carburetor jet sizes."

*And....:*

"A starting point for air/fuel mixtures for most race engines is:

Idle: 1 to 3-percent CO or a 14.1-13.4:1 air/fuel mixtureCruise rpm: 1 to 3-percent CO or a 14.2 -14.0:1 air/fuel mixture.

Power mixture and acceleration: 6.6-percent CO or a 12.0:1 air/fuel mixture for a normal engine; a high performance engine with improved combustion chamber design such as a Pro-Stock or a Winston Cup engine, in some cases, use a slightly leaner power mixture of 4-percent CO or a 13.0:1 air/fuel ratio."


----------



## Lemans guy (Oct 14, 2014)

I keep wide band 02 in each exhaust and have digital meter next to my vac gauge....when you want lots of power the AFR drops into 12 to .....13 some ...power 14.7.... Or 14 to 16.... Cruise......

These cars idle a little rich....1000 RPM will be near 12.5.....

To get my 550 idle with AT....I have to get about a little more into 11's.....now that is just off idle mixture screws.....

The race cars with hot cams won't idle at 550.... More like 900 or 1000... That will be 12's

I would check or change fuel filter first...as it sounds like what your throttle demands at 4000......is not there.....

Might need a new pump, could have a clogged fuel sock in the tank strainer, fix that and your 13.9 will fix itself......

As far as mixture not pressure.....

Also make sure throttle plate is opening as required, check the linkage....those tri powers have all that cool linkage....

But it all has to open the correct amount as well.......

Good luck


----------



## Nz66gto (May 28, 2016)

Cheers I have a new fuel pump lines are all clear I haven't checked my tank side yet , before I go any further could I run a fuel pipe straight to fuel can and test that if fuel pressure doesn't drop at 3500 rpm I found my issue?. Also is my dyno numbers about right on a stock 66 Gto it's just my torque is down 110 from stock I have dyno sheets above. Regards


----------



## PontiacJim (Dec 29, 2012)

Nz66gto said:


> Cheers I have a new fuel pump lines are all clear I haven't checked my tank side yet , before I go any further could I run a fuel pipe straight to fuel can and test that if fuel pressure doesn't drop at 3500 rpm I found my issue?. Also is my dyno numbers about right on a stock 66 Gto it's just my torque is down 110 from stock I have dyno sheets above. Regards



Putting a fuel line into a fuel can may work. If there is any restriction from the tank, it could show up doing it this way.

Could be a number of reasons why your torque numbers are down. Maybe the Pontiac simply made too much torque for the dyno and your tires were actually burning rubber on the dyno rollers. LOL

Many factors can affect torque - cam selection, heads, exhaust, but until you can get your fuel issues solved, I would not be overly concerned and then run another test to see how it changes both HP and torque numbers. Did you do the test with the air cleaners off?


----------



## Nz66gto (May 28, 2016)

air cleaners on also only went to 4600rpm and hp was still going up but he pulled out as it was getting to lean. I think it was probably only getting fuel from centre carb will do test this week what rpm does dumpers come in ? My guess is 3500rpm
Cheers for your help


----------



## PontiacJim (Dec 29, 2012)

Nz66gto said:


> air cleaners on also only went to 4600rpm and hp was still going up but he pulled out as it was getting to lean. I think it was probably only getting fuel from centre carb will do test this week what rpm does dumpers come in ? My guess is 3500rpm
> Cheers for your help



By "dumpers," do you mean the end carbs have the vacuum opening? The vacuum operated end carbs could be your problem if they are no opening correctly. I think most would say the vacuum operated end carbs are not the way to go. Never had one with a vacuum system, my experience was with the mechanical linkage and never had any issues with them. 

Personally, I would only use mechanical linkage on a tri-power. They do make a kit that will make a change over to mechanical linkage. You can find it here: 1966 Tripower Linkage kit | Pontiac Tripower


----------



## Nz66gto (May 28, 2016)

No I have mechanical linkage that works fine my problem is the 1-2 psi fuel pressure at 3000-4000 rpm not enough fuel I was thinking of putting a electric pump in I will change my pickup in tank also this weekend.cheers for your ideas


----------

