Hi all. My brother and I just got a motor built for his 91 bird. Specs are as follows.

-350 10.7:1 comp
-procomp 210CC heads
-howards 234/244, 110, .488/.510 lift cam
-edelbrock air gap intake
-Holley 4150 750cfm w/ proform main body carb
-vacuum advance dist w/ MSD 6AL box

Now we originally had full timing at 35* full advance around 3100 rpm with the vacuum line unplugged. The car seemed to run kind of rough and a freind said that on WOT he thought he heard pinging. Were only running on 91 octane so we backed timing to 29* full advance with vacuum unplugged. The odd thing is that once you plug the vacuum line back in, the timing seems to not change from when it was unplugged whether it's at WOT or idle. Should it change a lot whether the dist has vacuum or not?

Also, the car has been averaging 10 mpg and it's a T56 car. The carb is untouched from the old setup which was a similiar cam and heads, but a vic jr intake and only 9.0:1 comp, but the old setup got much better mileage. Any ideas on why this would be? I have heard that engines with more compression need less fuel under cruising because they are more efficient. Not really sure where to start here. Car runs pretty good and has an occasional pop from giving it throttle off idle but the accelerator pump cams are on the #2 position and I'm thinking that just going to the #1 position will fix that. I'm no carb expert but I know all the basics. Any help is much appreciated.

 

It sounds like the vacuum canister is bad. It could also be the port in the carb...but that seems unlikely. Regardless, 34* advance should be good (although a TRUE 10.7:1 compression may not like that on 91 octane). Also, the vacuum advance should only be active at idle and part throttle (not at WOT). At idle, you should see around 45* advance. Your poor gas mileage is directly related to the timing.

 

So you are saying it should be connected to a manifold source and not the ported source? I was thinking that it was the other way around.....

Please correct me if Im wrong, but dont both ports go to 0 at WOT but only the manifold source would give vac at idle? Ported is 0 at idle too right?

 

Exactly. I have found that the response is MUCH better using a manifold source. I belive the auto manufacturers only went to the ported source as a means to counteract emissions testing (NOx readings at idle)...not for better performance. And either source will drop to near zero at WOT. Here is how my car is currently set up:

Base: 18* Advance
Total Mechanical: 38* all in by 2800RPM
Vacuum advance at 1000RPM idle: 52* (My car idles at 12" vacuum)
Vacuum advance at light cruising speed: 55* (20" of vacuum)

Another downside to not having enough advance at idle and cruise is your engine will run hotter. I am not sure why this happens, but I have experienced it and I have read about it in many places. Perhaps it is due to unburnt gases lighting off in the exhaust port instead of staying in the combustion chamber.

 

The motor is a true 10.7:1 compression. We built it based on another companies crate engine matching everything identically so the compression is correct (has 58CC heads on it). I'm pretty sure when I checked it, the timing at idle with vacuum plugged in was around 10 degrees. Sounds like a deffinate problem though so I will look into it. We have a spare dist kicking around so I can try using the vacuum canister off that one and check it out.

I am still curious on the carb itself though as well. As I said, it came off a heads/cam 9.0:1 comp engine and is untouched on the new one. I have heard that higher compression engines require less fuel for cruising because they are more efficient. Will this throw the jet sizing off by a lot? We've done a WOT run in the car and then pulled a plug and it was a nice and brownish color. I'm sure the jetting isn't spot on but it seems close.

 

It is possible that the engine may require less fuel at cruising speeds. However, I would not be using the main jets to set WOT AFR. That should be accomplished using the power valve and the power valve channel restrictors. If you use just the main jets to set WOT, you will certainly be rich during cruising speeds.

What jets and PV's do you have in there now?

 

Well we fixed the timing issue. The vac advance on the dist was plugged into an incorrect port and wasn't getting vacuum. We plugged it into manifold vac and now it has around 40 or more degrees of timing under normal cruising. Feels much better too.

It's still rich though. The carb has a 6.5 PV but is only making around 6 HG of vac at idle however we havn't played with the idle mixture screws yet so we may be able to get more out of it. Not sure on the jets since we bought it with the old motor combo and havn't tore into the carb yet. If the vac at idle doesn't go up much then the PV is surely the problem for running rich. Any recommendations?

And as far as setting WOT AFR, I always thought the sec jets were for that, but your saying to do it with the PV and channel restrictors. Can you explain that a little more?

Appreciate all the help!

 

The jets play a roll in the mixture at WOT...but the PV is there for a reason. When the vacuum drops low enough, the PV opens allowing fule into that chamber and it enters the fuel circuit through some tiny openings in the metering block called the "Power Valve Channel Restrictors". You can see them if you take the PV out. If you count on the main jets to add all of the fuel needed at WOT, then you will be too rich for normal cruise speeds. Real light cruise speeds are almost totally fed by the idle/transitioin circuits.

For your combo, 6" of vacuum at idle seems low. I would think it would be more in the 10" range. Under light cruise, it will go up even more. Even if the PV is opening at idle, I don't think it will much (if any) fuel into the carb. Work on your mixture screws and base idle speed next. Check around for vacuum leaks too. One other thing...my Airgap setup really liked a 4-hole tapered spacer. It didn't net any gains at the strip, but at idle and light cruise it was much crisper.

 

If I am not mistaken, a motor with retarded timing runs hotter because the heat is not contained in the combustion chamber, and cannot be absorbed by the coolant. Instead the combustion happens later and is pushed out the exhaust immediately.

 

Thanks for the explanation! So basically what your saying is to go with a PV say 2 HG of vac lower than what the car makes at idle, and that should be good? I'm sure we can get more vac out of it at idle. We'll play with it tomorrow. I don't think there are any vacuum leaks honestly. The car does have a 1" carb spacer but it's not a 4 hole type. It's the open style.

Another thing. The car would never run over 170 or so degrees before we "fixed" the timing advance issue, but we took it for a ride and it got to 200 degrees with the advance working now. Not really sure why it's doing that since it smells rich.

 

I would play around with no spacer or a 4-hole type. My combo idled horribly and the low speed response was mushy with an open spacer. At the track, it lost so much in the forst 330' that it couldn't make it up at the top end (even though there appeared to be more top end power).

Let me know how it goes!

A PV 2" less than idle is a pretty good place to start. That is what I am doing with mine right now. As for the temp going up...make sure it isn't running lean! The rich exhaust smell could be blowby (your ring seal starting to go). Just be careful with it. 200F isn't bad...especially if it was a warm, humid day.