What heads do you have?
Duster running rich
- Thread starter lenky1
- Start date
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What heads do you have?
Then they are open chamber. The chance of you being much over 9.5:1 is pretty slim.
What carb options you thinking
So what does that tell you about the rich issue. Might not be the carb!
Timing and a vacuum gauge to tune the idle mix will go miles to get it sorted out.
Something more adjustable, like a Quick Fuel of sorts. Work with it first. You may luck up with the timing. Retarded timing can certainly cause it. Did you ever say where the initial timing is set?
Mattax
Just the facts, ma'am
You have to begin with the timing in the correct ballpark.
Do what Crackedback posted.
I'm going to change the order a little bit since this is all new and the engine starts for you.
A. Get a Timing Light, Tachometer, and buy or make timing tape/marks.
The timing tape or marks go on the damper so you can read timing more advanced than 10* Before Top Center (BTC). If youy happen to have a 'dial back' timing light, then you can skip this step and it will generally be accurate enough to get you by. Many also will show rpm. RPM is critical since the timing changes with rpm. You must know the rpm of each and every timing measurement.
B. With the engine running, get the timing at the slowest idle you can, and also at some other rpms such 1000, 1250, 1500, 2000. That's enough you can call it a baseline reference. You know the engine started and ran with this timing curve.
Side note: Normally timing is measured with the vacuum advance disconnected and the vacuum line plugged.
C. The 'idle speed' screw on the carb adjusts the throttle position. There's a limited range of adjustment where the throttle opening is in the working range. Too far open or too closed and the idle and off/idle will fuel mixtures will get out of wack.
Even without removing the carb you can get some idea of whether its in the working range. For example, if there is no or little response when turning the idle mix screws, then the throttles are probably too far open.
Do what Crackedback posted.
I'm going to change the order a little bit since this is all new and the engine starts for you.
A. Get a Timing Light, Tachometer, and buy or make timing tape/marks.
The timing tape or marks go on the damper so you can read timing more advanced than 10* Before Top Center (BTC). If youy happen to have a 'dial back' timing light, then you can skip this step and it will generally be accurate enough to get you by. Many also will show rpm. RPM is critical since the timing changes with rpm. You must know the rpm of each and every timing measurement.
B. With the engine running, get the timing at the slowest idle you can, and also at some other rpms such 1000, 1250, 1500, 2000. That's enough you can call it a baseline reference. You know the engine started and ran with this timing curve.
Side note: Normally timing is measured with the vacuum advance disconnected and the vacuum line plugged.
C. The 'idle speed' screw on the carb adjusts the throttle position. There's a limited range of adjustment where the throttle opening is in the working range. Too far open or too closed and the idle and off/idle will fuel mixtures will get out of wack.
Even without removing the carb you can get some idea of whether its in the working range. For example, if there is no or little response when turning the idle mix screws, then the throttles are probably too far open.
Last edited:
Mattax
Just the facts, ma'am
MoparLeo
NRA BENEFACTOR LEVEL LIFE MEMBER
A modified engine combo like yours will always run rich, and for good reason.
An engine that runs rich, just burns extra fuel.
An engine that runs lean, especially under load burns parts. eg pistons,valves etc...
Why a double pump carb for the street ?
To get the best possible A:F number. Find a shop that uses an exhaust gas analyser to tune with.
The biggest problem with home engine builds is that many parts tend to not compliment each other.
Think of 2 cooks each baking a cake.
One baker uses a tested recipe so he knows what to expect.
The other baker use similar ingredients but doesn't have a complete recipe.
Sometimes you need to back up and start over instead of tweeking a bad/unknown recipe.
And keep this in mind, " A good street engine/car generally makes a bad Race car, and vice versa ."
Street cars need to run well at various speeds and rpms for sometimes extended periods of time. The "race engine" runs at normally just high RPMs for a short period of time.
You need to decide what compromise you are willing to put up with. You can't have your cake and eat it on an old school type engine.
Modern, computer controlled cars can get crazy horsepower and decent gas mileage.
But that is because they are computer controlled with many sensors feeding info into a sophisticated computer that adjusts everything many times a second while you just control the go pedal.
An engine that runs rich, just burns extra fuel.
An engine that runs lean, especially under load burns parts. eg pistons,valves etc...
Why a double pump carb for the street ?
To get the best possible A:F number. Find a shop that uses an exhaust gas analyser to tune with.
The biggest problem with home engine builds is that many parts tend to not compliment each other.
Think of 2 cooks each baking a cake.
One baker uses a tested recipe so he knows what to expect.
The other baker use similar ingredients but doesn't have a complete recipe.
Sometimes you need to back up and start over instead of tweeking a bad/unknown recipe.
And keep this in mind, " A good street engine/car generally makes a bad Race car, and vice versa ."
Street cars need to run well at various speeds and rpms for sometimes extended periods of time. The "race engine" runs at normally just high RPMs for a short period of time.
You need to decide what compromise you are willing to put up with. You can't have your cake and eat it on an old school type engine.
Modern, computer controlled cars can get crazy horsepower and decent gas mileage.
But that is because they are computer controlled with many sensors feeding info into a sophisticated computer that adjusts everything many times a second while you just control the go pedal.
TT5.9mag
Two atmospheres are better than one
Seriously, no. I run a double pumper on just about everything. None of my junk runs “rich”. It’s all about optimization of what you are using. Learn how to tune a carb and give the engine what it wants. To the OP you should not have to compromise on your tune up. Learn what the circuits of the carb are, what they do, and how to tune them. It’s not that difficult.
It was built by an engine builder in Southern Missouri 20 years ago basically. My uncle only put on about 400 miles in almost 10 years. I remember him having issue back then. I dont know what they all were but I do remember it blowing a freeze plugs. When I got the car there were literally 3 carburetors 3 fuel pressure regulators and he couldn't get it to run. I have done a ton to it all ready to fix issues. Unfortunately for me carbs and tuning aren't my specialty. Im going to start with timing and see if I can get this figured out. Again I appreciate all input
Lenky,
The place to start in getting good idle quality & not over rich is NOT with the carb. It is with the ign timing at idle....& many do not understand this.....
The timing controls engine efficiency. When timing is correct, more HP is produced...& less fuel [ leaner ] is used doing it.
For this particular thread, forget about total timing or a timing curve. That comes later....
This engine has a sizeable cam &, with low-ish compression, will need a lot of timing at idle, perhaps 40* or more. Yes, 40*. Getting the timing correct will get the pri throttle blades in the correct position....or at least closer to the correct position. This is important because it affects the relationship with the transfer slot, & that has a BIG impact on rich/lean.
When the timing is correct & T-slot correct, you will be able to get a consistent idle [ not smooth with this cam ] which will be on the rich side, but not smelly &/or carboning up the spark plugs.
Many smart people use a dist that has an adj vac unit that connects to manifold vacuum [ MVA ]. With MVA, the VA unit adds timing at idle. Eg, MVA adds 25* & static initial is 15*. Total timing at idle is 40*
You will see in the examples below that idle rpm INCREASED with the extra timing from MVA. The increase in rpm comes from the engine making more hp from the extra timing.
The place to start in getting good idle quality & not over rich is NOT with the carb. It is with the ign timing at idle....& many do not understand this.....
The timing controls engine efficiency. When timing is correct, more HP is produced...& less fuel [ leaner ] is used doing it.
For this particular thread, forget about total timing or a timing curve. That comes later....
This engine has a sizeable cam &, with low-ish compression, will need a lot of timing at idle, perhaps 40* or more. Yes, 40*. Getting the timing correct will get the pri throttle blades in the correct position....or at least closer to the correct position. This is important because it affects the relationship with the transfer slot, & that has a BIG impact on rich/lean.
When the timing is correct & T-slot correct, you will be able to get a consistent idle [ not smooth with this cam ] which will be on the rich side, but not smelly &/or carboning up the spark plugs.
Many smart people use a dist that has an adj vac unit that connects to manifold vacuum [ MVA ]. With MVA, the VA unit adds timing at idle. Eg, MVA adds 25* & static initial is 15*. Total timing at idle is 40*
You will see in the examples below that idle rpm INCREASED with the extra timing from MVA. The increase in rpm comes from the engine making more hp from the extra timing.
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