Visit The Dirt Forum for More Information

Author Topic:   Engine Help
Dirt-Tracker
Dirt Full Roller

Total posts: 60
posted November 28, 2002 09:47 PM
Just had my engine refreshed back in Sept. In the last race of the season it would have a pop at idle. Just a real quick pop. Where do I start. Maybe in my Carb.....it was only about 35 degrees out. Had a hard time keeping it at temp. Any suggestions?

Dirt-Tracker
Dirt Full Roller

Total posts: 60
posted November 28, 2002 09:51 PM
Forgot.....i run a 347 Ford stroker with a Willys 2 barrel carb. Noticed some fuel on the outside of carb... and also the air mixture screws usually are about the same amount of turns out and we couldnt get it adjusted right that night. Thats why im wondering about carb....getting ready to mail back to Willys for winter refresh job!!!

Earl Parker II
Dirt Full Roller

Total posts: 39
posted November 30, 2002 07:55 AM
Have you worked with the idle mixture needles to see if making the idle system richer or leaner affects the pop? Is the base timing where it needs to be? Also, can you clarify what was 35 degrees out?

If you can supply a little more information I'll be happy to try to help you out.

------------------
Earl Parker II
www.parkercarburetion.com
704.482.7588

Wheels47
Dirt Maniac

Total posts: 127
posted November 30, 2002 08:31 PM
The ambient temp was 35 degrees....could only keep engine at 140 degrees while running the main. The idle mixture screws are way off. We cant get it to adjust. We usually run the timing on this engine at 31-32 degrees. hope this helps

Earl Parker II
Dirt Full Roller

Total posts: 39
posted November 30, 2002 09:04 PM
You'd think I would have figured out your reference to '35 degrees out.'

First of all, a water temp of 140 would imply an oil temp of 160 or so- it probably didn't run very well.

I think the pop and lack of idle needle adjustability are related. Remove the carburetor, turn it over and see how much of the transfer idle slot is visible beneath the butterflies- my guess is almost 1/8Ē. This means that the butterflies are way too far open at idle, which will really kill acceleration. Below is an excerpt from my CD, Performance Tuning to WIN, which describes how to setup an idle system. Itís written for a four barrel but youíll have no problem figuring out what to do:

Idle system setup- method 1

To start with, invert the carburetor and check the position of the throttle butterflies. Turn the primary idle speed setting ***** to set the bottom edge of the primary throttle butterflies about .020" from the bottom edge of the transfer idle slot. Don't worry about measuring anything - your eyeball is good enough. Repeat the process on the secondary side. You might want to open the secondary side just a little more so you can have a little adjustability when fine tuning your idle speed later. Positioning the throttle butterflies near the bottom of the transfer idle slot at curb idle is absolutely critical for maximum acceleration.

Next, whether you have two or four idle mixture needles, turn them all in until they are lightly seated. Excessive force here will damage both the needles and metering block and make the idle fuel mixture difficult to set with any accuracy. After seating them turn them out 1 1/4 turns, which is a good baseline setting. Now you're ready to reinstall the carburetor and setup your idle system.

Before you start the engine, examine the primary side of the throttle body. Hopefully you'll see a little tube, covered by a rubber plug. This vacuum port connects with a passage in the throttle body that 'sees' manifold vacuum. Remove the plug, attach a good vacuum gauge to the port and position the gauge where you can see it clearly.

Without touching the carburetor, turn the engine over until you have pumped fuel into the bowls. Work the throttle a few times then start the engine. If it dies, which is likely, you'll have the turn the primary idle speed setting ***** to increase the RPM to get it to idle while it's cold. Since throttle butterfly position is critical, count the turns and fractions of turns so you'll know exactly where you're at. The whole idea is to be able to return the throttle butterflies to the position you originally set them at. As the engine warms up it should gain rpm, so you should be able to reduce the throttle opening at least somewhat without the engine dying. Now the fine tuning begins.

With the engine idling, pick one of the secondary idle mixture needles and turn it in 1/4 turn while you're watching the vacuum gauge. Give the idle a few seconds to stabilize. If manifold vacuum increases repeat the process, letting the idle stabilize each time, until it starts to decrease. If turning it in decreases manifold vacuum then try turning it out. When you've found the 'sweet spot' (i.e. the manifold vacuum is as high as you can get it) repeat the process with the other secondary idle mixture needle.

How difficult it is to find the sweet spot largely depends upon the cam profile. With a mild cam it's usually easy- there is a very definite point where manifold vacuum is the highest and a small adjustment either way will reduce it. As the cam profile becomes more radical it becomes commensurately more difficult to find the sweet spot. With a really radical race cam very often the only way to set an idle mixture ***** in the proper position is to turn it in until vacuum starts to decrease, then turn it out (often a turn or two) until vacuum starts to decrease, then turn it back in until it's positioned between the two extremes.

At any rate, once the secondary idle mixture needles are adjusted the idle speed should increase to the point where you can adjust the primary throttle butterflies even closer to their initial setting. After you do that, repeat the whole process with the primary idle mixture needles. Presumably you'll be able to pick up enough idle speed by optimizing the idle fuel mixture that you can close the primary throttle butterflies down to their original position near the bottom of the transfer idle slots. As a final check give each idle mixture needle a slight turn in then a slight turn out. If any motion hurts manifold vacuum, you know that needle is set properly. At this point if the idle is stable and the engine responds quickly when you just crack the throttle, you should be good to go.

One final note: Make sure your timing is set correctly before starting this process.

The above should help get you going in the right direction. If you have any problems at all just get back to me on the board, email me at eparkerii@carolina.rr.com or call me at 704.482.7588.

------------------
Earl Parker II
www.parkercarburetion.com
704.482.7588

Back to the Archives