Engine Noise

[sondial]

Need some help troubleshooting the last problem in a longtime rebuild. The best way I can describe it - there is a spot in the pedal - just before the secondaries open, and just after they close, where I hear what sounds like poor combustion - kind of like a car that has too low a grade of gasoline, when the engine clatters and timing is off. I'm not sure that's what it is, but that what the rattle sounds like. The noise is faint a low speeds, but get's much worse (louder) at highway speeds. On the highway, it's right at crusing speed, so the only way to prevent the noise is to accelerate or decelerate. I can reproduce the sound anytime I want. Otherwise, the vehicle runs and sounds excellent. Smooth running.

Here's background on the vehicle:

This is a 79 Ford Bronco - bored out and stroked out the original 351M block to create a 400 - similar to the tech article I have read on this site around 2001. Changed out the original cam with Comp cam 32-206-3. Changed to TRW forged pistons 8.2:1 CR. Changed from original 2 BBl for a 4 BBL 600 CFM Holley. Changed to an MSD centrifugal advance distributor from original vacuum advance.

All of the internal engine work was performed by a reputable race shop in Lubbock, TX, so I have a high degree of confidence in it. Essentially they gave me a long block - the only internal thing I may have done was install the push rods and torque down the rocker arms.

Here's what I've tried so far:

I began to notice when I moved from TX to Oklahoma. But that may be just because I was paying attention more. I initially thought it had to due with a lower grade of gas. So I mixed a tank with racing fuel, such that I believe I had about 95 octane mixed. This did not solve the problem.

I tried to adjust the timing to get rid of the problem - this did not solve the problem.

I have change out to a bigger size jet in in the carb - this did not solve the problem. #1 plug in fact looks to be a little rich, but not bad.

I changed to the lightest spring possible for the secondaries - thinking if they came open sooner, and stayed open longer, it would alleviate the problem. This did not solve the problem.

I tried the lightest spings in the distributor, thinking more advance under load might help. This did not solve the problem.

So I've tried the less expensive things I can think of.

There's only one more thing someone has mentioned to me to try that I haven't yet. Go back to a vacuum advance distributor instead of the centrifugal advance. This obviously costs some $$ - so I though I'd ask if anyone has run into this problem before, or has any other suggestions.

Thanks in advance for any help. I am definitely a novice at this - this will likely be my one and only restore/rebuild - so don't hold back on even the simplest of questions.

 

[Seabronc]

I'm not familiar with the elevation statistics for the two locations, but if you have had a significant increase in terrain elevation, then your carb may have been running too rich with the original jets which would require going to a smaller jet rather than a bigger one. Also, your compression ratio really doesn't require 95 octane. Another possibility is the gas in your area, what is the percentage of ethanol in your area?

Good luck,

 

[sondial]

Moved from Midland TX - Elevation around 2800 ft

Moved to Tulsa OK - Elevation around 750 ft

I can definitely try a smaller jet.

Percentage of ethanol in Tulsa is 10%. What is your thought with the ethanol?

Thanks again for the help

 

[Bully Bob]

I think the "rule of thumb" is -- up in altitude=smaller jetting.

However, I think the issues is in the carb. Does that carb. have the squirter nozzles in the secondary.?

That "dead-zone" sounds like the rush of air is leaning mixture until the jets catch up...

Did you put all the tests back to "square-one" (springs & such).?

If that's a "new" Holley----they may have a clue., or they may swap it.

Isn't there a carb., cam., timing compatability chart for cam changes..?

Ignition/timing is always a possibility.. /emoticons/[email protected] 2x" width="20" height="20" />

 

[swampthing]

i would call holley explain the problem to them they can help you...is it mechanical or vacuum secondary

 

[sondial]

I did purchase a new Holley - 600 CFM - PN 80457S. It does have a squirter nozzles, although I wonder if the squirter is in the primary and secondary, or just one or the other? Unfortunately I won't be able to see it for another week and a half.

I have not put things back to "square one".

In essence of that question - let's go back to square one.

When I first cranked this engine up after the rebuild, I set the timing where I though it should be ~ 6-8 degrees BTDC, using the timing mark I thought was the correct one to use. I drove it down the road, and overheated in 20 miles. Eventually, I moved the timing further back, until it quit overheating, and sounded much better. As I got to looking the other day, I remembered this incident, and that there appear to be two timing marks.

The first marker, is at about the 10:30 position - hangs off the side of the harmonic balancer. It is triangular in shape, and has a definite line - which is perfect for shooting a timing light at, as you can clearly see the numbers pass by the marker. I'll call this the triangle marker.

The other is at the 9 o clock position, that hangs just off the harmonic balancer. It is lobe shaped, rounded on the end, with what appears to be a small circle - almost like a magnifying glass you might hold up to your eye for looking at maps - about the size of a nickel. However, this mark is not convenient at all for timing, as you can't see the number pass by the mark very well due to the angle, so you kind of guesstimate. We'll call this the circular marker.

I initially used the triangular marker, as this seemed like the obivios choice. However, this is the mark that caused the vehicle to run hot.

When I moved it back closer to the circular marker, this is where it ran much better. So, it's been there for a while. At the time I was using a digital timing light, which was nice

Years down the road, now I'm trying to solve this problem, thinking it runs great everywhere but this "spot". Now I've been making small moves with a different timing light. However, I just got to noticing this has a dial on the back of it - which apparently was dialed way advanced. When I moved it back to zero, it showed again, I was closer to the circular timing mark.

So, I'm not sure what mark to use to make these adjustments. I'd like to know what the dial advance function is used for too. Need some educating

 

[sondial]

Secondary is spring actuated - this is what I changed out when I attempted to get the secodaries to open quicker - so this is a mechanical secondary - correct? I would think vacuum wouldn't have a spring.

 

[Bully Bob]

sondial.,

Being a member for 7 yrs., you have yet to fill-in your info. pages on this board showing your specifics. This is a BIG help in getting folks to chime in with help & ideas.

If you don't feel you'll need any help beyond this thread., then disregard.

"All of the internal engine work was performed by a reputable race shop in Lubbock, TX, so I have a high degree of confidence in it."

This being "true"., I would talk to these folks as many eng. changes req. coordination. i.e. cam, carb, ignition, maybe even hedders, etc.

If there's any high perform. eng. folks on this board., I'm not aware. I certainly am not.

However., (I'm assuming) a mech. secondary would have a hard linkage rod runn'n to it & can be opened with the eng. off.

The vac. secondary is pulled open with eng. suction & may have a spring to close it. (not familar with that carb.)

Maybe with that cam., you should have a Mech. secondary ...???

There will be nozzles in the primary...sometimes they're in the secondary as well.

 

[sondial]

I joined back when I got close to finishing the project 7 years ago. Since then, I took on remodeling a house and never quite finished the Bronco. Now that I'm done with that house, I'm getting back to finishing what I started. So I haven't been active on this site since.

Where do I go to put in my information? Looked around, but didn't see where to input that.

 

[Bully Bob]

Cick on your name --upper right-- Profile.

You'll find a place to enter profile & signature. Sig. is pretty much all you know 'bout your rig.

Profile is about you, where you live (city state) , etc.

It's pretty easy. /emoticons/[email protected] 2x" width="20" height="20" />