Well this one is kind of hard to describe but it bothers me so im gonna give it a shot at describing this, although im not sure if its normal or not.

I have a 355 that i had rebuilt a couple of months ago. With it i got a comp cam, shorty headers and a 2500 stall.

When i lightly press my foot on the gas my engine sounds like its "surging" im not sure if surging is the right word to use but it sounds like what it would sound like if a cam was audible at open throttle.

It only makes the sound until i reach a certain RPM then it doesnt make it anymore and my car sounds normal after that.

Could it be that my carb isnt pumping in enough gas at lower RPMs?

Maybe a timing issue? (hear no pinging)

Could the stall have something to do with it?

What would be a good decent carb to run with my 355?cfm rating?
The one i have right now is a 600 cfm carb 4160.


Any help/suggestions appreciated.

 

Sounds really vague, but if your idle mixture screws arent set right they can cause it to act real funky at low cruising RPMs. But it could be anything... thats just an easy one to adjust.

 

I think I know exactly what you are talking about.

Go up 3 sizes on your main jets and see if that fixes it. Sounds like a lean condition. Are your plugs white?

And it is always a good thing to double check your timing. It can slip. Check that first.

 

Thanks for the reply guys.

Im kinda new to the whole carb scene, i have adjusted my timing before because i had pinging, and right now its set to where it doesnt ping. Although it could definetly be a timing issue.

The sound is really annoying me though its kinda forcing me to get to Higher RPMS faster to avoid the bogging/surging sound.

@3rdgenmaro I know people experiment a lot with jets, i havent had a chance to look into them yet. How would i know what size of a Jet i have?

Last time i checked my plugs they were clean.

i have a holley 4160 and since this whole issue im thinking about just throwing it off my car and getting another carb.

As far as the mixture screw ill have to look into that too, if its as easy as turning it it shouldnt be hard to check.

Any suggestions on what carb specs i should be running on my engine? any tips on adjusting the screw?

 

read the sticky above on carb tuning. That will help you some.

Checking the jets is very simple. All you have to do is pull the front bowl off the carb.

Loosen the 5/16 headed bolts on the bowl, remove one of the bottom bolts first (fuel will come out so either have a rag or cup or something to catch the fuel.

Then remove the other three bolts. Gently remove the bowl. The jets are right there. A flat screwdriver is all that is needed to unscrew them. There will be a number stamped on their side.

Reinstallation is just as easy as removal. Be sure to get the accelerator pump back in place. Also if it has never been pulled apart before, it is good to get a set of the blue non-stick gaskets to go back in. Remove all existing gasket material, but do not scratch the carb surface.

Read the sticky above. That will help you get the carb dialed in. And double checking the timing is always good.

Dont toss the carb, take a little time to understand them and you will catch on.

My s10 (specs below) had the same surge at low rpms while cruising. I pulled the bowl and found I had 66 main jets. I went and bought a set of 69s. Swapped them in and the surge disappeared.

 

oh and the sticky is in the "carb" forum

 

The 4160 is a little tricky to tune but it's extremely popular for a reason. Go adjust your idle mixture screws in 1/4 of a turn and see if that helps. If that doesnt, then out 1/2 of a turn. If htat doesnt help put em back where they were.

You know how to baseline the mixture screws, right? A 4160 you turn the screws all the way in until you can feel them lightly seat, and then you back them out 1.5 turns. From there you're supposed to use a vacuum gauge and so on, but you can just seat of the pants it really. A 1/4 turn farther out on my car makes it run like *** below 3000 RPMs under acceleration. Car kind of sputters a little and whatnot.

 

Update on my car.

Today i decided to mess with the idle mixture screws.

I turned them 1/4 outward, did both and noticed a decrease in the bogging/surging sound. However it is not 100% gone i still get the surging sound just not as bad.

I also noticed that my cam is much more quiet at idle now. Did i increase fuel or air by spinning the screws outward?

Another thing i noticed was that the screws were not evenly matched, they are both set at different settings. I plan on trying to set them to the base setting on my day off from work probably this thursday, simply because i dont want to mess something up and have to miss a day from work.

Whats the worse that could happen anyway?

Tomorrow i might experiment and try to set the screws inward 1/4 after setting them back to the original setting i started with.

Any tips on finding a setting for the mixture screws? Or should i continue to guess and check?

Thanks so far for the info guys, been a great help. Im happy i atleast noticed an improvement from messing with the screws.

 

Do a google search for baselining a Holley 4160. Theres a good one in the carb board too. Just read as much you can there are different methods and the more you learn the more you'll know... I told you mine.

Just turn the screws all the way in, back them out 1.5 turns. Adjust in 1/8 turn increments from there. Make sure you adjust both evenly. I think I rmember some guys turn them out 1.5 turns, start the car, adn turn them in until teh car stalls and back it off a half turn or a 1/4 turn or something... Im not really sure.

Since adjusting them DID help, that does tell you it's a tune problem... Turning them in leans out the mixture, unscrewing them richens the mixture. The mixture screws may not necessarily be the problem (you do want them adjusted the same though, so get on that) but the fact that they improve it a little tells you that your mixture is off, and you can work from there. If you can get it to idle and drive okay, then you work on throttle response in different situations. But focus just on mild cruise and mild acceleration for now.

http://www.bob2000.com/carb.htm

Take note of this section which is very informative:



The idle mixture being off will affect drivability. As soon as you get it in the ballpark where it drives okay start adjusting the systems farther up the tree there. As you put more load on teh engine it needs more fuel so if the harder you drive it the more it clears up, it's likely one of the systems lower down the tree is dumping too much fuel.