engine surging

monty76 · October 27, 2005, 09:05:05 PM

Ok, had the vapor lock issue about a week ago. Installed an electic fuel pump(8psi) by the tank feeding the mechanical pump. Hose from the mechanical runs to the fender then up and over the the fender brace to the carb. Sloppy I know, just trying to clear up the vapor lock. Now I get the car surging as if you were giving it gas then backing off. This happens after I get the tires spinning and then flooring it. It'll get about 3 good surges then takes off. Doubt it was vapor since the engine was at 180 degrees and it was cool out side, car had been idle for about an hour. Plenty of time to cool down till I started it up. I'm running an 750 Eldebrock and the accelorator pump is set to the max. Bigger carb maybe? Gotta line on a 850 double pumper.

derailed · October 28, 2005, 06:20:23 AM

This exact thing happened to my car under the same scenario and it turned out to be the filter on the sending unit plugged right up. After i replaced that and put new lines and tank in it ran fine. I was running an electric and mechanical pump as well but took the mechanical off for the time being. I think my pushrod is shot.

Chryco Psycho · October 28, 2005, 11:33:13 AM

8 PSI is high for a carb , it should be 6 max so the floats are not pushed down & open the needles up with the pressure

myk · August 13, 2006, 04:13:34 AM

Well, every ONCE in a while if my car is under very heavy load the engine will surge very badly. What other things are there to look for? My fuel sending unit, its filter and the vapor separator are about 3 years old. I don't recall how long I've had the stock fuel pump but it's at least 10 years old. I've got an Eddy 750 for a 'carb. I bought new plug wires but still need to replace the plugs themselves. Initial timing's set at 19*, but I forget what the total time is, and I keep monkeying with the spark advance because I can't figure it out-lol.

Any ideas? Thanks in advance...

Chryco Psycho · August 14, 2006, 12:14:33 PM

disconnect the vacuum advance & see if the surging stops

myk · August 15, 2006, 05:50:58 AM

Haven't had a chance to try that yet, but whenever I disconnect the advance the idle drops considerably and I'm assuming that the engine won't run as smoothly. Do you want me to just kick up the idle speed and try it anyway?

firefighter3931 · August 15, 2006, 08:00:03 AM

Quote from: myk on August 15, 2006, 05:50:58 AM
Haven't had a chance to try that yet, but whenever I disconnect the advance the idle drops considerably and I'm assuming that the engine won't run as smoothly. Do you want me to just kick up the idle speed and try it anyway?

I agree with Chryco, plug off the advance, re-time it to 16-18* at idle and reset yout idle mixture and idle speed adjutments.

Ron

myk · August 16, 2006, 03:43:33 AM

I'll get on that right away. Also, you guys have stated before that vacuum advance can sometimes cause problems when tuning engines, especially ones built with a nod towards performance-why is that?
Also, I really wish I knew which way to adjust the 'carb in regards to fuel mixtures. Monkeying with the mixture screws doesn't affect RPM's in any way-I only wish I knew what kind of 'cam I was running. Judging from the smell of the exhaust I'm pretty sure the car is running too rich, but looking at the plugs they indicate a lean condition...I don't know...

myk · August 17, 2006, 06:00:27 AM

Quote from: firefighter3931 on August 15, 2006, 08:00:03 AM
Quote from: myk on August 15, 2006, 05:50:58 AM
Haven't had a chance to try that yet, but whenever I disconnect the advance the idle drops considerably and I'm assuming that the engine won't run as smoothly. Do you want me to just kick up the idle speed and try it anyway?

I agree with Chryco, plug off the advance, re-time it to 16-18* at idle and reset yout idle mixture and idle speed adjutments.

Ron

Plugged off the advance and the port on the 'carb and set the timing anywhere from 12 to 19*, and I still had the surge. To make matters worse, as the timing was advanced, say around 12*, I got the hesitation from hell. I leaned out the Eddy 'carb as I'm pretty sure it's running rich, but couldn't make a difference in RPM's doing that either.

I'm still wondering if my fuel pump just isn't delivering at WOT, or my vapor separator/lines are clogged or something.

Can a 'carb be jetted too lean but still run too rich? I can recall someone saying a while ago that Eddy 'carbs are always jetted too lean out of the box...

Chryco Psycho · August 19, 2006, 02:54:27 AM

you may need to install lighter springs under the metering rods to stop them cycling up & down to kill the surge
no a carb cannot generally be jetted lean & run rich , unless the idle cicuit is set up tto rich & the jets are small

myk · August 19, 2006, 12:18:28 PM

I figured as much, as the jets in the Eddy's primaries are the 2nd richest that you can install. I will try the lighter springs and see what happens. All of this time I thought that I needed heavier springs...

Ghoste · August 19, 2006, 02:04:35 PM

It works exactly backwards from you initially think. A heavier spring will push the needle up out of the jet and allow it to run rich under all conditions. If the engine makes enough vacuum to overcome the spring, it can still pull the rod back into the jet. Many people mistakenly think the Carter style rod and jet has the spring working against a force on the opposite side of the little piston but it is the spring and engine vacuum both working on the same side of the piston and applying opposing forces to it.

Chryco Psycho · August 20, 2006, 01:16:32 AM

the metering rods need to stay down at idle , longer duration cams decrease the manifold vacuum at idle allowing the rods to rise & enrich the mix & can cause surging

myk · August 20, 2006, 02:02:00 AM

So, surging can be caused by a mixture that is not only too lean, but also too rich. I've got my work cut out for me...

Chryco Psycho · August 22, 2006, 02:14:53 AM

yup

myk · August 29, 2006, 02:55:34 PM

Well, I've gone up to the largest primary jet available for the 'carb, and I must say that I can feel the difference throughout the RPM range. Should I try and re-jet the secondaries also? It's hard for me to be certain, but I'm pretty darn sure that the surging is gone-at about 80-90mph I'm just not sure. I still have to mess with the idle mixture screws.

I'm messing with the color-coded step up springs as well, although the color stamping from Edelbrock is so weak I can't tell one spring from another; guess I'll just have to try them all, but you guys are saying weaker springs, right?...

myk · August 29, 2006, 03:03:45 PM

Quote from: myk on August 29, 2006, 02:55:34 PM
Well, I've gone up to the largest primary jet available for the 'carb, and I must say that I can feel the difference throughout the RPM range. Should I try and re-jet the secondaries also? It's hard for me to be certain, but I'm pretty darn sure that the surging is gone-at about 80-90mph I'm just not sure. I still have to mess with the idle mixture screws, although I'm convinced that I'm way too lean-my spark plugs tips are whiter than my last date.

I'm messing with the color-coded step up springs as well, although the color stamping from Edelbrock is so weak I can't tell one spring from another; guess I'll just have to try them all, but you guys are saying weaker springs, right?...

Chryco Psycho · August 30, 2006, 01:35:29 AM

up jet the secondaries about the same # of steps as the front
what is your vacuum at idle in gear now
you need a light enough spring to keep the metering rods down at idle in gear

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