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hirot
Ian Hart
Ashbourne UK
(88 posts)

Registered:
06/01/2011 05:15AM

Main British Car:
1971 MGB GT (conversion) Rover 3947 R380 gearbox

authors avatar
hunting/surging at 2000rpm with edelbrock 500
Posted by: hirot
Date: September 13, 2015 11:44AM

Hi, wondered if anyone has any suggestions on how to cure the hunting/surging I get when I am doing around 2000rpm. I have a rover 3.9l v8 using an edelbrock 500 on an edelbrock performer manifold. The car runs really smoothly accelerating through this rev point but if I just drive at 2000rpm (ish) the car just hunts, not badly but its noticeable.
I re-jetted ages ago which got all the plugs biscuit brown and my fuel consumption is good, and recently changed the air filter to breath through the vent by the screen. This has had the effect of making the car run really smoothly especially in traffic and improved my fuel consumption.
I can run the car in 5th at 1000rpm and its smooth, change down to get around 2000rpm and it becomes lumpy.
Is this likely to be a jetting problem, ie too rich or lean, or just the way the carb works ?
Ordinarily I have just lived with this but having just driven 500 miles it has started to bug me. So any help will be greatly received.


Nexxussian
Erik Johnson
Alaska
(62 posts)

Registered:
04/20/2015 10:32PM

Main British Car:
1974, MGB, Citroen Color Rover V8

Re: hunting/surging at 2000rpm with edelbrock 500
Posted by: Nexxussian
Date: September 13, 2015 03:20PM

Is this a new symptom with the "cold" air (I read that as the vent just forward of the windscreen)?

From your description it sounds like it's been doing that all along.

If that is the case, and you have eliminated the possibility of a vacuum leak, check your vacuum under the conditions you describe (4th gear 2000 RPM) and compare that to the color code of your step up springs.

it sounds like 2000 in 4th may be too close to the step up point of the springs installed.

Just out of curiosity, what vehicle speed is it that you are pulling 1000 in fifth or 2000 in 4th?


Another possibility, if you are running vacuum advance, is you may be right in the vacuum range the advance can shifts.

Let us know what you find.


hirot
Ian Hart
Ashbourne UK
(88 posts)

Registered:
06/01/2011 05:15AM

Main British Car:
1971 MGB GT (conversion) Rover 3947 R380 gearbox

authors avatar
Re: hunting/surging at 2000rpm with edelbrock 500
Posted by: hirot
Date: September 14, 2015 10:25AM

Hi Erik, thanks for the reply. I have always had the problem and it is a cold air feed. I removed the heater and the carb breathes through the 5" round hole in the firewall. Also the car runs for a few more minutes when stopped before the temperature rises and the fans come on. So it was worth doing the mod.
I will have to get a vacuum gauge, the one I have is too low pressure and will check.
At 1000 revs in 5th I am doing 30mph and it is probably 50mph at 2000 in 4th....but I will check.I think 30mph in 3rd is about 2000 revs which is where I keep ending up in traffic. The the annoying thing is that 2000 revs matches our main speed limits of 30 and 50.
I had wondered if it was step up point but will need to read and reread the edelbrock manual before I mess up.I worked out which jets I needed from the book but the springs seemed a whole new ball game.
I am running vacuum advance and the problem could be caused by the advance. Hadn't thought about that one as I had always thought/expected it was carb related.
I guess I could try blocking off the vacuum advance to see if it changes, although I assume the car might complain with no advance.

Note: Just read edelbrock book and I will check which springs I have and buy some stronger ones. Will let you know what happens.



Edited 1 time(s). Last edit at 09/14/2015 12:07PM by hirot.


Nexxussian
Erik Johnson
Alaska
(62 posts)

Registered:
04/20/2015 10:32PM

Main British Car:
1974, MGB, Citroen Color Rover V8

Re: hunting/surging at 2000rpm with edelbrock 500
Posted by: Nexxussian
Date: September 14, 2015 06:41PM

Last time I bought step up springs they came in a kit or assortment, for not much money.

One more thing I thought of, it could be your mechanical advance curve.

If you use a timing light, can you see if there is a significant change around 2000 RPM?

Depending on how forceful your surge is, it could be some oscillation or resonance in that system, possibly from sticking flyweights.

I would suspect the step up springs and vacuum advance first, but if you already have a timing light you could check the mechanical advance for "free."

If that is the issue, cleaning and lubricating the pivots should help.


hirot
Ian Hart
Ashbourne UK
(88 posts)

Registered:
06/01/2011 05:15AM

Main British Car:
1971 MGB GT (conversion) Rover 3947 R380 gearbox

authors avatar
Re: hunting/surging at 2000rpm with edelbrock 500
Posted by: hirot
Date: September 15, 2015 06:39AM

Looks like I have a small project.
Checked the springs and they are the original Orange spring. So I can buy the Pink and Plain and see what happens.
I have a timing light so next trip I shall check the advance. I did strip and rebuild the distributor 3 years ago when I rebuilt the engine, however, the weights could well be sticking.
The distributor is the original 35DLM8 which so far has seemed OK once I had a Lucas DAB118 amplifier as opposed to a Chinese replica which failed after 50 miles.
The surge isn't forceful just annoying which is why it has taken me 3 years to attempt to fix it.


DiDueColpi
Fred Key
West coast - Canada
(1365 posts)

Registered:
05/14/2010 03:06AM

Main British Car:
I really thought that I'd be an action figure by now!

authors avatar
Re: hunting/surging at 2000rpm with edelbrock 500
Posted by: DiDueColpi
Date: September 15, 2015 02:38PM

Ian,
If the surge turns out to be the step up springs.
Then going to a higher rated spring will only make it worse.
At 2000 rpm your combo isn't providing enough vacuum to keep the metering rod down. And or you're too lean on cruise.
Try a yellow or blue spring to keep the pistons down.
You want just enough spring to keep the pistons down under cruise and then let them rise under power.
A quick way to tell if it's the spring is to remove them and drive the car. Gently, as you will not have any enrichment under load.
If the surge is gone then your springs are too strong.

Live like you mean it
Fred


hirot
Ian Hart
Ashbourne UK
(88 posts)

Registered:
06/01/2011 05:15AM

Main British Car:
1971 MGB GT (conversion) Rover 3947 R380 gearbox

authors avatar
Re: hunting/surging at 2000rpm with edelbrock 500
Posted by: hirot
Date: September 16, 2015 12:45PM

Fred, thank you for the input. I have ordered a spring kit and will try what you suggest. I always thought that hunting was too rich a mixture (though I am not sure why) but apparently it can be rich or lean. So as usual I am a little wiser. I will now go up/down on the springs and not be disappointed/confused if the stronger one make it worse.
It could be running leaner now that it breathes cold air at a slightly positive pressure from my screen air intake as my mpg went from 26 mpg (UK gallons) to 27.5 mpg on a run.
Regards Ian



hirot
Ian Hart
Ashbourne UK
(88 posts)

Registered:
06/01/2011 05:15AM

Main British Car:
1971 MGB GT (conversion) Rover 3947 R380 gearbox

authors avatar
Re: hunting/surging at 2000rpm with edelbrock 500
Posted by: hirot
Date: September 25, 2015 11:19AM

Well I went to the yellow spring, which turned out to be green in my pack, and it has solved the problem. I will run it like this to check and then try the blue springs to see if it makes a difference.
Turned out to be a cheap fix for a change.

Many Thanks....Ian


Nexxussian
Erik Johnson
Alaska
(62 posts)

Registered:
04/20/2015 10:32PM

Main British Car:
1974, MGB, Citroen Color Rover V8

Re: hunting/surging at 2000rpm with edelbrock 500
Posted by: Nexxussian
Date: September 26, 2015 03:20PM

Glad to hear success was simple and inexpensive. :)


ex-tyke
Graham Creswick
Chatham, Ontario, Canada
(1165 posts)

Registered:
10/25/2007 11:17AM

Main British Car:
1976 MGB Ford 302

authors avatar
Re: hunting/surging at 2000rpm with edelbrock 500
Posted by: ex-tyke
Date: September 26, 2015 06:26PM

Just for the record:
Edelbrock step up springs:
3 in. Hg (Blue).
4 in. Hg (Yellow).
5 in. Hg (Orange).
7 in. Hg (Pink)
8 in. Hg (Plain).


hirot
Ian Hart
Ashbourne UK
(88 posts)

Registered:
06/01/2011 05:15AM

Main British Car:
1971 MGB GT (conversion) Rover 3947 R380 gearbox

authors avatar
Re: hunting/surging at 2000rpm with edelbrock 500
Posted by: hirot
Date: September 27, 2015 05:59AM

Yep thats what my book said too but the Edelbrock kit had green springs instead of yellow, all other colours were there. Maybe it was an old pack.
In any case green seemed to make a difference....perhaps its an eco spring....ho ho


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