part throttle surge problem

[HP2]

Okay, ran into a carb problem that has me a bit stumped, hopefully some of you can help with this.

Basics, 440, 11:1 compression, mild cam, dual plane intake, headers, Ebrock 750 carb.  Even spent some time on a dyno and found out my old off idle bog was a lean spot while the rest of the range was fat. Stepped up the squirters, leaned out the rods, changed the rod springs and saw big power and mileage increases.I've been running this combo for years very successfully.

Over the winter I noticed I was developing a surge at part throttle. Kind of ingored it for a while until an hour long trip made the problem real unbearable. Under accelleration, it runs great, idle is smooth, but at part throttle, high vacuum cruise mode, it surges.

Tore in to the carb laast weekend, found the needle screens pretty gunked up with crud and sludge in the float bowls. Soaked everything in cleaner overnight, blew it all out the next day. Put it all back together, reset all critical dimension, put it back on and the problem was better, but still there. Wondering if it was a ignition issue, I put a 650 on and it ran fine.

So, what should I start looking at now? It seems odd to me that ht ecombo would just go sour, but thing can wear out I suppose. Thoughts?

[71chally416]

Even my '90 Dodge truck has been stumbling lately. Crappy winter formulation gas + warm weather?   

[Bullitt-]

Edelbrock's instructions suggest this is a lean condition.....why would it change 


Anything changed about the car....or where you drive or the conditions you drive in??...fuel pump, tank, lines, air box/filter, anything that would change the volumetric efficiency such as headers or exhaust mods.

[ntstlgl1970]

Try a lighter spring and see if it changes

[HP2]

Nothing has changed on the vehicle its self (its my truck, btw,) but the winter blend gas is a variable.

I also was thinking that perhaps the spring tension may be an issue. I'll see if a change there alters things at all this weekend.

[Chryco Psycho]

are you running in cooler air ? If it is on the edge in normal temps & you are 10-15* cooler it could be lean for sure as more oxygen will be in the cooler air with higher density

[ntstlgl1970]

Actually now that I think about it (been a while since I messed with an edelbrock carb) I think the stiffer spring would be better. You are at a higher altitude than sea level there, right?

[HP2]

You are at a higher altitude than sea level there, right?

Oh yeah, 6000'.

It has been cooler through the winter than in the last few winters, but it has been decently warm lately so it should have gone away by now.

[71chally416]

6000' altitude + reformulated gas is the suck for carb calibration    I'd try getting gas somewhere other than where you normally go and avoid the ethanol mix if possible.

[Chryco Psycho]

you want the rod down in cruise mode so I would look at a softer spring under the metering rods

[HP2]

I've got a complete spring set, so I can mix and match.  I forget what spring is in it now, but it was the recommned one from the manual based on vacuum readings.

Unfortunatly, getting differet gas is not an easy solution. We have at least 5% ethanol all year round with any number os distributors stepping up to 10% through the winter and some using 10% all year round. Nearest station without ethanol is 45 minutes up a mountain pass. I can buy non ethanol race gas locally from a pump, but at $4 a gallon, don't really consider it a viable option for a semi-daily driver.

[71chally416]

You must really have your timing retarded to be able to run pump gas in a 11:1 comp 440 with a mild cam without detonation. 

A few gallons of race gas mixed with a fill is all you should need.

[HP2]

Nope, it is advanced a decent amount for cruise rpm mileage.

It is the lack of atmosphere up here that makes it all possible. My cranking psi is only around 150, so I'm well within the pump gas range. Ya know, the air force requires pilots to wear oxygen masks when operating at the altitude I live at. Back when we had a large nascar track trying to get a Cup date, the thing that really killed it was that almost every major team said they would skip a race at our track, even if it was a Cup date, because of what the altitude does to an engine program and the amount of research it would take to retain engine performance a single race up here was more than of any of them was willing to spend.

[71chally416]

The last 440 I ran on the street back in the 80's had the timing set at 30-32* to keep it from eating itself with pump gas. It was just 10-1 comp and had the small factory cam. The iron heads run best with 38 to 42* total timing because of the plug location and the chamber shape. It'll run fine with less timing, but not nearly as good as it can. 

[Bullitt-]

Don't want to send you off on a wild goose chase but something sounded familiar so I did a search and it seems a bad or out of adjustment vacuum advance can cause surges.