Surges at partial throttle. Used Eddy 600

[BillZ260]

I got rid of the bogging by changing total timing from 36 to 34, but now when I'm cruising at speed, any speed, the car is constatly surging. Meaning that it won't stay at a constant speed smoothly. It's like I'm on / off the gas just enough to take the slack out of the rear end and it gets all hurkey jerky.

 

Any slight acceleration is butter smooth and normal, but holding throttle steady and the car eventually approaches this ballance point where it's funky.

 

The carb is used but doesn't leak that I can tell. Do I need to rebuild the carb or maybe just tune it some more? I've adjusted it per the eddy manual, and before the car was finished we took the top end appart and replaced some gaskets and the rubber seats but not everything.

 

Is this simply be cause everything in the car is new and tight EXCEPT the rearend?

 

Thanks.

[jt1]

Try backing some more timing out. If you're using a vacumn advance, you probably need to work with it.

 

jt

[BillZ260]

First I plugged vacuum line and car ran fine, no surging, no pinging but wasn't as responsive.

 

Then when I started messing with the vacuum advance settings I ended up having to back the vacuum all the way out counter clockwise before it stopped surging. I can't tell if it's pinging or not. So does that mean it's 'seated' and the same as not having the vacuum advance or does that mean it's just the minimum amount of vacuum advance that my motor can tolerate at it's 'total' setting of 34?

 

I'm new to tuning, so this is a good exercise for me, and thanks for the help!

[jt1]

So does that mean it's 'seated' and the same as not having the vacuum advance or does that mean it's just the minimum amount of vacuum advance that my motor can tolerate at it's 'total' setting of 34?

 

It could be either, hard to say.

 

If your damper isn't marked, the best thing you can do is get a timing tape, then using your light and tach, check the timing every 500 rpm from idle until you reach max advance, 4K or so. Do this without the vacumn, then with it, and make notes or make a graph. Then you have a base line. It's best to plug the line to the vac port and work with the mech first. Generally the more initial you can run, you will have better idle and sharper throttle response. If you get too much initial, it will be hard to crank, especially when hot. This is determined by your cam and cranking compression, so you have to experiment to see what works.

 

Next determine the total mech you need. The best way is max power on a dyno. If you can't do that right now, stick to 34-36deg total. The trick is to keep the total around 34-36 and get enough initial for good idle/response. You normally need to limit the amount of mech advance by bushings(MSD), a plate kit(HEI), or by welding the slots in the plate (PITA). The weights and springs control how quickly the mech comes in, changing the shape of the advance curve.

 

I think there is a writeup on MSD's site that explains this much better than my idiotic rambling. You will have to hunt it up yourself.

 

Lastly their is the vac advance. It can do two things: Add in some advance at cruising rpm to help out gas mileage. This is important if your car gets driven much, not so important for a weekend hotrod. The second is to add some timing at idle to help throttle response. You just have to check and see what yours is set up to do. Some folks may use 10 deg inital at idle for easy cranking, the use the vac to pull in 10-12 more deg for good response off idle, then a total mech advance of 36 for max power with no vac, then maybe 50 deg for max mileage at light throttle cruise. Others may just use the light throttle advance, and not add any at idle. So there are some benefits to the vac for a daily driver, but not so much for a hot rod.

 

To sum up this bunch of crap, you want to run the max timing you can while maintaining easy cranking, no pinging under any conditions, and no surging or bogs.

 

John

[BillZ260]

Gotcha, will start playing with it and do some more reading reading reading!

 

Last night it wansn't surging but today it was, so I have some tinkering to go.

 

Thanks JT1

[BillZ260]

I read a couple of good articles last night and I think with those and your input I have a pretty good Idea of what I'm trying to achieve now.

 

I'll try to play this weekend and I'll post results.

[BillZ260]

I read even some more and this morning when out to TUNE.

 

First thing I noticed, and the reason my vacuum advance adjustments didn't do anything was due to the fact that I had the vacuum plugged into the wrong port! So once I changed it, the timing was way off.

 

Starting from scratch I did the following and the car runs SOO much better!

 

Un-hooked the vacuum advance and set total mechanical advance to 35 degrees.

Mapped non-vacuum curve every 500 rpm till 4000ish.

Re-hooked vacuum and mapped it through 4000. (I got lucky and ended up having the correct vacuum setting for my needs!)

 

Maps look like so

----------Idle-----1000-----1500-----2000-----2500-----3000-----3500-----4000

No Vac---15-------15--------28--------32-------34--------36-------36--------36

Vac Adv--36-------38--------52--------52-------52--------52-------52--------52

 

I put about 50 or 60 miles on it cruising around this afternoon and it doesn't bog, herky jerky, ping, nothin! It has way more power when I'm cruising on the highway, I notice my vacuum is way more with the new settings, which should mean much better mileage. Overall I'm really happy with the timing.

 

I do think I need to work on the carb settings some more but it's difficult with the lowered air filter on there, and I'd like to have a much more accurate tachometer to tune it with. The exhaust smells rich but there isn't alot of black smoke, so maybe it's just me.

 

Thanks JT1!!!

[jt1]

Bill, I've been on vacation a few days and haven't got to check in here. Glad to see you making progress. The map looks pretty good, and the best thing is you have a baseline and can evaluate changes.

 

Edelbrock has excellent tuning info on their site. A wideband O2 meter is a huge help when fine tuning the carb. The biggest eye opener for me was that the adjustments were so sensitive. Many times I was jumping back and forth over the right spot because my adjustments were too large, especially with the idle mixture screws.

 

Keep up the good work.

 

John

[BillZ260]

Thanks for the input I think a 02 sensor is probably a must for anyone that cares about tuning. I'm seeing that from the articles I've read, but not really sure which meter is best, even after reading the Ford Muscle Mag's compao article.

 

The carb is off, there are a couple of spots that it hesitates then comes on strong and is running rich enough to make the house smell any time I leave to take a drive. (that's what Crystal tells me )