cruise mode surge

[john h]

hey all,
probably a silly question and a shot in the dark.  I have been working on a cruise mode surge 38-45 MPH Edelbrock thunder 650.  I have the kit with t he jets rods and springs.  been through every combo.  Now the question.... can a poorly functioning vented gas cap be causing my issue?
 

Thanks
John

[cudabob496]

https://www.youtube.com/watch?v=gCGvcmR7R8M

[Strawdawg]

hey all,
probably a silly question and a shot in the dark.  I have been working on a cruise mode surge 38-45 MPH Edelbrock thunder 650.  I have the kit with t he jets rods and springs.  been through every combo.  Now the question.... can a poorly functioning vented gas cap be causing my issue?
 

Thanks
John

yes, or too much timing in the rpm band that it occurs in...particularly if you run vacuum advance

[john h]

yes, or too much timing in the rpm band that it occurs in...particularly if you run vacuum advance

I am running 14.5 deg base timing.  I do also have vacuum advance.  I have not checked the total timing and 38-45 would be about 2000-2400  RPM range under load.  Should I disconnect and plug the advance and take it for a run or just dial the base advance back to say 12 Degrees?

Thanks.

[V02Barracuda]

Quite a few years ago I had this exact same problem.  I mentioned it to a friend and he said that my primary jets need to be opened up a bit because it's running too lean.  So I removed the primary jets and drilled them out just a little bit.  Really, I mean just a little.   
 
Surge is gone and it runs very well.

[burdar]

What distributor is on it?  You need to find out what your total timing is.  Some stock distributors can have 27+ degrees of mechanical timing.  That puts you over 40 degrees with your initial at 14. 

I also have a surge at cruising speeds but haven't looked at it yet.  I'm using a MP distributor with 20 degrees of mechanical timing and have the initial timing set at 15.  I've read that a surge at cruising speeds is a lean condition.

[john h]

Quite a few years ago I had this exact same problem.  I mentioned it to a friend and he said that my primary jets need to be opened up a bit because it's running too lean.  So I removed the primary jets and drilled them out just a little bit.  Really, I mean just a little.   
 
Surge is gone and it runs very well.

I have a few jets to choose from in the tuning kit.  I have done all the combos available on the chart.  I can use a larger primary and the stock step up rod to see what happens.  First though I'm going to turn the mixture screws a 1/4 turn and test each way just like the link video posted eariler in the tread.

[john h]

What distributor is on it?  You need to find out what your total timing is.  Some stock distributors can have 27+ degrees of mechanical timing.  That puts you over 40 degrees with your initial at 14. 

I also have a surge at cruising speeds but haven't looked at it yet.  I'm using a MP distributor with 20 degrees of mechanical timing and have the initial timing set at 15.  I've read that a surge at cruising speeds is a lean condition.

I am running the standard Mopar distributor that came with the Crate 360 Motor.  I guess I don't know how much advance it has.  How do I test total advance?  The documentation I got with the engine recommends 12 degree base timing, but I have found that it runs best and I get the best most constant vacuum gauge readings at 14-15 Degree base.

[burdar]

There should be a number stamped into the mechanical advance plate but I don't know how easy that is to see with the distributor completely assembled.  You can put a timing light on it and see how far the timing rises when the RPM's increase.  The vacuum advance needs to be disconnected while you test it.

The video talked about "off idle" surge...not a surge while cruising.  The idle mix screws don't affect anything once the carb is off the idle circuit...I don't think.

[john h]

There should be a number stamped into the mechanical advance plate but I don't know how easy that is to see with the distributor completely assembled.  You can put a timing light on it and see how far the timing rises when the RPM's increase.  The vacuum advance needs to be disconnected while you test it.

The video talked about "off idle" surge...not a surge while cruising.  The idle mix screws don't affect anything once the carb is off the idle circuit...I don't think.

I guess you are right. Id did talk off idol.  The mixture screws would do nothing at 38-45 MPH. 
I will test the advance by plugging the vacuum line and bringing the rpm up and holding it at various points.  I can keep upping the srobe until I see the 0 mark line up.  the thing I'm wondering about though is when the advance pod is hooked backup, won't that further advance the timing?  If so would I want to test it with that hooked up to see total advance with the vacuum?

[burdar]

The rule of thumb is that the SB Mopar likes right around 34 degrees total.  That total is the initial advance + the mechanical advance...NOT including the vacuum advance.  Depending on what vacuum pod you have, you could end up with over 50 degrees with the vacuum assist.  You need to get it running properly WITHOUT the vacuum advance to start with.  Then, add in the vacuum advance for better fuel mileage.  The vacuum advance can be adjusted with an allen wrench inserted into the hose nipple. 

I'm no expert...this is just what I've read.  I would plug the vacuum advance until you get the surge under control.  Then worry about tuning the can. 

[Strawdawg]

just plug the hose and try the car to see if it goes away.  Sometimes when we kick the initial advance up along with a faster curve, the vacuum advance can cause it to be too much at cruising rpm....as Burdar said, some cans have more than others as well.

[john h]

Thanks.  I'll give it a shot.

[Chryco Psycho]

Put a slot or mark on the damper 2 3/8 " clockwise from the TDC mark this is 36* .
 disconnecting the vacuum advance is the first thing I would do , total timing is basically more important than base timing as the engine spends most of its time at 2500 rpm or higher .
 I time by ear so bring the engine up around 2500 -3000 rpm & advance the timing to where the rpm just stops rising back it off just a hair so you are on the cusp of where the rpm stops rising & goes flat & lock it down , this way timing should should be around 15* at idle .
 if the surge is still there the advance weigth springs could be too light or it could be fuel related .

[john h]

Put a slot or mark on the damper 2 3/8 " clockwise from the TDC mark this is 36* .
 disconnecting the vacuum advance is the first thing I would do , total timing is basically more important than base timing as the engine spends most of its time at 2500 rpm or higher .
 I time by ear so bring the engine up around 2500 -3000 rpm & advance the timing to where the rpm just stops rising back it off just a hair so you are on the cusp of where the rpm stops rising & goes flat & lock it down , this way timing should should be around 15* at idle .
 if the surge is still there the advance weigth springs could be too light or it could be fuel related .

After you set the timing the way you described, do you hook the vacuum advance back up or leave it off?