Surging At Steady Speed

[jkskinsfan 19]

I took my '69 out for its first significant road test ( Round Trip of 62 miles ) and for the most part it ran well. One thing I did notice is that it seems to be Surging when cruising at a steady speed. ( Between 45 and 65 MPH +- ) I can't hear it, but I can feel it if that makes sense. Any ideas what might be causing the Surging? ( 351W, Automatic, 2BBL )

 

 

 

JK

[S code 69 13]

Probably either a little lean or you're timing is off a little. check your plugs after a hard run and put a dial back light on and see where your at all in around 2500 RPM. to much attention is paid to initial timing and not enough to total timing, chances are one of these things is your problem.

[jkskinsfan 19]

Thanks for the response, S code. To make the Carb richer I should back out the Idle Mixture Screws, right? I can't do anything about the timing. The Distributor is frozen to the block. I've been trying to get it loose, but don't want to break it by beating on it too hard. The Initial Timing is set at 10* BTDC right now, and it'll have to stay there until (IF) I break the Dizzy loose. I don't have a "Dial Back" timing light, and I just returned the regular one that I had borrowed.

[S code 69 13]

adjusting the idle screws won't change your mixture at cruise speed, you'll jave to jet that if it's the problem.

 

One of these days you'll have to get your distributor to move but for now get your hands on a dial back light and see where you're at all in, you can change that without moving the distributor if needed. a quick way to see if you're too advanced is to pull your vacuum advance line and plug it, see if it makes things better or worse at driving speed.

[jkskinsfan 19]

I was wondering if a dirty Fuel Filter could be the cause. I replaced it less than 200 miles ago, but the Gas Tank is the "Original" and even though I drained it as best I could, could there still be some stuff floating around in there that is clogging up the new filter?

[SparkyGT 26]

see if you can get a clear inline plastic filter, also for the stuck distributor, use some penetrating fluid, and a rubber hammer or strap wrench

[ForceFed70 14]

Surging at light throttle/cruise is usually an indicator that the engine is running lean.

 

If it was a fuel delivery problem, it would show up during hard acceleration before you noticed any problems at light throttle.

 

Like mentioned, I'd try some larger jetting in the carb.

[d.reese 13]

Suggest reading this, Im having the same problem with my GTO tri-power. The two barrel ctr carb is basically starving my engine at RPM and causing small burps / surge!

You need to fatten it up a little (richer) go up in jet size or two, based on this:

 

http://www.enginebuildermag.com/Article/15762/carburetor_tuning_the_airfuel_equation.aspx

 

Sample of link:

 

The original equipment carburetor(s) that came on a muscle or classic vehicle’s engine was tuned for the leaded gasoline of the day, so in most cases the engine will tend to run lean with the reformulated and/or oxygenated unleaded gasoline of today.

 

The gasoline of today also has lower volatility than the leaded gasoline of days past, which will cause most carbureted engines to need a slightly richer A/F mixture at idle and light load part throttle driving conditions to have the same drivability as it had with the leaded gasoline of the ’60s and ’70s.

 

Back in the 1950s and early 1960s, the car manufactures tended to calibrate their carburetors on the rich side of the ideal A/F mixture needs of the engine with the leaded gasoline of the day. Then starting in the late 1960s, the carburetors were calibrated more toward the lean side of the ideal A/F mixture needs of the engine so the vehicle could pass the exhaust emission standards that were just coming into existence.

The modern reformulated conventional and oxygenated gasoline of today will cause the A/F mixture to shift leaner when compared to the leaded gasoline of the 1960s and 1970s. This means if the A/F mixture was lean with leaded gasoline it will be even leaner with today’s gasoline blends.

The high performance and replacement carburetors sold today are sold with an A/F mixture curve designed for a generic engine; therefore they must be tuned for both the specific engine and the blend of gasoline they will be used with. These aftermarket carburetors should be designed with an A/F mixture that is rich enough for a wide variety of engine packages with different exhaust systems, but this is not always true. Some of the aftermarket carburetors we see need a lot of tuning work to get the A/F mixture correct for the engine’s demands with the reformulated unleaded gasoline of today.

Edited July 11, 2009 by d.reese

[rockhouse66 22]

Are you running the vacuum advance? If so, disconnect it (and plug the vacuum line) and see what happens. This could "fix" the surge.

[ForceFed70 14]

Are you running the vacuum advance? If so, disconnect it (and plug the vacuum line) and see what happens. This could "fix" the surge.

 

And hurt the fuel economy. Best to figure out what the actual problem is.

[rockhouse66 22]

And hurt the fuel economy. Best to figure out what the actual problem is.

 

Well, if this fixes it, then I guess he knows what the actual problem is.

[jkskinsfan 19]

Thanks for all of the suggestions, everyone. I decided to drill out the existing Jets to make 'em just a little larger. I also replaced the Fuel Filter with a See-Thru plastic one so I can see if it's picking up any crud from the gas tank. Took it out for a test drive and it ran as smooth as silk. I would tend to think that making the Jets larger was the solution to the surging problem, but I'll have to wait to see if crud floating around in the gas tank was clogging up the filter. I'll keep you posted. Thanks again for all of the feed-back.

[S code 69 13]

I would maybe pull your jets and see what they are then get a couple notches up on them. drilling jets is a little like adjusting your headlights with a hammer.

[jkskinsfan 19]

drilling jets is a little like adjusting your headlights with a hammer.

 

:laugh::laugh::laugh::laugh:

 

It seems to be running pretty good right now, so I think I'll leave well enough alone for the time being. Like the old saying goes, "If it ain't broke, don't fix it".