Need help: Motorcraft 2100 Carb Surging

[rutkak]

Mustang Gurus,

I posted about an engine surge a few days ago and got some helpful recommendations but I probably muddied the question with mentioning white smoke when the engine is warm. That's a separate issue...but on to this one.

My engine surges at idle when getting warm and warm/hot but cold. I swapped out my Motorcraft 2100 (1970) with my Autolite 2100 (1966) and the engine no longer surges....hence the Motorcraft 2100 has an issue.

The carb was recently rebuilt by GottaFishCarbs but now that I think of it, I'm not confident it ever really ran right (i.e. not surging). It's certainly very pronounced now and borders on even being driveable.

Anyway...the bad surges from 14 to 17 psi vac but is pretty clean. The carb was fouled up excessively hence the reason I sent it to get rebuilt.

What on the carb can I check, tweak, focus on? I sprayed a good bit of carb cleaner in her but I'm guessing a jet or something is clogged. The gas tank is new and the gas is squeaky clean.

KR

 

[rutkak]

Just an update for anyone with the same problem. Yeah...so the carb was freshly rebuilt by Gotta Fish Carbs and clean as a whistle but always surged since the day I got in back. I figured other issues but eventually swapped with a known good and narrowed it down.

The surging was most notable once the car warmed up. I haven't driven her much because of multiple projects but she's been started everyday and sat idling (poorly). I took the carb completely apart and cleaned EVERYTHING. I had a mostly full rebuild kit to pick from and 2 other 2100 series carbs to use as parts bins if needed. I let most parts soak in Berryman's Carb cleaner and I blew it all out with spray brake cleaner once done soaking. I did find a little bit of crud hiding but it wasn't much and I cleaned it out. Anyway, I put her back on and NO CHANGE...STILL SURGING. I about lost my mind but my brakes were basically out also so I had to take it back to the place that did my brakes 2 months ago.

She kind of drove fine though for the 10 min ride to the brake place. She was still surging but not as noticeably. I picked my car up 2 hours later and it drove fine. I drove it a good bit around town...and once again...fine. No surging. Hmmm... Weird.

Could be whatever gunk in the carb got cleared as I started pushing massive amount of fuel.

 

[midlife]

Check the flatness of the carb base: it might be warped and would lead to a vacuum leak - hence the surging.

 

[rutkak]

I never thought about a carb base being warped but I sprayed starter fluid all around the engine & carb looking for vac leaks with no change.

I didn't find any other than the intake manifold leak (but I fixed that). I'll repost if the surging issue comes back.

But just for the sake of it...is there any fix or mitigation for a warped carb other than get a new carb?

 

[72HCODE]

It sounds like your idle adjusting limiters or limiter stops are not set 100% right too lean and you get surging.

The limiters are under the carb. Next to the accelerator pump.

Set the idle fuel mixture screws and limiter caps to the full counter clockwise position of the limiter caps. Then you are suppose to turn each screw inward until lightly fully seated then turn the screws out 1 1/2 turns to get a start. You can also try opening both screws 1/8 a turn and see if it stops surging but at some point you want to balance them to make sure they are open the same. These screws and limiters work like the idle air bleeds on holleys

I would recommend picking up a little book or online guide for the 2100 carb it will walk you through all the adjustments.

It's possible when they rebuilt it they set it wrong or you motor just needs a different setting now.

 

[Mustruck]

If You Have power brakes, A leak in the Vacuum break system could cause the problem.