198 surges at partial throttle cruise

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Mopar92

Mopar92

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1973 Duster 198 ci. 75k original mileage. So I believe all of the emissions crap is still on it. Canister and lines are for sure still on. It starts pretty easily after a few days of sitting. Immediately idles smooth. If you punch it quick, it's revs smooth. Full throttle, pulls smooth. So idle, transition, and full all feel good. Once you get cruising at say 1/4 throttle or whatever.... It's stumbling and surging. I guess it's more of a surge. It is not misfiring. It's just annoying as heck. What should I start looking for ? Or just buy a rebuild carb for it ?
 
Mopar92 said:
1973 Duster 198 ci. 75k original mileage. So I believe all of the emissions crap is still on it. Canister and lines are for sure still on. It starts pretty easily after a few days of sitting. Immediately idles smooth. If you punch it quick, it's revs smooth. Full throttle, pulls smooth. So idle, transition, and full all feel good. Once you get cruising at say 1/4 throttle or whatever.... It's stumbling and surging. I guess it's more of a surge. It is not misfiring. It's just annoying as heck. What should I start looking for ? Or just buy a rebuild carb for it ?
Click to expand...
Mopar92 said:
1973 Duster 198 ci. 75k original mileage. So I believe all of the emissions crap is still on it. Canister and lines are for sure still on. It starts pretty easily after a few days of sitting. Immediately idles smooth. If you punch it quick, it's revs smooth. Full throttle, pulls smooth. So idle, transition, and full all feel good. Once you get cruising at say 1/4 throttle or whatever.... It's stumbling and surging. I guess it's more of a surge. It is not misfiring. It's just annoying as heck. What should I start looking for ? Or just buy a rebuild carb for it ?
Click to expand...
I had a similar problem on a 62 Dodge D100 years ago and it was the distributor bushings were bad. Replaced the bushings and that fixed it.
 
I had a 318 Duster back in the day that did that. Turned out to be the pre-filter in the gas tank.
 
If it runs smoothly under hard throttle and at lower RPM's (like taking off from a stop), then the fuel delivery is likely A-OK.

I would be suspecting:
- The carb fuel level... the float setting may be off or the float a bit gas logged.
- EGR sticking open
- Weak spark due to a coil or ballast issue, plug wires, rotor, etc.
- Timing variations as suggested
 
Where is the EGR on this thing? Some parts of this emission system are missing or inop. The canister is hooked up and all the lines are good. So is the distributor line. There is some sort of vacuum deal not being used by the carb.
 
EGR is on the side of the exhaust manifold, facing towards the driver's side inner fender. It is a mushroom shaped metal device with 2 bolts in the base that thread into the exhasut manifold.

See the lower circle in this pix:
Carb Problems: Holley 1920:
 
Can I just eliminate the EGR and move on? Do you think the EGR would make this thing surge like this? I don't know much about it. Thanks.
 
Distributor bushings aren't causing your problem -- your car has electronic ignition, which is not subject to dwell and timing variation due to sloppy distributor bushings (which at 75k miles you almost certainly do not have).

Quick and easy way to check if the EGR is causing the problem: remove and plug the vacuum hose where it attaches to the EGR valve. Keep your ears open for pinging, though. (And while you are at it, bypass your OSAC valve—that's the widget perched on the passenger side of the firewall, with one hose to the carb and one hose to the distributor. Run hose directly from carb to distributor, not stopping at the OSAC valve, and your car's driveability will improve markedly without any negative side effects.)

It's likely your carburetor is over/due for some attention. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download.

Additional tune-up parts and technique suggestions in this thread.
 
Mopar92 said:
Can I just eliminate the EGR and move on? Do you think the EGR would make this thing surge like this? I don't know much about it. Thanks.
Click to expand...
It would effecting mixture if it is stuck or sticking. It is easy to test the EGR for sticking: put a vacuum hose directly on the hose nipple of the EGR and suck a vacuum on the hose by mouth; you should see the EGR valve's stem move as your reach the right vacuum level. And the above test blocking the hose closes should close the EGR valve... IF it is working properly. If it is stuck partially or fully open, then blocking the hose won't change anything and the EGR will be leaning the mixture all the time; it should lean the mixture just under certain operating conditions.

The EGR valve can be eliminated with a block-off plate IIRC.

Either lean mixture or weak spark is most likely the cause of the symptoms described.
 
Ok guys. The previous owner already bypassed that carb mandated deal on the firewall. The EGR doesn't even have a hose on it. It's frozen up. Can't move the rod with my finger at all. Much less move it with a vacuum hose. What next?
 
Lack of Vacuum advance,EDITas in busted diaphragm,or failed vacuum circuit;
or problem in the low speed circuit, specifically; the transfer slots,the idle bleeds, or the emulsion tubes.

There is also a slim chance that the total timing is over-advanced, that is to say that the combination of initial plus centrifugal,plus v-can, is too much.
 
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It can't hurt to go through the carb. Is there a preferred kit? One that can do the new gas?
 
The kit from www.daytonaparts.com has a superior inlet needle/seat design and for that reason is worth getting. No kits (no matter the brand) come with the correct float gauge any more, all you get is a useless strip-of-paper ruler -- good reason to also pick up a NOS kit off eBay or wherever to grab hold of the float gauge. The float in your Holley 1920 carb is Nitrophyll (foam); they tend to take on fuel and grow heavy over the years. New float doesn't come in carb kit but is still available. While you're at it, wise to do the Fuel line mod.

It is not unusual that you can't move the EGR valve with mouth suction; most of us are not able to generate even close to the amount of vacuum needed to operate the stiffly-sprung EGR valve. If it doesn't move even when connected directly to manifold vacuum, or to a vacuum pump, that's another matter.

A blockoff plate looks nicer, but isn't necessary; you can leave the valve in place and it'll sit there doing nothing, or if you want to make super extra double sure you can remove the valve, flip it 180°, and reinstall it (with a new gasket).

Also make sure your thermostatic air cleaner system is intact and working correctly, and your manifold heat control valve isn't jammed and its spring isn't broken.

(Whoever pays $140+ for a carburetor rebuild it, no matter the model or size of carburetor, and no matter whether we're talking about American or Canadian dollars, needs to shop more carefully)
 
Ok guys. Let me reclaim some of my observations here. After spending time anylizing the surging more... It's really clean running at idle and wide open. But after really driving it more it's more than that. The surging/stumble is just above idle. It's not 1/4 throttle. It's more like just at tip in or cruise. Could this be as simple as a carb adjustment. I found a receipt from 6 years ago where a new carb was installed.
 
The float level being low could do that. That is where I would start. If it a 1920, read up on 'wet float' adjustment. I would just rasie the float level 1/16" at a time, maybe 2 times at most (1/8" total float adjustment is a LOT) and see how it reacts.
 
If it is at tip-in, it is an accelerator pump issue.
If it continues after tip-in it is either a t-port sync issue( where because of a timing issue, the throttles are incorrectly indexed to the transfer ports) or emulsion-tube/air bleed issue,or if you have adjustable metering rods, they may need tuning.
 
It's at the tip in location on throttle. It's not just as you tip off idle, it's that same pedal location. If you are cruising at a tip in throttle pedal amount, and squeeze 1/16" more on the throttle... All the way to WOT it's clean and strong. It's that one little dead spot in the pedal. Taking off or cruising.
 
Repeat; post #22
#1)Start with the Accelerator pump circuit. The check balls may not be working, or the pump is faulty, or the nozzle restricted,faulty adjustment,or simply just too much slack or slop in the linkage system.
#2) Set the T-port sync (see below)
#3) Crank the metering rods up, if you got such. If no Mrods, raise the float level 1mm at a time.
#4) If you still got problems, you are gonna have to disassemble that baby and go through the emulsion system.
 
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