rough running ram50 1989 2.6 carb

[frogzmax]

Hi there new to the group, just got a 1989 ram 50 2.6 carbed a few months ago, the truck is in mint shape beautiful paint, engine bay is clean, low kms. engine runs a bit rough though. it starts up first try every time, when it gets warmed up it idles really rough but doesnt die. When I drive there seems to be a slight lack of power, if I am in first-third and i take my foot off the gas and apply it again lightly the truck will jerk. I started by having the truck warmed up at idle and parked, I pulled one spark plug out, no change to idle, pulled 2 no change, pulled 3 still no change. There is spark going to the spark plugs, the resistance of the spark plug wires are pretty low in the hundreds where the manual says it should be in the thousands. I am going to start with new wires,cap and rotor to start. I also did a dry compression test 150 straight across. Has anyone encountered anything like this? any input would be greatly appreciated. thank you

[FMS88]

Welcome to the club!
The compression numbers you obtained are reassuring and indicate the engine is sound. Most likely the problem is the ignition, fuel or other peripheral systems. Has it always run rough since you got it? Were any repairs made before you got it or since you owned it? If there were recent repairs, keep those in mind. However if none, you're smart to start with a tune up, including new plugs. Chances are it just needs a tune-up. But if that doesn't fix it, you'll then be on a firm footing to consider and look for other causes that an out-of-tune engine would otherwise hide.

[SubGothius]

If it only starts running rough after it's warmed up, that could indicate the choke isn't fully opening, or maybe the coil resistor going bad.

[xboxrox]

Hello frogzmax ~ welcome to this place :thumbup:

ROUGH IDLE (assuming good ignition good spark plugs good compression good fuel)

1) valve lash not correct ~ are the lifters noisy ~ Rislone engline flush & oil change & hy-per lube ZINC
2) carburetor air/fuel idle mixture adjust ~ shoot for highest reading on a vacuum gauge
3) visually check air cleaner thermo valve being open with warm engine & air filters not clogged
4) visually check choke plate is open with warm engine
5) check warm engine idle RPM with a tach
6) check warm engine initial timing with timing light
7) check for vacuum leaks with hand vacuum pump ~ especially test the distributor vacuum actuator & secondary throttle actuator
8) check the carburetor to intake flange connection for leaks ~ use teflon plumbers tape sparingly on threads of loose bolts
9) clean & remove any electrical silicone grease from the TPS & carburetor control wiring harness connections
10) it's possible the carburetor needs work ~ rubber diaphragms wear out over time regardless of mileage on the vehicle
11) DIY carburetor rebuild or buy a rebuilt carb from a vendor close by your location <~ keep going back for replacement warranty carbs
12) check out my member page with Mikuni carburetor photo albums (more photos coming soon)

Thanks for joining here frogzmax

[frogzmax]

hey thank you for the reply, I am not sure what was done to it. the guy I bought it off was not the original owner. He told me the owner before was an older lady that kept it well maintained. I believe he just bought it and sold it to me in a very short period of time. From what I can see it looks like it was well maintained. the timing chain cover doesnt have an oil on it so I assume that was replaced recently haha. it is running the stock mikuni carb there is a loud sucking noise when i take the air filter cover off so maybe there is a vacuum leak somewhere. Ill start with the tune up, it had those taylor spark plug wires, so I just ordered some denso ones. there is spark going to all the wires but it was quite orange not sure if that means it is a weak spark.

[frogzmax]

Hello frogzmax ~ welcome to this place :thumbup:

ROUGH IDLE (assuming good ignition good spark plugs good compression good fuel)

1) valve lash not correct ~ are the lifters noisy ~ Rislone engline flush & oil change & hy-per lube ZINC
2) carburetor air/fuel idle mixture adjust ~ shoot for highest reading on a vacuum gauge
3) visually check air cleaner thermo valve being open with warm engine & air filters not clogged
4) visually check choke plate is open with warm engine
5) check warm engine idle RPM with a tach
6) check warm engine initial timing with timing light
7) check for vacuum leaks with hand vacuum pump ~ especially test the distributor vacuum actuator & secondary throttle actuator
8) check the carburetor to intake flange connection for leaks ~ use teflon plumbers tape sparingly on threads of loose bolts
9) clean & remove any electrical silicone grease from the TPS & carburetor control wiring harness connections
10) it's possible the carburetor needs work ~ rubber diaphragms wear out over time regardless of mileage on the vehicle
11) DIY carburetor rebuild or buy a rebuilt carb from a vendor close by your location <~ keep going back for replacement warranty carbs
12) check out my member page with Mikuni carburetor photo albums (more photos coming soon)

Thanks for joining here frogzmax


Hey thanks for the response
thats a huge list to check much appreciated

1) valves arent noisey engine drives very quiet
2)adjusted this a little seemed to help a bit but without vacuum gage :(
3)have not checked thermo valve air cleaner replaced
4)have not checked
5)not checked
6)timing was a bit retarded i have it now between 10-7 ran better than before
havent checked the rest yet but will, also going to check out your page
thank you!

the old fuel filter was filled with dirt so I think I may need to clean out the gas tank and lines. I am hoping the rough running is partially due to bad ignition timing. is there anyway to check the ignition coil is working properly?

[FMS88]

h it is running the stock mikuni carb there is a loud sucking noise when i take the air filter cover off so maybe there is a vacuum leak somewhere. Ill start with the tune up, it had those taylor spark plug wires, so I just ordered some denso ones. there is spark going to all the wires but it was quite orange not sure if that means it is a weak spark.

The loud hissing/sucking noise you hear with the air cleaner top removed is completely normal. If you had a vacuum leak, it wouldn't idle well if at all. The spark does sound weak. If the new wires, cap and rotor don't improve the spark, check the coil, ballast resistor and ground connections. If you have the Haynes manual, page 5-4 tells how to test the transformer type coil you have using an ohmmeter. The primary coil resistance should be 1.3 Ohms, the secondary coil 27k Ohms and the resistor 1.35 Ohms. If it's okay, clean the battery's negative cable connections to the body and the engine, and the smaller ground wire between the head and the fire wall. Good ground connections are very important.

[xboxrox]

My spark is also orange color ~ except for the ballast resistor ~ my truck has all new ignition parts including the ignition module inside the distributor & new coil, cap, rotor, plugs, wires ALL new ~ not done all the ground cable cleaning tho (battery post ground is clean)

My engine backfired twice 1st time ever today going up the Koolau mountains Eastbound on the H3 Fwy to Kailua from Pearl Harbor ~ traffic was also fighting really strong Easterly winds about 40 to 50 MPH ~ had to put the truck in 3rd gear WOT to maintain 55 MPH speed, normally it can make it to the Harano Tunnel in 4th gear ~ the gas tank is 1/4 full with 2 bottles of CATACLEAN added ~ Google says backfire can be caused by a lean fuel mixture or a bad catalytic converter ~ Google also says a bad or failing O2 sensor can cause backfire ~

I have a new O2 sensor on hand ~ :explosion:

FOR frogzmax ~ Google says:
https://www.autoblog.com/2016/01/11/symptoms-of-a-bad-or-failing-oxygen-sensor/

Rough engine idle and misfires ~ As your oxygen sensor is going bad you may notice your vehicle is running rough, misfiring (https://www.yourmechanic.com/services/engine-is-misfiring-inspection) or running irregularly while idling (https://www.yourmechanic.com/services/car-idle-is-rough-inspection). Since the oxygen sensor output helps control engine timing, combustion intervals, and the air to fuel ratio, a bad sensor can disrupt these engine functions, causing a rough or irregular engine idle.

When to replace the sensor ~ If your vehicle was manufactured within the past 15 years, the oxygen sensor should be replaced every 60,000 to 90,000 miles (https://www.yourmechanic.com/article/how-long-does-an-oxygen-sensor-last). This sensor is a wear item and does fail over time. Replacement of a bad or failing oxygen sensor will reduce the level of emissions your vehicle puts into the atmosphere, while keeping your engine running smoothly and properly. As soon as you notice your Check Engine Light, poor gas mileage, or an irregular engine idle, book a service with YourMechanic to have your oxygen sensor replaced (https://www.yourmechanic.com/services/oxygen-sensor-replacement). This will ensure your vehicle is running smoothly and efficiently, and will help prolong the life of your engine.

[frogzmax]

I checked the cat with a digital thermometer and it showed it was working fine, also replaced the o2 made some difference. I will check the coil after I get the wires, I heard the secondary throttle vacuum thing can mess with these carbs too. Have any of you had this problem? I am hesitant to mess with the carb I have a toyota pick up also and that carb was a nightmare. Did a rebuild on it but it never ran close to how the weber 32/36 I replaced it with. I appreciate all the info guys ill keep you posted!

[FMS88]

A bad secondary throttle vacuum chamber won't affect idle or performance at lower RPMs and loads. Its failure causes power loss under load at higher (~3000+) RPMs. It's easily tested. Just apply a vacuum to its port. If it won't hold the vacuum, replace it, which can be done without removing the carb. Rebuilding the 2nd Gen Mikuni can be a challenge and some rebuild parts are no longer readily available. So if it's running fine and mileage is good, don't mess with it. If it starts to go south and it's not due to an external component like the secondary chamber, then consider installing a Weber.

If you want information on the Mikuni's functions and steps to perform diagnostic tests, look at the Chrysler Training book contained within this thread:
http://www.mightyram50.net/vbulletin/showthread.php/8147-1988-mighty-max-carburetor-vacuum-diagram-needed(actual-photos)?p=73354#post73354
The book is for 87-88 models, but the 89 should be very similar if not identical.