1983 Mitsubishi Mighty Max 4 cyl Two Wheel Drive Automatic 125K miles
I have a 1983 Mitsubishi Pickup – 2.6L; 4-cyl; 2wd; Automatic (125,000 miles) that I have just completed a major engine overhaul which included a installing a new cylinder head, rebuilt carburetor, and distributor. I’ve been able to get the engine running, but I’m having a problem trying to “tune the engine". With the idle rpm set correctly, the engine will start and run only if ignition timing is 7-10 degrees ATDC with the vacuum advance disconnected (spec is 5-9 degrees BTDC). When I adjust the timing to the specification, the idle speed increases beyond specification, roughly 1100-1300 rpm. This causes port vacuum to operate, which if connected to the vacuum advance on the distributor, would cause timing to be too far advanced. When I adjust the speed screw downward to get it in spec, the engine starts to idle erratically and then dies. It’s almost acting as if the truck is running in a sort of “open loop mode", but there isn’t an ECU on this truck. I’ve tried setting the timing using the procedure in the shop manual, but if I do that, it won’t start at all. Incidentally, I also had the same problem when I re-used the original distributor and control unit.
I’ve doubled and tripled checked the valve timing to make sure that it is correct and all the valve clearances are in specification. If I can’t get the idle rpm and ignition timing within specification, it will never pass smog here in California.
The original cause of the engine repair was an overheating event causing the cylinder head to warp and lose compression. During the overhaul, I didn’t take the crankshaft out or remove the timing chain and cam sprocket as I didn’t think at the time, there was a need to. It doesn’t seem likely that these would be the cause unless the chain is bad (it’s the original chain). I’ve also tested and ruled out vacuum leaks, both from external devices and the mating surfaces of intake manifold to cylinder and carburetor to manifold.
Part of the problem I have is that I do not have good information on the Mikuni carburetor. The shop manual I have does not cover what all screw adjustments there are on the carb and I haven’t found information on the internet.
I’ve also noticed on my particular truck, there is an engine speed sensor and relay as well as a speed control unit. I know a malfunction in the speed sensor can affect timing, but again, my shop manual does not cover where it is or how to test it. It is also silent on the speed control unit.
Any ideas or information you have that could help me would be greatly appreciated.
Hi TKennelly, Welcome to 2carpros and TY for the donation
Did you put the engine no.1 piston on its compression stroke/TDC and line the cam and crank sprocket as specified and also the timing marks at 0/TDC on the vibration damper and rotor pointing directly underneath the no.1 cap tower?
November, 4, 2010 AT 8:42 PM
Yes, I did. I've done that several times. I've also set timing quite a few times before, which is why it's baffling me. When I think it should be close, the timing ends up being more retarded than it should be. When I turn the distributor body to set the correct timing, that's when the idle rpm increases up to about 1200 rpm.
November, 5, 2010 AT 1:36 PM
Do you have vacuum at all times on the distributor vacuum advance unit and how many vacuum port does it have>
November, 5, 2010 AT 3:58 PM
It has two ports for ported vacuum, which is what the vacuum advance is supposed to run on. It also has two manifold vacuum ports. When the rpm was in spec, I didn't have port vacuum; however, when I adjust the distributor to the correct timing (vacuum advance disconnected), a connected vacuum gauge shows port vacuum about 12-15 inches. Of course the idle rpm is above spec at about 1200-1300 rpm.
This car has an engine speed sensor and an engine speed sensor relay according to the wiring diagram. However, it has no ECU. The shop manual I have has virtually no info on the engine speed sensor or how it plays into the running of the engine. I know in late model cars, engine speed sensor can affect timing and idle rpm, but I'm unsure of how if affects on these early model cars.
November, 5, 2010 AT 4:07 PM
A connected vacuum gauge shows port vacuum about 12-15 inches.
Your vacuum reading is low
What you got here is full intake manifold vacuum that comes below the throttle plate-A ported vacuum comes from above the throttle plate-when the throttle plate starts opening this is the time it suppose to feed vacuum to the vac adv unit
Right now its advancing the timing when its not suppose to-
There's another vacuum called Venturi
November, 5, 2010 AT 5:50 PM
Any chance the idle cam for the choke assembly is holding the throttle open a little and making it idle higher?
November, 5, 2010 AT 7:18 PM
I did not check the idle cam. I will check that next week when I get a chance to work on the car again.
But how would that explain why I was able to get the engine to idle correctly, then when trying to set the timing (with vacuum advance disconnected), the idle rpm increases as I turn the distributor to advance the timing?
November, 5, 2010 AT 7:50 PM
If the fast idle cam is properly adjusted and releasing to its normal position the curb idle speed should set, inspect throttle linkages if its sticking
You need to triple check the valve timing again-I'm puzzled here myself-something don't jive here-
Double check the A/F mixture adjustment and the routing of the vacuum hoses to the vacuum advance. Could also be a TVSV
November, 8, 2010 AT 2:11 PM
Is it possible that it is a sort of 3 way tuning between timing, idle speed, and idle mixture? It makes sense to me that if it the timing is set correctly that as I back out the idle speed screw to the spec speed, with the throttle plate closing, the engine may be starved of adequate fuel. As I do this, the engine begins to lope or surge before dying.
What is a normal number of turns for idle mixture? Right now I have it at 2 turns out from bottom (fully closed). Unfortunately, I do not have access to an exhaust analyzer to help me set the mixture correctly.
November, 9, 2010 AT 1:49 AM
A/F mix 2 turns out-set the specified idle speed or lock it at 700rpm-now set the timing again then adjust the curb idle speed 700rpm. Now start dropping 1 a/f mix till you can feel the engine missing-then turn it out 1/4 turn adjust to 700, do the other a/f mix the same -final adjust the curb idle speed to 600-700rpm-you're done