1983 Mitsubishi Mighty Max Timing Problems and Idle Speed

Tiny
TKENNELLY
  • 1983 MITSUBISHI MIGHTY MAX

Electrical problem
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.

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Sunday, October 31st, 2010 AT 6:48 PM

13 Replies

Tiny
RASMATAZ
  • MEMBER

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?

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Tuesday, November 2nd, 2010 AT 11:54 AM
Tiny
TKENNELLY
  • MEMBER

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.

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Thursday, November 4th, 2010 AT 8:42 PM
Tiny
RASMATAZ
  • MEMBER

Do you have vacuum at all times on the distributor vacuum advance unit and how many vacuum port does it have>

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Friday, November 5th, 2010 AT 1:36 PM
Tiny
TKENNELLY
  • MEMBER

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.

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Friday, November 5th, 2010 AT 3:58 PM
Tiny
RASMATAZ
  • MEMBER

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

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Friday, November 5th, 2010 AT 4:07 PM
Tiny
JACK42
  • MEMBER

Any chance the idle cam for the choke assembly is holding the throttle open a little and making it idle higher?

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Friday, November 5th, 2010 AT 5:50 PM
Tiny
TKENNELLY
  • MEMBER

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?

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Friday, November 5th, 2010 AT 7:18 PM
Tiny
RASMATAZ
  • MEMBER

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

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Friday, November 5th, 2010 AT 7:50 PM
Tiny
TKENNELLY
  • MEMBER

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.

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Monday, November 8th, 2010 AT 2:11 PM
Tiny
RASMATAZ
  • MEMBER

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

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Tuesday, November 9th, 2010 AT 1:49 AM
Tiny
TKENNELLY
  • MEMBER

Okay, I've done some more inspection and testing. Here's what I've got:

Valve timing was double checked and is okay. Since I didn't take the crank out or either sprocket or timing chain off, it's hard to get the valve timing out due to the fact that the cam is keyed to the sprocket so it only goes in one way. Anyway, it's lined up as spec.

I checked the choke cam and although difficult to see, it doesn't appear to be holding the throttle open.

After warming the car so that the choke was open, here's what I got as far as measurements go:

Manifold Vacuum = 20 in
Port Vacuum = 12 in
Timing = 5 deg BTDC
Idle RPM = 1150-1200 Backing idle speed screw out below 1150 causes surging and then ultimately stalling.

I also ran a test on the air switching valve and solenoid valve on the carburetor as per factory shop manual. The electrical test showed that the engine speed sensor is working. However, when turning off the solenoid valve, which causes manifold vacuum to open the ASV, the idle speed did not change. According to the test, if the idle speed drops, the solenoid and ASV are both good. What I'm not sure of is whether this is a valid test seeing how the idle speed is above spec.

It seems to me more and more is that there is too much air getting into the air/fuel mixture causing stalling if the throttle is closed too much. However, it doesn't appear from the vacuum readings that there is a vacuum leak.

What do you think? Is there any other test to run or anything else I should check at this point?

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Friday, November 12th, 2010 AT 11:59 PM
Tiny
GRUMPPA
  • MEMBER

I do not know if anyone is still interested, but I have just taken "hands-on" course on the Mik carburetor.

I recently purchased an '86 Dodge RAM 50 AKA Mitsu Mighty Max. The second thing I did was purchase the Dodge Factory manual from FAXXON. I will not say the manual was a waste of money, but seems as though the original Japanese was translated by a Hindustani and printed in a low-bidder shop in Nabibia. Be very careful when they tell you what screw does what and how the screws are labelled! My Matchbox Toy had a choke/ starting problem which some good old boy tried to compensate for by twisting screws until he got the idle WAY up (almost red line until the engine warmed up). I just conquered the choke, (little thanks to the manual), so I set about getting both the idles down.

I ended up buying a second used carb on eBay and playing with it to find out what was supposed to do what.

What this is all leading to is; there are three Speed Adjustment Screws in the lower right hand quadrant of the carb as you face it from the Driver's side fender. They go from left to right and using the manual labels are (Left high; SAS 1), (Center low; SAS 2), (Right low and "way back; SAS 3). SAS 1 is the Curb idle which the book says should be 800RPM +/- 100. The center screw SAS2
they say do not touch. The "High Idle" screw is misrepresented in my manual's pictures and is actually SAS 3. They give no figure for a High Idle speed, instead sayin to adjust SAS 3 to where the primary barrel main valve just closes on a 0.80 mm (0.31") drill. Like I am going to pull the carb just to do that! Right now I have the high idle at 1800 RPM and I am going for as close to 1500 as I can get.

I have taken care of my own cars for awhile (I am 63). As the timing comes in to spec, RPM will increase and vacuum will max out. Just like you describe. I am thinking SAS 3 got "bumped" out closing the valve. I would try bringing SAS 1 out as you approach 7 degrees BTC just as you have been. But when she starts to gasp, try screwing SAS 3 in a bit and see if she starts to breath better. It does not take a lot on SAS 3. That is how my truck was "compensated" for a bad choke.

Believe it or not; it worked for me and The Matchbox Toy! Thanks, Grumppa

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Thursday, January 12th, 2012 AT 1:03 AM
Tiny
RIVERMIKERAT
  • MEMBER

Wish we could have been more help. Glad you were able to figure it out.

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Wednesday, January 18th, 2012 AT 9:05 PM

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