Hesitation while driving at constant speed?? 88 d50

FINALLY got my shifter problem fixed, so on to the new one. First off I don't know the history of this truck, so it could be anything. I have a mikuni carb with a new fuel filter and pcv valve. Went for my first real drive, went to get gas (first full tank I've put in this truck) and dumped in a can of seafoam. Driving down the road at a steady speed could be 35 or 65 and the truck will hesitate and it stalled a couple of times at a light. It doesn't seem to like to restart well when warm (3 cranks) but starts fine when cold. The stutter seems to get worse the warmer it gets. Kind of intermittent, pretty sure it's fuel related as sometimes it runs just fine.

I don't have a fuel pressure gauge. Can I just crank it and measure how much fuel I'm pumping?

Ideas on the best place to start?
fuel pump
carb
vapor lock
gas tank debris

Vacuum leek in the brake booster. Mine would die every time I hit the brake some what hard.

The vacuum line to the brake booster had slipped down a little. I pushed it up and made sure the clamp was tight, but this happens while I'm driving and not braking. Where was your vacuum leak in the booster at??

I also found a couple of caps on my carb that were pretty sad and most likely leaking, but I've still got the stumble. I was driving it tonight and feel like it might be the carb. It only happens after it's warm, a couple of miles, and it seems to be on deceleration. So intermittently on the flat, when I hit a slight downhill, or slowing down for a stop.

The mikuni carb is super clean and I can tell the exposed gaskets are new and soft. Maybe I still have a vacuum leak, float or other carb adjustment?? Wish I could do the Weber carb, but the wife has cut me off and I don't have enough mad money for that right now. Might have to check out the local parts yard again as I still some credit form the tranny debacle.

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Originally Posted by camoit

Vacuum leek in the brake booster. Mine would die every time I hit the brake some what hard.

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mine dies when i go over 55 mhp and go on neutral hit the brakes and just cuts off.

replace the booster. it has a hole in it like mine did. I could hit the brakes and it would just die... If it's full of fluid then the master is bad also. but you would have noticed that already. no brakes aaaahhhhhh ------- =)

But as for stumble that tells me the accelerator pump may be having a problem. If it's driving down the road at 35 + or so and feels like it's running out of gas then the fuel filter is plugged.
Also if you have the feedback carb it could be the O2 sensor screwing up.

This is what mine does as well. Only time it doesn't stall at stopping, is if I have the idle set high, or if I put it in "N" and keep my foot on the gas and the brake. Everyone told me it was a vacuum leak. I replaced the carb gasket, found a line with a hole it in, still same issue. Driving down the road it hesitates unless my foot is on the gas, press in the clutch, and it smooths out. Mainly at 5-50mph. I think mine has a vacuum leak at the throttle butterfly shaft. When we took the carb off, it was all black by the spring.

Guess it is time for a new carb. I bought a new PVC (mine was clogged), no change. I also got a fuel filter, haven't replaced that yet, it is next.

Mine was doing this too, Panic stop I would have to side step the brakes and gas to keep running. Brake Booster? hmmm I need to check this for a leak. If it needs a Float adjustment it can also do this according to Weber. The force of the "G"s putting on brakes they said can cause float bowl to pour over edge and flood out carb. I adjusted floats on my Weber, which one float was tweeked down farther then other about 2-3mm. I have yet to try the float fix because i'm still doing list repairing.

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Originally Posted by cnatsu1

The vacuum line to the brake booster had slipped down a little. I pushed it up and made sure the clamp was tight, but this happens while I'm driving and not braking. Where was your vacuum leak in the booster at??

I also found a couple of caps on my carb that were pretty sad and most likely leaking, but I've still got the stumble. I was driving it tonight and feel like it might be the carb. It only happens after it's warm, a couple of miles, and it seems to be on deceleration. So intermittently on the flat, when I hit a slight downhill, or slowing down for a stop.

The mikuni carb is super clean and I can tell the exposed gaskets are new and soft. Maybe I still have a vacuum leak, float or other carb adjustment?? Wish I could do the Weber carb, but the wife has cut me off and I don't have enough mad money for that right now. Might have to check out the local parts yard again as I still some credit form the tranny debacle.

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I found this tidbit on hesitation with carb. Camoit's idea seems very likely and it's an easy check. Just throttle the carb and look into barrels for a squirt from pump.
Here is some other info about a lean setting causing the hesitation. I have the entire document which it talks about the Weber, but carb hesitation is pretty universal when it comes to diagnosing the problem.

8. The last thing I do after this is make sure that there is little to no off-idle hesitation which would indicate a lean mixture. The accel pump in the Weber should prevent that, but sometimes, it’s possible that your final mixture setting is on the lean side and so you might get some hesitation. If you do, turn the mixture screw out just a hair, like 1/16th of a turn and then run the engine at 2000rpm for about 10 seconds and then let it idle. Now snap the throttle open quickly. Hesitation? No, great, you're done. Still some hesitation? Make another 1/16th turn out and repeat.
9. Now when you're doing this, it’s important that you are able to tell if you are hesitating due to a lean mixture, or stumbling due to an overly rich mixture. There are several clues that will help you out here. The first is if its leaning out, you may get some backfiring through the carb. The second is if its over-rich, you'll see lots of black smoke when you snap the throttle(s) open. The third is if turning the idle mixture screw out a sixteenth of a turn makes it worse, then its over-rich. Ultimately, what you >want to see when you snap the throttle open is a hint of black smoke out of the tailpipe. No smoke, you're too lean. Just a little smoke is what you want. Now, if you've got a nice, perhaps *slightly* burbly idle around 900rpm, and an engine that snaps enthusiastically off idle, you're done! The main key to remember is that you want your idle screw having to move the throttle as little as possible, since, again, if you have to open the throttle very far, it will uncover the transition ports, and that suggests that your idle jet is too lean.
One other consideration is that if you have the throttle just about closed and the mixture adjusting screw pretty far in, then your idle jet may be too rich.
Another note: I keep saying that you want the throttle as closed as possible, but in actuality, having the idle speed screw turned in about a half to one turn (after making contact with the bellcrank) is about right. Any more than that and your idle jet are too lean, any less than that and your idle jet maybe too rich. Your plugs can also show whats going on inside the chambers. In my experience the trucks run a little rich anyways, but it could go either way.

Hmmm, should be able to disconnect and cap off the vacuum line going to the brake booster to check and see if this is the problem?? I'm assuming you should be able to still brake but it will be reduced braking. Not sure.

Thanks for the carb info bradmph. I haven't worked on a carb for a LONG time. Will try it out today. No carbs at all at the local JY.

Yes it will still stop. Just be ready to PUSH HARD,,,,,,,, Real HARD...

I replaced my booster a few months back :/ and it still cuts off

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Originally Posted by flbadrian

I replaced my booster a few months back :/ and it still cuts off

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Have you done the fuel filter? Also check to see that the accelerator pump is pumping the full stroke and the squirter's aren't plugged up on one side.

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Originally Posted by camoit

Have you done the fuel filter? Also check to see that the accelerator pump is pumping the full stroke and the squirter's aren't plugged up on one side.

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Yes I have changed the fuel filter... I have no clue what a accelerator pump is? Sorry

With the engine off and you look into the carb you will see a squirt of fuel when you turn the throttle.

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Originally Posted by camoit

With the engine off and you look into the carb you will see a squirt of fuel when you turn the throttle.

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Okay I'm going to do this later and hit you back up. I'm suppose see gas go in there right?

Yep use a flash light to see in there. Also in the manual section under over 216 manuals> look for the montero manual.It covers your truck and will help you understand how the carb works.

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Originally Posted by camoit

Yep use a flash light to see in there. Also in the manual section under over 216 manuals> look for the montero manual.It covers your truck and will help you understand how the carb works.

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alright i looked in to my carb and i seen the gas squirt, it squirted a good stream too alot of pressure ... what do i check for now?

What it your ignition timing set at? Also you might want to run a can of cleaner through the fuel system.

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Originally Posted by camoit

What it your ignition timing set at? Also you might want to run a can of cleaner through the fuel system.

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It's at 5. Like how it says under the hood, and I already did that 2 weeks ago ran some JB in it...

Try 8 BTDC and see how that goes.

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Originally Posted by camoit

Try 8 BTDC and see how that goes.

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The timing right?

seafoam it? good cleaner that might help while your diagnosing. sorry just saw your past post on cleaners. Try taking your distributor cap off and grab a hold of that rotor and turn it. See if it snaps back quickly or just sticks. The advance on mine had worn out springs and would just lay there after I turned the rotor imitating an advancing of it.

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Originally Posted by BradMph

seafoam it? good cleaner that might help while your diagnosing. sorry just saw your past post on cleaners. Try taking your distributor cap off and grab a hold of that rotor and turn it. See if it snaps back quickly or just sticks. The advance on mine had worn out springs and would just lay there after I turned the rotor imitating an advancing of it.

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The rotor don't move its kind hard, like theirs a little movement but won't do a 360 turn.

yes on the timing. 8 deg. As for the test bradmph was talking about the rotor should move slightly advanced when turned by hand. about 3 deg then move it's self back. Just a tiny amount of free movement thats all. It should move thats he is after.

you could try disconnecting the vacuum ine from the egr and plugging it. This will eliminate the egr as a suspect if nothing changes.

correct, should be some springiness in it and like Camoit said just a few degrees turn.

Are you sure your getting hesitation also. Maybe that's the way the truck just runs like and your sensitive to it? The smog device stuff could be increasing the hesitation, Andy's idea sounds like a good fix if you can leave it removed and block all the smog ports without emissions test looking under your hood it might be a good idea later on trying.
when the truck is running stroll back to exhaust pipe and if the exhaust burns your eyes within a second or two your rich, too rich. If its just heat, should be ok. Don't breath the crap, just do a quick little eye test. You get worse exposure daily just breathing our polluted air. It's not the normal way to check mixture, but it can give you a quick answer.

Had some time to play with the truck today and found a little nick in one of my vacuum lines. Thought I'd checked them all, but this was down at the bottom of the mikuni carb with two other lines. Amazing how many vacuum lines there are on this thing. Fixed my original stumble problem - YEAH!! Still on my first tank of gas, so I want to make sure any old gas is out of the system before saying for sure. Going to check my plugs and timing next and want to check out the advance as someone else mentioned.

New question: truck doesn't want to start now when it's warm. When I pulled my air cleaner off I could hear some bubbling (truck was hot and had just been driven) and there was a little smoke coming up from the choke side of the carb. Looked like a little witches caldron. I'm pretty sure this isn't normal. Any ideas what this could be? I was about to order a weber carb, but now I've fixed the stumble problem, I'd really like to hold off on that expense for now, especially with having to get the electric fuel pump to go along with it.

It sounds like the thermostat is not working or somthing along them lines. That was the sound of boiling water. Fix that before going any further, do not over heat the thing or you may need a head. If you don't have a fan shroud get one. Use a Montero one. There metal and easy to find at the recycle places. Any fan more then 2 inches away from a radiator must have a shroud. Not enough air to cool it at idle. This is just one of many Mitsubishi's mistakes.

Number one idea, Listen to what Camoit said and do not over heat these trucks. They are notorious for cracking the heads and do it with a slight over heating. Watch the temp gauge, if there is not one, Get one. I run a 180 degree thermostat. Lets them run cooler and gives a little extra time if you see a rise in heat out of the ordinary. Flush out radiator when replacing a thermostat.
I cringe when someone says their truck is over heating and didn't pull over immediately or seen fumes from heat off engine cause it reminds me of my over heating experience. I watch gauges and still couldn't catch it in time, next thing I know, I am making milkshakes in my oil.

I don't think it was water. I was thinking more along the lines that gas was going in the top of the carb (where it probably shouldn't be going) and burning off from the engine heat. The steam/smoke was coming from inside the carb and I couldn't see or hear anything until I removed the air filter. My temp gauge isn't working, but it didn't seem like the engine was overly hot. I'll put the temp gauge and thermostat at the top of my list though and look for a shroud.

In my Haynes diagram it looks like the enrichment jet is on that side of the carb. Maybe it's leaking down into the top of the carb without going down the intake after I shut it off??

The wisp of vapor coming from the carb is normal I think. Just hot air/fuel mixture evaporating from the motor. The bubbling sound could be the overflow from the cooling system. Once you turn off a hot engine the coolant stops circulating and cooling system pressure rises as the heat from the head is absorbed by the coolant. Your rad cap will let pressure in excess of 13 psi escape into the overflow bottle and then the coolant will be drawn back in by the vacuum created as the engine cools. If the rad cap is old it my not be holding enough pressure to stop the coolant from boiling. The bubbling could be the coolant boiling in the cylinder head. Make sure your rad is full when the engine is cold and the coolant is up to the full line on the overflow bottle. Check the overflow hose for cracks and leaks and replace it if defective or the coolant might not get sucked back into the radiator. Also might want to check the coolant with a hydrometer to make sure the water content is not too high. Water boils at a lower temperature than anti-freeze. What does the temp gauge say when you are driving? Both of mine run at just under half way to the hot line while moving down the highway and around halfway or just a bit better in city driving.
Hot starts have always been an issue with my Mikuni carbs also. I find that if I crank it over then wait a few seconds then crank it again, it starts on the second crank. Only in the summer. Sometimes depressing the accelerator just a bit while starting helps too.

checked my advance - working (put a spare tube on and sucked on it to see it move), timing was off - set to 8btdc, fixed broken thermostat wire, plugs were BLACK. Cleaned plugs and set gap. No more witches caldron in the carb and the truck seems to be starting good while warm now. Going to get a compression tester and check compression.

These trucks always seem to burn rich with all the smog stuff on them. At least mine use to, had blacken plugs around ring and tannish looking electrode. Just as long as it is not wet with fuel, oil or water.

Tan electrode is what you want to see on these trucks, the outer ring doesn't really matter unless it is really cruddy black or some other color.

An engine that is on the verge of overheating or just lightly overheating will make that sound in the intake and head. If it's blowing into the overflow bottle it would come from up front. Ikeepitclean's truck was making that noise just after break in because it had no shroud. Even though the gage showed half way up. But how hot is H or cold is C?

I believe the ideal operating temp. is 190-195 degrees F. so I would think the middle of the gauge would indicate around there. You could try a 180 degree thermostat for the summer but a fan shroud would be a better fix as Camoit says.