Horrible gas mileage and substantial lack of power.

[Skattergun]

Found a new door/window, so now I can start sorting out the other problems. Keep in mind I am NOT a mechanic nor could I even pass for one. I've only been doing most of my own work since 2011, and while I'm a pretty quick learner and usually successful when I set out to fix a mechanical problem, there is a LOT that I don't know. If I ever sound clueless, it's probably because I am.

My main problem with my little D-50 right now is the horrible fuel mileage. I'm not sure what these trucks usually pull, but I know it should be doing better than the 10-12 mpg that it's running right now. The first thing I'm gonna try of course is to replace the fuel filter, but what else could be causing that kind of mileage?

Again I'm not particularly an expert, but I'm kinda feeling like it's something with the carb, it was running rich as hell before (strong gas smell, flooding, etc) but I had a buddy fiddle with it and the gas smell and flooding are gone at least. The carb is new, but the last owner DID admit he didn't know what he was doing and it probably wasn't set up correctly. It's just got a Mikuni carb that I'm gonna replace eventually, but how would someone that only vaguely knows what he's doing tune it to a somewhat acceptable point as to eliminate that as an issue?

The other big issue I'm having that may or may not be related to the same issue that's killing the fuel mileage is a serious lack of power. Overall it's just sluggish as hell, and I've noticed one thing with it that has me somewhat concerned. Mine has the 3spd auto, and it's my understanding that when the pedal is pushed to the floor or near it that it downshifts to 2nd for hills or whatever. Most of the time this seems to be how it works, but it's not consistent at any speed or RPM. Good example is one specific hill that I hit daily, it just stays in 3rd the whole time and it's lost about 10mph by the time I get the top even with the pedal to the floor. Does this sound like an issue or is it normal or do I just have the whole system confused?

[geezer101]

Hi Skattergun, I'm going to put money on your carb being past its rebuild time. This stuff gets asked a lot by members who are noobs when it comes to nuts and bolts - read through the threads here covering carb woes. If you haven't pulled down a carb before, the Mikuni on these engines is not the carb you should trying your hand at first. They are complex. Try blasting it with carb cleaner, then purging the fuel bowl by taking off the air filter and with the engine running cover the throats to starve the engine of air. This will ramp the vacuum inside the carb and pull anything that might be blocking jets out. It's worth a try and normally yields an improvement. These carbs have common issues when they get tired like auto choke failure and/or no secondary throat function. Find another complete Mikuni and try swapping it but whatever you do, don't take the only carb you have apart in an attempt to fix it. The alternative is swapping the Mikuni for a Weber - this is extensively covered on mightyram and is a worthwhile performance improvement.

[finalfighter]

if you floor it quickly or relatively fast and the transmission kicks down than that sounds ok. if you gradually work the pedal to the floor it will most likely not kick down. it depends on the loud and speed. sounds like your carb. i had an auto 4 speed, (same gear ratio as yours just has overdrive) and it could pull any hill in my area in 3rd no problems. i just got a 87 with a g54b and will be swapping to the weber 32/36 dgv just to get rid of all the vacuum lines, might be what you need.

[Skattergun]

I'm going to put money on your carb being past its rebuild time.
Thing is, the carb that's on it was put on brand new by the last owner just a couple of months before I bought it and he only put about 2000 miles on the truck after that.


Either way the two things I've seen repeated multiple time now is to not really fool with those Mikuni carbs and to slap one of those Webers on it, so I spose that's what I'm gonna do in the near future when I can scrape the money together.

[geezer101]

Thing is, the carb that's on it was put on brand new by the last owner just a couple of months before I bought it and he only put about 2000 miles on the truck after that...


Sorry, should've been more thorough when reading your initial post - my bad. What was the idle rpm and start up like? There are only 3 screws that should need any adjusting on these carbs - air/fuel mixture screw, throttle stop and fast idle (if your idle speed isn't right straight off the mark). There are adjustments for the auto choke and secondary throttle stop but both of those need to be set prior to installation (the secondary throttle stop screw is impossible to adjust when the carb is bolted down and the auto choke needs to be partially disassembled to set the choke butterfly spring tension - both should've been set when it was assembled). The fuel bowl float level should've been set correctly from assembly too but you never know...

Check all the bolts and screws on the carb first. If they're loose it can cause air leaks into the carb body. Then look at the throttle cable adjustment to make sure it's not loading up the throttle. Hope this link helps you - http://www.mightyram50.net/vbulletin/showthread.php/5005-Carburetor-issues-xp?p=43407&viewfull=1#post43407

[ratesheet]

Could be a few other things...check the timing, these trucks are very timing sensitive....check to see that the catalytic converter is not plugged...my 86 G63B has the catalytic converter as part of the exhaust manifold with a mfg. recall for a second cat added....the cat in the manifold can be taken apart, I'll bet you find the cat material has melted solid across and there is very little exhaust flow...keep up posted.

[camoit]

Also check the choke and make sure it's opening up. Just pull the air cleaner top and look. It should be closed when cold and open when warm.
Also we have a factory first gen manual with a nice flow chart to look through in the manual section.

[Skattergun]

What was the idle rpm and start up like?
Idle is probably something I should have mentioned to begin with since I have noticed some issues with that. When you first start it up it idles pretty high, usually somewhere between 1500-2000. It eventually levels out once you start driving it, appears to generally idle somewhere around 700rpms where it in theory should be, but it's generally pretty damn rough, only smooths out if you give it a little gas and get it to about 1000rpms.

I'll have to check on everything else that was mentioned, I'll try and check it all out tomorrow if I get the time.

[camoit]

Could be the needle and seat have crap in it. Or the float is sinking.
Also check the vacuum at an idle and when it's driving. If the cat is plugged it will have good vacuum for about 3 seconds then drop down.

[BradMph]

I like a little higher idle so when I turn on lights, heater, or the electric fan kicks on, it doesn't chug the motor. 700 seems low to me IMO. .



Keep in mind I am NOT a mechanic nor could I even pass for one.

when you get done fixing everything and working on all of it, you will be an unofficial mechanic, I guarantee it, lol

[geezer101]

^what he said. You would need at least 850-900 rpm idle when the engine has warmed up.