carb freezing

[albertsanchez84]

Hey yall what's up just picked up a 85 king cab 4x4 720 from my cousin a couple days ago. Truck was idleing high so did a quick search on google and found this site have been hooked since. Like i stated earlier the truck was idle high about 2500-3000 rpm so I did away with the vacum lines capped them all except for dis advance like lalsimmons thread still idles high tho didnt seem to help the issue. Also through the help of my cousin adjusted what i thought was the idle screw turns out it was not thanks to fastboatmans awesome thread it was the fuel flow one ( cant think of the proper name) any who was messing with that today when i noticed the carb mount plate were it hooks up to the engine was frozen. Litteraly white and frosty and cold to the touch has anyone ran into this. Not sure if this has anything to do with the truck idleing super high and not accelerating past 55mph. Thanks guys any help would be much appreciated

[XavierBK]

Does your truck have the carb heater? If so, is it working?

[albertsanchez84]

Since this is my first vehicle with a carbureator not to sure i will post a pic later to see if it is or not. It might be tho since it has a couple of wires going to it would this affect the high idle as well?

[albertsanchez84]

also would this issue affect my MPG im getting around 11 MPGs or is this a whole nother issue ?

[fastboatman29212]

Welcome to the 720 Forum!
I could be wrong, but most carbs idle too high because someone tinkered with the idle mixture and throttle adjust screws. They get adjusted because the engine was cutting off at a lower idle, most likely due to a blocked passage in the carb. The other likely cause is a vacuum leak somewhere, thus causing a decrease in vacuum coming down thru the throat of the carb resulting in fuel starvation. The adjustment is not really a FIX, just a way to get more fuel down into the engine so it won't stall.

The solution to your problem is going to be a thorough cleaning of the carb or spending your cash on a Weber. Since you have deleted the vacuum lines anyway, you have done most of the work needed in preparation for a Weber.

Your Hitachi carb is rebuildable, but it will take time and a lot of patience to get it pulled apart and back together. I took a spare carb apart 2 or 3 times before I attempted rebuilding my actual truck carb. Lucky for me it worked out fine.

Your RPMs should be under 1000 if it's running properly. You should get anywhere from 25 - 30 MPG depending on how you drive.

[albertsanchez84]

Appreciate all the help. I have read your post fastboatman on rebuilding the hitachi for refrence on mine. So i took a couple pictures to try and explain my situation or for refencing what im trying to explain how do i go about doin that and can i do it off my phone?

[albertsanchez84]

This is me driving down the road

This is at idle


This is the carb at idle as well

[albertsanchez84]

I have adjusted the fuel flow screw many ah time was first thinking that it was the idle screw after reading here later found out it was not. I have not adjusted the idle screws since i have not been able to locate them. Adjusting the fuel flow screw does nothing to the idle issue tho i have screwed it in all the way unscrewed it and it also does nothing.


I did however notice this linkage was laying on the carburetor housing looks like it needs to be attached to the plastic part above it on the teapot type thing. Could this be the culprit

[albertsanchez84]

Should the mechanical linkage (yellow) be attached to the plastic plunger (red). This pic was taken while truck was @ idle

[fastboatman29212]

The small linkage you are pointing at may be broken. Mine was. However, it will not cause the engine to idle high.

The idle set screw is located at the base of the carb and is factory sealed. It determines how much fuel flow thru the idle jet and is set at the factory. If the plug was removed by a prior owner, then they probably adjusted the screw (which has a needle point end). That can all be reset, so don't worry. That screw all by itself will probably not cause the engine to idle at 2000 RPMs.

The throttle adjust screw is merely a fine tune for the throttle cable. Turn it in or back it out to increase or decrease the fuel to the engine with the foot off the pedal.

This photo shows where to find the throttle adjust and factory idle screws.


The other thing that COULD be a problem is that your throttle cable might be adjusted or set up wrong. You will need to disconnect it and see if the idle can be changed by moving the carb's throttle linkage by hand. Of all the possible problems, that would be the easiest to fix.

[albertsanchez84]

Thanks again today a i unscrewed the throttle adjust screw all the way off then attached it back on maybe by a thread or so to hold it in place still nothing rpms would not come down i bought the carb rebuild kit and will do that this weekend be sure to keep you guys posted. As for the throttle control cable that was the next issue i was gunna bring up is there a nominal lenght it factory setting it need to be at i unscrewed it at the throttle linkage messed with that for a bit also tried to adjust the "pedal positioner " at the gas pedal its a screw with a rubber stop im asuuming to allow the rite amount of tension on the cable. So ill probably have to adjust that after i mess with the carb. Well wish me


**BTW** The carb mount plate or base were it meets the manifold still freezes turns frosty white any idea why it's doing this hoping the issue clears itself up after i rebuild the carb

[fastboatman29212]

The frost on the base of the carb is not a problem IMHO.

You need to disconnect the throttle cable from the carb to see if the throttle linkage is stuck or not. If it is stuck, then you have found your problem. Move the throttle linkage by hand once the throttle cable is disconnected from the carb.

An engine is a giant suction device that will draw as much fuel down through the carb as possible. So basically, a carb is designed to LIMIT how much fuel the engine gets. At idle, the lower butterfly is almost competely closed. That means the engine only gets fuel delivery from a tiny orafice called the idle jet. Fuel delivery is just enough to keep a steady idle. When you step on the pedal, you are opening a much larger passageway for the engine to suck through. That increased suction draws fuel through the main jet and the RPMs increase rapidly.

Based on the above paragraph, something seems to be keeping the lower butterfly open and allowing the engine to suck too much fuel. The frost on the carb base is caused by fuel being vaporized. When fuel vaporizes, it will cool off anything in the area.

[albertsanchez84]

Well here i go getting the carb tore apart got my step by step guide thanks fastboatman


i did not see this part on your tutorial is this the carb heater ? It had a wire going to it . If it is should i assume it doesnt work since the carb kept on freezing

[fastboatman29212]

The carb rebuild photos were made with a spare carb and I agree that I do not see the carb heater part you talk about.

However, this is my actual carb and you can see the metal prongs on the spacer plate.

[albertsanchez84]

I have finished the bottom section of the rebuild so far done for the night need to get up @ 4 for work . Thinking i should b done this weekend depending how long xmas shopping takes. The port on the bottom next to the prongs on the spacer plate, is that the hookup for the vac advance

[albertsanchez84]

Just finished the rebuild gunna install tomorrow lets hope i put evrything back together rite. Thanks fastboatman for the carb rebuild post you did without it dont think i would have thr nerve to try and take on a task like this. The only issue i ran into was rebuilding inside hahahaha the wife wasnt to happy but besides that went pretty smooth

[albertsanchez84]

So I installed the Carb Yesterday didn't help much was pretty frustrated . Called my pops happens he had an extra carb we installed that one and wallah got her to idle at around 900 not bad i would say. WE had to modify the wire harness the new carb didnt have all the wires as the old one did. This one only has the one wire to the choke and one wire to the anti dieseling solenoid. Also had to splice the wires together wasnt getting any power for the solenoid so i spliced it with the choke one seems to work fine now tho.
So is the new carb i installed an older version and will it lead to any issues seems to be working fine now.

[fastboatman29212]

Just drive it and see. I'll bet it works fine. Is it a Hitachi?

[albertsanchez84]

Drove her to work runs fine 40+ miles did realize when i got to work tho i forgot to install the air filter and housing hahaha. But anyways really appreciate the time and effort you guys took to help me out really appreciate it