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Thread: Newly aquired 86 g63b with Mikuni carb

  1. #1

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    I.D.K.?

    Newly aquired 86 g63b with Mikuni carb

    OK guys so I just was given a 1986 Ram 50. It has had 2 owners but hasn't been daily driven in 10 years. I have so many questions but I am starting here. There have been modifications done by would be mechanics. So to start It was pulling fuel from the old gas can in the bed with two hoses ran too an aftermarket electric in line fuel pump. That being said I always like to have a baseline to start my trouble shooting from. I replaced the tank and fuel line but assumed that the in line pump was installed due to the mechanical pump not working. After making the fuel system as close to factory as I could it runs for about two or three minutes then acts like it is starved for fuel and dies. But this is only with the inline pump running. I shut it off and it will run fine at idle and will also run fine a steady 3000RPM. The issue now is that when I first hit the gas there is the classic fall on its face for a sec before grabbing fuel. What I am here to ask is if the weber conversion is the where I should head from here? Is rebuilding the Mikuni worth it? I plan on driving it daily for a while before performing an engine swap so I'm interested in what the best method is. Thanks for any input.

  2. #2

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    Hi and welcome to mightyram. Sounds like the accelerator enrichment pump on the carb has failed. Easy to check (engine off) - remove the air filter, manually hold open the choke butterfly and twist the throttle linkage open and closed a few times. While you do this, look down the primary throat for a jet/stream of fuel coming from the side of the throat. No fuel stream or a dribble of fuel indicates the pump diaphragm has failed. You don't need to remove the carb to fix this (a little fiddly to work on while the carb is still mounted but do-able) but as the pump diaphragm is part of a kit I would say it's time to overhaul the carb anyway seeming it has been sitting for so long. They probably installed the electric pump thinking fuel supply was part of the problem but be aware that if the engine starves out of fuel while at highway speeds or being under hard open throttle it may indicate the mechanical pump is struggling.

    As for your question - yes the Weber swap is worth the effort. Not a straight forward deal but the minor mods that need to be done aren't complicated to pull off. The Weber will be happiest with accompanying engine mods but will work fine just as a change over. Depending on the electric pump you are already part way there for the conversion (the Weber needs a high volume, low pressure pump to operate correctly) You have 2 Weber carb options - 32/36 DFEV or the more modern designed 38DGES. The 38 has more scope for engine performance tuning. Both will offer better power delivery and economy than the stock carb under daily driving conditions.

    *engine type - if your engine has an alloy timing cover it's a 4G54/G54B 2.6 otherwise it'll be a belt driven G63B 2.0 engine.
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  3. #3

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    I.D.K.?
    Thank you for the welcome! And also so much for the advice. I am excited to start the journey with my truck. I am ordering the rebuild kit today as my carb is also leaking a small amount of fuel from the front side upon further inspection. I might just replace the fuel pump while I have the carb off. any specific information that might be helpful? My engine has a plastic timing cover.
    Also I have driven her around the block a few times and it does seem as though it is acting starved for fuel after giving her a bit off steep throttle. After climbing a steep hill it coughs a bit, this problems seems to go away if I keep it in low gear. It has the 3 speed automatic 2wd. That being said keeping it in low gear seems to lessen the dying out. When it does die out sometimes she wont start back up easily. With or without the electric fuel pump running. I apologize for throwing a lot of information at you at one time but I want to properly diag. and repair. Thank you again for your help!

  4. #4

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    There may be another issue other than the carb/fuel delivery. There are a number of things that can cause engine stumble or random engine shut downs (not specific to these trucks/engines - just treat it as normal fault diagnosis...) Stick to the basics first - engine tune, vacuum leaks, blocked EGR, carb function (auto choke, fuel bowl vent, accelerator pump *which looks like a possible fault etc) fuel filter/hoses/pump then look at things like distributor advance function, ignition coil. If you plan on doing the Weber install, there is a specific kit for automatic trans optioned trucks (there is an additional linkage for the transmission kick-down cable) My money is on something minor that is nerfing your engine like poor engine tune or a tired coil, lazy fuel pump or a clogged filter.
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  5. #5



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    Change the fuel filter and timing belt if it has them. Do not pass go tell you do.
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  6. #6

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    And this - http://www.mightyram50.net/vbulletin...and-do-it-ASAP

    This thread will help you 'Dodge' a hail of bullets when you first get your hands on a pre-loved Mitsu truck.
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  7. #7

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    Ok. Just finished rebuilding the carb. Just did the full rebuild as we had discussed before. acc. pump no longer leaking. But of course she is doing the same thing as before. I'm thinking of trying the ignition coil now. There are no vacuum leaks (well none that change when doing the basic leak check. It does seem to worsen when the truck is heated up. It idles smooth and will remain idling as long as I want, it still stumbles for that first bit of throttle increase. but once past that point it will run at holding 2000RPM and above smoothly. When I do acc. hard up a hill she acts a little starved for fuel. Here is the information that I think is key, When it does cough and die during driving I have to hold the throttle open and crank for a min before it begins to pop and start again. This happens with and without the aftermarket in line fuel pump running. So I do not think that it is a pressure issue even if the mechanical pump is failing. And if pressure is the issue why will it run at high rpm sitting still fine but not after opening the throttle while driving. I'm frustrated as I cannot nail down what is causing the issue.

  8. #8

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    I refer you to Camoit and Geezer's post just before you

  9. #9


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    Srr, thanks for sharing your progress & troubleshooting results... I dont have an answer but I have somewhat similar issues with my truck... Some of my threads have carburetor info links... I hope to begin rebuilding my Mikuni carb very soon; so far just buying tools & parts to throw at it...

    It might be another 2 years or more before small trucks are for sale in USA so keep at it brother... The FSM has a bunch of DMM checks with carb on the truck, worth a look...

    Aloha--
    George

  10. #10

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    Srr7172,
    Add 5 gallons of fuel (baseline) remove fuel filler (Gas) cap, roadtest & post back!
    Thank You.
    Last edited by claych; 07-12-2020 at 12:43 PM.

  11. #11


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    Srr7172

    Hmm, well the rebuild fixed the gas leak, good job Assuming the stock Mikuni is on your truck, my totally unprofessional advice is to remove the electric fuel pump and use a good original type fuel pump, thanks O'Reilly's & NAPA... My truck ran better (less starving for fuel) when a new stock fuel pump was installed... Assuming your ignition system is OK (look at the spark plugs ON A COLD ENGINE) use dielectric grease SPARINGLY it causes misfires... I must clean out the dirt & dielectric grease on the main electrical harness connector on my carburetor (possible goof on my part)... All my vacuum lines are tight & correctly connected but I have not cleaned or tested EGR or Thermo Valve yet... No start Grrrrrr... My spark plugs as black and as coated as can be, never seen any worse... Running a lilttle rich perhaps..? Well not running he he he...

    Maybe try these checks: (all gaskets are good & fasteners snug BUT not over tightened, lithium grease is friendly for actual torque read)...

    There is a round metal vacuum diaphragm at the rear of the carb (FSM names it Depression Chamber) which opens the secondary butterfly throttle valve, does it open the throttle? There is a short approx 3" long piece of vacuum hose (white stripe coded) from that diaphragm to a nipple at the bottom rear of the carb... Maybe you could use a brake bleeder hand vacuum pump to see if it opens the secondary throttle blade (the primary throttle needs to be opened for the secondary to open/there is a metal to metal interference of throttle pieces otherwise) The depression diaphragm should be able to hold vacuum without having to keep pumping the vacuum pump... I used a DMM innova 3320 to test resistance of the carb's selenoids FSM reading requires 49.7 ohms (more ohms = more electrical resistance = less work being done by the selenoids) Open the carb's main harness connector and test on the male ends attached to the carb side of the harness... You can also DMM test resitance of the TPS (yes the Mikuni has one, shocked me) test at it's harness connector which is a round black connector abt the size of a little finger... FSM shows how to adjust the ohm readings of the TPS if out of spec -- throttle open 4.9 kilohm & closed throttle 1.2 kilohm... Check that the choke plate is open when the engine is warm (a choked engine will lack power as bad as a weak fuel pump)... Might be the float level is out of adjustment but that requires carb disassembly (I have not done that yet) OFC you changed the fuel filter & removed the gas cap for a spin up the hill too, right..?

    Do yourself a favor (if not too late) DO NOT adjust any screws on the carb EXCEPT one named (SAS1) idle speed adjusting screw... Too late for me, I already made that mistake... I have just started down the road of trying to rebuild my Mikuni carb (my truck won't start/flooding?) possible dirt in the needle seat..?

    If you read this far & did the above BUT still have problems; people say a Weber carb conversion is an option (not for me)... If you decide to convert to a Weber, please be advised, I am wanting the phenolic plastic fuel bowl/throttle body center section of a 1985 or 1986 Mikuni; mine is cracked... My truck a California emissions & assuming a Federal carb of those years is same enough... Your engine "U" 2.0L & mine "W" 2.6L might be same enough there too..?

    Well, if you did, thanks for reading this far & best of skill 'n luck to you... If anyone can fix it, YOU CAN..!
    Aloha --
    George

  12. #12


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    Srr7172 what choke cover screws or rivets did you use to rebuild your carburetor..? anyone..?

    SCREWS.jpgSCREWS2.jpg

    ANSWER discovered at my local hardware store ~ all 5 screws are male threaded & size is Metric M5 x 80
    Last edited by xboxrox; 08-06-2020 at 12:27 PM.

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