Results 1 to 10 of 10

Thread: What modifications can you make to a gx390?

  1. #1
    Join Date
    Jan 2019
    Location
    Stevenage, UK
    Posts
    21

    What modifications can you make to a gx390?

    I am currently building a gx390 as my first engine project and I would like to know about what modifications I could do to the engine. So far I have done:

    (Loncin GX390 Engine)
    Degoverned & Low Oil Sensor Removed,
    Aluminium Top Plate & Fuel Pump,
    Fire Ring Head Gasket,
    Skimmed, Race Ported, Diamond Cut Seats, Big Valve Cylinder Head,
    Stainless Steel Valve Train Kit,
    Roberston Race Exhaust,

    and I am also looking to add

    Billet 4.510'' +.100'' Conrod,
    Lightweight Flat Top Piston,
    0'' Deck Height,
    .307 Race Camshaft,
    ARC Lightweight Flywheel,
    6* Advanced Timing Key,
    34mm Flatslide Mikuni Carburetor Kit,

    what else should I change? should I go for a bigger carb with an accelerator pump? I am not too concerned with how long the engine will last, I just want to push the limits of the engine. Mounting this to a 2008 Tony kart chassis clutched.

  2. #2
    Join Date
    Oct 2013
    Location
    iowa
    Posts
    6,942
    Looks ok.
    No concern for engine life is open ended answer, more more more.

  3. #3
    Join Date
    Oct 2013
    Location
    Linden, IN
    Posts
    9,716
    Quote Originally Posted by John_OSU View Post
    I am currently building a gx390 as my first engine project and I would like to know about what modifications I could do to the engine. So far I have done:
    34mm Flatslide Mikuni Carburetor Kit,

    what else should I change? should I go for a bigger carb with an accelerator pump? I am not too concerned with how long the engine will last, I just want to push the limits of the engine. Mounting this to a 2008 Tony kart chassis clutched.

    You didn't mention what size valves you're using. From my experience, even a well ported head and larger valves, a 33 pumper carb is more than adequate. Now, if you've got 1.5 inch IN valve and welded up port runner, you may be able to go bigger on the carb. That carb will easily handle .400" lift. You didn't mention ratio rockers, but if you were to use 1.3:1 rockers with your current cam, that would make a lot better use of the 33pumper carb.

    We've built quite a few engines like this if you need some assistance (and have the carb as well,) just give us a call at the shop.


    -----
    Thanks and God bless,
    Brian Carlson
    Carlson Racing Engines
    Vector Cutz
    www.CarlsonMotorsports.com
    Carlson Motorsports on Facebook
    30 years of service to the karting industry
    Linden, IN
    765-339-4407
    bcarlson@CarlsonMotorsports.com

  4. #4
    Join Date
    Jan 2019
    Location
    Stevenage, UK
    Posts
    21
    35mm inlet valve, 31mm exhaust valve,
    .356 lift cam
    reinforced 1.1 : 1 billet rocker rocker arms

    I don't understand what you mean by welded up port runner though, is it an american thing?

  5. #5
    Join Date
    Oct 2013
    Location
    Linden, IN
    Posts
    9,716
    Quote Originally Posted by John_OSU View Post
    35mm inlet valve, 31mm exhaust valve,
    .356 lift cam
    reinforced 1.1 : 1 billet rocker rocker arms

    I don't understand what you mean by welded up port runner though, is it an american thing?
    Have you welded the intake runner and re-ported it to change the entry angle?
    The 33 pumper carb would make a good choice for what you've got it sounds like.
    If you're staying on gas/petrol, the 34 flat slide will be alright too.

  6. #6
    Join Date
    Jan 2019
    Location
    Stevenage, UK
    Posts
    21
    Just a disclaimer, I am in hertfordshire, UK so international calls or meeting in person is somewhat impossible with american users.
    --------------------------------------------------------------------------------------------------------------------------------------------------
    I am actually having another engine builder(Daniel Papworth GXTUNINGSTORE) I know do the head because I lack the machining equipment or the experience that he has but its a custom job.

    GX390 Full race ported & polished cylinder head, milled .100''. The head has a custom valve job with a 40mm inlet and 32mm exhaust stainless steel high flow valves, 65Lb valve springs and an aluminium retainer & Split keeper set.

    with the reinforced 1.1:1 rockers being 50, 1.23:1 billet rockers being 90 and gage 1.3:1 rollers being 190, is it worth going nearly 4x the price to gage roller rockers?

  7. #7
    Join Date
    Oct 2013
    Location
    Linden, IN
    Posts
    9,716
    Quote Originally Posted by John_OSU View Post
    Just a disclaimer, I am in hertfordshire, UK so international calls or meeting in person is somewhat impossible with american users.
    --------------------------------------------------------------------------------------------------------------------------------------------------
    I am actually having another engine builder(Daniel Papworth GXTUNINGSTORE) I know do the head because I lack the machining equipment or the experience that he has but its a custom job.

    GX390 Full race ported & polished cylinder head, milled .100''. The head has a custom valve job with a 40mm inlet and 32mm exhaust stainless steel high flow valves, 65Lb valve springs and an aluminium retainer & Split keeper set.

    with the reinforced 1.1:1 rockers being 50, 1.23:1 billet rockers being 90 and gage 1.3:1 rollers being 190, is it worth going nearly 4x the price to gage roller rockers?

    Absolutely. And there's likely a better cam to work with it as well.
    I guess a lot depends on your level of competition and how serious you want to get with it.

    No problem on the international calling issues. I'm just so used to everyone having cell phones and international calling plans that I guess I assumed everyone has one now, (even though I don't.) I've got a great customer (and friend) in Bermuda that calls me regularly to chat and I take for granted how convenient it has become.

    BTW, y'all's English is tons better than our 'Merican English.

  8. #8
    Join Date
    Jan 2019
    Location
    Stevenage, UK
    Posts
    21
    The GX390 engine is not very popular in the UK, so there is basically no series for it and because of that, no competition. If anything, its a competition against myself and any other engine builder. I simply want to do it because its getting me out of my comfort zone and definitely helps me in my work(BMW Apprentice).

    What cam would be your personal recommendation for the setup I have chosen so far?

    BTW, y'all engine part prices are tons better than our 'Queens Highly Taxed Imports'

  9. #9
    Join Date
    Oct 2013
    Location
    King, NC
    Posts
    3,148
    A somewhat common modification to the "industrial" engines used in karting that are converted to true OPEN modifications is to take the stock block, fill in the intake passage with weld material, then re-bore at an angle that allows the intake air a straighter passage. http://www.mikeclementsracing.com/enginespage.html has some examples where you can see, esp. on the flatheads, that a 90* inlet has been modified to be angled... This is expensive, but worth it if you're chasing power. Those intake passages can be filled with epoxy to keep the air from getting trapped in corners and causing turbulence. Valves can be relocated.... Your imagination (and pocketbook) are the limit. The key to remember is that engines are air pumps with a hot-air process occurring in the middle of the cycle. If you can get it to pump more air, more quickly, you have achieved more power. Often the intake and exhaust processes are interdependent, so if you change one side, you must also change the other for full benefit. I've seen some GX390 thumpers on youtube videos from your side of the pond, and looks fun. Here in the states the Mini Cup cars and Swamp boats use GX390s or their varients, and some open kart builders like Brian Carlson and Jimbo and others have stocked some performance parts. Because those engines are in no-mans-land on the unlimited dirt side (most are bigger, or smaller) and virtually non-existent on the sprint side, I'm sure you see that some parts are harder to come by. good luck, and please post pics. we like odd projects here.

  10. #10
    Join Date
    Jan 2019
    Location
    Stevenage, UK
    Posts
    21
    I didn't even think about taking photos or recording it as I thought people wouldn't be interested, i'll start from now on.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •