125 Engine Build - Help Spend My Money
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125 Engine Build - Help Spend My Money

This is a discussion on 125 Engine Build - Help Spend My Money within the Grom Performance forums, part of the Honda Grom Garage category; Background I bought a Grom early this year and started racing it with friends. As bought, the bike has 7,000 miles and a header, pipe, ...

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Thread: 125 Engine Build - Help Spend My Money

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    125 Engine Build - Help Spend My Money

    Background

    I bought a Grom early this year and started racing it with friends. As bought, the bike has 7,000 miles and a header, pipe, PCV, tail tidy, LED signals kits, and a few other odds and ends. I installed Öhlins front and year, MNNTBX bronze swing arm bushing, sticky tires, and a few other odds & ends. Had a ball this summer racing and my bike is very evenly matched with my buddy's.

    BUT

    Last week while chasing each other around at high speeds (for Groms) my engine seized. Trailered home. Magnetic drain plug covered in shavings. No rattling when it went so I suspect bottom end. At this point I'm not sure whether the seized engine is worth rebuilding or if I should just go with a used engine (which I have a line on).

    ANYWAY

    My Grom friends and I have come to a gentleman's agreement that big bore kits aren't allowed. Max displacement is 125cc. Help me spend my money to make this track-focused Grom make more power.

    Recap: Bike has header, pipe, PCV with unknown tune, and a seized engine. I upgraded the clutch this summer.

    So, I'm looking at your various heads, pistons, cams, etc. My goal is the most power I can make with 125cc displacement without decreasing longevity too much. I look at all the options and I have a hard time deciding which would be the best combination of everything.


    • Which head? Or OEM head with maybe some home porting?


    • Is a 4 valve head possible? Is it even worth it?


    • Which piston? Is a piston even necessary?


    • Which crank? Or, stick with stock crank?


    • Which cam? Or, stick with stock cam?


    • Does converting to an oil pump add power to the rear tire because the oil spinner goes away, or is it just to improve lubrication, or both, or neither?


    • With a higher output 125cc motor is an oil cooler a good idea? I don't want to add unnecessary weight, but will if it's functional.


    • I see that heavier duty starters are offered. Necessary or good idea for 125cc built motor, or BBK only?


    • Any other things I should be considering?


    Again, I'm looking for a balanced package of upgrades to do to my Grom engine for power for road racing while keeping 125cc displacement.

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    Old1Eye's Avatar
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    I can only offer my experience with building the original monkey engines up.
    I haven't fully modded my 2018 monkey yet.
    Only larger air intake and exhaust with Efie mod so far
    115km/h indicated.

    So if you want to modify the standard cylinder you want to increase your fueling, most accurate way would be with a power commander and an auto tune O2 sensor module, or go the cheaper way but not as accurate way with an Efie device.

    Stick with the stock throttle body and injectors as they are enough for stock bore.
    Bigger air intake and filter.
    Free flow exhaust.
    Upgraded cam, version 1 or 2 depends and where you want the power, and again brand will also effect this too.
    High compression piston will help a fair amount too.
    There are ported and bigger valve heads that would also make a good difference, but no point in trying to get a 4v head to fit with the stock bore.
    The clutch case for removing the oil spinner is more of a convenience thing and adding the oil filter than anything else, though it might help the engine spin easier, can't say myself for sure on that.
    There are a few little things you could do to but I guess that's the main things.

    Oh and an oil cooler depends on your local climate and your type of riding, BBK would need them for the bigger kits, some people run the smaller bore kits without.
    You can add one if you want and probably easier if you did change the clutch case, but not necessarily needed.
    Last edited by Old1Eye; 11-03-2019 at 10:42 PM.
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    does your motor still turn over?
    2015 Grom - High Compression 125 Build
    2018 Grom - 143cc BigBore
    1986 Spree - 70cc BigBore
    2017 DRZ400SM

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    Nothing wrong with rebuilding that engine. If you want to be sure you got out all the metal you can split the cases.

    If you need to stay with the stock displacement your money would be best spent on porting and perfect tuning.

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    Quote Originally Posted by alienbogey View Post
    • Which head? Or OEM head with maybe some home porting?
    Ported stock head
    Quote Originally Posted by alienbogey View Post
    • Is a 4 valve head possible? Is it even worth it?
    Not on 125cc
    Quote Originally Posted by alienbogey View Post
    • Which piston? Is a piston even necessary?
    12:1 compression or greater - aim for 11.5:1 effective compression with your cam choice
    Quote Originally Posted by alienbogey View Post
    • Which crank? Or, stick with stock crank?
    Stock
    Quote Originally Posted by alienbogey View Post
    • Which cam? Or, stick with stock cam?
    Many good options, and none of them are the stock cam
    Quote Originally Posted by alienbogey View Post
    • Does converting to an oil pump add power to the rear tire because the oil spinner goes away, or is it just to improve lubrication, or both, or neither?
    Just get the Kitaco right-side cover and call it good
    Quote Originally Posted by alienbogey View Post
    • With a higher output 125cc motor is an oil cooler a good idea? I don't want to add unnecessary weight, but will if it's functional.
    A 3-row is a good idea if you run aggressive timing but in your area, it's not *needed*
    Quote Originally Posted by alienbogey View Post
    • I see that heavier duty starters are offered. Necessary or good idea for 125cc built motor, or BBK only?
    The Kitaco starter is a good idea with the high comp. piston
    Quote Originally Posted by alienbogey View Post
    • Any other things I should be considering?
    aRacer or Power Vision and a good tune, considering that's probably why you blew the first motor
    High flow oil pump
    Porting the throttle
    5 speed kit and a set of sprockets

    To be honest, I got bored pretty quickly with a 12.2:1 125cc motor and the TB cam. I have a 181cc NEO kit sitting in the garage; just waiting for my intake manifold to show up to have the head ported and matched to the manifold before I install it all.

    Last edited by mrsaturn7085; 11-04-2019 at 01:04 PM.
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    7K miles on the engine so I assume you bought a used grom from someone or you bought a 2018-19 grom new from a dealer and put on 7k miles.

    First off drop the engine to inspect the broken parts, if engine stop working normally the piston went "lack of oil" or the stock OEM rod broke "lack of oil" or too many wheelies with out a modified oil pump with copper pipe extender.

    split the case and check out the parts, it is EZ to rebuild on a stock 125cc.

    Check out my posting on organizing your parts as you remove them from the crank.

    If crank/rod failure you will have to buy a new rod kit, "cheap" on OEM parts.

    Your stock head should be removed and inspected and if in good order do a head porting, valve cleaning and polishing and valve lapping. Open up your intake port to about 26-27mm and also your inlet tube for your throttle body to 26-27mm including your plastic spacer.

    get rid of that PCV and get yourself a Arace mini2+ or mini5, this will get rid of you piggy back ECM unit.

    Clean out the case after splitting and check for damage.

    Get yourself a Kitaco clutch side cover to remove the oil spinner and get yourself a external oil cooler, lots of 5 fin ones on ebay from China.

    Look up my old posting and read and re-read my post about organizing the parts being taken apart.

    check the bearings for the crank and transmission to make sure the bearings spin freely with no lagging or ruff spots when spinning the wheel.

    Use the correct assembly grease.

    good luck

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    WV_Grom's Avatar
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    https://store.hardracing.com/finbro-...tegory_id=2690 ???

    While your in there rebuilding the engine, throw in a 5 speed and watch your machine walk away from your friends grom!
    2018 Grom
    Koso 170 4V/34mm TB/Koso injector/ DIY intake/ aRacer Mini5 + AF1 + DG1
    Koso pump & chain tensioner/ Stock clutch ring / 60% stiffer springs
    Takegawa Crank Support Bearing / Takegawa Catch Can / Kitaco Clutch cover
    Clear cam sprocket cover/ Egay oil Cooler
    Ohlins F&R / Hindle exhaust w DB killer / Galfer brake line w EBC HH Pads
    Pro Taper grips, G2 throttle tube / Gromfatherz axle spacers
    Kepspeed alum. swingarm / Bar end mirror, RAM mount, USB Port
    Seat Concepts / Opmid V2


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    Quote Originally Posted by WV_Grom View Post
    https://store.hardracing.com/finbro-...tegory_id=2690 ???

    While your in there rebuilding the engine, throw in a 5 speed and watch your machine walk away from your friends grom!
    Faster acceleration, if you're a good shifter, but the top speed remains the same, correct?

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    Quote Originally Posted by 50-50 View Post
    Faster acceleration, if you're a good shifter, but the top speed remains the same, correct?
    I was wondering the same thing.....
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    I like the nature of the OP's thought-process regarding this idea, and I many times found myself thinking along similar lines: port the head, get a higher-compression piston, and add a cam -- all while still keeping it ridable and reliable and 125...…but it is the whole deep, dark, and confusing world of the fuel-controller requirement which leaves my head hurting and halts me in my tracks.

    I will be really interested to see where this thread goes.
    50-50 and Davey K like this.

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