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Discussion Modification for Honda Singles

Discussion in 'Honda 250cc Singles' started by Gen, Jun 4, 2023.

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  1. Gen

    Gen Well-Known Member

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    @Andych

    I have a couple of toys you could use to unleash the beast, the cam on the left peaks out @ 9.000, the other,,14,000 :D

    tighe.jpg
     
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    Gen, Jun 4, 2023
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  2. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    Get the behind me Satan... :)
    Hmm, could be interested in the 9000 one.
    14000 ... OMG the poor little Honda.
     
    Andych, Jun 4, 2023
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  3. Gen

    Gen Well-Known Member

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    A bloke named Nigel Cammon tuned junior solos, the 356 grind had it's turn on the dyno, at 14,000 he had to walk away , it was still developing power, I put into my LSR postie with a 125 top on it, it still had the auto clutch, which took up @ 3,000, the cam just laughed at me, no movement in the take off department, the 399 grind went on to live a happy life in the postie, & a 175 I built for a mates son, after My missus barred me from "wasting my money"
     
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    Gen, Jun 4, 2023
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  4. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    Well let me know if you want to move the "little " cam on... :)
     
    Andych, Jun 4, 2023
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  5. Gen

    Gen Well-Known Member

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    I've become attached to it Andy, and the "closed chamber" head it likes, 'ol Ivan Tighe made the valves for it too , I'm sure they are still available from https://tighecams.com.au/
     
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    Gen, Jun 4, 2023
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  6. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    No worries... I doubt I will go down that track. It all depends on what it is like when finished... My memory of it from 50 years ago is that it was a lovely little bike... now I am a touch larger etc it may end up being a right pig and I will want to sell it.
    Then again my memory could be correct and I might decide to keep it for some fun... then I might look at the engine.
    For now it is enough just trying to get finished or to a point where I am happy to leave it and get back on the SRX.
     
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    Andych, Jun 4, 2023
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  7. Allan

    Allan Well-Known Member

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    I took a different approach to improve the performance of my SL125.
    I swapped the crankshaft for a stroked crankshaft that took the displacement to 145cc.
    I then fitted the cylinder liner from a 305 Honda and installed a 750 Honda piston. That took the displacement to 175cc.
    Since the valves were originally designed to feed 100cc, the engine was running out of breath at around 6000 rpm instead of 9000 rpm.
    A hotter camshaft from the USA solved that problem and running on alcohol with a compression ratio of about 13:1 gave a further boost. I tried higher compression ratios, but the head gaskets didn't survive.
    I used to race the bike at local short circuit meetings and I could out-accelerate 125cc Honda Elsinores that were supposed to develop exactly twice as much power as a standard SL125.
    It was such an easy bike to ride. Unlike the two strokes I raced against, it had a power band a bit like a steam engine or an electric motor. It felt as if it was a case of "if the engine is running, it is in the power band."
    The SL 125 in standard form has one of the flattest torque curves in existence. As a 175cc, the torque was phenomenal.
    In fact I built and fitted a side car and still had performance similar to a standard SL125
     
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    Allan, Jun 4, 2023
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  8. Gen

    Gen Well-Known Member

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    Memories,,

    mistress pics 001.jpg

    assembled crank.jpg

    mistress pics2@more 082.jpg
     
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    Gen, Jun 5, 2023
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  9. Allan

    Allan Well-Known Member

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    Did that version of the stroked crankshaft use a spacer and a longer cam chain?
    The version I had heated and shrank the conrod so that the piston only came up to the original position. That meant it went down a long way, so most of the skirt was cut away apart from the little bit between the flywheels.
     
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    Allan, Jun 5, 2023
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  10. Gen

    Gen Well-Known Member

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    The stroker crank lives in a Postie bike, with a cb/xl 100/125 barrel, modded XR200 head. the oil pump drive shaft had to be machined to clear the big end, it uses a postie cam sprocket on the crank, & the cam sprocket, also postie, needed to be modded to fit the 200 head (different tooth count) , when the motor was stripped down after a week of salt lake racing, I found the cam sprocket had fractured & was in pieces, but it still ran,,,
     

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    Gen, Jun 11, 2023
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  11. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    Andych, Jun 12, 2023
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  12. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Are those cams as asymmetrical as they look in the photos?
     
    ruckusman, Jun 12, 2023
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  13. Gen

    Gen Well-Known Member

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    Yes, like a pair of DD'd on an older jogger, lift smoothly, drop rapidly.
     
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    Gen, Jun 12, 2023
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  14. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Interesting, wonder if they float off that nose at high RPM.
     
    ruckusman, Jun 12, 2023
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  15. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    At 14,000 rpm anything could be happening


    Sent from my iPhone using Tapatalk Pro
     
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    Andych, Jun 12, 2023
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  16. gregt

    gregt Well-Known Member Premium Member

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    If you plot the lift curve at the valve it'll be symmetrical. The leading and trailing cam followers have a constantly varying rocker ratio as the contact patch with the cam moves in and out.
    Which is why, with this type of cam and follower, the lobe center figures should be checked at running clearance.

    And, with springs of the correct strength, it shouldn't valve bounce. A very similar Suzuki I built,- destroked GN125 with 200 bore for 150cc - pulls 14,000 rpm. 200 valves in a 125 head Around 9mm lift.
     
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    gregt, Jun 13, 2023
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  17. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    Seeing as I asked for this thread I should add to it... I decided to go Big Bore (150cc) on the little SL125 and bought a 61mm piston kit from the UK, cheap enough and good feedback.
    Disaster struck when the barrel was bored and the lower skirt of the sleeve ended up breaking through in one section and was shim material thin on the rest... not a good result.
    IMG_7300.jpeg

    Once I knew that had happened I started searching for alternative sleeves to put in the 125 barrel... Most information came back that the CB77 sleeve was the way to go but I wanted to have some dimensions to make sure it would work.
    In the meantime I have purchased a "Big Bore 150cc kit" from Ali Express which should arrive around the 8th August.

    Today I managed to get info back from a supplier of raw sleeves (not bored to size) to suit CB77 305cc twin. It seems it may well not be suitable either based on the numbers I have been given which are below.
    These are from LA Sleeve and in Inches... MM sizes are next to each dimension in brackets.

    Part number is H-21 and sizes are:
    2.836 FLG OD (72.0344)
    .145 THK. (3.683)
    2.522 OD. (64.05)
    2.431 LOWER SKT OD. (61.74)
    3.800 TOTAL SLEEVE LENGTH. (96.52)
    2.180 ID SEMI. (55.372)

    While I havent measured the length of the sleeve or popped the old sleeve out to measure the OD in the barrel and flange the indicated Lower Skirt is too small for a 61mm bore. It only leaves .74mm (0.37mm per side) of material.
    If the sleeve actually fits into an SL125 Barrel a raw sleeve would be better and then have the lower skirt machined to just a hair under 64mm so it goes into standard crankcase opening.
    I am not going to bother with that as the cost of the kit from China was less than the cost of bore and hone let alone adding in a sleeve etc... unless of course you really wanted it to be a sleeper and still show 122cc on the side
     
    Andych, Jul 28, 2023
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  18. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Andy, go through the catalogue on the Advanced sleeve website if you get stuck - you can search using millimetre dimensions
    This one comes straight up searching for 65mm, for a 61mm-65mm bore 69mm sleeve O.D.
    https://advancedsleeve.com/advanced-sleeve-3618fa-for-honda-cb750k-61-65mm-powersports-engine/

    Also there's all purpose sleeves from LA Sleeve - there's a tab for flanged sleeves if that's what you need
    https://www.lasleeve.com/tech/all-purpose-sleeves

    You may need to have the upper case opening bored larger in any case
     
    ruckusman, Jul 28, 2023
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  19. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    Cheers for that... hopefully I wont need to worry as my high specification Ali Express Big Bore kit will be bulletproof.. .:neo:
    I guess the info I posted was more about dispelling some of the myths around going big bore on these little engines. Lots of people saying you can bore them to 61mm or you need this or need that. The same as saying a CB750 pistin works.. and it may well work but the ring grooves are in a different spot and is the offset on the gudgeon correct etc.
    Back in the day when these were heavilly modified, more for Trials using the TL125 there was little opportunity to catalogue the info and it was all passed around via word of mount / experimentation etc.
    Plus there was no Ali Express either... I mean for a smidge over A$100 i get a new barrel, piston, gudgeon pin, retainers, head gasket and base gasket with a 61mm bore. If it wears out I would just buy another one as it is more expensive to bore it out and get a new piston and rings lol
    Let alone having the sleeve removed, reboring the barrel, machining the sleeve skirt to suit and then boring and honing to suit... that is more like a $250 plus machining job, then the cost of the sleeve and pistin etc. Just not worth doing on this old bike.
     
    Andych, Jul 28, 2023
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  20. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    I've bought a few bike bits from aliexpress, not heaps, but enough to have an informed opinion - the quality is very, very good to excellent, it seems that the days of junk have passed.

    China is now, more or less the world's factory and they're paying attention to quality control
     
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    ruckusman, Jul 28, 2023
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