Honda TL-125 FAQ
This is the TL 125 FAQ page as it was on the Thumper Pages, retrieved with help of the Internet Archive Wayback Machine!
Return To The Thumper Page

Honda TL-125 FAQ

Maintainer: Jarmo Kaila
Version: 2.4
Date: 12/16/2001

1) What engine did Honda put in the TL125? Is it the same engine that was used in the CB/CL/CT/SL/XL?

Jarmo Kaila:
All of these small (100 and 125 cc) Honda engines are pretty much the same. Even when the parts numbers don't match they are often interchangeable.
Proper caution must be excercised if trying to attempt cam/piston swaps for increased displacement. Some of them could result in negative piston-to-valve clearance, which will have non-desired effect on the engine performance .

Bob Ginder:
All these engines are very similar. Comparing the TL engine to other older small Honda thumpers the cases, ignition and crank (heavier on the TL) are different. Barrel and head are the same, although there are differences between different year models.

2) How do I identify TLs? What variants are there?

Jarmo Kaila:
ModelYearSerial noBore & StrokeDisplacement
TL125 (K)1973Frame TL125-1000003 onwards
Engine TL125E-1000001 onwards
56.0mm x 49.5mm122cc
TL125 K11974Frame TL125-1100001 onwards
Engine TL125E-1100001 onwards
56.0mm x 49.5mm122cc
TL125 K21975Frame TL125-1200001 onwards
Engine TL125E-1200001 onwards
56.0mm x 49.5mm122cc
TL125S 761976Frame TL125S-1000417 onwards
Engine TL125S-1000001 onwards
56.5mm x 49.5mm124cc

Generally the TL125 engines can be divided into one-piece-head (1973 to 1975) and two-piece-head (1976). In the latter the cylinder head breaks horizontally into two, camshaft in between.

The TL125 was re-introduced in the '80s in Japan as model "Ihatovo". It was made for a few (3?) years, starting 1981. The main changes were that front forks were changed to semi-air assisted type and rear shocks used were different (up-side down). There were also a number of cosmetic changes, such as silver frame, black hubs etc. (Thanks for TL owner Sinzi "Sin" Okayasu for this info).

3) Are other parts on the bike from other bikes (besides the immediate "family" of Hondas)?

Jarmo Kaila:
Shift lever splines are the same as many other bikes. For example, MSR part #07-0735 for YA-5 (for CR-80) makes a nice folding tip shift lever after a little bit of bending. This mod will get rid of the shift linkage, but will of course make the shaft more vulnerable since any hits to the shift lever are transmitted directly to the shaft (most bikes have the shift lever directly mounted on the shaft so I don't consider this a very serious risk).

4) What are the common problems/weaknesses with the TL?

Jarmo Kaila:
1.) The worst weakness must be the infamous 4-stroke off-idle cough and hesitation. When suddenly opening the throttle off of the idle position the engine "coughs", hesitates for a while and then goes (well, goes as well as you can expect from 4.5 HP). This feature certainly rewards a smooth riding style. Big bore or stroker kit should help. Note that pumper carbs are not allowed in the AHRMA "modern classics" class (although I think they should be).
2.) If you experience any difficulties in stalling the bike in slow speed sections you may want to try retarding the ignition. This will result in slight loss of top end power, but you won't notice anything in trials riding (maybe different for street riding where you really get to use the top gears with WOT). My recommendation is to set the static ignition timing correctly and scrape a mark on the ignition points back plate. Then set the ignition to fire at TDC (there is a mark on the flywheel for TDC) and make another mark. Set the ignition between these points and start experimenting, changing the timing little bit at a time until you are satisfied. On my big bore bike I'm running at almost exactly the half way point.
3.) The camshaft bearing surfaces are machined directly on the cylinder head without any sleeves. Luckily there are companies that can machine and fit proper bearigs (see question 5 below).
4.) The cam chain adjustment is very counter-intuitive, even if you have access to the Clymer service manual. You have to "unscrew" the tightener to tighten cam chain (manual just tells you to "turn the tightener until resistance is felt"). When I got my bike the tightener was screwed in all the way, resulting in so loose cam chain that it had jumped 2 teeth on the sprocket.
5.) TL has a tendency to "fall into a turn", i.e. in tighter turns the bike wants to increase the steering angle and one has to hold back against the bars. For more discussion on this issue see item 7.
6.) Sticking clutch is a common phenomena experienced by many bikes with wet clutches, but TL seems to be especially sensitive. It is caused bu clutch plates (both friction and metal) wearing extremely smooth, so the oil in between them is squeezed to a very thin film and acting almost like glue. Differencies between bikes are mostly attributed to different metal plate materials, although there is anecdotal evidence of different types of oil causing more or less sticking. One known cure is to roughen the metal plates so they retain more oil. I used to do this with coarse sandpaper, the effect would last for a few months of very active riding. Then Bob Ginder of B&J Racing performed his "knurling" on the plates and I haven't had to touch the clutch since. Both operations also improve the clutch action, the start of clutch slip begins more progressive. If you decide to do plate work do it so that the plates are not warped! If you warp a steel friction plate it is almost impossible to get it straight again.

Bob Ginder:
Don't use silicone sealant on the top end rebuild. There is a small oil passageway that is very easily plugged if sealer is used, resulting in immediate top end failure. Also, occasionally remove the clutch cover and clean the oil slinger. TL has no oil filter.

5) Are there any companies in the US or elsewhere that sell hop-ups, additions, etc?

Jarmo Kaila:
TL125 is so new bike that your friendly local Honda dealer < grin > should be able to help you with most of your standard parts needs. At least 1997/1998 such unusual items as rear shocks (w/o springs), fork tubes, seat covers and side covers were still available.
Here is a list of some companies that seem to have considerable experience with the TL125:

B&J Racing Products
Big bore kits (150cc), head flowing, various vintage and modern trials parts and accessories, almost any part, accessory or work you would need for the TL (new, aftermarket and used parts, fork reconditioning etc. etc.). B&J Racing also makes custom control cables and custom foam air filters.
Telephone: +1-615-789 5956
Fax: +1-615-789 5725
Address: 1204 Big Bartons Creek Road, Dickson, TN 37055

Best Motorcycle Repair
Big bore kits (134/146/160/175cc), cam bearing repairs with precision bearing inserts, Kong cranks, strokers using Honda 185/200 cranks, full service CNC machine shop services, some used TL parts.
Telephone: +1-540-552 0722
Address: 3676 Old Creek Road, Blacksburg, Virginia 24060

David Silver Spares
Parts supplier in the UK. They also manufacture special exhaust systems for the TL.
Telephone: +44 1728 833020
Fax: +44 1728 832197
Address: Unit 14, Masterlord Industrial Estate,Station Road, Leiston,Suffolk, IP16 4JD,United Kingdom

Powroll Inc
Performance stroker cranks, big bore kits, head porting.
Telephone: +1-541-923 1290
Fax: +1-541-923-5637
Address: 4211 SW Mountain Pkwy, P.O.Box 920, Redmond, OR 97756

Sammy Miller Motorcycles
The original TL tuner. Lots of parts, spares and accessories. They also manufacture new TL125 tanks and side panels as well as special exhaust systems.
Telephone: +01425 616 446
Fax: +01425 619 696
Address: Bashley Manor, Bashley Cross Road, New Milton, BH25 5SZ, England

Speed & Sport
Speed & Sport has commissioned a batch of fiberglass replicas of the original TL125 fenders. They run $100 for the pair, and are now in stock. They also have in stock grey plastic universal fenders, the Sammy Miller front fender mount, and various other trials accessories (h'bars, grips, twistgrips, chain tensioners, tires, etc.)
Telephone: +1-562-490 0012
Fax: N/A
Address: N/A

For more suppliers see the TL125 Parts Page at the Honda TL125 Home Page.

6) Since the engine seems to be the weakest part (performance wise, not on reliability) of the TL125 package tell me a little bit more about the available engine modifications?

Pam Falcioni (Powroll Inc.):
a short run-down of what we have for the early style (one piece head) 125's:
135cc Bore Kit - Bores into stock liner
145cc Bore Kit - Requires Resleeve, no case machining
9.5mm Stroker - we do the work on your stock crank assy
Other things - hardweld mid to top end cam - we regrind your stock cam, oversize header with supertrapp - down pipe, heavy duty valve springs - recommended for race engines using our cam. We can also do any machine shop services, plus performance head work (porting, oversize stainless-steel intake valves, etc).
Basically, what we do on a stroker is this:
-Move the crank pin location outward on the flyweights (using an offset bushing)
-Shrink the stock rod assembly
-Reassemble the whole thing
This makes the piston travel further down in the cylinder (this is where you get the increased displacement). The stroker also makes it easier for the crank to "push" the piston through it's stroke, making it easier to generate torque.
Since we shrink the rod - everything goes back together just like stock parts. You can use any piston you wish - the only thing you'll have to check is to be sure the skirt of the piston is short enough to clear the flyweights at bottom dead ctr.
The 9.5mm stroker is the only one we offer for the 125 - If you do the stroker alone, with a stock piston, you'd end up with about 150cc. Probably about 25% more horsepower, and 30% more torque than stock. You can also put the stroker with either one of the bore kits (135cc w/ stroker = 160cc, 145cc w/stroker = 175cc).

Hoyt McKagen(Best MC Repair):
Stock-Parts Big Bore combos for TL125:
Bore, mmDisp, ccPistonLinerH GasketNotes
58.5134CB550125*/CB5501251, 2
61146CB750CB550*/750Special/1853, 4
63156XL185'180' jug1853, 5, 6, 7
651663Wheeler'192' jug2003, 5, 6, 8
65.25167XL/XR 200'194' jug2003, 6, 8
65.5168XR200R'195' jug2003, 6, 8, 9
1Bore-out or may use CB550 liner if cases bored. Deck hgt slightly less than 125, use .010" base gasket.
2Re-cut int valve pocket.
3Re-cut both valve pockets.
4Uses bored CB550 liner. May use CB750 liner if cases bored.
5Pull-start motor uses low-compression piston.
6Cases must be bored. Jug number is found on rear of cylinder base. These have cast-in liners, are heavier construction and have more fin area. All cast-ins can be repaired with CB350 and/or CB750 liner.
7May use CB750 liner in 125 block.
8May use CB350 liner in 125 block.
9No factory overbores available.
Common cause of scrapping these motors is burnt cam bngs in head; repair service is available for this problem for above heads and others. Generally, rebalancing crankshafts is not necessary to use these bits with the TL, as revs are usually not excessively high, but cranks can be re-done if desired for smoother running with the bigger bores.
Entire 185/200 top end swap-ons require case machining only, plus longer camchain tunnel head-bolt. You may use shortened 185-200 cylinder blocks with 125 heads and cases, and use shortened camchain to put the bigger heads on modified 125 cylinders. Cams and rockers do not exchange between head styles. Valves are compatible between styles and those from bigger motors may be used for oversizes for smaller.
Compression ratio with original head will go up in proportion to increased displacement assuming no other changes, but in swapping on 185-200 top ends it will decrease. You can figure it from original size and CR, and new size. For example, a 168cc kit on 125 motor is running ~10.6:1 if it started as 8:1 and the smaller, low compression option offered by the 166cc piston would make it ~9.1:1. But a 194 jug and head starting with 9.5:1 would run ~8.4:1 on the shorter stroke. To help make up the latter or to increase compression in any motor, .025" annealed copper head gaskets are available. These should be used with a sealer such as Coppercoat gasket paint. Compression ratio directly affects torque and in motors which need to be responsive at lower revs it's desirable to have it fairly high; this can also help with the TL125 'hesitation' problem.
Above combos may be used with 100cc motors; all except 134cc need case boring, all need re-lining and/or bigger jug, and all need small rod end bored for the listed pistons. A 156cc motor may be made with no rod mods, by using 63 mm Suzuki GS750 piston and rings.
All base gaskets will physically fit any parts in above series. Those for smaller motors will need spigot opening enlarged for some big-bores, or suitable gaskets from bigger motors can be used. BG from larger motors will overhang some smaller crankcase bases but are still functional.
Honda has long held that a cylinder liner must have sufficient thickness to withstand on its own considerable loading; this is because at operating temperatures the liner is essentially retained only at the flange and more or less floats freely along its length. If it's thin, it can vibrate and cause ring leakage, and if it's too thin it can break. I realized long ago I wasn't going to hold the displacement record for this motor, as I liked to finish races. Though all of those above are reliable, sizes marked '*' leave little room for repair re-bores and none for additional displacement regardless of whose kit it is. Hence it's advisable to re-line even in cases where not strictly necessary and in any case to use largest liner available. It being about as easy to fit a 550/750 unit into the 125 as to bore stock one for size, re-lining is also cost efficient.

Al Johnstone:
There has been comment recently on oversize TL engines. I do not wish to unnecessarily prolonging the debate, but I would like to illustrate a point I made on the Message Board:
When overboring with the intention of using a CB750K2 piston the liner MUST be replaced with one of a larger outside diameter. I know that not everyboby agrees with this, the implication being that the old liner will suffice. This is untrue, it will fail. Sometimes it is claimed that a 122cc barrel wont do but a 124cc barrel will. This is untrue; both liners are of the same outside diameter and when bored to accept a 750 piston the wall thickness is left at 0.018" (less than 0.5mm). On the TL125 site there is a picture of my cracked standard barrel bored to take a 750 piston. As I bought the bike in this condition I cannot say how many miles or hours the engine ran for before it failed. However, it does not appear to have run for long as the piston is relatively unscuffed. In addition, because it is not possible to put a proper 'lead in' at the bottom of the liner there is a high risk of breaking the piston rings on assembly.

Jarmo Kaila:
I now have a big bore kit in my TL125, and the difference in engine response is amazing. IMHO nobody should try competitive riding on a TL without one. Engine response improved greatly, practically no "advance" is required to lighten up the front wheel. Generally any big bore kit will increase the compression ratio, exact amount depends on the piston used. I retarded the ignition and run only slightly over 0.02" spark plug gap (0.6mm). This makes the engine less "snatchy" at low rpms, making it more difficult to stall. I can't notice any loss of top end from the retarded ignition (now even tried to compare on 4th gear WOT hill, less difference than I could time with a stopwatch). Wit a big bore mill the engine braking naturally increases, this takes some getting used to.
The piston in my TL is 65.5mm, making the displacement about 167cc. I've received a lot of e-mails about modifications people used to do to TL's, seems that in the mid to late 70's a lot of people tinkered with the TL. The biggest displacement anybody pushed the TL in the past that I've personally heard of is 199cc (=65.5mm bore x 59.0mm stroke). There are rumours of over 200cc bikes being run in the 70's, if anybody can confirm this please drop me a line.
I think the absolute max for the TL engine is 223cc (=67mm bore x 63.3mm stroke). For more info see the Story of the Big Bore TL.
IMHO if you decide to add a big bore kit that requires opening up the cases go all the way and install the largest you can fit/find, ie. something with at least 63mm bore. Also, if you decide to go ahead with the stroker kit I'd have the cases bored to accept a larger liner while you have the engine all torn apart, this way adding displacement in the future will be rather painless operation.

7) How About Frame and/or Suspension Modifications?

Jarmo Kaila:
As mentioned earlier TL has a tendency to "fall into a turn", i.e. in tighter turns the bike wants to increase the steering angle and one has to hold back against the bars. During turning the bike is also very sensitive to obstacles, and has quite a bit of "bump steer". These handling characteristics are probably a combination of "tall" triple clamp (ie. one where fork tube plane is offset substantially from the steering stem), high rake angle and relatively high center of gravity and foot peg location.
I been experimenting with various modifications to alleviate this problem while still keeping the bike in AHRMA "modern classics" class. Easiest improvement is raising the fork tubes up in the triple clamps. Raising the tubes (ie. tubes sticking up from the top triple clamp) just 10mm to 20mm (1/2" to 3/4") makes a difference and doesn't reduce ground clearance to any noticeable effect. The next step is changing the rear shocks for longer units. Original Honda units are 14 1/2" long (370mm), I have swapped them for 15 1/4" (390mm) Works Shocks units from B&J Racing. These modifications together changed the measured rake from 28.5 degrees to about 25 degrees (note: these were measured so they may not be absolutely correct, however the decrease in rake is about 3 to 3.5 degrees).
To address the triple clamp offset issue I actually changed the whole front end for a unit from mid 70s Yamaha TY250. TY forks are slightly shorter than TL, so they had the same effect as raising TL forks in the clamps.
Of course I'd like to hear from anyone who might have tinkered with the TL frame geometry etc.

8) Any must-have accessories and/or modifications?

Bob Ginder:
For competition bike the following items are recomended: good aftermarket shocks, good tires, good airfilter and Renthal hadlebars. A big bore kit obviously helps.

Jarmo Kaila:
TL has a little spring that can be used to "lock" the left footpeg to "up" position, however in most bikes I've seen this spring is lost or broken. So although TL125 is quite easy to start it is probably a good idea to remove the spring from the right hand footpeg. This lets you lift up the footpeg, you get almost 1/4 turn more kick start movement. Especially advisable if you install one of the big bore kits.

If you plan on doing any engine work yourself get the "Honda oil filter spanner wrench", the special socket used to get the oil filter nut out in all these small engines. This is required just to access clutch, for example. The nut is torqued pretty tight and it won't come out with needle nose pliers without ruining the pliers and damaging the nut (don't ask). Honda part number for "Locknut Wrench 20x24" is 07716-0020100. Motion Pro also makes this tool as their part #08-015.

9) How about oil and fuel requirements for the TL125?

Jarmo Kaila:
Due to the 4-stroke engine of the TL you can't experiment with the engine oil as freely as with a "normal" 2-stroke bike. Also for this reason my favorite trials bike lubricant (synthetic ATF fluid) is absolutely out of the question. The challenge is to find a good lubricant that gives smooth clutch action without dragging or slipping.
My first tries were with "traditional" high quality engine oils, such as Mobil 1 and Castrol Syntec. Both caused clutch slippage in the big bore engine, probably due to the anti-friction additives (and occasional drag despite having knurled clutch plates). I have now been using Silkolene Racelube (20W/50) for some time, and been very satisfied with the clutch action (due to unavailability I have not been able to try my first choice, the MC version of Mobil 1). If you are interested in MC oils see QuackFAQ for a good article on MC oils.
Big bore TLs tend to be a little bit finicky on their fuel requirement. Especially during hot weather my TL pings quite easily when I try to use all of its low end torque at low rpm (ie. WOT from off-idle), especially if the fuel is not fresh. This is not really surprising, as these are the most likely conditions for pinging to occur. I just make sure I use fresh high octane fuel (93 M+R/2), and occasionally add a cupful of octane booster. Standard TLs should be perfectly happy with just pump fuel. For a comprehensive write-up on the subject of gasoline plese see autos/gasoline-faq.

10) What magazine articles have been done on the bike?

Ian Smith Information lists there being 5 articles in their Motorcycle Reports database. For more information see their homepage at

11) What is the going price for a good condition TL-125?

Jarmo Kaila:
Varies a lot, but I have seen decent examples for about US$800-1200. Expect to pay more if the bike is in cosmetically very good condition, or if there are extensive, well made engine modifications.

12) What resources are available on the net?

Jarmo Kaila:
The THUMPER web page
The THUMPER mailing list is home to some TL owners and lively conversation on single cylinder four stroke motorbikes. The Thumper Page has instructions on how to subscribe etc.
The Honda TL-125 Home page is by and far the most complete resource specifically devoted to the TL125 and has a TL125 message board for TL125 related discussions.


Pam Falcioni
Bob Ginder
Al Johnstone
Jarmo Kaila
Hoyt McKagen
Mark Worsfold


Honda TL125
Fantic 309
Suzuki RV90