Vintage Dirt Bike Q & A

Sooooo, here's the deal.

Several years ago I bought the bike to go vintage racing...from the original owner...very cool! Anyway, the bike has absolutely NO bottom end power. I'm not talking "it's weak down low" I'm saying I cant even move the thing in first gear if it's not pinned WFO at redline.

See, what happens is the bike bogs horribly and "double carburets" badly. I know because I took off the air boot and can see the gasoline spray coming back OUT the carb and getting sucked back in a second time.....BOOOOOOoooooogggggg .

Pressure tested the motor and it held 6psi for 5 minutes. (per Motion Pro tester specs)

Piston looks good and clean, rings all loose and free...

No gnarly scars on the cylinder, just the usual wear spots.

I even made the mistake of ordering a replacement Mikuni 28 from those total idiots at Carb Parts Warehouse. Who sent me the entirely wrong carburetor and took over a YEAR after I returned it to get me my money back....but that's beside the point.... And it made no difference at all.

SO... Now I'm looking at the cylinder, and it appears as though the bottom of the intake port has been massaged with a porting tool at some point in it's life... Is that the "hot set-up" for top end only power?

Interested in hearing your thoughts...

Skiddy

When I briefly ran my '74 CR engine with a modified piston (@ 6mm shorter rear skirt length) it had these same symptoms - my air filter would actually get wet with gas, it was pretty scary - so I went back to stock. The shorter skirt drastically reduced low end, while adding almost nothing on top.

If your intake port floor has been cut away and it's running the stock piston-skirt length, you might try an MT125 piston (not an MT125R piston!), as I believe most of the Honda 125 pistons use slightly different skirt specs and the MT125 should be the longest. BTW, the MT125R road-racer engine used radical intake timing & wouldn't run at all below 500 rpm, it had to be push-started (no kickstarter).

You can also try jetting the carb very lean on the pilot circuit (#48 - #52 pilot, unworn needle jet), which should at least give you some initial throttle response to get the revs up; more compression & ignition advance wouldn't hurt either. The best fix would be another '75 CR cylinder with standard intake timing.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Upon discussion with a "old timer" racer who raced back then, he inspected the cylinder and thinks it may be stock with the suspected porting being done from the factory .

I admit, it's not a very polished port job, kinda looks rough and OEM quality.

I wish I could get my hands on a stock port spec sheet.

I'm also looking for the stock jetting specs for the 75 cr carb...I'm taking off the Mikuni as the bike ran better with the stocker....

Skiddy

The lowest point of the bottom edge of the intake port windows on my bone-stock '75 boost-port CR125M cylinder is 23.5mm from the bottom edge of the cylinder liner. The ports & passages look like every other stock 70's Honda 2t cylinder I've seen; minor casting flaws, mild surface roughness, sometimes evidence of touch-up with a grinder at passage-liner transitions only (I've never encountered visible OEM grinding on the port windows themselves, but YMMV).

The '75 056A carb specs I have are 128 main jet, 50 pilot, 200 air, 271003 needle, 3.5 throttle valve. I didn't realize Honda went from a 60 pilot in '74 to a 50 pilot in '75 - you might want to try 42-48 pilots in your carb just for grins.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Mahalo Ray !

Ok, my carb had a
125 MJ
40 SJ
271003 Needle
3.5 slide

Keyster kit contains
"D42" needle
128 MJ
50 PJ

I'm installing now... I already put motor together so I can't check your port specs...but I have a sneaky suspicion it will be apart again tonight....not expecting miracles with just a jetting change..

Skiddy

Well...that didn't take long......still runs like shit.
Plugging along......

I wasn't expecting much from the jetting change either (which it looked like the previous owner had done already anyway - a #40 jet is super lean), but it was worth a try just in case.

If the problem is too-late intake port closing and replacing the cylinder isn't an option, the easiest fix would be to build up the ground-away port areas with JB Weld, then file & level them to the desired timing (I'd suggest 26mm above the liner bottom edge to start; you may really like the added bottom end and quicker throttle response, and if not you can always remove material). This will work - if the port/passage surfaces are prepared properly JB Weld will get the grip-of-death on them (I rub a thin initial coating hard into the metal, then add add'l coats to that as needed) the piston skirt will barely touch the JB Weld after initial break-in, temps are well below the @ 400F max for JB Weld, etc.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Maybe not recommended but I cut 10 mm of the bottom of my piston, use a 81 CR125 rod kit with a 30mm Mikuni & FMF pipe, slightly ported cylinder and the bike flies. Yes I lost some on the bottom end but it hits hard in the middle & on top. On the lower end it's not so much of a bog but a very slow response meaning to keep a finger on the clutch when needed. Weird but I tried a 15 counter tooth & 53 tooth rear & it was almost like the symptom you talking about. Went back to the stock 14/49 for the 74-75 CR125M.

Just sounds like carb or cylinder issues. I would do what Ray says and try to find a clean 75 cylinder with the boost port.

Good luck