Vintage Dirt Bike Q & A

I never had time to tune correctly my rebuild CR125. When I get it out more than a year ago, the first time I tried to ride it, I had problem with the crank seal. I bought them on internet. It was the good size but it was cheap one lip seals and they didn't resist the first ride. Last year I found good crank seals and replace the cheap blow one.

Yesterday I get the CR out of the basement. It start on the second kick. I adjusted the idle and it is running with a good idle. But when I open the throttle, it run like it was on the choke and can't rev correctly. If I try to open the throttle to rev it, it rev a little bit and then stall.

The main jet is a 160 and I tried to run it on the 2, 3, 4 and 5 clip position on the needle. It seem to run a little bit better on the 5 position. The air screw is at 2 turns and the idle screw is at more or less 5 turns.

I cleaned all the jets and the floats valve, I recheck the floats level to 20 mm.

I forgot to test if the boot is sucking air, I'll try that tomorrow or on the weekend.

Any idea of what can be wrong?

_________________
1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i

If it's the stock carb (stamped 005A behind the choke lever), a #160 main jet is way too rich; stock is #125 (Honda P/N 99101-357-1250). I'd check the pilot jet too (stock was #60, I'd use a #55, Honda P/N 99103-360-0550). If the pilot jet is 28mm long, it may be hard to find.

Jets 'R Us has the main jets:

http://jetsrus.com/a_jets_by_carburetor ... 25-xxx.htm

Ray

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

Thanks Ray, I'll check for new jets! And yes, it is a 005A.

_________________
1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i

Ray, you are the best!!!

I found a 128 main jet in my old parts stock. I replace the 160 with this 128 and the bike is running very good now. Can't wait to try it!

I can't ride it where I live but in 3 weeks there's a big Vintage Off Road Festival in Ormstown, Qc. I'll bring it there and I'll do a test ride... and maybe a race if it run fine.

Thanks again!

_________________
1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i

That's great!

When you do your test rides definitely try different needle positions, you should find that needle adjustments have a lot more effect with the smaller main jet. I usually start with the #1 (top / leanest) clip position followed by the #5, to get an exaggerated idea of the lean/rich performance difference, then try the remaining ones.

Ray

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

I'll try that Ray and I'll let you know the results.

Thanks again!

_________________
1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i

I went for a test ride today.... not very good.

I try with a 128, 125 and 122 maint jet and with different needle position. The best result was the 128 with the needle at position 1. It idle good and when I blip the throttle it rev crisp and look ok. But when I try to ride it, the engine chocked and want to stall. When I pull the clutch and try to rev it, again it is like an engine on choke, no power and want to stall.

I did like a race start, rev it a lot before relase the clutch. It run good for a couple of second but as soon as I release the throttle a little bit, it bugged again.

Today the wheater was sunny and hot, around 80° - 85°F.

I checked the spark plug and it is not very wet. Dark brown but relatively dry.

There's a thing with the pilot jet. The one in the carb doesn't have anything writen on it and the only other one I have is a #50 but 32mm. Too long I need a 28mm

I spray some carb cleaner to see if the boot is craked or take some air somewhere. Nothing I spay a lot and the engine keep running at the same idle.

When I raced those bikes, 40 years ago, I don't remember having any problem with the jetting. Is it possible that this old card is have to much wear? Wrong throttle valve? Do those old carb are good or it's better to buy a brand new Mikuni?

_________________
1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i

40 years ago, the carb on your bike surely worked perfectly. If it's in the same condition as the rest of your bike, it can probably be made to work perfectly again - I've done it with 4 of these 28mm Keihins (004A, 005A, 060A, 092B). Or you can throw a Mikuni on that otherwise all-original bike; I wouldn't do it, but many would.

Here's a test - start the bike, then turn the fuel petcock off and keep it running until it runs out of gas (5 minutes max). Keep your hand near the kill switch. Your 'mystery pilot jet' could be way too big, like the main jet was. You may also have a worn needle jet, incorrect slide (stock will have 3.5 stamped on the bottom) or some parts missing - without a picture I can only guess.

There's a correct #55 28mm pilot jet on eBay right now. The float bowl can also be modified slightly to accept the longer 32mm jets; I've done this with two of my carbs, it takes about ten minutes with a Dremel tool.

What is your compression pressure?

Ray

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

Ray, I agree with you, I also prefer to keep the stock carb if possible.

I checked with a local Honda dealer in Montreal and he said he can have a #55, 28mm pilot jet for a Keihin from a distributor (not Honda) and they have one in stock. I will get it in a few days, $5! So I'll wait for this before doing any other testing. This way I would eliminate this possibility.

I didn't check the compression. It's a brand new 1rst oversize with a new Wiseco piston/rings. I checked the piston and rings gap when I rebuilt it and everything was on specs.

What is the purpose of the test you propose to do with the fuel petcock close? Usually on the last drops of fuel, the engine start to rev higher.

I will also check the slide to see what size it is. At the same time I'll take some picture to show to you.

Thanks!

_________________
1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i

Ray, I just noticed that you are in Connecticut. 2 Weekends ago, I was in Rhode Island to see the World Trial Championship. We went to Rhode Island via Connecticut. We rode there on our street bikes from Montreal. That was the first time for me in Rhode Island, and the place for the trial was really nice. The trial was really amasing!

_________________
1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i

stou,

I hope the Honda dealer gets the right 28mm pilot jet for you - Keihin makes at least four different types in the 28mm length. In case there's a problem, here's the correct type's dimensions:

http://jetsrus.com/a_jets_by_carburetor ... _slow.html

...and here's the one on eBay:

http://www.ebay.com/itm/NOS-OEM-Honda-S ... 9e&vxp=mtr

This seller has a number of these jets, in different sizes - the 99124-076... / 99124-300... / 99103-357... / 99103-360... / 99103-361... part numbers will all work...

http://www.ebay.com/sch/m.html?_odkw=ke ... t&_sacat=0

Yes, a properly-jetted carb on an idling motor will increase idle speed a bit as the last gas runs out, then go ying! ying! and stall. A too-rich carb on an idling motor will scream its brains out as it runs out of gas, which is why I mentioned the kill switch. I think a too-lean motor will just stall.

Thanks for the info on the trials championship! I don't know if I could have made it, but I would have liked to see one - and I don't think there will ever be one closer to where I live than Rhode Island.

Ray

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

Unfortunately, this was not the right jet! Right size 28mm, #55 pilot jet but the thread size is wrong.

This is what I received:


And this is what I need:


This weekend I'll look to modifiy the bowl to used the #50 32mm I already have. You said that you did it to two of your carb, so there's no reason I can't do it on this carb.

If for any reason it doesn't work, I'll order one on the internet. Or maybe I'll wait for the weekend of August 22-23, we have a big of road festival with trial, hare scrambler and motocross race. Last year I saw many guys with Elsinore. So maybe they will have some jets for sale.

_________________
1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i

stou,

I don't think either of the jets in your pictures are correct for the 004A / 005A carbs.

Look at this (correct) one carefully:

http://www.ebay.com/itm/NOS-OEM-Honda-S ... 46255f6f9e

The threads are small-diameter and fine pitch, and the tube with holes is long and thin. The most important measurements are the thread diameter / pitch / length, thin-tube diameter / length, and overall length. The location of the orifice inside the jet is also important, but not really critical in my experience.

If you want to modify the bowl, just install the 32mm jet & put a drop of paint on the end of it, then put the bowl on with correct alignment until it contacts the paint. Then grind away an area about 6mm diameter x 6mm deep around the paint mark. This will be where the threads of the brass drain plug and bowl meet, so you won't be able to use the drain plug anymore (I never found it very useful anyway).

The 32mm jet pulls a little less fuel than the 28mm for the same jet number, so a #55 32mm is like a #52 28mm, etc.

Ray

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

Yes, you're right... again!

I didn't have the time to look at the 32mm mods today. Anyway, I bought the good one for the 005A on eBay.

But if I have the time, I'll give a try at the 32mm. And thanks for the hint about the paint drop!

_________________
1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i

Ray, I looked at the pilot jets this morning. With my eagle eye, looking carefuly the hole size with both jets side by side, I can see that the 38mm #50 jet is smaller thant the wrong #55 I bought this week. But the wrong #55 look the same size that the one in the carb. So from eye checking it look to be the right pilot size, a #55.

The thottle piston is a #3.5.

About the needle, the one in the carb as the number 271303. I have a new one with number 042 (see picture).

The size of the needle are:
271303: straight part 2.41mm lower - taper part 1.77mm
042: straight part 2.50mm lower - taper part 2.12mm






I noticed that I bought BR9ES spark plug and the manual said B9EV. I don't think it can make a big difference... ?

I begining to think that it can maybe be a timing problem. I never checked the timing with a timing light. I only aligned the marks of the stator and the crankcase. As long as I remember, 40 years ago, I never had to check the timing with the timing light. What do you think?



I did some tests with for the float level. I removed the carb from the bike, removed the bowl, I put some cleaner in a temporary fuel tank and measure when the fuel start to flow. At 20mm, there's no flow, the valve is closed. It start to flow at around 22mm.

Do you think this is enough to go out of gas? Can 2mm on the float level can make a big difference like that? Enough for the bike starting and idle fine but not running properly?

_________________
1974 Honda CR125M - 1977 Honda CB750 - 1982 Yamaha XT550 - 1986 Honda TLR200 - 2016 Honda CRF1000L Africa Twin- 2012 Ossa TR280i