Vintage Dirt Bike Q & A

With your help I've been able to get my 75 250 elsinore up and running my issue now seems to be a poor throttle response from idle and lower rpms but after that it seems to run great. Would like it to be more crisp from from take off any ideas?

Dave

Fuel, spark or compression.

Check carb. I ended ordering a FL250 (oddysey) carb kit from ebay (thanks user rayrivers)and replaced everything I could from that kit. It is not exact kit but the best we can get and all of the important parts are in there.
You may he running rich due to 45 year old "O' rings letting fuel pass.

Check timing. If points are old and pitted I suggest you replace with a new set and go through the hassle of setting them correctly.

Do a compression check. There is a manual that you can download from this site that will help you decide if low on compression or not.

Rick

Thanks for the thoughts I did replace the points and your right setting timing is a hassle I settled for probably 21 to 22 degrees BTDC I have a Keyser carb kit I haven't installed because the carb looked very good, also checked compression and found 150 psi. Wondering about the correct jetting to use or carbon build up? It starts first kick with no choke but will give me some kick back at times is that timing again?

Thanks Dave

Dave,

Some kick-back while starting seems to be normal for non-CDI Hondas of this era. Starting easily without the choke is NOT normal, though, and indicates a very over-rich mixture at idle.

I'd try a 50 or 52 pilot jet (a 55 will hardly do anything, and a 48 may be too lean). I think your carb uses the easily-available 32mm-long pilot jets (link), but if it uses the shorter 28mm ones you'll have to try eBay.

My friend and I cleaned out his pipe with an oxy/acetylene welding setup, it really improved throttle response and top end.

Ray

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

If you have the carb kit please install it. 40 year old "o" rings seep fuel and may also be the reason it starts without the choke.
My MT runs so much better after the carb kit was installed.

My silencer was plugged up. I fixed that one.

Rick

Thanks guys I thought I should install the kit I have but still want the most from it so jetting and cleaning up the carbon stays in the back of my mind. I'll keep u informed on my progress.

Dave

Sorry to take so long on updating my progress mainly because I've been trying to fine tune my 75 mt 250 on my own. I cleaned the exhaust in a parts cleaner whiched helped alot running now with a 52 slow jet new intake gasket have the timing set at about 21 BTDC , try adjusting the air jet but I don't see much change when doing it so I leave it at about 1 and a half out from bottom, I do have to choke it when cold but very little. It runs better but fouls plug once in a while and still is not responsive enough at take off, also only get 25 mpg. As always your help is greatly appreciated I just can stand the thought of bringing it to a shop to let them figure it out.

Thanks again
Dave

If the bike is still so rich down low with the #52 jet that the air screw has little effect, it could be your needle jet and/or needle are worn (my MR had this problem, a new needle & jet really woke it up). You can test this by moving the needle clip to the #1(top) position and see if that helps at mid-throttle. I'd also leave the air screw at 2.5 turns out, it may run a little better just above idle. You can try a #48 or #50 jet too.

There's a guy on eBay now selling the hard-to-find needle / jet set, for a good price too:

http://www.ebay.com/itm/252460038626?_t ... EBIDX%3AIT

Ray

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

The Keyser carb kit I installed has a new needle valve and jet needle I installed a 50 slow jet now no different, moved the needle clip to top notch no differance. Adjusting air screw make little or no difference, also now idle screw is turned all the way in just to get it to idle at around 1600 still fouling the plug.I also tried spraying contact cleaner behind the flywheel while it's running looking for a change pointing to a bad seal. I have to be missing something in the equation or not going about this in the right way trying not to give up and have to pay somebody to look at it. As always any help is greatly appreciated.

Dave

The Keyster carb kits have the fuel-inlet float valve (for the main gas flow from the tank) and the slide needle with the clip. They do not supply the needle jet, which is the long hex-head brass collar that the main jet screws into (the needle jet uses a fold-over locking washer to keep it in place). If the needle jet is worn even a small amount, a whole lot of fuel can dump into the carb throat from idle on up, often making the air screw / pilot jet have little or no effect with normal-range jet sizes & air-screw settings.

If you're feeling brave and want to do something easy that should make a very noticeable difference in how the bike runs, try replacing the pilot jet with a small screw (of the same thread, of course) or sealing off the pilot jet passage in another way. In a properly-running bike this should make the bike unable to idle (ever) or start without the choke even when hot, but in your bike I'm guessing it will have a very different effect. :^) I would definitely turn the idle speed screw back out a few turns before starting it, and make sure your kill switch is working.

A bad LH crank seal will result in excessive leanness, not the richness you have.

Hopefully you were able to get one of the MT250 needle jet sets in the link I posted. The needle jet can be hard to unthread from the carb body sometimes; I use a sharp chisel to unfold the locking washer, then numerous quick low-torque wrench twists to break it free (a little heat helps too).

If a new needle jet is not an option, a much leaner than normal pilot jet (35 to 45) may help; if you go lean enough, it will help. A leaner 3.5 slide will help a lot (Honda P/N's 16022-373-405 or 16022-360-004) but won't be easy to find.

Obviously there are many things that can cause super richness too; leaky choke, plugged air jet or internal air passages, carb body cracks, etc. etc. I think the last time I counted there were around 16 separate passages inside this carb, and it's real easy to miss a problem in one or more of them even if the carb 'looks good' inside.

Ray

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

Just thought I'd let u know my progress, I sort of threw in the towel found a honda mechanic with 40 years of experience and let him take a shot at this bike. He put the carb. back to stock including a 128 main jet (the keys ter kit came with a 160) replaced a grounded stator and replaced the wiring harness not trusting the previous owners alterations, lucky all from a donor bike I had. It runs way better EXCEPT from idle I have to keep it from loading up by blurping the throttle he wants the exhaust pipe cleaned by cutting it open but would rather I handle that. So I guess I need input on best approach to clean this pipe out.
Hope you have some idea's
Dave

Just an FYI, the 75 stock jets were 115 main and 58 slow with a 16012-358-034 needle jet set, a 128 main sounds rich to me. These bikes do not like rich conditions at all, how does your plug color look?

Mark

Plug is dry and a light tan color it runs alot better with this jet set up that's the reason I want to clean this pipe out

Dave

10-4, my thought process on this is the closer you get your bike back to stock condition the more it makes sense to run stock jets in your carb. Cleaning out your pipe is getting it closer to a stock condition. I have a 74 and a 75 totally stock with stock jets and exhaust and they both run very well. Swapping jets is fun at first but it gets old pretty quick. You probably know this but the main can be changed by loosening the clamps and twisting the carb to gain access to the bottom of the bowl. Anyway, good luck.

Mark

Your right that's my goal that's why I'm hesitating cutting into or burning my pipe but if that's what it needs so be it,thanks for your input.
Dave