Bottom end dead spot

[skiracer]

I need some help.  My 76 250 MC5 has a HUGE dead spot on the very bottom end.  The bike starts okay, but when I give it some gas, it wants to stall out.  If I play with the trottle, it runs at mid to top end, and high top end.  I thought I might have dirt in the carb in one of the jets, so I took the carb (Mikuni 36mm) apart, cleaned everything, with the same results......  Any suggestions?
Thanks, James

[Greg Settles]

James:

I am new to all this but I have a 74 250 that was doing about the same thing. I found a crack in the case behind the mag. I also found a crack in the mag coils.

Greg

[Ernie Phillips]

Sounds like a lean condition as the carb transitions to the needle/needle jet circuit.  Try full rich on needle (clip in lowest groove).  If this helps but doesnt't give desired result, try richer needle/needle jet combo.  Could be air leak, could be flakey ignition, loose wire ...

Think back to when it last ran successfully.  Then retrace all your modifications/changes and reconfigure the rig to a known starting point.  If it ran good before, then it will run good again.



Ernie P.
Chattanooga, TN

[skiracer]

Ernie, thanks for the input.  The last thing I did prior to cleaning the carb, was replacing the main seal on the mag side.  Maybe the seal is bad?  Or maybe I did something wrong with the instalation, though it is a simple process.  What do you think?
Thanks, James

quote:Originally posted by Ernie Phillips

Sounds like a lean condition as the carb transitions to the needle/needle jet circuit.  Try full rich on needle (clip in lowest groove).  If this helps but doesnt't give desired result, try richer needle/needle jet combo.  Could be air leak, could be flakey ignition, loose wire ...

Think back to when it last ran successfully.  Then retrace all your modifications/changes and reconfigure the rig to a known starting point.  If it ran good before, then it will run good again.



Ernie P.
Chattanooga, TN

[skiracer]

Greg, thanks for your input.  As I told Ernie, I replaced the main seal on the mag side.  I didn't see any cracks in the cases.
James

quote:Originally posted by Greg Settles

James:

I am new to all this but I have a 74 250 that was doing about the same thing. I found a crack in the case behind the mag. I also found a crack in the mag coils.

Greg

[Ernie Phillips]

James,  Raising the needle (richen) is easy to do.  I'd try this first.  If the mag seal (or other air leak) was problematic, I would think that you would have idle issues.

Has the existing 36mm Mikuni run successfully on this bike?  If not, maybe someone here can post baseline jetting specs.

For some resent history: I just completed rebuilding a 125 Sachs.  Prior to the rebuild, it was running rich just off bottom. So, while I had everything apart, I decided to disassemble and clean the carb (30mm Bing).  Since it had been rich, I went full lean on the needle.  When I fired the bike up, it hit a dead spot just above a fast idle.  I could clutch it and rev it and make it go, but it sure felt like it was super lean.  I pulled the slide and went one notch off full rich.  The bike perked right up.  Now, I did a whole lot of other changes to the engine during the rebuild (GS cylinder, fresh bore, port matching, minimum squish, + compression...)  but, changing the needle clip 3 spots made a world of difference.  Good luck -EP

[Ron]

Hi James,
You might try a larger pilot jet.
You'll know the pilot is to small when the engine quits before the air screw is turned out 1 full turn at a normal idle speed.
If it turns out more than 2-1/2 turns and the engine speed keeps increasing, the pilot jet is to large.
Ideally, you want a smooth idle at 1-1/2 - 2 turns out on the on the air screw.
Much more than that and the spring tension on the idle screw won't hold it in place.
That's the way a Mikuni will respond, I assume a Bing will react the same way.
RonW

[firstturn]

Stick with what Ernie has said.  If you want to test the seal and do not have a leak test set up just put som baby powder beind the MotoPlat and run the engine.  If it is leaking you will see what gas leakes in the powder.  But I still agree with Ernie on the irratic idle if you have a bad seal.

Ron Carbaugh

[slvrbrdfxr]

James,
Make sure you take a good look at the seal retainer behind the magneto too because they are very prone to corrosion which can leave some deep pitting where the outside of the seal contacts the retainer. The engine can easily suck air around the outside of the seal if pitting is present. I know this from personal experience. I created a homemade leakage tester from some common plumbing fixtures, a trailer valve stem and an old vacuum/pressure gauge. I use a hand tire pump to pressurize the top end to 6 psi. Once the top end is pressurized I use a spray bottle of soapy water to look for leaks. This homemade contraption was inexpensive to build and proved to be a great troubleshooting aid.  A tight top end should drop no more than 1 psi in ten minutes. When I got finished with my bike it held 6 psi for an hour so I knew it was good and tight. If you are interested in making this tool just shoot me an email through the site and I will send you some pics. Anyone else wanting to see the tool is also welcome to email me for the pics.
Dave McCullough

[tomale]

cracks may not be visible, I had one on a bike in a strange place and I would have never guessed it if I had not done a leak down test. the reality is that these are old engines and cracks can form just because they are old... I always grease my seals before I put them in. it is possible if there is just a little bur on a sealing surface, it could cause a problem... Seals are cheaper than engines and I have heard of bad new seals.... I once made the mistake of using a NOS Seal, bad Idea.. they loose their suppleness and they will not last. If you still suspect there being a jetting problem, do a search I believe I posted a few years ago the jetting I am running on my 76 250 with a mikuni carb.... I am basically at sea level so you can take it from there...

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)

[skiracer]

Okay, I raised the neddle.  It helps a lot, but it's not there yet.  So, I need to make the bike richer.  To do so, I should install a larger(?) main jet, or pilot jet???  Since I am raising the neddle, I am guessing the main jet, right?  Thanks, James

 quote]Originally posted by Ernie Phillips

James,  Raising the needle (richen) is easy to do.  I'd try this first.  If the mag seal (or other air leak) was problematic, I would think that you would have idle issues.

Has the existing 36mm Mikuni run successfully on this bike?  If not, maybe someone here can post baseline jetting specs.

For some resent history: I just completed rebuilding a 125 Sachs.  Prior to the rebuild, it was running rich just off bottom. So, while I had everything apart, I decided to disassemble and clean the carb (30mm Bing).  Since it had been rich, I went full lean on the needle.  When I fired the bike up, it hit a dead spot just above a fast idle.  I could clutch it and rev it and make it go, but it sure felt like it was super lean.  I pulled the slide and went one notch off full rich.  The bike perked right up.  Now, I did a whole lot of other changes to the engine during the rebuild (GS cylinder, fresh bore, port matching, minimum squish, + compression...)  but, changing the needle clip 3 spots made a world of difference.  Good luck -EP
[/quote]

[brian kirby]

quote:Originally posted by skiracer

Okay, I raised the neddle.  It helps a lot, but it's not there yet.  So, I need to make the bike richer.  To do so, I should install a larger(?) main jet, or pilot jet???  Since I am raising the neddle, I am guessing the main jet, right?  Thanks, James

If you raised the needle to the most rich position and you think you need more, you need to change the needle jet, not the main or pilot. Remove the main jet, turn the carb upside down and the needle jet will fall out. Read the size it will be a letter-number, some thing like Q-2. Find out what is in there now and get one or two sizes richer.



Brian

'72 Berkshire

[t20sl]

Did it ever run fine?  Reread your answer and still not clear.  Was changing seal trying to fix the dead spot or did it run ok before seal change?  If it never ran fine then go with larger slow (pilot jet) first.  Don't change main.  Main effects 1/2 throttle barely and really doesn't have major effect until 3/4 to full.  Pilot effects idle transtion to 1/4.
If my memory serves me correct I read somewhere you can remove needle completely from a Mikuni and it should start and take a little throttle.  If it doesn't and falls flat (lean) then go with bigger slow jet.  Then reinstall needle.  I have never tried that method tjhough.  Ted
Ron:  So much for my memory.  That makes more sense.

[Ron]

Ted,
I believe your refering to running the bike without a main jet installed.
It's a good way to sort out a starting point for the needle/needle jet combo.
If the bike doesn't start to blubber at 3/4 throttle the needle jet is to small and becomes a restriction to flow.
With a restriction in the needle/needle jet combo, a larger main jet will have no effect at all.
Needle tapers are for fine tuning at specific but different throttle positions and of course the main jet controls mixture at wide open.
RonW

[tooclose racing]

I'm just gonna use this thread to jump in and say HELLO to everyone and mention that I just got my '76 MC-5 running and boy does it rip!!!  After bringing my new purchase back from VMD, I parked it while my body healed.  Tried to get it going late August with no luck and - mistakenly, thought I was getting gas and spark so I was baffled.  Yesterday, a former racer friend stopped by to help me figure this Big Orange Boy out and we quickly figured out that the cylinder was NOT getting any mixture.  Heck, the thing was not even tickling.

First comment about an MC-5: getting to that Bing 54 is NOT a maintenance friendly experience. Awful! But...that aside, I put in a call to Mr. Gary Ellis to leave a message while my buddy and I worked on loosening up and extracting the Bing for disection purposes.  Gary calls back right while we are staring at the right side frame mount removal attempt in utter defeat and first thing he says is "you haven't pulled the carb yet have you?"  Uh, no..but not for lack of trying.  [}:)]  "Great- well, just give the carb bowl a couple of raps".  Huh?  We proceed with what the doctor ordered, and...ta-da...gas flow when tickled.  My racer-friend, who went very fast on big bore Huskys back in the day, was kicking himself for not considering this "unstick the float/float valve" activity. Tightened everything up, fire her up and SHE IS STUCK OPEN. Can I hear an Amen for a functioning kill button? (Amen).  Shut her down and realized we (as part of pulling Bing's slide and needle effort) had probably not "threaded the needle" when putting slide back in and buttoned things up.  Fixed that, fired her up again and....sweet music.

THANK YOU Gary Ellis for this wonderful machine and your timely tutoring yesterday!  I'm still pretty amazed at the power, and find myself short-shifting for now until I get a bit more acclimated to it.  I've never had a dirt bike over 175cc...

Anyway..good luck with YOUR MC-5 jetting/throttle response efforts. This thread is just SO typical of the great trouble-shooting advice you get here at POG.