6B Misbehaving

[KJDonovan]

I got a bike that has me going in circles.  Last spring, I built a 72 Six Day with a 6B engine, Mikuni VM-28 carb, MX pipe.  For everyday trail riding I have zero complaints, but after running on the pipe for a while then backing off, when you try to get on it again, it momentarily just cuts out for a fraction of a second then after doing this a couple times it will load up and is nearly impossible to clear out.  

I let a buddy use it in a Hare Scramble a couple weeks back and half way through the second lap (3-1/2 mile loops) it started doing this.  I have sorted out more than my share of these, but this one has me scratching my head.  Here's how it's set up;

Fresh Engine – 55MM Wiseco Piston, Fresh Bore, New rod, bearings, seals, ect.  After building the engine did a leak down test at 6 PSI, only dropped 1/2lb PSI after 8 minutes.  So, no leakage.  Axial play on crank bearings is about .001".  Compression is at 140 – 145.  No binding or slop on the transmission shafts.

Carb – Brand new Mikuni VM-28 (not a Chinese knock-off, a real Mikuni) Jetting:  Slide #2.5, Main Jet #150, Pilot Jet #50, Needle Jet 169N8, Jet Needle 5F21 3rd slot from top (it likes this rich position).   I initially started with a larger main and smaller pilot but evolved to this.  Nice even transition from the pilot to the needle jet with great mid-range hit and the plug (NGK B7HS gapped at .022") is always a nice chocolate brown.

Ignition – NOS Motoplat mini-6 tested by Vance Smith with red wire coil.  I have the timing set at 2.5MM BTDC as it seemed to run a little hot at 3.2 BTDC, so I retarded it slightly.  

Fuel Flow – New OMG high-flow petcocks with plenty of fuel flow.  Replaced cork in gas cap and tank is venting fine.

Fuel – CAM2 110 Octane E0 with Platinum Spectro 32:1. Same stuff I use in everything else.

Pipe – Zero obstructions, I built the pipe myself out of others, so I know there is nothing obstructing it and I keep the end plugged when it's not in use so critters can't store anything in there.

So, as I mentioned above the problem I am having is after running on the pipe for a while then backing off, when you try to get on it again, it momentarily just cuts out for a fraction of a second then after doing this a couple times it will load up and is nearly impossible to clear out.  

I initially thought this cut out was electrical and the old Motoplat I had, tested out of spec.  Vance Smith provide me a with a new Motoplat and I have tried 2 different HT (red wire coils) with it and no real difference.  Same issue with old Motoplat.   I also tried completely removing the kill switch and wire just in case there was a bare spot and same deal.  As for grounds, all the engine attachment points have no paint on the frame, and I also added a grounding strap between the head the coil.  When I check with a multi-meter I have continuity between the coil, head and the stator mounting plate.  Tried a new plug too, same problem.  Last thing I tried was pulling the air filter, just in case it was starving for air, no difference.
 
So air, fuel, spark is basically all a 2 stroke needs, so why is this thing misbehaving?  I don't think I am missing anything gross and obvious, but at this point I am going in circles.  Ideas / recommendations appreciated.

Thanks,

Kevin

[454MRW]

I am not a racer, attested to by Larry Perkins which his expertise might shine a little more light on the solution to your problem, but what I have read about running racing fuel is that they don't color the plug like what we expect pump gas to. Taking that into consideration, I would tend to think that on the main jet it's a little rich or the position of the needle is a little high, but once it's fully on the main jet the needle does not come into play, so my recommendations are based on the plug color at least when running pump gas should be light brown, not chocolate. Try careful selection of slightly smaller main Jets and see if that makes a difference. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1978-1979 MX-6 175 & 250 KTM's
1976-78 125-400 RM's & 79 PE250 Suzuki & 2012 DR650
2007 CR125R Honda
1977 MC250 Maico

[Daniel P. McEntee]

Check the fuel line nipple and opening in the carb and make sure it's completely open. I will guess that you are "out running" the float bowl volume, and perhaps the floats are sticking down. My thinking is if you don't keep fuel flowing enough to maintain float level, the floats of course drop. If they get stuck in the down position as you slow down, then the bowl gets over whelmed with fuel and it floods. I have a 250 Hercules that was giving me some hesitation when run wide open in 7th gear. Once I backed off, it was fine. I read later on that back when they hit the country and the desert racing scene, this was a problem. The solution at that time was to drill open the carb body at the fuel line nipple, like with a 3/16" bit, and control the fuel flow during stops with the petcocks. I think you are having a similar issue, but your floats are sticking in the down position. Like Mike suggests, maybe try some straight pump gas with no alcohol and do some plug test runs and see how that behaves and read the plug. If that all looks good, check the float operation. On second thought, maybe check the float operation first. May want to check all the carb vents also. Just my uneducated wilds a$$ guess.
  Type at you later,
   Dan McEntee

[Larry Perkins]

Mike,

You don't have to be a racer to fix or understand probs.  I was pissed off when I said that.  You are plenty smart.  Sorry for getting mad then.  

Kevin,

I had a Mikuni prob a bit similar and could not figure it out on my 250.  Gary Ellis finally got to the bottom of it and a gasket was missing under the seat needle mechanism.  It would load up and never rev properly.  With the gasket it was good to go.  Not sure if that is it but make sure your Mikuni is not missing anything don't assume it was all good just because it was new.

Larry P

[KJDonovan]

Guys,
Thanks for the responses, much appreciated.  Like I said, I have reached the point where I am going in circles so this is helpful.

-Mike, I have tried smaller main's and a leaner position on the needle but when I did I had pinging up high and no decent mid-range.  As for fuel, unfortunately in New England, the only ethanol free fuel is race fuel.

-Dan / Larry, I had not considered the possibility of running out the bowl quickly, then it getting overwhelmed with fuel or the possibility that I may be missing the gasket under the float needle seat.  I had looked at fuel flow to the carb, which is good, but I'll take another look for obstructions starting at the fuel nipple and check the float needle seat to be sure nothing it leaking by it.  I will also make sure the vents are actually venting and there is nothing that may be gumming up the floats and keeping them down.  I will let you know what I find.

Thanks,

Kevin

[Paul Danik]

Any chance you could borrow a carb from a good running 6B and see if the problem persists? Is the piston you used a brand new one or a NOS one from "back in the day" ? The gentleman who did my topends back in the day hated Wiseco as he felt the quality of the rings was not up to the OEM ones as they did not provide proper wall tension once they got hot.

Good luck,
Paul

[KJDonovan]

Paul,

Great idea on borrowing a carb, but I don't have another running 6B to try one from.   As for the piston, it's a new Wiseco (not NOS) I purchased from Al B.

Thanks,

Kevin