Engine missing intermittently

[Bob Wood]

I am still hunting down a problem on my 27HP Mosler (been going on for 2 years). I changed plug wires 5 flights ago ( seemed like it was solved) and last night the stumbling, or hesitation or multiple miss occurred again on takeoff.  it did not happen during run up. I did get it to do it again after I landed. I took off climbed to about 500 feet and it felt like 1 jug was cutting out. No bubbles in gas lines, more that just a miss....

This comes out of nowhere. I thought it was carb shake related, so i dynamically balanced my motor, so running really smooth now. EGT's are more consistent since the carbs are not shaking. 

I was going to focus on the ignition because I have changed barbs, rerouted fuel. changed plug wires. It always happens at 3/4 to full throttle so maybe the advance may be off?

I am going to check timing and the advance and see if it looks ok. also will change plugs..

You guys have any other ideas??

Thanks,
Bob

[Dan_]

You probably already have checked it, but the float level being off could be causing the float bowl to not be able to keep up at the higher rpm.  Look for a lean condition.

I recently replaced a needle and seat on a briggs and dint get the seat pushed in all the way.  The motor runs fine at low rpm, but not good at high rpm.

Also the float vent tubes can cause problems when out in the slipstream...  Look for a rich condition.

Intake leaks...

If it is the same cylinder each time you may want to switch the carburetors side to side to see if the problem follows.

FWIW.

[Bob Wood]

Dan
Thanks for reply. I did check floats and had set them at 16mm per the manual. I am running 180 mains, I wonder if it is possible that the powerloss I am getting is a rich baugging condition? I was going to try to go to 170 main jets and see what happens. I did recheck my fuel flow and changed to a different fuel filter before the Tee split. One carb had a flow of 3.6GPH and the other at over 5GPH. After the filter change they were both up over 5gph. I thought that may have been it....

What happens is : At run up all ok for maybe 20 seconds and then it sounds like you switched off the fuel and it is starting to quit. sometimes a throttle change will help it. I have just let it run and it comes and goes.....

Seems rich because EGT down around 900 or so.

My vent tubes are just 6-8" hoses all 3 tied together and pointing slightly to rear......Would that be picking up a suction?  I could try to reconfig those maybe......

Thanks for the help!
bob

[Dan_]

If it is rich enough to cause a miss, it should show soot on the plugs.  Or at least darker color.

A lean condition will "sputter in" when you open the throttle, a rich condition will "blubber in"...

Here is a way to set the float without measuring.  Also the link at the bottom of the page shows a way to tee the vent tubes.

[Bob Wood]

Read over the float setting non measuring method. this is not even close to what Mikuni specs in their manual...I get the reasoning, but not tried it.

Has anyone out there set them at this height?

The other thing I noticed is on the vent plumbing article, the 3rd vent is not talked about. I read once in this forum that the float vent for overflow can be capped off. I tried it both ways and did see less fuel venting out when my carbs shook.Ran fine capped off. with non shaking carb uncapped is fine too.

I was thinking of plumbing all 3 vents together so there would be only 1 vent tube hanging out in the slipstream. With an angle cut on the downwind side to prevent any suction my the slipstream.

Anybody go through this?

Bob

[charlierainman]

  I have a motorcycle that I had problems with a few years back, I know it's not an airplane, but I will get to the point. I owned the bike for a few years, never a problem and then out of the blue it began to lose power and quit for no reason, it left me on the side of the road 50 times. I checked everything and came to the conclusion one day, it was getting a vapor lock. I would pull over to the side of the road and watch the fuel filter, it would be empty and about ten minutes a bunch of air bubbles would appear in the filter and then the gas would run in and it would start again. I was ready to tip it over and light it on fire. I somehow read an article on a forum about this motorcycle running out of fuel at higher speeds and that you needed to change the needle and seat to a larger one, I think my stock one was 2mm and I installed a 2.5mm. It cured the problem, in fact it worked so good, I bypassed the fuel pump and just run a gravity feed now. I still don't know what happened, but haven't had it quit in 3 years now.

[Dan_]

I think there is a fuel pump needle and seat and a different gravity feed one for Mikunis...  It would certainly pay to check into it.

[Aaron]

This is an interesting thread. Did you end up figuring out your intermittent miss? I have experienced the same miss on my 1/2 VW with dual mikunis. It is almost like you blipped the kill switch. Nice shot of adrenaline to make sure you don't have any residual cholesterol hanging around... I was going to tear down my slick mag and ensure that there is nothing wearing out in it as I have seemed to check everything you have checked as well.

FYI I have also experienced the float bowl vents pulling a vacuum (I started a thread on it at one point). My engine seemed to settle in nicely with my bowl vents routed into snorkel like ends pointing into the slipstream, however I pinched off the ends so it vents, but doesn't get direct ram air, rather partial positive pressure. She came alive with that arrangement. Ran good for a couple years, however recently, now and again at random times I get the same anomaly of a random rather large miss every now and again. Can never recreate it on the ground which is never what you want Hoping you have found a smoking gun which is why the thread died, and hoping you aren't going to say you sold your legal eagle over the matter :p

[Bob Wood]

Aaron
I did figure it out. It seemed to be carb vibration. For some reason I get a harmonic carb shake at 2850. If I avoid 2850 all is well. Those mikinis do not like vibration

[Aaron]

Interesting. Thanks for updating!