Engine hesitation in level flight

[Bob Wood]

Looking for some suggestions.
 After dynamically balancing my 37hp motor. It is running lots smoother and lots of power , however,I  get a cough or miss or hesitation in level flight.  I took off Friday made one circuit in the pattern and was high so decided to do a level pass and go around.  I was at at about 3/4 throttle and I think i gave it a little throttle to start a climb, and had the hesitation, or miss or cough, just enough to think it may be quitting.  I did not touch the throttle until I picked out my runway, pulled throttle and landed.

So i had no issues on run up after warm up, climb out, and after i landed did a long full power run up, no issues......

I am running a ram air gas cap on my Team minimax plastic tank.  Also a 1" diameter fuel filter on way down to the Y split to both carbs.  My fuel lines are bubble free after balancing my engine. My fuel flow test I did before my first flight last year flowed 16 oz in 2.5 min. (3 gallon/hour)

I did the fuel flow test above the Y at the filter point before the filter. Thinking I should redo the flow test down at the carbs on each side and add together for total flow.?? Maybe the filter is restricting flow.  If this were the cause you would think it would always happen at full throttle.

I am running VM28 Mikunis. i am at 130 jet on one side and 150 jet on the other ( carbs came with 200 jets).  This gives me 1050-1100 EGT. Any smaller jets result in power loss at full throttle ( cutting out).  So I kept these jets in. All this testing & set up was done before the engine balance.

Running Premium MOgas (no ethanol)   Comp Ratio 8.1 :1

Any thoughts on other things to check?

Thanks,
Bob

[s johnson]

Bob,

Do you have a slick 2220 mag or the 007 distb.? Scott Casler has mentioned in the past that on some  surplus mags the condenser are bad and should be replaced if it shows   signs of missing or runs ruff.   Not sure if this symptom happens all the time or intermittent. I asked him years ago if I should replace mine and his reply was that if it still runs good there was no reason to replace it. 
Just shooting in the dark here and most likely not your issue here.  Also have heard that on the 007 distb ( Blue coil ) that they need cooling air to keep them from over heating. Could be a problem if mounted in the foot well possibly but have never seen or heard of a problem from anyone here in the Eagle community.
Sounds more like a fuel flow issue as you suggest.

Sure wish I could help. Good luck  and hope to see you and your Bird  here in Oshkosh this year.

Scott J.
Oshkosh

[Dan_]

No problems at full throttle indicates you may have a lean spot at 3/4 throttle... 


When lean the engine will sputter in when adding throttle, when rich it will blubber in. 


You may need to raise the needles... 


Try to diagnose this with ground running @ 3/4 throttle and egt gauges...

[Bob Wood]

Dan,
interesting point.  I will test the EGT.  I am fairly sure my EGT goes down then I come off full throttle, but i need to run extended at 3/4 to see. It could also be Rich.  The hesitation I felt could be either I think...Your blood pressure goes up so much hard to concentrate on the exact nature of the hesitation.

Thanks will dig into this.

[Bob Wood]

Scott,
I am running a 007 total loss Ignition.  I have never felt my coil temp after running. it is mounted on the firewall between my feet. Will keep it in mind. Easy enough to run a cooling hose to it.

I need to get this solved to get to EAA in July.!!

Maybe i am rich or lean on 1 jug at 3/4 throttle as Dan mentioned. I ended up with my needles in the middle slot on both carbs.  My EGTs only get to 1050 or 1100 at full throttle and that drops off to below 900 as I throttle down. (my Belite EGT's start at 900 degrees)

My jets are 130 & 150 and seem to be good at full throttle, & plugs look clean and non sooty

Maybe it is the carb transition between 3/4 & full throttle before the mains kick in...I wish I would have pushed in full throttle when I had the issue, but was too busy looking for a landing spot!!!

[leshoman]

Something else to check would be float settings.

[Bob Wood]

Thanks Les, set per the Mikuni Manual spec? I will recheck

Bob

[Bob Wood]

Ran up and calibrated my throttle to zero in on where the hesitation is. At 1/2 throttle my Rpms are 2850. At 3/4 throttle my rpms are 3120. When going to full there is no additional rpm increase. My hesitation did happen on the ground for the first time and happened at about 2600. Just below 1/2 throttle. Since my max egt during these settings is 1050 I am assuming my hesitation/ stumbling is being too rich. Do I need to move the clip one notch up from center? 

Any thoughts on next steps much appreciated.
Bob

[Dan_]

Raising the clip on the needle will drop it lower and take you leaner..

I would try raising the clip on the needle...
Did you get the blubber sound..?

[Bob Wood]

Dan,
I guess it is a blurbble....I only heard it twice but makes sense. just so darn scary to hear in the air.... Can not believe it can be lean, due to the EGT at 1050 max....I never can get good feedback from the Mikuni guys on there he needle clips normally run. Seems like most guys are in the middle (slot 3 I think)

Is it normal to max out the RPMS on the VM28's at 3/4 throttle?

I also wonder if only one jug is rich...I am running a 130 main jet on #2 and a 150 on #4.  I maybe need to revisit that and see if I can lean out any more on the mains.  When I did all the testing to lean out to increase my EGTs my motor was running lots rougher (before the dynamic balance)  The most I was able to get was 1050 EGT on both sides, any leaner and the enging would cut out at full throttle....1200 would be lots better I am told.

Thanks for your help!
Bob

[Dan_]

The location of the EGT probe is important.  They should be a certain distance down the exhaust pipe from the valve seat for everyone to get the same numbers.


The Sonex people recommend 4 inches from the exhaust flange for their . (pg.56)


It could be that you are already too lean. Lean of peak. 



This post is for a 45 horse power Cassler engine with 150 and 160 main jets.


Do you have the


It is perhaps best to start at WOT and get max rpm and peak EGT and work your way down through each throttle range.



The last page on the manual shows (by width of the diamonds) that the needle has the most influence on 3/4 throttle, and at full throttle none...

[Bob Wood]

Well I figured out the hesitation and cutting out at 1/4 to 1/2 throttle. I kept reading on how one carb system leads into the other and to set mains last so I moved my jet needle to slot 5 full rich. I also went back from 130 & 150 on the main jets to 170. Guess what happened... the motor smoothed out. My static went up from 3090 to 3190. I only get about 100 rpm gain between 3/4 and full throttle. The egt's went down below 900 ( where my belite gauge starts)from my previous 1050. No cut out or hesitation at the 1/4 to 1/2 setting. I went up and flew and it has never run better. Now I plan on moving the mains down to around 155 to see if I can get my EGT's back up to 1000 or 1100. Currently my number 4 plug is a little sooty and the number 2 looks good and brown.  Thanks for everyones help. This carb science is an art in itself!!
Bob

[s johnson]

Bob

That's fantastic news. Way to stick with it. Nice job. Eaa bound I hope. Looks like we might have three this year if you can make it. My self, Les Homan I believe is going to come a day early or so and set up where the Slug is kept and do a little flying locally then fly in to Airventure together. I think that's the plan anyway. Denny Sleen might be coming  from Minnesota as well. Could be another good year.

Scott J
Oshkosh

[Bob Wood]

Scott,
Yes I plan to fly over early and tie down and go back home and get all my Camp Scholler stuff, I stay for the whole week. I have watched the you tube on flying in, i just need more time on the stick. I have had so many unscheduled landings I am good at landing short but need touch and goes and nail procedures to land just over the fence.  I still need to do stall testing etc.  I have just been flying in the pattern trying to get my engine to run right for the last 12 hours.  It is really going to be fun flying this thing without always sweating an engine cut out!!

Good news on Les and Denny, both of them have been a big help to me, as have you,will be good to see you all. Stay in touch on your fly in plans.

Bob

[s johnson]

Bob,

That's good news. Just be aware that the ultralight arrival procedures per NOTAM's are for specific  dates and times. When do you plan on flying over? Check the EAA for Notams regarding the ultralight arrivals. Not difficult at all.    You should have a copy of them with you. Not that it would do you much good while you are flying in an ultralight but required. I can send you a copy if you need. :-)


Scott