Engine cutting out at level cruise.

[Bob Wood]

I  have had 2 occasions in the last 3 test flights where run up was fine , take off fine, and when in level pattern cruise at 2800 or so, get a big cough and RPM decrease , like it wants to quit. Both times i reduced the throttle dropped the nose and set up to land. goes to like 1200 rpms and lands and taxis fine, and will not do it on the ground....

I am also spitting gas and as per my other post just discovered that my floats may be set really high....

Is it possible i am too rich and choking out due to too much fuel.....

Any ideas much appreciated!

Bob

[Aerodude45]

I would think that it had something to do with flight attitude... Maybe try running it on the ground with the tail lifted and attached to something very sturdy to replicate level cruise?

It's been a while since this post. Have you since resolved the issue?

[Bob Wood]

Still trying to solve the vibration issues, the vibrating float bowls are the source of my issues I am convinced....  Since my last post i corrected the location of my hub counter weight, (1 bolt hole off) and it REALLY smoothed things out below 2000 rpm.  at 2600 or so the carbs are really shaking and venting fuel...Right now i am searching for a person to dynavibe my motor. Based on the dynavibe seminar I attended at EAA and their case study on the 1/2 VW i think i am looking at an easily solved issue. I will report results.

[s johnson]

Bob,

You might be rich. What main jets are you using? Do your carbs bounce around a lot? When I had my dual carb set up I tried bracing them up pretty rigid and seems to me it made the fuel spitting worse. Not sure how yours are set up.  If I remember right you are in Wisconsin. If so.. It has been real humid lately and could be carb ice. Maybe, maybe not. I am running Englemans single carb with a long intake tubes and they sweat really bad with this humid wx we have been having lately.

Scott

[Bob Wood]

Hi Scott,

I have the VM-28's and am running a 130 main one one side and a 150 on the other.  I ended up there to get my EGTs up. any lower and i get cutting out at  the top end. any higher and  I get no EGT readings..

I cant believe these carbs could get icing, running behind the engine like they do....

I really think it is vibration related.  I can see the power bands (one around 2600) there the carbs start to shake and gas vents out of the vents.  Below 2000 my motor is silky smooth...

I have a line on a dynavibe owner in Milwaukee who is going to balance my motor on the Dynavibe unit.

I went to their seminar at Airventure as was really impressed with what they did to smooth out a 1/2 VW motor.

I will report back on results. Hopefully next week or 2.

See attachment for Dynavibe info.

Bob

[joecnc2006]

How was the Crank balanced? Just curious.

[Bob Wood]

It is a Dimalo (spelling) crank that Leonard uses all the time.  It was balanced along with the rods and pistons by Scott Casler. It has the Mosler hub and counter weight.

I had to re orientate the prop to position for hand propping and mis aligned the counter weight so it was not at 9:00 at TDC it was more like 7:00 position.  This alone smoothed out the motor perfectly below 2000 RPM.. It was like night and Day.. I am thinking it still may not be balanced correctly.

This is the reason my next step is Dynavibe balancing.

A friend of mine built the motor and ran it for about 40 hours smoothly he reported.

I swapped my TN prop for Franks Prop and vibration was the same ( higher static however)  So i ruled out props as the cause.

Thanks for the input!
Bob

[s johnson]

Bob,

That's quite a difference.. Hope they can do that with yours..

Scott J.

[stevejahr]

Note that carb ice is only a function of temperature and humidity and has nothing to do with carb location.

You still have a vacuum causing temperature drop and you still have fuel atomization causing temperature drop.

Now the design of the Mikuni-style carb may be less susceptible to icing since there is nothing downstream (like a throttle butterfly) of the vacuum dropping and fuel evaporating area.

Dynamic balancing really is the best way to go.  Measure the effects and coordinate with operation to figure out the actual balance weight needed for that specific setup.  Everything else is just a guess.

[Bob Wood]

My Progress update, Mikuni 28 spitting fuel and rough running:

I had my engine dynamically balanced and it made a world of difference. The A&P I worked with noticed I had air bubbles in my lines down to the carbs,. that were having a head time getting up & out to the tank because I did not have a dip free route to the tank. Nothing major but it did make my motor smooth out when we got them out. We could not balance it completely due to this problem, so we are going to do it again this week. My bouncing carb on #4 seems so be generating air bubbles which may be causing it to run rougher than the #2 side.  I can see the vibration difference from the cockpit....#2 (left side) seems to be alot smoother ( I see less vibration on that side)

We added 2 grams of weight not quite opposite the counter weight, as I said we are not done I expect to add a little more weight someplace.

It seems that the counterweights are sometimes too heavy.  The dynavibe case study,( I saw at AirVenture)showed this as well as Scott Johnson's motor , which this same A&P did the day after he did mine. 

Scott maybe you can chime in here, Jakob told me you added 50 grams opposite your counter weight....What did you use for weights??

In my case the bouncing float needles are really a problem. I did carb bracing , but balancing is really the key here I think....

More after my second balance this week

Bob Wood

[joecnc2006]

I got all my parts from Bob Demello, he got my heads, rockers and valve covers from Scott Casler, Bob also sent my crank to Scott and Scott machined the end to acept the Mag shaft which Scott makes and sent it to Bob all together. When Bob got everything he rechecked the crank for balance. and then sent me all the VW parts along with a case that he (Bob cleared) for the 82mm crank.

Taking to Bob he mentioned that he is the one which Scott gets his cranks from or use to, he said Scott has not ordered any cranks for a little while, he use to do batches of 10 or more, maybe he found a different source not sure. But Bob was telling me the makes the cranks and balances them, magnaflux checked and then sends to Scott.

We also have discussed other plans which call for cutting a certain size steel plate and welding it yourself to the crank, this i a very bad idea without getting the crank balanced. He has seen those cranks which people have sent to him after they have built the engine and the whole thing just shakes and wants him to fix it, He said that he sugest to them to get another crank to have it done right in the first place.

That being said, is the reason I asked if the crank was indeed balanced correctly which sounds like it is from what you are saying.

How do they actually balance the engine? I have seen talk about weights added to the prophub to cirect for its balance along with a proper Propeller balancing.