trouble with BME 116

skipm

Member
I have a problem I have been fighting for 1-1/2 years. Often on start up it acts like a cylinder is missing making it hard to start and unable to throttle up without bogging. Usually after 10-15 seconds of stumbling it will catch and run fine for the flight. Let is sit and it starts over. Typically the left exhaust is cool during this time. I have replaced plugs & ignition. I believe the tuning is good, smooth idle and transition. Enriching the idle does not help. I have RE2 pipes on 14" headers. I have tried header lengths from 12 to 16" with inconsistent results although I can't say it has ever comes up on the pipes. At one point I thought the pipes were savaging the fresh air/fuel mixture at idle so there was nothing to burn but as I shortened the headers the RPM dropped. I am fresh out of ideas. Any thoughts?

Thanks, Skip
 

Tired Old Man

Staff member
Have you tried running the engine without pipes at all to remove headers from the problem equation? I am not aware of anyone that has ever truly nailed down tuned pipe performance for the 116. OTH, I have seen and heard of a lot of people hanging stuff proven on DA's and calling it good with little to verify the call.

Try the pipe/header removal run test first, with ear protection of course, and get back to us.
 

skipm

Member
Awhile back I did try it open exhaust, lost 500 rpm which according to Tom @ BME was to be expected without any back pressure. Unfortunately I never got stock mufflers as they were extra and I knew I was going to use pipes and the exhaust flange is special so the mufflers I have won't fit. As for the miss I don't remember if it was an issue or not. It was only one short run. It definitely wasn't a A HA moment.
 

psycho-patch

Well-Known Member
I had this problem with an older Brison 6.4. After thinking about this for a minute I remember now, it ended up being a ring that was just starting to stick in its land with carbon build up. I tore it down, cleaned it up and reassembled with no new parts. Ran fine after that and the dead cyl. on cold start went away. Just my 2 cents, my experience.
 

Bunky f. knuckle

Cover shot, MA 10/09!!!
Since you don't fully remember about the 116 trial from a year back, I'm sure we all would suggest doing this gain. Checking to see how it starts.

You could check for stuck piston rings relatively easy. Using some onboard thermocouples (onboard temp gauge) on the backside of the cylinder head. Those will tell you if you have a stuck ring. 1 side will run hotter than the other, by a long shot.

I had a 157CSTS that would stick the rings. Never knew they were sticking until I put onboard thermo's on it. One side was normal temps, around 225F, and the other side was closing in on 400F. Never missed on start up. Actually started like clock work, idled great, transition was awesome! I used both an IR gun and the thermos. The thermos were much more accurate.

I would try using thermocouples (on board temp gauges) and taking it around the patch a few laps. Then monitor the temp readings. You would only need to do this once. A difference of 30F is no great deal. Seeing a difference of 50F plus is.
 

Tired Old Man

Staff member
A cylinder runs cold because it is not producing power. The more power it makes the hotter it becomes.

Lack of compression will, in the event of a near total loss of compression, cause a cylinder to run cold. That is a rare event though. Typically a cold cylinder is caused by a lack of ignition event to trigger combustion.

In the original post it was said that plug wires had been swapped and the ignition had been changed. What ignition was used after the change? Was it a new ignition, one known to function perfectly or one that had been laying around from another engine?

A weak ignition will not trigger under compression. It is very possible to have a twin ignition that is weak on one side only. A more likely culprit, is having a poor cap to plug fit, which does the same thing since the plug and cap are not securely grounded. A dead hole also happens if there is a break in the plug wire, or if one has been pulled free of the ignition case.

There's too much missing from this picture to make a call, but I have serious doubts the use of a thermocouple or heat gun is going to provide much in the way of enlightenment.
 

skipm

Member
thanks for the replies, the replacement ignition and plugs were new. I can try swapping the leads to see if the problem moves hopefully this weekend if the snow melts. I also have access to a temp gun I can try. Whether the replacement was defective, I don't know. This is my only twin. I have tried different plug gaps. Right now I am set at .020. Using Stilh HP ultra oil @ 42:1 in premium pump gas from day 1. When I have looked in the exhaust ports, everything looks clean. The engine is baffled. Never had a problem running hot that I'm aware of. I did have to open up the high speed jet .003" because it wouldn't go rich. Even now you can't make it gurgle. What header length is commonly used with RE2's? I have resisted going real short as I have heard that can damage the engine and make performance peaky. I recently got a MVVS 58 with a ProFlow smooth pipe & 14" header spinning a Fuchs 24x10 prop and it runs great, accelerating verticals. Thanks again. If there is other background info you need let me know. Skip
 

Bunky f. knuckle

Cover shot, MA 10/09!!!
When mine was almost healthy, on stock mufflers, I ran 3 props, back to back to back. Falcon 28x9.5, Mejzlik 27x12TH and 28x10. Tom suggested that I run prop to make the engine spin up easier. All 3 props ran within 200 RPM of each other. But mine had issues, but won't get into it here. But it does make great power, and everyone I talk with about theirs, loves them. Mine was a lemon from day 1. My original ignition wouldn't fire. Luckily, I had a spare RCExl on hand, swapped it in, and away we went.

I do have Trevor Brum's cheat sheet handy. He says a Falcon 28x10 CF on 16.5" headers with RE2's, pulls 6600 RPM. Mine was about 100 stronger on stock mufflers, that I remember.
 

Bunky f. knuckle

Cover shot, MA 10/09!!!
A cylinder runs cold because it is not producing power. The more power it makes the hotter it becomes.

Lack of compression will, in the event of a near total loss of compression, cause a cylinder to run cold. That is a rare event though. Typically a cold cylinder is caused by a lack of ignition event to trigger combustion.

In the original post it was said that plug wires had been swapped and the ignition had been changed. What ignition was used after the change? Was it a new ignition, one known to function perfectly or one that had been laying around from another engine?

A weak ignition will not trigger under compression. It is very possible to have a twin ignition that is weak on one side only. A more likely culprit, is having a poor cap to plug fit, which does the same thing since the plug and cap are not securely grounded. A dead hole also happens if there is a break in the plug wire, or if one has been pulled free of the ignition case.

There's too much missing from this picture to make a call, but I have serious doubts the use of a thermocouple or heat gun is going to provide much in the way of enlightenment.
I understand all that. But what I don't understand is why I was sticking piston rings on the rearward most cylinder??

The big difference in cylinder temps made me dig into the engine, not the excessive performance. :)
 

skipm

Member
I'm getting around 6000 on a Fuchs 29x10. I had 16" cross over headers. I have chased the length down from there. Never seemed to make a difference until it started dropping off which was not what I was expecting to happen. Went back to 14" as a middle ground. The top end is fine. Pulls a vertical in a AW 100cc Ultimate as long as I need it, just not the pep of the MVVS/proflow combo. The real issue is the starting. Trying IMAC this season and the last thing I want to be fighting is a hard starting engine. I got a few other props I could try, Mejzlik 28x12 & 29x10S. Might help the top end but don't how it could effect the start up.
 

psycho-patch

Well-Known Member
A sticking ring will run cold on that side. The problem with my scenario was the absence of port pull ( lets call it, produce a vacuum, but that is not really what it is) from that sticking ring cyl. to fuel fill the chamber sufficiently enough to ignite(to some degree). Too lean a mix to combust even with the pressure from the crankcase (produced by the bottom side piston pumping action) just enough cold to make it miss fire. The healthy side was trying to go fat. That's called shorting a cyl. Once run for maybe a minute the piston pumping action trying to build pressure would push the ring out enough to seal the cyl. then initiate better compression, thus start to also assist pull from the port sufficient fuel to support combustion. Then run fine.
Carbon build up behind the ring in the lands prevents compression pressure build up quick enough to push the ring outward against the cyl wall and seal. This also provides the depression to help pull fuel from the crankcase to the chamber via the intake port. It was just ~lazy!
But like T.O.M. said, swap ignition lead sides to see if the problem goes with it to the opposite cyl. you have an ignition problem. Swap the plug next if it stays the same. See if it follows.
 

Tired Old Man

Staff member
Skip,

A couple friends and I (RTK and Altavillan) were out at a field one day for a little fun flying. Altavillan and I were having a great time but RTK was cussin' up a storm and just flippin' and flippin' and flippin trying to get his engine to start. When it did start it just didn't sound and perform right. RTK, he was going to change out the ignition as soon as he got that miserable beast back to the house. The end of the story has a conversation taking place where it was suggested his headers were the problem, not the ignition.

A little foolin' around and guess what? The ignition worked perfectly but the headers were preventing the engine from starting. The refraction timing (header length) was totally wrong, sucking the charge out of the cylinder. Now Altavillan, being the budget engineering specialist he is, always made his cans out of empty aluminum Bud bottles and legs sawn off a lawn chair. Damn things always worked.
 

Bunky f. knuckle

Cover shot, MA 10/09!!!
I'm getting around 6000 on a Fuchs 29x10. I had 16" cross over headers. I have chased the length down from there. Never seemed to make a difference until it started dropping off which was not what I was expecting to happen. Went back to 14" as a middle ground. The top end is fine. Pulls a vertical in a AW 100cc Ultimate as long as I need it, just not the pep of the MVVS/proflow combo. The real issue is the starting. Trying IMAC this season and the last thing I want to be fighting is a hard starting engine. I got a few other props I could try, Mejzlik 28x12 & 29x10S. Might help the top end but don't how it could effect the start up.
Thats a bit too much prop. That engine will be happier in the mid 6's. I flew a MenzS 28x10 on my 116. It was a turd. Engine never sounded happy. Sounded decent on the ground, but was ehhhhh in the air.

28x12 might be a bit better.
 

skipm

Member
I have a couple of proFlow cans, 1 in another plane. Would it tell me anything if I pulled the pipes and put them in with the 14" headers? The plane its in now has a shorter standard length DA header. Even with the longer length the cans should not pull the charge out of the cylinder. It would be interesting to see how the top end runs to determine if the pipes are actually working.

As for prop size, I knew it was lugging some but for my 3D skills it was ok. Hovers 1/2 to 3/4 throttle with solid pull out so I left it alone.
 

skipm

Member
An update, I put a set of canisters in the plane using the existing headers leaving the ignition & plugs in place. After retuning I got flights on the setup. Cold starts were much easier, sometimes 1 flip w/out choke. One time it did bulk for about 5 seconds on throttle up but quickly cleared. The pipes were apparently working as I lose 400 rpm in the setup. I'm guessing I need to make new headers that I can shorten until the starting clears. Any thoughts?
 

closetguy

Well-Known Member
that's weird that you have starting problems with the pipes.never had heard of that happening before.
I would at first, guess that your pipes are clogged,but that's not the case sense the rpm gain.
at lease you found your problem.
 

Tired Old Man

Staff member
I've seen it happen with a friends plane.

Pipes are like magic if you don't understand and do the math, and have all the dimensions that are related to the different parts inside a tuned pipe. Tuned pipes work on exhaust refraction and if that refraction timing is incorrect can and do cause a loss of power or inability to start. The exhaust charge departing the cylinder encounters a reflector inside the can that bounces the exhaust gasses back at the cylinder, causing a pressure wave. There is a period where both intake and exhaust ports are open inside the cylinder and if the exhaust wave hits a portion of that time period perfectly the engine will not be able to accept an incoming combustion charge. They can also draw the combustion charge out of a cylinder before the ignition point, leaving nothing behind that will burn.

Done perfectly you gain power. Done wrong and there will be no end to your problems, and diagnosis will be tough. Everything you do with the engine when using tuned pipes influences their performance. How you tune it, the prop you use, how that affects RPM and the way the engine is loaded, the diameter and length of the headers, the number of bends in the pips and the total amount of bend radius. Everything impacts pipe performance, which is why I won't use them. The engine manufacturers do not provide good reference points for pipe tuning so everyone thinks that if they worked on that engine they will work on their engine, and that one brand of pipe is better than another. I don't buy into that because pipes work only when the user gets everything else right. They don't and their small RPM gains typically reflect their errors. But everyone will take your money to sell them to you.

In this case I believe there is a combination of a prop that's too large inhibiting the engine from spinning up to a level that will utilize the pipe, some likely header length issues, possible pipe diameter error, and probably some pretty tight bends that are causing issues.

My perspective on pipes, power, and noise levels;

If you need more power, buy a larger engine.

If you want less noise, buy an even larger engine and muffle the hell out of it. You'll lose power but make less noise. You can throttle back to reduce RPM and prop noise. Note all the yard and wood cutting engines don't have pipes and just about every engine by any maker runs pretty good with a basic muffler/exhaust diverter. None of them were designed to run pipes, and a well set up pipe makes for a screamer engine you won't be able to use for anything but racing. We can be our own worst enemies.
 
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skipm

Member
i would like to thank everybody for considering my problem but I am afraid things have gotten off track with the prop discussion. I understand the prop is not allowing the engine to rev up but I'm not sure how the prop selection can be effecting the hard starting and cylinder missing until the motor warms up. For help with further discussions I have attached some pics of the headers in question, installed and removed. They are made up from 25mm KS header parts so size & bend radii should not be a problem. I still feel the length is the issue but in earlier experiments things got worse as I shortened so I went with the 14" length that works well on the 58 MVVS/smooth pipe combination. I was figuring both are high compression engines with the same size cylinder. I know, apples and oranges. Maybe I have to get below a hump. The intent of the longer headers was the minimize the peakiness the the throttle curve. The 29" Fuchs prop is intended to be able to be cut down to either 27 or 28" so Saturday night I cut down to 28" and regained the 400 rpm lost using the cans. Will see what it does back on the pipes. Also I was surprized the can setup was louder than the pipes. Anyway, thanks again. Skip
 

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Tired Old Man

Staff member
Pipes look nice.

For a cylinder to fire it only needs three things, fuel air, and a spark. One of the three is not working the way it should. A pipe most definitely can impact how the fuel enters a cylinder. If you have the pipes off, look at the side of the piston. If it's black you have header issues that are screwing with your engine's performance.

A cold cylinder says it's not getting fuel, spark, or enough air to combust the mix, or a combination of the three. If you have changed spark plugs, if you have changed ignitions, if you have swapped the plug leads from side to side with both ignitions and the problem persists, you only have fuel and air left. The problem IS NOT the engine, it's something working with the engine that's supposed to be in balance to make it run.

People always want to blame the engine, but rarely is the engine itself to blame for performance issues unless relatively severe damage has occurred. The carb does not deal with a specific cylinder in a twin, nor do the reeds. If only one reed is function the entire engine will run poorly, not just one side. A cylinder could be leaking from loose base screws or stripped spark plug threads, or it could even be cracked. You'd see a wet area if it was cracked, and likely traces of fuel leaking out the base of the cylinder if it was leaking there. BME has been taking quite a hit in this area of late, with three of them being dumped for all the wrong reasons, with the two that have been mounted in planes with the new owners running like a perfectly spinning top. One hasn't been mounted yet. So take a step back and through the process of elimination determine where the problem is originating.

OTH, you could have the engine tuned so rich the hot side is not hot at all, but just warm and the cold side is cold because it's too wet. If you can hold your finger on the hot cylinder after the engine has been running any length of time it's waaay too rich. Unless you're JediJody. He can grip a welding torch tip and not feel it. Tough hands:yikes:
 
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