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BollAero1.8 Running Gremlins

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Tim Wescott

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I wouldn't worry about fuel blow-back at low speeds. It's absolutely typical, and not a problem. It's a product of the intake being open for just as long after TDC as it is before TDC. As the piston comes down, a puff of gas comes out of the venturi -- as long as more goes in than out, the worst that'll happen is that the engine will be a bit inefficient, and will leave a trail of raw fuel behind the airplane as it goes.

That blow-back is typical even of crank-timed intakes; to attain higher top speeds the intake is left open after TDC to give the inrushing air time to stop moving. Only engines that are timed for very conservative running (i.e., easy-starting beginners engines) don't do this. Piston-timed engines can't avoid it, because the thing has to close the same number of degrees after TDC as it opens before TDC.

Now -- having said that have you checked the primary compression? What did you find? Have you put your hands on the engine and tested for leaks? If you put your face up to the exhaust port and slowly pull the engine through a rotation, you should be able to hear a "puff" as the transfer port is uncovered, and feel it on your face (your lips and cheeks are the most sensitive here).

A certain amount of leakage between piston and cylinder is normal (although perfect is always good). If you can rotate it at one to three rotations per second and feel or hear that "puff", then your bottom-end pump is doing its job, and you can stop worrying about it.
 

pat_pending

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OK. Status update. Spurred on by the helpful comments in this thread, I am taking it that there is nothing fundamentally wrong with the timings etc and I should keep going! My thought is that I had the hole in the spray bar facing in the direction of the flow rather than 90deg.

Since writing however I have b*ggered something else up and lost compression leading to it not firing at all anymore. I think in hindsight I rushed the lapping process and the fabrication of the contra-piston such that after a bit of playing, I scored the top of the bore. I will take my time and re-do and make a new piston using the excellent instructions from Adrian here:


This shouldn't take me long and hope to be reporting back soon.

Thanks again for the replies. Patrick
 

johwen

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In small engines dimensions are very critical for instance even if your porting is dead accurate a slight variation in the piston length both above and below gudgeon pin centre changes the timing of events. Have you checked the crankcase compression can leak through surface finishes on mating parts. A little shellac on mating parts can provide a good seal here. Crankcase volume can cause problems in small engines also keep all clearances to a minimum. Check crankcase compression by removing the cylinder head but not the cylinder and you should be able to feel some compression. To check for any leaks if you are a smoker/or friend who can blow some smoke through the carb vis a tube and then turn the crank to compress the smoke in the crank and observe if there are any leaks where there should not be. I would also try a new spry bar with just one hole pointing in the engine I would say with a number 70 drill because if the hole is too big and a flow through hole it probably won't create enough vacuum to draw fuel in. Another way to try would be to put a cap over the carb with a smaller hole the choke the air supply and see if it will pull fuel and run the gradually if it does, increase the choke hole slightly until you reach a point where it become temperamental. I that happens you may need to make a new barrel with a smaller opening. Even if you made carb to drawings a drill cutting slightly oversize could cause the problem Hope this identifies some areas you have not tried. Cheers.
John
 

Nibby2226

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Lapping is crucial. It took me three evenings to "creep up" on the piston fit - patience is key. I lapped until the piston was just binding just below TDC then cleaned everything down and made the final fit by gently rotating the oiled piston in the bore.
At this point it is very easy to get the piston stuck ☹
Final fit was just on the verge of sticking at tdc and a free sliding fit at bdc, the bore has a very slight taper.
 

KenC

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I think your engine is just running for a few seconds on residual fuel in the crank case and it is not sucking fuel when running. You said in a recent post that your cylinder is loose against the crankshaft. I think you must mean against the crank case. Your cylinder must not be loose or even a drop-in fit, it must be a very close fit that needs a good thumb press to get it in place. If it is loose the crank case can never be sealed because there will be a permanent leakage path between the cylinder outside diameter and the crank case and out through the hole in the case for the exhaust. This is something beginners often do not realise. If your cylinder is loose I am afraid your only solution is to make a new close fitting one. Sorry about that.
Ken, builder of 50+ engines
 

Rodrigo Castellanos

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I think your engine is just running for a few seconds on residual fuel in the crank case and it is not sucking fuel when running. You said in a recent post that your cylinder is loose against the crankshaft. I think you must mean against the crank case. Your cylinder must not be loose or even a drop-in fit, it must be a very close fit that needs a good thumb press to get it in place. If it is loose the crank case can never be sealed because there will be a permanent leakage path between the cylinder outside diameter and the crank case and out through the hole in the case for the exhaust. This is something beginners often do not realise. If your cylinder is loose I am afraid your only solution is to make a new close fitting one. Sorry about that.
Ken, builder of 50+ engines

Hi Ken!

Yes, that seems to be the problem, and even worse... in my first two tries, the carb was not screwed against the cylinder, just against the case.... so think al the internal pressure got lost between cylinder and case...

you don´t feel sorry as i don´t! i been learning a lot with the boll, and feel very happy about, as i was expecting a worse behavior.. for sure i will make a new cylinder/piston to get a runner engine.

Kind Regards
Rodrigo
 

KenC

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The carb only needs to be screwed into the crank case, not against the cylinder itself. If your cylinder is loose, in the case of the Boll Aero, you will not only have a case leak around the cylinder and out of the exhaust, the crank case will also leak permanently out of the carb, and the intake will simply not be timed by the piston because the carb will in effect be permanently open to the 'case.
Don't worry, many beginners do not realise on an engine like the Boll Aero or the Lubbock "Midge" of which it is a blatant copy, the cylinder to 'case fit is very important.
Ken
 

Ramon

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Hello Patrick, I don't visit here often these days but as a builder of diesel engines myself I always take an interest in others who do and especially if they are having problems.

I have read through all the replies, all of which offer good sound advice but I think there may be one small thing that no one has mentioned that may possibly be the root of your problem.

You say that the fuel will draw when you choke the engine but that it then runs back toward the tank. This usually indicates leakage in the fuel line somewhere (usually a loose fitting fuel tube) and looking at your picture of the venturi notice that the spray bar - on the fuel tube side - is threaded for quite a length. If this runs the full length of that side of the spray bar are you pushing the fuel tube directly on this threaded part ? If so that is probably where your leak is - there will be a leakage down the thread - it is much better for the fuel tube to mate with a smooth surface and preferably with a increase in diameter at the end to ensure a good seal.

A bit late with a reply but I hope this will help you solve the problem if you already haven't made the discovery.

Todate all of my engines have rewarded me with decent running but I have had a few commercial engines that haven't so quite share your frustration.

Good luck with it

Ramon (Tug)
 

pat_pending

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Hello Patrick, I don't visit here often these days but as a builder of diesel engines myself I always take an interest in others who do and especially if they are having problems.

I have read through all the replies, all of which offer good sound advice but I think there may be one small thing that no one has mentioned that may possibly be the root of your problem.

You say that the fuel will draw when you choke the engine but that it then runs back toward the tank. This usually indicates leakage in the fuel line somewhere (usually a loose fitting fuel tube) and looking at your picture of the venturi notice that the spray bar - on the fuel tube side - is threaded for quite a length. If this runs the full length of that side of the spray bar are you pushing the fuel tube directly on this threaded part ? If so that is probably where your leak is - there will be a leakage down the thread - it is much better for the fuel tube to mate with a smooth surface and preferably with a increase in diameter at the end to ensure a good seal.

A bit late with a reply but I hope this will help you solve the problem if you already haven't made the discovery.

Todate all of my engines have rewarded me with decent running but I have had a few commercial engines that haven't so quite share your frustration.

Good luck with it

Ramon (Tug)
Hi Thanks Ramon, ill remove the threaded part lower down and try again. I have made a little fuel bowl that screws on to that and the spray bar is that long to reach the bottom. I could shorten it and use a bit of tube too.
 

gadabout

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Are you sure the spray bar fuel hole is 1mm ? You say the spray bar is 3.4mm in dia but the hole looks far smaller than 3.4 times smaller, might just be the pic .
 

Ramon

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If you have a bowl type tank screwed directly to the spray bar with the threaded portion covered in fuel then my thoughts are invalid. However the symptom you describe is definitely one of leakage of air into the fuel system. If it will draw the fuel up with a finger choke then the 'internals' appear to be sound but you are losing the vaccum created for sure. You cound try as I suggest but using an isolated fuel tank - ideal starting point is that the horizontal centre line of the tank should be level with the venturi. If you still have the problem with a tight fitting fuel tube then you need to look elsewhere. Is there any air able to get in around the fit of the spray bar in the venturi for instance.

Having had another look at your image I notice the inlet hole is in the wrong position. A two hole spray bar set up should have the holes directly across the venturi ie 90 degrees to the airflo and a single hole set directly ( or very slightly off) pointing down the venturi. In either set up you should not be able to see the holes. Moving it may improve your running but it won't be the answer to your problem.

Usually, for the most part that is, if an engine will fire and run on a prime it will, given the correct timing, run continuously.

What size prop are you using?

Keep on picking away at it - you'll get there soon I'm sure
 

pat_pending

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Ok, a status update on this. Like all things as a beginner with this hobby/passion, sometimes when you think you're really close you have to back up again to get to the finish. Here's where I am at right now:

I mentioned above that I lost compression while I was testing. I suspect that its because didn't follow the advice for cleaning off diamond lapping paste properly and it kept lapping during the test firing. Lesson learned. My blast it with some WD40 and clean with a paper towel approach is just not enough.

I think the fit of the sleeve to the crank case is not tight enough. It's a light push but, as I thought the crank-case opening wasn't that critical from a finish perspective, I didn't take that much care and left it 'as bored'. I have since lapped it as best I could and will remake the cylinder liner with a tighter fit perhaps requiring a bit of heat applied to crank case for it to push in.

So I have re-made a new liner and piston and am in the process of lapping it and have a piston blank ready to lap to the bore when its done. I'm following Ramon's excellent guide to lapping here Lapping (and honing) - some techniques for discussion

I will get her back to life in no time and will have ruled-out a couple of the things on the list.

Update soon. Thanks for all the replies
 

pat_pending

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Are you sure the spray bar fuel hole is 1mm ? You say the spray bar is 3.4mm in dia but the hole looks far smaller than 3.4 times smaller, might just be the pic .
Hi, i think it was the pic and that was just a spec of crud. Here is a better photo. Thanks, P
IMG_0190.jpg
 

Ramon

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There is nothing wrong in using diamond paste to lap with but if the parts are unhardened then the particles will remain embedded in the surfaces of both parts. Even thorough cleaning with conventional solvents will not remove these particles quickly leading to a running in and equally fast if not faster running out situation. Sounds like you have just experienced that Patrick.

Diamond particles on soft materials can really only be effectively removed using ultra sonic cleaning.

It's been quite a time since I made a diesel - all this talk is proving quite infectious!

Tug
 

KenC

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Hello Patrick, I don't visit here often these days but as a builder of diesel engines myself I always take an interest in others who do and especially if they are having problems.

I have read through all the replies, all of which offer good sound advice but I think there may be one small thing that no one has mentioned that may possibly be the root of your problem.

You say that the fuel will draw when you choke the engine but that it then runs back toward the tank. This usually indicates leakage in the fuel line somewhere (usually a loose fitting fuel tube) and looking at your picture of the venturi notice that the spray bar - on the fuel tube side - is threaded for quite a length. If this runs the full length of that side of the spray bar are you pushing the fuel tube directly on this threaded part ? If so that is probably where your leak is - there will be a leakage down the thread - it is much better for the fuel tube to mate with a smooth surface and preferably with a increase in diameter at the end to ensure a good seal.

A bit late with a reply but I hope this will help you solve the problem if you already haven't made the discovery.

Todate all of my engines have rewarded me with decent running but I have had a few commercial engines that haven't so quite share your frustration.

Good luck with it

Ramon (Tug)
Ramon. The Boll Aero is a sideport engine in which induction is timed by the piston. As such when turned by hand the carb will both suck and blow. So one can expect that with a finger blocking the carb, as the engine is turned over the fuel will be sucked along the line and then it will be blown back again as the poster has experienced. In a properly constructed engine this in no way shows that suction nor timing is a problem. Of course when an engine is actually running the system is dynamic and therefore the suck and blow no longer applies to such an extent. I suspect the poster does in fact have a whole range of issues with his engine and not just one cause of it not running. He has not used the dimensions from the drawing but has adapted dimensions to suit his tools, he has a cylinder which is rattling loose in the case with permanent leakage around it and out of the carb so there really is no induction timing at all, he has not understood which fits are important, and finally he has clapped-out the engine by lapping with diamond compound and then not ultrasonic cleaning the parts. He appears to have a whole bunch of troubles best remedied by starting again, but he deserves high marks for persistence.
Ken
 

Ramon

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Hello Ken,

Yes I am aware the engine is sideport but last year I spent a frustrating afternoon trying to get a friends Amco 3.5BB to run in a model. It was doing exactly the same thing - fuel drawing up on a choke then running back. Investigation finally revealed that the tube had hardened on the short stub of the spray bar and due to it's tight location in the model had slightly stressed itself to allow air in between tube and spray bar.

I can see that Patrick is struggling with this hence happy to help if possible and would agree that there may be other issues that need to be overcome. I'm sure you would agree however that there is nothing more satisfying than seeing an engine just built burst into life so to help someone overcome difficulties with their early building of these little engines is something I'm more than happpy to do if I can.

Havn't made anything since the Olivers - did you find that GMA article in SIC?

Regards - Tug
 

KenC

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No I didn't find that actual article, but knowing George well enough since he was a founder member of our little group, his subriquet is rather like the one that I would apply to a well known personality on the Barton forum, "Always certain, often wrong". George was the master of what he did at the time, but I do not regard him as a reference. I think this discussion came about because of a comment that I made about never lapping a piston and cylinder together, always apart. I was trying to be helpful to beginners in engine building because there is general misunderstanding about what lapping a piston etc actually means. But of course there are times when a final fit can be achieved by lapping piston and cylinder together usually just with oil, in fact I have seen world champions in tether car racing do just that at meetings in Germany.
Nothing made since the Olivers? How about a bunch of 5cc Bugls. That would be interesting!
take care and stay safe
Ken
 

Ramon

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Hi Ken,

Ha ha! If the person on Barton is who I think you mean then I'm afraid I'm not able to share the same opinion. In my dealings with both I found George to be very helpful which is not something I can say about the other - 'nuff said on that one!

Yes, George was a 'master' at the time but his knowledge that he was always happy to share is still relevant today in it's place and certainly in the area we are discussing - that of building small diesel engines. That the more esoteric high end competitive motors have moved on a huge pace since his day is without question but I'm sure you'd be the first to agree that were he still alive George would be up there amongst it.

Personally I value the information he published and shared over the years - it's still as valid today (for me) as when I first read it. I think I said before sometime - cat skinning is a vast subject. I'll offer my thoughts but always try to ensure the recipient knows that whilst it's 'my way' it's certainly not 'The Way'.

You are not the first to suggest the Bugl :), Ian Russell had a similar thought - I have to agree with both of you, it would make a cracking subject but the desire isn't quite there at the moment. Currently finishing off a Corliss valved steam engine that's been around a bit too long.

Regards Ken - stay safe too

Tug
 

KenC

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My suggestion of the Bugl wasn't really a suggestion at all, just a poor attempt at a joke. Sorry.
 

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