Not signed in (Sign In)

Vanilla 1.1.5a is a product of Lussumo. More Information: Documentation, Community Support.

  1.  
    Hey guys, first off I would like to thank you for letting me join the group. I built Bob Shores Little Angel over the past couple months, and have not been able to get it to fire at all over the past couple weeks of trying to get it started. I believe I have narrowed this down to a compression issue but I wanted to get everyone's opinion. I do not believe that the engine has fired one time in my almost 30hrs of trying to get it started. I have messed with just about everything that I can but the compression, which includes relapping the valves so they are now very smooth moving and have a great fit with the carb, experimenting with valve springs (now I have it back very close to the plans just enough to keep the intake closed, but it is easy to open and a little stronger spring on the exhaust valve), and I have experimented with timing as well (now set very close to the print firing at about 5deg BTDC, exhast valve opening at 140deg on Bob's degree wheel with 0 at TDC, and closing at 10 deg). I can see the motor pulling fuel up the fuel line both with my finger over the intake hole and off, I have also built the intake reducer and tried this as well. I am using the Hall effects sensor and have verified that I have good fire at the right time as well. Right now I cannot feel a great amount of compression when turning the flywheel, and what I do feel does not seem to stay for very long if I stop movement on the compression stroke. My next step is trying to make a piston ring over the weekend to see if this improves compression, if that doesn't work I believe I will be rebuilding the whole front end to try and get better fits. Any help you have to offer would be great.

    Thanks,
    J.R.
    • CommentAuthorgbritnell
    • CommentTimeJan 12th 2012 edited
     
    Hi J.R.
    The issues you are going through are the same that almost everyone starting out building IC engines has. The 3 basics of operation are fuel, compression and ignition. Without any of the 3 the engine won't run.
    Let's start with the compression. There are 2 elements that need to be addressed for good compression. The first is the piston\ ring fit and sealing within the cylinder bore. Assuming that your bore is round and the piston has about .0015-.002 clearance you should be able to turn the engine over on the compression stroke and get a good bounce back on the rod. The best way to check this out is to remove the cylinder head and just hold the heel of your hand over the bore. Now turn the engine over. By doing it this way you eliminate any leakage from the head. If you don't have good compression with this test then you have to find out what is wrong, bore concentricity, piston fit etc. If you do have good compression then the next thing that needs to be checked is the valve sealing. When checking the valve seal the best way to do this is by some means of sucking in the exhaust and intake port. On a small engine sometimes you have to make a tube with a slight taper on it to push into the port. Now attach a piece of tubing to the tube and then suck with you mouth. If you feel any leakage then the valve isn't sealing properly and the fit at the seat should be checked. Once you have this checked out it's time to move to the ignition.
    The ignition can be anywhere from 5-15 degrees BTDC. With these engines it's not critical but should always be set somewhat before TDC. This is the practice with full sized engines and it applies to these miniatures as well.
    The last thing to check out is the carburetor. To try and get the engine to start you need to open the needle valve just the slightest amount, 1/4-1/2 turn. Now turn the engine over. If there is no response then choke the engine by turning it over and putting your finger over the intake for just one stroke. You should see fuel being pulled through the line. Now turn the engine over again several times. It should try to make a pop, if not then open the needle no more than 1/8 turn more and repeat the above. It should get to the point where the engine will pop. If it does but won't keep running then open the needle a little bit more. With these carbs and short taper needles you have to sneak up on the fuel settings. It doesn't take much to go from lean to flooded. Once the engine starts and runs the carb can be adjusted for optimal running.
    It sounds like a lot of work but as I stated at the beginning you need all 3 of the elements of operation working before the engine will run.
    gbritnell
  2.  
    Thanks for the quick response and all the helpful hints. Just an update, at this time I have made a good piston ring, weakened the intake spring, and made some other adjustments to the engine so that I am now getting it to fire on ocassion. I have ran the carb up from the closed position in 1/8 turn steps like you suggested with both the intake fully open and with the option intake restrictor from the plans. I have found that it hits best if I choke the carb after adjusting the setting and then it will hit for about 3 or 4 times while I am turning the flywheel, but has not yet run fully on its own. I believe that this is probably a timing issue, but I have some questions about the proper timing of the engine. The plans say the exhaust valve should open at 140 after top dead center. I have seen vaious posts regarding if this is the actual "140" degree mark on Bob's compass or if this is actually 140 degrees ATDC (220 on the compass). Does anyone know the correct answer to this? Also does this sound like a timing issue, or is it possibly in the carb adjustments? I have also noticed that on occasion I will get some exhast blowback through the intake of the carb, but this only happens ocassionally, do I need to make the intake spring slightly stiffer? Also I have not put a check valve in the fuel line, I have read some posts over the internet about this, but is this an absolute nessecity to get the motor to run. Thanks in advance for all the help.

    JR
  3.  
    Try setting the spark at 25 deg. BTDC. This may be somewhat early for hand cranking but if you can spin it a little faster it should run. I have had the same problem with a couple of my engines. If compression is a little low they are hard to start with low speed cranking. Advancing the spark and cranking by a rope the low compression does not matter so much. After I ran my engine some to break it in the compression improved to where I can start it by hand.
    • CommentAuthorricmazzo
    • CommentTimeJan 17th 2012 edited
     
    void
    • CommentAuthorricmazzo
    • CommentTimeJan 17th 2012 edited
     
    void
    • CommentAuthorricmazzo
    • CommentTimeJan 18th 2012 edited
     
    void
    • CommentAuthorricmazzo
    • CommentTimeJan 18th 2012 edited
     
    void
    • CommentAuthorricmazzo
    • CommentTimeJan 18th 2012 edited
     
    My apologies I had posted on the wrong place my issue.
    • CommentAuthorgbritnell
    • CommentTimeJan 18th 2012
     
    Hi JR,
    Timing is given in crankshaft degrees so 140 degrees after TDC would be 40 degrees before BDC. This point is not critical, what's more important is that the valve closes near or just slightly after TDC. As far as ignition timing I would say that 5-15 degrees before TDC should be adequate.
    gbritnell