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D1 engine running probem
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TonyF
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May 13, 2019 - 9:33 am
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I have a D1 Bantam recently purchased from a person who only used it for display.  It is absolutely immaculate, but I am having difficulty in getting it to run properly.  I have very little 2-stroke knowledge but competent with classic and vintage cars.  So I am asking for some advice please.

It starts OK, usually 2nd or 3rd kick.  It runs but does not rev happily and I doubt fast enough.  It sounds like it isn’t firing on every cycle.  It pulls OK round the back lawn but I haven’t tried it on the road, but I suspect I would be lucky to get 20mph out of it! Whatever position you have the choke it doesn’t affect how the bike runs!! (Not sure if it is over rich or weak!!).  It runs a bit smoky so I assume it isn’t burning the fuel mix properly.

Previous owner recognised the problem and had the carb and mag overhauled but to no avail (I have the invoices for this work).

I have done the following which has made no difference:

  • Drained and replaced the fuel using a 32:1 mix with good mineral 2-stroke oil
  • Check the fuel line and tap – both are new from the previous owner.
  • Stripped the carb and blown it though with an airline – I know the previous owner had had it overhauled but I did this just to make sure it hadn’t picked up a bit of dirt. Jet is correct 75.
  • Checked points, HT lead which looks new and NGK spark plug which looks new.

The one thing I have spotted is that there is a slight fuel seepage from the crankcase just below the carb. It is where the barrel screw goes though.  A little seeps up from the casing and a little seeps round the screw.  This is only slight but would this crankcase leakage be enough to cause the problem?  If not any other ideas?

Thanks,

 

Tony

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cocorico
Central France
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May 13, 2019 - 9:46 am
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Hello Tony. The smaller Bantams don't normally need a choke unless it's very cold.  Pressing the tickler gently until you get a weep of fuel from the overflow hole is the normal procedure for a cold engine. It may be worth removing the crankcase drain plug (the small one on the generator side of the lower frame) as you may have accumulated some unburned fuel there. Your petroil mix should be 25:1 for a D1 (to start with) and the smokiness could also be due to the fact that you have not managed a long enough run to burn off the residues. Have you removed the spark plug and checked for a spark with it outside the cylinder?

1956 D3 running, lights to sort. 7 other bikes in the Barn, 6 runners (when I get time!) and a still in progress Morini 250

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TonyF
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May 15, 2019 - 8:07 am
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Hi,  I don’t think I explained clearly the affect of the choke.  With the engine hot and running if I close the choke it makes no difference to how the engine is running.   I would expect that closing the choke would make the engine slow and labour a bit.  Also removing the choke/filter unit makes no difference.  It infers to me that the engine is running very rich or very weak.

I have removed the plug and kicked the engine over with the plug resting on the crankcase.  There is a very strong spark.  The needle was on position 3 (from top) and I have moved it to position 2.  However, it makes no difference to how the engine is running.  So I have still got the problem.

I have also tried a compression test and I get 80 PSI which I guess is OK.

Anybody with ideas to try?

Thanks,

Tony

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Sponge
Lancashire (A chip shop somewhere near Preston)
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May 15, 2019 - 9:00 am
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Hi TonyF, 

If it doesn't care if the choke is open or closed then its getting its air from someplace else (leak ?).  

Sounds like carb or air leaks - possibly both.  

Things to check:

Drain the crankcase and see what comes out. 

Dismantle teh carb and check for any blocked or obstructed jets, also check the float to see if it is actually floating.

Get some washing up liquid around all the carb and crankcase joints and run the engine. If you see bubbles anywhere then you have a leak. 

Hope this helps and keep persisting - change only one thing at a time and eventually you will get there.

Sponge   

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cocorico
Central France
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May 15, 2019 - 9:39 am
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If you look in FAQs, check the index to find a topic on the overhaul of the AMAL carb used on the D1, it's a very thorough description. 80psi sounds a little low to me, have you tried after adding a little oil through the spark plug hole to improve the seal around the rings?

Have a look ** Please log in to view **.

1956 D3 running, lights to sort. 7 other bikes in the Barn, 6 runners (when I get time!) and a still in progress Morini 250

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TonyF
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May 25, 2019 - 8:21 am
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I have again stripped down the carb and made sure that it is all clean. Blown through again with an airline and gently put some plastic “wire” though the 2 small passages just to make sure.  Remember this was an overhauled carb (in Feb) to try and overcome this problem so I think it isn’t the carb causing the problem.

Put a stroboscopic timing light on it and it gives very regular fast flashes as I would expect.  I think, as best as I can tell by ear, that the engine fires properly only every 2 or 3 flashes.  Is there anyway with a strobe that you can accurately check the timing, ie with the side cover off what would you align the flywheel to with a strobe to check it is firing at the correct time.  I appreciate I could do it very crudely by finding top dead centre down the plug hole with a bit of wire then putting a white mark on the flywheel. 

Next time I get a few moments I will try a bit of oil in the cylinder to see I get a better compression ratio.  What should the compression ratio be on a Bantam – I would have thought 80psi would be OK?   

Any thoughts would be appreciated.

Tony

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