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Thread: Sr 50 Ditech correct fuel and air pressures?

  1. #1
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    Sr 50 Ditech correct fuel and air pressures?

    I have a Sr 50 Ditech 2002 H2O I am trying to fix.

    Can someone tell me what the correct air pressure and fuel pressure would be when cranking and during operation?

    Can anyone please describe me how pressure should change in both lines during cranking and when it should finally start (at what pressures)?

    I am looking for as much info as possible because I'm having some problems with air pressure I think.

  2. #2
    apriliaforum expert Snugg's Avatar
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    Air pressure is going to change depending on altitude and what crank you have.
    Air pressure should be between 60-70psi

    Dead head fuel pressure should be around 140psi
    Regulated fuel pressure is 36psi
    Running fuel pressure = 36psi + air pressure

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    Hey, thank you for the reply.

    Thing is, I've bought a new air compressor and while cranking, the pressure won't go higher than max 3 bar(43 psi) no matter how much I crank it. If I hook the air line to an external compressor at 5-6Bar(72-87PSI) the scooter starts up witout problems and the engine works as it should.

    So I don't know where the problem lies. Should I be testing the fuel pressure?

    Also is there a video of how fast the air pressure should be rising normally when cranking?

  4. #4
    Moderator (ADSUM) Grenadiers's Avatar
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    Quote Originally Posted by pingo View Post
    Hey, thank you for the reply.

    Thing is, I've bought a new air compressor and while cranking, the pressure won't go higher than max 3 bar(43 psi) no matter how much I crank it. If I hook the air line to an external compressor at 5-6Bar(72-87PSI) the scooter starts up witout problems and the engine works as it should.

    So I don't know where the problem lies. Should I be testing the fuel pressure?

    Also is there a video of how fast the air pressure should be rising normally when cranking?
    If you are able to run your scoot from an external Air source, then chances are good that your Fuel Pump is OK. To verify, “T” in a pressure gauge and see what readings you get.

    Your Air Compressor should be producing more pressure then that. You either have a leak, perhaps the large Compressor “O” ring is shot/damaged, or the smaller “O” ring on the Air Line connector, or the leak could be from the hose clamps you are using.

    The other thing is that perhaps the Air Compressors small wheel isn’t engaging the Crankshaft properly.

    Our Air Compressors are High Pressure, Low Volume pumps. As such, they will build up pressure fairly fast providing that there are no leaks in the system.

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  5. #5
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    I've also bought a new Compressor O ring. The smaller O ring I have not checked, I'll try to find it and inspect/replace it,

    I will connect the air compressor directly to the gauge only and monitor what pressure it read and if the pressure is stable and report back.

  6. #6
    Moderator (ADSUM) Grenadiers's Avatar
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    Quote Originally Posted by pingo View Post
    I've also bought a new Compressor O ring. The smaller O ring I have not checked, I'll try to find it and inspect/replace it,

    I will connect the air compressor directly to the gauge only and monitor what pressure it read and if the pressure is stable and report back.
    If the Air Compressor Line is more then 2 or 3 years old, you may want to replace it with a new hose... Specifically, Fuel Injector hose and secure it at both ends with Fuel Injector Hose Clamps and not the standard worm-screw style as they tend to bite into the hose and cause a potential leak site.


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    The Air compressor line was 8+ years old and I've bought new hose but that didn't help.

    I ran some tests. I connected the pressure gauge directly to the air compressor and cranked. It took it 16 seconds to build up to 5 Bar is that ok? There was no pressure loss after so I take it that there are no leaks on the compressor's side.

    Some form of leak must be on the other side - the injector's side since the pressure never goes over 3Bar when everything is connected and cranking, right?

    I am inspecting the outer side of the injector, here is the schema is the o-ring number 11 supposed to fit tigthly?

    Maybe I should pull out the whole injector and replace all o-rings. Also for reference, in this and this threads I've found some useful info and photos which I'll use to compare my injector against.

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    Any comments?

    Is there a link where the whole injection is explained in details?

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    Moderator (ADSUM) Grenadiers's Avatar
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    Quote Originally Posted by pingo View Post
    The Air compressor line was 8+ years old and I've bought new hose but that didn't help.

    I ran some tests. I connected the pressure gauge directly to the air compressor and cranked. It took it 16 seconds to build up to 5 Bar is that ok? There was no pressure loss after so I take it that there are no leaks on the compressor's side.

    5 Bar is good from the Compressor... For bests results, you should "T" in your gauge rather then a direct connection.

    Some form of leak must be on the other side - the injector's side since the pressure never goes over 3Bar when everything is connected and cranking, right?

    Did you do a Soapy water test to see if there are any leaks? It dosn't have to be a big leak for problems to happen... Most likely places for leaks (on new or newer hose) is at the clamp site.

    I am inspecting the outer side of the injector, here is the schema is the o-ring number 11 supposed to fit tigthly?

    No... but to hlp during instlation, I used some gasket sealer spray to coat one side of the "O" ring to help it to stick to the Injector and hold iot in place.

    Maybe I should pull out the whole injector and replace all o-rings. Also for reference, in this and this threads I've found some useful info and photos which I'll use to compare my injector against.

    Removing the Air and Fuel injectors is something that I would do only as a last resort. THe reason being is that on the Air Injector there is a Teflon Ring and if you damage that on reinstalation, you will be in for a lot more trouble as a damaged Teflon Ring can cause hot burning gasses to get buy and cook the injector.
    Keep in mind that you will want to have a new "O" set on hand if you decide to open up the Fuel Rail and the injectors as you DO NOT want to re-use old "O" rings. Especially if they have a LOT of miles on them...

    If you remove the Air Compressor barb from the Fuel Rail, you will notice a small “O” ring on the end… That one may need to be replaced.

    But before you do anything else, take a spray bottle and make yourself a solution of soapy water, then, with the engine running or with the help of someone cranking the motor (NO MORE then 10 seconds as you can burn out the starter motor if you crank for extended periods of time!!), spray the solution on the Air Compressor line from the Compressor to the fuel Rail and see if there are any bubbles being produced.


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  10. #10
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    To clear something up, if i pump let's say 5 bar of pressure to the injector's side of air line, should it keep that pressure when the engine is not running? Is that line supposed to be shut and air tight when not running or not?

  11. #11
    Moderator (ADSUM) Grenadiers's Avatar
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    Quote Originally Posted by pingo View Post
    To clear something up, if i pump let's say 5 bar of pressure to the injector's side of air line, should it keep that pressure when the engine is not running? Is that line supposed to be shut and air tight when not running or not?
    As long as the air line is tight with no leaks, it should hold for anywhere from 1 to 4 days...

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  12. #12
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    Thanks for the advice. Here is my report:

    Just checked, both sides of the air line keep pressure (5bar) fine when I'm not cranking the engine. There were no bubbles on hose connections. I even removed all hose clamps out of curiosity and there were still no leaks/bubbles on connections at 5Bar and the gauge showed no leaking. So I'd say the hose fits pretty tight and does not leak even without clamps.

    At this point I should provide some more info:
    I am trying to start an engine that has been rebuilt recently (50 cc piston + cylinder kit, crankshaft bearings, head gasket, base gasket, crank gasket) .

    The shop where I bought the parts at offered the service for free so I let them do it as I don't have much experience.

    Since the air pressure does not raise over 2, max 3 bar when I crank the engine even for long periods, could it be that there is a leak inside the engine?

    Also on the head and base gasket there seems to be some black silicone sealant applied is this normal?

    Maybe I should do a leakdown test? What do you think? What should I try next?

  13. #13
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    ANY sign of silicone or any other type of common aerobic sealant any where on or around the base/head is a sure sign of an I-d-i-o-t. I build/repair marine Two strokes every day, some that go to 10k Rpm plus. I can't tell you how may times ive seen this on consumer vessels- its sad really, and a SURE sign of TROUBLE. you better take compression readings real quick. and thanks for your post, as i have a ditech I rescues that i Just put together, and couldnt find the air specs til now.- Dave

  14. #14
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    Just a quick question... I'm buying a new water trap today as the trap I have now isn't good. The new trap can handle 6.3 bar maximum. Is that enough? And sorry for posting this to your thread. Thanks!
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    apriliaforum prov-nov polepeach's Avatar
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    If it is the Original one that came with the bike, it more than likely too old, so if i was in your shoes i would replace it. just a hunch



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