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Discussion Starter · #1 ·
Hey guys,

I'm new to motorcycle engines so please bear with.

I am building a custom trike and using a CBR500R engine. I have spark and fuel pump is running when I press the start button but the engine won't fire. When the engine is cranking but makes a thumping sound. Video is here

I don't have the exhaust fitted and have replaced the air box with two small air filters.

Any ideas?

Thanks
 

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The "dub-dub" sound is likely the normal intake noise which is usually muffled by the OE airbox.

Do you have a can of ether (aka "Quick start spray")? If so, remove the air filters and spray a bit into the throats of each intake while cranking. If it fires and runs momentarily you probably don't have fuel injection happening even if the pump is running.

If you don't have ether, you can try brake clean but don't breathe the exhaust produced if it starts (i.e. do the test outdoors in a well-ventilated area.)
 

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The engine is designed to start with the throttle closed. Just in case you are opening the throttle wide.

If you have the engine up on a workstand and a container of gasoline on the floor with a hose to the fuel pump,
there might be a problem with the fuel pump not priming (not able to draw fuel up that far into itself).
If that is the case, try raising the gasoline container up a foot or two while not letting the hose dip back down near the floor.

Could you have an air leak in the fuel hose connections leading to the pump?

Given what I might try myself if frustrated, the shop manual warns: don't use commercial carburetor cleaner in the throttle bores.
 

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Discussion Starter · #5 ·
@Blackfin

Ok bought some of the quick start spray and will try it later but just wanted to check if i should be spraying it with the throttle closed or open the throttle a little and then spray it?

@vt500 I have a short fuel line (around 40cm) going directly to the pump which is submerged in the fuel and is only about 1ft below the carb, when the fuel pump starts it does look as though there is more than enough pressure to push the fuel to the carbs.

Thanks Guys
 

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All you're trying to do is put something combustible (ether) into the combustion chamber; to that end, it won't matter much if the plates are closed or cracked a bit: You're just looking for signs of life.

It'll be easier to get a usable quantity in there if you crack the plates a little.
 

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Discussion Starter · #7 ·
@Blackfin

OK so tried the spray and it sounds like the engine really wants to fire and start but just wont, the engine popped a couple of times but didnt fire, im starting to think it might be a fuel supply issue.

Ive checked the spark plugs and there is still a spark and have cranked the engine with the spark plugs out to clear it and then tried again but still the same thing. A thought i did have was that as i have taken the air box off would that be now throwing the air/fuel mixture out of wack and causing it? maybe not enough fuel?
 

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I know it might sound stupid, but have you checked if the plugs would like to ignite in the right order?
 

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Discussion Starter · #9 ·
I have tried switching the plugs to see if that makes a difference but nothing. While i am on this is there anyway to determine which plug goes to which cylinder? One of the plugs has a little number 2 on it but i cant find a way to see which is number 1 and which is number 2.
 

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Long shot but can you try swapping the connectors at the ignition coil in case you have 1 on 2 and 2 on 1 (as alluded to by @Oyabun)?

The engine should fire and run for a second or two on a shot of ether even with zero fuel.
 

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Discussion Starter · #11 ·
@Blackfin

Ok so figured out the way I had round I had the coils was right.

There seems to be more happening each time I try. The engine really sounds like it wants to fire.

Now I'm getting what sounds like a bubble popping. Video is here


Just to mention I don't have an exhaust attached if that makes a difference.
 

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Can't tell from the vid: Are you spraying ether into the intakes while cranking? I see a filter still in place...
 

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Well, ether should light off if the engine has good compression, a spark at the right time and air/fuel is making it into the chamber.

Can you try with the throttle cracked to ~50%?

Can you remove the plugs and check them? Are they wet/soaked? Or dry?

Do you have the tool to check compression?
 

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Discussion Starter · #15 ·
@Blackfin

Have checked the plugs and they are both dry. Have tried with the throttle open a little also. The annoying this is that it sounds like it wants to fire!

I dont have a compression tool no, so cant check that unless there is a way to do it without the tool.
 

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It does sound like it wants to fire. Try:

- while not cranking, hold the throttle wide open and spraying ether into each intake -- a two or three second shot per hole. Then hold the throttle open a bit and attempt to start it.

- another old trick to test-fire an engine; remove the plugs and dribble a little bit of fuel (preferably a 50:1 two-stroke mix but straight gas will work for a quick test) right into the cylinder; re-install the plugs and attempt to start it while holding the throttle open a bit to allow some air in.
 

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You have had some cracking advice on hear. This is just my two penneth. All that squirting you have done I would have thought it would have started by now. Did you hear the engine running before purchase ? If not I would go back to basics, check valve timing, compression test, valve clearance. If all is ok then some way of checking spark at correct time and new spark plugs.Maybe major air leak at inlet manifold. From the video it sounds like a compression problem but that's easy for me to say. Good luck.
 

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Discussion Starter · #18 ·
Ok still got nothing. I'm going to get some new fuel tomorrow just incase the fuel has gone bad and I'm going to take the carb off, clean and check the inlet manifold. I've ordered a compression tester too so will check that when it arrives. That way at least I've tried everything I can. If not, anyone wanna buy an engine? Haha
 

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The reason I'm suggesting a compression test is because ether is called "quick start" for a reason; it lights off extremely easily. Vacuum leaks and old fuel wouldn't really matter with ether: If the engine won't fire properly on that stuff there may be a problem with the spark, its timing or the engine may be very low on compression.

Of note is that you said the plugs were dry: If the fuel pump is pressurizing the system but the plugs are dry after all that cranking it may be that the injectors are not firing. This wouldn't affect the ability of the engine to start and run for a few seconds on ether but it may point to problems that may need to be solved after you get it running on the laughing gas.
 
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