Strange spark test

[ianyoung77]

I have a 1990 50Hp Yamaha and it was running rough so I did the spark test and found #2 had no affect removing HT lead. I then swapped #1 & #2 plugs and the engine would not start. I then replaced the faulty plug (now in #1). I repeated the lead test and found #1 & #3 ok but #2 makes the idle rougher (fitted with original "good" #1 plug).<br /><br />Obviously the spark in #2 is now doing something but what can make it rougher? I thought maybe timing but that seems ok. I think this cylinder drives the fuel pump but that is also new. Can a failed fule pump affect #2? I remember reading in one post that you shouldn't be able to pump past the fuel pump with the primer bulb. Is this correct?<br /><br />I have checked all of the fuel system I know of and even swapped #1 & #2 carbies and the problem remains on #2.<br /><br />Thanks for any help

 

[ianyoung77]

Re: Strange spark test

I'm thinking the problem may be due to carbon build up on #2 cylinder (it had an intermittent ignition coil). <br /><br />Unfortunately we do not get any of the normal products like seafom or ring free in Australia. Does anyone know what a suitable product is?

 

[bigbrownbuku]

Re: Strange spark test

if you remove the plug lead and the idle is rougher thats good. if you remove a plug lead and the idle makes a scant difference thats not so good. check your throttle sync.

 

[ianyoung77]

Re: Strange spark test

Cheers Ben,<br /><br />What's happening is when I remoe the plug lead it runs better. As soon as I reconnect it the engine runs rougher. I'm hoping there is maybe some carbon in the cylinder.<br /><br />I have checked the synch visually and it looks good. Is there any other way to check it?

 

[ianyoung77]

Re: Strange spark test

OK,<br /><br />I have done the spark test and the squirting petrol test. The spark is ok but squirting petrol made no difference. Rechecked the compression and it was all good. Checked the fuel and oil pump pressures all good. <br /><br />Following Rodbolts suck squeeze bang blow I am thinking the vacuum (suck) may not be good. Tried checking the vacuum and found it was very little at the oil intake (running on 50:1). How is this normally checked. Can a poor connection cause this problem. Would it more likely be the reed gasket or the carby gasket? Any other ideas?

 

[bigbrownbuku]

Re: Strange spark test

do a compression test, should all be even and above 100.<br /><br />kitting the fuel pump and cleaningthe carbs isnt a bad idea to start.<br /><br />the ONLY engine ive ever seen run better with a plug lead disconnected had a piston that had separated from the rod which was clanging up and down the cylinder as it idled.<br /><br />the sync is easy, the screws are left handed threads, note carefully the arrow on the throttle butterfly shafts where the screws are. disconnect the oil link rod(if there is one) back off the throttle stop screw completely and the cam follower screw(the little roller), then undo the sync screws on the throttle butterfly shaft. light pressure on each butterfly(closed) starting at the top do them up again, i try to do this part in stages. reconnect everything and adjust the idle stop roughly, leave the cam follower loose. this is all dependent on your idle mix screws not being damaged, check them visually then reset them roughly to 1.5 turns out.<br /><br />if it still wont idle true do it again, if it still wont idle true you have a carb or a reed problem, maybe a bad crankcase seal. if you remove the carbs again you can see the reeds, use a torch( an australian flashlight to all you americans that just muttered wtf?)

 

[ianyoung77]

Re: Strange spark test

Ben, Thanks for the comments it is much appreciated.<br /><br />The compression test was fine - all above 120. Slight variation but less than 10.<br /><br />The idle is rough but the motor runs WOT ok if I can get through 2000rpm. The compression looks consistent every pump so I hope the rod is still connected.<br /><br />I have synched as suggested but it still runs the same. I even tried having #2 slightly more open than #1 and it made no difference.<br /><br />I took the reeds out and inspected them. They looked fine. I was going to try a gram gauge test to check their spring and swap #1 & #2 to see if it makes any difference. When I have the reeds out I should be able to check all pistons are connected to their rods.<br /><br />Which crankcase seal are you referring to? The one between the reeds amd the crankcase? I would assume most of the other seals are covered by the compression test.

 

[bigbrownbuku]

Re: Strange spark test

the piston connected to the rod comment has no bearing on your engine, it was an anecdote.<br /><br />my gut is telling me you havent cleaned the carbs well or there is still a problem in that area you cant recognise.<br /><br />try messing with the idle mixture adjustments to see if that helps. the cam follower(the little roller that moves the butterflies) pickup timing is very close to idle timing so set it very close so the throttles open very soon after the throttle lever moves. if it has a flat spot on it replace it.<br /><br />shift the idle mixture screw in slowly in small increments and wait for change after every adjustment. the engine will increase in rpm slightly at the point you want to stop and back it out 1/2 turn from that point.

 

[ianyoung77]

Re: Strange spark test

Cheers Ben,<br /><br />I tried swapping the carbs between #1 & #2 and the problem remained on #2. From that I assumed the carby could not be the problem otherwise it should have followed the dirty carb.<br /><br />I will replace all the gaskets around the intake and see what happens. Thanks again

 

[Grant S]

Re: Strange spark test

Are you able to swap the coils over in the same way you swapped the carbs?

 

[ianyoung77]

Re: Strange spark test

Grant,<br /><br />Thanls for the comments. I tried swapping the coils as well. Obviously I could not swap the CDI outputs though.

 

[bigbrownbuku]

Re: Strange spark test

Originally posted by ianyoung77:<br /> I'm thinking the problem may be due to carbon build up on #2 cylinder (it had an intermittent ignition coil). <br /><br />Unfortunately we do not get any of the normal products like seafom or ring free in Australia. Does anyone know what a suitable product is?

you can use a mercury product called powertune.<br /><br />do you have any fuel filters betwen your tank and engine?<br />have the carbs been cleaned recently?<br />did you back off the throttle stop screw during the throttle plate sync?

 

[ianyoung77]

Re: Strange spark test

Ben,<br /><br />I tried the Yamaha product which is called combustion chamber cleaner. It had very little effect.<br /><br />No, I do not have any additional filters. Since I only run the portable tank I'm not sure I need it since I change the petrol often.<br /><br />Yes I backed off the throttle screw and oil contol rod. Thanks for the comments - I will keep trying.

 

[bigbrownbuku]

Re: Strange spark test

did you remove the carbs and clean them?

 

[ianyoung77]

Re: Strange spark test

Ben,<br /><br />Is there another way? Originally I took the motor to a workshop and noticed no real difference after they did the carbs. I then pulled them down myself a couple of months later and found gunk under the pilot cap. I then fully cleaned them and changed the needles. This has led to a distrust of some repairers.

 

[bigbrownbuku]

Re: Strange spark test

if you have good spark and you have good compression. your spark plugs are new and your reeds are good. you have to rule out dirty or incorrectly setup carbs. once thats ruled out then you look for the odd stuff like bad crankcase seals etc.<br /><br />pickup a genuine service manual and have at it yourself. they are very simple engines.

 

[ianyoung77]

Re: Strange spark test

Thanks again Ben,<br /><br />Unfortunately I have now found the problem. I pulled out the reeds and looked at the pistons as I turned the flywheel. It is clear #1 & #3 travel straight where #2 flops about. It is as though it has thrown a ring. Interesting the compression test was ok.<br /><br />Is it worth fixing? At this stage I am thinking of removing the powerhead and taking it to a dealer for fixing. If it is easy I will obviously do it myself - any ideas what I'm looking at?

 

[bigbrownbuku]

Re: Strange spark test

when i worked in perth i knew of at least 1 rebuilder that could pull powerheads of the shelf. you might want to look around and see whats available. if its just a straight swap it may end up being cheaper. a dealer will pay attention to all the finer points of refitting a new powerhead and sparking it up.