Everything2Stroke Forum banner

1 - 15 of 15 Posts

·
Registered
Joined
·
6 Posts
Discussion Starter #1
My 79 DT175 was running great until a source coil failed. I replaced it with a terrible reproduction that barely worked before putting in a used OEM Yamaha source coil. Bike ran fine for a day, but would die out every couple blocks and then restart after ~15 seconds of sitting. I thought my float level was off, but it was within spec. Then all of a sudden the bike wouldn't start at all.

Even with starting fluid, there's no explosions or coughing...Nothing at all EXCEPT that it will sometimes backfire when using starting fluid. I'm getting super frustrated trying to solve the issue. I've tried everything I can think of...

-Bike easily makes a nice blue spark across 1/8" gap when tested
-All ignition parts tested with multimeter and are within spec
-No timing to adjust, the CDI ignition stator has no adjustment
-Two different plugs tried, both tested and gapped correctly
-Old carb was fine besides leaking a little, but I put a brand new one on just in case (same jetting)
-No reason fuel should be bad, but I flushed the tank and put fresh gas in anyways.
-Reed valves look good. Bowed out a tiny bit in the center, but they're supposed to be, right? No chips or warping.
-Petcock flows freely
-Compression is fine, pulled the head off and the bore is looking great. No holes in the piston or anything.

Please help me thing of what to try next!
 

·
Registered
Joined
·
32 Posts
Obviously not all is fine as you have stated, otherwise the bike will run. . . unless your timing is kucfed because of the different coil you are using.

You say you have spark, Fuel is good, Carb. fine, Plugs good. . .bla bla bla. Never heard of reed valves supposed to have a bow? Get a real number with a compression tester.

Check where the CDI magnetic pickup behind the rotor is in relation to the rotor magnet when the piston is close to TDC. You will need a dial indicator to see the height of the piston. The pickup and the magnet should be crossing paths around that point. If it is not you may be firing at the wrong time, in which case will cause the backfiring you have been experiencing. I'm thinking that the used Yamaha coil is not the right one? I don't know, but start there. Get a timing light also, it is needed when working on electronic ignition CDI units.

By the way the timing is adjustable, however you might have to convince the bike also. Modification may be required, either to the stator plate or the pickup mounting hole.
 

·
Registered
Joined
·
6 Posts
Discussion Starter #3
Should have mentioned before, I checked the compression with a gauge and its at 125psi.
I have a timing light but I'm just not really understanding how the timing could be off (though you're right, that's definitely what it seems like...), there's only one way to mount the source and pulse coil do the stator plate, and one way to mount that stator plate to the engine. One other thing I forgot to mention is that I also checked that the flywheel is lined up correctly on the woodruff key. So if everything lined up correctly, how could the timing be off?

The coil I replaced is not the pulse coil (the one that triggers the SCR in the CDI), its the source coil that charges the capacitor in the CDI. In other words the triggering side of the ignition is essentially untouched.

I agree that it seems like a timing issue but I guess I'm the kind of dude who isn't really satisfied that I fixed something unless I understand how it broke. I'll do some poking around and see if I can figure out when it's firing.

Do I need to like dremel out a channel in the stator plate if the timing is somehow off? Why would Yamaha make the timing un-adjustable if it could randomly get out of wack like this?
 

·
Registered
Joined
·
52 Posts
i know you did a compression test but have you had a look at the piston rings, remove exhaust and look up the exh port to check that the rings are good and not stuck into the piston ring gaps. sometimes you can get good compression but the fuel just goes down in to the crank. check for air leaks in the crank,joints,seals etc.
If the timing was off it could have caused piston ring damage.
Lathemad Kev.
 

·
Registered
Joined
·
6 Posts
Discussion Starter #7
One (maybe) interesting thing - out of frustration, I installed a used CDI box that I had lying around, thinking that maybe the cdi was somehow messing up the timing, even though everything I know aboud CDI's means thats sorta impossible. The bike fired over when I kicked it, but only for a couple seconds and then wouldn't start again at all.

Is the flywheel key good? In addition to compression test have you done a leakdown test?

Yep flywheel key is present and the flywheel is aligned correctly on it...checked and double checked this because it was the only think I could imagine would have an effect on the timing, as everything else can only be mounted one way.

Haven't done a leakdown, unfortunately I don't have the tools or facilities to do one right now, though I bet I could make something out of an old bike pump and some fittings...hmmmm....


Also, have you ohmed the ignition coil? Plug cap sepreately?
I have, forgot to mention that too. I measured everything in the ignition system when I was originally tracing down the bad source coil. Just for the hell of it though, I went and re-measured today. Primary winding was 1ohm, secondard is 6k, which according to a sheet I have, is in spec.

Plug cap was measuring higher than spec by a good margin when I re-checked it today, so I pulled a good cap off a running Puch moped I have in the driveway and tried it on the Yamaha to no avail. Granted, the one off the Puch was NOT a resistor-type plug cap (ie it measures ~0ohms while the Yamaha spec is 5.46-7.34k ohms) but as I understand it, resistors are really just to cut-down on electric noise when the plug is firing near other electronics. So, it should still fire right?


i know you did a compression test but have you had a look at the piston rings, remove exhaust and look up the exh port to check that the rings are good and not stuck into the piston ring gaps. sometimes you can get good compression but the fuel just goes down in to the crank. check for air leaks in the crank,joints,seals etc.
If the timing was off it could have caused piston ring damage.
Lathemad Kev.
I had the top end off pretty recently before this problem arose, but just for the hell of it I pulled the exhaust and looked inside. I'm not sure what you mean by "stuck in the piston ring gaps," but they are in the groove in the piston and look OK to me. Here's a picture I took looking up the exhaust. Those things that look like scoreing on the piston are probably not the best, but they've been there for a while and you can't feel them with with your finger at all.
Here's the picture:
http://imgur.com/Qq6Rfpv


Crank seal on magneto side is new within the last ~200 miles or so, as are the base gasket and reed valve gaskets. Intake manifold isn't dried or cracked at all.
 

·
Registered
Joined
·
32 Posts
Are you still getting spark? If not I have to back up the idea of ohm(ing) everything out. Check coil resistance and resistance to ground. Perhaps the CDI is bad. Do not measure the CDI with a multimeter.

What plug Brand/part number and heat range are you using?
Plug / boot / wire OK?
Plug condition. Wet, oil fouled? Should be dry and clean looking.
 

·
Registered
Joined
·
6 Posts
Discussion Starter #9
Are you still getting spark? If not I have to back up the idea of ohm(ing) everything out. Check coil resistance and resistance to ground. Perhaps the CDI is bad. Do not measure the CDI with a multimeter.

What plug Brand/part number and heat range are you using?
Plug / boot / wire OK?
Plug condition. Wet, oil fouled? Should be dry and clean looking.
I've said it a couple times now. I have measured everything I can measure in the ignition system, including the wiring itself. It is all within spec and the bike will give a nice blue spark over a large gap when I kick it over.
I've tried two different CDI's and neither solved the problem.
Plug is NGK B8es, I've tried two different plugs, both are fairly new and look fine but I'm gonna go buy another one just in case.
 

·
Registered
Joined
·
32 Posts
Ok, then for now we can assume the ignition system is functioning properly. Let's focus on the fuel system.
How certain are you the engine is getting fuel? I know you tried starting fluid already and what the results were. Nevertheless,

Buy a set of reeds, I do not believe they should be bent anywhere. You can purchase Yamaha part number 345-13613-70-00 Stainless, or a set of fiberglass ones for around $30? Regardless, reeds should not be bowed in the middle.

You should also make sure the reed block is flat also, or the reeds will not sit flat. The reed needs to form a seal and if they are bowed they will leak.

Also, How does the plug look immediately after attempting to start the bike? If it is too wet well then . . . If it is completely dry . . . I mean, it should be a little wet, I suppose, considering it will not start, but not so wet that it cannot start due to being overly wet.

-Good Luck
 

·
Vendor
Joined
·
1,500 Posts
125psi isn’t a spectacular compression 150psi would be much better. Perhaps your compression on the low end along with incorrect timing may be a problem.
JT
 

·
Registered
Joined
·
6 Posts
Discussion Starter #12
Ok, then for now we can assume the ignition system is functioning properly. Let's focus on the fuel system.
How certain are you the engine is getting fuel? I know you tried starting fluid already and what the results were. Nevertheless,

Buy a set of reeds, I do not believe they should be bent anywhere. You can purchase Yamaha part number 345-13613-70-00 Stainless, or a set of fiberglass ones for around $30? Regardless, reeds should not be bowed in the middle.

You should also make sure the reed block is flat also, or the reeds will not sit flat. The reed needs to form a seal and if they are bowed they will leak.

Also, How does the plug look immediately after attempting to start the bike? If it is too wet well then . . . If it is completely dry . . . I mean, it should be a little wet, I suppose, considering it will not start, but not so wet that it cannot start due to being overly wet.

-Good Luck


Boyesen Reeds are on the way, and while I definitely agree that it would make most sense for the reeds to lie flat, I found a section in a Yamaha PW50 manual about how the reeds are supposed be bowed out slightly in the center, and that bow should be facing out, just as mine are. Obviously a PW50 and a DT175 are completely different animals, but I just thought it was worth noting.

Plug always looked a tad wet when I pulled it out, but not crazy wet. I'm certain its getting fuel now, because I finally got the fucking thing going. How? I bought a brand new plug and put a tiiiiiiiiny gap. Spec is .028, I have it at like less than .020 I'd say. Good news right? Well Sorta....

Now, the bike runs, but will only run for about a minute. Then it shuts off, but will start again after a minute or two.
First guess was that maybe my gas cap wasn't venting. Took it off and there was no change.

Next guess was that my float level was off, so I checked that, and it was spot on (measured from the gasket surface, with the gasket uninstalled, to the bottom of the float. I tried that with both my carbs...the original Mikuni, and the replacement chinese one I have. Even tried increasing the float level to be sure, and it kept cutting out after a minute or two.

Then I thought it was a restriction in the fuel filter, line, or petcock. Got some brand new fuel line and bypassed the filter. No change.

Pulled the petcock and cleaned it best I could. No change.

So I said fuck it and made an auxiliary tank, with a free flowing fuel line straight to the carb, and there was no change.

So now I'm coming back to ignition. I've done a fair bit of works with electronics and if this isn't a fuel restriction problem, it sure sounds like some sort of thermal cycling problem to me. Given that the CDI box is fairly far from anything that could get very warm, that basically brings me back to the stator coils and ignition coil.

I tested my source and pulse coils when cold and then again RIGHT after the bike cuts out. Pulse coil is fine, but the source coil rises by like 60 ohms when it gets warm! And then gradually drops back down to where it should be. Talk about a pain in the ass! That also might explain why I needed such a small spark gap, maybe the source coil isn't charging the capacitor in the CDI enough to give a nice strong spark (Oh, and yes, I tried both of my Yamaha CDIs, plus some modern one off one of those new chinese bikes that I adapted to work...no change)


TL;DR - Fuck source coils
 

·
Registered
Joined
·
29 Posts
Hmmmmm resistance rises when it gets hot? Shorting internally when it gets hot and not producing enough power to charge the capacitor. Interesting find and sounds like a complicated bit of sleuthing to figure it out.
 

·
Registered
Joined
·
290 Posts
what year dt175? i have 2 77 /78 mono shock bikes here, one i bought a stator oiff ebay and it had a bad solder joint.
i hung that and it started to fire first kick m then the other shows up...
the top end is bad some kid that could break an anvill had it before me i like never took the top end off it i looked through the exhaust port at the karnage of no oil...
anny way , im almost thinking this is the same as an it 175..
i had a prob once with a skidoo i bought a 1977 cube nose with 950 miles on it. abd the dam thing wouldnt fire the 1 cylinder right had brand new points and condenser stator etc in this, i remember seeing a kawi engine in a sled and they used ford ignition caps on the coils doubling up the caps wouldnt you know it that was the problem..
personally i think your ignition side coil is going bad , i have seen bothe source coils and ignition coils screw with me, its why i used to have so much junk as the neighbors would call it, dirt bike ppl called it willy wankas motor cycle factory.. and thanks for the tour i will be back xxxxx or xxxx as i didnt bring enough wampum.......
nice to see you broke loose the ohm meter , ive actually bought the one dt175 stator off ebay , and it had a bad solder joint i touched up the solder joints with fresh solder and fixxed it..
thanxz and there is a difference between mechanics and parts changers. a mechanic figures out hwat part is bad, a part changer bolts something on and charges the shit outa somebody till they go elsewhere , my neighbors were getting me back in 2013 and 2014 on a chevy truck , the computer was bad they musta spent 10 weekends on this truck changing everything , prob had a few burnt valves to its throttlebody 4.3 , that from 2013 when they bought it till 2015 when they got rid of it. of corse these mechanics are all better then me .
 

·
Registered
Joined
·
2 Posts
hi did you ever figure out what the problem was? i have a 1980 dt175 yamaha which ran ok in the past and over the last year or so has been going through some trouble starting up..I feel like this post is very very similar to what im experiencing. Im trying to figure out where to post about this and get some advice from some master 2 strokers.
thank you..
 
1 - 15 of 15 Posts
Top