DR250 piston hitting spark plug

I have a 1990 DR250, my piston is hitting my spark plug. I am working with the jetting, trying to get it right. I pulled the plug to check the color and noticed the gap was wrong. I gapped it and put it back in. Started it, it ran, then when I took the plug out to check it again, the gap was to small again. So I figured the piston must be hitting it. I verified, it is the right plug for that bike. The only way I could get the gap to not be affected was to put 3 spark plug washers on it? I dont get it, could someone before me have put a dome top piston or something? What should I do? Any help?

you could pull the top end off to see?

What brand/number plug are you using, what is the gap you are setting the plug at initially, and what is the gap after the suspected interference with the piston?

using the NGK the book calls for, gap is .035mm, after the problem the gap is 1/2 that distance at the most. when gapped right the tip is almost 90 degrees from the top of the plug. after i run the plug its bent down to about 45 deg. ??

right. you are an idiot :bonk:

1) please make sure its the right plug

2) NO PLUG GAP IS 0.035mm?!!??!?! 0.8 is more realistic

3) buy a new plug, unless the plug has visible SMASH marks on it, id say the electrode is weak

4) if you have the correct plug and the head hasnt been skimmed, there is no way it is physically possible for the piston to hit the plug, i would say the electrode is weak, and the spark plug washers are lowering the compression ratio to a level which is weak enough to create a weak enough combustion to not bend your weak plug.

5) sorry to be so harsh but you really shouldnt be working on a bike if you believe a plug gap should be 0.035mm. thats miniscule.

He means .035 inches, which converts to about .8 of those millimeter things.

Yes, thats right, im not a complete idiot, Hes right when i said .035 i ment inches. The book calls for .03-.04 inches, or .8-.9 mm. .035 according to my spark plug gapping tool .035 inches is .089mm, which is the right gap for the ngk plug that goes on a 1990 DR250. Now that we have that cleared up does any one have any usefull info, other than pulling the head off(which is a last resort). I thought the head could have been shaved or there was a high copression piston in it. Does any one know if the piston has a raised area on one side or the other? It also occurred to me that the guy who "rebuilt" the engine before me may have put the piston in backwards.

are you certain its hitting the piston, as said, the washers could just be easing off the combustion conditions. take the plug out, have a good look at the plug, make sure its been hit, and get the piston to top dead center and have a good look at where the spark plug would have been hit through the holes aswell

when gapped right the tip is almost 90 degrees from the top of the plug. after i run the plug its bent down to about 45 deg.

I am confused, the way that sentence reads in my head, makes it sound like the arm of the plug is sticking straight up? Obviously this is not correct as .035" is closer to the width of thicker construction paper. Could you take a picture perhaps of the plug before/after running?

If you are indeed gapping this plug correctly, I would say that the head needs to be pulled. There is just no other way around it, and sadly may be the easiest solution. Let us know what you find.

*EDIT - Sorry I didn't not drink my morning coffee fast enough. I understand what you mean now by 90 degree's :bonk:

Max, Did this just start happening? Is the bike new to you?

Whats the history with this thing?

With the plug out you can look right at the piston where it will interfer with the plug - you might need an inspection mirror and a flashlight. If it's interfering (and it sure seems that way to me) you will see the mark (dent) on the piston.

I don't know anything about the DR250 but if you are lazy (no insult intended :bonk: ) and just want to keep it as is you should be able to find a plug having a shorter reach and simliar heat rating. Or use another crush washer or two.

wondering why you started fooling with the tune anyway, was it pinging? Maybe it has a high compression piston. Have you done a compression test?

Pictures & better background story would be a help but as suggested I think maybe we are dealing with a seriously decked head &/or a high comp/incorrect piston.

Try to get a good quality photo of the plug before & after & down the plug hole with piston at TDC or (ideally) with the head off.

The suggestion of a compression test is about the best non-invasive way to find out if it's a modded motor & I'd do that & compare with standard figures before going any further.

P.S. I hope I don't need to tell you this but nevertheless I will just in case-: When you get the problem sorted fit a new plug regardless of whether the one you have is right as the action of bending/re-bending/bending yet again will have fatigued the electrode & the last thing you want is that breaking off & jamming down the side of the piston.

I'm thinking it might be carbon build up on the piston. I've seen lawn mowers bend electrodes from carbon build up, so why not a bike?

If it is carbon, swing by the drugstore and pick up some 90% (not 70% or 75%!) rubbing alcohol and some long cotton swabs. The alcohol will melt the deposits away with almost no effort. You can pour some into the plug hole and leave it in there for a while too.

I'm still suspicious that he has the wrong plug in the bike...like a projected tip plug. Occam's razor, people.

Until he tells us the plug number that he's using, I remain skeptical...


The plug I am using is an NGK,DPR8EA-9, which is what the manual and the local dealer told me was the right one. I was also suspicious about carbon build up on top of the piston. Last night it was dark when I got home and my shop light burned out and my flash light was too weak, but with the plug out and the piston at TDC all I could see was black. I think there might be a lot of carbon on the piston, I used a little seafoam ( cleans carbon out of intakes). Want to make sure you guys under stand, it's not Slamming in to it. It seems to just barely hit the plug, It closes the gap on the plug to about half of what it should be. You cant even hear or feel it hit, I held the compression lever in and kicked it slowly 3 times with the plug wire off and it was still bent. So I dont think it has anything to do with a weak electrode. I have reproduced this 3 times, with the correct plug. I do have a new one to go in once I get the problem corrected. Its almost Turkey day so I will have some time off and will be able to look at it more (long as I can get some garagetime without the wife killing me). I will definately keep you guys posted on what I find.

Now why couldn't I think of that? (carbon buildup).... :bonk:

I kinda doubt the carbon theory because carbon builds gradually & doesn't accumulate where there's not room, like in a tight squish area or where the spark plug goes.

BUT! I've been wrong before.

if u think carbon build-up is causing the piston to

hit the spark-plug,

brother ,

you need to take your medication.

take it to a suzuki dealer before you grenade the engine.

Seafoam sucks, I've not had any luck with it cutting carbon unless the engine is up to temp and running. Use the 90% alcohol, it's cheap and works like a charm in a cold engine.

If it's not carbon and the engine isn't making any bad noise when running, then I'm with Thumpzuki.. it's had to have had machine work done..

I just thought of something else... if someone stripped the plug hole in the head and fixed it with an insert... maybe the insert has screwed down into the head too far? That should have been obvious when you looked, but if you're not looking for it....

I doubt it's carbon for the same reason - carbon builds up over time. Unless the plug is a new one and it's longer than the one that was in there letting the carbon build up. Techron additive (in the bottle) will rip that out in one tank

Good suggestion - check for a plug thread repair.

Seems like the interference is about .015". That's less than one crush washer.

0.015" interference isn't much when it comes to the electrode - the piston is moving pretty slow close to TDC and the contact just pushes the electrode away.

I still have the feeling the topend might have been altered somewhere along the way. Valves are the concern - it is bad for the piston to interfer with those. In the end it would be good to see it there is evidence of that. If there is then the valve stems are likely bent and the seating and guides are on their way out. A compression check would be very interesting.

Take it to the Dealer??? I don't know - so some half trained mechanic with a hangover adds a crush washer and charges two hours labor? I'd do the work myself, know for sure what's going on, and save the money.

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