250R wooo's again

[Wi200x]

Been fightint my 84 250R for some time now. Upon the suggestion of the bike mechanic I know he suggested putting some tape on the throttle to know where exactly the surge is happening, etc. Well, went out on a test drive about 1.5 miles from home and the thing died when accelerating moderately. Couldn't get it to fire but a few times and then it would just bog out. Happened for my son the other day but he was able to get it started. We pulled it home, wouldn't start at the shop so I took out the plug to check spark. Only had spard at the center electrode at first. Stuck in another plug and it fired right up, after it warmed back up ran stronger than ever. I did have the air screw in 1/2 turn from stock to richen it up. I did have it die intermittently for me too a few times but always started right up. Running an NGK 8 plug. Running Yamalube at 32:1 but got a deal on some BelRay MC1 and will run that at 50:1 so maybe that'll help plug fouling. How long do your plugs last? I never replace the plug in my chain saws, etc.

Interesting discovery. I found out that I cruise (30mph or so) at 1/4 throttle so when I get the surging when letting off the throttle (see previous posts) it's at less than 1/4 throttle. The mechanic suggested I run a 55 pilot instead of the stock 50. I also put a vacuum gauge on the trans vent to see if the trans crank seal was sucking air and could only get a pound of positive pressure-never vacuum so I'm confident the seal isn't leaking vacuum (it never uses any trany oil.

[3leggeddog]

i would go up in the pilot jet,set your mixture screw at stock,and run your oil mix at 50:1 regardless of brand.

[Wi200x]

Just got back from running the R at a local riding area that only allowed 1 & 2nd gear half throttle and once in awhile 3rd gear. Surged real bad when letting off the throttle-ran good otherwise. Pulled the plug and inspected it when I got home and it looks perfect. Tan color and no signs of detonation. Haven't tried the BelRay oil yet as I haven't recieved it yet.

[toocheaptosmoke]

When the engine is surging, shut down the engine while keeping the throttle in the same position, then check the plug. Checking the plug after your ride may be giving you a false reading.

[Dirtcrasher]

Just got back from running the R at a local riding area that only allowed 1 & 2nd gear half throttle and once in awhile 3rd gear. Surged real bad when letting off the throttle-ran good otherwise. Pulled the plug and inspected it when I got home and it looks perfect. Tan color and no signs of detonation. Haven't tried the BelRay oil yet as I haven't recieved it yet.

Gee, that sure sounds like a fun riding area!! LOL...

Anyhow, you said air screw and I thought the R's had a fuel screw? In other words turning it out would let in more fuel.

Maybe I'm crazy (or just wrong) but I thought air screws worked opposite from fuel screws.... just make sure your turning it the right way to get the desired result you want.

I'd make sure that screw has a good oring and spray carb cleaner around the boots and intake while it's running to check for leaks. The one bike I had crazy surges with had a piece of the old reed gasket under the new one and it was letting outside air into the mix....

[Liquid-Darkness]

Been fightint my 84 250R for some time now. Upon the suggestion of the bike mechanic I know he suggested putting some tape on the throttle to know where exactly the surge is happening, etc. Well, went out on a test drive about 1.5 miles from home and the thing died when accelerating moderately. Couldn't get it to fire but a few times and then it would just bog out. Happened for my son the other day but he was able to get it started. We pulled it home, wouldn't start at the shop so I took out the plug to check spark. Only had spard at the center electrode at first. Stuck in another plug and it fired right up, after it warmed back up ran stronger than ever. I did have the air screw in 1/2 turn from stock to richen it up. I did have it die intermittently for me too a few times but always started right up. Running an NGK 8 plug. Running Yamalube at 32:1 but got a deal on some BelRay MC1 and will run that at 50:1 so maybe that'll help plug fouling. How long do your plugs last? I never replace the plug in my chain saws, etc.

Interesting discovery. I found out that I cruise (30mph or so) at 1/4 throttle so when I get the surging when letting off the throttle (see previous posts) it's at less than 1/4 throttle. The mechanic suggested I run a 55 pilot instead of the stock 50. I also put a vacuum gauge on the trans vent to see if the trans crank seal was sucking air and could only get a pound of positive pressure-never vacuum so I'm confident the seal isn't leaking vacuum (it never uses any trany oil.

It sounds to me that the surging from letting off the throttle is a lean condition. Could be the needle setting. Try the needle on a stock setting. That surge might also be overheating. When my bikes overheated they acted the same way and wouldn't idle. I thought it was a jetting related issue, but in the end some higher octane and lower compression fixed what you are describing. Try some higher octane fuel. What mods does your bike have right now?

[Wi200x]

I've tried changing the needle from the 2nd position to most rich and it didn't make any difference. Just haven't been able to pick up any vacuum leaks with carb cleaner and have replaced the base gasket, reed gaskets when redoing the cylinder and piston. Everything is stock on the engine. Replaced the right crank seal and that didn't make any difference. Doesn't use any trans oil so I don't think the left side is leaking but may replace it this week just to keep it from giving any trouble in the future. I was thinking that detonation was igniting the fuel prematurely but haven't had any trouble killing engine with switch. Running the highest octane fuel I could find at the pump in a midsize town (91 octane?). The slower pace of the riding area probably did lead to higher cyl. temps and it would surge repeatedly at idle whereas at home would only do it when really winging it out. I thought that maybe the pilot jet was getting plugged with dirt but don't think this would plug so predictably.

[NINJA]

I know it sounds crazy, but test your ignition coil and coil wire or spark plug boot. It almost sounds like an electrical issue, hence none of your carburetion changes have helped. I could be wrong though.

[Wi200x]

I went ahead and ordered a left hand crank seal and cover gasket and will get them replaced hopefully Saturday if my parts make it. I adjusted the air screw out slightly halfway between were the speeds drop off but didn't change a thing. Reworked the bottom of the choke to make sure it wasn't intermittently allowing gas and air to bypass (didn't make any difference but is real sensitive to leaking if the choke lever is moved ever so slightly up). Engine always starts on the second kick if using the choke when it's colder out and will idle right away without too much throttle manipulation.

[toocheaptosmoke]

Maybe a sharp edge on the head gasket??

[SYKO]

Maybe a sharp edge on the head gasket??

WHAT??! please explain.....

[toocheaptosmoke]

WHAT??! please explain.....

I've heard that a sharp edge or point in the combustion chamber can get hot and start to cause preignition. I'm out of ideas of what else it could be.