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Old 07-26-2014, 12:49 AM
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Default CR 250 Fouling Plugs

     

Hi Guys,

This is my third time back here looking for help, and Ive tried all the previous suggestions, except for one...
I haven't checked right side crank seal.

My 2002 CR250 runs very sweet. I have modified jetting, Ive installed the BR8EG plug, Ive checked and cleaned air filter. Ive checked the spark plug cap and lead. Ive tried running and riding the bike hotter.
It still fouls a new plug in a matter of minutes, or sometimes hours.
There is a small amount of oil coming out the exhaust, nothing hectic.
I mix 50:1 100% Synthetic with 95 fuel.

It either fouls minutes after starting up for the first time, or if i ride down a long gradual slope with the throttle shut off. I run the bike down the hill under its own compression, when I get to the bottom ill open the throttle and it fouls immediately.

The plugs are coming out very black, wet, and oily not crispy.

Do I run a hotter plug, or check crank case seal first? Engine oil level appears to stay the same?

I only do trail riding so, the bike does lug from time to time, I do my best to clear it out when I get the chance.

Pleeease help! I'm about to sell it out of frustration!
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Old 07-26-2014, 01:05 AM
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Sounds like rich jetting to me.
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Old 07-26-2014, 01:19 AM
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How is the float height setting and float valve sealing? This would be my first port of call in the event that a bike is fouling plugs.
If you can identify if the plug is 'fuel fouled' that would be helpful!
Jetting leaner to compensate for incorrect float height or a leaky float valve ( and/or float valve seat ) is simply a band aid to the actual problem. It'll be best to check the above mentioned.
A hotter plug won't help. I would also check the compression.

The right side crank seal is likely to be the culprit-it's easy enough to fix, I would replace the seal and collar to be sure ( the collar can groove out and still leak with the new seal... If I'm not mistaken the seal rides on the collar not the crank shaft )
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Old 07-26-2014, 01:38 AM
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Yes, the seal does ride on a collar. Running 50:1 instead of the mfg recommended 32:1 will also make your jetting richer.
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Old 07-26-2014, 03:41 AM
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Quote:
Originally Posted by stevo450 View Post
How is the float height setting and float valve sealing? This would be my first port of call in the event that a bike is fouling plugs.
If you can identify if the plug is 'fuel fouled' that would be helpful!
Jetting leaner to compensate for incorrect float height or a leaky float valve ( and/or float valve seat ) is simply a band aid to the actual problem. It'll be best to check the above mentioned.
A hotter plug won't help. I would also check the compression.

The right side crank seal is likely to be the culprit-it's easy enough to fix, I would replace the seal and collar to be sure ( the collar can groove out and still leak with the new seal... If I'm not mistaken the seal rides on the collar not the crank shaft )
Thanks for the great advice...

Surely running more oil in my fuel mix will worsen the problem of fouling plugs?
The plug is quite wet when I pull it, and it is greasy and black, not crispy... Is that "fuel fouled" ?
Please explain a little more about float height and valve, and how to check and set... If you don't mind please.

I run one size leaner on the jets than I should, due to my type of riding... So i doubt its over-fueling... We ride trails in the mountains, and can go up 1000m in a single ride. So we run on the lean side, also due to the slow trails, the bike doesn't get to blow out often...

I will order seals and see if that might be the source of the problem...

Thank you
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Old 07-26-2014, 05:36 AM
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You're right. You are fouling plugs and they are soaked with fuel. Couldn't possibly be a jetting problem.... And yes, adding more oil will lean out your mixture.
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Old 07-26-2014, 11:02 AM
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Quote:
Surely running more oil in my fuel mix will worsen the problem of fouling plugs?
This has been adequately answered.

More oil is less fuel=LEANER fuel-to-air mix
Less oil is more fuel=RICHER fuel-to-air mix

Oil isn't necessarily what will foul a plug-unless excessive oil from the tranny is getting into the crankcase-you stated that you've seen no change in the tranny oil so I think your carb is the culprit ( of corse most pedantic guys change the seals more regularly than most .... so it's not a bad idea only I don't see it as the root problem ).

Quote:
Is that "fuel fouled" ?
Generally, yes.

Quote:
Please explain a little more about float height and valve, and how to check and set... If you don't mind please.
Certainly! I'll do my best, but there is also a VERY good write up with illustration pictures in the 'Knowledge Section' on ATM.....

How To Set Your Motorcycle Carburetor Float Level (more commentary).

The float and float valve are responsible for keeping the correct FUEL LEVEL in the carb. Contrary to popular belief that the float and valve ( sometimes called float needle ) just stop flooding isn't the only purpose and in fact flooding is a sure sign that the carb is WAY off.
Carbs wear out and MUST be serviced and vital components checked more than you would think ( and most... )
Back to your question, it is very simple to check and the link I posted has all detailed info.
Setting the floats is the start point of any carb maintenance, this affects how deep the carbs sit in the fuel and ultimately governs how much fuel can be drawn into the cylinder ( this is of corse a very simplified version )!
Most carbs will, by nature of design, have the float height gradually getting higher ( allowing more fuel into the carb-BUT perhaps {and most often} not enough to 'flood' or correct term 'overflow' thus without inspecting the float height regularly you won't know until it starts overflowing or running poorly ) this due to many obvious factors.
The float valve and valve seat need to seal to shut off the fuel feed at the right time. The valves/needles also slowly wear ( usually getting hard and grooved ) also some carbs have a small O-Ring to seal the float valve SEAT ( removable brass part that houses the valve/needle. )

Once this has been set it's then time to start jetting and taking plug readings!

Now, I just remembered this year has the Mikumi........ They are NOTORIOUSLY hard to dial in ( jetting ) and I find they overfuel mostly if they aren't run at WOT or ridden aggressively ( they aren't impossible ) this is partly due to the carb design and it's fuel delivery system-I won't bore you with the details (high pressure/low volume verses low pressure/high volume etc )
A common modification is to ditch the old Mikumi and install a Keihin PWK Air-striker, a much better carb for these bikes and generally improves the engine performance and is easier to tune.

---------- Post added at 02:32 AM ---------- Previous post was at 02:00 AM ----------

Off topic, but I see your in South Africa... Couldn't help but wonder if your anywhere close to the Drakensberg judging by your user name .... 'Berg'Rider.

Last edited by stevo450; 07-26-2014 at 10:56 AM.
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Old 07-26-2014, 02:00 PM
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Start by going back to 32:1 with the fuel mix. You are too rich when using the 50:1.

Paw Paw
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Old 07-26-2014, 08:57 PM
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Quote:
Surely running more oil in my fuel mix will worsen the problem of fouling plugs?
Less oil is less lubricant! Listen to stevo.
Think of your plug fouling problem as more of a fuel/air issue and not a fuel/oil issue.
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Old 07-26-2014, 09:32 PM
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There are plenty of folks that elect to run their premix 50:1 (I am not one of them) and don't have plug fouling issues. If you are dead set on running 50:1 for some reason, then you can certainly resolve this issue by going through the step already discussed earlier (verify your crank seal isn't leaking, set float height, verify your needle valve is working correctly, then jet accordingly). Running 50:1 is going to require leaner jetting than the Honda recommended 32:1.
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Old 07-27-2014, 02:02 PM
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To add to this if you don't know how long your float needle and seat have been in the bike for, its a good idea to just replace them to eliminate it as a problem.

I was having some tuning issues on my 01' CR250. I checked the float needle and height often but changed it even though it didn't really "look" worn to me. With the new needle in I had to significantly lower the float height (because the new needle had so much more material on it). Now the bike runs excellent again. I wasn't fowling plugs but it just wasn't running the way it should.

I also ride very tight hare scramble trails and run 32:1 oil. Your plug fowling is definitely NOT going to be from running more oil. A friend does use 50:1 in his KTM but thats what is recommended and jetted for straight from KTM. Your Honda is jetted from the factory for 32:1.
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