300ex running problem?

[f3joel]

I just bought a 300ex and it starts up fine, sounds good. but it won't get into the higher rpms. it has been completely rebuilt ( or so i was told, you can plainly see the new gaskets everywhere ) but the guy i bought it from said the exhaust had come off and then it just stopped revving up all the way. told me he thinks it's a burnt exhaust valve. i really don't think it's a burnt vavle as there is absolutely no smoke. it doesn't have any power at all though, i let the clutch out in first and it will move at about 5 mph, i switched to 2nd and got about 8-10 mph. i'm going to check all connections and throttle cable in a minute but wanted to know if anyone has any thoughts or has experienced the same.
thanks, joel

[Exrider434]

Ok theres a little black cover down behind the rear brake pedal. Take that off and then pull the two wire off and clean em up a bit then put em back on and see if that makes a difference

[f3joel]

Originally posted by Exrider434
Ok theres a little black cover down behind the rear brake pedal. Take that off and then pull the two wire off and clean em up a bit then put em back on and see if that makes a difference thanks for the reply. i'll check that out tonight and post up with results.



edit: no luck, but thanks for the suggestion

[f3joel]

am i correct in assuming that it's not a vavle if there is no smoke?

[bwamos]

Burn't valve, won't necessiraly smoke.

Rent a leak down tester from your local auto parts store. That will tell you quickly if you have a burnt valve.

Did the guy rejet when he replaced the exhaust?

Check the jetting in your carb. If you're runnin with the airbox lid on w/ an aftermarket exhaust you should be in the 125k-128k Main Jet range. No lid, in the 130k-135k main jet.

[f3joel]

it's got an aftermarket carb and is supposedly jetted for turbo blue (by local shop that built engine). he said it ran like a beast till the exhaust came off while riding.

I guess my next step will be the leak down tester.

thanks, joel




edit: also it's supposed to have a 12 to 1 piston and hardened rocker arms, new crank ect... totalling 1800 worth of engine work

[bwamos]

So you are running the race fuel, correct? (Turbo Blue). 12:1 won't run on pump fuel. I'd estimate you'd need at least 96 octane.

[f3joel]

Originally posted by bwamos
So you are running the race fuel, correct? (Turbo Blue). 12:1 won't run on pump fuel. I'd estimate you'd need at least 96 octane. yeah, it's turbo blue in it. might be alittle old though.

[honda250xrider]

lol wanna bet 12:1 wont run on pump gas did this in my 250 for about 2 years no problems, ran the webcam 320 grind cam, ported head, had no knock suprised me but heck i know i wasn't gonna waste the extra money on it if i didnt have to. also check the air boot did this come off at all make sure all the connections from the airbox to the carb and motor are all tight, I've experienced this problem and it was the cause of the boot slipped off. I wouldnt think it would be timing but bwamos is right check with your local store and see if it is an exhaust valve.

[AtvMxRider]

Check and make sure the slide hasn't came loose in the carb. There is alittle plastic clip that holds it on, check and see if it is still connected

[f3joel]

Originally posted by AtvMxRider
Check and make sure the slide hasn't came loose in the carb. There is alittle plastic clip that holds it on, check and see if it is still connected i'm going to take the carb apart tomorrow night.


i wasn't able to rent a leak down tester at any of the auto part stores around here. but i did use a compression tester on it tonight. when i bumped it over it went to 60psi then jumped to 90 then 120 and maxed out.

[f3joel]

Originally posted by f3joel
when i bumped it over it went to 60psi then jumped to 90 then 120 and maxed out. since stock compression psi should be about 170 i'm guessing it is a valve?

[bwamos]

Originally posted by honda250xrider
lol wanna bet 12:1 wont run on pump gas did this in my 250 for about 2 years no problems, ran the webcam 320 grind cam, ported head, had no knock suprised me but heck i know i wasn't gonna waste the extra money on it if i didnt have to. also check the air boot did this come off at all make sure all the connections from the airbox to the carb and motor are all tight, I've experienced this problem and it was the cause of the boot slipped off. I wouldnt think it would be timing but bwamos is right check with your local store and see if it is an exhaust valve.

That's a 250x. The 300ex wont normally run at 12:1 on pump fuel. The 250x is known to be able to run on premium pump up to 12:1. Don't aske me why.. lol. The 300ex must have a slightly higher dynamic compression due to the increased stroke, or run hotter from the added CC's, I dunno.

My 330 11:1 is borderline detonation. In very hot weather, or very cold weather I'll usually have to toss in a bit of Toulene to keep it from detonating.

I agree with the rest though. ;) It it was cam timing, it would feel like you were towing another quad behind you. It doesn't bog down though.

[bwamos]

Originally posted by f3joel
since stock compression psi should be about 170 i'm guessing it is a valve?

Could be. Or it could be rings, etc...

That's why the leakdown tester is nice instead of a compression tester. A leakdown tester is pressurized and you can hear where the air is escaping.

Either way, you're gonna want to pull the top end off and do a good inspection. He probably did do damage to the valves though. Not to hard to fix. Once you figure out the damage, I'd just ship the head off to a good engine builder like Dasa, Sparks, etc.. and get it all cleaned up because the valve seats are probably gonna need to be reground etc... and that's not something we have the tools for.

[f3joel]

i ordered a manual for it, was hoping to tear it apart this weekend but i'm being sent to california in the morning for 5 days. guess it will be next weekend.

[bwamos]

Good call. The Honda Service Manual will be your new best friend. ;)

[f3joel]

Originally posted by bwamos
Good call. The Honda Service Manual will be your new best friend. ;) finally got a chance to take it apart tonight. everything is very clean and looked right, but i put some gas (didn't have any kerosene or solvent) in the exhaust port and noticed it leaking very slowly from one of the exhaust valves.

so i'm thinking about ordering some kibblewhite black diamond intake and exhaust valves and springs. do i also need to order the valve guides? anything else? everything else in the motor looks good, no marks on the cylinder wall or anything.

[not2hi]

just regrind them yourself...tools cost under $10 (if you already have a spring compressor) and it takes less than 30 minutes to do.

[f3joel]

forgive my ignorance, do you mean regrind the valves or the guides?

[not2hi]

sorry...I meant the valves themselves...

When I rebuilt our 300EX for the second time, it fired up and sputtered for a few seconds then quit...did the compression test and it was actually sucking, not blowing...I must have had the timing 180 out and bent the valves. Bought new valves and guides, put them in and then reground the valves...it was kind a trial and error thing, grind - reassemble - leak test - disassemble - grind - etc...

stick with it you'll get it!

[250X_project]

Now, what do you mean "grind"? Did you just lap the valves into the seats with the gritty compound, or actually use a valve grinder with the 45* angle?

[f3joel]

i went ahead and ordered the kibblewhite valves and spring kit from lukesracing.com they had a really good price on them. hopefully they are fast at shipping.

[f3joel]

ok, so i put in the kibblewhite valves and springs and put it back together. the ******* runs exactly the same. take the top cover off the carb and discover the linkage ( little plastic piece with a spring ) is laying in the bottom, the spring is jacked. Put it back on and ran for 5 minutes before falling off again. i called the honda shop and they want 57.99 for the parts (it comes as a set with the throttle arm and everything) so i'm either going to rig something to hold it on or get a used carb off ebay or something. probably a 400ex carb.

[not2hi]

try servicehonda.com they are typically alot cheaper on OEM parts than your local Honda dealer

keep us posted

[f3joel]

Originally posted by not2hi
try servicehonda.com they are typically alot cheaper on OEM parts than your local Honda dealer

keep us posted thanks for the site. they are cheaper, .72 cents cheaper.but for 60 bucks to get a spring out of the kit. i might as well get the 400ex carb off ebay and rejet. i'm thinking about using some safety wire and just going around the plastic piece with it in place and twisting it up tight. should work

[not2hi]

yikes! typically they are 1/3 the cost of my local Honda shop...

[f3joel]

Originally posted by not2hi
yikes! typically they are 1/3 the cost of my local Honda shop... oh man, i decided to try and rig it up. took the top cover off the carb again and found the spring and plastic piece was still holding. but the 2 screws in the bottom had come out. so after putting it back together i went and rode it for about an hour. i think i may need to rejet tho. i couldn't find any turbo blue so i went for VP c12 and it's a little less octane but highly oxygenated. i hear your supposed to run it a little fat.