High RPM, runs like dodo!

[bob393]

I got one for you carb guys that may be new to ya?

Its a 1994 750, stock exhaust, jets, air box, filter, all that stuff.

First I'm kind of a spirited touring guy and as far as I'm concerned the bike is running perfectly. I cruse at about 6000rpm's, I shift at about the same, the engine pulls from low 1500rpm, when I don't want to shift, life is good at 45-50 mpg.

Now here is the thing, my son is the exact opposite! He is not happy if it's under 10k, and shifts at 12k, rides like his hair is on fire! He is telling me that the bike just started to go soft at about 8k rpm. Twist the throttle and it goes slower. He described it as running on two cylinders. He thinks the jets are clogged because I put some New Jersey cheep regular gas in it.

He tells me that it just started after I returned from a nice 250 mi trip down to NJ last Saturday, yes the HOT last Saturday and yes the bike was hot on my way through Jersey City. Hot enough for me to think it might have been a bit soft too.

I haven't dug into it myself yet, it's getting late, but I had a computer in front of me so I thought I'd through it out there and see what comes back.

[scottynoface]

Have you ridden it hard to verify his claims? I find it hard to believe the mains would be clogged if the pilots are not...i think he's feeling the difference between a katana and a gsxr...

[bob393]

Have you ridden it hard to verify his claims? I find it hard to believe the mains would be clogged if the pilots are not...i think he's feeling the difference between a katana and a gsxr...

No, that's a good point! I'll try that tomorrow.

However this is the only bike we have and the only bike he rides.

[bob393]

OK so I took the bike for a spirited spin on my way home from work today. Shifting at around 9k. First gear was fine at full throttle. BUT in second it broke up badly at around 6500, 7000, like it hit a rev limiter and the throttle was dead. The bike actually started to slow down! When I up shifted it was fine. My son was right, runs like crap in the upper ranges. Any thoughts?

[arsenic]

It's probably an electrical issue. In my opinion, it has zero to do with the fuel delivery aka carbs.

[bob393]

What would you guess would be the first thing to look at. I admit it hasn't had new plugs in 5k mi. just changed the oil though at 15k haha.

[gsx]

try some new plugs in it. Im a compression nut, every time i take a plug out i always stick a compression reader upper down in there. just check and see(if you can) what you get for compression(but it is unnecessary, like i said i like to know what the motor is doing) For the $12 you will spend on plugs its worth a try, if that dont fix it, at least you got fresh plugs reason im saying that, im a marine mechanic, and while an outboard is a whole different game than a bike, iv seen plugs cause some odd running issues before. Over time they seem to break down(especially the resistor types). if new plugs dont work, got to go up the chain and see if any of the coils have a crack or whatnot in them

[scottynoface]

"Hasnt had new plugs in 5k miles"

Why would you swap plugs out that early anyway?

[Badfaerie]

I'm thinking a fuel delivery problem. If it's not getting enough flow out of the tank it'll cut out at higher rpms as well. Check the likes are not kinked. If you put a fuel filter on, take that off, pre's don't use them, and check your petcock. You might try it again with the petcock on reserve or prime.

[arsenic]

It's not going to feel "like it hit a rev limiter" because of a fuel delivery issue.

[bob393]

Well on the way back from buying plugs I think I just lost 2 cylinders entirely, Still to hot to look at but I'll know tonight, hopefully, when I pull it apart. Now I'm quite sure its electrical!

[arsenic]

I already was. Glad you are.

[bob393]

OK well it's fixed. New plugs, and checked the resistance on the coil primary and secondary , and the signal generator, they fell within specs. checked out. The only thing I found out of the ordinary was some corrosion on the plug from the signal generator to the ignition module. I cleaned it up, put on a dab of silicon dielectric grease and put it back together. Strange huh?

[arsenic]

Not really. Glad you got it all fixed up, nice work.

[bob393]

No thank you

Hey Arsenic, while your right there, you might know this. When I pulled the cover off to get at the signal generator oil came out of the bottom of the opening below the oil pressure sender. Is that right or do I have a seal failure?