84 honda vf700s problem

[drthv8r]

alright everybody, tricky problem. here's the situation

i own an 84 honda vf700s (sabre) that i bought towards the beginning of last summer. When i bought it then, it rode very nice...not perfect but very very well. good enough to where i was extremely happy with the purchase.

over last summer i put 6k on the bike and stored it for the winter in a heated garage. pulled it out this summer and did the regular maitenance on it; carbs cleaned and adjusted, new tires, oil change, new fluids, cleaned the plugs, yadda yadda

this is the issue i'm having: after about 65 mph, the bike starts stuttering and acting like it is being starved of fuel. the thing is though i've had the carbs apart, adjusted, readjusted, recleaned, and resynchronized multiple times, thinking it was just being starved for fuel. i've extensively cleaned the pet**** and the entire carb system. i've also been running gumout and seafoam through it because the pilot jets are non-accessible unless you drill out the carbs - something i haven't done yet. the additives have had no success.

the mood that the bike gives me is weird though. like i said, at highway speed it really starts dying off. it will usually continue to accelerate depending on the gear, but not all the time. right at 4500-5000rpm where i should be getting a nice power curve, it flattens out every gear. there is no pull in the upper rpm ranges. it isn't consistent from gear to gear though and even isn't consistent every time i'm on the bike. to top everything off, i'm getting roughly 90-100 miles on a tank just running around town though in lower rpm ranges, everything runs perfectly.

i'm running out of ideas and options. i was thinking maybe the timing or valve clearance is off, but idk if that makes sense. another friend suggested maybe the pulse generator or a coil is getting weak, causing the constant supply at higher rpms to dwindle.

any help or suggestions that anyone may have to help me would be greatly appreciated! i will be checking back and posting with progress if i find something out or try something new

thanks ahead of time!

[Easy Rider 2]

Originally Posted by drthv8r

any help or suggestions that anyone may have to help me would be greatly appreciated! i will be checking back and posting with progress if i find something out or try something new

My guesses:
Float level is too low
OR
Tank vent is plugged.

P.S. You probably should NOT have taken the carbs apart for cleaning.
If a machine is running at all, a good carb cleaner in the gas is often all you need.

Also, drilling out the plugs over the pilot screws is not big deal........but that is an IDLE adjustment, not hight speed.

[drthv8r]

i didn't think of the tank vent, i will have to check that out. the floats are right where they need to be, i've had a caliper on them to make sure they were all at the same height and that height was where it needed to be.

the carbs were so gunked up that they would start leaking through the overflow within a minute of the bike choking to life. imo, those carbs aren't that bad to work on - i've done much worse. imo the hardest part was getting them out and back in the bike lol

[Easy Rider 2]

Originally Posted by drthv8r

and that height was where it needed to be.

No offense intended but the thing that you are absolutely positively sure is NOT the problem.........often turns out to BE the problem.
I have a lot of experience in that area !!!

Do you know the little trick about checking the actual fuel level in the bowls by using a piece of clear tubing attached to the drain nipple ??

And did I suggest using some carb cleaner in the gas even though you have done the manual cleaning??

[drthv8r]

when i took the carbs apart, i did check the floats. a manual i have recommends 8.3mm (which is pretty lean) while other places recommend 7mm. 3 of the floats were pretty close to the 7mm mark when i got the capiler on them, and the one that wasn't i adjusted to match the rest.

yup i did know about that trick. that's how it's recommended to check to see if your carbs have gas so you aren't cranking on the carbs forever after cleaning them. i have been using additives in the gas hoping that i missed something and trying to knock it out of the system.

[seruzawa]

Originally Posted by drthv8r

when i took the carbs apart, i did check the floats. a manual i have recommends 8.3mm (which is pretty lean) while other places recommend 7mm. 3 of the floats were pretty close to the 7mm mark when i got the capiler on them, and the one that wasn't i adjusted to match the rest.

yup i did know about that trick. that's how it's recommended to check to see if your carbs have gas so you aren't cranking on the carbs forever after cleaning them. i have been using additives in the gas hoping that i missed something and trying to knock it out of the system.

Since you have the carbs off you've already been introduced to the maintenance nightmare that is the early Magna.

If the choke is even slightly on after the engine warms up it won't run at all. I rigged a little spring on the choke lever on my 84 Magna to endure it stayed completely shut after I almost got stranded once and had to ride home with a length of string tied to the lever to hold it closed.

BTW, those old Magnas had weak trannys and camshafts. There are some remedies for the cams that you can find in a google search. It involves re-routing the oil flow. The tranny will simply blow at around 30K miles usually.

[The_AirHawk]

If the trouble persists, and you just can't find anything else wrong - try stepping-up one jet-size all-around on the Mains. Jets are relatively cheap, try a shop that specializes in MotoX bikes (they'll probably have them in bulk).

I suggest this, as I had a friend with a VFR750 that had similar problems, and the previous-TWO owners also combatted it (including the original, I assumed). He ended-up going one size larger on three carbs, and two-sizes on the last one (after experimentation). That fixed the problem. I'm not sayin' this will fix yours, but you seem to be competent-enough to manage a jet-change and tell if it's "good" or "bad".

Occasionally, even the factory will eff one up. (it had stock airbox, stock filter, stock exhaust, stock-size jets - you name it, it was "stock")

[drthv8r]

Originally Posted by The_AirHawk

If the trouble persists, and you just can't find anything else wrong - try stepping-up one jet-size all-around on the Mains. Jets are relatively cheap, try a shop that specializes in MotoX bikes (they'll probably have them in bulk).

I suggest this, as I had a friend with a VFR750 that had similar problems, and the previous-TWO owners also combatted it (including the original, I assumed). He ended-up going one size larger on three carbs, and two-sizes on the last one (after experimentation). That fixed the problem. I'm not sayin' this will fix yours, but you seem to be competent-enough to manage a jet-change and tell if it's "good" or "bad".

Occasionally, even the factory will eff one up. (it had stock airbox, stock filter, stock exhaust, stock-size jets - you name it, it was "stock")

i'll keep this in mind if i ever run into the problem again. but i figured out today, almost completely by accident, what the problem was

so i've been recently experimenting with the idea of a forced air intake on my car and/or bike, and i was wondering how to do that. that's when i realized that the original air filter had a huge hole in it (i think factory original, never got replaced before i bought the bike), so there was nothing hampering the air flow. pictures below


original air filter


original air filter vs replacement air filter (bought from a honda dealer)


this is why i wanted to clean the carbs, look at all the rotted air filter ini there

so just as an experiment, i took the air filter out and took the bike for a ride. what a difference. not even like the same bike. but what should i do? i shouldn't be running around without an air filter?

my family had an old dehumidifier laying around with a foam that was very similar to the replacement, except about half the thickness and not nearly as dense. i traced it and cut with an exacto knife. picture below.



problem cured. the replacement air filter i got from a honda dealer wasn't letting in nearly enough air, hence why it was choking at highway speeds. the bike has not run this well since i've had it in my ownership. my day got significantly better

so if any of you are having similar problems, take a short ride without the air filter. if that cures it, find some thinner, less dense foam and give that a shot.

[Easy Rider 2]

Originally Posted by drthv8r

so if any of you are having similar problems, take a short ride without the air filter. if that cures it, find some thinner, less dense foam and give that a shot.

What this really means (probably) is that it was running rich with the air filter in place, maybe due to a previous jet change or other malfunction.

While your home-made filter is better than nothing, it's not much better.
You really should try to find a reusable, low restriction "real" filter......like a K&N maybe. (ouch, ouch.....my fingers hurt from typing that !!)

[drthv8r]

Originally Posted by Easy Rider 2

What this really means (probably) is that it was running rich with the air filter in place, maybe due to a previous jet change or other malfunction.

While your home-made filter is better than nothing, it's not much better.
You really should try to find a reusable, low restriction "real" filter......like a K&N maybe. (ouch, ouch.....my fingers hurt from typing that !!)

i thought about it running rich, but there would be a lot of evidence on the spark plugs and it would have been very bogged down in lower rpm ranges - neither of which was occuring. it was running its best in the lower rpms

i also did check for a k&n performance air filter yesterday after making my own. they don't carry one for this model bike. i have been keeping my eyes open though for some sort of replacement. maybe it could lead into a forced air intake project...