1984 VF500F power/surging at highway speeds

Sorry for the duplicate post....I may have put it in the wrong place to begin with....

Hi Guys, once again desperation is forcing me to the well of knowledge. I just bought an '84 500, the bike has 1200 original miles on it. It fortunately was stored correctly and is in very nice shape runs just as a guy would expect for its age. I have had the carbs cleaned and adjusted, oil and water changed. The bike when first started and/or driven for a short period of time shows no problems, as the weather was hotter and i rode farther I began to get a "surge" a "miss" ... something..... at normal highway speeds that was not evident in town or in lower gears. Once the surge started at speeds approaching 75mph the bike would not go any faster given any amount of throttle as if it were starving for fuel we changed the fuel filter, no change, air filter, no change. The mechanic found a bulletin VF500 #4 from august of 1985 addressing vent hose rerouting hot start problems / poor driveability at highway speeds due to internal venting of the carburetor float bowls. Among the recommended fixes are rerouting of the vent lines away from the center of the carbs [presumably to cooler air?] and replacement of a spring, jet needle and vacuum piston/diaphragm in the vacuum chamber as well as the new main jet and slow jets that are included with the "kit" This kit is apparently not available anymore and seemingly the problem was addressed on the '85s
I have 2 numbers on the Honda bulletin H/C 2082873 P/N 16020-MF2-004

Would anyone know anywhere I might find such parts, or better yet would anyone have dealt with a similar problem that wouldnt require another 400 dollar foray into the carbs.

Thanks in advance for any advice/tips anyone could offer, I really like this bike....the 30 miles i have spent on it ....and would like to put about 1000 more before the snow flies...and it was 32 degrees here yesterday morning so it coming
Trig

 

Sorry, don't know about the kit you're looking for, but FWIW I can co-miserate with you as I had a 500 Magna for a while and even after an ultrasonic carb cleaning the bike refused to have any power when you cracked the throttle wide open. I sold it before trying to figure that one out.

I heard the rubber diaphragms could get pinholes, but didn't see any on mine.

 

I found the kit I was looking for....another 400+ bucks, thats why I had hoped to find someone with a trick or two up their sleeve....ill hold fast!

 

just the jets alone, which are dirt cheap, might well fix the problem.

also, try to completely disconnect float bowl vents and see if it improves.

 

Anyone find a solution to this? I'm dealing with the exact same thing with my '84 V30 Magna as well.

 

It's probably not the jets, but I would look at the diaphragms. It sounds like they are not opening enough which is choking out that cylinder. Some cylinders are probably still trying to pull, that's the surging. A clue was the low mileage - the diaphragms probably got dry-rot. You'll need to pull the carbs and clean them thoroughly.

The kit you are referring to was to repair a hot start condition and only addressed the idle. Once your are up to highway speeds (like you describe) you are not using the idle circuit anymore.

 

Ok, I just tore apart my carbs, and checked all the diaphrams. they are all in one piece and the slides are sliding. I checked all jets and removed and recleaned them. I polished up the needles too. Bike still runs the same.

It always used to get over 50 mpg, this first tank got 46. It feels like it's running lean, but at times I get a whiff that smells like it's running rich, but, there's no backfiring or popping (which I would expect with a rich running bike).

Choking it doesn't change it. Cool weather makes it run better. the hotter the weather, the more surging to the point of refusing to go above 65 mph.

Is there any chance it's a spark issue? I've changed plugs, but is there any history of the coils having issues?

 

Per my previous post, it's not the jets. The jets almost never clog up, but the idle circuit does quite often. That is where the smallest passages are in the carb body. It's good that your diaphragms are ok, but you really needed to remove the idle needles and clean out those passages really well. I'd try that first and move on to ignition later. Remember: if you think it's the coil, then it's not the coil. Just like the main jets, it's almost never that part which is bad.

 

I did take the larger jets AND the smaller jets out and clean them. I didn't clean "the idle circuit" that I know of though. What is that, and where do I clean that? One year I had forgotten to drain the gas out, and the idle jets were all plugged solid. I had to dismantle it and clean them to make it idle.

The bike does idle fine, but does the idle circuit stay involved while the engine is running at higher speed? It's the high speed (engine at 6k rpm or above) going down the highway were the surge is horrible.

If I fire the bike up, ride the 1/4 mile to pavement, and then jump on the throttle, the thing will run to 90 in seconds. If the weather is warm, and the bike warms up to normal running temp, the surge starts and it'll barely run at 65.

Tom

 

Do a Fuel Pump Flow test. You may not be getting enough fuel at high RPMs.

Also read these as they may help
Carburetors/Tuning - SabMagFAQ

 

You need to remove the idle mixture screws. These are the ones that you can adjust with the carbs installed on the engine.

Yes, it does. It's really more of a function of throttle position, but there is some overlap between the different carburetion stages.

That almost sounds like a vacuum leak. Have you checked for that? Those carb insulators can be tricky during the install procedure. I'd check that out just to be sure.

 

Or it could be rich...that would explain better cold running. When it's running crappy, try engaging a little choke. If it gets better, it's lean. If it gets worse or makes no difference...you might be rich...or something else is going on.

No disrespect intended, but it doesn't sound like a very thorough carb clean was done. They can be tricky devils sometimes as a clog can come where you don't expect...such as in an air bleed circuit.

I would also check carefully the needles, are they attached as they're supposed to be? Are they stock or from a jet kit?

I wouldn't dismiss masonv45's suggestion either.

 

Absolutely, that's an excellent test to see if it's rich or lean because the symptoms are often the same.

That's kind of what I was thinking.

I don't know, if it's working at all that's good enough. If the pump is working it will run, if not it won't. I run gravity feed on my bikes and the flow is plenty enough to supply full throttle runs indefinitely. I'm going out on a limb here and say that it's not the pump.

 

As for the fuel pump...No doubt that's a long shot.

However, I worked on two 84 GL1200's this Spring within a two week period that had intermittent fuel pump failures. On occasion they would just die. I remember thinking strange coincidence...both 84's...both similar failures. Gotta say though the symptoms nor the bikes are the same as this one.

BTW...both GL pumps failed in a somewhat strange manner...they clicked audibly as if they were working OK. But when a clear fuel line was hooked up, You could see lots of air as well as fuel at first (marginal operation)...then a eventually more air...and not everytime...but on occasion...they would quit pumping completely.

If gravity has to push fuel through original "plumbing" , including a fuel pump that's inoperative, I don't think it would work too well. Masonv45 suggests doing a "flow" test on the fuel pump...which I don't think is gonna solve broderick's problem, but is easy enough to do...and sometimes eliminating even a "longshot" is a step in the right direction.

 

No worries, I know the carb cleaning wasn't very thorough, I was only making sure the jets weren't plugged, and that the diaphragms weren't torn. Not knowing what else to check, I just put it back together before I screwed anything up. :smile:

I did try a little choke, and it didn't make any difference. BUT, then I tried Full choke, and it smoothed out! It must be lean. So, I guess I'm down to "something is plugged" or "Something is leaking vacuum". right?

One more note, when it starts surging, it doesn't matter if you have the throttle barely cracked, or if you roll the throttle on full, it surges the same. I guess that might eliminate the vacuum leak a little. The fact that the choke fixes it would mean the carbs have enough fuel being delivered (so fuel pump is ok?).

Bye the way, I sure appreciate you guys helping me out here!
Tom

 

To test for a leaking carb boot, spray WD-40 around the carb boot when it is idling. If the idle changes, you've found your leak.

Do all 4, you may have more than one leak.

 

You could still have a marginal fuel supply. I wonder if your bike runs well for a "short" period of time like the original poster? I don't remember if the v30 Magna has a vacuum operated petcock like the 'ceptor...but if it does, you may want to test it with a vacuum pump. I've had to replace at least a couple of those on 'ceptors over the years.

 

No Vacuum leak to be found. I tried the WD40 around all possible leak points and the idle never wavered one way or the other.

 

Hi, in my opinion you have to test fuel pump. And/or fuel pump relay. A fuel pump seems to run well till you try to increase RPM range. The device (under the black cap of the pump) tends to block (it is a bridge with electrical contacts). Well, it happens that this little bridge gets blocked after few minutes or the electrical contacts are not good. If the mechanical part moves freely, Clean the electrical parts (if you remove the black cap you'll see a spark between these contacts). Do you hear a noise around the pump when you turn on the ignition key ? If not, the pump is blocked and can not send gasoline to the carbs, so you run with gasoline that arrives by means of gravity and that is not enough to feed carbs at higher RMP.
Hope to be useful to you.
Lamps to all VFR owners !

 

You know, I've never got this method to work. I've even tried spraying (I use carb cleaner) on an open vacuum port and the idle doesn't even flinch. I suppose what I'm getting at is that there could still be a vacuum leak.

I'd say that it's either to idle circuit (I think I mentioned that before) or one of the slides.

It's been mentioned several times now, but I'm VERY certain that it's not the fuel pump. I could be wrong, but in all my years of working on VF's I've never seen a pump, relay, etc fail in this manner. As I mentioned in a previous post - they either work or they don't. The pump doesn't produce and appreciable pressure, it just dribbles the fuel up to the carbs. If it's working, fuel gets to the carbs. If it's not (and they fail a lot) no fuel gets there and the engine won't run at all.