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-   -   83 VT750 stalls above 3700 rpm (http://www.motorcycle.com/forum/showthread.php?t=16602)

ranchwrench 10-04-2010 05:35 PM

Quote:

Originally Posted by seruzawa (Post 250218)
It's an '83.

yeah, I gotcha....just haven't seen one for a while. :p

aartd 10-04-2010 07:02 PM

I checked the oil level and it has not risen and does not smell like gas. I only ran the engine for about 30 seconds after I shut off the fuel.
Do you think I could fix this carb problem with a new set of float seats and float needles? The floats are plastic and appear to be good. I'll also
check the float height and adjust as required. Should I be looking at anything else?

seruzawa 10-04-2010 07:29 PM

Quote:

Originally Posted by aartd (Post 250221)
I checked the oil level and it has not risen and does not smell like gas. I only ran the engine for about 30 seconds after I shut off the fuel.
Do you think I could fix this carb problem with a new set of float seats and float needles? The floats are plastic and appear to be good. I'll also
check the float height and adjust as required. Should I be looking at anything else?

Well, that's what I'd try next.

ranchwrench 10-05-2010 11:07 AM

Quote:

Originally Posted by aartd (Post 250221)
I checked the oil level and it has not risen and does not smell like gas. I only ran the engine for about 30 seconds after I shut off the fuel.
Do you think I could fix this carb problem with a new set of float seats and float needles? The floats are plastic and appear to be good. I'll also
check the float height and adjust as required. Should I be looking at anything else?

I'm too slow for you guys....I'm still stuck trying to diagnose this thing. :) I'm just curious....did you check the carb sync on these bad boys? V-twin carbs inbetween the cylinders are nearly IMPOSSIBLE to keep in sync when you pull them off. And you'll get some really weird running problems if the manifolds are hard and old and leak a bit, while the carb sync is off.

If you DO pull the carbs again, I wouldn't mess with the float height on those shallow little float bowls. (assuming they're new anyway) And I would never do a carb set like this without changing the rubber manifold gaskets because it's too hard to get them in if they're stiff and you'll develop leaks from torquing on them to get the carb set back between the jugs.

aartd 10-05-2010 03:34 PM

The only sync check I did was to visually note that both carb throttle plates (butterflies) open at the same time. The Clymer manual I have does not address the synchronizing of the carbs. I'll look into getting some new intake manifolds while I'm waiting for the new float seat and needle. Thanks for the help. I'll let you know in a few days if I have any luck with the new parts in.

ranchwrench 10-09-2010 08:05 AM

Quote:

Originally Posted by aartd (Post 250288)
The only sync check I did was to visually note that both carb throttle plates (butterflies) open at the same time. The Clymer manual I have does not address the synchronizing of the carbs. I'll look into getting some new intake manifolds while I'm waiting for the new float seat and needle. Thanks for the help. I'll let you know in a few days if I have any luck with the new parts in.


Can I give you a bit of hard won advice? I'm a former well-paid m/c and auto tech, and when I say "well paid", I'm speaking in relation to my former peers who made even less than I did.

I tried owning a shop, then two shops, then a franchise, and the more money I made in motorcycling, the further I got from the one thing I liked doing....diagnostic troubleshooting. So now I do something else, but still enjoying fixing bikes.

Now, if your bike was sitting in front of me, fixing it right would be trivial. I'm not bragging, just stating the fact that any good tech could fix your particular problem while arguing with his girlfriend about cleaning the cat box. But if you (on the other hand) rode into a shop where everyone started shouting solutions to your problem and bringing out parts to "try", you'd probably be wishing for an excuse to leave without hurting anyone's feelings.

If you start changing parts as a diagnostic method, you might just as well start by checking your front tire pressure, because it's just as likely to fix the problem.

Now, the garden variety CARB SYNC check is so mundane, that most good techs do it without even thinking about the aforementioned cat box. The reason? You can check engine condition (a dead cylinder, no compression, will not draw a vacuum), and a running bike eliminates the ignition question) That leaves a/f mixture and carburetion on your particular machine. (there could be multiple problems, of course, but it only makes sense to start with the most likely and work from there.)

You mentioned that you 'eyeballed' the carb sync and felt that was good enough. What you probably don't realize is that you can't accurately compare the space under the throttle plate on VT's or magnas because the carbs face different directions (unlike an inline 4, etc.) and you will never get accurate results this way. (in a pinch, you *could* use a known diameter like a drill stock and use it as a feeler guage (no, you can't use flat feeler gauges-they're too wide) Also, the VT linkage is somewhat hinky and is difficult to get a good carb sync, even when everything else is PERFECT, which is why they run so badly when slightly out of adjustment.

But you're inviting disgust at least and disaster at most, by simply replacing carb parts because you think that'll 'help'. This is the way larger problems always START at dealerships with beginning techs.

My advice? Stop and think, write down a flow chart problem diagnosis, and be disciplined enough to follow your plan and not skip steps.

Good luck. :)

aartd 10-13-2010 04:09 PM

I put the new float needle and seat in but they did not solve the problem.
Now the bike starts well but only idles and runs at low rpm for about a minute before it runs rough and dies out (due to flooding of the engine). Does this sound like a weak ignition problem? If so what can I do to determine what needs to be done to fix the problem? Also I noticed that there were large dark particles in the new fuel filter. I put another new fuel filter in but do I need to remove the tanks for cleaning?

aartd 11-21-2010 11:16 AM

After cleaning the small tank, replacing the main jets, which were .005" oversize, and trying a couple of used but good spark units the bike still did not run well. I discovered that if I removed the air cleaner the engine would run nearly perfect so I determined something was causing an overly rich condition in the carbs. I knew the choke piston springs were not very strong so I tried replacing them with stronger springs. The stronger springs solved the problem and now the bike runs, with the air filter in, as good as it looks (like new). Thanks for pointing me in the right direction so that I could fix this great old bike.


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