Fuel delivery or electrical?

[JVince]

So we gave up trying to tune the carbs and took the '84 VT700 to a dealer to have the carbs rebuilt and tuned. The shop doesn't normally work on bikes that old but the mechanic was confident. After the rebuild the bike seemed to run like a champ. There was a slight stumble in the higher RPMs but it seemed to work great. Then it started running poorly and the shop didn't want to let it go in that condition. I had used an aftermarket fuel pump in place of the OEM one so they were concerned that the fuel delivery might not be quite up to par. That seemed a little confusing since the aftermarket pump actually pushes more fuel but they were giving me a good price so we went with it.
Today they called and said the new OEM pump and filter aren't really making a difference and it's still running "so-so". The shop has pretty much given up on fixing the issue and they're refunding us money. I have nothing bad to say about the shop, but my question now is: Where do we go from here?

The mechanic's last comment was that he had trouble getting power to the new OEM pump and he suspects something electrical. Could the pump be losing power intermittently? Is there a fuel relay that could be failing? I know the bike could run on gravity feed so that might explain why it ran great, then poor on occasions.

Any thoughts? All new fuel line and the tank is clean. There IS something that rattles around on the inside of the tank. I noticed it when we were prepping for paint. Should that be there? Any ideas are much appreciated, thanks!

[byggd]

Can you run power directly to the pump and take it for a ride? If it doesn't help at least you will have eliminated power to the pump as the problem.

[John Hopkins]

no nothing should rattle about inside..but why are you painting the tank before you have fixed the bike..and you do know that there are two tanks don't you..

John.

[JVince]

The motor needed to be completely resealed. While I was working on the motor my wife repainted all the tins. Yes, we know there are two tanks, so now that I think about it even if the main tank was plugging up it shouldn't affect flow.

Now the dang thing doesn't even want to start. Tons of backfiring. When it does start it has a wicked stumble when you crank the throttle really fast. Could the ignitors have gone bad? They did get wet at one point when we had the bike apart...

[John Hopkins]

Backfiring is good, it means you have some spark, what do you want to do..start at the beginning..spare new sparkplug..unplug any plug and plug the lead into the spare, using a piece of wood, press the thread against the cylinder fins and turn the engine over...nice blue spark..yes..then try the same trick with the other three..good sparks..electric's ok..problem is with either the fuel or air supply..

John.

[JVince]

I'm getting good spark (the last time I checked) and the bike likes to fire up if I'm on a charger or jump battery. I'm thinking maybe the battery isn't getting charged enough so I'll test the stater next. I just took a little ride and it also backfires when I let off the throttle but I think that might be carb adjustment.

Thanks for the replies, we'll keep working! I was worried about the coils and ignitors though. I was curious if they could be starting to fail and giving a bad or inconsistent spark.

[vanshee]

look for an intermitint short

[John Hopkins]

Originally Posted by JVince

I'm getting good spark (the last time I checked) and the bike likes to fire up if I'm on a charger or jump battery. I'm thinking maybe the battery isn't getting charged enough so I'll test the stater next. I just took a little ride and it also backfires when I let off the throttle but I think that might be carb adjustment.

Thanks for the replies, we'll keep working! I was worried about the coils and ignitors though. I was curious if they could be starting to fail and giving a bad or inconsistent spark.

Coils are just what the words says..a length of copper wire rolled into a coil and varnished to separate the loops from touching each other..when they break down it is usually because the varnish has cracked and voltage jumps from one loop to the next...this changes the inductance of the coil, in effect making it a link..in other words the coil is very unlikely to become intermittant... wires to and from any componant can break down, becoming high resistance or open circuit, if you still feel that the spark is failing you should first change the sparkplugs, the HT leads and the plug caps..

John.

[JVince]

We've changed plugs a few times when we were fooling with mixture but I haven't changed the wires yet, thanks for the though! I'm getting this sinking feeling I might have a short somewhere. I hate shorts. I might look at a new harness if that's the case, eventually .