Where to start with 86' VFR700F that hasnt been touched in over 5 years?

I just got off the phone with a gentleman who runs a carburetor shop and he told me that he doesnt think its the carburetors at all. He said the carbs wouldn't know what engine speed the bike is running, and that the carbs operate purely off the amount of throttle thats being rolled on. So basically, since my issues were at very specific points on engine speed, (3.5k and 4.5k), the carburetor wouldn't make the problems be that consistent. He told me its probably an electrical issue.

What do you guys think?

 

Have you tried a full-strength dose of Techron or other fuel-system cleaner through the bike? I did it about yearly as preventative maintenance (Redline, Amsoil, and Techron are all pretty much the same stuff, PEA in a bottle).



I'd be a bit more detailed than "works or not" with the electrical, but I'm putting my money on the carbs as well. More complicated -> more to go wrong -> more likely to be a problem.

As to the electrical possibilities:

The CDI is probably not an issue. The remaining suspects are coils, plugs, and wires. All of which are highly unlikely to have a frequency-dependent operation. But, a weak spark may only manifest itself at certain frequencies of engine operation due to the engine itself (timing changes, momentum of the engine, etc). Check the plugs, and check the wire/plug contacts for corrosion. ground the side of the plug somewhere out of sight, and start up the bike and check for spark (and expect to get a few shocks along the way). But it will probably check out ok.

Just to be sure, put a multimeter on the stator wires, and check to make sure that output is within spec through the ranges (full diagnostic procedure is in the FSM).

Then, time to go back to the carbs (assuming that all checks out ok).

While yes, carbs are not aware of engine rpm, they operate instead on airflow. And airflow is dependent on 2 things: engine speed and throttle position. A slow roll through those RPMs is going to be at a higher vacuum, lower-velocity than a WOT run through them. So, if throttle position changes it, and especially if WOT makes it nearly go away, I would suspect the carbs more than anything else.

I mentioned the fuel bowl vents earlier, because I had the same sort of issue. A low speed roll up past 5500-6000 rpm would stumble badly, so badly it wouldn't go over that rpm. But I opened up the throttle wide enough, it cross through the stumble region fast enough to continue on (but I stayed well shy of redline since I knew something was wrong, suspected too-lean fueling, and didn't want to blow things up).

Now, given that you've got essentially the same setup as me, you're probably too lean of a mixture to start with, at least at idle.

So here's what I would do to troubleshoot the situation:

- remove the airbox lid and filter, and inspect everything
- double, and then triple-check that all hoses attached to the carbs are correctly routed. There are a lot of them, and if you get them wrong, odd things can happen.
- if the emissions equipment is in place (purge valves and charcoal canister), isolate to simplify the problems. There are test procedures for everything in the FSM, but they may take a couple less-common tools (like a hand vacuum pump).
- check for vacuum leaks - These will make the engine run lean, and lean means misfire and no power. If you find a leak (carb to head boots are highly suspect, especially if old), then fix and start over. Same goes for all those old rubber hoses. Make sure to check that the vacuum ports on the heads are properly capped, too (especially if you've sync'd the carbs).

Now, if that's all checked out, and it's still not happy, dig deeper.

- start bike with filter removed, and watch operation of the slides. They should all move in concert. rap the throttle, do they all pop open in concert? if you rev it up over 3.8K, do any of them act differently than the rest? The goal here is isolating a problem cylinder (or two) from the others.

Everything look ok? dig deeper.

- drain and pull the carb assembly. Buy new carb boots if yours are still original, they won't survive the removal. If you have problems draining any of the bowls, you have a suspect there (crud)
- **DO NOT REMOVE CARBS FROM THE PLATE that forms the bottom of the airbox!!** Very bad juju. The plastic air and fuel 4-way pipes are unobtainium. Break one and you're hosed.
- flip them upside down on a clean bench, and pull the bowls.
- inspect everything
- very carefully remove the slide spring/diaphragm covers, and VERY carefully inspect the diaphragms for tears. They'll do weird stuff when torn (but you should see that one as an slide that doesn't open with the rest). I've torn them during assembly/dissassembly, they are VERY fragile.
- clean everything. Compressed air and a soft wire through every passage you can find/reach.
- And again, the FSM has all the details on tearing things down. Just be very careful, since many parts are no longer available.

My experience has been that the local fuel is pretty good, so long as you stick to the "top-tier" stations (76, chevron, shell, etc.) But you might have picked up some gunk.

 

I bought some Techron and put it in last night then went for a 20 minute ride and it was okay. Then today I rode around for another 20 minutes and towards the end of the ride, it started acting up. The bike was pretty warm. Maybe about 60% to the red zone on the temp gauge and then it started to act up. It even died once. Its strange, I can hear the bike actually not fire on all cylinders temporarliy. You can clearly hear it change its exhaust rhythm for a few seconds at a time then switch back to sounding normal, then back to an odd sound. Like going from 4 cylinders, to 3, back to 4, then 3 but maybe a different 3? Who knows.

I talked to some local Honda mechanic, but hes wants 95 an hour. Forget that...

 

Unemployed atm

 

Haha. I'm totally fine with the unemployment, but the bike stuff makes me sad

 

trailer it up to toecutter with a truckload of beer.:biggrin1::biggrin1:

 

OR, if TOE's too occupied or disinclined, send the boxed carbs to me for inspection/ repair if you've decided your problem is due to carb issues. i love challenges, and a Judge said i need to keep my hands busy.....

 

So I took a long drive up to Toe's and the bike ran fine. Who knows? Maybe I blew out whatever problem was in there...

But on my way home, my chain broke and wreak a mild havoc on the drive sprocket cover and who knows what else...

I'm just curious - when removing that cover and the clutch slave cylinder, am I going to have an oily mess on my hands? Mainly from removing the slave? The service manual just says to pull the clutch lever in but doesn't say anything about clutch oil or anything like that. Does it just come up with no mess?

I just want to know what kind of project I might get into this afternoon.

 

No oil, but inside that cover is the most horrible filthy-dirty place on any bike !! If you tore out any bolts from the engine case that mount the cover and slave, you're kinda screwed. Why were you riding on a bad chain, you didn't have any clue ??

 

There's something, looks like part of the chain cover that mounts to the engine, thats torn off, which if you look closely, you can see in the picture (right in the middle; the white part). Hopefully thats not a deal breaker but it looks to be out of the way of the chain and its not affecting the slave.

 

As you can see, it tore it up pretty good in there. The top of the drive gear cover is totally torn open...

 

Two words for ya "JB weld"

 

That is no bueno, best you can do is extract the broken bolt and try e new bolt and see if it holds. passed that it gets serious.

 

Toe, you saw his bike but didn't see that he was at the end of his chain ?? :behindsofa:

You won't be riding that bike anytime soon, Scott, sorry! The mounting points that hold the clutch slave bolts are gone. JB won't help. I've dealt with that problem before on a damaged '87 750 i bought cheap and on a customer's bike. Repair would be difficult or impossible and expensive, so you now have a parts bike. Good example why never to ride on worn out parts.

 

The slave cover actually still holds pretty well. The bottom two mounting bolts are still intact (in the first pic, you cant see the 2nd bottom mounting bolt because the angle of the shot). The top two are totally maimed though.

And to Toe's credit, he did say that my chain was needing to be replaced.

 

Once you get it running, be careful riding it. It will need new tires.