91 VFR 750 Backfired once, now running on 3 cylinders

Much better setup is a single vac gauge with a 5 outlet connector with valves on it from a fish tank, costs about $5. Good vac gauge about $30, Check your base carb so you know what it is reading, then work your way through the other cylinders so they all operating at the same vacuum

 

That makes sense...on other bikes it seems like the cylinders interact with each so adjusting one messes with another. Does that happen with your VFR?


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Dang now that's a dialed in bike


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Good try but........

Of all sudden malfunctions that's among the least likely. Once set, that shit don't change. :sour:

I'm puzzled as to any good answer to the op's problem, but i'd point out that a plug may spark perfectly when out of the engine but fail to spark under cylinder compression. In that case an electrical--probably ignition coil or defective plug--problem would be responsible. It's always good to switch the plug from a firing cylinder to one non-firing as a test.

 

It was way off beforehand too. I had the throttle bodies overbored 2mm and when I installed them it was really rough at 2-4% throttle and 6000 and up. One cylinder was showing 9:1 AFR and another was almost 16:1. After balancing the throttle bodies and some street tuning, everything cleaned up nicely.

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Still haven't been able to make it out to Brooklyn to pick up my manometer, but I was able to get it firing on all cylinders by just adjusting the front carb's sync screw.

It's more or less now running how it was before the backfiring...maybe even slightly smoother (I don't get that clutch rattle at idle anymore).

So I'm guessing that the entire past month I've owned it the carbs have been horribly out of sync and I just didn't realize it.

Really interesting that it was teetering on the edge of being so bad that the bike wouldn't even run and I didn't even know it.

Later this week I will hook the manometer up and get all the carbs dialed in. Thanks for the help!


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I wouldn't be surprised if a really bad backfire could throw the carbs out of sync.

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Kowa is the official Honda Tool. I remember ordering some moon wrenches also known as manifold wrenches and when I received them, was bummed out to see they were made in Brazil and were'nt even Kowa badged.

 

Couldn't get the screw off the #3 intake vacuum port so soaked in pblaster for the night, but synced the #1,2,4 cylinders. The motion pro tool is a real piece of crap and the fluid broke up, but I got them close.

Engine runs pretty well and accelerates freely (same as it did before this issue). It backfires pretty regularly on decel if I accelerate high (up to 8k) while engine braking. I will try to sync the #3 cylinder to the rest and see if that helps. Also have a new pipercross air filter coming in a day or two as I have a suspicion that it's so clogged it's screwing up the vacuum and causing the engine to run rich.

I noticed that when syncing when I would increase the idle the fluid level would actually drop. When I would slowly rev the engine would bog and try to die. Seems like it would be the opposite.


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Backfiring on overrun is a lean condition. Usually when synchronizing the carbs, one cylinder is fixed and the other three are synched to the fixed one.

Less vacuum = higher idle

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I've had the lean decel pop on other bikes, but this one is a more forceful and loud backfire, not arguing with you, but this one seems stronger. Either way I've still got some more work to do with the air filter and getting that last carb synced before troubleshooting the backfire (if it still exists)


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Refresh my memory, which carb is the base one? Its the one without an adjuster - so with this is mind, you could'nt get the number 3 screw out of the plenum, that means that you only had two adjusters to adjust, with the base carburetor in mind not having an adjusting screw. Just asking ok. Established that the "tool" sucks out loud. Another question, do you still have the PAR equipment on this bike? On the 93 I get a slight pop on light de-cell coming home after a ride. Never really heard anything out of the ordinary on rides in general. Cheers

 

I believe the #1 (front left) is the base carb on my 91.

Got the screw out of the #3 (back right) carb using my 23" extra long screwdriver that I bought and got them all synced reasonably well.

I was able to get all the air bubbles out of the motion pro syncpro by shaking it from the top like a thermometer, so I got it to work out okay.

What I found interesting is that with the sync tool hooked up to the carbs I could not rev the bike, if I tried it would sputter and die. Seems like I should be able to rev it, no?

Anyway, put it all back together and idle isn't rock solid (it has never been since I've owned it, probably some small vacuum leaks), but it runs pretty strong. Backfire is still there if I run it up to 8k or 9k in 2nd and let it wind back down/engine brake. Maybe there's an exhaust leak.

My pipercross finally arrived from the UK but they sent me the wrong damn one, so now I've got to wait another week.

Anyway, consider this case closed and the problem fixed with a carb sync, hope it helps someone in the future.


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