91 VFR 750 Backfired once, now running on 3 cylinders

Hey everyone - if you'll recall my first post ever was about my "new" VFR's RR wires catching fire on the day I bought it.

That was a month ago, and all has been well...until last night.

I was 1 block from home, accelerated pretty hard in 1st gear, let it wind back down and around 4k rpm on the way down it backfired once or twice and immediately started idling lower than usual.

I checked it out again tonight and the front left (#1 cylinder?) is not firing, exhaust didn't get hot. I pulled the plug wire and it's getting spark.

I suppose next step is compression test? What are some possibilities here? Valve problem? Carb issue? The bike was running well before to redline, starts easily every time (actually still starts easily, but idles rough and sounds flat).

Would appreciate insight into what this sounds like to you guys. Could it be something serious? I have to ride it 6 miles through NYC to get to my workshop in Brooklyn, so if I can't get it firing before then it'll be an interesting ride.




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Pulling redline is always going to cause things to bust

 

check float level for that cyl

 

Is the plug fouled? That could indicate that it got drowned in too much gas, and the backfire was the excess gas igniting in the exhaust.

If it recently ran fine, do not torture yourself with worrying about a serious problem. Relax and take it a step at a time.

Take a taxi if you can. If the cylinder is flooding you do not want to be creating a flamethrower for six miles. You could barbecue a pedestrian or blow the exhaust off the bike. I am somewhat exaggerating...kind of...

 

Just remembered that I forgot to turn the choke all the way off until about 20 minutes into my ride the other day. That could have fouled a plug...will replace those first and see where that gets me.


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Thanks for the update. That would be great if this is the full extent of the problem.

 

that would do it :eek-new:

 

Changed spark plugs, still have the issue




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Plug from the dead (front left) cylinder is on the right, good (front right) cylinder on the left. Visibly they look the same, doesn't look like any obvious sign of fouling.

Right rear cylinder is firing as well, so I think that rules out a coil issue.

Looks like the next step is a compression test on that cylinder?

I suppose I could pull the valve cover off tonight as well and take a look to see if anything is amiss.

All the rest of my tools are in Brooklyn though so compression test will have to wait.

Any other ideas for now?


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Recall RJGTI'S post. Is it getting gas?

 

Pulled the valve cover (that was fun...) everything seemed in order.

Tried to get at the carb bowl to crack the drain screw and see if it had gas in it, unfortunately that seems damn near IMPOSSIBLE. To get to with the carbs on the bike. Just that one carb bowl...its facing the wrong way, not sure if someone mixed them up when they put them on.

Anyway, I wanted to take a look at the intake just in case maybe a critter had crawled in there and gotten lodged in or something...who knows.

Took the air filter off, it is very dirty, started it with no air filter and it fired on all cylinders! Verified by putting my hand over that snorkel, it had suction, and ran lean quickly when I covered that one. Exhaust also got hot.

Put it all back together, started it, still on 3 cylinders, same one still dead.

Seems very weird to me that a dirty air filter would cause this one cylinder to die and not the others. Maybe it's causing some kind of weird vacuum activity?


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Carbs could out of balance just enough that the dirty air filter had more of an effect on that carb.

 

I'm thinking something along those lines as well. I'm going to try to get my motion pro sync tool on these guys sometime within the next few days. Also just ordered one of those flexible screwdrivers that someone posted in a different thread for the sync.

I'm going to order a new air filter while I'm at it. There seems to be quite a debate on which is the best.

On a side note - I can already tell that any carb related work is going to be a cold awakening for me as my other bikes are a Honda cm400 and a naked Yamaha Seca II, both of which I can disassemble the carbs with them still on the bike if I wanted to.


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you said it went lean when you covered it...that is wrong, it went rich. check float level

 

John, glad to hear you got a handle on this.

Filters: My mechanic highly recommended the BMC. But also, there is contention over whether the input flow on these bikes is truly limited by the filter. A higher flow filter risks more contamination reaching the engine. So you may use an OEM spec filter just as well, and may not notice a difference in power. Better for the budget too and can replace it more often.

 

I was a little tired when I wrote that last night, but yea you're right it would have gone rich when covering it. It responded to me covering it (the idle raised), so I assume that means it was firing.

Since it looks like I will have to pull the carbs to get at the floats I'm going to try syncing first and see where that gets me.

Carb overhaul is in the cards either way.

Thanks for the responses and support guys.


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MotionPro synch tool is a waste of time. You have to synchronize the tool off the base cylinder before each use, its total :bs: get a Morgan Carb Tool or something with analog faces. side bar: Used to be that they were using mercury not this man made stuff. Totally hate that tool btw, far as a tool for adjusting synchronizing screws, its a bent angle one with a control on the end like your describing, MotionPro might sell one of these, the one I have is from Triumph. The tip and the way its guided into the screw if key, mine has a plastic cup, so you get the bike up as high as you can with a good headlight on (on your head) and use this tool, takes me under ten minutes to do it now.

You could unscrew the fuel mixture screw on offending cylinder and spray some carb cleaner in there. Oh, do not mess with the float levels what ever you do. Each over haul kit from Honda is about 30 dollars maybe more, oh you have to buy two style kits, they sell the float needles with seats seperately, then you need the bowl gasket kit. You may be able to do K&L but I can't help you out.

 

What do you mean?

 

If you have the new style motion-pro tool with the man made blue mercury, as opposed to the old unit that was yellow and used real mercury. You have to calibrate the tool before each use off the base cylinder of the bike your working on. On my gen 3 bike its carburetor 2? Its the carburetor that does not have a screw adjuster, (can't remember which one without looking at the manual, I know the gen 3 and gen 4 bikes are different base cylinders.)

Anyway getting back to the "tool" its basically a user un-friendly piece and I would give it to you for free if you were my neighbour :loco: I use a morgan carb tool and its very nice. Everything is still the same as the above posting. MotionPro has a good customer service base and sells decent bike stuff, they will even sell you more fake mercury when your engine sucks it into the plenums :frusty: don't ask...

 

Haha yea funny you should mention it about the motion pro...I've used it several times on different bikes, but my friend's Nighthawk ate all my fluid last time I used it. Just got the refill kit in today so I'll have to go through that refill process. If I could go back in time I would have bought a better tool, but I already dropped $100 on it so might as well live with it.

Along with the bendy screwdriver I also ordered a 22" screwdriver set. Figured with the way the V engine is packed in there the extra long screwdriver will come in handy.

Thanks for the tip on the floats. I'm really hoping I don't have to pull the carbs right now since I would rather do a bunch of stuff to them at once (rejet, clean, etc) rather than do a one-off troubleshoot like this


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