Backfire Through Carbs

I have gone back and forth on this bike so many times and it has sat for quite time now. Over the winter I pulled the engine and did head gaskets, valve seats, valve seals and a valve adjustment to the factory specification, I installed the engine again and then it sat for about 4 months. Now the weather is warm, and there is a vintage show coming up, and I'd really like to get this thing going. So, since you guys have always given great advice in the past, I was hoping I could ask you again. This is the scoop on my 87 VFR700...

With cleaned carbs, idle screws currently set to 4 turns (I started at 2.5 and worked my way out to 4 as I was not having success), fresh plugs and a fully charged battery I am consistently seeing backfiring through the carbs on all cylinders and the bike will not start. Before I put the engine back together, I triple checked my timing and my valve clearances. I am pretty confident that I have all of that right. I do not have a compression tester small enough for my bike, but I was once told (this may be wrong) that if you cover the hole with your thumb and cannot prevent your thumb from blowing back off the hole while cranking, that you should be good. I did this makeshift test and it seemed to be ok.

Also, I'd like to note that I had issues like this before I did any engine work. The bike would backfire through the carbs when idling, but not so much while riding. I would really appreciate any help. Thanks!

 

I am posting this really in the hope this will prompt others better qualified to chip in on this one.

The following rather left field input may be way off target. I stress I don't own a 700 or indeed a bike with carbs - but in the bad old days when I had a car which did, I guess the basic causes of backfires will still apply and may be similar for a VFR.

However to have a backfire through the Carb (not the exhaust), I would have thought means fuel is being ignited when the inlet valve is open :kaboom: [sorry always wanted to use that one] which points to you either have the timing badly wrong, or the plug leads have been connected wrongly. Obviously sparks should only be reaching a cylinder when both valves are closed.

Another more worrying cause would be damaged or stuck valves - which a compression test will eliminate.

As for backfires in the exhaust they are mostly down to improper air/fuel mix - generally down to the carb/float/throttle adjustment,
But equally this could be down to a range of other things. Especially if you have been working on the carbs, vacumn leaks can seriously weaken down the fuel mix, and you may need to check for clogged fuel lines/fuel pump issues.

My first port of call would be to check for bad/wrong ignition timing issues. If it has a distributor - check for cracks, rotor arm arcing? and obviously wrongly connected plug leads.


Hope that helps




Skimad

 

Thanks Skimad,
I agree that the symptoms would lead to valve timing, but I haven't really checked plug leads. These bikes have individual coils per cylinder and I know for a fact that the wiring from the coil to the plugs is correct, but from the coil back to whatever it connects to could be wrong I suppose. I'm really trying to avoid dropping the engine again to check my timing because I am pretty confident it is right, but who knows.

 

If your mixture screws need to be out 4 turns to idle OK, that suggests that your idle jets or other small passages are partially plugged. (If screws are too far out, you can lose the spring pressure, and they may vibrate looser and eventually fall out !) If you let the bike sit for anything over a month or two with fuel in carbs, you may need to try a fuel system cleaner or go back into the carbs to clean manually. What setting did you apply to the float heights, 7 or 9mm ? Did you poke a thin steel wire (.012") completely through the idle jets ??

 

Yeah, I actually bought a set of jet cleaning wires and pushed through the biggest that would fit. What about float bowl height? I've never attempted to adjust this because I have no idea how or what it does. Can someone give me an overview of the process? This may be an issue as these are not the original carbs from the bike.

 

Any and all tuning efforts are useless unless the float height settings are on-spec FIRST. FSM shows the adjustment, and there are home-made tools that can help with accuracy, like the credit card cut to the float setting #.

 

So, to understand the process correctly, the credit card would sit across the bowl and the measurement of the portion of the float sitting below the credit card (with the carbs oriented in the normal upright position, not upside down on the workbench) should match factory spec, correct?

 

Sorry, did not notice the picture before, that answers the question

 

Just be sure the position when you measure doesn't allow the weight of a float to compress the needle................................................................................................except a very discreet, tiny, tiny amount like about .25mm.