stumbles and backfires

[Black Sled]

I could use some help diagnosing my problem.  When I bought my Tourer 2 years ago, it came with Cobra 6-6 pipes and needles and, apart from gummed slow jets, was running fine.  I installed 38s/105s/K&N and desmogged and it was running good.  At Sturgis I had my Valk dyno'ed and it was running great up to 4000 RPM, but beyond that was pretty rich (as low as 10 AFR).  It seemed like the performance had been deteriorating a little, but I figured it was probably the hot weather and higher elevation.

This week I went back to the stock mains and stock air filter as a starting point to retune.  Also finished a few odds and ends on the desmog, finally cutting the tubes off and filling the plugs with JB Weld - I just had rubber caps on before.

Now the bike runs fine up to about 1/2 to 2/3 throttle, but beyond that it stumbles, and at WOT it actually backfires.  I have been over it thoroughly looking for leaks and found and fixed a few things.  Made some progress on the stumble, but it is still there, and the backfiring is actually getting worse.

Is it possible that, since the bike was relatively new (5,000 miles) when I installed the larger jets, that now, two years later (16,000 miles) it would run leaner with 100's than it did originally?  Also, if I did have a vacuum leak somewhere, is there a way to identify which cylinder?  Honestly, all the plugs seem to be running darker, but I think I had the fuel screws out too far initially. 

Could this be a timing issue?  I put my Dyna away and am back running the stock ICM.

I welcome any helpful comments or thoughts.

[Black Sled]

I forgot to mention, I have a Pingle, so I know it isn't a bad petcock problem.

[Ricky-D]

Backfire at high RPMs with the motor under strain is sign of an ignition problem or sticking valves. 

***

[Mr.BubblesVRCCDS0008]

I think I would check the slides in the carbs as they are vac. operated  and one might be sticking or have a hole in the diaphram. If you changed the needles you might have one that's not sealing good.

[DFragn]

I think I would check the slides in the carbs as they are vac. operated  and one might be sticking or have a hole in the diaphram. If you changed the needles you might have one that's not sealing good.

Yep, that's what I was thinking. Careful inspection of the diaphragms with a backlight might locate a pinhole or small tear.
Or maybe the carb sync is way out.

[Black Sled]

okay - I looked everything over and haven't identified any problems.  I have isolated the problem to the left bank though.  Engine seems to run nice for first few minutes while warming up.  But once I put a load on it the engine stutters and spits on the left side. 

Does this sound like a coil?  How would I verify that? 

Any other thoughts?  Thanks again....

[DFragn]

okay - I looked everything over and haven't identified any problems.  I have isolated the problem to the left bank though.  Engine seems to run nice for first few minutes while warming up.  But once I put a load on it the engine stutters and spits on the left side. 

Does this sound like a coil?  How would I verify that? 

Any other thoughts?  Thanks again....

Not the coils because each controls 2 cylinders and on opposing banks. i.e. 1-2, 3-4, 5-6.

[Robert]

If you didn't remove the baffle in the air box that could be one reason why the mix was to rich on full throttle with the mains done. It doesn't pay to go half way Honda did their research when they made the fuel system on this. The right bank problem sounds again more like carb problem than anything. Like was posted the coils do one cylinder each side. I would also check the fuel supply hose that goes into the bank I have heard of problems with them. But I would put my bet on bad float needles. The mileage doesnt make a difference in fuel mixture. I would also check to see if the choke is staying on. I would also like you to pull the plugs again on that bank ideally the plugs on the Valk seem to run to the white/lean side and maybe the plugs may be fouling if to rich. Also you need to do a trigger wheel to really get the most out of your bike.

[9Ball]

what notch are the e-clips on in your cobra needles?  Sounds like a lean condition to me.

[Patrick]

Have you checked each carburetor for proper fuel flow?? Are they all getting enough fuel supply?? I think I would look at diaphrams and the needle settings as well as 'fuel screw' settings.. I'm not quite sure what 'fuel screws' are ,but, I'm thinking you're referring to the pilot screws which should be at about 2.25 turns with 35 jets and 1.75 turns with 38s..These screws should be turned in gently so as to not damage the seats and needles themselves..If they have been damaged then they should be replaced..You also didn't state[or I didn't remember it] whether or not the air box has been modified,, I kinda hope it hasn't been..If it has been, then I'd recommend replacing it..These engines do tend to run a bit lean in the low and middle engine speeds with the 35s and they do run a bit rich at higher engine speeds with the 100s..

[Black Sled]

I did try raising the needles one clip but didnt seem to help much, if at all.  I could try raising the needles another clip but the plugs are running dark.

I have the 38 slows and the fuel screws are set at 1.5 turns.  I started out at 2 full turns and turned them in 1/2 turn after seeing the plugs dark.  Not much difference in the symptoms, but could explain why the dark plugs.

Could I be too rich?  I cant imagine being too rich on the mains or needles but it does seem to get worse fast as the engine begins warming up.  Right out of the driveway it seems to run smoothly. 

Perhaps I should change to new plugs.  I did clean them with a wire brush as they were pretty dark.  They don't look fouled but perhaps one is missing?

[Michvalk]

I had a little problem like that. I took a can of starting fluid and sprayed around the carb and vacume points and found an intake leaking at the head. There is o-rings on the intakes that can and do leak,especially when the intakes are removed for carb service. If they are the least bit squashed they Will suck air. And look at the clamps on the carbs both at the airbox and where the carb attaches to the intake. If these are loose you can get the same type reaction. Air leaks will cause a lot of these kind of problems. Good Luck

[9Ball]

I did try raising the needles one clip but didnt seem to help much, if at all.  I could try raising the needles another clip but the plugs are running dark.

I have the 38 slows and the fuel screws are set at 1.5 turns.  I started out at 2 full turns and turned them in 1/2 turn after seeing the plugs dark.  Not much difference in the symptoms, but could explain why the dark plugs.

Could I be too rich?  I cant imagine being too rich on the mains or needles but it does seem to get worse fast as the engine begins warming up.  Right out of the driveway it seems to run smoothly. 

Perhaps I should change to new plugs.  I did clean them with a wire brush as they were pretty dark.  They don't look fouled but perhaps one is missing?

moving the clips towards the top of the needle (flat part) is taking them in the lean direction.  Try moving in the other direction.  Typically, the cobras are set in the third (middle) notch.

[Black Sled]

when I say I raised the needles, what I mean is that i dropped the clips to a lower notch.  Thanks for the confirmation though...

[Ricky-D]

I had a little problem like that. I took a can of starting fluid and sprayed around the carb and vacume points and found an intake leaking at the head. There is o-rings on the intakes that can and do leak,especially when the intakes are removed for carb service. If they are the least bit squashed they Will suck air. And look at the clamps on the carbs at the airbox and where the carb attaches to the intake. If these are loose you can get the same type reaction. Air leaks will cause a lot of these kind of problems. Good Luck

I disagree with you on this particular idea.

Any air leaks upstream from the carburetor will have no effect upon the motor performance or power.

Simply recognize that: air is air, where ever it comes from.  That's all the motor requires, an air source. I can remember as a kid, taking the air cleaner off the car to go racing with my buddies.

You could most likely remove the air box and all the connected hoses that go to the carburetors and get a still, good running motor. I wonder if anyone has tried that?

Only air that finds it's way into the motor and affects the fuel/air ratio will have any affect on the performance/power of the motor.

***

[Madmike]

I had a little problem like that. I took a can of starting fluid and sprayed around the carb and vacume points and found an intake leaking at the head. There is o-rings on the intakes that can and do leak,especially when the intakes are removed for carb service. If they are the least bit squashed they Will suck air. And look at the clamps on the carbs at the airbox and where the carb attaches to the intake. If these are loose you can get the same type reaction. Air leaks will cause a lot of these kind of problems. Good Luck

I disagree with you on this particular idea.

Any air leaks upstream from the carburetor will have no effect upon the motor performance or power.

Simply recognize that: air is air, where ever it comes from.  That's all the motor requires, an air source. I can remember as a kid, taking the air cleaner off the car to go racing with my buddies.

You could most likely remove the air box and all the connected hoses that go to the carburetors and get a still, good running motor. I wonder if anyone has tried that?

Only air that finds it's way into the motor and affects the fuel/air ratio will have any affect on the performance/power of the motor.

***

Bigger problem is the risk of running unfiltered air through the engine and "dusting" it.  It would also seem that from those that have tried air box inlet mods without jetting mods have seen limited success - that said though  I don't know whether any difference in air box pressure produced by loose carb inlet side clamps would have an effect on the engine or not.  But then I was wrong about Flat6's air inlet sponge ................ and was surprised that it would produce a difference.

[fstsix]

I have a 45 Mikuni flat slide and been there done that, Here is what you should try before jumping off cliff. You stated runs good at cold start and gets worse' that is rich condition. The problem is this undo all jets back to stock except needle mod. Only do 1 change at a time IE slow jet main jet or you will never know where you are and and replace plugs. I have found that  the slow jet is a major change and 3/4 on the risk is into the main, back fire is loaded up. You should be happy with just needles with pipes and 2-1/2 out on pilot screw. BTW My Valk is Supercharged 5 years 31 MPG. Ride safe. Ps. Before Magnachager i had put on Viking Headers and his needle kit and this is a large tube exhaust 99.6 HP 100 TQ. on Dino.

[Black Sled]

Well, after doing everything else I finally pulled the mains jets on the sputtering side.  As it turns out I had 2 mains partially clogged.  Even though I had cleaned them with carb cleaner and blew them out with air, I hadn't actually inspected them before installation.  With a magnifying glass I could see some crap plugging the jets. 

I tried to clean them out with a sewing needle (those holes are small) and dumped a bottle of Techron in the tank.  Running (almost) like new now!  Getting better every time I ride.

Thank to all who offered help.