Fueling issue

akadaryl33 · July 25, 2013, 09:57:03 PM

So my "87" with 9400 miles has some sort of issue with fueling between approx. 5200 and 6000 rpms. If I have it wide open it breaks up in that range, but if I back the throttle off just a bit, it rips. I have recently rebuilt the carbs with RPM kits, mostly because of this issue, but since the rebuild it did not go away. It only ever does it between those rpms and has me very frustrated. It's also worth noting that since the rebuild my gas mileage has dropped dramatically. I installed the fuel/air screws to the same setting as the previous ones were ( 1 1/2 turns out if I remember correctly) but my mileage went from around 42-46mpg to around 35.... Anyone have any thoughts?

Arnie · July 25, 2013, 10:11:39 PM

My thought is that your mainjet is just a bit too large or fuel height a bit much.
I'd want to see results of a plug chop done at that full throttle, rpm speed though before I changed jets.
Fuel economy is probably due to you 'exploring' the engine response:-) Your 42-46mpg reading sounds like a steady state cruise figure to me.

Arnie

akadaryl33 · July 25, 2013, 10:14:38 PM

Forgive my ignorance but what's a "plug chop" ? When you say fuel height are you saying my floats are allowing too much fuel in the bowl?

andyb · July 26, 2013, 05:34:21 AM

Sounds to me like too big of a main as well, though it's possible that a needle or two got assembled in the wrong order.

akadaryl33 · July 26, 2013, 09:09:43 AM

Not that I'm doubting anyone about the main jet, but I used the RPM stock rebuild kits when I swapped everything out and prior to that it was stock. Even in stock form when I bought it (with 7100 miles) it was doing this. It had been sitting for a good many years and the carbs were crapped up really bad. I meticulously cleaned them and it ran pretty well the first year besides this oddity between 5200 and 6000. If not for this I wouldn't have even rebuilt the carbs. If the needles were installed incorrectly what would I be looking for? I didn't realize they went in a certain way other than dropped into to seat and the wire bracket slipped over the tongue of the float. I guess maybe I may have to pull them off again ( insert sound of head beating against wall). Thanks for the advice guys!

FJ111200 · July 26, 2013, 09:52:18 AM

Quote from: akadaryl33 on July 25, 2013, 10:14:38 PM
Forgive my ignorance but what's a "plug chop" ?

These are Plug's Chop's

But seriously, a plug chop is performed when you want to check what is happenning inside the cylinder at a given amount of rpm.
There are 3 or 4 places in the rev range that you check it at, and say it's full throttle, 3/4 throttle, half throttle and 1/4 throttle.
So there you go burning down the road at 3/4 throttle say, then all in one go, kill the engine, pull in the clutch, come to a halt and whip out the plugs. Look at the colour of the plugs to get some sort of idea of the fuelling at those revs.
Don't do the chop at idle 'cos all you'll get is what the carbs are doing at idle and it won't represent the true definition, so to speak.
Then try it at other rpms to get a good overal idea.
Hope you can understand this.

andyb · July 26, 2013, 10:14:17 AM

Quote from: akadaryl33 on July 26, 2013, 09:09:43 AM
Not that I'm doubting anyone about the main jet, but I used the RPM stock rebuild kits when I swapped everything out and prior to that it was stock. Even in stock form when I bought it (with 7100 miles) it was doing this. It had been sitting for a good many years and the carbs were crapped up really bad. I meticulously cleaned them and it ran pretty well the first year besides this oddity between 5200 and 6000. If not for this I wouldn't have even rebuilt the carbs. If the needles were installed incorrectly what would I be looking for? I didn't realize they went in a certain way other than dropped into to seat and the wire bracket slipped over the tongue of the float. I guess maybe I may have to pull them off again ( insert sound of head beating against wall). Thanks for the advice guys!

If you cleaned things when you were in there, you can change the sizes of the orfices if they're plugged up. You can also screw up and swap the air jets for the mains, which will make you comically rich.

The float needles don't have an adjustment, and you're right.... they're pretty hard to install wrong. I meant the slide needles. On the plus side, they're easy to get to, as you don't have to pull the carbs to get to them.

That said, if it's always done this when running, it's possible that a prior owner had screwed with something (or had a shop do poor work). What's been done to the breathing of the motor, anything? (Exhaust, pods, etc?) What were the jet sizes when you were in there? Were the slide needles the adjustable type with multiple grooves, or a single groove for the E clip? How many washers were under that E clip?

When doing a plug chop, you need to be at the throttle position as well as the RPM you're having troubles at. WOT chops done at 3krpm won't be accurate, as the main jet isn't the limiting factor until after the 6k mark.

red · July 26, 2013, 10:30:06 AM

Now, I'm no expert on carbs, but you opened this discussion to "anyone."

First off, does the stumble happen at those RPMs in every gear, or just the top gears? Do things change on a day when the wind is strong and steady? Is a run into the strong wind the same as a run going downwind? Do you have the stock air cleaner? Is the airbox altered, in any way? I once had a bike that ran well, but the right amount of wind would make it "stumble" in power at high RPMs. Airflow into my engine was being cut, by certain wind velocities. In your case, you may have the opposite problem (too much air pressure, caused by wind), but it's still a problem. If this is the case, you would not see this problem when running on a dyno, but only when riding. Now I know these answers won't fix anything, but they will give clues about what may or may not be the problem. Since the problem survived a carb cleaning and rebuild, I think we need to look further than the carbs for the right answer.

One possibility is in the spark plugs (the heat range, or even the brand that you have).

Another possibility could be the power available from the ignition system (a weak spark), which may be fine for some RPMs, but not giving you enough spark for all RPMs. Maybe somebody here knows what voltage the spark plugs should be getting, for your bike.

The problem there may also be "mechanical," if a wire can vibrate too far at certain RPMs and go intermittent (or short to ground). Any wiring that you can move by hand is a suspect, then. If so, then plastic wire ties (TyWraps) are your friend, and they are cheap. Nuts-And-Bolts stores (a separate heading, in the phone book) probably have them by the bundle. Home improvement stores and auto parts stores may have them, also.

racerrad8 · July 26, 2013, 12:14:34 PM

Give me a call and I can help you sort this out without doing a bunch of emails/posts.

Randy - RPM

FJmonkey · July 26, 2013, 07:15:23 PM

Quote from: racerrad8 on July 26, 2013, 12:14:34 PM
Give me a call and I can help you sort this out without doing a bunch of emails/posts.

Randy - RPM

A real time saver but we loose the chance to read how it was solved. Feeling left out now....

akadaryl33 · July 26, 2013, 08:20:44 PM

Wow, thanks for all the responses!!!! I am heading out of town until Monday so I won't be doing anything until then. To answer some of the questions, the air box, filter etc. are all stock. The problem happens in all gears but is more noticeable in the higher ones. It was my understanding that the carbs had not been touched before I bought it, but I cannot verify this. Randy I will definitely take you up on your offer when I get back, thanks!!!!
For those who are interested I will update how the problem was fixed when it gets sorted out, so no one feels left out. Lol

Josh

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Fueling issue