Bogging and spluttering when throttle opened more than 50%

[Llewelyn05]

Hey guys. Just joined the forum hoping to get some advise from the knowledgeable fellas here.

So about a month ago i bought this 93 Vulcan en500. The previous owner told me it was spluttering after he had it standing for a year or two, and he gave me a fairly good deal on it. With me thinking, with a background of many many motorcycles owned, repaired and worked on (and a few cars inbetween too), that ill just need to clean out the carbs a bit, clear some gunk out and itll be all good. And i was right... temporarily.

The first night i got the bike, she wouldnt go over 35mph. Spluttering and "bogging down" if the throttle was opened past 50%. Eager and excited of a cruiser (this is the first time ive owned this type of bike) i ripped of the carbs, gave the a quick clean, one pilot jet was blocked with some gunk and slapped them right back on. Took it for a test drive, and my oh my was i suprised at the difference. Very quickly reached 100mph, and very very chuffed with the bike, i parked her for the night and started planning many many joyrides and weekends ahead.....

I ordered some new plugs, oil filter and gave it a nice service, you know, just so it would be good to go for these planned joyrides.

Started it after the service, and the spluttering and bogging down was right back. Non plussed, i figured some more gunk mustve come through the fuel lines and caused some more crap in the carburetors, so i pulled them of and gave it a thorough cleaning this time round. To no avail however, as this problem persists.

My planned joyrides have been put on hold, as i can not for the life of me find whats wrong. Below ill describe the problem as best i can, and list everything ive done to try and resolve the issue.


The bike start almost on the first swing when cold, and choke pulled out. 20 seconds of high idling and i push in the choke and it purrs like a kitten, idling away perfectly. Take her down the street without opening the throttle more than 50% and its a joyride. As soon as i open up though, there spluttering and backfiring, and it wont go over 50mph in 5th gear.

I started by adjusting the pilot/idle mixture screw (measured original position at 3 turns out) no change.

Thoroughly cleaned carbs, submerged in seafoam, blown through all ports with air etc. No change.

Order carb kit of ebay, sceptical when it arrived as it had parts for 4 carbs and no jet sizes stamped on, but installed anyway, no change.

Ordered carb kit from Amazon, suppplied by all balls racing, and this came with 2 x 115 main jets, when the manual calls for a 112 left carb and 115 right. It came with new pilot/idle mix screws, new pilot jet, new main jet, new needles, and new float valves/needles. It also had the new o-ring and washer for pilot/idle screw which i installed for good measure. Ive ran it with both new 115 jets, and with the old 112 in left carb and new 115 in right, no change. Id like to mention that it came with 4 more o-rings, 2 differnt sizes, that i honestly dont know where to install...if someone knows? Maybe this could be the issue?

Ive checked the valve clearance on inlet an outlet ports, they were still perfectly set.

Ive opened the side cover where stator coils and magnet pickups are, and they look perfect. (If theres a way to conclusively test these, id gladly do it, as im currently waiting for a new gasket before i can put the cover back on.

Ive balanced the carbs with a home made device, some glass bottles and tubing.

Ive checked for vacuum leaks, sprayed some starter fluid around the intakes, it doesnt rev up, so im pretty sure it doesnt have a vacuum leak.

Ive ran it without an air filter, just to make sure that wasnt constricting it.
All these things has not mad the slightest change.

If ive missed anything, please ask me, i would appreciate any help. Im contemplating on taking it to a qualified mechanic, but im not that kind of person lol, i know i can fix this, and im pretty sure its something im missing here. Comon sense and some, not a lot, of carburetor experience tells me that because its an issue past 50% throttle, that the slide/needle/ main jet has something to do with this. Whats mind bogling is why did it run so splendid after the first carb clean, and if ive taken everything back to stock, why isnt there even the slighttest improvement?

Oh, and i need to mention the bike has straight exhaust pipes, and stock air filter.

[bikerbill]

Welcome Llewelyn05.

With the sputtering coming on at 1/2 throttle or higher you need to look into the main jet circuit instead of the pilot jets. Make sure that all parts of the airbox are intact and properly sealing. The airbox lets in a limited amount of air with the carbs calibrated to that airflow. Any extra air will affect this calibration. I have the C model and there is a 'snorkel' present that, when removed, makes the bike bog severly at over 1/2 throttle.

Is the exhaust system clear?

Put the new spark plugs in. I use the next 'hotter' heat range plugs, DR8EA to minimize fouling.

Hope this helps.

[Triangles]

Just so I understand. You have the stock air intake on the bike, right? I had a '94 where the PO had replaced the stock intake with pod filters. I think I got the bike because he couldn't get it to run right. I got it acceptable but not as good as I would have liked after a year of tuning. That was nearly a decade ago, I'll try to dust of my grey matter and remember what I did. I'm asking about the intake because the carbs are tuned from the factory with the intake air box restriction. This restriction pulls more gas thru the carbs. If you eliminate this restriction not as much gas gets sucked in and you go way lean and stumble. Also these bikes tend to stumble all over themselves when parked for extended periods of time. If this latter cause is your problem, the easiest solution is to run several tanks a gas thru the carbs. Gasoline is a pretty powerful solvent and will remove most gunk as good if not better than carb cleaners. Okay I'm done rambling for now... LOL

[Triangles]

Another random thought is that you might have rust or trash in the gas tank. There is a screen over the petcock inlet that could be plugged up starving the engine of gasoline.....