4-5K RPM Stumble -- Stumped

[Nateshoe22]

I'll start w/ the background. Bought an '85 VMax as a garage-find, hadn't been on the road for 5+ years. Along with a variety of other items the carbs were stripped down to individual bodies -- ultrasonic cleaned and rebuilt w/ Keyster kits. The CV diaphragms and intake boots both looked rough so both got replaced w/ebay items. Carbs synced multiple times. Float level checked both ways (w/ carb off the bike and the fuel level check w/ carb on bike) Bike starts, idles and runs well EXCEPT in 2nd gear under moderate to heavy throttle (3/4-full) around 4-5k rpm the bike hesitates and stumbles but then cleans out and pulls hard in the remainder of the rev range and gears. With less throttle the stumble is not there. I switched back to the original pilot needles, tried moving needle position with washer (neither keyster or original needle has slots). Tried adjusting idle mix screw -- no change. Jets are stock size, rest of the bike is stock. One thing I thought is maybe the air bypass in the aftermarket CV diaphragm body (ebay items were diaphragm and body together) is not the correct size -- I threw the original ones out w/o checking. The bike runs well and pulls hard everywhere else, I'm stumped, the carbs have probably been off 10+ times. Other relevant parts replaced: fuel filter, air filter, spark plugs.

Thank you in advance for the help!

-Nate

 

[CaptainKyle]

If you used any hard parts out of the keyster kits put all if the stock hard parts back in. Keyster kits are junk !

 

[Fire-medic]

What Kyle said about the Keyster kits. I'll use OEM or K&L. Not Keyster.

 

[Mr. Max]

2nd gear slip? Its somewhat common problem. Search forum, it has been discussed before.

 

[Nateshoe22]

Good to know about the carb kits, they were included with the bike. I did use the jets, float needles and seats.

After more reading I think it may be the dreaded 2nd gear slip. My initial thoughts were that it sure felt like misfire, but the fact that it only occurs in 2nd gear under heavy load makes it seem more likely to be the gear issue. The issue has also been almost identical regardless of any carb adjustments I’ve made.

Don’t have the space for the rebuild right now, I think I might just let it go. It’s an ‘85 w/ 18k miles, new tires, Corbin seat. What’s it worth as is? It’s still very rideable

 

[CaptainKyle]

Unfortunately 2nd gear is not a cheap fix & needing carb work to I think you would be doing good if you get $ 1500 out of it but that is just my opinion. Where are you located ?

 

[Fire-medic]

Captain Kyle is correct, $1,500 for a blown 2nd gear engine would be a good deal for you, for an otherwise stock bike. I personally wouldn't pay that-much for an otherwise decent-shape bike. The only way I can see it being worth more-than that, is if it was loaded with desirable accessories, or it was a big-bore engine build, and nothing-else the matter with the engine.

You know, you can ride that for years, just shift from 1st to 3rd.

 

[Nateshoe22]

Yeah the bike is still very rideable, 1/2-3/4 throttle in second gear is no problem. With this new hypothesis in mind I need to go take it for a long ride.

I understand $1500 would be great. It wasn’t I bike I was planning on keeping, it was a clean-up, fix up, ride for a season and sell. Just don’t have the space this winter to tear it apart.

I’m in West Michigan

 

[Fire-medic]

Yeah the bike is still very rideable, 1/2-3/4 throttle in second gear is no problem. With this new hypothesis in mind I need to go take it for a long ride.

I understand $1500 would be great. It wasn’t I bike I was planning on keeping, it was a clean-up, fix up, ride for a season and sell. Just don’t have the space this winter to tear it apart.

I’m in West Michigan

I was just in the area, depending upon where you are I could have looked at the bike for you, and offered suggestions. If you can give the bike > 1/2-throttle in second gear, your dogs and meshing holes in the gear body aren't as-bad as many others I've seen. Still, the only way to fix it is to replace the gearsets having worn components, it's only going to get worse.

Here are shots of where I was, let's see if you've been to the same places. Bumblebee was hanging-out at one bike dealership, kind of a more-elaborate sculpture Kyle does, he's about eight feet tall; and AJ Foyt's Indy car was from this era of Foyt's Coyotes, power by Ford. This is a Ford-Cosworth DFV-V8. It's a 1974 build, and it was good on a closed circuit for > 217 mph, not-bad for 255 cubic inches. The last is a 1916 Packard Twin-Six racer, capable of 104 mph, raced in Argentina for ten years, and stored for another seventy years, until it was restored and put on display in western MI. Behind it is one of the familiar cars in many 1950's and 1960's movies, a Checker Marathon taxicab. Who knows where they were manufactured?

 

[Nateshoe22]

I like the quiz. Checkers were made in Kalamazoo, don’t know about the transformer but I’m guessing the other cars were at the Gilmore museum ( never been but heard great things)

 

[Nateshoe22]

I’m farther north, spitting my time between Grand Rapids and Ludington

 

[Zeus36]

My second gear is going. It will hold in steady cruise, but pop out under load. I just skip from 1st to 3rd when taking off (so I really wind out 1st...). It will hold downshifting if the revs are not too high.

 

[Fire-medic]

Yep, right on both.

Checkers were made for decades in Kalamazoo. I used to go by the test track, entering the city. 'Bumblebee' was in a powersports dealer on Sprinkle Rd. in Kalamazoo, Zeigler. The Big 4 Japanese brands, Indian, KTM, Can-Am, Polaris, and winter sports equipment.

The Gilmore Car Museum, I'd estimate the grounds, with all its buildings, and the inventory, is probably $40 million, at a WAG.
Home | Gilmore Car Museum

Anyone who finds they are in southern MI, or close-to it, this is a great place to spend a day. In Kalamazoo is Air Zoo, where they have an SR-71! A lot of other planes, too. Air Zoo Aerospace & Science Museum | Kalamazoo, MI

Just-over the MI-IN state line is the Auburn-Cord-Duesenberg Museum, in Auburn IN. It's about 100 miles from Kalamazoo (I-94 east to I-69 south).
Historic Automobile Museum | Auburn Cord Duesenberg Automobile Museum

 

[Nateshoe22]

Well got a chance to take the bike for a longer ride tonight. After reading more on the 2nd gear slip it looks like I am at the beginning stages of its failure. It is an interesting sensation ; like a violent clutch slippage without a noticeable spike in RPMs.

$1150 is my firm number if anyone is interested. Otherwise it gets stored until I buy a house and have a place to work on it.