Dies at 5500 rpm

[ride314]

Ok, here it goes. I have been rebuilding a 1986 Vmax which sat for approximately 8 years. I know the previous owner and have good knowledge of what has been done to the bike. I have redone almost everything at this point and have been working on it for over a year.

When the bike was originally parked, it had super trap exhaust and no jetting changes. It ran good. I have put stock Vmax exhaust on and rebuilt the carbs with Keyster Carb Kits (maybe a mistake!!!!). Drilled the end caps on the stock exhaust, installed a UNI air filter, and upped the main jets two sizes from stock. Compression on all cylinders is good.

The bike starts great and runs great till I hit 5500 rpm. At 5500 rpm it feels like the bike hit a rev limiter. If I can get it to about 7000 rpm, it takes off like a shot (with the vboost on or off). Most of the time I cannot get the bike to rev through the bad spot.

My vboost control unit is not working and is disabled.

I am out of ideas. I am thinking the Keyster Kits are crap and might be causing my problem but not sure.

Summer is approaching and I am ready to ride this thing. Any ideas would be appreciated.

 

[gamorg02]

Ok, here it goes. I have been rebuilding a 1986 Vmax which sat for approximately 8 years. I know the previous owner and have good knowledge of what has been done to the bike. I have redone almost everything at this point and have been working on it for over a year.

When the bike was originally parked, it had super trap exhaust and no jetting changes. It ran good. I have put stock Vmax exhaust on and rebuilt the carbs with Keyster Carb Kits (maybe a mistake!!!!). Drilled the end caps on the stock exhaust, installed a UNI air filter, and upped the main jets two sizes from stock. Compression on all cylinders is good.

The bike starts great and runs great till I hit 5500 rpm. At 5500 rpm it feels like the bike hit a rev limiter. If I can get it to about 7000 rpm, it takes off like a shot (with the vboost on or off). Most of the time I cannot get the bike to rev through the bad spot.

My vboost control unit is not working and is disabled.

I am out of ideas. I am thinking the Keyster Kits are crap and might be causing my problem but not sure.

Summer is approaching and I am ready to ride this thing. Any ideas would be appreciated.

does it want to stall at 5500 or does it just not want to go higher. what if you immediately let of the throttle?

does it do it if you just rev the bike sitting there or when its being run on the road or both?

 

[ride314]

Thanks for the quick reply. The bike does not stall, it just won't go any higher. When I let off the throttle, it will accelerate slightly then just power down like normal.

At idle the bike runs awesome. It only does it under power. In fact, until it hits the 5500 rpm range it feels normal and powerful.

 

[KJShover]

Sounds like gummy carbs. Run some seafoam through your system and also look up shotgun and pea shooter carb cleaning techniques.

http://home.comcast.net/~onlyone12/carbs1.htm

Then when you get them all clean, throw in a sync and take her for a spin.

http://www.vmaxforum.net/showthread.php?t=3959

 

[maleko89]

Main jet is too large. Put 150's or stock 152.5's in.

 

[gamorg02]

Thanks for the quick reply. The bike does not stall, it just won't go any higher. When I let off the throttle, it will accelerate slightly then just power down like normal.

At idle the bike runs awesome. It only does it under power. In fact, until it hits the 5500 rpm range it feels normal and powerful.

no problem.. i was going to let mark chime in but figured i'd ask some of the simple questions to get her going..

 

[ride314]

Thanks. After reading some posts here, I learned I am a retard in thinking that opening the exhaust will lean the bike slightly. I was thinking backwards. I will put the stock jets back in and test it. I am at an elevation of 2,500. Do you think the 150's would be better.

Mark, I am sure this has been gone over before but what are the mechanics of my problem? Why does opening the exhaust richen it and why is it crapping out at 5500? Why does it run good at 7500? Just trying to learn all I can. Thanks in advance.

Do you think the Keyster Kits are good or bad?

Thanks again Mark and Garrett.

 

[maleko89]

Hey, the reason opening the exhaust richens is because the engine is allowed to breath more. General rule is opening up exhaust richens and opening up intake leans. At your elevation I would try 150's....147.5's would be better. That's what I ran with slip-ons at 1000 feet.

The reason why you hit a wall at 5500 is because that's where you start transition to main jet. Think of it like this. Someone is pouring a can of beer down your throat at a constant, manageable pace. Then, he suddenly pours faster...you're going to cough and choke. Okay, let the jokes fly. :biglaugh:

Keyster is okay if all you use is the jets. I installed a keyster kit in a customer bike and found the jet block gaskets are very rubbery and thicker than stock. Since it's thicker, the jet block, emulsion tube, etc have a longer path to the carb body and all sorts of carburation issues are the result. What I would do is keep the brass jets and such but order stock Yamaha gaskets and o-rings.

Hope this helps.....

 

[ride314]

Got it!! I will do some wrenching this weekend and see if I can make it run better. It has been in the low 90's here for three days. I am dying to get on that bike.

 

[gamorg02]

Hey, the reason opening the exhaust richens is because the engine is allowed to breath more. General rule is opening up exhaust richens and opening up intake leans. At your elevation I would try 150's....147.5's would be better. That's what I ran with slip-ons at 1000 feet.

The reason why you hit a wall at 5500 is because that's where you start transition to main jet. Think of it like this. Someone is pouring a can of beer down your throat at a constant, manageable pace. Then, he suddenly pours faster...you're going to cough and choke. Okay, let the jokes fly. :biglaugh:

Keyster is okay if all you use is the jets. I installed a keyster kit in a customer bike and found the jet block gaskets are very rubbery and thicker than stock. Since it's thicker, the jet block, emulsion tube, etc have a longer path to the carb body and all sorts of carburation issues are the result. What I would do is keep the brass jets and such but order stock Yamaha gaskets and o-rings.

Hope this helps.....

mark.. great little write up.. thanks as always.

 

[ride314]

Is there any danger of being to lean with the 147.5 main jets?

The areas I generally ride have many elevation changes from 6,000 to sea level.

Thanks in advance.

 

[Rhoy Carter]

Also check the timing advance

 

[ride314]

I think my last question got lost in the mix. Is there any danger of being to lean with the 147.5 main jets.

I ride between the elevations of 6000 and sea level. Thanks in advance. This forum is great.

unk:

 

[maleko89]

You'll be fine, the Vmax likes it lean. I ran 147.5's from 600 - 3000 feet w/o any issues.

 

[MONTREALMAX]

Hey, the reason opening the exhaust richens is because the engine is allowed to breath more. General rule is opening up exhaust richens and opening up intake leans. At your elevation I would try 150's....147.5's would be better. That's what I ran with slip-ons at 1000 feet.

The reason why you hit a wall at 5500 is because that's where you start transition to main jet. Think of it like this. Someone is pouring a can of beer down your throat at a constant, manageable pace. Then, he suddenly pours faster...you're going to cough and choke. Okay, let the jokes fly. :biglaugh:

Keyster is okay if all you use is the jets. I installed a keyster kit in a customer bike and found the jet block gaskets are very rubbery and thicker than stock. Since it's thicker, the jet block, emulsion tube, etc have a longer path to the carb body and all sorts of carburation issues are the result. What I would do is keep the brass jets and such but order stock Yamaha gaskets and o-rings.

Hope this helps.....

love this site...........awesome stuff

 

[4gasem]

Let me add that Maleko is talking Mikuni numbers NOT Dynojet numbers when he's throwing out 147.5 and 150.

Chris

 

[ride314]

Mains changed to 147.5 today. Still stumbles badly at 5500 rpm. Low end power is awesome and after about 8 seconds of stumbling, I am able to nurse it into the higher rpms then...hold on!!! 115 mph when I shut it down (got scared).

V-boost is still disconnected and in the off position. I am begining to think the "Keyster" brass is causing me the problems. Also have not changed the jet block gasket which is also "Keyster".

If it is the brass...which ones are giving me the problems???:bang head:

Help!!!!

 

[ride314]

OK....after changing one part at a time here is what I found. They Keyster Carb kits are crap. The jet block gasket is so rubbery and thick that the jet block cannot ceat properly. Changed it and the bike still ran bad.

Finally changed the needle jet (the one the needle slides in) and hello, fixed the problem.unk:

I would not recommend that anyone buy the Keyster Carb kits!!! I learned a valuable lesson for sure.

I am getting a very strong odor of unburned fuel now (mostly at idle), running the stock 152.5 mains. I think I will lower the main jet and see how that works. Any suggestions??

 

[one2dmax]

The keyster kit is junk and well known by many in the forum here. I have pics showing the problem with the emulsion tube you noted. If you look at it compared to the stock one the bleed hole is in the wrong place.

Sean

 

[vmaximum]

love this site...........awesome stuff

same here, appreciate all the senior guys that take the time to help the guys learning the ropes. i have been learning a lot from many of you on here thank you. -Rob-