1978 XS750 high speed problems

[bookem]

I really need some help with this one. I have cleaned the carbs (more than once), checked the timing advance, and checked the valve clearance. The center intake valve is maybe .050 tight. The problem I'm having is with top speed and high rpm. The bike doesn't seem to want to go over 130 kph/ 5700 rpm when being ridden. When reving in neutral the bike will go past redline for a sec then drop to 5000 rpm then holds around 7000 rpm sputtering a bit. All cylinders seem to be firing as all pipes get hot. I've pretty muched ruled out the carbs, valves, and timing advance. Any new ideas would be great. Thanks

[DGXSER]

I know you say you have ruled out the carbs, but have you ruled out the jetting? This sounds very much like to small of a main jet for your set up. If you opene dup the air box, or replaced the exhaust or something you might have leaned out the top end. JAT

[bookem]

When I purchased the bike it came with the airbox removed and 3 pod filters on. After tuning it up and taking it for a drive I wondered the same thing as I have run into this before as well. I tried the old trick of applying some choke which usually works for too lean a mix. Unfortunately it didn't help. My aircraft mechanic brother has suggested too rich may be the problem as the bike seems to have great power all through the throttle but just tops out like it has a rev limiter on it. Tomorrow will bring another Carb tear down.

[DGXSER]

Try running it up at the higher rpms and then shutting the bike down using the kill switch before letting off the throttle. Then coast to the side of the road and pull the plugs to see what they look like. If they are black and sooty it is to rich, if white or grey ash looking, to lean.

Not sure what the stock jetting was for the XS750. But it is typical to need to jump up a size or so when switching over to pod filters, especially if you have a non-stock/hi-performance exhaust.

[bookem]

Ok, I've tried the high speed shut off and the plugs looked more or less alright. Since then I have taken the carbs apart and found the needle clips were 1 notch below center. Center being the factory default. I moved the clips down 1 more notch, bottom and found very little ifany differance. I then moved the clips 1 knotch above center, again not really any differance except when reving in neutral it seemed to go higher. At this point I'm thinking maybe the ignition box. I am going to set timing and take a compression test. The only conclusion I can come to is that th hp just isnt there.

[DGXSER]

I'd at least try jumping the main jets up 1 size. Jets are about $4 a piece at the local yammarobbers, so for $12 you may fix it.

[bookem]

gonna give the next size bigger jet a try. I found that leaning the mixture out by moving the clip 1 notch higher allowed me to rev in neutral smoothly from idle into the red. the richer the mix didn't seem to make a differance. so far I've had the carbs apart 4 times today. They should be clean I think, lol. I'm not blowing oil out the overflow so I don't think I have piston, ring, or cylinder problem. I hope it is the jets, right now I have power all the way through the throttle twist. Thanks for the help.

[ManagerMike]

I'll chime in with the 'sounds like fuel starvation to me too" bit.

[bookem]

well I gotta say I never would have believed that the jets would make that much differance. I've chopped yamahas and hondas for a few years now and have seen pod filters make a 10 mph diff but never 30mph. Couldn't get new jets till Mon. So I drilled the jets, I know a cardinal sin. The differance was amazing! Topped out at 8000 rpm, a 2000 rpm differance. Thanks for the help guys, I would never have believed it if I hadn't seen it.

[DGXSER]

Glad it worked out, and you should see it better with the true jets in it. And yes, a future owner will really be ahppy if you put the right jets in it, so when they buy new ones marked the same as those and it runs like crap they will nto be chasing their tail for weeks!!!

[IanDMacDonald]

1978 Yamaha XS750E

Hi guys, similar problem with my bike as stated in this thread. Aftermarket K&N pod air filters installed on bike, The bike seems to run decent on the road (chugs a little), but on the highway, it goes no faster than 70mph on the money @ 3500 rpm. I suspected too small of jets, making the bike run too lean on top. My question is, how do I know what Jet to get (Main?) to fix this issue, and the proper size. My local dealer will not touch the bike, so I am forced to go to a small repair shop that wants close to $300 for the carb. work. I would love to do the work myself and save some money. Also, the carb joints going from the carb to the intake are dry rotted. How much of a performance issue is this with regard to my problem? Thanks for the help.

Mac

[DGXSER]

For the jets, you need to open up the float bowls and see what jets are in there. The size should be written on the face of the jet as you look at it installed. Then, go the next size up to see if it cures your upper end issues.

As to the carb boots, use some unlit propane and point it around the boots while the bike is running. If it sucks it in, the rpm will jump and you know you have a vacuum leak (leak in the boots or their flange points). If not, then you can use some plasti-dip or the like to reseal the exterior of the boots.

[IanDMacDonald]

DGXSER's,
Thanks for the reply. I have every intention of draining the tank tomorrow and pulling the bowls off to check the jet sizes. My question is, do you know of a reputable person/company that I may purchase larger jets for this bike? How does it work? Can I go to a Dealership and ask for a specific size jet, or does it have to be specific to my bike/carb, etc.?
You guys are saving me a boat load of money. Thanks for all your help.

Mac

[DGXSER]

Your local yamarobbers or even and independant bike shop can get the jets. They are "Mikuni Large Round jets" for the mains. Just tell them that and the size you want. They should be able to order them. Cost about $4-6 per jet.

There is a guy on ebay named Georgefix, has been very helpful and informative on carb parts for me over the years. You could try him if your locals seem lost. But I would be VERY surprised if they can't get you what you need.

If your really lucky and know a friendly service department, you might ask the guys in there and bring them a cold refreshment or six and they may find some in their stores of old parts.

[IanDMacDonald]

DGXSER-
Ironically, I just bought a set of Carb joints from Georgefix on ebay last night for $42. They started out at $32, but the other bidder wanted them as bad as I. My joints look bad, so I figured my as well buy them and I will not have to worry about it.
Thanks for your help. I will definitely keep you updated with regard to me getting this bike performing decent.