New Member - Top End Power Issues

[smXS400]

Hello,

I am a new member to the forum, not a new Yamaha owner but new to the XS400 market. Current bike is a 1982 Yamaha XS400R SECA. I bought the bike for $600.00, was having issues with it not starting properly, discovered previous owner had forgot to gap the plugs 0.o.

Other yamaha's I've owned:

1986 Yamaha FZR 400 Genesis
1986 Yamaha TT#350

2002 Suzuki DR650SE

It is running but I am having issues with top end power.

So heres whats going on.

the bike starts and idles fine, seems to have good low end power and midrange punch till about 6000 RPM. In first second and third gear it is not as noticeable, but in fourth fifth and sixth gear the tach barely reaches past 6000 rpm. At first I though maybe because I am a bigger guy this was the issue but same thing even on the downhill. when in neutral the bike revs out to 10000 no problem, just seems to be under load. It surges and sputters a bit.

I have cleaned the carbs, ran a de-carbon fluid through it, one of my exhaust valves is .03 mm out of spec. I have yet to do a compression test on it, still waiting for the rain to stop.

Are there any suggestions on things to check for this problem>

SM

 

[16VGTIDave]

Any modifications to the bike? Exhaust? Air filters? Regardless, it sounds to me like the the bike is running lean on the top. This could be caused by emulsion tubes that are plugged, or that the main jets are too small. I'd start with a complete carb teardown and cleaning.

 

[smXS400]

The only modification I can see is the 2 into 1 exhaust with a slip on megaphone muffler. It does have me wondering though if they didn't think to gap the plugs, would they have re-jetted after installation. Also could the out of spec exhaust valve cause some of the problem? or could it be exhaust leaks?

Once I am able to I will pull the carbs and do a good once over on them again.

 

[16VGTIDave]

If the muffler doesn't have baffles, then that may be the culprit. Our engines are setup to have some exhaust back-pressure, and require a fair bit of tuning to work without it. Let us know what jet sizes you find when you tear down the carbs.

0.03mm out of tolerance for the valve lash is insignificant and not likely an issue. If you still have the engine apart, I'd get the correct shim just for the sake of not having to do it in a few months. Exhaust leaks cause more issues at low RPM, but they also should be fixed.

 

[smXS400]

JUst pulled my carb out. Turns out that one of the boots that is on the engine side of the carb is cracked and the lobe that the bolt secures the boot to the engine is broken... I assume a vacumm leak on the downside of the carb could cause my top end power issues?

SM

 

[16VGTIDave]

Nope. A vacuum leak would make the bike hard to start and idle poorly. You found issues, just not the ones you are needing to find.

 

[smXS400]

Just letting you know that this is the first time I have pulled the carbs on this bike. I ran a cleaner solution the first time as the people I bought it off said they just did a full clean...never believe people apparently. So I have a small pinhole in one fone of the CV diaphrams. I am just gonna keep posting more and more issues as I find them...

SM

 

[smXS400]

So 127.5 main 117.5 main and both pilots are 45.

The two main jets are stock and the pilot jets should be 42.5 stock so they have changed the pilot jets.

Since the mains are stock is it possible that I need to go up a size in the mains to give myself better performance on the top end?
Could this be causing my lean top end?

SM

 

[smXS400]

I have decided after much thought to run a 130 and 120 main jet respectively get them next week and I will report back.

SM