Red line

[sandmanred]

What's normal for a 78 XS400E as you approach red line rpm? Mine is pretty solid and powerful from about 5000 to 8000 rpm once it's all warmed up. It begins to falter a bit as you press over 8000 rpm, power falling off and sounding like it's miss firing a bit. Should that power continue to red line or is it normal to falter a bit as you approach red line?

 

[XSLeo]

Most of my bikes reach redline without faltering. If they do I figure out why. Sometimes it's an ignition issue, others it's fuel delivery issue. It may even be a mechanical issue.
Check and adjust the mechanical aspects of you engine. Cam chain tension, valve clearance.
Check and adjust any ignition issues, Points gap and timing. Check full advance timing as well as at idle.
If those don't cure it then it's into the carbs.
www.amckayltd.com/carbguide.pdf has a "Tuning for Mods" section. The guide was written with the XS650 carbs in mind but the XS400 BS34 Carbs are vary similar to the XS650 carbs.
Leo

 

[xschris]

My stock 79 2-f which is the same as a 78 will pull red line easy without issue even 9.5. Your muffler mods would have a great impact on performance. You will have to try fuel adjustments to try and fix some of it. But you may find it still wont be perfect and just have to live with it. Mufflers like the stock ones are designed for this motor and carb setup. Once that has been changed you would have to make others to compensate.

 

[sandmanred]

Thanks for the input, it's good to know smooth power to red line is achievable.

Performance actually improved a bit with the new exhaust compared to the old. The old exhaust had corroded so far as to not really be stock anymore. In fact, the left side rear end blew off completely just before I took them out of service. That made the faltering even worse and added a higher level of backfiring on decel.

Carbs are synced, air screws are optimized, valves are set, points are cleaned and gapped to spec. But I have only set the timing by using a conductivity lamp across the points. Today's task is to set it by inductive timing light.

 

[sandmanred]

Timing is now set by inductive timing light. It was a maybe 4-5 degrees off.

It will now run to red line at roughly 1/3 throttle with smooth power. It starts to falter around 8000 rpm at higher throttle opening. It might even falter sooner when it's wide open throttle.

From what I can understand it sounds like xschris is right, I need to look at jetting? From what I've said here can you make any suggestions for how much to change? I've no experience with re-jetting so it seems like I need to up in jet size but how much?

 

[sandmanred]

Reading a bit on jetting sounds like it's mostly the main that's my issue. Call my local shop to find out if the had the sizes above stock. Ran over and there to 145s and 147.5s and found only the stock size. Bought two just because I was there. The went home and ordered two sets of proper 145s and 147.5s from jetsrus. Noticed the estimated 2 wk delivery time!!! and decided to drill out the spares I just bought in the mean time. Drilled to 0.058, just a tiny bit bigger than stock and put them in this morning.

I still have to do a proper run out in the country but made a short run with a few seconds of WOT after warm up. It's a rocket all the way to red line at WOT now. Power ramps up all the way to red line.

 

[xschris]

Great! Do so plug chops to see if your other stages are good.

 

[sandmanred]

Ok, spoke too soon. Got out for a better test this afternoon. It's much better than stock jetting and I think the main jet is the main item but I'm still not there. Gears 1-2-3 are pretty good through from closed to WOT but 4-5-6 bog a bit from 3/4 throttle to WOT. I may wait now for the real jets to arrive to do more as I really don't know how much I changed the mains from stock.

 

[NewHavenMike]

Just curious if you checked to see if the timing fully advances equally on both sides? Rare chance, but something I have needed to adjust for. Probably from worn contacts or unequal gap??

Does it run worse as engine temps go up?

On my 400cc block, Im using carbs from the original 360cc block, stock air filters and tapered exhaust. Jets are 130 mains with a stock pilot and the needle dropped one notch lean...Rips every gear.

 

[sandmanred]

Just curious if you checked to see if the timing fully advances equally on both sides? Rare chance, but something I have needed to adjust for. Probably from worn contacts or unequal gap??

Does it run worse as engine temps go up?

On my 400cc block, Im using carbs from the original 360cc block, stock air filters and tapered exhaust. Jets are 130 mains with a stock pilot and the needle dropped one notch lean...Rips every gear.

Yes, checked both sides, at low and high rpm. Low rpm is spot on both sides. High rpm is between the marks on both sides. It runs a little better as engine warms up.

 

[sandmanred]

I think I got the jetting right.

Ended up with 147.5 mains, 1 notch down (leaner) on the needle than stock and everything else stock in the carbs. Went the wrong way on the needle and got to the richest notch before I figured out I was going the wrong way. Screams to red line in gears 1-4. Smooth power at all throttle positions in all gears. The only casualty so far is a diaphragm that got punctured wrestling that snap ring out. Pic below shows the baffle. The only escape for exhaust is the gap between the baffle and the muffler body, the slots are welded shut.

Any body find a snap ring pliers that fits down in the bottom of the carb diaphragm assembly I'd like to know. Mine had to be filed down a bit and it still doesn't fully compress the clip.

Pic of baffle assembly

 

[buztabuzt]

Not sure about that exhaust. Would a needle nose work or curved needle nose?

Sorry for temporary thread Jack/ignorance, but I had no idea that there were needle notches on the XS (knew my Honda did). Is that the case on the stock carbs? What notch is stock positioning and suggested? I didn't recall reading about them in the many, many carb threads I've seen. Photo cropped from when I last had mine apart.

 

[sandmanred]

Yours must be a different model carb. This is not my needle but it looks like this.

 

[xschris]

77-79 carbs use a different type slide and needle than 80-82 bikes.

 

[XSLeo]

The notches in the needles are numbered top to bottom. Lets say your bike should be in #3. Count from top down.
Putting clip in #2 leans the needle. #4 richens.
Leo