Won't rev over 5000 rpm and how to test solid state reg


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Ontario, Canada
Won't rev over 5000 rpm - Solved

I've been trying to figure out why the bike won't rev over 5000 rpm. I can't get onto jetting the carbs properly because of it. Won't rev over that in neutral or on the road, no matter if I pin it or try to feather through it, just won't go. It doesn't bounce off it like a rev limiter, just kind of loses power and bogs, like it's missing on a cylinder. Tried adding a frame ground to the battery, no help. Read that someone had reversed the plug wires and that was his problem but didn't solve it for me.

Tested coils
New plugs
New wires
New gas
carbs 100% clean - diaphragms tested and good
Can't seem to find any exhaust leaks at the head
New carb holders and gaskets, both have nipple capped off
Float heights set to 2mm below the mating surface using clear tube
Valves in spec, going to double check now that it has ran for a bit.
180 compression in both cylinders

Up next is to sync carbs on the bike, double check pickup coil position and try another TCI I have.

Any other things you can think of I can try?

The bike will idle all day, revs great in neutral up to 5k and you can drive it with some feathering of the throttle and popping out right exhaust but I think that's the carb settings I've got right now.

Second, is there a way to test the solid state regulators on these?

I only ask because the bike may be overcharging or charging strangely. I've read through plenty of charging threads on here but they're all about non-charging. One thread was asking how to test solid state reg's but said replacement was only option so that may just be it. I'm going to order an XJ650 reg/rec combo for the future anyway so I have it when it's needed.

The battery had limited use and worked fine up til yesterday where I believe it developed a dead cell. Shows 11-ish volts with key off, 8 with key on and 3 when trying to start. Just find it strange how it had no problems up til then but I guess that's just how they go. Wanted to make sure the charging system didn't kill it before I fry another new battery.

I've tested my rectifier which tested good. Tested the alternator and they're all at 0.7-0.8 ohm after subtracting my lead resistance (most accurate I can get with my meter) so it's in spec and 4 ohms across field coil. When running I could smell what I believed was acid and I get 14.5 volts at the battery at idle. Now this seems high, however the only load aside from the motor at idle is an LED taillight and small LED speedometer so it is quite bit less load than stock bikes. Headlight was not on. When I was revving the engine to 4k - 5k the voltage dropped to around 13.5 but this was only a few seconds worth of testing. Didn't get to test much more than that before the battery crapped out completely.
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What main jets are you running?

Any exhaust or intake modifications?


14.5 volts at idle is a bit high; you may have just boiled your battery off to where there aren't enough electrolytes in solution anymore. At a certain point you can't just add distilled water anymore; you'll need more acid if it is a lead acid type.
Battery is a sealed AGM. I've ordered another one. Only $25. I'm going to try another regulator and test voltages between the two.

145 mains. 45 pilots.


UNI filter pods directly on carbs. Exhaust is 12" shorty's welded right to the headpipes just after footpegs.

Don't have both pipes on in this pic, but there are 2, not a 2 into 1.

Main and pilot should be fine for that setup. I ran those same mufflers in the past with a 2-2 setup like that.

I don't think the battery was the primary cause even though it died for whatever reason. A weak or low battery usually won't keep the engine from revving high since the alternator is making decent power at high rpms and can sustain spark (though you will feel a weaker top end for sure). A weak battery will definitely cause problems with idle; the bike will die. Since yours didn't have that issue I'd look elsewhere.

Electronic ignition rules a lot of things out, but not the ignition box.

Coils can be deceptive. I have had some that tested within the normal range but didn't work too well. Definitely check your other ignition box before changing them out of course. Too much current running through the coils will fry them though; I killed my last set when my regulator grounded through the frame and passed full current through them for a while.

Do the electronic bikes have a separate condenser like the points models?

Do you have another air filter you can try? Those UNIs are good but might be causing an issue if the carbs have not been synced.

120 psi compression isn't a huge deal; it should still work fine.

Intake or exhaust leaks won't have a huge effect at higher rpms. Unless there is a gaping hole at the exhaust ports on the head this wouldn't disrupt performance as much as you're describing; you'd see symptoms at idle mostly.

Which reg/rect are you using? Are they stock parts?

Also, are you using resistor spark plugs with resistor caps? You should use one or the other, but not both.
Thanks for taking the time, really appreciate it.

While syncing the carbs just now it blew the headlight fuse I believe (Headlight is out, haven't checked why yet, but assume the fuse). It's only a 10 A, going to put a 15A in anyways but I'm also going to throw another reg on to double check. When I turned the headlight on at idle when it was working it dropped down to 13.5 volts but slowly crept back up. I still think the reg is not working correctly.

I just synced the carbs at idle, still won't rev up there.

I'm just about to try another TCI box.

I also have some other coils I can try as I was also thinking it's possible one of them is not sparking well at higher RPM. One thing to note is that the bike came with the older white coils. Everything I had researched pointed me towards those being for the points bikes, which this is not. The bike was not a runner when I got it so I can't say whether or not those coils were just thrown on there at some point.

I believe the only parts in the electronic models is the TCI box and coils.

I can remove the filters and see if that makes a difference but I have synced the carbs just now. I also have stock filters I can try.

Using stock rectifier and regulator. Rectifier works correctly as does the alternator. Only regulator I can't test. Going to order a combo reg/rec for the future.

I am using resistor plugs (NGK BPR7ES I believe) with new non-resistor caps.

Off to try another TCI.
What do the plugs look like? Have you tested the pickup coil for the ignition? Has it been moved at any point. If it is not set to the correct position the bike will be out of time. Test specs are in the back of the manual.
It seems like we're closing in on an answer; there are only so many things it can be and you're about to knock out the last few question marks. Keep it up and it will get solved in no time!
The left plug was a nice brown color, right plug was black when I checked. I adjusted the mixture screw to 4 turns out from 2.5 but didn't recheck because that was when I realized it wouldn't rev over 5k.

The pickup coil I put back in the same position when I rebuilt the engine. I'm about to go check it again as well as the valves.

Another TCI did not fix it. Another regulator provided the same readings. The headlight fuse keeps popping as well.

The right pipe is throwing alot of smoke when trying to rev past 5k and a little at idle. Looks whitish/grey, smells like fuel so this all may just be a carb issue somehow.

I've got a stock set of carbs I'm going to throw on it and see what that yields as well.
If it were oil burning you'd be able to smell it as well; it is probably fuel; fits with the rich plug as well.

Maybe double check that float height? You measured originally externally based on the bowl/body lip, but maybe get an internal measurement for giggles? Are they brass or foam/rubber floats? You could also closely look at the float action: hold the carbs right-side-up with the bowl off and push the floats up and down by hand. I inadvertently bent a tang that rests up against the tower one time and that screwed up the way it opens and closes even though the height was correct. Here is a post I made on that long ago: http://www.xs400.com/forum/showpost.php?p=53262&postcount=121

Are the float needles new and firm/springy?

When my regulator was pumping out too much current I was also popping fuses like crazy. It also killed the battery I had in there at the time.
That float I had to bend the tang on quite a bit as it was very low when I went to set the float heights. I remember that side being wonky.

They are not new needles but I picked the best 2 I had which I believe to be just fine. The floats are plastic or whatever material it is, not brass. I don't recall my floats having the tang you pictured.

It is possible that this regulator is also no good but the bike it came off of was a runner when we got it last year. Really wish there was a definitive way to test it. I don't have another to try. It never goes above 14.5v though with either regulator but the headlight fuse popping has got me thinking something must be wrong with it.
If it isn't going over 14.5 I think it is ok. That is the max that is normal I believe, but usually that is at 2,000 to 3,000 rpms. If your idle is low, like 1,000 to 1,200 you shouldn't be getting much action; at least mine doesn't. 14.5v is within the normal operating range and should not (I don't think), by itself, pop a fuse; the bike should normally see this voltage while running at higher rpms for long periods of time.

It is possible these problems you are having are unrelated. You may have a short somewhere causing the problem with the head light and the regulator might be fine. The issue with the running could be carb related.

When is the last time you replaced that fouled looking plug? One thing I noticed when I was tuning my bike was that once I got a fair amount of oil/gas/carbon on a plug no matter how much I cleaned them they never fired quite as well as a fresh one. If there is enough carbon you will definitely not fire properly.

If we assume the regulator is working well enough and also assume the short is a separate issue and that the ignition box is working fine (since you tried two) this leaves the carbs, coils, and the pickup coils. These would be on my radar. In terms of carbs, if anything, I'd focus on diaphragms/slide (does it slide up and down smoothly, does it fall somewhat slowly), emulsion needle/tube (is it egg-shaped and worn?), float needles, float height, etc.
Sorry, retested compression because I didn't open throttle, 180 psi in both sides so that's good.

Tried a stock set of carbs with stock jetting and it still stumbles but seemed to go a little higher. I can't test ride cause the battery won't fit that I'm using in the meantime. I am going to test out these carbs with a 140 main and a 45 pilot when I can to see what happens.

However, the smoke now looks bluish or so I think. I replaced the valve seals during the rebuild. It has me worried that it's the oil ring. I had to reuse the factory oil ring as the new ones would not work that I got with the kit. The factory ones looked mint though as did the pistons they came off and I am using those pistons in this engine.

Im thinking it may be burning oil though because the plug pulled from that side is black (though it had ran with other carbs before so it may have been black then) and had a section that looked wet, coupled with the smoke out that side.

The plugs were brand new after rebuild, have not used another set.

One thing I did notice between the other set of carbs and mine are that my floats have more 'hang' to them than the others. Meaning that when the carbs are sitting like they would on the bike, the floats on mine hang much farther. Not sure it means anything though as that would only affect it if the level is really low. The right side float on mine has the tang for the needle at a much bigger angle than the other but that's what the float height tubing test says it needs to be at...

I tested the slides on both sets of carbs before and they work just as they should. They fall nicely and quickly, but if you hold your thumb over the top hole they stay or fall very very slowly.

I don't think I'll be able to test much more until I get the battery that I can install so I can test it out.

I too think the headlight issue is it's own problem, I'll look into that once I get this sorted out.
Took it for a short ride and goes almost to 6k now with those carbs so I am thinking it may a coil issue rather than a fuelling issue. I have another set of coils I'll try.

Waiting for the head to cool to pull the plugs and see what it looks like.
Don't forget that a fresh engine build may also burn some assembly lube depending on how liberal you were with it. If you aren't sure if the color is blue trust your nose. Oil burning smells pretty distinct from gas burning. It's like smelling old milk. If you smell it and you aren't sure if it is bad it is fine. When it's bad YOU KNOW.

Your compression is very good, so I wouldn't worry too much about mechanical issues; to me it seems like everything is right. I also think mechanical issues would rear their head at all ranges of the rpms and not just at 5,000, which, incidentally, is just about exactly where the main jet is supposed to kick in.

If it were me I would also replace those coils no matter what. You have put so much work into this build that it seems silly to leave a liability like unknown coils on that bike. I'm probably more liberal when it comes to recommending the electrical system get replaced than most other people, however. I don't mess around.

Keep us apprised. I think you're almost there :)


That other photo I linked earlier of the brass carbs shows the tang that controls the "hang" of the floats. Since yours are different and not brass I am not sure how or if yours can be adjusted. It did have a pronounced effect on my carbs when mine were hanging too low; when the floats hang too low the float needle is allowed to come far out of the float needle seat; this means it can start cocking diagonal and get stuck or otherwise hang up due to the long travel. This is the mechanism I suspect, anyway.

I found that I had the best fuel control when the float needle only travels 1 or 2 mm from open to fully closed (not counting forceful compression of the spring inside the float needle). Far beyond that resulted in sloppy control; I was able to actually feel it getting hung up when I lifted the floats with my hand while holding them right-side-up as they are when installed.
I have rode this bike over 40 miles on the rebuild and started and idled numerous times so whatever oil may have been in the cylinder when assembled would be gone now. I also don't recall it ever smoking before yesterday or the day before.

It goes to 6k now. Fuel in the lines don't run out or even drop so I don't think it's running out of fuel that way.

Tried another set of coils, no change.

Don't have a strobelight, going to see if I can borrow one, but my pick up coil has the tab lined up in the middle of the mark which I believe is correct according to another thread I looked at.

I'm afraid I cooked the oil rings by revving it trying to test this out because it definitely lets out a puff of smoke after being revved up. I let it warm up before doing it but perhaps not enough? I'm quite upset with myself about that but I could also just be paranoid and frustrated right now, haha.

I'm going to put my carbs back on with the bigger jets and take it for a longer spirited ride to see if that helps with the smoke out of that exhaust.

But i'm lost on what else I can try.

So far have tried:

- 2 sets of carbs throughly cleaned carbs and float height set (one stock jetting, one with 145 mains, 45 pilots, stock needle height, 2.5 - 4 turns out)
- Synced carbs
- 2 sets of coils (one with new wires and caps)
- 2 regulators
- 3 TCI boxes
- Pick up coil is positioned correctly (yet to attach timing light though)
- Pick up coil ohm tested, 700 ohm on both
- Valves clearances set
- 180 psi compression
- Good fuel flow
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Tried new plugs, no better.

When I put the timing light inductor around the plug wire you can hear the engine tone change as if you put a load on it. Never used a timing light so is that normal?

It also wouldn't seem to rev as well when the inductive piece was around the wire.

When riding it feels like it's missing on a cylinder. I now remember when first checking for spark before starting the motor for the first time it seemed like a very dim or little spark.

What all would cause a weak spark if it is infact weak? I'm running a brand new ground cable from the engine directly to battery. I also have a ground strap running from the head to the frame and another ground running from the frame to the battery.
My alternator rotor is marked up from what looks like a PO trying to remove it? I have another rotor from a spare motor I can put on and clean everything up in there, but all my alternator tests have been perfect.

Thanks for being patient with me. Definitely appreciate it.
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Well it has plenty of spark.

I'm going to take the top cover off the motor and recheck my cam timing and cam chain tensioner.

ANY ideas? No matter how far-fetched? I'm out of ideas and things to check.

Valve float from weak springs? Though that is not really something I can check.
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Before I realized it was a tci bike I was wondering about the automatic timing unit. But your bike doesn't have one.
Anything on TCI bikes that changes behavior at different RPMs other than the TCI?

The changing behavior with the strobe light might imply that something is amiss with the low voltage side of the coils. Or it might be nothing.
Double-check all the connectors? Especially the ground connection all the way from the TCI to battery.

If you take the head off you could check that the valve springs are the appropriate length.

Not sure if it's a good idea but is it possible to check spark through the full RPM range?