Low HP and slow throttle response


81 yam
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So my 1981 XS400SH I have had 20 years (old reliable), got a crack in the manifold boot, ran lean and hot, melted a plug, and broke a valve. Now after a head rebuild and new valves and guides (and a new tank), and several valve adjustments, it idles great and roars when I get up to speed, but the throttle response is slow - and acts differently when warm and choke off vs. on - and definitely missing some HP. Also gets very hot very fast.

From this helpful site, I learned that order is :
Valves (done, 3x), runs great
Carb synch (done)
Carb mix (never touched)
Carb idle (nice 1100rpm)

No compression leaks I could find – seems all nice and tight and sounds great. Maybe exhaust leak at manifold output (tightened as much as I could before I feared breaking the mounts off) – but I don’t see this being the reason for throttle delay/HP.

Every article on this site talks that about this says carbs clean/adjust.

One thought is messing with the fuel mix screws (which may have been adjusted to account for the leak I had – and I need a richer mix without the leak) - but as far as I know it has only been factory tuned and am afraid to adjust it. I feel like a kid playing with heart surgery messing with them - and would rather have someone who knows what is up.

Wish I knew a carb nut to test adjustments before I go there – it ran GREAT until the day I broke down, and I fear that taking it apart to clean will not end well - and dont wanna touch it if I dont need to - I fear it wont end well.

Anyone have any thoughts??? No one around hear will touch a bike over 20 years old unless it says Harley on it!

I just adjusted and balanced my carbs on Saturday, following careful cleaning. You've given some good info, but I have a few questions:

- After reassembling everything, did you set the timing...with a timing light for your year, I believe?
- When is the last times the carbs were cleaned inside and out? Can't tune reliably without clean carbs.
- Are your idle mixture screws exposed? In '81, weren't they under a cap originally?

If it were me, I'd check the timing and make sure the carbs are clean (if you're not sure) before trying anything else. After that, I would set the screws at about 3 1/2 turn out from a LIGHT seat, and see how it runs. That's exactly what I did Saturday, and the bike starts and runs great, though mine is a '79, so the idle mixture screws were never covered and the ignition is points (I just set static timing with the bike off). After I got it up and running, I hooked up the manometer and balanced the carbs (intake tract completely assembled including filters).

Your issue could be carb or ignition related, but I would guess it's carb adjustment or dirt. lean mixture makes for a hot running bike.

Oh, and why did it take several valve adjustments?
Thanks for the comments!! to your questions - one thing at a time:

- No timing set. The 81 (and later) does not have points, but an electronic timing device that slides on a groove at end of crank shaft (see pic) so no way to screw that up - or so I have read. it is not showing me issues here that I would expect for timing which is not firing at all/rough idle, or inability to get high RPM - I think

- Carbs never cleaned - bike has been driven 300 miles a year last 15 - never ANY problems, reliable as a swiss watch - and I've owned this bike since 1991 with 1200 miles on it. Not saying they don't need it, but 1) it ran literally perfect up to the day I got hot from the lean mix and cracked the valve, and 2) I fear I would just mess it up (kid doing heart surgery) and create too many other variables here - before I do heart surgery I'd like to rule out anything else it could be? I have heard about cam chain tensions falling in/grinding up and a bunch of other things that could be producing the same symptoms and want to rule out everything else - carbs ran perfect the day i took them off to replace valve.

- idle mix screws are exposed and recessed - but much like the carbs and carb cleaning - I hesitate to turn them because they were likely set at the factory or by a mechanic and were previously working perfect. I'd want to know this is a likely reason (i.e. need to compensate mix for the now-absent hole in the intake boot) and rule out other options/causes before I introduce more variables - Id be stabbing in the dark trying to turn these screws, not knowing which one or which direction (or if needed) - and I have heard horror stories about people stripping the brass...

So I like your advice on what you did and that may be my next move (reset idle mix screws and re-synch carb). Can you lmk about when you set the idle screw (last?) and also any good links to clean a carb/what to replace).

But....my hesitation is that given all I describe above - and no need to do timing - are we sure enough about causes of my symptoms that I should start disassembling the carbs and cleaning them just because it is a good idea, even though they never gave me trouble before - or do I start turning idle mix screws? It seems that "dirty carbs" is a catch-all solution here, and not something I know how to do/what to replace so i want to be sure, since I had no prior problems. Meantime, I did test and synch the carbs with a nanometer (weren't too far off, but also bike wasn't real hot), and turned the idle screw to where it idles at 1100+/-, but not sure what else I may have set up/changed that would cause the symptoms. Moral is i'd hate to jump in and disassemble a perfectly good set of carbs, or start turning idle screws at random - if they have been set up/were working - if it could be something else....

For example, one thing I wonder about and have heard mentioned (like the chain tensioner) is exhaust - I put in new copper rings, and tapped in the shims and tightened them down to where the studs are bending the collets where they attach to the engine.. I dont feel that I am losing compression out of this area, but there is some smoke after engine heats up - several folks here said to expect this when you replace a head, because the oil is getting splashed around/burning off.

Also I found this small tightening nut after putting on the carbs and tank - sitting on the left side of the bike - on the engine block left side, under the petcock. Doubt it is the solution to all my problems but it is a clue...(see the 2 pics below) - any ideas what it is?

Last question you had was about valve adjustment. This is required after replacing a head/valves - and I never did it before so first time I got close and it ran some, but clearly sounded not perfect - sputtering and backfiring - but it ran. I read here and in the manual that after replacing valves/head that even if done perfect, you need to adjust valves, run it a bit, and then go back and test/re-adjust, because as they run the metal heats, moves, loosens a tad. The second adjustment found them off by .001 -.002 from my original setting, and the third time showed them tight where they should be, so that seems to be dialed in....

Thanks for the input!

IMG_20110913_183716.jpg image.png image (1).png
Mikuni BS34s aren't heart surgery. I think yours need to be cleaned, well and adjusted. All you need is to take your time, be gentle, and keep track of the parts.
The coil has a notch in it and only fits on the cam one way- where the nub is - cannot be changed. Open to carb cleaning 101 advice or links if everyone believes that is the key. I just cannot understand how it was fine thru the day the valve broke, and looking for other causes before I dive in, as it is 40 years old and feel like leaving what I know worked alone, until all options are tried. Could be as simple as the fuel mix or bad job mounting the exhaust - I dont know. nothing has changed, except a bad valve and hole in rubber intake boot gone. Hmm...
I understand not wanting to screw up what worked before. Why don't you try a compression test. Do it dry first., then with a teaspoon of oil. If it is low dry, and wet, you still have a valve issue. If it rises with the oil, it may be rings. Getting really hot can screw with piston rings in addition to valves.
The coil has a notch in it and only fits on the cam one way- where the nub is - cannot be changed.
The thing with the notch is not the pickup coil. The pickup coil is the black plastic thing mounted behind/around the thing with the notch. It is held down with two screws at about 1 o'clock and 7 o'clock. Loosening them allows the plate to be rotated.

I'm not staying that timing is necessarily your problem, but it is something that can be adjusted.
Sounds like timing to me, when they replaced your valves they had to pull the pickups and cam. Thus the reason you have to reset timing. like stated above rotate the pickup plate to adjust. What do your plugs look like, and again like stated above check compression. Also check your plug gap.
Good point on moving the pickup plate - but I can only move it like a couple degrees either way, right?... Any good threads on how to dial this in (sometimes i miss points!). Maybe we are onto something - it would explain a lot. I assumed automatic meant automatic!

I know the cover had a nub screw to keep people out of there I had to take out to get to the head. I have seen lots of threads about not adjusting or no need to adjust (below), and only a few that touch on how to do timing with the CDI unit I have (no points). I am gonna give this a go this week - if any ideas or threads or info on how to do this please share!

I will also check my compression and my plugs and post. Problem/concern with compression is that I worry about an accurate reading from turning on the throttle to check it, as this issue (carbs or timing) may give me a bad reading (or maybe that doesn't matter?). Any tips on getting a good compression test with my situation?

Ironically this all started by the intake valve boot crack - led to lean mix/hot engine - led me to check compression with/without oil (thinking I'd melted the rings) - and found low reading one side - used an optic camera and found pistons/rings were great - which led me to remove the head and found the cracked valve (pic below) - which led me to re-machining the head and all new valves/guides and a total rebuild - and here I am again.

Compression good to check, but haven't run the engine long enough since rebuild that i would a destroyed the pistons and rings if that is the thought...

Any ideas on the random part below?

cracked valve.jpg

To be clear, you always have oil in the crankcase. I was referring to oil in the cylinder through the spark plug hole to see if the rings were sealing, but if you've done all that you describes above, it's back to timing and carbs...though I'm unclear how a visual inspection of the rings tells you whether the are sealing, but i suppose if you're not blowing smoke, it's not horrible.

You will need a timing light to check the timing. Download the factory manual from the sticky and look up the timing for your bike.
The thumb screw is what I was referring to - sorry! I found it after I put in my carb - sitting on the left side of the engine housing below the choke...


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Thanks Hough. gonna do the timing next and check compression and plugs - will post. My only point was that my lack of compression was easily seen here and due to the cracked exhaust valve. True I could have ring issues (even bought them), but I was surprised how beautiful my 40 year old pistons were and the cylinder walls were like perfect. Having blown rings before, there is usually some good evidence - metal shavings, etc. - that are rather obvious here. Not proof positive, but keeping my fingers crossed!



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OK my bike had the issue of it idling great, but wont get up to speed or does so very slowly. Valve timing is dead on and I have synced carbs them and played with the idle mix screw some, but not too much.

All my chats have led me to the fact that the ATU needs adjustment (other posts on this SAME issue got into the carbs and the cam chain/tensioned, but most found it to be timing in the end - so these areas are on hold for now because of progress with timing adjustment). So I adjusted the timing forward and back and had a little success but not consistent (choke on or off and 1 degree matters) - but at times I did have it responsive and roaring, only to turn off the choke (or on) and have it change. Still not warmed up.

A couple questions:
- Best practices on ATU adjustment - not sure how to - just found improvement through trial and error. Comment in this forum mention using a light? Others simply suggest matching its position to the LF mark (see http://www.xs400.com/threads/ignition-timing-on-a-1981-special.11608/)
- Fuel mix screw - what should I do to ensure adjusted properly - or look for on my plugs to check this?
- Best way to check compression - people mention full open throttle (I want to double check it)

Thanks - Appreciate any ideas here!
OK had a nice day and thought I'd dive back in. Before pulling the exhaust, engine, and so on to get at the chain, and before looking at the timing, I figured I would do a compression test. Remember that I completely tore down the head and valves and reassembled everything after finding the the broken valve (see above). Once again, the engine got hot and a valve broke (pistons looked fantastic) and had head and valves totally redone.

Here are the plugs - very black and oily.
View attachment 27862

So I then did the compression test and found:
Dry: L - 75, R - 95 and Wet - :L -160, R - 175?

So now I am worried about the rings - whereas I thought I just needed to maybe fix the timing chain. Wondering if the symptoms (slow throttle response, lower power) are due to the potential timing mentioned above - or the rings or something.

Any thoughts/ways to rule this out? I am starting to get to the end of my rope looking at pulling the engine out again after total reassembly - thinking timing issues - and now it seems like I need to get all new gaskets and a complete tear apart. Anyone have any thoughts as to what I can do to verify my issues before I poen the patient up?


I finally found a guy in town that works on older asian bikes - and he was going to pick the bike up and do some testing for me yesterday. However the day before, I went back and:
- re-checked valve clearance (not much adjustment needed)
- adjusted the timing (this is where I gave up)
- reset my idle - dialed it back down

If you remember - I had slow throttle response and was chasing down timing, until I got the very low compression readings above. I have 100+ hours into my disassembly and reassembly + a few bills $$ into the head reman./valves replacement and tank repaint...

The last 2 things I did were key - and I cannot stress this enough:
- put a lot of fresh gas in
- new, solid charge on the battery

HFS if the bike didn't start right up and GOOOOO! Just like before - all the HP was working for me and had immediate throttle response! Yahoo! I drove it to my kids school for a band show that night - it was 74 degrees in Cleveland. Just too bad the Indians didn't have the same luck!

So - I was surprisingly baffled. here is what I think happened - please learn from my errors (Remember - I was about to tear into the timing chain, carbs, and potentially the lower unit to replace the rings and so on):

1) Doing my compression test, I believe that the combination of some worn threads on the nearly new compression tester, some slight valve mis-adjustment, a weak battery, and lack of lube in the cylinders/valves (until i did the wet test) gave me the misleadingly low compression readings

2) While adjusting the timing, I was so focused on the idle and timing, that I really was not aware that with all the stops and starts my battery was really low (and a no-start made me think I needed to re-adjust, but sometimes it would catch), and also with low to no gas in it, I would get fooled when it stalled or wouldn't throttle up, thinking the idle or timing was bad - leading to more adjustment and more use of gas and electricity

In the end, the issue WAS my timing, and the advancement of the ATU (by rotating the unit slightly) made the difference - but I couldn't see it because of low gas and battery power used during the adjustment - and a bad compression test mis-lead me down another path. Stupid? Yes! Happy? Very! The way it came about was like magic - Thanks to everyone for their help.