77 xs400 starting problems

The old valve cover that I inherited has the tach cable stilll attached while the one on the bike isnt there at all. I looked at seeing if there was a way to reuse the old one but it seems like the two are mated and it would involve some work to install it on the bike.
The tach cable is secured with a screw. To remove the cable from the top cover, remove the screw completely and pull on the cable to extract it. In my experience, the cable is a tight fit, so you may have to use some force. Putting the cable into the new cover involves cleaning and lubing the cable, inserting it fully into the drive hole (may have to push hard to get it to seat) and then reinstalling the screw.
Done done and done. Runs like a top now, kicks over real easy and the electric start turns over immediately. Thanks to everyone who gave me pointers and advice. Im gonna ride this thing in its current configuration for a while and see what speaks to me. I do wanna upgrade the headlight now though from the incandescent to led. Anybody care to suggest a drop in replacement ?
Alright gentlemen yall deserve an update so here it is.

Rode it to work a couple days in a row to see what else would shake out and some things immediately jumped out to say hello.
Hanging throttle at 3k rpm. Figured it was a lean condition due to vacuum leak so looked at the usual suspects, carb boots were leaking through a small crack, replaced. Pulled carbs and ordered a couple rebuild kits. Diaphragms are good but the left carb had a needle broken in the mixture screw hole. With some very gentle convincing was able to get the tip out (shh dont tell xschris lol). Up next were throttle shaft seals, basically crumbled as I pulled them out. Petcock was fine, I think its mechanical anyways. Gave the carbs another good clean and some things that I wanted to note and see what yall think, 145 main jet size and 42.5 pilot jet. Kinda makes sense seeing as how this thing runs pods with baffled exhaust, not sure but if someone wants to chime in I'd love to hear your thoughts. Carbs are still on bench so I have to do some more tuning once they are back on.
On to the next problem.
Kills batteries. Started with a new fresh battery and was left stranded at the convenience store so battery ain't charging. Haven't checked resistance at stator but rectifier checks out and regulator doesn't. When I was trouble shooting, at 2k rpm voltage at battery was reading 12.76 and falling slowly. The regulator was a cheap and easy fix to rule out before getting into stator, seems unlikely to me that a bike with low miles like this one would have a burned out stator but we shall see. Ive searched this forum extensively for this specific issue and it seems to boil down to those 3 components but I might be missing something. Found a used oem regulator on ebay so thats gonna be fun lol. I know there was a fellow here who had an extensive write up regarding regulators and aftermarket replacements so if this isn't the key I'll look closer at that thread. Thats all for now, if the weather permits I'll put everything back together this weekend and see what else catches fire lol. Thanks again for all the support and knowledge, this place is an invaluable resource for beginners and I can't be more grateful for it.
I thought that was your write up tstidham. Here's the one I got off a seller on ebay. Claims it worked when taken off the bike but we will see. Cost 15 bucks so if it doesn't it isn't a total loss. If it ends up being busted I will go ahead and get the transpo. Wasnt able to put it all together today due to carb rebuild kit having wrong mix screw. When I pulled up the parts diagram to order an oem one I noticed that there was nothing about rubber seals and washers that I so commonly see on this site. I'll attach some photos of the stock and the new screw and if anyone has any insights I'd be grateful to hear them.


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Yeah that looks like the one I've got. Unfortunately I went through a local yamaha dealer and paid through the nose for two of them. Seemed to me that it would be the only way I could ensure that it was the right fit. Never used part zilla before are they pretty reputable as far as getting oem parts that fit?
Had some time today to get a little work done on the bike so here's an update for yall.

Carbs bench synced with feeler gauge so should be close enough to give me a steady idle, freshly charged battery and got that voltage regulator replaced with a used one of questionable quality. Carbs cleaned and rebuilt, fitted with oiled up airpods directly on the back. Started with mix screws at 3 turns out and throttle pretty close to being fully closed. Ill spare you details but the only way I could get it to idle off choke was 5 turns on the mix screw and idle as high as it would go which only showed 1200 on the tach. Im a little stumped because the plugs look like a rich condition.

There are so many unknown variables with this thing but I think its coming down to air. Engines are giant vacuums and carbs are just venturis that use airflow to pull gasoline in. Having said all of that and with my current air flow setup I still believe it's too much air so I'd like to talk about this h pipe. This thing creates some restrictions in air flow and given that it hides the filters beside the battery box and behind some plastic bits, it kinda makes sense if you run it without the stock box. Rebuilding the carbs helped with the hanging throttle but didn't get rid of it so I'm thinking this h pipe that came with it might be a thing.

I'm gonna take it to work tomorrow so I can have a proper place to put it on the center stand and take a look at the stator and some other unrelated things. Can't wait to get into the rest of the wire harness lol


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the only way I could get it to idle off choke was 5 turns on the mix screw and idle as high as it would go which only showed 1200 on the tach. Im a little stumped because the plugs look like a rich condition.
Regardless of the plugs right now, the mix screw position and the hanging throttle comment sort of point to a lean condition and maybe an air leak. I my opinion, just putting pods on with the H-pipe shouldn't make that drastic of a change of idle fueling, so I would look at an air leak (carb holders and especially the butterfly shaft seals).

Also, I see you have a small pod on the crankcase vent. The tube off of the H-pipe crossover goes on that vent. I suggest you put the vent back on and use the H-pipe until you get a handle on the pilot circuit mixture. Too many changes at one time only lead to a troubleshooting quagmire.

I'd like to talk about this h pipe. This thing creates some restrictions in air flow and given that it hides the filters beside the battery box and behind some plastic bits, it kinda makes sense if you run it without the stock box
I don't see the H-pipe as causing any restrictions as it is a straight shot from the filter to the carb with only a slight bend. In fact, it has been suggested that the H-pipe helps provide laminar air flow to the CV carbs which do not do well with turbulence at their inlets. The H-pipe also has a cross over which can even out pressure differences in the two carb inlets. Ultimately it is your call, but as I said above, I would troubleshoot your issues with the pipe in place. I don't believe it is causing the fuel mixture issue, but it can complicate troubleshooting.
Alright I'll give it a closer look this afternoon. When I rebuilt the carbs I replaced butterfly shaft seals while I had them off, diaphragms looked good too. The carb holders are new but could still be leaking so I will look into that as well. Im digging your suggestion though because I had the same thought last night, this thing is too far away from its stock configuration so I need to head back in that direction to find a baseline. It didn't come with any air system outside of the h pipe so I'm kinda making this up as I go along.
I'm wondering if I should spring on building a stock air system. Looks like I can find the box on ebay and all the little clamps and screws on partzilla.

I should also clarify the throttle hanging condition, it doesn't hang anymore, its slow to return to idle but it does settle down. It also doesn't creep anymore. A couple weeks ago when I was troubleshooting the charging issue I would bring it to 2k rpm and after a second it would creep up to 3k rpm on its own. Doesnt do that anymore which makes me think the seals and gaskets are good.

Some other things to note, float height was checked with calipers off the carb body and are within tolerances. The carbs also have 145 main jets and I'm assuming matching needles.

I also wanted to ask why does the crank case breather need to be under vacuum? I thought it was a reed valve that vented out when under pressure?
I also wanted to ask why does the crank case breather need to be under vacuum? I thought it was a reed valve that vented out when under pressure?
The crankcase to carb intake line is mostly to reduce emissions. It is not a reed valve, but simply an opening straight to the crankcase with some baffles in the path. This line doesn't need to be on the crankcase, but if you are not using it, plug the carb side of things so that uncontrolled ingestion of air is not occurring which may very well impact performance of the carbs.
Cap you were right again. I started heading back to the original configuration and kept getting warmer, sprinkled a little bit of tstidham's carb tuning knowledge into it and ended up with a bike that runs alright with h pipe and pods. Tried with and without covers, makes no difference. No hanging idle no creep so vacuum issues look to be solved. As its tuned now, it doesn't produce a lot of torque or high end power for that matter, seems to peg out at 3k rpm. My question for you, where would you start in terms of tuning to get it back to where it was? When i first got the bike running i rode it a couple times with nothing in the carbs and it produced some startling torque.

About the regulator, it does charge the battery but its very slow and forget about lights you'll lose 1-2volts immediately. When I metered it at 2k rpm it started at 12.86 and made it to 13, is this normal operation?

Anyways thanks again for all the knowledge, I've got like 5 of tstidham's posts bookmarked at this point. Reading a couple different posts you made about tuning carbs helped me wrap my mind around how those circuits function so big thanks man.


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Glad I could help! Carb tuning is a bit of a lost art these days, but my first vehicle was a 70 VW squareback with a 68 dual carb setup that the previous owner swapped in to replace the old factory bosch fuel injection that broke. So at 16, I was thrown into the world of mixture screws and carb syncing right from the start. Mind you, that was 1986 and anything I could afford as a teen was carbureted.
For the high-speed, check where the clip is positioned for the needle on the diaphragm. If you have multiple grooves that is. I've seen some needles with only one groove. Stock needles get the middle groove, to add fuel use the next groove down and see how it runs.
Yeah you aren't wrong tstidham it is a lost art. I had a similar experience as child rebuilding carbs for my dad on an 80s suburban lol.

I'll check tonight to see if there are grooves on the needle but if I recall correctly there aren't. I guess I've stepped into the world of tweaking and tweaking a little more and then a little more lol. If there aren't other grooves on the needle I guess I'll look to buy a couple. I know mikesxs is shit for carb parts so I'm guessing go straight to mikuni? Any suggestions?

I like how you describe power across the rev range capt lol so I will look into ignition timing again. Thanks for the pointers guys I have no problem turning wrenches but every now and again I need someone to point me in the right direction
Limped it into work but didn't have much time to tinker with it. Really is a struggle to get this bike to 35mph lol. Went ahead and checked sync and that was acceptable, fiddled a little with mix screws to make sure the circuits were clean and they had an affect. Readjusted idle and gave it a spin to see if I could get closer. Hit a hard rev pulling into shop before killing it and then pulled spark plugs. Image attached is of the left cylinder spark plug. Right still looks rich but left looks lean. Let me know what yall think. Tomorrow ill do ignition timing and maybe pull carbs off.

Sure is getting tempting to rebuild an old air box. Anyone know how I can page xsxhris?


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