Idles and cruises well, breaks up under heavy load, not sure which carbs I have.

[donn29]

Hey all, just got my first bike. 1980 XS400SG 3W5. 33k kms. Got it as a non runner, but was able to get it together and it seems to idle well and it holds speed okay, usually gets up to speed okay too(new rider, so I'm taking it easy). At mid to high RPM or high throttle it seems to suck the bowls dry, or something. When I try to get the bike into the higher RPM/throttle range, I can hear one cylinder cut out sometimes, and sometimes I can lose both cylinders if I stay deep in the throttle. Usually by the time I have slowed down a bit, it's back to running and doesn't ever stall, but I've been slowing down in gear, giving it time. I'm no carb pro, but I've spent some time on the manual and youtube as well, but can't really get this nailed down.

When I got the bike, the carbs were installed so I took them off to clean them, and they were very clean. So I sprayed them with carb cleaner and reassembled. That was a few months ago and the bike is plated now, so I've had it out a few times now and have taken the carbs off again. They still looked great, this time I removed the float needle and seat, the main jet and emulsion tube and the idle jet(the small one beside the main). I saw on youtube that the pilot jet(might be mixing the pilot and idle up) that is inside the intake of the carb doesn't need to be removed and is damaged easily, so I left it in place. I also pulled the needle out of the slide/diaphragm. I was able to blow cleaner through every orifice I could find. These carbs seem to have no way to adjust them it seems, other than swapping jets, or adjusting the float height. I checked the floats and they are perfectly set to 26mm, but in some manuals I've seen they say 26 or 32mm. So I'm not sure which is right. Main jet is a 130, needle position is 3 for the slide needle.

Looking at some manuals, the 26mm, 130 and 3rd position seem like I might have XS360 carbs, or maybe I haven't come across the right manual yet. One cylinder is a little low on compression, but I will do another ride tonight and hopefully the compression will come back a bit when I recheck it. It also has almost straight pipes with no pipe connecting the exhausts together. Stock airbox/filters. Have not worked out a way that I'd like to do a carb sync yet.

Any advise?

 

[CaptChrome]

Welcome to the forum. Those look like the stock carbs for the bike, Mikuni BS34 Mk !!!.

A couple of thoughts that I know can cause your issue even with super clean carbs;
1. Have you checked the ignition timing? If the timing is retarded or the advance isn't working, then you will get a bog when you give it the beans.
2. Set your float bowl levels using the plastic tubing method that is spelled out in the factory manual, not by using a caliper on the float. Download of the manual is pinned to the top of the Garage.

 

[donn29]

So I was out again and it almost seems like there is a rev limit at 5k RPM. I tried leaving in half choke and it seemed to cut out sooner, and I tried turning the fuel off, and it started to backfire. Does that make sense if it's running too rich? This bike doesn't have points, so I will check the timing and see if its at 10(LT) at 1200 and 36(Double Dash) around 3000rpm?

 

[donn29]

I can only do the float method with the one bowl. One of them has a stripped screw on the drain. I do have a clear filter and it's fuel level stays close to the top of the bowl. Not sure if that means anything.

 

[GuyS]

Did you get your bike running yet? The attached are apparently the jet sizes for your 80 XS400SG. These jet sizes are different from what you stated above (you said jet was 130 but Yamaha says it should be 135). that’s slightly richer than yours. I bought an old Maxim and found it quite difficult to get the proper specs. There’s so much ambiguity out there. Also over the years owners have messed with the carbs/jet sizes etc. The original Yamaha engineers played with all kinds of variables finally finding the best of the best. It might be an idea to go back to stock as much as possible as a place to start. If your float bowl drain screw is stripped,…. at some point you’re best to deal with it. What happens when you want to drain the carb for winter? Might as well deal with it now and use the tube method to nail the float level 100%. It will save you time at the end of the day. I used the tube method with mine and set the float level to be 3mm from the joint at the bowl/carb. I have exactly the same carbs as you do BS34’s.

 

[donn29]

Thanks for sending those over! I did put in a few rides as it is. I did a half turn out on the only adjustment screw(was at 2.5 turns). I've discovered that when the bike runs poorly, if I let it warm up longer, the issue goes away, I think the plugs are not hot enough perhaps, or like you said, the jetting is too rich, and the higher temp from longer idling helps. It also has basically no mufflers, so perhaps the jetting is for the exhaust? Since my last post, I've ridden it twice where the bike wouldn't get over 3k at first(choke makes no difference), but I stopped somewhere safe on both of those rides and let it warm up more and when I left again, it ran great. On one of the rides, I let it idle longer at home and it was great from the get go. Once it runs clean, it never gives me an issue for the day. I plan on pulling and cleaning the plugs and maybe trying a heat range hotter. They are NGKs with I think a heat range of 5.

 

[GuyS]

Do you get any backfiring when you quickly let off the throttle?

 

[donn29]

While cruising, there is sometimes some deceleration backfire, but more like popping. When it's not warmed up it will sometimes backfire while the throttle is open. Are you asking more about cruising or idle revving with a quickly closed throttle? I should be taking the bike out again this weekend.

 

[GuyS]

Well I guess it’s more curiosity because mine did it the other day…..a popping sound a few times when I let off the throttle. Oddly I took it out yesterday and no popping? It just be an artifact of the carbureted engine and the age of the bike. Since yours is more or less an open pipe you may not have the issue, although from what I read the backfire popping occurs just past the exhaust port when the unburnt fuel gets some oxygen.

 

[donn29]

Actually, I'm pretty familiar with tuning automotive engines with standalone computers. I have an older 80s Toyota Supra with a Megasquirt ECU. On those they use a throttle sensor to shut the fuel flow off when the throttle is closed and the engine under a set point, say 2000rpm. If you disable the decel fuel cut, you will get popping with a closed throttle and higher RPM, but that usually only happens with a stoich fuel mix or leaner. If the engine is in decel with what would be a good or safely rich fuel mix, there is little to no decel rumble or backfire, unless the exhaust is very hot and there is raw fuel(spark is cut and fuel is left on or leaking past injectors).