'81 Hanging idle & refusing to starting when hot - Need a plan of attack

[sometimesnever]

Hey guys,

My brother and I recently finished the very long build of my '81 XS650SH down here in Aus and I'm now trying to get it running reliably so that I can actually start riding it!

A bit of background first:

The bike when we bought it was in good running condition and 100% stock.

During the build, we fitted a Pamco (E-advance), Mikes/HHB PMA kit with the dual output coil, deleted the battery with a Sparx cap and removed the starter gears, so kick only and rewired from scratch.

It also has Unifilters and a set of Gordon Scott "fat chopper pipes".

Timing and valve clearances were set during the build but carbs at this stage were untouched from stock.

The problems:

After initial assembly and wiring, the bike was a bit difficult to start. On the somewhat dubious(?) advice of a friend and reading some threads on here, I swapped the BP7ES plugs for the hotter, resistor type BPR6ES and that actually made it start much easier.

After doing some light test runs in the streets around my house, I found it was stumbling off idle (sometimes stalling) and breaking down for a second here and there but pulling pretty well through the rev range (restricted to 2nd and 3rd gear and max 60km/h due to inner city streets).

The real problems start when it gets hot...

The idle starts hanging quite high, getting progressively worse the longer I ride (within 10-15mins). Engaging the clutch brings it back down temporarily but it hangs again soon after taking off.

Once this starts happening, if I shut it off (or it dies) it is impossible to start! Even after waiting 30 mins+ it won't start (also tried using starter spray) and after wearing my leg out, I've been putting it away and leaving it until the next day, when it starts second or third kick.

I assumed the bike would probably be running lean due to changing the pods/exhaust and some advice on here that a lean condition could lead to hanging idle. However every time I have pulled the plugs (often during testing), they have had wet black deposits around the circular outside edge. This black wet stuff wipes off pretty easily with a rag or finger and isn't what you would call "sooty", more like "tacky". Electrode looks light brown and relatively normal (?), if a little uneven in it's colouring.

Things I've tried:

- I checked carb balance with the manometer method and they were out of sync. I got them balanced.

- Set the mixture screws 3 turns out (and various half/quarters turn adjustments either side of that, when the bike was running).

- Pulled the carbs, checked for jet obstructions and general cleanliness, set the float heights (which were out of spec). Inspected the diaphragms and tested for leaks by blocking the vent.

- Replaced the cracked old intake boots and gaskets.

None of this has yielded any noticeable improvement... So my quest continues but from here on out, I'm not sure exactly what to check/adjust next and in which order of priority.

I have a timing light on the way to check that. I also assume valve clearances will be a good place to start, even though they were set when we put the engine back together?

I'm also very happy to revisit the carbs as I really only did the stuff listed above, hoping that would fix things.

Other than that, I'm not sure what to do next.

Any advice would be greatly appreciated!! I love the way this bike has turned out and if I can get it running properly I will be a very happy camper.

Sorry for the wall of text. Just wanted to be thorough straight off the bat, hoping that one of the experts here will see the big picture clearer than I currently do.

TLDR Version: The bike runs sort of ok but won't start when it's hot. Like at all. What should I check and in what order?

 

[TwoManyXS1Bs]

My gut feel is that the idle circuit is a bit too rich, and/or compression is a bit low...

 

[XSLeo]

www.amckayltd.com/carbguide.pdf
Leo

 

[weekendrider]

Throttle shaft seals?

 

[scabber]

Other than removing the electric start was the motor rebuilt ? When you had the carbs apart did you record the jet size ? Fixed or adjustable needle ? How far off were the float levels to need adjusting ? Carbs can look clean and still be dirty. The piolet circuit especially. Often overlooked is the orifice at the bottom of the float bowel and prone to clogging. The carb guide is a wealth of information. Cleaning, Tuning and how they work is all covered.

 

[retiredgentleman]

The air/fuel mixture is too lean. You'll need to go larger with the pilot jets and main jets.

You may also have air leakage around the carbs. Even though the carb boots are new, they could still be leaking air.

Timing may be advanced too much. Use a timing light.

Try replacing the capacitor with a battery.

 

[DogBunny]

Normally on a hot restart you have to hold the throttle open a little bit. If it doesn't re-start after a couple of kicks then it is flooded and you have to hold the throttle all the way open.
That wont cure your hanging idle, but try it, and if the bike still doesn't start hot it is a further symptom of the problem and might help someone figure it out.
Not sure why your plugs are wet, but a hanging idle is one sign of a too lean pilot jet, and you should have gone up at least one size on pilots with your pods and pipes. If the electrode looks okay I'd go up one size on pilots and see what happens, it's an easy test, and is the correct thing to do when you change to pods and open pipes.

 

[5twins]

I think you're running too lean, way too lean. The BS34s were lean from the factory even on a stock machine. Your mods have aggravated the problem. You could be on the verge of over-heating the motor and that may be why it is reluctant to start hot. The hotter #6 plugs aren't helping matters either. Nor is the fact that they are resistor types. You need some resistance in the ignition system but you already have that in your plug caps. You've added more. Personally, I wouldn't run the bike until you get bigger jets. You could damage the motor.

There were two float types used in the BS34s, brass and plastic. They have different settings due to their shape. The brass were used one year only, the first year (1980) of the BS34s. The plastic floats came along in 1981. However, Yamaha didn't update the setting spec in the manuals for several years. That makes it pretty easy to set the plastic floats wrong while thinking you did it right because the manual said so. Brass floats are set to 27mm, plastic to 22mm.

 

[sometimesnever]

My gut feel is that the idle circuit is a bit too rich, and/or compression is a bit low...

Thanks 2M, my jets are stock 42.5 pilot/132.5 main. I would've thought with the new uni's and straight exhaust there's no way it'd be running rich? Also, it idles pretty well for extended periods when I first start it.

Compression on the other hand... I have noticed that it feels significantly less when kicking whilst hot. Not sure how much difference is normal though?

www.amckayltd.com/carbguide.pdf
Leo

Thanks Leo, I read the carb guide before setting my float heights and learnt a lot. Not saying I wouldn't benefit from another read through though! And I'll admit that when I had the carbs off and found my floats were out of spec, I sort of stopped there.

Throttle shaft seals?

Could be, but I checked with some starter spray around the shaft on both sides, while running and no change to the idle. There is definite in/out movement but no up/down. That's acceptable, right?

Other than removing the electric start was the motor rebuilt ? When you had the carbs apart did you record the jet size ? Fixed or adjustable needle ? How far off were the float levels to need adjusting ? Carbs can look clean and still be dirty. The piolet circuit especially. Often overlooked is the orifice at the bottom of the float bowel and prone to clogging. The carb guide is a wealth of information. Cleaning, Tuning and how they work is all covered.

Thanks Scabber, I didn't do the engine rebuild, my brother did so I'll ask him exactly what he did. But I know that at least, he did the rings and had the bore honed by an engine shop.

As for jets. I checked pilot (42.5) and main (132.5). Didn't check which needle I have though. Will pull them off again to confirm and give them a really good clean while I'm there. RHS float was out by over a mm (23.89mm). LHS float was 21.40 and I adjusted it to exactly 22mm. They were also a little different on each side of the individual floats (twist) but I got them very close by the end. I used a digital caliper, measuring from the gasket surface as per instructions on here and in the carb guide.

The air/fuel mixture is too lean. You'll need to go larger with the pilot jets and main jets.

You may also have air leakage around the carbs. Even though the carb boots are new, they could still be leaking air.

Timing may be advanced too much. Use a timing light.

Try replacing the capacitor with a battery.

Thanks RG, It would make total sense to me if my mixture was lean but the plugs don't seem to suggest that? They're tacky/oil if anything, at least around the outside. Or is it possible that I'm giving too much weight to my (probably) inferior plug-reading skills?

I'll double check for leakage when I replace the carbs this time but I made sure to tighten the clamps until the ends touched and sprayed starter spray around the boots to make check that the new gasket was sealing ok as it was different to the original (fatter).

Timing light should arrive tomorrow, so that's my next stop! After that I'll try to borrow a known good battery from somewhere.

Really appreciate all the help and ideas so far guys. As much as I want to ride, this is actually a good incentive to learn about this engine. I'll definitely post my results when I get a chance to do some homework, hopefully this week!

 

[5twins]

That little bit of wetness around the outside of the plugs is probably some oil from inside the cylinder. It can be common on a well used motor. When you read plugs, you really don't look at that outer edge, you look at the electrode that bends over the top and the center porcelain part. What you "read" mostly is the ring of color down near the bottom of the porcelain .....

 

[sometimesnever]

I think you're running too lean, way too lean. The BS34s were lean from the factory even on a stock machine. Your mods have aggravated the problem. You could be on the verge of over-heating the motor and that may be why it is reluctant to start hot. The hotter #6 plugs aren't helping matters either. Nor is the fact that they are resistor types. You need some resistance in the ignition system but you already have that in your plug caps. You've added more. Personally, I wouldn't run the bike until you get bigger jets. You could damage the motor.

There were two float types used in the BS34s, brass and plastic. They have different settings due to their shape. The brass were used one year only, the first year (1980) of the BS34s. The plastic floats came along in 1981. However, Yamaha didn't update the setting spec in the manuals for several years. That makes it pretty easy to set the plastic floats wrong while thinking you did it right because the manual said so. Brass floats are set to 27mm, plastic to 22mm.

Thanks 5twins. Ok that's at least a couple of well respected votes for "too lean" and I know better than to disregard carb advice from you guys...

I'll order some larger jets this week and buy a new pair of BP7ES. Any idea on where I should start with sizing? I'm in Melbourne which is close to sea level and has an average annual temp of around of around 20C. Although we are about to go into the colder months.

I have the plastic floats for sure and they're set to 22mm as per the carb guide.

That little bit of wetness around the outside of the plugs is probably some oil from inside the cylinder. It can be common on a well used motor. When you read plugs, you really don't look at that outer edge, you look at the electrode that bends over the top and the center porcelain part. What you "read" mostly is the ring of color down near the bottom of the porcelain .....

Thanks for that 5twins. I didn't realise that the ring was on the porcelain was so important.

That would explain why I haven't been seeing what I thought lean plugs should look like, when conventional wisdom would suggest I've been very lean since day 1!

I hope I haven't cooked her and done damage...

 

[5twins]

These CV carbs are sort of altitude compensating so it usually doesn't play into jetting changes much. Besides, you're not at any to speak of. I can't tell you exactly what jets to use but the usual routine for mods like yours is one to two up on the pilots and two to four sizes up on the mains. Many find their totally stock BS34 equipped bikes run better with one up on the mains so I wouldn't even bother with that. I'd start one up on the pilots and two up on the mains. I'd also get three up on the mains to try. Jetting for mods is experimenting. You will need to test a few sizes and see what works best.

Yes, do check your needle. The U.S. spec BS34s had a fixed one but many of your "Down Under" models got an adjustable one. That would be better for tuning purposes. If you do find the adjustable version in there, you could drop it a step or two right now. That would richen things up and may make it run better until you can get the bigger jets.

 

[sometimesnever]

These CV carbs are sort of altitude compensating so it usually doesn't play into jetting changes much. Besides, you're not at any to speak of. I can't tell you exactly what jets to use but the usual routine for mods like yours is one to two up on the pilots and two to four sizes up on the mains. Many find their totally stock BS34 equipped bikes run better with one up on the mains so I wouldn't even bother with that. I'd start one up on the pilots and two up on the mains. I'd also get three up on the mains to try. Jetting for mods is experimenting. You will need to test a few sizes and see what works best.

Yes, do check your needle. The U.S. spec BS34s had a fixed one but many of your "Down Under" models got an adjustable one. That would be better for tuning purposes. If you do find the adjustable version in there, you could drop it a step or two right now. That would richen things up and may make it run better until you can get the bigger jets.

I know you can't speak in absolutes but this is a great place for me to start!

Thanks a lot for the detailed response man.

Much appreciated.

 

[scabber]

It appears you have stock jets. A 45 or 47 piolet usually works to cure the hanging throttle. 5twins two up on the mains should be close.

 

[sometimesnever]

It appears you have stock jets. A 45 or 47 piolet usually works to cure the hanging throttle. 5twins two up on the mains should be close.

Cheers mate, I ended up ordering 45 and 47 pilots and 137, 140 and 142 mains.

They should arrive this week, so I'll report back with some results when I've had a chance to pull the carbs and change them.

 

[5twins]

Do inspect the needle assemblies to insure they've been put together correctly. There should be a thin metal washer above the needle e-clip and a thicker plastic washer below it. If these have been switched around, the needle will sit too low and be too lean .....

 

[sometimesnever]

Do inspect the needle assemblies to insure they've been put together correctly. There should be a thin metal washer above the needle e-clip and a thicker plastic washer below it. If these have been switched around, the needle will sit too low and be too lean .....

Thanks, I'll definitely be checking whether I have the adjustable needles and making sure they're assembled correctly.

 

[sometimesnever]

These CV carbs are sort of altitude compensating so it usually doesn't play into jetting changes much. Besides, you're not at any to speak of. I can't tell you exactly what jets to use but the usual routine for mods like yours is one to two up on the pilots and two to four sizes up on the mains. Many find their totally stock BS34 equipped bikes run better with one up on the mains so I wouldn't even bother with that. I'd start one up on the pilots and two up on the mains. I'd also get three up on the mains to try. Jetting for mods is experimenting. You will need to test a few sizes and see what works best.

Yes, do check your needle. The U.S. spec BS34s had a fixed one but many of your "Down Under" models got an adjustable one. That would be better for tuning purposes. If you do find the adjustable version in there, you could drop it a step or two right now. That would richen things up and may make it run better until you can get the bigger jets.

Thanks again for your advice guys.


5twins, it turns out I do have the adjustable needle, so would you recommend dropping it a step in addition to the larger main and pilot? I'm planning to start with a 45 pilot and 137.5 main like you suggest and go bigger from there if necessary.

 

[5twins]

I'd leave the needle where you found it to start, only moving it if the larger mains require you to. Many times larger mains will bleed over into the upper midrange making it too rich and causing a stumble under heavy throttle. If that happens then you need to lean the needle setting. Test for this with the larger mains by using full or near full throttle in about the 3.5 to 6K RPM range.

 

[sometimesnever]

I'd leave the needle where you found it to start, only moving it if the larger mains require you to. Many times larger mains will bleed over into the upper midrange making it too rich and causing a stumble under heavy throttle. If that happens then you need to lean the needle setting. Test for this with the larger mains by using full or near full throttle in about the 3.5 to 6K RPM range.

Absolute legend!


Thanks man!