Help with miss please????

[Biggles]

I'm new here - I have a 1980 2FO model with BS34 carbs. The symptoms are a huge miss like a dead cut at 2500-3500 rpm on small throttle openings. I researched here and pulled the carbs apart and cleaned etc. The needles were worn, the pilot jets were out of the BS38's, one diaphragm was leaky, the main jets were dodgy, the mixture screws misssing rubbers and the air filters disintegrated.

So new diaphragms, new Canadian needles and needle jets, new mix screws, set floats, 42.5 pilots, 135 main jets and mikes pod filters. Still the miss! Carbies balanced on my water manometer, mixture set using dead cylinder method.

This bike has the TCI ignition. The coil is off a GTR1000 Kawasaki but is a 2.5 ohm one and a substitute Honda one gives same results.

I am stumped! What next?????


Biggles (from Darwin, Australia)

 

[5twins]

Do you have the rubber plugs over the pilot jets and what # of turns out are your mix screws at?

 

[Biggles]

Hi, thanks for fast response!

Bought new rubber plugs. I started screws at 3 but I think they are at about 1 now from screwing them in using the dead cylinder method. There is a very definite peak rpm and they are very sensitive. It seems lean to me however it seems that this miss is independent of the screws as the old incorrect pilots and worn screws exhibited it, I also put in 45 pilots and it did it, now with the 42.5 pilots and new screws it still does it ........

Biggles

 

[5twins]

Well, it does sound more electrical related than carbs then. Do you have an advance unit or is your ignition factory electronic?

I would put the mix screws back out near 3 turns, maybe 2.5 with your Canadian needle and needle jet. I don't feel the dead cylinder method works all that well on the BS34s, they're too pollution mandated which seems to limit response from the screws. On the older carb sets, going a little past the ideal adjustment spot in either direction produces a noticeable change in motor speed or roughness. You need to go way past the good spot on the BS34s for things to start happening.

 

[retiredgentleman]

You could unplug the rec/regulator unit and run the engine to see if that helps. The small magnet on the rotor could be getting weak.

Perhaps a good idea to measure the resistance of the pick-up coils.....................2 coils both should have around 700 ohms.

 

[Biggles]

Thanks to you both!

I will do one thing at a time - check pick up coils, then run without rectifier/reg, then try mix screws. My carbs are very sensitive to 1/8 turn on mix screws at about 800 rpm. I have factory TCI. Will post a photo when I work out how.....

Cheers!

 

[Biggles]

Ok diagnostics done. Pick up coils 735 and 756 ohms. No change with reg/rectifier disconnected.

I tried winding out the mixture screws and the effect was a slower idle and less engine braking with a marginal effect on the miss. There is a slight improvement though.

Full throttle roll on in 2nd gear results in the miss through 2500 - 3000 some surging or flat spots until about 5000 then pulls to redline reasonable smoothly. Should I start upping the mains and fiddling with needles? Will this have an effect on the 2500 part throttle range as well?

Or should I start looking at the TCI box? I have no readily available substitute box unfortunately....

 

[Biggles]

5twins and retired gent thanks for your advice.....

Nearly sorted now - 137.5 mains and needle raised one notch. Careful fiddling with mixture screws and making sure bike is fully warmed up and the previous miss is just about gone. I was amazed that the carbs should make such a difference!

Only issue now is a slight hesitation that feels like a lean cut at 4000 rpm. It pulls ok up after that at full throttle and max rpm. What is next?


I think I need a signature block.......

1979 Special just with low bars and pods with semi-sorted carbs....

Photo attached of the bride on my last project - donor bike Yamaha Scorpio 225

 

[5twins]

Get rid of the Mikes pods and switch to UNIs. That will probably cure the 4000 RPM issues, maybe the others as well.

 

[Biggles]

Thanks 5twins I will bite the bullet and cough up $100 for a couple of bits of rubber and foam........

Thanks for advice

 

[Biggles]

Ok - Uni filters fitted with improvement in 4000 rpm issue. Still the occasional dead cut at 2500-3000. It's frustratingly intermittent. Readjusting the mixtures caused (I thought) this to disappear. However the next outing after that the miss came back. New plugs no difference.

My question now is what happens in the TCI ignition unit at 2500 - 3000 RPM? Is there a switch in advance? Could that be the time the unit is intermittently failing? I do not have a substitute so I have to commit to trying to repair it or waiting to source a suitable replacement......

Help!!!!!!!

 

[angus67]

100 bucks,eh?
http://www.motorcycle-superstore.co...ID=AvantLink|NA|55963_d3fe7c65&WT.MC_ID=54020
these are sold in pairs.

 

[Biggles]

Got away with $70 Australian....grrrrrr....

Still struggling - TCI looks good with no obvious faults. Coil next but I am lost here.......

 

[Biggles]

Still need advice. I can almost eliminate the miss or at least move it around with carb tuning. So I have ruled out ignition. However I now realize that I have a 1979 2FO engine with later BS34 carbs. Is there some massive conflict here?

I am running 42.5 pilots, 137.5 mains, Canadian needles at 2nd from top, mix screws adjusted for smooth running. This seems reasonable according to the consensus but why is it proving impossible to sort????

 

[Biggles]

More work and something of a result!

I have the Canadian needle at the leanest setting, 137.5 mains, 42.5 pilots, mixture screws at 2.25 turns out. It's a bit fluffy off idle, but no more savage miss at 2500! From 4500 to redline there's some flat spots. This seems to suggest 140 mains but then I can't go leaner with the needles to compensate. Should I go back to the fixed clip arrangement? I don't fancy leaner in the mixture screws as well....

(I have Uni filter Australian model single stage pods and factory special exhausts)

I read every word of 5 twins responses to the saga of evermore Xs's troubles and found it excellent advice.

 

[5twins]

If pods are your only mod, you shouldn't need to go too rich with your re-jetting. Let's confirm a few things. First, how did you oil the UNIs? For the black foam pods, UNI recommends a 50-50 mix of gas and plain motor oil. The normal foam filter oil is too thick and will clog them. Next, 1980 BS34s would have brass floats. If that's what you have, what did you set them at? The spec is 27.3 ± .5mm. This differs from the 22mm spec for the later plastic floats. Last, with all the carb "tuning" the P.O. did, you may want to check the air jets and verify they are correct. Maybe he changed them. They are another way of tuning the idle or pilot circuit. If he put smaller ones in, that may explain why you seem rich there even with stock pilots. A #135 small round main is used as stock for the air jet.

 

[blknoel]

I'm not sure you have this (very) common misconception but carb tuning is done with respect to throttle position, NOT RPM.

There was a guide I read a long time ago, it basically gave this rough guidance:

- if you remove the traditional airboxes on most UJM (universal Japanese motorcycles) and replace them with high flow filter (pods), you up the main jet by 2.5.

- if you replace the stock UJM exhaust silencers with freer flowing exhaust muffler, you up the main by 2.5.

So if you do both, you up the main by 5 (like from 135 to 140, or the next closer size)

Now that guide gives you just the ballpark region so you can fine tune when preventing over-leaning the top end (detonation is detrimental there). Knowing how to read the plugs helps as well. So if I were you I would set everything back to stock jettings and up the main first according to the mods you have, once you have the main in the ballpark, you can fine tune the needle position and the pilot.

PS: 5twins is correct about the float level though, if this is set wrong everything else won't work. So set this correctly first before attempting rejetting.

 

[5twins]

With these CV carbs, you have to take both throttle opening and RPMs into consideration. The slide works and lifts according to engine demand. If you whack the throttle wide open, that doesn't necessarily mean the slide is open all the way. It will only open enough for what the engine can take and use. This makes these carbs easy to over-jet, especially on the mains. Even if your main is too big, the bike may still run halfway decent because the slide isn't lifting all the way and that large main isn't flowing at it's full potential.

 

[Biggles]

Thanks guys! I understand the principle of the CV system and have had success before on Yamaha singles, XJ650 and XJ750, and also Kawasaki GT750 with these sorts of carbs and mods with pods and straight through pipes. However this one is baffling me.

5-twins - the Unis are the horrible single foam thing without the end cap. They are Australian made. The instructions say immerse in the Uni brand oil and squeeze out. I have tried this on other cycles and it is exceptionally messy and feel that it over-oils them.. On these ones I sprayed filter foam oil evenly over them and squeezed etc until they appeared and felt somewhat dry. No oil drips out of them. In any event conventional wisdom says clogged filters would richen the mixture across the spectrum and help my lean midrange.

Brass floats set to 27mm. I will check the air jets however the crappy Unis are so smooth in the throat that I had to cut grooves with the Dremel and glue them on with gasket cement otherwise they pop off!

Blknoel - copied this - I started with 132.5 mains and used full throttle - high rpm runs to increase the main size to pull clean at 5000 - 7000 rpm as suggested in the carbguide document. I couldn't get 140's but 137.5 seems slightly thin and flat at about 6000 and I get stumbles with the 142.5. So when the 140's get here I might try them.

Trouble is I have no more clip positions on the Canadian needles without shimming. Regarding the main air jet, if it feeds the main only, it will have an effect of reducing the effective main jet size wouldn't it? I can grasp the bleed-over principle and if this is so then a larger air jet will lean out the needle and might solve my issue......

Or do I need to toss the Canadian needles and needle jets and go back to the worn fixed clip ones?

 

[blknoel]

When you get the 140 main check top end runs with it, don't touch needle yet. It's important to change only 1 variable at a time and observe the effect.

5twins I agree with what you say about the nature of the CV. However its recommended to leave RPM out of the equation because when you tune carb there must be load on the engine. And RPM when loaded is gear dependant, RPM at 5th gear will be much different to at 2nd gear (even if it has the same throttle position)

If you can do plug chop tests for the top ends (main jet) by all means do, it will be the next most accurate reading apart from a reliable dyno.

One excellent advice I had when doing these is to mark your grip position so you can see which throttle position you are currently at:

Good luck!