Carb tuning advice

[JamesDavies]

Hi, need some advice for getting my bike running right. It’s an xs400se
The main issue right now is the rev hang it has on idle. And occasionally won’t idle will just stall, or when idling and going to throttle it cuts out. On warmer days it will idle but have rev hang. Guessing it’s running lean?

It has all new gaskets, tubes (of which both vac pipes are blocked off or routed to the Fuel tap)
Diaphragms has no tears or holes in them.

The bike has free flow exhausts, and pod filters directly to carbs
The carbs were striped and cleaned and rebuilt stock from what came with the bike. With these it wouldn’t even start, so I upped the main to what came with the rebuilt kit and it got it started but not perfect.

The question is what jet sizes should I go to?

stock jetting was
Main 125
Pilot 42.5

Currently
Main 140
Pilot 42.5

i’m thinking?
Main 140-142.5?
Pilot 45-47.5?

needle height has been set to 4 as per manual and never changed from that.

the idle screw has been set to 4 turns out. The only way I can get it to idle.

 

[xschris]

Ditch the pods you have and get some black foam uni's. The type you have are the worst. Use the stock intake H-pipe if you still have it as it will make things even easier. Floats set to 26mm? Your pilot mix screws should be around 3 turn out or so but with such a modded bike that may also not work. Sync the carbs with a manometer? Your idle screw is set to where the bike needs to idle correctly never a set number of turns. Once you fix the bike to get a good idle you will need to do some plug chops to see if your mains and slide needle need adjusting.

 

[JamesDavies]

Thanks for the reply,

I’ll try a set of foam filters, I’ve also had no luck with any filters fitted.
Can’t use the H piece due to space of where the new battery box now is.
Yup floats set to 26mm
Carbs are synced.
move had them apart again blowing cleaner through all the passages checking for blockages and finding nothing.

my bad I meant the pilot mixture screw, its set to 4 turns out, anything less the bike will cut out once off choke or not start. the idle screw is set from the last time it was running so to let bike idle at 1200rpm

Any way to rectify the rev hang? Some times it bad enough that once the throttle is blipped it’ll just stay at 3000rpm until I turn it off.

 

[xschris]

It’s the intake setup. These type of vacuum carbs run poorly without the stock setup. H-pipe with uni’s is the next best thing.

 

[JamesDavies]

It’s the intake setup. These type of vacuum carbs run poorly without the stock setup. H-pipe with uni’s is the next best thing.

Ok I’ll try get some sort of h pipe configuration going, just curious how others have managed to get rid of the h pipe and had no issues?

 

[JPaganel]

just curious how others have managed to get rid of the h pipe and had no issues?

They mostly haven't.

Some just learned to live with the issues, some abandoned the project. Check out how many "almost complete", "needs a little TLC", and "needs adjustment" projects bikes are in the classifieds. Probably "lost interest", too. And see how many of them have pods.

It is theoretically possible to trial and error this with jets and needles, if you spend enough time and money. There are some recommendations you can find, if you search, but they might not apply to you because of climate where you live.

 

[JamesDavies]

They mostly haven't.

Some just learned to live with the issues, some abandoned the project. Check out how many "almost complete", "needs a little TLC", and "needs adjustment" projects bikes are in the classifieds. Probably "lost interest", too. And see how many of them have pods.

It is theoretically possible to trial and error this with jets and needles, if you spend enough time and money. There are some recommendations you can find, if you search, but they might not apply to you because of climate where you live.

you have a fair point with the projects.
if it’s something as simple as needing the h pipe set up with correct jetting I’ll persist and hopefully find a good medium ground.

thanks for the advice

do your self or anyone know if it’s the tuning issues are specific to H branch piece between the carbs that is key or just the length of piping to the filters that make the big difference?

 

[xschris]

It's all about balancing air pressure to the vacuum operated carbs. The stock exhaust also will help correct this. Going with VM style carbs will fix those issues but other problems come up as those type carbs were never designed for this bike. Remember engineers at yamaha set these up to run for best overall performance with the motor it had. Making changes will harm usually more than it helps. But if it's a look your going for it shouldn't matter. I went down the mod road years ago and tried many intake setups and exhaust systems. They all took some part of the performance range out of them. Some added but usually only top end were I don't ride ever and I am not racing any of these bikes. Soooo

 

[JamesDavies]

It's all about balancing air pressure to the vacuum operated carbs. The stock exhaust also will help correct this. Going with VM style carbs will fix those issues but other problems come up as those type carbs were never designed for this bike. Remember engineers at yamaha set these up to run for best overall performance with the motor it had. Making changes will harm usually more than it helps. But if it's a look your going for it shouldn't matter. I went down the mod road years ago and tried many intake setups and exhaust systems. They all took some part of the performance range out of them. Some added but usually only top end were I don't ride ever and I am not racing any of these bikes. Soooo

I understand that, I’m mostly hoping to just fix the rev hang, if it’s under powered or has flat spots while riding I could live with it, just not the rev hang on idle.
Thank you for the help really appreciate it.

 

[xschris]

A 45 pilot jet might help. Make sure to use real mikuni jets. The aftermarket ones can be wildly off in size and quality. Be sure your pilot mix screws still have the spring, washer and o-ring in place and the o-rings are in good shape.

 

[JamesDavies]

A 45 pilot jet might help. Make sure to use real mikuni jets. The aftermarket ones can be wildly off in size and quality. Be sure your pilot mix screws still have the spring, washer and o-ring in place and the o-rings are in good shape.

Thank you,

just ordered some new jets in a few sizes to try along with some tubing to try replicate the H pipe if I need to or at least a longer intake pipe, might help a better air flow.
I’ll pull the screws out and double check everything is there and good.

 

[Denverdean1]

I'm going through the same issues right now trying to tune. I suspect the Rev hanging is because your too lean with the 42.5 pilot jet. I went up to 45 and it helped. That will atleast get your air/fuel crew to the proper turns out. I made some filter extensions using 2" rubber pipe couplers. Make sure your timing and everything is perfect as well.

 

[JamesDavies]

Update, so issue fixed

I ordered new jets and filters. Fitted the 45 pilots, foam filters. Also gave the carbs a clean out once more to be sure. This did make starting the bike much easier and the reduced the effect of the rev hang.
Upping the jet size didn’t stop the rev hang so I assumed I must have a air leak. Managed to smoke test the intake and found the rubber boots to head were leaking. now these are brand new with new gaskets. However when tested against a straight edge have a slight bow in them. I’ve double up the gaskets and issue fixed.

No rev hang, nice idle.

can now get onto testing the bike while ridden.

Thanks for everyone help and suggestions.

 

[xschris]

Were the intake manifolds from Yamaha? I never had any issues with fitment with them.

 

[JamesDavies]

Were the intake manifolds from Yamaha? I never had any issues with fitment with them.

My local dealers said they weren’t available from Yamaha so I went for eBay or classic Yamaha specialist to get my parts.Couldn’t say where these came from as I bought them quite a while ago.

 

[xschris]

I see a lot of Chinese ones out there for cheap. My guess is those are hit or miss on quality. I know the ones made for the xs650 can be off also. The biggest thing was being able to hold up to fuel without deforming. I have been lucky to find nos ones for cheap over the years. Even got a new set for $30 one time.

 

[JamesDavies]

I’ve seen some nos but with cost of shipping to the uk wasn’t worth the cost… well maybe with the hassle I’ve had. Least I know to check them before fitting in the future