Fine tuning an 850

[psyguy]

So, here's the baseline:

• individual foam filters and V&H 4-1
• rebuilt top end with new o/size pistons/rings and lapped valves, valve clearances in spec
• carbs ultrasonically cleaned with all new o-rings, fitted with DJ Stage 3 kit and set as per DJ instructions incl pilot mixture screws at 4.5 turns out, pilot jet Mikuni #45 (this is a size up from USA models), DJ needles on the 3rd notch from the top, no leaks at the carb boots
• Dyna coils and ignition, coil-relay-mod, new sparkplug leads
• exhaust tested for leaks at the headers - no leaks

The problem:

• pops on decelaration quite a lot, through the exhaust
• occassionally pops through the carbs: At a steady speed only, at small throttle opening like 1/8 to 1/4 turn.

Bike idles well and pulls with no coffing / sputtering in any gear at any rpm and from any throttle position (i.e. no noticeable issues while riding other than the two mentioned above).



I had the ignition timed only statically but I would think it's spot on as per Dyna instructions. Though, I'm wondering if the above problems may have to do with the ignition timing rather than carbs settings.


Any pointers?


Thanks for any help

[49er]

So, here's the baseline:

• individual foam filters and V&H 4-1
• rebuilt top end with new o/size pistons/rings and lapped valves, valve clearances in spec
• carbs ultrasonically cleaned with all new o-rings, fitted with DJ Stage 3 kit and set as per DJ instructions, pilot mixture screws at 4.5 turns out, pilot jet Mikuni #45 (this is a size up from USA models), DJ needles on the 3rd notch from the top, no leaks at the carb boots
• Dyna coils and ignition, coil-relay-mod, new sparkplug leads
• exhaust tested for leaks at the headers - no leaks

The problem:

• pops on decelaration quite a lot, through the exhaust
• occassionally pops through the carbs: At a steady speed only, at small throttle opening like 1/8 to 1/4 turn.

Bike idles well and pulls with no coffing / sputtering in any gear at any rpm and from any throttle position (i.e. no noticeable issues while riding other than the two mentioned above).



I had the ignition timed only statically but I would think it's spot on as per Dyna instructions. Though, I'm wondering if the above problems may have to do with the ignition timing rather than carbs settings.


Any pointers?


Thanks for any help

Psy, make sure that you don't have any air leaks at the header. I know you had to modify that area, so re check for good sealing. That is one cause of backfiring on decel. The other is running too lean a pilot jet/setting. If you have shimmed the needle, you may now be too lean on the pilot to effect a clean transition to the new needle position.

Don't rely on the static settings for your ignition timing. Recheck it with a timing light and also the rpm that full advance occurs. Your timing won't be causing the pops on decel though.

[hikermikem]

I think you need bigger size pilots... 4.5 turns out is a lot, should be around 2.5-3.0 - try bigger pilots & then re-set the mixture screws again.

Wouldn't hurt to check the timing with a light either - then you'll know if that's right or not...

Good luck!

[psyguy]

Forgot to say it's CV carbs.

Psy, make sure that you don't have any air leaks at the header. I know you had to modify that area, so re check for good sealing. That is one cause of backfiring on decel.

Ian, I fitted the header, run the bike for a while, then taken the header off and looked for any leaks (on the gaskets/header mating points) but they seal perfectly. Being the perfectionist I added some exhaust sealing paste nonetheless...

The other is running too lean a pilot jet/setting. If you have shimmed the needle, you may now be too lean on the pilot to effect a clean transition to the new needle position.

Pilot screws are at 4.5 turn out as per DJ instructions. They are seating so very clumsily at that many turns out and undoing them even further looks like they'd just pop out.

Pilot jet is #45 which is already a size larger than what DJ kit is made for (as USA bikes have #42.5). When I take the plugs out after idling, they are rather black, so not lean.
Should I try another size up on the pilot (I think I have a set of 47.5s somewhere)?


Would lifting the needle another notch do anything to popping on decel?
Or, is it only the idle circuit that's involved when the throttle is closed and needle position doesn't play a role?

Questions... questions ...

[49er]

If your plugs are black on the pilot circuit, you're too rich. However, this contradicts what is happening on decel, when the engine pops.

Did you get the black plugs after a pilot circuit plug chop? If not, you need to do this properly to eliminate the possibility of a rich needle influence on the plugs colour.

Also, those individual foam filters will be adding more restriction to air flow than pods. I believe the suggested settings in the DJ kits are for when using pods.

Are you still running the stock carbs (BS32SS)?

[psyguy]

Yep, BS32s. Yep, checked the plugs after idling. I can double check... but I also can start the engine from cold with very little choke and on idle I can smell the fuel more than before - all suggesting added richness I would think.

If your plugs are black on the pilot circuit, you're too rich. However, this contradicts what is happening on decel, when the engine pops.

That's why I'm pulling my hair...

[49er]

Try 40 pilots, setting idle screws to highest rpm method and test again.

[psyguy]

Thanks guys.

Any other thoughts from anyone else?

[hikermikem]

So, here's the baseline:

• individual foam filters and V&H 4-1
• rebuilt top end with new o/size pistons/rings and lapped valves, valve clearances in spec
• carbs ultrasonically cleaned with all new o-rings, fitted with DJ Stage 3 kit and set as per DJ instructions incl pilot mixture screws at 4.5 turns out, pilot jet Mikuni #45 (this is a size up from USA models), DJ needles on the 3rd notch from the top, no leaks at the carb boots
• Dyna coils and ignition, coil-relay-mod, new sparkplug leads
• exhaust tested for leaks at the headers - no leaks

The problem:

• pops on decelaration quite a lot, through the exhaust
• occassionally pops through the carbs: At a steady speed only, at small throttle opening like 1/8 to 1/4 turn.

Bike idles well and pulls with no coffing / sputtering in any gear at any rpm and from any throttle position (i.e. no noticeable issues while riding other than the two mentioned above).



I had the ignition timed only statically but I would think it's spot on as per Dyna instructions. Though, I'm wondering if the above problems may have to do with the ignition timing rather than carbs settings.


Any pointers?


Thanks for any help

When I first read your problems, it sure sounded like a lean condition however, when you then posted your plugs are black, I now "recall" my above advice . I still think your mixture screws are turned out too much & perhaps that's what causing at least some (all?) of your issues.

I would try this first:

- turn each mix screw in one full turn & go for a ride
- adjust from there, perhaps 1/4-1/2 turns depending on your plugs.

Of course, you might want to try adjusting them using the highest RPM method first but in any event, I think your richness has something to do with your screws out too far (especially if your pilots are already bumped up).

Good luck,

mike

[Suzuki_Don]

If you have shimmed the needle, you may now be too lean on the pilot to effect a clean transition to the new needle position.

PSY - excuse me butting in here but I have a similar problem, not popping back but a flat spot between closed throttle and the needle phase.

IAN - I have shimmed my needles and have a flat spot as mentioned between closed throttle and when the needle comes into operation. As you have said the shimmed needles could mean the pilot is too lean now. What is the best way to fix this situation: with the mixture screw or a larger size pilot jet or a smaller size air jet. Does the pilot jet only feed fuel to the mixture screw or does it also feed fuel to the venturi once the mixture screw has no effect any more? In other words does the pilot jet have a part to play between the mixture screw stage and the cut-away stage?

The issue with mine is not a huge one, not a huge flat spot, just a lag when accelerating out of a corner before the needles "come on". On tighter, slow speed corners I do not feel it because the throttle is snapped right open, it's more noticeable on high speed corners (50mph) where more gradual opening of the throttle is happening.

Thanks.

Sorry for hijacking PSY, but I did not get many responses to my thread on this issue.

http://www.thegsresources.com/_forum/showthread.php?t=178151

.

[49er]

PSY - excuse me butting in here but I have a similar problem, not popping back but a flat spot between closed throttle and the needle phase.

IAN - I have shimmed my needles and have a flat spot as mentioned between closed throttle and when the needle comes into operation. As you have said the shimmed needles could mean the pilot is too lean now. What is the best way to fix this situation: with the mixture screw or a larger size pilot jet or a smaller size air jet. Does the pilot jet only feed fuel to the mixture screw or does it also feed fuel to the venturi once the mixture screw has no effect any more? In other words does the pilot jet have a part to play between the mixture screw stage and the cut-away stage?

The issue with mine is not a huge one, not a huge flat spot, just a lag when accelerating out of a corner before the needles "come on". On tighter, slow speed corners I do not feel it because the throttle is snapped right open, it's more noticeable on high speed corners (50mph) where more gradual opening of the throttle is happening.

Thanks.

Sorry for hijacking PSY, but I did not get many responses to my thread on this issue.

http://www.thegsresources.com/_forum/showthread.php?t=178151

.

Don, you PM'd me on this, but I will reply to the group as it may help others.

I read your other post, so here are my views on and a number of points.

Because you are still using your airbox and original carbs, don't change the diaphragm springs or the pilot air jets.

Don't be anal about the Colortune flame results. They are a guide only. I couldn't get my VM's to transition cleanly after setting a blue flame.

Your plug pic indicates a slight richness to me.

You have shimmed the needles. Did you do that to lean it off?

To correctly adjust your pilot mixtures, set all carbs at 3 turns out. Now individually turn each screw in until the engine starts to drop rpm and run roughly. Now open the screw until the engine starts to smooth out. Turn another 1/4 turn out to see if it changes. If it doesn't, you have found the right spot. Repeat for each carb. If you continue opening these screws and the engine doesn't increase rpm, you have started to get too rich. When you end up with this situation, you will experience the same results that you are describing, a flat/fat spot, just off idle position.

All the circuits overlap, so it can take some time to get perfect transitions from 2000 - 10000 rpm You don't need to consider the cutaway on your CV's!

Hope this helps.

[psyguy]

If you continue opening these screws and the engine doesn't increase rpm, you have started to get too rich. When you end up with this situation, you will experience the same results that you are describing, a flat/fat spot, just off idle position.

I have found out just this when attempting to get rid of the popping - I ended up with a fat spot just off idle (but the popping remained...)

[Suzuki_Don]

You have shimmed the needles. Did you do that to lean it off?

Hope this helps.

Ian thanks for the reply. Very helpful indeed.

I shimmed the needles to raise them a tad to richen the mid range. Every part of the range of opening the throttle from idle to full throttle is really, really good except for just off idle, probably about 2000rpm I would say. Although it is also the throttle position more than the revs the motor is at. the mixture screws were set at 2.5 turns out.

I have also just read in the Mikuni Manual that if there is a manifold air leak this will cause a lean condition and part throttle detonation. This is also a symptom that I have. I did not relate it to my carburation problem. I changed from 95 octane to 98 octane and still have detonation on opening the throttle initially. I thought the ignition timing was just a bit advanced. So this is something else for me to investigate. It could be a bit hard to check the tightness of the 6mm carb boot bolts with the carbs in place, but I will give it a go.

[Darin Jordan]

Have the carbs been Carbtuned balanced?? My 850G did this same thing, and had a strange "burble" at 1/8th throttle opening or less, and would pop through the exhaust, occassionally popping back through the carbs even.

After I did a good valve adjustment and balanced the carbs with the Carbtune, it's smooth as silk now.

My setup, for reference: 1980 GS850G, Stock Motor, K&N Pods, Mac 4-1 with competition baffle. Dynojet Stage III, 165 Mains, Needles in 3rd slot, 4.5 turns on Pilots.

[psyguy]

Have the carbs been Carbtuned balanced?? After I did a good valve adjustment and balanced the carbs with the Carbtune, it's smooth as silk now.

My setup, for reference: 1980 GS850G, Stock Motor, K&N Pods, Mac 4-1 with competition baffle. Dynojet Stage III, 165 Mains, Needles in 3rd slot, 4.5 turns on Pilots.

Lucky you Darin!
Yes, the carbs are freshly balanced and valves in spec - and my setup is exactly like yours (except the filters are foam filters...) but it still pops quite a bit.