Topic: Air Fuel Ratio for 836 engine?

[scondon]

   I took my bike in for a Dyno run yesterday. Got it all hooked up and ready to test but a software "glitch" prevented them from getting any readings and the test has been postponed until next Saturday. It actually worked out better for me as I was able to jot down the AFR readings and see how my bike was burning fuel through the rpm range. Now I can make adjustments to the carbs before the Dyno run.

   The tech said that an optimal ratio would be 13 parts air to 1 parts fuel, or 13:1 AFR, coming out of the exhaust. I am making changes to the carbs today in order to get closer to the 13:1 mark but would first like to hear from you guys that have experience with these "hopped up" motors as to whether 13:1 is the optimal ratio and also whether I should be trying to achieve that number throughout the entire throttle range, or more specifically, whether I would want to be slightly "richer" or "leaner" at certain rpm's.

      As always, information is key and I'll try to provide as much as I can.

Bike: '78 CB750F

Engine: 836 bore
           Stage 1 porting
           Megacycle 125/65 cam

Carburetor: stock '78 750F
                Main jet 120
                pilot jet 38
                two washers under each needle (needles raised approx. 1 1/2 clip positions)
                Fuel idle screws 3 1/2 turns out (stock setting 1 3/4 turns)

Air filtration: stock airbox with high flow K&N filter
                  twenty six 3/16" holes drilled in box lower(rear)

Ignition/Coils:  Dyna S, Dyna 3.0(green)
                    Timing spot-on "F" and exactly between advance marks
                    NGK D8EA plugs, .030 gap

Exhaust:  4-1 Kerker w/ medium competition baffle


     With this set-up I am getting these AFR readings:

    Bike in neutral(no load):

RPM:

Idle           21:1

2000         18:1

3000         15:1

4000         13.5:1

     Bike in 4th gear(load):

RPM:

Idle           N/A

2000         16.5/1

3000         14:1

4000         14:1

5000         13.5:1

6000         13:1

7000         14:1

8000         15:1

9000         15:1



        If 13:1 AFR is the goal then this is the plan I have for the carbs today:

Increase Main jet from 120 to 130

Richen the low rpm's by either

a) more turns out on the fuel mixture screws. I'm afraid I may already be maxed out on increasing fuel through the 38 pilot jets.

b) raise the needles


         I know that I will have to live some ranges being either too lean or too rich. Replacing the carbs with CR's, Mikuni, or other is not in my plans for this bike because it is my Daily Rider and because I have another bike in the works that does have CR's and I can't afford two sets. I could use help from you guys with experience in this area.

Note:  I tried to make this as "readable" as possible. Hope the info isn't too jumbled

[mrbreeze]

Hey Sean,Your numbers indicate you are mostly on the lean side so jacking the main jets is good.You are running big bore,cam,etc as compared to mine....stock. We are at the same altitude basically. You have stock airbox although opened up with holes drilled compared to mine with pods. I tried different main jets and needle settings and finally settled at 138.5 mains and clip at the 2nd from top (almost max lean). I tried it in the middle and 2nd from bottom(almost max. rich) with lower mains but where I am now is the best running through all ranges.I would like to see my A/F numbers just out of curiousity.Keep us posted.

[scondon]

  Thanks MrBreeze. I had the same carbs w/pods as you are running, the needle clip was in the same position as yours, and I was running 135 mains. I got to those settings after many plug chops, jet changes, and clip jumping. Try as I might I could not get my set-up to run well through the rpm range and was always sacrificing one area for another. Granted I have always been trying to get a 30 year old,well used carb set with 30 year old internals to function as I want. If I could source new needles with different taper I would probably have continued working on them since the pods really helped the bike JUMP in the power band(6000-9000 rpm).

   In desperation, and with a Dyno run looming,  I yanked the carbs and installed the best set of '78 carbs I have and put together the set-up I currently have on the bike. These carbs have their limits as well but the overall performance through the throttle range was much better and even though I'm lean right now the initial test run through the twisties was far more enjoyable than it has been in recent memory.

   The stock airbox was a difficult choice for me since it only increases the hassle of future jetting changes and there was the worry that I wouldn't get enough air at high rpms(hence the drilling). But with the AFR showing that I'm running leaner and leaner as I climb up to 9500rpm I am encouraged that I have more air available than I had originally anticipated.

[Jerry Rxman Griffin aka MuthaF'er]

Sean,

Stroke of luck that you get 2 runs for the price of 1, er, too bad you have to do it twice! (what is the going rate these days?) How much time do they give you on the dyno? What do you have to do with your carbs to up the slow jets? Are the screws going to vibrate out if you keep turning them out? Will the combination of raising the needles and upping mains 2 sizes get you too rich in the mid range? 5K - 6K you are very close.

Sure will be nice to see how you do and it'd be great to get that jetting on the money. I think if you guys post all these graphs with engine mods and before and after carb mods it will really help all the guys especially when it comes to carb tuning.  

[scondon]

Sean,

Stroke of luck that you get 2 runs for the price of 1, er, too bad you have to do it twice! (what is the going rate these days?) How much time do they give you on the dyno? What do you have to do with your carbs to up the slow jets? Are the screws going to vibrate out if you keep turning them out? Will the combination of raising the needles and upping mains 2 sizes get you too rich in the mid range? 5K - 6K you are very close.

Hey Rxman, stroke of luck indeed. Not only do I get some good knowledge in advance, I also get to check out the hot counter girls for a whole 'nother hour   There are two shops just opened that have Dyno's. One charges $90/hour and has the most imaculate and well thought out shop space and equipment(the fan they placed next to my motor could handle a row of 750's). The other shop charges $65/hour and is a "hole in the wall" and has bikes of every size shape and make overflowing onto the street. The cost of getting a "footprint" of my bike run on the Dyno is 1 hour flat rate. Tuning the carbs and fiddling with the ignition while on the dyno has been quoted to me at a starting rate of $600 (from the $65/hr guy).

    Screws are now at 4 turns out and don't appear to be anywhere near loose enough to begin vibrating out. In order to up the slow jet size I would have to pull them out, drill them, and press them back in. I have been unable to follow the procedure for removing these pressed in jets, outlined by others on this forum, without mangling them. Mine are stuck in pretty good.

    As far as upping the mains and raising the needles go, that is what I was hoping to get some help with here. From the above AFR info I get that I am "perfect" at 6000rpm and increasingly lean in both directions from that point. Bike is pretty responsive though. I went ahead and put in 130 mains and opened the idle circuit 1/2 turn and took it for a ride. I did nothing more to the needles, just left them where they were. Here is what I noticed:

    No change in low rpm riding(1/8 throttle?).

    Only change in mid rpm's(1/4 to 1/2 throttle?) is that the engine does not slow the bike as fast when I let off the throttle. I prefer the engine to slow more rapidly.

    Holding the throttle wide open on an incline while in 4th and 5th gear(above 5000rpm):

   With the 120's there was a definite lag(more than a few seconds) before the engine "caught up" and began accelerating

   With the 130's there is NO response, neither acceleration nor deceleration, unless I have the rpm's at or above 7000.

   What I gather from this is that my jump from 120's to 130's was far too much. I have some 125's to put in and it's possible that 122.5 mains might be the best compromise. It is a real pain to keep removing the carbs and airbox in order to unscrew the float bowls and change jets and am hoping that anyone with more experience can help me know if I'm guessing correctly. The needles can be moved with the carbs on the bike.

[Jerry Rxman Griffin aka MuthaF'er]

I hate taking carbs off too. With my older style carbs I can pop the bowl off, hold the float up, and using the tool from my kit with a round handle and a U shaped opening that fits the main jet holder tube, remove the tube and main jet together. They didn't identify what tools are used for what in the owners manual but it must've been intended for this. Replace the main and put the assembly back in. If it were me, I'd put the 125's in, take a plug reading, and hope the needles are OK. I have to remove the carbs for the needles. On yours too? Sounds like the 130's in conjunction with your needle settings are making it a little rich before 7000rpm. Did you read your plugs at mid range and top end with the 130's?

OR perhaps your 130's are ok but the increase in the overlap area in mid range is making it too rich and you need to lower your needles.

SO, not that I'm much of a carb tuner, I'd read the plugs at top end first, get them set, and if running clean I'd probably pull the @#$%^& carbs and adjust my needle a little leaner.

Did I say I hate carb tuning? Damn needles.

[scondon]

If it were me, I'd put the 125's in, take a plug reading, and hope the needles are OK.Sounds like the 130's in conjunction with your needle settings are making it a little rich before 7000rpm. Did you read your plugs at mid range and top end with the 130's?

OR perhaps your 130's are ok but the increase in the overlap area in mid range is making it too rich and you need to lower your needles.

 Thanks Rxman. I did not take any plug readings during todays ride. I did pull plugs with the 120's and they were white through idle and mid. with only a hint of tan showing on #2 at mid range. I did not have enough road to do WOT chops.

  I'm in full agreement that the 130's muddied up my mid range which was pretty sweet before. Pretty sure I'm attempting the impossible in trying to richen the 1/8 and WOT without throwing my mid completely off. A comprimise is the best I can hope for.

  I am raising the needles in the attempt to compensate for how very lean I am at 1/8 throttle. When I first installed thes carbs I was so lean that I had pre-ignition at 1/8 throttle and again at 3/4 throttle. I installed the two washers under each needle and the 3/4 throttle "rattle" disappeared and the bike became much more responsive. The 1/8 throttle "rattle" diminished in severity but is still present when I start up from a complete stop or if I try to accelerate from 3000rpm or lower in 4th or 5th gear. I can prevent pre-ignition by just sloooowly rolling on the throttle and it only becomes noticable after 20 minutes of High rpm riding. For the first 20 minutes of riding it is not present at all.

I have to remove the carbs for the needles. On yours too?

   The 77-78 carbs are very different from the earlier style. You do not have to remove them from the rack in order to get the needles out. But you do have to remove them in order to get the bowls off and change jets. Completely the opposite than the '76 and under carbs.

Did I say I hate carb tuning? Damn needles.

Me too. Damn jets.

[mrbreeze]

You are correct Sean. The earlier(like mine...75K),You have to pull the carbs to move the needle position but you can change mains with carbs installed. Thats what really pissed me off and I'm glad I finally got it sorted out.If it was the needles I wanted to move....it cost me time. If it was the mains...it cost me money for jets. You have the new and improved(?) carbs so you have a whole different ballgame to play.I have faith in ya bro........you will get it ......talk at ya later.

[Jerry Rxman Griffin aka MuthaF'er]

Get it as close as you can then the dyno will get you where you need to be. Can you describe your pre-ignition rattle? Wish I could hear it in person so I'd know what to listen for. Since you can adjust your needles without removing the carbs why not remove 1 of the washers and try that with the 130's. No luck there, then go with the 125's with 1 washer then 2. Trial and error. Mine would probably be on the money if it weren't for removing carbs. Close was always good for horseshoes, hand grenades, and MY carbs.

[scondon]

Can you describe your pre-ignition rattle?

    Honestly, to me it sounds like the primary chain is rattling. Like all the tension is taken up on one side and the loose side is bucking and vibrating about. I would have just assumed that was what it was but I took it by my buddies shop and asked him to ride it and tell me what he thought. "Definitely pre-ignition"

    I've heard it described as having sand in your motor. Yeah, it kinda sounds like I have sand in my motor.

[eldar]

Have you retarded the timing a bit? That is usually required from what I have heard unless you got premium going, then it may still be needed depending on your cam.

[MRieck]

That's why the Dyna2000 works well Sean....first you have an accurate ignition map and second....an truley adjustable non mechaminical one. 13/1 is nice bit in honesty I've found carbed bikes a little better at about 12.5/1. Fuel injected bike are different. Now what are you going to use that igniton, carbs and head on??

[scondon]

That's why the Dyna2000 works well Sean....first you have an accurate ignition map and second....an truley adjustable non mechaminical one. 13/1 is nice bit in honesty I've found carbed bikes a little better at about 12.5/1. Fuel injected bike are different. Now what are you going to use that igniton, carbs and head on??

   All that stuff is going on the "Von Rieck Express"(rolling frame in my avatar)    Just waitin' on some rods and for APE to finish my crank, then the motor's going together and into the bike. Terry's gonna ship me some nice "bling" to finish it off and I hope to be burning fuel in it by Christmas.

    That Dyna 2000 ignition is sitting on the shelf........mocking me. "Use me now, use me now....you fool.....you FOOL!"

[scondon]

Get it as close as you can then the dyno will get you where you need to be. Can you describe your pre-ignition rattle? Wish I could hear it in person so I'd know what to listen for. Since you can adjust your needles without removing the carbs why not remove 1 of the washers and try that with the 130's. No luck there, then go with the 125's with 1 washer then 2. Trial and error. Mine would probably be on the money if it weren't for removing carbs. Close was always good for horseshoes, hand grenades, and MY carbs.

   Rxman, you may not consider yourself much of a carb tuner but your above advice is pretty spot on I opted to make just one change to the carbs, either jet or needle, before my dyno run in order to get an accurate reading on how that change would affect the rpm range. I went with changing the mains from 120 to 125 and left the needles unchanged. Surprising to me was how much the midrange was changed by one jet size. Here's what happened:

 4500-5500rpm dropped to 10:1 AFR

 Ratio makes a steady climb after 5500 and hits the 13:1 mark around 7500rpm(up from 6000rpm)

 It continues to rise and spikes at 14:1 at 9000rpm and drops back to to 13.5:1 at 9800rpm


So......Needles down, by one or two washers, and main jets at 125 or 128 is where I'll probably end up. 130's made the midrange too damn sluggish and I don't do much riding above 8500rpm.


   I've got the dyno results in my hands but won't be able to scan them for posting until I get to work Monday

[Jerry Rxman Griffin aka MuthaF'er]

Thanks for the kind words. I'm still learning. You can't usually make 2 changes at a time and determine what did what. I'm drooling just waiting to see the scan. Maybe I'll get mine right on one day.

Are you going to tease us and not tell us the HP figures?

[eurban]

In all likely hood, if you take your idle mix screws(fuel control screws in the 77/78s) more than 3 turns out, they will not function properly.  Switching to larger pilot jets would be the proper way to handle this although on the late carbs your options seem to be either drilling or buying a set of screw in types that are a very close match.  . . .All this said I suspect that you have some issues with your idle circuit if you are needing that many turns out. I'm running an 836 in a 78K motor with pods (4-4 exhaust tho) with a similar cam etc and have settled on about 1 1/8 turns out.  Are your float levels equal and correct? You can put clear plastic tubing on the carb's brass overflow nub, hold the tube up along the carb body and open up the drain screw a bit.  (this won't work on the earlier carbs!) Fuel will flow out the nub and up to the actual level of fuel in the bowl.  Compare all carbs. Did you verify that your pressed in idle jets are clear.  In general, improper fuel delivery, low float level or plugged idle jets etc will have a significant effect on the mix at wide range of throttle settings and RPMs.  It woud be a shame if you are trying to tune a set of carbs that aren't really working correctly.  Good luck 

[scondon]

In all likely hood, if you take your idle mix screws(fuel control screws in the 77/78s) more than 3 turns out, they will not function properly. Switching to larger pilot jets would be the proper way to handle this although on the late carbs your options seem to be either drilling or buying a set of screw in types that are a very close match. . . .All this said I suspect that you have some issues with your idle circuit if you are needing that many turns out. I'm running an 836 in a 78K motor with pods (4-4 exhaust tho) with a similar cam etc and have settled on about 1 1/8 turns out. Are your float levels equal and correct? Did you verify that your pressed in idle jets are clear. In general, improper fuel delivery, low float level or plugged idle jets etc will have a significant effect on the mix at wide range of throttle settings and RPMs. It woud be a shame if you are trying to tune a set of carbs that aren't really working correctly. Good luck

   Thanks for the input, eurban   My idle screws are turned out so far simply because of the AFR readings I got. They showed idle to be very lean. If I just go by what "feels" best then 2 1/2 turns is PLENTY. Much lower than that and I get a lot of popping from the exhaust when I close the throttle at speed.

    Carbs have been "dipped" and sonicly cleaned. Float levels correct and idle jets clear. I actually have two sets of these carbs, both have been cleaned and properly set, and both get identical results from the idle circuit so I am assuming that they are in proper working order. I believe you are correct in stating that going to larger size on the idle jet is the proper way to handle this.

     

Thanks for the kind words. I'm still learning. You can't usually make 2 changes at a time and determine what did what. I'm drooling just waiting to see the scan. Maybe I'll get mine right on one day.

Are you going to tease us and not tell us the HP figures?

   Just pulled one washer from the needles and am going for a test spin. I figured that I'd wait until I got the Dyno sheet scanned and could post ALL the relative data, but teasing isn't nice is it?

I got almost exactly what Sparty got:

Max Power-  74.9@8756rpm/ 103.7 mph

Max Torque-  50.1@7341rpm/ 86.0 mph

Note: Max torque also coincides with where the AFR is about 12.6(Thanks Mike).

[MRieck]

The pleasure is all mine Sean. Your rods are at the shop. Heaviest was 3.5 grams from the lightest....not bad. Are those F2 rod???

[scondon]

The pleasure is all mine Sean. Your rods are at the shop. Heaviest was 3.5 grams from the lightest....not bad. Are those F2 rod???

    Those rods came out of engine #31xxx and the top end was definitely stock F2. Rattle can paint over everything but I'm sure the base is factory black. Are the bolts not as big as you remember?

[eurban]

Perhaps you should consider whether or not the AF readings from the dyno are accurate particularly at idle.  Was the bike properly up to temp when these readings were taken?  Is the meter accurate at the gas flow rate at idle?  Is the meter accurate at all?  The bikes performance (both to your seat of the pants and the HP/torque #s on the Dyno) is the real guage of whether the mix is right.  You say 2 1/2 turns out give you good seat of the pants results?  Does this mean that idle speed and performance drop once you go beyond 2 1/2 turns? If so I would say the dyno's AF meter is not accurate at least at idle.

[scondon]

Perhaps you should consider whether or not the AF readings from the dyno are accurate particularly at idle. Was the bike properly up to temp when these readings were taken? Is the meter accurate at the gas flow rate at idle? Is the meter accurate at all? The bikes performance (both to your seat of the pants and the HP/torque #s on the Dyno) is the real guage of whether the mix is right. You say 2 1/2 turns out give you good seat of the pants results? Does this mean that idle speed and performance drop once you go beyond 2 1/2 turns? If so I would say the dyno's AF meter is not accurate at least at idle.

    I removed one washer from each needle last night and the midrange(4000-6000rpm) was much improved. However I am now starved for gas on at least one cylinder at idle. I believe that taking the slides in and out a couple times has unsynched my carbs but I won't take that for granted so today I'll recheck everything.

    I'm going to resynch the carbs and reset the idle screws to 2 1/4 turns(initially). Also, I'm going to attach some clear line to the float drains and see what my actual fuel level is and if any floats need adjusting rather than just assume that all initial settings were correct.

   The lean AFR readings correspond with my plug readings(white), with my engine temp(hot), and with the popping from exhaust and pre-ignition rattle. A nights sleep on the matter,and after reading your suggestions, has me thinking of the possibility that one or even two of my bowls may have low fuel level even though float heights were measured initially.

   In answer to your Q's:

   The bike was fully up to temp on the dyno run. I believe the AFR meter is accurate and am checking to see if one, possibly two, carbs are throwing the readings so lean. 2 1/2 turns gives good "seat of the pants" results, turning futher out makes the idle "chug" a bit but also lessens the "rattle". I have not been able to increase, or decrease, the idle speed using idle mixture screws between 2-4 turns out.

    I think your settings of 1 1/8 turns on the idle screws is exceptional. 77-78 K and F carbs have different part #'s and I'm curious as to whether there is any difference in our idle circuits(jet size or other). I haven't seen a 77-78 750F that doesn't burn lean at the factory setting of 1 3/4 turns.
 

   

[old750]

Hey just a quick question. Isn't a 20:1 mixture really really bad even at idle?

[scondon]

Hey just a quick question. Isn't a 20:1 mixture really really bad even at idle?

  Had to go back and see where ya got that # from. OH! it's right there in my first post     Yeah, that would be really,really bad. I took that reading manually with the AFR meter jumping all about and the bike out of gear and not fully warm. Yesterdays Dyno run only measured between 3200 and 9800rpm but I took a peak at the AFR on the screen and the readings were closer to 17-18:1 at idle. Dropped to 10:1 by 4000rpm.

   Perhaps I should remove the first four "no load" readings from my first post as they probably aren't much good.

[eurban]

When I put together my HiPo project bike for the first time I assumed that I would need to richen up all the carb settings to accomodate the various mods (836 kit, Pods {for looks}, performance cam, Dyna III and Dyna 3ohm coils etc)I am using essentially an NOS 78F carb set btw.  All rubber carb parts were replaced. I actually drilled a set of the stock #35 idle jets to an aproximate #40 size, and I raised the needle setting one step richer (I ordered a set of adjustable needles specced for the 77 models from David Silver Spares which come with a new needle jet as well) used a 120 main and set the idle mix screws for 1 1/2 turns out.  Carbs were bench synched first and then synched while running.   The accelerator pump was visually confirmed working and I used new intake rubbers. I also installed an oil cooler with a thermostat and use 93 octane gas with the 10:1 cr of the Wiesco 836 . .. . . Well the bike would start nicely with minimal choking but was very lumpy at idle once warmed up.  The idle mix screws showed increased performance each time turned them in (clockwise) After a good bit of experimentaion I am now using the original size #35 idle jets with1 1/8 aprox turns out and have remarkably smooth running bike that as best as I can tell is well carbureted. Idle mix screws can now be adjusted for max idle speed at a given throttle setting (turning the screws should make a difference!)  I also "tested" the performance with various mix screw settings and the max idle screw setting seemed to work the best.. . . . .I honestly think that even with the mods that you are running that the stock idle jets should be big enough.   If you are indeed running a set of 38s (where did they come from?) then you should not need to go any larger.   If something is amiss in your carbs, fix it before you make any more attempts at tuning.  Make certain that you don't have an air leak (could explain many of your symptoms).  Confirm float levels and synchronization.   That accel pump is critical so make sure it works!  I would suggest an oil cooler with the 836 kit as it pretty much eliminated the very faint pinging I was getting during hard acceleration on hot days.  Retarding the timing a hair is certainly an option as well.  . . .

[scondon]

    Hey eurban, Thanks for taking the time to write that up   Seems that we are running near the exact same set-up with the only difference being my drilled airbox. I just assumed the pilots were 38, don't remember where I got that info from. And I think your correct that the stock pilot jets should be enough regardless of the engine enhancements. I initially ran '76 carbs and jumping up the pilots on those did not help at all.

    Thanks to your persistence I am starting from scratch and throwing out all assumptions that things are set and cleaned properly in the carbs. I did some tests while synching the carbs today. I put a clear line on the float drains to check actual fuel levels in each carb. 1,3,4 carbs have the pilots immersed in 1/4" of fuel but the level in #2 carb is almost 1/8" inch lower even though the floats measured the same when assembling the carbs. All 4 pipes get equally hot when starting the bike and letting it idle(1100rpm).

     I'm not going to pull the carbs today, but I'll practice pulling the pilot jets on my parts carbs. Although dipped and sonicly cleaned I am not going to assume that the passages are 100% clear on the carbs I'm running.

     The accel pump shoots fuel pretty high when I took #2 float bowl off. When I do get these carbs off again I'll check that it flows to each carb equally.

     I got the deraile oil cooler and run 92 octane.

    Mixture screws are at 2 1/4 right now and am taking the adjusting tool with me for a ride right now.

   Thanks again eurban. Really do appreciate your help