Engine sputtering around 5K RPM under LOAD only

[squevill]

Hi everyone,

I am getting a recently acquired 79 back into shape. Now that the 2nd gear fix has been applied, and the clutch is running reasonably well, I am turning my attention to how well (or not so well) the engine runs. The carbs have been synched, the fuel-air mixture adjusted with a colortune (what a nice piece of kit that is to have!), the advanced mechanism has been cleaned and wires checked, one wire had to be fixed. The coils are original and all measure ok, installed pod filters, cleaned and re-jetted carbs to match pod setup, new manual petcocks (no octopus), but....

The engine runs really well when static, it's smooth all the way up to the red line. The problem is when I ride the bike, the engine stutters at around 5k RPM no matter what gear I am in.

Looking suggestions for what could be causing this problem while the engine is under load but not when static?

Another thing to mention, the choke is not working right, I have to hold some throttle with no choke to warm it up... just throwing that in the description in case it might be relevant to the problem.

I do have inline fuel filters but same problem happens without them as well.

[Ken Talbot]

The problem with the enricher circuit suggests to me that that circuit is not clean, and if it is not clean, chances are good that other circuits may be in the same condition. I don't see anything in your note indicating a complete strip down of the carbs for a thorough cleaning. Until that has been done, all bets are off.

[HalfCentury]

......and you have pod air filters. Which brand of pods?

What are your jet sizes? If you don't know, then you must write them down when you perform a thorough carb teardown and cleaning. There are members here that have pods and can offer suggestion.

Lastly, did you check carb float heights?

[BA80]

......and you have pod air filters. Which brand of pods?

Very good point.

[TopCatGr58]

As the others have noted, you have Pod filters. If they are the Generic EMGO brand, they have a prominent mounting lip around the inside edge and when they are mounted, they obstruct the airflow into the ports on the inlet bell of the carbs. A quick test is to just take the pods off and run the bike and see if you still have the 5K stumble, but I suspect you won't.

Next, review this tech tip on how to mount pods without causing the airflow restriction.

http://www.xs11.com/forum/showthread.php?t=8269

T.C.

[redonkulos366]

are you using jets from a carb rebuild kit, or are they stock Mikuni?

[spectra]

I agree with Rednonk above^^^^^^ what jets are you using?

I rebuilt the carbs in mine with aftermarket stuff and it ran like crap.......I swapped out to Mikuni stuff and all is better. I also run pods on mine they are Uni Filters though.

[squevill]

Thanks for all the great suggestions,

The carbs have been cleaned this summer, I just pulled them out again and re-cleaned the enricher circuit but that made no difference to the choke operation. Unless it is related, I won't worry about that for now... want to get rid of the hesitation problem first.

The pods are cheap generic pods from Amazon (seller is E-ting), the main jets came with a "kit" for pods, outside carbs are running 147.5 and inside carbs are running 150. It also came with a couple of 152.5 that I could install in the inside cards and put the 150s to the outside cards... what do you all think?

I drilled the slides as per the kit instructions and raise the needles to the second position... perhaps I need to move the needle up some more?

The funny thing is that once I get past 5.5k RPM, the bike takes off like a bat out of hell!!! the hesitation is from around 4.5k to 5.5k RPM.

Next I am going to try running without the pods and wait for some advice from the collective wisdom of the forum.

[BA80]

Thanks for all the great suggestions,

The carbs have been cleaned this summer, I just pulled them out again and re-cleaned the enricher circuit but that made no difference to the choke operation. Unless it is related, I won't worry about that for now... want to get rid of the hesitation problem first.

The pods are cheap generic pods from Amazon (seller is E-ting), the main jets came with a "kit" for pods, outside carbs are running 147.5 and inside carbs are running 150. It also came with a couple of 152.5 that I could install in the inside cards and put the 150s to the outside cards... what do you all think?

I drilled the slides as per the kit instructions and raise the needles to the second position... perhaps I need to move the needle up some more?

The funny thing is that once I get past 5.5k RPM, the bike takes off like a bat out of hell!!! the hesitation is from around 4.5k to 5.5k RPM.

Next I am going to try running without the pods and wait for some advice from the collective wisdom of the forum.

Some here will disagree but don't flog a bike like I do.

Every 78 - 79 engine I've had (including the one on my SG now) has had that aggravating stumble somewhere between 3 and 4K. I've tuned trying to rid the bike of it but it always remains.

The best I've had an early engine run is with the MAC headers wide open. It's non existent then. The bike will try to spin out from under me. The noise will make your ears bleed though.

[TopCatGr58]

THis is much better information. I thought you were hitting a wall and not able to go above ~5k rpms, and that is symptomatic of the Pods. THey may still be adding to the air flow and throttle response? I didn't see anywhere as to what your exhaust setup is? Stock, or aftermarket, 4/2 or 4/1???

Okay, you must have gotten a DYNA JET kit, they are the ones that recommend drilling the vac. slide to allow it to rise or respond quicker.

As for the jet sizes, they are NOT Mikuni, so it's hard to compare them and their sizing vs. Mikuni. SO....you will need to do some throttle chopping checking to see what the plugs look like to first ensure that the mains are big enough, but you say you can get to redline once you get past the stumble zone, so they may be right, but coloring them would still be good to help eliminate that.

Motoman has decades of carb tuning, and could possibly provide some more guidance about how to reduce or eliminate the 5k stumble. From my understanding, the stumble is a lack of fuel/air needed at that particular rpm zone?? Or...is it that it's getting too much fuel/air and once it catches up and gets/builds to the higher rpm's then the engine can handle it??

Raising the slide needle gets it to supplying fuel/air quicker/sooner, and possibly TOO soon for that rpm zone? When making tuning adjustments "they" recommend changing just 1 thing at a time and retesting to see what kind of change it made.

SO...I would throttle chop check at the ~6k rpm zone and see what the plugs look like, if decent color THEN I would try lowering the slide needle a notch and see how it responds. If the stumble is less severe, then try adjusting another notch, etc.!

Good Luck.
T.C.

[motoman]

Carb float height would be my first suggestion to check that it IS correct, and that all floats are identical height...very important. Other than that, I have to say I've never had experience working with the earlier carbs, specially with the mods to them that have been done. The cross-over passage DOES make them easier to tune in order to run correctly...not as finiky as the later carbs.
The enricher circuit issue is due to the either one or both of detent balls/springs missing. Therefore, enricher pivot shaft won't stay in full on or partial position until manually pushed in.
Other than that, sorry I can't be of more help...at this point anyways. I just refrain from making suggestions unless I've personally had some experience myself.

[motoman]

Also, another thing to check and eliminate as a possible issue is the mechanical advance assembly. That assembly under the left ignition cover needs to be floppy loose and free movement. If it isn't, it needs to be removed, cleaned and re-lubed. Another area to eliminate as a possibility is the pilot circuit jets. They need to be INTERNALLY clean, and usually takes a magnifying glass to inspect internally. And mostly important, the main jets and pilot jets HAVE to be Genuine Mikuni. If they are RD or K&L jets, this WILL be an issue when running and tuning. Pilot jet designations are located on side of jets and WILL require a magnifying glass to see the numbering on them AND the curly Mikuni designation. Of course, the main jet numbering is on top of jets, but also MUST have the Mikuni curly designation beside the numbering. Problem with aftermarket jets is Mikuni still has copyrights in effect, and aftermarket jets do NOT use same flow testing. Also the aftermarket jets CANNOT be copied exactly internally. Flow differences are also dramatic compared to Genuine Mikuni jets. For example, since I've checked size differnces, 110 hole size of aftermarket main jets are equilalent to 115-117.5 hole sizes of Genuine Mikuni main jets.

[motoman]

BTW, reporting back here as to the main/pilot jet brands used would be helpful to others here, and helpful to direction of diagnoses.

[red bandit]

I don't remember anyone using mains any bigger than 145 in a 79. What exhaust do you have?

[BA80]

I was not aware of a Dynajet kit available for 78 - 79 carbs.