Topic: Jetting help. Up and down in jets?

[Scott S]

 Bike is a CB550 with stock bore. Fresh rings/hone, pistons coated with heat dispersant and oil shedding coatings. I had the head milled .020" and used a CycleX .040 MLS head gasket and stock base gasket. I used new valves, with the intakes 1.00mm over sized. I had a 5 angle valve job done.
 Cam is a CB650 and I used an adjustable gear to find true TDC and then set it up stock. Head is home ported and polished with matched manifolds, but I think we did a pretty good job (pics to follow). Exhaust is the Passolini/CR replica with baffles, also ceramic coated inside and out.

 After reading Flatlander's build and how he ended up with stock jets on a much more modified motor, I left the jets stock. That's 38 pilot and 100 mains. Clips are stock.

 Here's what I'm finding:

 I have to set the mixture screws at ~3/4 turn from seated or else the idle is too high. I have them maybe the width of the slot on the head past 3/4 turn. Any more than that and the idle is too high and I can't adjust it with the main knob. Even at this setting, the bike is cold natured for a couple of miles, settles into an ~1,100 rpm idle, but if I touch the main idle knob at all it will jump to 2k RPM. All this tells me I'm lean on the pilot.
 There's a slight stumble or bog around 3-4K, but it runs pretty good in the mid range. When I check the plugs after keeping it to 5-6K and just bopping around the neighborhood, they look good.

 BUT....
 Today I took it out for the longest ride yet. It's slow to pull past 6K and seems to bog and break up. All signs of being too rich up top. It does not want to pull to redline. I pulled it to around 7K and killed it and coated home. The plugs were dry, black and sooty.
 This tells me I'm rich up top, even with stock jetting. Here's what a friend that runs a dyno had to say:

 "Think of it this way. If the carbs are the restriction which in this case they probably are, as the now more efficient engine pulls air thru it can only pull so much so then it creates more vacuum in the carb which pulls more fuel. Very common for me to see taking jet out of bikes with head work. Must have done a good job!!"

 I'm trying to schedule some dyno time. Is it possible that I need to go BIGGER on the pilot (I have a 40 and 42 to choose from) and SMALLER on the main (I have 90, 98, 100, 105, 110 and 115 to choose from)?
 Oh...and stock air box and paper filter.

[PeWe]

All that wool can not be good for the flow. That is something for a vehicle check that miust be done for a valid admittance to be used. Remove direct after.

[Scott S]

 They're pretty loud as-is, and definitely free flowing. Flatlander is using the same pipes on a more modified motor and his dyno runs are showing that he needs to go richer up top. I have less cam, less displacement and less compression than he does, though.
 The exhaust would be unbearable without packing!

[Scott S]

 BTW, I tried to get pics of the plugs but my phone is essentially worthless for close ups. Just take my word for it: The plug chop showed rich, and it didn't want to continue pulling to red line. Another sign of being rich.
 Plugs 1 and 4 still had a tiny bit of color on the ceramic, but definitely rich, being sooty and black on the tips. Plugs 2 and 3 were just black and sooty.

 What do you think of trying the #40 on the pilot (or would you go all the way to 42)? And since I don't have a 95, go with the 90 on the main. Mid-range feels pretty good. And when I was keeping it in the neighborhood, around 3rd gear and around 3-4K, the plugs didn't look too bad.

[flatlander]

your dyno friend has a point. it is quite possible that you engine is more efficient to a point that it does need less fuel in the mix, in the high rpm range which is where you get the most benefit from the porting. still a bit surprising as i'm now on 105 mains and need some bigger ones still, i didn't expect there to be such a difference between our setups.

it does sound counter-intuitive that it wants a larger pilot jet but could be that there you don't get anything from the porting yet, but the exhaust makes it want more fuel. i'm not saying that's what it is, just trying to guess.

if you plan to get it on the dyno i would just cut the chase, make the changes you identified now and do the rest there. it's much quicker than with plug chops and a good dyno guy will be able to tell you almost straight away what you need.

i'll be away now for almost a month so there will be a break in my tuning. have now 105 mains, stock 38 pilot jets and the air screws 1.5 out which is a little on the lean side but it pulls well and idles well.

[turboguzzi]

the usual advice.... sort first your main jet at WOT &  high rpm and then work down the rev range from there.


and make sure that your ignition is healthy, many ign issues can feel like carb problems.

[Scott S]

 The 90 mains I have must be for the 350/400 carbs. They're physically too small for the carb bodies. Think going from a 100 to a 98 is even worth it? Or should I just order jets?

 I am a little surprised that it seems rich up top, but that's sure what it felt like and what the plugs looked like after a chop.

[Scott S]

 And I don't know what could be a problem, ignition-wise. I checked battery voltage when I got home and it was 13+, so it's charging. Coils are good. Plugs and caps were new. Pamco ignition and timing was set with a light (albeit a crappy one).
 Tell me about timing advance and retard, though. If the crappy light has my timing off, will that cause problems with higher revs?

[Scott S]

 Another quick observation: I was using an old but clean looking, genuine Honda paper filter. I blew it out prior to installing.
 I don't know how the Emgo filters compare, but I can hold the new Emgo filter up to the light and see light passing through. The Honda filter doesn't. Is it possible that the clean "looking" Honda paper filter is clogged and causing the top end bog?
 I should have tried running the bike with the filter out, but now I have the bowls pulled to change jets. I just don't know which way to go on the mains.

[turboguzzi]

well, wrong ign. timing will cause general havoc, so get that sorted/checked. suggest you use also resistor plug caps and "R" plugs, many think that non resistor caps and plugs give better spark, but it works the other way round.
if rich at top is what you think then go down and try. it's tedious trial and error regretfully, no amount of hand holding over the internet will solve your jetting

[Haybus]

Google search "KZ slide carb tuning". It's a decent set of instructions for general tuning of all the circuits. Don't worry, the tests apply to Honda's too . Also, check the voltage at the coils and the battery. If you have 1V drop at the coils try to fix that before tuning. It will act like carb issues.

Alan

[scottly]

  Is it possible that the clean "looking" Honda paper filter is clogged and causing the top end bog?
 

Yes, a clogged air filter will cause an overly rich mixture, which manifests itself more at higher RPMs. Been there, done that.

[flatlander]

yes, try a fresh air filter.

[PeWe]

well, wrong ign. timing will cause general havoc, so get that sorted/checked. suggest you use also resistor plug caps and "R" plugs, many think that resistor caps and plugs give better spark, but it works the other way round.
if rich at top is what you think then go down and try. it's tedious trial and error regretfully, no amount of hand holding over the internet will solve your jetting

+3
I have also bad experience with R plugs. Used with NGK 5kohm caps, dyna 5 ohm coils. Both Denso Iridium and NGK DR8ES-L in my CB750 felt and looked like a too cold plug.

Ignition MUST be OK before finding correct carb setup.
My ignition was too late (retarded) and jetting felt OK. Plugs indicated that ignition could be more advanced which ended up in more power and need of more fuel. I increased main and fuel screws, twice. Last update not tested yet.

[Scott S]

 Using stock coils that ohm'ed out OK and are providing spark. I trimmed the wires back a couple mm's. I'm using the correct NGK resistor caps; VD05F and XD05F, all new. Plugs were new NGK D7EA.

 It's pouring rain here for the next few days so I won't be able to do any testing until probably the weekend.

[turboguzzi]

one afternoon at a dyno with a good range of main jets could sort out most issues, rain or not, highly recomended.

forgot to say all along, very neat bike!

[scottly]

When I was 15 years old I had a Honda 90cc C200. It began to crap out at higher RPMs, and became progressively worse, lowering the RPM where it happened, and the plug was always black. I took the carb apart countless times, blew the stock paper filter out multiple times, and the symptoms never really went away. I finally threw in the towel and took the bike to the local Honda shop, where they told me it needed new rings. After paying all my hard earned lawn mowing and paper route money, I was instructed to "take it easy for a while so it can break in", so I did as told. After the break-in period was up, and I started letting it rev, the same damn thing started happening!? After several more unsuccessful goes at the carb, I tried a quick test ride without fitting the rubber hose the connected the rear of the carb to the air duct that went to the air filter, as it was a bit of a PITA to fit. Whoohoo! It ran great!! The last try must have fixed the carb!?? I hurriedly refit the rubber hose, strapped on my helmet, and was going to go for the first ride in months with the full 6.5 horsepower at my command! Wrong. It started missing and crapping out 50 feet out of the driveway. I ended up removing the paper from the stock filter, and replacing it with oiled foam, and the problem was solved. Two things I learned; don't trust anyone to work on your bike, and don't trust paper filters. That was a hard earned lesson that I won't forget soon.

[Scott S]

 Well, the weather cleared but my head didn't.  I've been sick as a dog for a week. Missed two days of work.

 I swapped the jets for 40 pilots and 98 mains. I installed a new aftermarket air filter (I just got a used UNI drop in filter in the mail today. I have some genuine UNI foam and I'm working on replacing it now).
 The bike seems to run a little better in the low and mid range, but I can tell it's still not right. I still get a bit of a hanging idle effect at times. I DID manage to get the mixture screws out to about 1.5 turns, which is where they're supposed to be. That seemed to add some smoothness.
 It cranks immediately, requires little or no choke, but is slow to warm up  to where it will idle on it's own. No popping, burbling, snapping, etc., that I can hear anywhere in the rev range. Either from the carbs or the exhaust.

 I've checked for air leaks and found none. I checked to make sure the advancer snaps back and it does. The springs are on the posts like they're supposed to be. I checked timing with a new light and both 1/4 and 2/3 are dead on at advance.
 I'm wondering if I have weak springs and the advance is maybe sticking at times? I also need to try the clear tube method and check fuel level. Those are the only other things I can think of that would cause idle issues.
 And there's still a chance there's some electrical oddity going on.

 Pulled the #4 plug...not after a chop or anything, just after messing around with the bike....and it's still definitely rich. I can see a little color on the porcelain, but it's also black and sooty around the electrode.
 At the very least, I don't feel bad about putting a few more break in miles on it like that. No chance of holing a piston or anything that way!
 I still plan on getting it to the dyno. Maybe that will reveal something, too.

[Scott S]

 I started a new thread with these questions, but it sort of pertains to this post. Please take a look and respond on either thread. Thanks!

http://forums.sohc4.net/index.php/topic,166083.0.html