Topic: Jetting Questions... *new issues

[Ravie]

Posted this in my build but thought it deserved it's own thread where it can be seen.

I know that jetting isn't exactly a science...more trial and error...but I wanted to get some advice from y'all on my setup.

Here's what I've got:

- Airbox modified the way Mark describes in his book - opened up intakes, holes for the carb breather tubes.
- Polished and ported head
- Crusinimage 836 kit
- Probably slightly higher than stock compression due to machining both the head and cylinder block flat
- Carpy's Sidewinder 4-1 Exhaust  I had planned on buying this but actually have a more restrictive set on it now than this. They're 2-1 pies on 1-2 and 3-4 and I've drilled out the baffle holes fairly large to reduce back pressure)

When I rebuilt the carbs I left the 105/40's in it and if I remember correctly I've got the needle set second from bottom.

I feel as though there is a good chance I'm going to need to go bigger on the main jets. I purchased some 110's but left them out for now. It's got Dyna coils, stock points.

I would love some thoughts as to what is the probable end-point here.

Thanks!

[Garage_guy_chris]

I have a very similar setup. If i were you i would stop running it till you get it up-jetted I would think its very lean with 110's Your profile says Topeka Kansas which is about 1000ft above sea level so your not far off me in that regard either

My setup

-airbox modified openings cut 3/4" taller with K&N drop in filter
-Ported / De-shrouded head with stainless valves
-Crusinimage 836cc kit
-Small bump in compression due to 836cc increase
-Ripple Rock Racers Hindle 4-2-1
-Dyna coils on stock points
-Live / Riding @ 250-750ft above sea level

My jetting is still a work in progress but its rideable and not too far off

Pilots #42
Mains #130
Needles #27201 on the 4th clip position from top
Air screw 1.0 turn out


Pilots: seems like not to many people need to up the pilots but it seems to help mine

Mains: I have tried #128 and #130's with similar results the #130's seem

If i were you i would order up some pilots in #40, #42, #45 and mains in #115, #120, #125, #130, #135, #140 and if you want get half sizes as well like #118, #123,#128, #133, #138 and a bunch of spark plugs for plug chops and foul outs

Start messing around. you will probably find yours isn't too far off my bike

To buy the jets try:

the mains are type (99101-393) they aren't so hard to find
 
the pilots are oem # 99124-076-0400 for a #40
                              # 99124-076-0420 for a #42
                              # 99124-076-0450 for a #45
They are a fair bit harder to find

http://www.jetsrus.com/a_jet_kit_street/honda_750_CB750_1973.html
-expensive but have pilots and mains, a 1 stop shop

http://www.vintagecb750.com/products/6/fuel-system
-have pilot jets for $11.95 for 4 and some main jet kits but they arent well setup for the sizes you need

https://www.z1enterprises.com/store/category/honda/carb-fuel/jets
-Carry the mains for $1.75 each (99101-393) type jets

[Ravie]

Sweeeeet. Thanks!

[70CB750]

Moving needle up or down is about 1.5 jet size change.  For Red I started with 120 and when I run out of needle positions I went to 125 and fine tuned it with the needle clip.

But she is stock 78F

[Ravie]

Ordered 42 pilots, a couple more sets of plugs for clean chops and 115, 120 and 125 mains. That should get me started.

[Ravie]

With the 42 Slows in...When I am idling (and especially when cold, but choke off) It's getting quite a bit of gas...running rich enough to smoke. When I rev I get a nice cloud of gas. I am assuming I should change those back to the original 40's. Affirmative?  Seems like a silly question I'm sure but I like to confirm things with more experienced folk

Haven't done a plug chop yet as I am (trying to be) patiently awaiting a carb tune gauge, So I am unsure of the 110 mains in it now. Time will tell.

[Ravie]

bumping...  anyone? 

[Davidov]

I would make that decision based on the plug condition.

With the 42 Slows in...When I am idling (and especially when cold, but choke off) It's getting quite a bit of gas...running rich enough to smoke. When I rev I get a nice cloud of gas. I am assuming I should change those back to the original 40's. Affirmative?  Seems like a silly question I'm sure but I like to confirm things with more experienced folk

Haven't done a plug chop yet as I am (trying to be) patiently awaiting a carb tune gauge, So I am unsure of the 110 mains in it now. Time will tell.

[flybox1]

I would make that decision based on the plug condition.

With the 42 Slows in...When I am idling (and especially when cold, but choke off) It's getting quite a bit of gas...running rich enough to smoke. When I rev I get a nice cloud of gas. I am assuming I should change those back to the original 40's. Affirmative?  Seems like a silly question I'm sure but I like to confirm things with more experienced folk

Haven't done a plug chop yet as I am (trying to be) patiently awaiting a carb tune gauge, So I am unsure of the 110 mains in it now. Time will tell.

+1
carb tune wont tell you anything.
hot engine, new plugs, 5 min idle under a fan.
if your plug tips are any darker than Jessica Alba, you're too rich.

[Ravie]

My point is not that carb tune will fix it. My point is that I don't want to be driving an uneven bike around trying to tune the fuel mix.

Will do that. The ones in it now are black as night. I've got several new sets ready for chopping. I did some more research and found that if anything usually big bore guys need less slow jet rather than more. I put the 40's back in and rechecked my float levels. Will report back once I play with it more.

[seanbarney41]

If you properly bench synced your carbs (and it sounds like you did by how well it idles) a vacuum sync will have very little effect on anything but the smoothness of the idle.

[Ravie]

Yeah it was pretty close. Got it synced tonight. Gonna do a short ride in a bit. I put the 40 slows back in and it settled down the idle consumption quite well.

[Ravie]

Okay, for those following along here are tonight's follies and some questions:

- Synced the carbs
- Reinstalled tank and went for a ride.
- Forgot to turn Petcock on (yes, I'm a dolt.)
- When it ran out of gas, I turned the petcock on, waited a minute, and restarted the bike. It would NOT stay running. Tried several times. Was obviously not running on all four.
- Pushed it for half a mile before I decided to try it again. It restarted and ran well enough to get me home (only about 5 small residential blocks).
- Cylinder 1 (left side) was noticeably cooler than the others. I pulled the plug. It was fouled. That explains that. Put a fresh one in and restarted...still not running right...so I changed out the number 2 plug as well. It wasn't fouled but carboned up...

Okay. Restart bike. Seems to be running better so I do a test run. Something is still not right. I come back and feel the pipes. 1 and 4 are so cool within a minute or so that I can hold my hand on them.

NOW!  Here's the...weirdness. Follow along.

I installed Dyna coils. The directions clearly state that it does not matter which lead goes to which side. The OCD in me would have made the color wires each go to the top and the black wires to the bottoms, but I managed to bypass my OCD somehow and installed them backwards from each other. TECHNICALLY according THEIR directions...that shouldn't matter. However when I put a test light on it...if I connect the lead to the wrong side it kills the bike... maybe that's just a duh. I don't claim to be a coil expert but I know that opposing charges shouldn't meet...

It seems to have run ok all of the other times I've messed around. I'm not sure how today is different.

The fact that there seem to be issues with 1 and 4 make me think it is coil and/or timing related. I'm done for tonight so I'll have to delve into that in the next few days.

Guidance?

[Ravie]

Some days I feel like I've never worked on anything before.

I switched out the wires on that other coil so they match now. It makes me feel better.

Decided I ought to check the timing. Amazingly, 1 and 4 are right on...but 2-3 are OFF THE SCALE!  Crap. I gotta go to work but I'll putz with that later. Last time I check it it was NOT OFF THAT FAR!  Weird.

[flybox1]

Keep after it.  double check everything 

[Ravie]

Got out there a bit ago and fixed the timing. As  said, 2-3 was WAY off. Got that remedied and fine tuned it a bit. Everyone seems to be playing along now, so I guess we'll see when I get to test ride it again. Will have to wait for later tonight or tomorrow.