Topic: retarding timing

[jasper88]

I've been hearing some pinging when I open my throttle 1/2-WOT in higher gears at anything less than 6K or so, especially under load, uphill. I tried to increase my main/needle jets to the point where things are over-rich with no results. I haven't plug chopped in a while, but my plugs are looking tan overall, except for one that's slightly lean, but not terrible. I know someone is going to recommend I look elsewhere before retarding timing, so my set up is as follows:

10.25 836
41A
stage 2 port, shaved head
4-1 with drilled airbox
D8EA
91 octane
valves .004, .005
oil cooler
warm compression 200 across the board

float levels set and verified with clear hose. Currently 122.5 mains, 40 pilot, needle in middle position. New boots without air leaks.

Full advance at 2500

So I'd like to try retarding timing to kill this pinging. How many deg should I try?

I don't have a programable ignition, just a Dyna-s, so how do I measure deg on the plate? Do you just set up a deg wheel and transpose a mark on the plate left of the timing mark?

Thanks
 

[PeWe]

CB750 with the round top carbs, right?
- Raise needles 1 notch to 4th notch from top.

+5000 rpm is the typical pinging rpm.

My 1000cc setup had 200PSI when testing with a cheap compression tester.
No pinging, Dyna-S ignition timed at 1200rpm idle with case mark, full advance at 2600-2700. Advancer has both springs cut 1/2 wound making Idle and up stable.

If it still ping after raising needles, cam might need to be retarded 2-3 degrees to release cylinder pressure.

Is cam timed or just bolt-in with stock cam sprocket?

I think one step richer needles will fix it.

[jasper88]

Yea, I have the round style carbs. (K5)

I couldn't exactly hit the degree numbers of the 41a, but it was pretty much squared up.

I raised the needle with the 122.5s and it was rich mid to WOT. Plugs were way to sooty. I guess I can try raising the needle and going down a jet size, but I figured I'd try retarding timing as well.

Hard to believe I'd have pinging by the looks of those plugs.

[scottly]

There are 2 degrees between the two full advance marks, and 10 degrees between the "T" and "F" marks, just to give you an idea.

[jasper88]

There are 2 degrees between the two full advance marks, and 10 degrees between the "T" and "F" marks, just to give you an idea.

Ahh. That's helpful!

I tried retarding it arbitrarily and it seemed to lose power.

[scottly]

How much was the head shaved?

[jasper88]

I'd have to ask Mike Riek, but it wasn't to aggressive from my understanding.

[grcamna2]

Could installing D9EA plugs help ?

[PeWe]

Is the 4-1 restrictive? ( Not loud when twisting)

My K2 with stock airbox, K6 carbs, a little bit more compression with K7 +0.50mm pistons, stock K7 cam and rather good flowing 4-4 (Yamiya no numbers) had very bad response at mid range when I had finished the build and jetted carbs to stock K6 jetting.

Increased main jets, 110, 115 to 120 and got much better response but stumbling rich at higher lifts.

Understood that carbs needed same needle height as they had with 836, ported head, cam and 4-1 back in the 80's on my K6.

Needles 4th, main 115, air screw a little less than 1 turn out. Runs fine.

I guess your bike will like that too if sooting plugs with 122.5.

[jasper88]

My 4-1 can is a cored-straight-through-fiberglass-packing jobber. Very little restriction there.

[TwoTired]

I vote for raising the needles and using smaller mains.  Feel Free to ignore...

My theory; a larger engine displacement will suck harder on the carbs and produce a deeper vacuum in the carb throat.  More vacuum differential applied to metering jets, will effectively make the jets provide more gas volume.

So, I'm not sure how you justified going to larger main jets, without any testing to see if it was needed.

I assume it's pinging while accelerating off idle, or everywhere in throttle position?  Raising the needle should help richen up the off idle response.  I confess I don't know what the stock pilot jet size is/was.

I suppose a dyno run isn't available?

Cheers,

[Don R]

[quote author=TwoTired

My theory; a larger engine displacement will suck harder on the carbs and produce a deeper vacuum in the carb throat.  More vacuum differential applied to metering jets, will effectively make the jets provide more gas volume.

 I have seen this in action. Adding a .563" stroker crank to a jr. Dragster engine always required smaller jets.

[grcamna2]

I vote for raising the needles and using smaller mains.  Feel Free to ignore...

My theory; a larger engine displacement will suck harder on the carbs and produce a deeper vacuum in the carb throat.  More vacuum differential applied to metering jets, will effectively make the jets provide more gas volume.

So, I'm not sure how you justified going to larger main jets, without any testing to see if it was needed.

I assume it's pinging while accelerating off idle, or everywhere in throttle position?  Raising the needle should help richen up the off idle response.  I confess I don't know what the stock pilot jet size is/was.

I suppose a dyno run isn't available?

Cheers,

Excellent  thanks for this information TT

[scottly]

There are 2 degrees between the two full advance marks, and 10 degrees between the "T" and "F" marks, just to give you an idea.

Ahh. That's helpful!

I tried retarding it arbitrarily and it seemed to lose power.

Have you verified that the timing is the same on both 1-4 and 2-3? Does the advance go to the full advance marks at 2500, then back to the F mark at idle?
Yes, retarding the timing will generally cause a power loss. My high compression 836 seems to be OK with 91 octane, and the only time I've noticed pinging was on a dyno run with too lean of jetting, but otherwise I can roll the throttle on starting from about 4-4.5K RPM reaching 5K @ WFO.
 

[Tracksnblades1]

[quote author=TwoTired

My theory; a larger engine displacement will suck harder on the carbs and produce a deeper vacuum in the carb throat.  More vacuum differential applied to metering jets, will effectively make the jets provide more gas volume.

 I have seen this in action. Adding a .563" stroker crank to a jr. Dragster engine always required smaller jets.

🎯

Stroke increases piston velocity at the same rpm.

[jasper88]

Could installing D9EA plugs help ?

It's worth a try.

Have you verified that the timing is the same on both 1-4 and 2-3? Does the advance go to the full advance marks at 2500, then back to the F mark at idle?

Yes, full advance at 2500.

So, I'm not sure how you justified going to larger main jets, without any testing to see if it was needed.

I assume it's pinging while accelerating off idle, or everywhere in throttle position?  Raising the needle should help richen up the off idle response.  I confess I don't know what the stock pilot jet size is/was.

I justified going with larger jets as a result of plug chopping years ago. Recently, I started noticing pinging in the 1/2 to full throttle openings, as I said above, so I started going though everything, including jetting, trying to hunt down the cause. I did raise needle jets at one point, with less pinging, but sootier plugs, so I went back to the old needle position and tried a larger main jet, that neutered my top end. I haven't had time to plug chop, but in my book, if you keep jetting richer and it runs worse, you're not headed in the right direction.

 

[PeWe]

You mix up in which throttle area you have too lean or too rich.
It is possible to run too lean and get sooty plugs when needles are too lean and main too rich.

Carbs have a pilot area, needles, mains. They all need to be right at the same time.

Twisting throttle a little in around 4-5k is needle area.

Way too rich mains can make up for lean needles on these carbs ending up in too rich at higher lifts.

These carbs are very simple to setup on a CB750, stock as modified with pods or other exhaust system. Only needle height and main jet to change. Air screws 1/2- 1 1/8 turns out

Other carbs more complicated.

[jasper88]

I need to get my floats straightened out before I do anything else or I'll just be chasing my tail. I had them set perfectly to 26mm for years. I checked again a few weeks ago, measuring off the side body cutout and it was perfect, but once mounted, the clear hose method showed I was 3-5 mm low across the board, (I started a thread earlier about this). I bent the float tangs to compensate for this discrepancy, and fuel measured 3mm below the carb body. Plug readings got more consistent, but I still had one lean and one sooty plug, though the later, is from cylinder one, that has an oil leak, making it hard to read. I checked levels again via clear tube and they were off. I had to set one float so close to the body, that there is only like 4-5 mm or travel before it bottoms out. (3rd carb)

So I need new float needle valves apparently, but strangely, the levels in the tubes with the carbs installed, are sensitive to the position of the fuel hose and gas tank height. I installed all new (6mm?) gas line, with no kinks. The hose routes down from the petcock, through the manifold hole, to the T-piece of 1-2 carbs and just about straight over to the 2-3 T-piece, like a K5 should. (I'll post pictures when I figure out how to.) I double checked all T-pieces, passages, running wire through the little carb vents and blowing air through them. If I turn on the petcock with dry bowls the tubes fill up within 10 seconds or so. But, carb 2-3 are finicky with tank/hose position. I had to reposition things a few times to get them to fill up completely.