Topic: The great epic carb battle of '05

[bistromath]

Hi guys... I used to hang around a lot when the Greenspun system was up, and now I'm back (with questions, of course!) This is a long one, but hopefully a good puzzle for you all....

Now, a little background: when I bought my '75 550F two years ago, the PO told me he and a "Honda Certified Mechanic" went through the carbs and tuned 'em up real nice like. When I put a borrowed Morgan Carbtune to 'em, though, I could never get #4 to sync quite right. Seemed to run OK though, a little rich (ran great with the air filter out), got worse w/altitude (took it up to 11,000 feet here in California, stopped when it wouldn't develop enough power in 1st gear to keep moving uphill). =D

So I got it into my head to see what was up with them, with a spare Labor Day weekend to look at them and all... got them off (good god is that ever a pain), stripped down, cleaned, rebuilt (which turned out to be a 4-hour job, no big deal at all), and put back on (I'm still swearing about that).

As it turns out, carbs 2&3 were running stock 98 jets with the K for Keihin on them. #4 was running a 100 jet, and #1 was running a 105! I was mystified to say the least. I guess the PO meant "Honda Certified Ape". And the 105 jet was chewed up around the bottom a bit like it'd sat on a shop floor for a while. So I reused the #4 jet, the O-ring looked okay but that might bite me in the ass later on (so keep that in mind!). My rebuild kits were 100s, which are 2 sizes too large but since it was all I had figured I'd go for it (100 is stock for the 550, 98 is stock for the 550F). Bench synched the carbs (for what it's worth). I set the float heights to 22mm like a good boy, as best I could with a micrometer and near-infinite patience. I set the heights so that the float arm JUST touched the needle pin at 22mm. And I kept the stock needle clip position at second from the top.

I put it all back together (just of note, it's infinitely easier to put the carbs back on if you take the intake rubbers off the intake manifold and put them on after the carbs are mounted, with the manifold attached to the airbox but the assembly loose from the frame -- but seal around the rubbers with Pliobond or a good sticky RTV to stop intake leaks).

They all took gas, none of them overflowed, and I made sure there was gas in each bowl before I tried starting it, like a good boy. Started it up and it ran! Holy crap! But it was sort of lumpy (which I guess is to be expected before synching) and bogged down when I rolled on the throttle hard. Bogs down enough to where it stalls if I dump the throttle on it. I took it for a spin, and it shows the same behavior, and is definitely running rich. Not so rich that it's awful, but richer than before, and bogs if I give it more than 1/2 throttle.

So the questions:

1. Does it make any sense in any situation ever to install main jets of different sizes in different carbs?
2. Would the difference between 98s and 100s really make that much difference in how the bike runs (keep in mind my test was w/air filter out)?
3. Can I really ask any questions until my Carbtune comes in the mail from the UK and I have a chance to sit down and synch them? IE, is bench synch good enough to start off?
4. Is there anything that I've overlooked (probably) that would cause this?

It is definitely firing on all four, though, so I'm at least sort of proud that I got them back together in any sort of shape at all. Let the fun begin!

EDIT: I do have one more question: I'm guessing raising the float height (lowering fuel level in the bowl) will lean out the mixture, but is this true? And if it is, does it lean it out all 'round or just at a particular throttle range, or just when rolling on the throttle, or just during the full moon?

Thanks!
Nick F

[Sam Green Racing]

Realy good post, but I am online at work and am about to go home, so will reply tonight when I have more time.

Welcome to the forum.

Sam.

[heffay]

bistomath~
i wish i coulda been here for the greenspun you lucky devil (or imp as it may be)

question 1 answer:  yes.  cylinders pull at different volumes sometimes i.e. minute differences in honing, boring, clearances, etc. can make enough of a difference to install different jets... think of it this way... you want to get the best performance from each cylinder... then get them as close as possible.

question 2 answer: yes, but you are the only one that can really answer that... if it runs better with a garden hose instead of a jet it runs better.  see my point?   

question 3 answer: well, you asked more than one question.... soooo, bench synch to start!

question 4 answer: 11,000 ft?  I can barely function at that altitude without proper training i.e. (jetting)... 11,000ft is insane to compare when you are from california (essentially sea level)  jet it for your altitude... not where you might happen to travel every now and again. 

[TwoTired]

1. Does it make any sense in any situation ever to install main jets of different sizes in different carbs?

Not if the air jets are equal across all four carbs.  Check the top right of the mounting flange on each carb.  The F models were all stamped 069A.  If you have stampings 022A or 627B, they didn't come with your bike from the factory. Worse, if you have more than one number set up type, then someone assembled a bank from mismatched sources.  The air jets are drilled into the carb bodies and there is some variance between the set up numbers.  Further, the needles and needle jets are different between setups as well as the slow jets.

2. Would the difference between 98s and 100s really make that much difference in how the bike runs (keep in mind my test was w/air filter out)?

Certainly.  But, mostly at WOT which is where they dominate the mixture control.  And, the lack of an air filter would likely make your mixtures leaner than when one is installed.

3. Can I really ask any questions until my Carbtune comes in the mail from the UK and I have a chance to sit down and synch them? IE, is bench synch good enough to start off?

Depends on how good you are at bench syncing.  But, then mechanical sync doesn't allow for differences in individual cylinder efficiences.  Of course, if your compression check shows all cylinders exactly the same then a perfect mechanical carb sync should do fine.

4. Is there anything that I've overlooked (probably) that would cause this?

Exhaust and induction changes from stock can also effect the mixtures provided by the carbs.  Stock pipes and filter?  What spark plugs are you using; Heat range, resistor type, gap?

Also, make sure all the major tuneup items are dialed in before tweaking carbs to the engine. Ie, timing, tappets, cam chain tension, etc.  For tuning purposes, learn to read your spark plugs for combustion balance. See:
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

EDIT: I do have one more question: I'm guessing raising the float height (lowering fuel level in the bowl) will lean out the mixture, but is this true? And if it is, does it lean it out all 'round or just at a particular throttle range, or just when rolling on the throttle, or just during the full moon?

While fuel level does have an effect on mixtures, it's really more direct to adjust the mixture relative to the throttle position.  The chart below may help you to visualize.  You might also wish to read the carb FAQ.

[bistromath]

TwoTired, thanks for the response. I'll take another look at it tonight, but just FYI the bike is bone stock except for a Dyna electronic ignition and coils. The carbs are 69A's.

To be honest I don't know whether it's rich or lean. It sounds rich to me, with that bogging sound when I load it, but I haven't run this bike lean yet so I don't know what it sounds like. It wasn't popping on deceleration at all like I would expect it to, and I didn't notice any surging.

I'll do a couple of plug chops tonight and have a look-see. I like your idea of running with the fuel shut off and watching its behavior as the bowls drain.

[TwoTired]

When you shut off the fuel and run the engine, the carbs do lower the fuel level.  Be aware the carb draws fuel from two sources in the bowl; the main jet and the slow jet.  The main jet reaches lower in the carb bowl than  the slow jet.  So, as the bowl empties, the slow jet starves first.  It will depend on throttle position, which jet is primary contributor during run time.  At mid throttle, for example, starving the slow jet will indeed make the overall delivered mixture lean since it flows it's little bit at any throttle posotion.  But, when you close the throttle (and, in effect, the main jet) the engine dies because no fuel is delivered to the engine.

The plug deposits should tell you if you are running rich or lean.  But, with your elevation issues I suspect it is too rich.  Since there is less oxygen at higher elevation, the mixture needs leaning to combust properly.


Did you get to see the chart I posted?  I saw it there last night after I posted, but today its gone. Strange, that.

Cheers,

[Sam Green Racing]

Well that's that sorted! I knew the old Timer would save my fingers.

Be it right or wrong, a mate of mine turned in some very respectable lap times in the IOM TT races some years back with odd main jets.
It may be an easy fix if you can't find the problem that is causeing the need to do it in the first place but, hey if it's runing so what.

On the altitude point,  Heffay had to wear his breathing equipment But old Timer is used to being out of breath, some of his pin point accurate replies are that long I myself get out of breath reading them.

Anyway, back in 1976 I entered a two man team in the 250 IOM production TT. The riders, local Chris Hart and a young guy I brought from the States,  Brian Zeky.
The bike, a 250 RD Yamaha  had already placed 2nd the year before with fastest lap and fastest through the Highlander speed trap at 112mph.
Unlike the highley tuned TZ Yamaha's the RD did not require any alterations to the jets dispite the Islands torturous mountain climb and deccent.

A few weeks back we entered a drag meeting at a strip we had not been to before.
On arrival we found it to be on top of a mountain, a WW2 dissused air field.
I wandered if the altitude might be a problem and asked a mate who holds several class records at this venue.
He said he had had to jet his two strokes down three times the first time he ran there, but said the four strokes would be OK.
Our CR750 with stock motor ran a full half second quicker than it had ever run, but some four strokes in higher states of tune suffered.

It would seem that the higher state of tune the motor is in, the more it is affected by altitude, two strokes in high states of tune suffer earlier.

Now what was this about 11000 feet ?  I bet you can see the end of the world from up there, or is that next week.

Sam.

[bistromath]

Well, I took it out again after work, found a flat stretch of mostly unused road, and did a WOT plug chop. Guess what I found? It's running lean, not rich. Guess I can't tell the difference by sound after all. It accelerated quite a bit harder when I gave it 1/3 choke, so that was the kicker. So I took it back home and put the filter and airbox cover back on, and brought it back out. It went like STINK! No hesitation on takeoff and wide open it hauled ass. So I guess I got that much right. It might still be a little lean but a WOT plug chop looked OK. I'll finesse that later. It's still pretty lumpy at idle, so I guess my bench synch skills aren't quite up to par. I'll wait for that Carbtune to come to fix that one =)

One other thing: I've got a flat spot in accelerating, very consistent around 1/4-1/3 throttle. Can't tell if it's rich or lean, guess I'll chop it later. Sounds like needles need to be raised or dropped?

[bistromath]

Oh and this is what 11,000 feet was about:


Went up the side of White Mountain in the Sierra Nevada with some friends. They all had fuel-injected BMW's and KTM Adventures. I had a thirty-year-old Honda but I wasn't going to let that stop me. I made it to 11,000' (top of road is 12,000) on the dirt and then it just wouldn't go any further. Would have made it all the way, probably, if I'd taken the air filter out, but no way was I going to do that on dirt.

Good story from that trip: on the way up, I hit some article of clothing on the highway & it wrapped itself around my rear axle. My rear wheel locked, so I skidded to the side of the road. It threw my chain and sheared the tensioner bolt on the left side. So I wedged a pair of needlenose pliers in the tensioner and nursed it to a gas station, where I bought about a dozen things that looked like they might fit and stuffed them in. The toothbrush didn't work, but the nail clippers worked like a charm. =D They got me 300 miles to Fresno, where I found a machine shop the next morning and the nice Mexican guy let me use his tools to extract the old bolt and put a new one in.

Remember, it's not a road trip unless something unexpected happens!

[Sam Green Racing]

Yes! you can see the end of the world from there,  it was this week after all.

[TwoTired]

One other thing: I've got a flat spot in accelerating, very consistent around 1/4-1/3 throttle. Can't tell if it's rich or lean, guess I'll chop it later. Sounds like needles need to be raised or dropped?

Hard to say without seeing your plug chop results or knowing where your needles are currently clipped.  But, At that low a throttle setting the partial choke test will give you and indication on which way to go.  If partial choke improves it then raise the needles.  If it makes it worse, then drop the needles.
Cheers,