Author Topic: Quick brainstorm (Read 3913 times)

jessezm · « **on:** February 17, 2012, 06:24:16 AM »

Tell me, what could cause one cylinder to run lean if I have verified that floats are set exactly the same, carb settings identical, idle and main jets free and clear, and carbs synchronized perfectly on a Morgan Carbtune? If I can't hear or feel an exhaust leak, either, but is there a finer method to detect one? I have a newly installed aftermarket exhaust, and I don't recall having this problem before, but I made a lot of other changes at the same time... (bike is a cb400f/466cc)

ekpent · « **Reply #1 on:** February 17, 2012, 06:42:18 AM »

Maybe the porting is a little different in that cylinder/head ?? Just a toss out---

bollingball · « **Reply #2 on:** February 17, 2012, 06:44:55 AM »

jesse Lot of other changes? In the carbs any metal parts? Tell us more. Lots more info. needed about all the changes. Not paint or shine stuff.

Ken

jessezm · « **Reply #3 on:** February 17, 2012, 07:06:49 AM »

Quote from: bollingball on February 17, 2012, 06:44:55 AM

jesse Lot of other changes? In the carbs any metal parts? Tell us more. Lots more info. needed about all the changes. Not paint or shine stuff.

Ok:
--head was just ported by JMR (stage one, not too radical)
--JMR replaced valves and installed bronze valve guides, new valve job all around
--HD valve springs and hardened alloy retainers
--carbs bored out by Rick Denoon
--New cylinder block freshly bored to 466cc, new pistons, coated crowns and skirts (ceramic on top and DFL on skirts)
--still running same Web camshaft (#2)
--running velocity stacks now, but same lean condition on that cylinder when I switch back to pods

The reason I thought about an exhaust leak was that right after I got the bike back on the road, I noticed the #4 pipe bluing. Then I went too fast over a bump and bottomed out the exhaust, and I immediately heard an exhaust leak. I rode home, ordered up a new set of exhaust gaskets, and reinstalled the exhaust later. But plugs still show the same lean condition. I was thinking that perhaps the #4 header isn't seating well in the head, but I can't actually feel or hear a leak there now.

RFogelsong · « **Reply #4 on:** February 17, 2012, 08:30:50 AM »

Maybe an old rotted rubber piece connecting the carb to the intake on that cylinder is letting air bleed through it just enough that you notice lean plugs. Also, valve adjustment, are you sure everything is at the exact same tolerance? If you could put the original air box back on so that all carbs drew from the same source it would probably be easier to narrow down to something like this. Currently it could be simply a different flowing filter or something.

jessezm · « **Reply #5 on:** February 17, 2012, 08:49:37 AM »

Carb rubbers are pretty supple but I'll run her and spray some wd40 around them to see if I detect a leak on that side. I just checked and double-checked that valves and they are all spot on...

just as a side note, I noticed this week a seller out of Japan had produced a new replica airbox for the 400f, and I'm thinking about trying it out with a fabbed up filter section behind it (something like the "anti-pod" perhaps)

TwoTired · « **Reply #6 on:** February 17, 2012, 09:03:45 AM »

Quote from: jessezm on February 17, 2012, 06:24:16 AM

Tell me, what could cause one cylinder to run lean if I have verified that floats are set exactly the same, carb settings identical, idle and main jets free and clear, and carbs synchronized perfectly on a Morgan Carbtune?

1 - Check fuel level in each carb instead of strict mechanical height. The float valves have spring pins. If they are sticky or the springs of different strength, it can alter the fuel level even with the same mechanical adjustment. You didn't say if all the carb internals were genuine Honda or aftermarket. Quality seems to be more variable with aftermarket.

2 - Also scrutinize the emulsion tubes, hole count, size of holes, and clean holes. Each make a contribution to fuel mixture.

3 - At the end of the emulsion tube is the Needle jet. Check that for size and shape variation compared to other carbs.

4 - The jet needle itself can be measured dimensionally (aftermarket or Honda?)

5- Have you verified there are no air leaks in the intake runners?

6 - Your report would indicate your plug reading came after operation at all throttle settings. This makes it hard to diagnose any particular section of the carb.
Have you done any "plug chops" to further isolate a particular throttle setting that dominates the problem?

7 - Have you checked for camshaft wear that may have distorted one lobe on the cam, changing the way one cylinder breathes?

8 - Were the carbs bored out before or after this "one lean" cylinder issue surfaced?

jessezm · « **Reply #7 on:** February 17, 2012, 09:24:00 AM »

TT--
I need to mock up a set of drain screws with nipples so I can verify fuel heights. I put a new float in the offending carb/cylinder with no change. float needles are all new aftermarket. Jet needles and needle jets are Honda, closely cleaned and inspected. I could pull them apart and look again as a last resort after checking other variables.

I will double check intake runners for leaks.

I did a full throttle side of the highway plug chop with new plugs, and all 4 plugs looked good, while #4 was slightly more lean-looking than the others (pure white vs tan). However, when riding around town, 1-3 look sooty and #4 is definitely lean.

I measured cam lobes for wear (it's only got 1500 miles on it) again before buttoning up motor, all within .001" of each other.

Thanks for the great ideas!

Lean condition didn't appear with bored carbs, but only after the most recent changes (exhaust, porting)

TwoTired · « **Reply #8 on:** February 17, 2012, 09:58:51 AM »

Quote from: jessezm on February 17, 2012, 09:24:00 AM

I did a full throttle side of the highway plug chop with new plugs, and all 4 plugs looked good, while #4 was slightly more lean-looking than the others (pure white vs tan). However, when riding around town, 1-3 look sooty and #4 is definitely lean.

In my mind, this sorta points to the throttle valve circuit: Jet needle, Jet needle orifice, Jet needle profile, emulsion tube parameters, air jet, or carb passageway for the air jet.

Quote from: jessezm on February 17, 2012, 09:24:00 AM

I measured cam lobes for wear (it's only got 1500 miles on it) again before buttoning up motor, all within .001" of each other.

Ok. I'll just note that HD valve springs do increase cam follower/cam lobe contact pressures. Those parts either "marry" themselves into comfortable coexistence, or wear rapidly after mating. Does the timing of the symptom show an increase in severity since the time of introduction? Or, was the symptom apparent immediately upon reassembly?

Cheers,

grcamna2 · « **Reply #9 on:** February 17, 2012, 10:01:54 AM »

Quote from: jessezm on February 17, 2012, 08:49:37 AM

Carb rubbers are pretty supple but I'll run her and spray some wd40 around them to see if I detect a leak on that side. I just checked and double-checked that valves and they are all spot on...

just as a side note, I noticed this week a seller out of Japan had produced a new replica airbox for the 400f, and I'm thinking about trying it out with a fabbed up filter section behind it (something like the "anti-pod" perhaps)

Jesse,
I like your idea of giving an airbox a go

+1

754 · « **Reply #10 on:** February 17, 2012, 10:16:40 AM »

Y ou have 4 ...115ish cc engines coupled together on a common crankshaft. Under ideal conditions, they will all be identical..
.. but in our lives they are not always,,if you can get them all the same..great.. If not just ride the darn thing..next week it may all change to a different one being out a tad..

lucky · « **Reply #11 on:** February 17, 2012, 10:23:35 AM »

Quote from: ekpent on February 17, 2012, 06:42:18 AM

Maybe the porting is a little different in that cylinder/head ?? Just a toss out---

No it is not the reason.

jessezm · « **Reply #12 on:** February 17, 2012, 10:24:30 AM »

TT, as to the timing of the problem, it all came on just after reassembly. First I had that issue with the obvious exhaust leak after I noticed the pipe bluing, then I bottomed it out and must have made it worse because I heard it right away (classic tick tick tick). But after installing new exhaust gaskets the problem persists, which is why I was thinking maybe the new exhaust is off-angle just a tad where it contacts the head...

Bill, I'll let you know if I do it, but I gotta take some cash I had set aside for it and fix my car instead :-(

754--I just don't want to put a hole in my piston is all!

lucky · « **Reply #13 on:** February 17, 2012, 10:33:41 AM »

Quote from: jessezm on February 17, 2012, 06:24:16 AM

Tell me, what could cause one cylinder to run lean if I have verified that floats are set exactly the same, carb settings identical, idle and main jets free and clear, and carbs synchronized perfectly on a Morgan Carbtune? If I can't hear or feel an exhaust leak, either, but is there a finer method to detect one? I have a newly installed aftermarket exhaust, and I don't recall having this problem before, but I made a lot of other changes at the same time... (bike is a cb400f/466cc)

You are using the wrong thing to check for intake leaks(WD40).
Use carb cleaner.
By the way all of this happened after you hit that bump.
The carbs could have got knocked loose on the rubber boots.

It is not an exhaust problem that causes a lean condition.

I have no idea WHY you would bore out the carbs.
How much was removed, and now that they are bored out, one of the slides could be installed backwards???

Also NO mention of needle position on the slides.

Remember all of this started when you hit that bump according to your post.
What evidence is there that the cylinder is lean??? Pipes can turn blue for many reasons. There is no evidence that blue exhaust pipes are from lean mixtures. What about all the owners with BLACK exhausts, are all of those suspect for lean cylinders too? NO.

jessezm · « **Reply #14 on:** February 17, 2012, 10:51:53 AM »

Lucky,

I used gibbs penetrant, thinking that it would quickly seep into any intake leak and make itself known, but if you say that's the wrong stuff, I'll do it again with carb cleaner...

As far as the bump--I noticed the bluing, which from my understanding can be a sign of excessive heat, on the #4 pipe and not the others, BEFORE I hit hit that bump. I noticed the obvious exhaust leak immediately after.

I have had the carbs off the bike since that time in order to recheck and adjust the floats, and re-synchronized the carbs at that time as well, so if anything came dislodged it has since been re-lodged :-)

As for the carb boring, please see this link: http://www.denoonsp.com/services.htm

My understanding is that the carbs for the 350 were not modified for the 400, even though the 400 had larger displacement and larger valves. Is this right? who knows! but I heard many good reviews so I did it, and am happy with the results.

Needle position is center. Slides are in correct position, too.

In my mind, bluing pipe and a greyish/white plug was evidence of a lean mixture. How else, besides a Dyno/gas analyzer, would you diagnose a lean condition? Thanks!

scottly · « **Reply #15 on:** February 17, 2012, 06:00:03 PM »

Make sure you support the rear of the carbs, rather than just letting them hang off the boots. 1+ on checking the float levels with clear tubes.

tlbranth · « **Reply #16 on:** February 17, 2012, 06:49:22 PM »

+2 on the clear tubes.

bwaller · « **Reply #17 on:** February 17, 2012, 07:23:53 PM »

It's easier to check fuel levels by the tube method but not impossible to remove the bowl to check either, just use two screws. You do need to check the level though.

It's not likely, but check the ID of the mainjet against another one. If they are aftermarket, stranger things have happened. If it seems lean through all throttle positions check the sizes of the other jets too for an oddball.

jessezm · « **Reply #18 on:** February 17, 2012, 08:04:41 PM »

Is there a standard set of gauges you can get somewhere to size the main jets? The #95's I have in there at the moment are from that kit from Sirius Consolidated off of ebay--I haven't heard anything bad about them, but I guess "trust, but verify" is the name of the game. I suspect the leanness is in the midrange, though. The pilots are #38's from the Honda.

scottly · « **Reply #19 on:** February 17, 2012, 08:22:48 PM »

Use the blunt end of drill bits as a gauge.

bwaller · « **Reply #20 on:** February 18, 2012, 04:03:41 AM »

The best option are gauge pins but not everyone has a set I realise. Check slide cutaways, jet needles and needle jets too. It's a shot in the dark but as a first step...

754 · « **Reply #21 on:** February 18, 2012, 08:30:12 AM »

ground dowel pinsa or turn and ground shafting should be cheap..

Tim2005 · « **Reply #22 on:** February 20, 2012, 02:22:05 PM »

Rather than trying to size the individual jets, an easier approach would be to swap the mains, pilots and needles from #4 with another carb's and see if the lean mixture problem moves to that cylinder.

Historically I have found that some 400 engines will run leaner on #3 cylinder, can't explain why but I have worked around it with different jetting on that cylinder & even running a #9 plug.

Finally, it might be worth running a compression test across all 4 to ensure you have a good baseline & that there's nothing more significant occurring.

jessezm · « **Reply #23 on:** February 20, 2012, 02:28:29 PM »

I've seen that same phenomenon with the #3 cylinder showing evidence of running lean on heads that I've taken apart--the valve faces will all be carboned up but the #3 valves will be chalky white. I thought I read something somewhere about it having something to do with the tach drive and cable so I've always disengaged it when I adjust the valves. I don't see the logic in it though...

Tim2005 · « **Reply #24 on:** February 20, 2012, 03:09:08 PM »

I don't see the logic in that one either - but getting the tach cable out of the way does make it easier to check #3 exhaust tappet anyway, so no harm in that...

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Author Topic: Quick brainstorm (Read 3913 times)

bollingball