Topic: 400F won't rev past 5000 rpm

[76400F]

OK, so after a long 30 year rest, my 76 400F is back on the road. It's been quite the learning experience, with most of the learning coming from you fine folks on this forum. The final stage is to get her running the way she did when I was 16 - hanging on for dear life while she pulled past 10000 rpm. Right now, I've worked some of the bugs out of her, but here are the behaviours during the various carb circuits:

idle to 1/4: seems to respond fairly well. sputters a bit when first starting out in first but has no issues in any other gear
1/4 to 3/4: seems to accelerate well, bog a bit, pick up decent then max out at 5k
3/4 to WOT: blubbers and bogs and is generally not happy. Won't rev past 5-6k

I've read every carb FAQ and thread but find myself unique in some way or another to the problems described. Or maybe the whole thing is just a bit too overwhelming because there's so many different possibilities in each carb circuit. I'm hoping someone has had a similar experience and can pin point a 'next step'.

Here's the dirt:

- I'm running K&N pods *collective gasp*
- Stock header with an EMGO shorty exhaust
- Complete 'overseas' carb kit purchased, installed, removed and thrown in the garbage
- New #38 pilot (seemed to help) and 80, 85, 90 main jets tried with no improvement (from SCI)
- Float levels checked and rechecked 3 times and all bang on
- Carbs balanced with my homemade vacuum meter (have a new gauge type coming)
- Emulsion tubes removed and cleaned meticulously
- Mixing screw 1 3/4 out (seems happiest there)
- Compression check last night (with a cheap tester i.e. probably low) showed 135 across all cylinders
- Tappets checked and rechecked - 0.002" all around
- Cam chain adjusted - runs fairly quiet
- Timing set statically with a timing light - bang on in the advanced state, slightly retarded in the idle (can't set both to be bang on - cam chain stretch? advancer?)
- New Daichi points properly gapped  *second collective gasp*
- New petcock on the tank
- New coils and wires
- New AG battery
- New R/R combo (no electrical problems, thank the gods)
- Plugs are D8EA

Last night when I synched the carbs again I realized that I was turning the screws out too far and that's what was causing the surges and generally poor idling, so now it's idling fairly well around 1200. That said, I had a hell of a time tightening the locking nuts on the adjustment screws without drastically affecting the synch. Which meant a lot of throttle blipping and starting all over again.

Did a WOT plug chop (with a new plug) and ran it up a slight hill in 6th for a good solid minute. Replaced the plug with the original and rode home. Cut the new plug apart and the entire electrode was still completely white (lean?). Looked at the original plugs and they looked to RANGE from lean to rich (see pics). 

I'm completely at a loss. The only thing that really makes sense is timing? Could it be the crappy points? Or should I still be looking at carburetor issues.

Thanks in advance for reading this long painful post...

[flybox1]

Fuel supplied is cumulative.  Your carbs dont have an internal mechanism to shut off the pilot circuit when the needle jet starts to kick in....
So...get your idle circuit mixture correct FIRST.
New D8EA plugs.  Idle plug chop, and no you dont have to cut the plugs back on this one.
I suspect your #38s are too lean for your pods/exh setup.  This will be confirmed if you cant get tan plugs before bottoming out your IMS.
 I'll only buy from jetsrus.  They are original Keihins, and yeah, theyre expensive at $11ea 

[76400F]

Thanks for the reply flybox. I changed the pilots from the stock 40s to 38s based on a lot of comments from guys experiencing the same issues I had in that circuit.  It actually seems to be running much better from idle to 1/4. I performed the recommended test going 15 mph in 6th and going through to 1/4 throttle to see if it would pull. It seemed to be fine.

Just discovered that jetsrus no longer ships to us Canucks. Is there another source in Canada other than Siriusconinc?

What I didn't mention is that it seemed to run the best with the 80s but was showing lean, so I went up to 85s.

[76400F]

Oh, forgot to ask what you meant by 'bottoming out your IMS'...

[flybox1]

You're awfully trusting of your butt-dyno 
I prefer looking at plugs to be sure I have it right 
Idle plug chop.  Get after it 

With 38 pilot jets you probably are lean at idle.
You will know for certain to go to 40's if you cant richen your idle circuit enough (bottoming out your IMS=turning them IN=richer) to get a tan color on a set of new plugs....

[76400F]

OK, awesome. You've given me that starting point I was looking for. (IMS=Idle Mixture Screw. Duh.) Getting some new plugs tonight.

So how precisely would you go about doing an idle plug chop. I've only ever done them for WOT.

[flybox1]

Ride your bike to operating temps.
swap in new plugs.
put a fan on your engine.
start your bike, let it idle, no blipping of the throttle, and go get a cold beverage.
after said beverage is gone, or after 4-5 minutes...pull plugs.
post a picture of all 4 plug tips in a row....1-2-3-4

[76400F]

Got the plugs. Will post as soon as I can. Thanks.

[caretta]

You did not mention anything about the needle clip positions or if you changed it from the stock middle position. My 1977 400F with individual air filters would not pull cleanly, at WOT, between 5000 to 7000 RPM with the stock needle position. It would get thru that rev range but not cleanly. I was running #38 pilots and #80 mains. I lowered the clips one notch which raises the needle and the 5-7K hesitation almost went completely away but not entirely. I then lowered the clips to the last position and the engine pulls very cleanly from 1400 to 10500 RPM with no hesitations. It also runs slightly rich which was expected but it runs very good and I would rather have a slightly rich running engine than a slightly lean one. From doing numerous WOT throttle plug chops I find the plugs to be on the darker side of tan but still in an acceptable range for my piece of mind.

The tiny carbs on the 400 are very finicky about air flow changes caused by the less restrictive individual air filters. I have never been able to get the off idle crispness I would like but I can live with the tiny hesitation right off idle. Most people would never notice the hesitation it is so slight.

Your 135 PSI compression seems to be in the ballpark for a 12" rubber hosed compression gauge. My very high quality 18" long hose and gauge, used for auto engines, reads around 130 PSI on my 400. My cheap gauge with a 8" hose reads around 135 PSI. All the readings depend on a number of variables so be happy if all cylinders read within 10% or so of each other.

You should buy a combo screwdriver/wrench from Motion Pro or another vendor to make setting the sync screws easy. It will save you lots of frustration.

Retarded timing at idle can give some off idle problems but is probably not a big deal if it is within a degree or so. The ignition advance on the 400 is pretty quick and by 2800 ? RPM the ignition will be full advance. I am not sure if my engine ever runs under 3000 RPM except for when at idle!!

No reason to destroy spark plugs to 'read' them. Get a 2x magnifier if you are having trouble. I use a magnifier and light to really see what is going on.

My engine liked to run the air screws at 2 turns out but there was not much difference between 1 3/4 and 2 turns.

Another though is that if you have not put many miles on an engine that has sat for 30 years you really need to run it for a few hundred miles to get all the internals sealing up again. Doing fine adjustments before getting the engine back in shape will probably result in you having to do some stuff again after the engine frees up.

[76400F]

Thanks for the reply caretta. #38 pilots and #80 or 85 mains are pretty much the consensus from what I've read. The only thing I haven't messed with is the needle position. I didn't want to frig with it if I didn't have to, but it looks like I probably will. If there was a problem with the needle position, would that cause it to stop revving at 5-6k even at WOT? I know flybox made a comment about fuel delivery being cumulative, so maybe that makes sense.

Not touching the crankcase was always in the back of my mind throughout the entire build. I know it had no issues when it was parked, but 30 years is 30 years. Thankfully apart from a few rough shifts initially, it's been shifting very nice. No leaky seals and no bearing issues. I changed the oil and filter a few days ago after the first 75 miles or so and it was not pretty, but I'm keeping my fingers crossed that it doesn't develop issues. 

I now have another avenue to explore once I get the idle plug chop done.

[Bodi]

WOT is not affected by the needle or the airscrew: it's all main jet, emulsion tube, and some needle position.
38s are usually good pilots for a 400 with pods. I can't explain why smaller is better.
Do a WOT chop on clean plugs, see what you find.
Tuning always goes from WOT down... if it will run at low throttle that's good enough to start. Once you have WOT correct you work down through the throttle position adjusting the needle position - and probably its taper, for pods - then the pilot jet and finally the airscrew.

Confirm your slides are in the right way around. You must see the cutouts from the pod side.

[jonda500]

+1 If you don't get the order right you will be chasing your tail as changing things that are above what you have already done will affect the things below!
After first doing the float levels, the correct order is:
1. Main jets
2. Needle clips
3. Pilot jets
4. Idle mixture screws

[flybox1]

I disagree with the order for tuning jets, and my reasoning is, since there is no mechanism to turn off the pilot or idle circuit  from supplying fuel during mid or upper throttle ranges, any change in jetting to a lower throttle position circuit,  done after a wot and main jet tuning, would change the overall mixture ratio  at all throttle positions above that circuit.

Get your main jet right for wot, and then change your pilot jet, you will have a small but noticeable change in overall mixture at wot as there is nothing to prevent the pilot jet to continue supplying fuel.  Vacuum is vacuum.....fuel and air will flow from all areas.

[flybox1]

We are all after the same thing.  A great running bike.  My butt dyno and plug chops after tuning this way show im pretty accurate.  If im way off base, convince me otherwise...

[76400F]

Thanks for the replies guys. Much appreciated.

Bodi/Jonda500: So when I did my first plug chop at WOT with 38 pilots and 85 mains, there was absolutely no colour on the plugs (see previous pic). I rode it cranked for a good minute or more straight. Maybe hit 60mph. So I assumed lean. Bought some 90s from SCI (jetsrus won't ship north of the border anymore) and nothing changed. I haven't WOT chopped with the 90s, but I'm assuming because it's not running any better, that there is something else at play. Plus I haven't heard of anyone with a 400F requiring 90+ jets, even with the large bore kit. So what else can I do wrt WOT plug chops?

Fly: What you're saying about cumulative fuel delivery totally made sense to me yesterday and still does.

I'm now wondering if I should have just followed the status quo and moved the needle position? Or maybe a 1/4 to 3/4 plug chop at this stage would make the most sense?

It was raining cats and dogs here all last night so I had to postpone plug chops and extended idling in my garage (need some new weather stripping on the garage door)...

[jonda500]

I have trouble doing plug chops - with WOT ones, it's hard to find a suitably long, straight, cop free section of road where I live, plus with any plug chop I nearly always burn my fingers trying to extract the plugs out of the pre-warmed up engine to put the new ones in for the plug chop.

So I have been using other clues like my butt dyno, the top speed attained cruising flat out up a certain section of road and trial and error much like yourself.

Also I try running with no air filter fitted- if it helps then I suspect it's rich, if it is no better then I suspect it's lean.
-and I try taping up the air filter or air intake area with masking tape to reduce the area by a third or more - if it helps then I suspect it's lean, if no different I suspect it's rich.

Just as the idle circuit has a little effect on throttle positions above that circuit, the main jet size also has a noticeable effect on the mixture at throttle positions below full throttle... yes we are all after the same thing

John

[scottly]

- Float levels checked and rechecked 3 times and all bang on

How did you check the float levels? You need to check the actual level in the bowls with the "clear tube" method.
Have you tried applying the choke slightly at higher revs? If the bike runs better, you are lean, and if it runs worse you are rich.

[76400F]

Like I stated originally, nothing is really making sense at this stage.

>the WOT plug chop that I did left absolutely no colour on the plugs = lean
>tried choking it a bit at WOT, full speed and it bogs it
>tried 80s, no rev, tried 85s, no rev, did a plug chop and it was still showing lean, tried 90s, no difference
>rechecked point gap and timing
>rechecked float levels
>resynched carbs
>recleaned carbs
>rechecked compression - 135 across all 4
>tried it without pods, no noticeable difference
>looking at the plugs after running it for an hour or so, two looked lean, one looked perfect, one looked rich (see original photo)

So last night I had a look at the original carbs that were on the bike while it sat in the field for 30 years. I had them soaking in my dip tank over night. The slides were seized when I first took them apart at the start of the build, along with a bunch of other issues, so I bought a used set and put them aside. When I took them apart last night, the slides had freed up and the slides, the needles, and the emulsion tubes were found to be in pretty decent shape. I lowered the clip to the 4th position and threw the emulsion tubes back in the carb cleaner tank to sit until tonight. I'm going to put them back together tonight with the 85 main jets, original slides and needles and emulsion tubes, and do a plug chop with the new plugs. Hoping for some improvement. I know it's best to only do one change at a time, but it's so out of whack I need to do something drastic.

Something interesting I noticed: the emulsion tubes from the original carbs have two sets of 5 holes and two sets of one hole. The ones in the carbs I bought have two sets of 5 holes and two sets of 2 holes. Wonder why? Seems odd.

Anyway, thanks for the support guys. I'll hopefully have some good news tonight.

[01Thomas]

As Scottly said, the choke is your friend. Whenever you feel the bike not performing, at any speed, without endangering yourself or anyone else, apply the choke progressively.
If, with some choke, the performance is worse, you're running rich.
If, with some choke, the performance is better, you're running lean.

It is best to perform this test in top gear, in your case at about 5000 rpm.
It is then up to you to diagnose why you have the condition that you have picked up.

cheers
Thomas

[jonda500]

Unless it's running super lean, you need to let a little air in when using the choke to check the mixture. Close it completely then crack it open just a little bit.
John

[76400F]

OK, so I needed to take a little break after becoming dangerously discouraged. I'm back at it now.

>I ordered and installed OEM 80 main jets.
>Ditched the aftermarket float needles and seats and reinstalled the originals (with new o-rings).  I actually received a new OEM set today that I'll be installing tonight.
>reset the floats to 23mm as per 100 threads on 400F carbs overflowing

I have a proper synchronizer arriving today as well, so the plan tonight and over the weekend is to install the new float needle sets, synch the carbs with the dials, and try it with the OEM 80 mains. First order of business will be to see if the overall performance has improved, then do some plug chops.

Here's the question I have at the moment: I noticed that the needles are loose on the slides, inside the emulsion tubes. Is this normal? 

Thanks again for all the past responses. I could never have made it this far without them.

[76400F]

Btw, tried it last night after the above mentioned tweaks and it fired up immediately. After a quick synch with the homemade manometer I got it idling perfectly at 1200 rpm for the first time since 1985. SO there's hope...

[nvr2old]

Boy..after reading this entire thread..I think at this point I'd put everything back to stock..carbs, airbox, pipe.  They run great that way.  Idle at 1,000rpm, smooth power delivery from there to 10 grand.  I know, I know..where's the fun in that..?

[Bodi]

Yes the needles should rattle around in the main jets. No problem. The slide hole is big enough to allow the needle to self-centre in the jet orifice.
Vital - clean orifice tube cross holes.

[HondaMan]

I'll offer up my "usual" 2 cents' worth: have you changed the now-hard O-rings in those flanges where the carb castings bolt to the head? They are very old now, and leak vacuum...also, I spent 6 weeks with carbs on a 400F three years ago, only to discover in the end that it had a bad coil on one side, despite prefect ohms readings (and new plug caps)...started easily, idled fine, absolutely refused to run past 5000 RPM, but otherwise was perfect. After a new coil, I had to undo the other tweaks to get it back on track...

[76400F]

Bodi: Thanks for the feedback on the slide needles. I figured as much seeing how there really isn't any way to stop them from moving. Wanted to make sure.

Hondaman: I originally decided to use the old boots, did the wintergreen thing which softened them up nice, and replaced the o-rings. Once I started having all these issues I decided to just buy new ones. Sirius had them in stock. No difference. The coils are new and I have not tested them. Definitely something to look at. 

So with the needle in the 4th groove, Keihin 80 mains and 38 pilots, floats set at 23mm and everything perfectly synched, the bike ran like an absolute bag of crap. From idle to 1/4 was a lot of hesitation and sputtering, 1/4 back down to idle in gear it would just quit. 1/4 to 1/2 was sluggish and more hesitation, 1/2 to 3/4 was a little better, but 3/4 to WOT seemed to be a lot better. Actually hit 9500rpm in 4th and pulled decently.

So this is the perfect example of why you only change one thing at a time, because now I'm going to have to go back and figure out which of the 3 changes I made has had a negative effect. At least I'm now fairly certain that the 80 main is right. Won't know for sure until the plug chop.

nvr2old: I've decided to stop annoying my neighbours (and myself) and replace the megaphone exhaust with original. It's arriving tonight. I'm hoping it also alleviates some of these tuning issues.

So the weekend plan (with the wife and kids out of town until Monday  ) is to install the stock exhaust and start doing plug chops. Beer intake levels will be directly proportional to success levels. Success>Beer.

[MoMo]

I had a somewhat similar problem with a MAC exhaust, original header and new muffler from DSS cured the ailment

[76400F]

Well, my new pipe didn't arrive yesterday so I'm still tweaking with the megaphone.

But there's some good news!! It seems like my problem the whole time has been my floats and needle sets. This morning I went back to my bin with all my original carb parts and pulled out the floats. They looked to be in fantastic shape other than being very dirty. I soda blasted them and straightened them out a bit and installed them with my new float needles and seats. The ones I was using had huge divots where the pins were contacting the tab. Bench set them back to 21mm. Filled them on the bench and not a drop of overflow. OK, getting excited. Reinstall the carbs and start it up. Getting more excited. Sounds great and seems to have new life. Slap the tank and seat back on and take it out. It's a totally different bike. Sounds great and is pulling decently. Other than a feeling of being a bit under-powered it's revving nice and running well. Stop and pull a plug and it's super rich. Take it back home still excited because I'm thinking the final tweaking should be easy peasy. Get home and realize that I forgot to put the pods back on! Put the pods on and take it back out for a good run. Other than a little hesitation transitioning from 1/4 throttle up, it's running great. Ride it for a while and take it home and pull the plugs. So NOW I'm in a good position to start plug chops and recheck timing, points, etc. Thanks to everyone who helped me get to this stage.

[76400F]

Sorry, that should have read 'running super lean'.

[76400F]

I still don't understand how float levels can play such a critical role in how the bike runs. I can see if the level is really low or really high but 1mm off and the bike runs like crap?

Need someone to explain that to me.

[BomberMann650]

I still don't understand how float levels can play such a critical role in how the bike runs. I can see if the level is really low or really high but 1mm off and the bike runs like crap?

Need someone to explain that to me.

A local honda legend reminded me once how everything about these motors is relying on a Vacuum to operate.
There is nothing about this fuel delivery system to aid the cylinder in receiving fuel to burn.  So as the cylinder goes through its intake stroke, it's literally sucking fuel from float bowl.  Thru the jets and down to the combustion chamber.
If the float levels are out of spec, the vaccum effect is negatively altered.

This is how we have fun with physics!

[76400F]

When you put it like that, it should be so simple to tune these. Make sure there's fuel and provide delivery at the right rate!

Here are the pics from the WOT plug chop I did this morning. Looks like it's still very lean. HIT a top speed of about 65-70 mph. It was still climbing slowly but far from it's potential.

80 mains
38 pilots
needle clip in the 4th position

Going to do the idle chop now.

[BomberMann650]

When you put it like that, it should be so simple to tune these. Make sure there's fuel and provide delivery at the right rate!

There will always be that missing link between science and the mystery of life!


Definitely need to richen that fuel mix.

[76400F]

So here's the idle chop results. Just synched the carbs so either they're not staying synched or the mixture screws need to be adjusted individually. I always thought that you set the mixture adjustment screws the same.

[BomberMann650]

I don't believe there is a rule against using the AF mix screw for fine tuning.  That is what it's there for!

Double check the sync if you're worried about it.

1/2 got sooty

[76400F]

So I'm thinking 1/4 turn out on 1 and 2. 1/2 turn in on 3. 1/4 turn in on 4.

[76400F]

By the way, I broke a new record on my last WOT plug chop: 2 cars, 3 motorcyclists and a guy on a lawn tractor all stopped to make sure I was OK while changing my plugs on the side of the road. Gotta love rural Ontario. Still good people left in the world.

[BomberMann650]

By the way, I broke a new record on my last WOT plug chop: 2 cars, 3 motorcyclists and a guy on a lawn tractor all stopped to make sure I was OK while changing my plugs on the side of the road. Gotta love rural Ontario. Still good people left in the world.

Just the lawn tractor alone is impressive enough.

[HondaMan]

So here's the idle chop results. Just synched the carbs so either they're not staying synched or the mixture screws need to be adjusted individually. I always thought that you set the mixture adjustment screws the same.

Yes. If you keep trying to adjust the color/mixture with varying air screw settings, you will chase your tail!
Set them at the factory setting, all the same. If the plug colors vary, look for the reasons elsewhere. After 40+ years with these bikes, I find this is the only sure way to track down the issues involved.

I have also found that the O-rings in the carb flanges, the clamps, the tiny (aftermarket, too stiff) float valves that were used in these bowls, and leaking exhaust valves with bad guides all have variously caused plugs to show different colors. It had nothing to do with the air screw settings.