Author Topic: 400F carb rebuild questions (Read 7373 times)

.RJ · « **Reply #25 on:** February 04, 2007, 08:16:32 AM »

Air leaks were from the covers over the slides - replaced the gaskets, problem solved. They "looked" ok, obviously not.

Bike is running great at idle, part throttle, up to about 1/2 throttle - then going from half to 3/4-Full throttle it completely falls on its face, wont accelerate at all. Plugs are nice and sooty - so is it running rich? Seems odd that it would. Only change is the air filters, jets are all stock and the only change to fuel delivery is the raised needle one notch.

750goes · « **Reply #26 on:** February 04, 2007, 12:22:43 PM »

put the needle clip back one position - lower it one spot - too much fuel going in by the mention of sooty plugs and falling on its face at that throttle position..

.RJ · « **Reply #27 on:** February 04, 2007, 12:33:49 PM »

Quote from: 750goes on February 04, 2007, 12:22:43 PM

put the needle clip back one position - lower it one spot - too much fuel going in by the mention of sooty plugs and falling on its face at that throttle position..

It runs great, with good response at any RPM up to 3/4 throttle or so. Its that fuzzy 3/4-Full throttle, any speed, any rpm that it completely falls flat - it wont even accelerate at all. Wouldnt the stock main jets be too small with the pod filters?

750goes · « **Reply #28 on:** February 04, 2007, 12:38:27 PM »

Try the clip back to its original position...then see how it runs...if it's still crap its your filters, and you need to start changing your jetting to accomodate these..
start from a baseline and slowly make one adjustment at a time - it could possibly also be your float heights still playing up...have the carbs been balanced??

.RJ · « **Reply #29 on:** February 04, 2007, 02:03:24 PM »

Bench balanced only - I've got the tool to balance them, just havent done that yet.

aptech77 · « **Reply #30 on:** February 04, 2007, 02:37:00 PM »

RJ,
750 beat me to it. Balance the carbs on the bike. Move the needle back. You might need to play with the jetting a bit. Pod filters change it up a lot. How clean is your tank? I would make sure it is very clean/flushed or run an in line filter. You might be getting junk into the bowls/ seats. With carbs, everything needs to be clean or they will give you hell. With pod filters you usually need to jet up? Something is not adjusted right. Make sure the air correction jet are clear. The little holes after the butterfly.

.RJ · « **Reply #31 on:** February 04, 2007, 03:36:18 PM »

I'll set the clip back to stock - gas tank is new and clean, nothing inside.

One other thing i noticed, when checking for spark I held the plug up against the frame and got a barely visible spark - and the plug has to be just about touching the frame before I get anything. Cause of concern, or is that how it is?

aptech77 · « **Reply #32 on:** February 04, 2007, 07:38:20 PM »

No, you should get blue lightning. What is the story on the coils, timing, etc.....

.RJ · « **Reply #33 on:** February 04, 2007, 07:57:24 PM »

Quote from: aptech77 on February 04, 2007, 07:38:20 PM

No, you should get blue lightning. What is the story on the coils, timing, etc.....

The points, contacts, etc all look good - it did run great up to 1/2 throttle so I would guess the timing is not that far off if its right. Is there a way to test the coils with a multimeter? Both manuals I have said to have someone else test them, or replace them.

Bodi · « **Reply #34 on:** February 05, 2007, 07:26:21 AM »

Weak spark can be low voltage at the coils, and with these old bikes it usually is. Measure the voltage on the black coil wires with the ignition on, slowly turning the engine over. It should be battery voltage with the points open (no load) and about 1 or 2 volts less with the points closed - any less can be a problem especially when starting - the starter motor pulls the battery voltage down quite a bit and even kick starting you'll be at a low battery voltage compared to running with the alternator providing power.You'll find lots of threads about wiring issues, basically the bullet connectors and switch contacts get corroded and cause the voltage drops. Cleaning and tensioning the bullets does wonders.
Teting the coils properly requires a coil tester. You can check the coil resistances and get a (poor) idea of coil isolation with a multimeter but most coil problems are due to high voltage breakdown internally which only shows up with a coil tester or a megger. Most "coil" problems on these machines are actually cable decay causing arcing to the frame or tank anyway. Good luck finding someone with the proper tester for a lost spark coil!
I'm not sure what you mean by "the plug has to be just about touching the frame before I get anything" - the plug has to touch the frame, you look for spark in the plug gap. Trying to jump a large gap just begs the spark to arc somewhere else - ie to the tank or frame as mentioned above.
Power droop at higher throttle could be spark related but probably you need bigger main jets with the pods. You can correct for the changed vacuum using the needle clips up to about 3/4 throttle where the mains are dominant.
The overflow problem is all about the floats and valves. The overflow tubes CAN leak but what you describe doesn't jibe with that. Make sure your float level is set properly. If you have brass tip float valves you can bed them in using a bit of brasso on the tip and turning the needle in the body for a few dozen turns. Wash the brasso out thoroughly. Rubber tip floats - don't try that. I've had very poor luck with aftermarket float valves, cleaning and polishing the KeiHin ones has worked best for me. Make sure your floats are completely free on the pivot pins. Polishing the pins using brasso or solvol and the inside of the float pivots using string soaked in the same pulled back and forth is a good idea. The float must pivot on the pin, not the pin in the support towers (although they should be free turning too).

.RJ · « **Reply #35 on:** February 05, 2007, 07:39:03 AM »

Anyone have a good set of ignition coils they want to get rid of?

I found a set on ebay for $75... I hate throwing parts at a problem though. i'd rather diagnose and replace as necessary. Tonight I'll change the needle clips back to the stock setting and go from there.

.RJ · « **Reply #36 on:** February 05, 2007, 04:00:47 PM »

Slides were in backwards. Go figure. I figured this out when reassembling them - I guess a good cleaning cant hurt either way.

Starts, idles, revs a lot smoother... go figure. I never thought much to check, since they're slotted, but if you put them in the wrong carb, they're backwards. I knew it was something stupid. Bike still was fussy to start and I havent ridden it yet - its about 12 degrees outside.

number13 · « **Reply #37 on:** February 05, 2007, 04:13:11 PM »

Quote

Anyone have a good set of ignition coils they want to get rid of?

I found a set on ebay for $75... I hate throwing parts at a problem though. i'd rather diagnose and replace as necessary. Tonight I'll change the needle clips back to the stock setting and go from ther

Hi RJ -

I am presently working on a 400 myself. I quickly found that a lot of
parts from a 350F, like coils for instance, are identical. Since the 400 is soooooo
"collectible" you will find 400 coils for 75 bucks, but the same part
listed for a 350 for 10 bucks. People, sheesh.
Best of luck!!

.RJ · « **Reply #38 on:** February 06, 2007, 02:36:16 PM »

A balmy 27 degrees outside, time for a test ride!

It misses and dies horribly under load. The higher the load, the worse it is. Does not seem throttle position dependent - it will miss just as bad at 1/4 throttle in 3rd gear going up hill as it does at 3/4 throttle in 1st get on flat ground.

Ignition coils seem the likely culprit but I'll check the ignition timing too. Found a set of 350 coils for $25 shipped on ebay, thanks for the tip. If thats not it... I have no idea.

aptech77 · « **Reply #39 on:** February 06, 2007, 06:14:07 PM »

Something is going on with that ignition system.....what is the reading of the coils?

.RJ · « **Reply #40 on:** February 12, 2007, 04:58:03 PM »

I found some time to do a little diagnostics this evening - first off, it wont start at all.

What I know:

Battery is new
Its getting fuel
Spark Plugs are new
I am getting spark from the points
I had to swap in the points/condensers from the spare engine I have, wasnt getting spark for cyls 2/3
Wiring from condensers to coils (blue and yellow wires) checks out
Coils have power at the black/white wires
Swapping in the spare coils I got off ebay made no change

All fingers point to both sets of coils I have being busted. Does anyone have a known good set I can buy? Or is it possible that the coils arent even the problem? Could the plugs have gotten fouled from all the poor running and cranking over?

750goes · « **Reply #41 on:** February 12, 2007, 05:42:24 PM »

Step back and take stock of what you have done, then get one thing done at a time - you seem to be jumping from one problem or perceived problem to another item................

1. check your timing thoroughly, from points gap to timing, then check it again...

1st problem solved

2. check for continuiy to your coil primary leads from the points - check for pinched wires or any joins...

2nd problem solved

3. if you have a test light or multimeter, check for 12 votls at the coil primaries. are all the plug leads secure

4. check for fuel flow from the tank to the carbs - running freely and clean - install an inline filter anyway - another problem sorted

5. check each carb for fuel flowing - from drain screw - - clean --ok - if not clean the jets again....

6. install the factory air box - even if you have to tape it together.....

7. set your idle screws to stock setting...

8. clean your spark plugs - check the gap - make sure the plug lead caps are all screwed on properly do you need to trim them back a bit

9. check you plug leads - are they cracked or brittle..

10. have a fully charged battery in the bike

11. check each of the battery connections - tight and clean
12. check the starter solenoid -connections tight and clean
13. check the starter motor connection - nice and clean

try it now

.RJ · « **Reply #42 on:** February 12, 2007, 05:57:42 PM »

Yes, I am jumping around a little... although I'm trying to cover everything I can. Originally, I knew the carbs were a problem - so I sorted that out.

3. if you have a test light or multimeter, check for 12 votls at the coil primaries. are all the plug leads secure

I've got 12V at the coils - both from the black/white wires feeding power to the coils, and with the coils connected, I have power from the blue and yellow wires

4. check for fuel flow from the tank to the carbs - running freely and clean - install an inline filter anyway - another problem sorted

Tank is new. Carbs are cleaned/rebuilt, everything is new - jets, o-rings, gaskets, etc.

5. check each carb for fuel flowing - from drain screw - - clean --ok - if not clean the jets again....

Done that - carbs fill up (and will drain) just fine

6. install the factory air box - even if you have to tape it together.....

I have not done this - but it shouldnt keep the bike from starting

7. set your idle screws to stock setting...

Carbs are all stock settings - idle screw, jets, needle clip, etc.

8. clean your spark plugs - check the gap - make sure the plug lead caps are all screwed on properly do you need to trim them back a bit

Plugs are new

9. check you plug leads - are they cracked or brittle..

Plug leads arent cracked, plug wires intact

10. have a fully charged battery in the bike

New battery

11. check each of the battery connections - tight and clean

Yep, clean, tight.

12. check the starter solenoid -connections tight and clean
13. check the starter motor connection - nice and clean

Starter turns over no problem

If I pull the plugs and hold them up against an unpainted part the frame/engine case i DO get some spark from both coils. I'm about to start pulling my hair out.

TwoTired · « **Reply #43 on:** February 12, 2007, 05:59:43 PM »

750goes has a good methodology for getting you running again. I wrote this while he was posting FYI...

Haven't heard of anyone who got a CB400F to work right with pod filters. They came close, but always at 1/8 to 1/4 the throttle response was poor with a wheeze/ stumble. This seems to indicate a need for a different cutaway ramp for the slides, which is not an easy adjustment. Most either lived with this foible or went back to the stock airbox to get the nice street performance back.

Do check your plugs. If you have changed the main jet and slide needle for a richer mix in order to get a better low throttle response, you could have easily sooted up the plugs so they short out on the carbon deposits instead of jumping the spark gap. New coils won't help carbon fouled plugs. Either clean your plugs or replace them. Spark plugs aren't new unless you just took them out of the box.
Remove one of the plugs and lay them on the engine to see if they spark >>across the gap<<. If so, look elsewhere for your starting problem.

Check your plug cap resistance and your plugs. If you have resistor plugs then replace your caps with non-resistor types.

To improve your low throttle response, turn your Idle Air Bleed screws in (1/8 at a time) to try and compensate for the lackof vacuum the pod filters provide for the carb bores. That is where the carb sucks on the jets. If the vacuum is lacking, less air should be provided by the Idle Air Bleed screws. If the Idle Air Bleed screws are hollow tipped, you may run out of effective adjustment range and then you must get larger slow jets in order to get more fuel into the carb bores at idle, which improves low throttle position pickup.

Hope you have a test track to determine your new Main jet size and slide needle settings. And, learn to read spark plug deposits for combustion conditions under actual loads. The pods or no air filter mean all the factory settings and adjustments are unlikely to be correct.

You are aware that ALL the basic tune up items must be spot on before dialing in the carbs, yes? Also, pretty futile without a carb synch. But, if you like doing the work over several times, then tune the carbs first.

Good luck!

.RJ · « **Reply #44 on:** February 12, 2007, 06:10:28 PM »

Quote from: TwoTired on February 12, 2007, 05:59:43 PM

Spark plugs aren't new unless you just took them out of the box.

Very true...

Whats the best method to clean them? I have to say, I've never really cleaned plugs before... I raced cars for a long time and i'd change them every winter, and sometimes during the season if I needed a different heat range, thats it. Most of my street bikes have been FI.

TwoTired · « **Reply #45 on:** February 12, 2007, 06:28:47 PM »

Quote

Whats the best method to clean them?

Sand blaster or bead blaster works very well. High carbon tool steel can do a decent job without leaving a layer of conductive steel in place of the conductive carbon soot.
PJ recommended a blow torch to burn off the soot. Sounds good, but I have yet to try it.

Cheers,

.RJ · « **Reply #46 on:** February 13, 2007, 09:16:38 AM »

Day off today.... double checked the wiring and picked up some more spark plugs - go figured, the old ones were fouled and with a new out of the box set she fired right up. :hammer: on me... never assume anything.

With 2" of snow on the ground I didnt take it anywhere, but I could "load" the engine with the rear brake and it didnt fall on its face like before. It has a bit of a stumble going from 1/4 --> 1/2 throttle. Still need to sync the carbs, set the timing, and make any carb adjustments, but i think I'm on the way.

Thanks all for the help and encouragement

.RJ · « **Reply #47 on:** April 04, 2007, 07:40:40 PM »

Just an update on the pod filters - perhaps a dead horse... but, I think I've got it figured out well enough. Bike has been running great, no issues - what I've found to work pretty well is stock needle clip position, #78 mains (OEM part) and 1-1/2 turns out on the air screws.

wardmoto · « **Reply #48 on:** April 04, 2007, 08:56:37 PM »

for what its worth...Open V stacks and open 4 into 4 meggas on a 350F...1 1/8th on the air bleeds, stock needles and 84 mains.

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Author Topic: 400F carb rebuild questions (Read 7373 times)