Topic: Accelorator pump question

[KowalskiCraig]

  Hey guys Im new to this forum and I have an 81 Honda CM400 which I believe is the same as a CB400 basically. Well my problem Im having is my bike runs perfect at idle and when Im cruising at higher rpms but if Im just toolin around in the city it feels like it wants to die off but still manages to stay running. It does this for the first 5mins during warmup when I rev it while its sitting in the driveway. Once its warmed up it revs just fine all the way thru, but under load it just bogs down. I have rebuilt EVERYTHING in these carbs, cleaned and new jets and everything. The only thing I didnt replace was the accelorator pump but the housing flows freely when i blow compressed air thru it. If you have any ideas please help me.
  My question is...do the holes in the accelorator pump diaphram have to be clear and open? Should they both be holes or should one be plugged with rubber? When I got a used one one hole was plugged the other was open so I took a small drill bit and opened the other one as well. Also how pliable should that rubber diaphram be? I mean it only moves up and down a fraction of an inch and its mechanical so it didnt seem to me that it would matter that mines a bit on the stiff side.
HELP! Ive taken these carbs in and out 5 times now and still same problem!

[Inigo Montoya]

How sure are you that your timing is right and the advance is working properly?

[KowalskiCraig]

Well I cant say Ive checked the timing at all. I didnt think the timing could get out of whack on these bikes. It seems to run awesome at high rpms and idles smooth no misses what so ever. As for the advance I thought that was not adjustable either. Im new to this old of a bike. I have a Clymers manual and a decent amount of knowledge. Will I find how to check/adjust this in there?

[KowalskiCraig]

Bump. I havent checked timing yet but any other thoughts on carb issues?

[Inigo Montoya]

Well the replacement pump I got only had 1 hole. It seems to work just fine though. if you pull off the airbox and twist the throttle while looking into the carbs, you should see a stream a gas shoot into each carb.

[crazypj]

Timing is electronic and not adjustable,
 I dont remember accelerator pumps on those carbs, accelerator pumps are in/on float bowls
 You don't mean air cut valves do you ? (about half way up side of carb)
 Any holes in diaphragms are usually a bad thing.
 Do you have stock airbox fitted?

[KowalskiCraig]

I will check the fuel stream when I get home. The holes in the accelorator pump are supposed to be there for gas passage I believe. And no Im not mistaking for the Air Cut off valves. I replaced those with brand new ones as well. My carb has a manual accelorator pump attached to one of the bowls on the outside that has a shaft that rides on the throttle linkage. There are no holes in the actual diaphragm. I do have the stock airbox and stock air filter and stock exhaust so I have stock jets as well.

[KowalskiCraig]

Ok so I checked for a spurt of gas out of that little nozzle in the carb...I did it with the bike off and just twisted the throttle and didnt see a visible spray. So how do I clean that? From what I can tell its part of the fuel rail and you cant really take it out to clean. Any suggestions? Ive soaked these things in vinegar and carb cleaner and used a compressor to blow out all passages  and thought it was all clear. But apparently not...

[crazypj]

clean out passages in float bowl and standpipe (use carb clean aerosol with tube)
 It's a mechanical pump, if operating rod is fitted properly you can check operation off bike.
 should be same/similar to 1977/78 CB750.

[kirkn]

IIRC, there's a passage off the float bowl that supplies the accelerator pump diaphragm chamber thru a one-way check valve.  Gravity keeps that chamber full, just like the bowls. 

When the accelerator pump rod is pressed by the throttle linkage, the diaphragm causes fuel to squirt up into the little opening in the throttle throat.  Typically, there is a pipe that spans the gap between the two carbs so that the one pump feeds both squirters.  If you completely dismantle the carbs, all that volume can take a bit of time / pumping action to fill. 

But, you're on the right track as far as using carb cleaner spray and/or compressed air to blow 'em out.  If you have an air nozzle with a rubber tip, you can press it hard and feel the air blowing out the tiny nozzle with the tip of a finger.

On the other hand, I'm not convinced that that's your trouble.  The accelerator pump only activates when you open the throttle.  At the steady-state tooling around that you're describing, the accelerator pump is not really moving. 

Under load, that might be a different story, but even then, if you open the throttle under a load and the AP is not working, EVENTUALLY, the carbs catch up and supply the proper fuel.  The AP was just an add-on in an attempt to eliminate the hesitation that would otherwise develop.  The way you're describing it, you're not experiencing an HESITATION as much as a complete lack...

As for timing, are you sure that the pulse generator isn't mounted on a mechanical advancer mechanism?  In the early days of point-less ignitions, the spark was indeed generated electronically, but the ADVANCE function was still handled mechanically.  It was only later that the advance function went electronic as well.  I guess it would depend on the exact year of your bike.  And I could be wrong for that model anyway.  It could be that they came along AFTER the migration to full-electronic.

Just a thought...

Kirk

[KowalskiCraig]

Well thank you for explaining what the accelorator pump even does! Ive been tryin to figure that out for a while haha. So its in a 1981 Honda CM400E if that helps. basically the same bike as the CB400 from what Ive heard. I know the accelorator pump housing allows gas or atleast the check valve flows freely. This carb does have a tube that connects between the little spray nozzles in the carb throat. Not sure if that helps.
When I first fire it up I cannot rev it at all unless the choke is on because it just falls flat on its face. Once it warms up for 5 mins then I can start to rev it and adjust the air mixture screws on the bottom enough that I can get a nice full rev for the most part. Then when I put it in gear and try to take off in first I get to about 5mph then it just wants to die out unless I feather the clutch and give it gas to get past that dieing point but then I gotta hold on because it takes off! The advance would sound like the cause. I have never taken off the side engine cover to check this so I guess thats my next step before I take those stupid carbs out yet again to clean them for the 6th time.
Thanks guys you are a big help

[crazypj]

All the 3 valve CB/CM 400/450 had similar generators/ignition systems
 They have a high speed and low speed circuit without mechanical advance.
 I have a schematic for it somewhere, (I was at Honda UK doing a course on CB400T)
 Officially, the timing is not adjustable, but, by filing the holes on pulse generator mounting, you get maybe 5 degrees movement which is enough to fine tune it.
There is a 'button' on flywheel to trigger things.
 It has fuel screws not air screws, further out is richer.
 I think stock setting is about 3 or 3.5 turns out (really should have had a bigger pilot jet)
 Unless you used genuine Honda jets you may have a generic 'fits nothing properly' jet in there
 Can you remember sizes in carb kit?
 The accelerator pump diaphragm is good? (you removed it before blowing air through housing?)
 I haven't done much with them since 1980

[KowalskiCraig]

Yup that sounds just like what my bike looks like under the left engine cover...I couldnt find any sort of timing to mess with just that knob that passes the electric pickup. The jets were 72/118 and you are right the full rebuild kit was not Honda. I ordered it from www.siriusconinc.com but it all seemed to fit snuggly. As for those little screws under the outlet side of the carb I thought those were air screws...hmm Ive never had them backed out THAT far. Ive gone to about 2 - 2-1/2 I was pretty sure the book called for 2 turns out as stock. Do you suggest I take the carbs out and maybe run to my local honda shop and buy honda jets and try to clean out those gas nozzles?

[crazypj]

I don't have carb to look at but I think pilot jet is around 35~38?
May be pressed in on some models
 Sounds like it's clear or it wouldn't idle with choke off.
 Can you get/post pic of carb inside float bowl?
 There may be something missing.
 The fit of jets isn't usually a problem, the accuracy of hole size is though

PJ

[KowalskiCraig]

Alright I will probably get around to tearing them out tonight or tomorrow and post a pic of the carbs inside and out. I do not believe I changed the original pilot jet as I think it was pressed in. Like I said it does idle perfectly once warmed up.

[KowalskiCraig]

Alright I hope these can help. Also before I ripped them out I warmed it up revved it a little then shut it off and checked the plugs. Right side is totally lean Left is just a tad rich. I backed out those two air/fuel screws to 3-1/2 like you suggested but it didnt change much.
First pic with AP still on

Second both bowls off

Third with jets still in 72/118 and the one under the rubber cap is pressed in and I did not replace them and do not know how.

Next is the jets taken out they all looked perfectly clean still.

This is the AP diaphragm with the spring. The bump outs on the left are open holes to allow gas I am assuming. It came with 1 plugged so I opened the hole.

Last is just a shot of the nozzles I did not see gas come out of.

Im going to take these to a buddies who has a stronger compressor than I do to blow all the passages out but your help after that is appreciated!

[crazypj]

I thought that was carbs you had.
Pilot looks slightly oxidised but it runs fine so not worth messing with.
 Is that a second jet screwed in under primary main jet?
 I use a strand of copper wire to burnish jet holes, they can have oxidation reducing size.
 Did you polish vacuum pistons?
 Tops are not distorted? pistons can be lifted easy and drop free (can cause pistons to stick)
 The alloy plug above mixture screw can probably be removed with high pressure air (some are staked in place)
 There are 3~5 bypass holes under it

[KowalskiCraig]

Yea it idles great and all the jets are brand new and wide open and the “second jet” under the primary you see is the nozzle jet the primary sits in to atomize the fuel?. It has several holes on the side of a hollow tube which the needle rides inside. I did polish the pistons and they move with very little air so they slide nice. Im assuming you are talking about the piston housing being distorted? but when the carb is all together they move very freely. Where would I blow high pressure air in to get those alloy plugs to pop out? I will do everything I can over the next days to clear all passages in the carbs. Any idea why the right cylinder would be so lean and the left would be slightly rich? I checked the float setting and its right on target at 15.5-16mm when the book says 15.5mm stock float height.

[crazypj]

The jet under primary main.
Sounds like its blocked or partially blocked or maybe the discharge drillings?
It deals with the transition from around ~2,500 rpm until secondary main starts delivering fuel (somewhere around 4,500) That's probably why, when slide lifts, it runs OK (ish) at higher rpm
The secondary main is the one with needle/needle jet, the emulsion tube is the thing with holes in the sides screwed into carb body with the 118 main jet

[KowalskiCraig]

Hmmm ok I will have to take a closer look at that jet. Is that a piece I can take out to clean? I do not remember there being anything inside that hole to pull out but I could be looking right past something. I will check that tonight. If thats what it does it makes complete sense because that is exactly when my problems start happening right around 2500-3000rpm

[crazypj]

Isn't there a screwdriver slot in it?
Looks like edge of one in pic

PJ

[KowalskiCraig]

Yup I broke my screw driver blade tryin to get it out too! Now what? Maybe apply a little heat to the body and buy a new small screw driver haha. Should be able to work on it tonight a little. I didnt have much time last night.

[crazypj]

I have a modified Snap-On electrical screwdriver that's a somewhat snug fit in hole. (edges ground down, snug fit in slot)
 May as well give it a tap before trying to unscrew, may help jar it loose?
 Probably want some type of release oil in there and some heat (not too much heat, die cast zinc melts pretty easy)

PJ

[KowalskiCraig]

Its alive!!!! Thanks to all your help I was able to really dive into the carbs and find the clogged passages and got those pilot jets out to clean too! Its not perfect but it doesnt stall out anymore. Now when I have the spare cash I will replace the exhaust ring gaskets around that stupid plenum underneath and it should be great. Man what a relief Ive been without a bike until today!
Thanks again

[KowalskiCraig]

Well I let it sit for 3 days after I said ITS ALIVE and it started doin the same thing again. So i spent the last week taking it apart cleaning it adding an inline fuel filter got ALL passages unclogged in the carb. Gas comes out of all the right orrifices haha. It still bogs down when I try to get goin. Revs pretty decent in neutral. I do have exhaust leaks but Ive seen worse. Do you think I need to maybe up the jets? Can a bike grow lean over the years? The only thing might be causing any bogging I can see is the boots from the airbox to the carbs might not be seating 100% anymore, but there are no cracks in them. Any thoughts would be appreciated! Have you ever seen a grown man cry?! hahaha