Topic: A fistful of throttle (stalls my bike)

[drumstyx]

Continuing my finishing touches in my rebuild ('78 750K), and it's time to troubleshoot the carbs

She's all stock except a Delkevic exhaust and replaced hard parts from http://www.vintagecb750.com/products/6/fuel-system  (I know, I should've kept the old hard parts, but when I went to swap back the needles I buggered up the old collars. Oh well)

So when it's all warmed up (oil reservoir warm to the touch), idle at ~1100, choke off -- a fistful of throttle, that is, as fast as I can twist it, makes it bog and stall. Just a bit slower/easy on the open, or a double tap (wee bit of throttle before goosing it) and it's fine, because of course the accelerator pump is kicking in. Also, a fistful from, say, 2000rpm and up works just fine.

Since I know the accelerator pump is working, I thought maybe it's not delivering fuel fast enough, since these symptoms seem to me like it's much too lean momentarily. So I held down firmly on the accelerator pump actuator (where the spring attaches) and made sure it moved quickly with the throttle. It wasn't the smoothest rev, but it was much more powerful. Pretty much what I'd expect from 40+ year old technology (come to think of it, why don't modern carbs need accelerator pumps?).

So the question is, is there an adjustment to be done to the accelerator pump system? Do those springs wear out? Maybe I buggered it when removing/replacing? Or am I missing something?

I find it rather odd because I'm pretty sure it's otherwise running quite rich at idle -- I intentionally set the pilot out quite a bit more than stock, as I plan to dial it in.

[TwoTired]

Had a similar issue with my 750F.  Fixed by tuning up the accelerotor pump circuit.
The pump gaskets look like mickey mouse with holes in the ears.  Squashing the gasket, closed up the holes partially which reduced the pump's output volume.  I drilled/ cut the holes in the gasket so they matched the holes sizes of the mating parts.  This was after I super cleaned the check valves and proved they would both flow and then seal when back flushed.  Lastly, there is a clearance setting for the actuator rod.  Check this as some have been sold a similar pump diaphram with a shorter rod.  The clearance is listed in the CB750 shop manual supplements.

My bike has the stock air box with a K & N, and it also has the stock 4 into 1 and muffler.  I can twist the throttle as fast as possible and the engine just picks up and goes.

[Bodi]

Even years with the pump, I don't think I've found a 750 that would not bog/stall when cranked to WOT at idle. Not saying they can't, but I don't think I've come across one that will rev up cleanly like that.

[drumstyx]

Even years with the pump, I don't think I've found a 750 that would not bog/stall when cranked to WOT at idle. Not saying they can't, but I don't think I've come across one that will rev up cleanly like that.

It's not something you'd *ever* do in practice on the road, I just thought stalling from it might not be a great thing, and might mean something is up.

Of course, the concept of an accelerator pump circuit invariably reduces throttle response, so I'm not surprised these carbs are rather lazy.

I let it warm up for a good bit longer and found I could no longer stall it with WOT from idle, but it was very much bogging.

I expect it'll run better once I have the idle circuit tuned better. I've positively determined it's impossible without a purpose built right angle screwdriver. I could tune carb #1, but no way I was getting to #2.

@TwoTired: Yeah, I made extra sure of those holes when I cleaned the carbs (like 4 times over lol). Took a small drill bit to em just to be safe. Tested check valves, confirmed action on my bench, etc. Actuator rod clearance might be an issue, and would explain taking too long to kick in. It's an OEM (as far as I know) actuator rod/diaphragm, so unless there's an adjuster (let me look in the book...) I don't know what I'd do.

[Keith]

The auto industry, except for some offroad/drag racing, does not use carburetors, everything is now fuel injected. VW pioneered their MPC fuel injection in 1968 in their "type 3". In the late 70's the VW beetle used air-flow control(AFC) which used the now familiar air flow sensor. I can't think of many carbs that didn't use an accelerator pump. In my drag racing days, I modified four barrel carbs so primary and secondaries opened simultaneously, with accelerator pump spraying in all four. Having my license revoked ended my street racing days.

[drumstyx]

The auto industry, except for some offroad/drag racing, does not use carburetors, everything is now fuel injected. VW pioneered their MPC fuel injection in 1968 in their "type 3". In the late 70's the VW beetle used air-flow control(AFC) which used the now familiar air flow sensor. I can't think of many carbs that didn't use an accelerator pump. In my drag racing days, I modified four barrel carbs so primary and secondaries opened simultaneously, with accelerator pump spraying in all four. Having my license revoked ended my street racing days.

No motorcycle carburetor I've ever worked on other than these 750's had accelerator pumps. Of course, all the other carbs I've worked on have been CV carbs, and being that there's 'continuous vacuum', I guess it's not necessary.

[HondaMan]

No motorcycle carburetor I've ever worked on other than these 750's had accelerator pumps. Of course, all the other carbs I've worked on have been CV carbs, and being that there's 'continuous vacuum', I guess it's not necessary.

That's correct: the CV carbs were the emission-'friendly' way to reduce complexity while meeting emissions requirements of the late 1970s and thru 1986.

Normally the accelerator pump will pick up the PD carbs fully to about 1/2-throttle snap, if they are set right and if the little O-rings that tie all the carb's pump holes together are tight and soft. I have found in Keyster kits that these O-rings in particular are about 0.2mm too thin, resulting is poor delivery due to compressibility in the whole string of them. I also had one "off brand" accelerator pump (2006, I think it was) that had a too-stiff diaphragm material, so it acted just like an old one. Still another one had a check valve that wasn't sealing right, being parly open for part of the stroke, so it barely squirted at all.

A few other things can cause this, though, not directly carb-related. One is: less-than-perfect seal of the hoses at the head, from being hard or having stretched-out hose clamps. Another is: too much spark advance at idle, or the 2-3 side timing "jumping" back-and-forth from a bent points shaft (look with a timing light). Another: one or more float bowls being too low: with the tiny float levels of the PD carbs, this is a near-critical adjustment. In particular, the K8/F3 carbs "B" type (California emissions) are 14mm level: they work MUCH better at 12.5, wetting the idle circuits about 10% more.

[drumstyx]

No motorcycle carburetor I've ever worked on other than these 750's had accelerator pumps. Of course, all the other carbs I've worked on have been CV carbs, and being that there's 'continuous vacuum', I guess it's not necessary.

Normally the accelerator pump will pick up the PD carbs fully to about 1/2-throttle snap

You're saying it's normal for things to bog beyond that? 1/2 throttle snap is definitely easy as it sits -- If I'm even the slightest bit delicate with the throttle, it's perfectly responsive, just that I can kill it if I'm really, really hard on it.

With other bikes, I'm used to people telling me that if it bogs at all in any position/rpm that something's wrong. But again, I'm used to 'modern' CV carbs.

Ignition is on the advanced side, but no more than ~1mm advanced at idle, and it's pretty stable with the timing light. All carb boots are new (non-oem, so we'll see how long they last, but for now they are *brand spankin' new* so they're fine).

Floats are within a couple mm when it comes to actual fuel level (clear tube method). I'd say 1-3mm below the seam...

[HondaMan]

No motorcycle carburetor I've ever worked on other than these 750's had accelerator pumps. Of course, all the other carbs I've worked on have been CV carbs, and being that there's 'continuous vacuum', I guess it's not necessary.

Normally the accelerator pump will pick up the PD carbs fully to about 1/2-throttle snap

You're saying it's normal for things to bog beyond that? 1/2 throttle snap is definitely easy as it sits -- If I'm even the slightest bit delicate with the throttle, it's perfectly responsive, just that I can kill it if I'm really, really hard on it.

With other bikes, I'm used to people telling me that if it bogs at all in any position/rpm that something's wrong. But again, I'm used to 'modern' CV carbs.

Ignition is on the advanced side, but no more than ~1mm advanced at idle, and it's pretty stable with the timing light. All carb boots are new (non-oem, so we'll see how long they last, but for now they are *brand spankin' new* so they're fine).

Floats are within a couple mm when it comes to actual fuel level (clear tube method). I'd say 1-3mm below the seam...

You shouldn't notice any big bog when riding: typically the largest "snap" a wrist can do from the seat is about 1/4 throttle range. I wonder though: one of the biggest 'bugs' I've fought lately with the PD carbs is in the rebuild kits we are getting - the springs in the float valves are so strong that I've had to deeper the bowls as much as 2mm (static setting) to get them to fill up properly. The OEM springs were quite soft, and stiffer ones like I see today cause the bowl to fill slowly up the last 2-3mm of depth. Likewise, on aftermarket accelerator pump I had (this was 3 years ago) installed on a K8 had such a stronger spring under the diaphragm that it didn't let the pump fill up all the way. I took that one out and put the OEM spring back in under the new diaphragm and it worked much better.

Also: if this is not a new, rebuilt engine, but has been sitting a long time, OR if it is a brand-new top end, don't expect a crisp throttle snap yet. It takes some time for the compression to balance across the cylinders. They must be within 10 PSI (5 PSI is better) in the later 750 for them to be snappy with these lean-burn carbs. If just one cylinder is lower than the others, that one will spit back at the carbs when the throttle is snapped, making the whole engine stumble.

[drumstyx]

You called it, brand new rebuild. Also very strong strings in the needles -- I was able to set the heights with the carbs straight upside down, with no compression on those springs... That said, the fuel levels still seem acceptable to me....even a little rich maybe. If I still have them separated by cylinder, I'll swap the old ones back in if I have the carbs off again.

[HondaMan]

You called it, brand new rebuild. Also very strong strings in the needles -- I was able to set the heights with the carbs straight upside down, with no compression on those springs... That said, the fuel levels still seem acceptable to me....even a little rich maybe. If I still have them separated by cylinder, I'll swap the old ones back in if I have the carbs off again.

Yeah, I've struggled with those before (lots more than once!). It's harder to "fix" in the PD carbs because they already run lean. I add +1mm to the float height, at the least: a little more if it stays dry. I've also found the Keyster idle screws didn't act quite right in the K8 carbs (PD42/a), making it stumble (no clue why: they measured the same, as far as I could measure). I ended up "fixing" that one in the end by changing the idle jet from the stingy #38 pilot size to a #40 (stolen from some old PD41 carbs), with their emulsifier holes severely "cleaned" to make them slightly larger. That worked out pretty well. Don't tell the EPA...