Engine stumble when revving from idle

[Wilcey]

Hi all, bit of an issue with my old girl.
Its idling beautifully, and once it increased rpm from idle it also runs well.
The issue I'm having is when i first hit the gas the engine stumbles, unless I slowly let it on. Sometimes it also backfires through the carby at the same time.

I've changed the points, condenser and spark plug leads and also recently changed the exhaust/intake manifold.

Do any of you have any suggestions of what the problem might be? I really don't want to have to rebuild the carby, as this uni student cant easily afford the $400.. Thanks!

[hsv-001]

Start by putting a finger full of grease either end of the throttle shaft to stop any airflow caused by wear in the shaft or housing . Check for any air leaks around the carby base as sometimes the Bakelite spacer can warp . I use water in an atomiser for this .Play in the distributor or ignition timing incorrect can also cause this .Might be worth checking the by-pass jet for foreign matter or an obstruction . Just a few ideas ,its been a long time since I played with Stromberg's . Cheers Haydn 

[Harv]

My guess is that the accelerator pump plunger in the Strommie has shat itself.

With the engine off, remove the air cleaner, and look down the carb throat as you manually pump the throttle linkage. You should see a good mist of fuel being pumped in every time the throttle is opened.

If there is no mist/squirt of petrol, you need to replace the accelerator pump plunger. They come in the overhaul kits. Sometimes you can get away with giving the leather plunger seal a bit of love and reusing the old one.

Let us know how you go.

Cheers,
Harv

[DCE80]

I was about to write something then i realised Harv was all over it👍. Carby kit is about $35 fuelmiser is the brand. Cheaper at local parts store than ebay. If you can gap some points you'll have no problems putting the plunger in.

[DCE80]

[Wilcey]

Hi again fellas thanks for your replies! I had a look at the accelerator pump and it seemed to be working as normal. Ended up pulling the whole carby off and noticed one of the gaskets was stuffed. Changed that over, put it back on and it seems to be good! One more question, idle mixture adjustment... Do you turn the screw in or out, and how do you find the optimal position for it. Thanks again!

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[Harv]

This may help for the Strommie tuning:
FB/EK Holden Stromberg Carburetion Enthusiasts Guide

Cheers,
Harv

[Wilcey]

Hi Harv,

Thanks for that, thats a brilliant guide, very detailed!! I sorted the idle mixture, but it seems the stumbling issue has returned... I had false hope!

Any other ideas?

Thanks,

Wilcey

FYI If i slowly increase the throttle it doesnt stumble at all, with moderate speed it does,and if i hit it hard it will usually backfire or stall entirely...

[Wilcey]

Hi Harv,

Thanks for that, thats a brilliant guide, very detailed!! I sorted the idle mixture, but it seems the stumbling issue has returned... I had false hope!

Any other ideas?

Thanks,

Wilcey

FYI If i slowly increase the throttle it doesnt stumble at all, with moderate speed it does,and if i hit it hard it will usually backfire or stall entirely...

*backfire through the carby*

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[Harv]

Really does sound like the accelerator pump is not working. Check the carb throat for that squirt/mist of fuel again.

When you open the throttle moderately fast, the carb lets in more air, and it goes lean on one or two cylinders. The engine then hesitates (stumbles) until the fuel catches up. If you open the throttle up real quick, lots of air gets in and more cylinders go lean. Unburnt fuel (with lots of air) passes out the exhaust valve, until it hits the hot exhaust manifold. It mixes with the fuel/CO in the exhaust, and detonates (you get a nice bang).

If you add a little more fuel to the carb throat (a very small amount via a syringe or eyedropper) whilst opening the throttle, I bet the problem doesn't happen.

If however the pump is working, the pump shot (fuel volume) may be present (you can see a mist), but too small (not enough mist). Check:
a) that the pump link is in the middle pump link hole. The inner hole (closest to the throttle shaft pivot) delivers less fuel than the middle hole. The outer hole (furthest from the throttle shaft pivot) delivers more fuel.
b) that the pump stroke is 13/64”-15/64” (unlikely to be the cause).
c) pull out the the pump bypass jet and check it is operable and clean. It is unlikely to be "undersized" as all early Holdens used a No. 56 drill (0.0465”) accelerator pump bypass jet.

Did you reset the timing with a timing light when you did the points?

Cheers,
Harv

[fcute]

Had this happen to my HR, I thought it was the accelerator pump and changed it still no good. Turned out to be a worn hole on the link that went to the accelerator pump.

[Wilcey]

Only just got a chance to have a look now. The linkage is definitely on the middle hole. It definitely seems the pump is working, there really is a decent spray of fuel coming through when i pump it. Im thinking maybe the three gaskets that connect the throttle body to the rest of the carby might be buggered also. Maybe causing a vacuum leak?

Thanks

[Wilcey]

Adding to that, at the front of the carby (part closest to the front of the engine) where the accelerator pump linkage is, if i spray start ya bastard up in that area, the engine nearly stalls... Would that be that gasket leak i was thinking, or potentially something else?

[Errol62]

Any things possible. Tried a length of hose to your ear and listen for sucking around the suspect area?


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[mcl1959]

More likely to be the gaskets either side of the Bakelite spacer than any other. But while you have the carby off check for sideways movement in the accelerator shaft. Any sloppiness here will be the cause of the problem

Ken

[Wilcey]

I've had some form of success, I put a kit through the carby and overhauled the entire thing, I'm certain it was a gasket somewhere, but long story short the hesitation has stopped. Now however, I think the 'vent valve' ?! Is stuck open, which I don't recall being the case before. Refer to image.
Secondly the choke being pulled on doesn't seem to affect the idle speed so the engine just stalls. So somehow I've connected the fast idle adjustment arm incorrectly... Looking at the carby guide among other things online I cant seem to work out how it goes.
Thanks again all!


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[mcl1959]

Yes vent should be closed when engine is off.
Hard to tell what's wrong just looking at a picture though

Ken

[Wilcey]

I'll have to rip it tomorrow and have another look. Thanks

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[ardiesse]

Just off the top of my head -

If the fuel bowl vent valve is open that far at closed throttle, I think you may have connected the accelerator pump link to the outside hole on the lever.   Normally, it's supposed to be open only about 1/16" at idle, and then closes once the throttle's opened.
And if the choke has no effect on the idle speed, it may be that you've installed the fast idle cam upside down (it's the bit that the idle speed screw bears on).  Been there, done that.

It'll also help to straighten out the choke cable with a couple of pairs of pliers.

Rob

[Wilcey]

I sorted the vent valve, the tip part of the metal plunger rod had to be broken off as it is not required for my model of carby. So I think that's okay now. With the fast idle cam, this is the current set up.. As you can see the idle screw doesn't even make contact..

Thanks!


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