General Hesitation on acceleration

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General Hesitation on acceleration

jtrashy

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Jan 19, 2019
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SF, CA
recently installed 650cc, ~300 miles in. Starts and runs great.
Has begun to hesitate on acceleration. ASSuming it's a fuel issue.
Fuel Filters? Carb? Jets? Air filter? valve adjustment?

here is video of what is sounds like
[ame]https://youtu.be/5RFJkha60oU[/ame]
 
Also.... it doesn't do it if the idle is greater than 1200 or if I rev the engine prior.
 
This is common on most of these engines due to the carb not having any form of accelerator pump on them which gives an extra squirt of juice to avoid the 'hesitation' you speak of.
 
My 650 behaves similarly. I’ve tried to adjust it out with timing and carb adjustments, but it’s pretty much the same. I may increase my idle speed a little (mine is around 800 rpm), as I also find that there’s little to no lag like that when blipping the throttle from a starting higher rpm.
 
The transition from the idle circuit to the main circuit is not exceptionally good on these carbs. Not very obvious with the 18 HP 500cc engine, but with the 650cc its probably more obvious.
 
Initially when I had a 650 engine in my 500, I found that going up to the next idle jet helped---it gives just a little bit more fuel until the main jet comes into operation.
 
recently installed 650cc, ~300 miles in. Starts and runs great.
Has begun to hesitate on acceleration. ASSuming it's a fuel issue.
Fuel Filters? Carb? Jets? Air filter? valve adjustment?

here is video of what is sounds like
https://youtu.be/5RFJkha60oU

If the engine didn't hesitate on acceleration initially but now it does, then something has changed. 300 miles isn't much so I'd check valve clearances, idle jet (accessible on the side of the carb - so no need to strip it) and ignition timing. If everything checks out ok, I'd try advancing the ignition timing a little just to see if it helps. I worked in a Fiat dealership back in the day and sometimes had to do this to cure a problem child - as long as the engine doesn't 'ping' when accelerating in top gear, it shouldn't do any harm. (ymmv)

As 'The Whitakers' has (have?) said, the lack of an accelerator pump in the carb tends to result in the type of hesitation you describe, occurring just above idle, and not present if idle speed is raised significantly.

However, single choke Weber fixed-choke carbs have what is called a progression circuit. When the throttle valve is opened slightly from the idle position, there is a tendency for the mixture to lean-out and thus cause the engine to hesitate/bog down. To overcome this problem, one or more transition orifices (aka progression holes) are drilled in the inner wall of the carb barrel, intersecting with the idle fuel circuit, just above the edge of the throttle valve (aka butterfly valve). As the throttle valve is opened from the idle position, these holes are now subjected to engine vacuum and allow some extra fuel to flow from the idle circuit, hopefully :) preventing the mixture from going too weak and thus avoiding hesitation. As the throttle is opened further, fuel is supplied from the emulsion tube/main jet and finally from the main jet.

These progression holes don't pass any fuel at idle speed as the throttle valve is almost closed, thus they don't experience any engine vacuum. Also, both the idle jet/hole and progression holes don't pass any fuel when the throttle is more than say 1/4 open as they are no longer experiencing enough vacuum to cause fuel to flow, the engine is now running on fuel supplied by the emulsion tube and main jet.

The reason the engine doesn't hesitate when the idle speed is raised to e.g. 1200 rpm is that the progression holes are now in action.

On the larger twin-choke 'sporty' Weber carbs, there can be up to 5? staggered progression holes in each carb barrel. Sometimes there's a removeable plug, so you can drill more/mess with things/mess things up and then re-seal with another plug.

On less 'sporty' carbs, you might befriend a Dentist, these guys have brilliant little high speed right angle rotary burrs which are ideal for drilling/enlarging progression holes. Another option is to modify i.e chamfer or notch, the edge of the throttle plate (butterfly valve) so that the progression hole/holes are uncovered a little earlier and subjected to engine vacuum so that they flow fuel a little earlier.

Just bear in mind that while it's easy to drill holes, it's not so easy to undrill them. :(


Any time I hear of someone having carb problems and they mention that they previously fitted a new butterfly valve and spindle, I can't help but wonder if they got the new valve exactly centralised - it doesn't take much misalignment to really mess things up.

I vaguely seem to recall that one of the 500's or 126's was fitted with a carb that had an accelerator pump? Hopefully, the cognoscenti will chime in to confirm or refute.....



Al.
 
I don't have a tuned lump. I've got a standard 594cc and 28mm Weber carb installed. I run a higher idle, around 1000rpm for two reasons.


The first is this keeps the oil pressure a bit higher, especially when the engine is very hot in the summer months. Although its fine to have the oil light flicker when hot and at idle, for the small additional fuel burn, I'd rather have good oil pressure at all times.


I prefer to have the engine tick faster over so less lag and to ensure good fuel flow available. Maybe not needed but this is just how I roll.


Regards
Rob
 
This is common on most of these engines due to the carb not having any form of accelerator pump on them which gives an extra squirt of juice to avoid the 'hesitation' you speak of.



Hi Whitakers

Would a electronic fuel pump solve this problem? I ask this as I have recently had my 652cc completed and started for the first time with said fuel pump and mine seems to rev quick nicely.
 
Hi Whitakers

Would a electronic fuel pump solve this problem? I ask this as I have recently had my 652cc completed and started for the first time with said fuel pump and mine seems to rev quick nicely.



Frankly no...…..the accelerator pump is merely a diaphragm type affair on the carb that when you press the throttle the diaphragm pump squirts a shot of gogo juice into the carb which stop the temporary bog down that you are experiencing.....
 
Hi Dan; what carb have you fitted to your '652'? Most of the bigger carbs fitted to the 500/126 engines have an accelerator pump. What sort of pump delivers the fuel to the carb is almost an irrelevance, because when the carb has the fuel, how it delivers it is entirely down to the design of the carb.
 
Hi Dan; what carb have you fitted to your '652'? Most of the bigger carbs fitted to the 500/126 engines have an accelerator pump. What sort of pump delivers the fuel to the carb is almost an irrelevance, because when the carb has the fuel, how it delivers it is entirely down to the design of the carb.



Hi Hobbler I still have the standard 28imb Webber at the moment. IMG_9073.JPG
 
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Something like this happened to me. What I did was to check all (both) jets and also removed the emulsion tube. I found at the bottom of the tube and partially covering the lowest holes a bit of the tiny rubber seal that had partly softened and been drawn down the tube. I believe that it is the very small seal which is supplied with the carb rebuild kit and would normally sit at the top. Now I’ve got good acceleration - on a standard 499cc setup admittedly.
 
I had a similar problem or even worst after i installed my overhauled 650 on. Removed the carb and found it blocked with paint chips. Maybe you can start from there.;)
 
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