Accelerator Pump Question

[NJBronk]

I tried searching but couldn't get a clear answer for this question:

Does the accelerator pump "activate" only when accelerating from a stop/idle or does it also activate when accelerating from partial throttle/cruising?

I have a 500 CFM Summit carb that seemed to work pretty well out of the box, and I did originally tune the idle mixture based on vacuum, etc. but I have struggled with a hesitation/bog when accelerating from cruising/partial throttle. It seems to accelerate pretty well from a full stop/idle.

I keep trying to mess with the accelerator pump, assuming that's the issue, but I can't seem to get it to take up all of the slack without it also bottoming out at WOT (can't really get .015 feeler gauge in without the top adjusting nut moving up/separating from the arm). I feel like the adjusting bolt is almost as long as possible / running out of threads to adjust longer, which can't be right...

So that lead me to my initial question - am I chasing the wrong thing for a bog when accelerating from cruising? Maybe it's timing and not the accelerator pump?

I did unplug my vacuum advance and plug that port and it seemed to maybe run slightly better, but I'm not sure if that was an illusion.

Any ideas?

Here's a pic and video of my accelerator pump - is this still too much slack?

 

[justintime51]

The accelerator pump is just for the initial hit of throttle. It sprays more fuel to accommodate more air. Sounds like a jetting issue, or possibly vacum leak leaning it out. But my guess is the jets may be to small. Check the burn on the plugs, that should tell you exactly what's going on.

Sent from my SM-G998U using Tapatalk

 

[NJBronk]

The accelerator pump is just for the initial hit of throttle. It sprays more fuel to accommodate more air. Sounds like a jetting issue, or possibly vacum leak leaning it out. But my guess is the jets may be to small. Check the burn on the plugs, that should tell you exactly what's going on.

Sent from my SM-G998U using Tapatalk

But does that initial hit of throttle include from cruising or just from idle?

I will spray and check for vacuum leaks and pull the plugs as soon as I get a chance, thanks.

 

[ared77]

Includes from cruising. If you were going 30mph and decided you wanted to go to 50 (for example) as you mash down on the gas pedal the accelerator pump is shooting gas to prevent a bad "bog".

 

[NJBronk]

Includes from cruising. If you were going 30mph and decided you wanted to go to 50 (for example) as you mash down on the gas pedal the accelerator pump is shooting gas to prevent a bad "bog".

Thanks, that’s exactly when I have an issue…

Will still check vacuum leaks and plugs, but will mess with accelerator pump as well.

 

[Wild horse 75]

From looking at the manual online it looks like there are accelerator pumps for both primary and secondary circuits. So you may need to look at how the secondary one is adjusted as well. There are also vacuum power valves on each circuit. This add fuel when the vacuum changes such as when accelerating at a cruise. You should be able to actuate both accelerator pumps with the engine off and see them squirt into the venturis. I do agree though that it may be under jetted. Start with the plugs. They won’t lie. You should be able to google what colour to look for and what they mean pretty easily.

 

[pcf_mark]

What is your timing set at for idle and cruise rpm? Stick or automatic? What is the vacuum reading when you are cruising and try to roll on the gas? Each of these has impact on how the vehicle will pull away when you roll on the throttle while driving along. Generally if it pulls away clean from a stop you have done all you can with an accelerator pump. The secondary is important and should be adjust too but most likely your timing is lazy or your power valve is coming on too late.

How is your vacuum advanced hooked up? Ported or manifold vacuum? As a test I would try manifold vacuum. If it picks up you know where to start.

Shoot a video, engine off, looking down the carb while you gently open the throttle a bit. This will show the stream of fuel relative to throttle movement.

 

[cldonley]

When you open the throttle any amount, it draws air in much faster than fuel can get through the jets and into the air stream. For that reason, accelerator pumps push an extra squirt of fuel in at almost any throttle position. You can test that by openning the throttle slowly and seeing where it begins to depress the pump actuator. If your pump is working (or pumps, in this case) the next thing to check is timing before going into the carb and changing jets, IMHO.

 

[NJBronk]

There's no accelerator pump for the secondary (vacuum secondary - carb is SUM-M08500VS). It's a manual (3 on the tree)

Initial timing was at 12 degrees btdc and I went to 14 degrees to see if that would help (no pinging, so I left it there). vacuum at idle is 18-20 psi, I didn't look at vacuum while cruising and now my proportioning valve just started leaking so I don't want to drive until that's fixed (always something...).

vacuum advance is hooked up to ported vacuum ("Timed Spark Vacuum Source" as Summit calls it).

I can see the fuel squirt from the accelerator pump when I open the throttle.

I'm guessing it may be timing-related since the behavior changed slightly when I unplugged the vacuum advance. The PO installed a later Duraspark II module by splicing into the wiring from the firewall and to the distributor. The distributor is looking a bit worse for wear (replaced cap and rotor, but I wouldn't be surprised if other things are loose/worn). I've been considering starting fresh from the firewall plug to a new distributor with the connectors to convert to Duraspark II, so maybe it's time to just bite that bullet and go from there.

Hopefully between tonight and tomorrow morning I can get the plugs out and take a look.

 

[pcf_mark]

WIth a stick shift I prefer manifold vacuum not ported. Swtich to that and see how it feels. That will give you good advance at part throttle and it will go away when you floor it so it should not ping. Do not be afraid of the timing number until it either pings or your exhaust get a bad smell (excess NOx emission)

What i found on Broncos is the vacuum is really good (high) at cruising speed like 14" or so. This hurts the partial throttle performance because you are cruising at 14" or so and try to get on the gas. The power valve may not open until 8" so you have a fair amount of time before the extra fuel you need to move the truck gets going. Bumping this to 10" power valve can make a slug really come alive. This assumes you idle mixture screws are correct and you are not just lean in general.