Vacuum test results help? (1 Viewer)

[andrewbridges1]

Absolute newbie here. I have a stutter as soon as I hit the throttle. I did a vacuum test from the little hose in the carb. These are my readings. It actually seems like it runs better without it connected to the vacuum advance. Less of a stutter but still not gone. What would y’all do next?

 

[brian]

first thing, the vac advancer should be connected to a ported vac source....one that only has vacuum after the throttle plates open off idle.

 

[DeskJock]

On my rig, I use a port on the other side of the carb. That port your using I have a plug on it.

When the engine is at idle - I get nearly vacuum to the vacuum advance and with a timing light it shows no difference on the fly wheel —- until I throttle up the engine.

I am using the City Racer carb.

 

[Grayscale]

The F-carb in pic likely has no 'port' vac source, just manifold.

 

[fyton2v]

If the gauge is steady, you're pulling enough vacuum IMO. Perhaps your accelerator pump needs replacing? That could cause hesitation/stutter when you first accelerate.

See the disclaimer in my sig.

 

[andrewbridges1]

If the gauge is steady, you're pulling enough vacuum IMO. Perhaps your accelerator pump needs replacing? That could cause hesitation/stutter when you first accelerate.

See the disclaimer in my sig.

I have already replaced it and it’s working perfect. I’m about to pull the valve cover to re seal it so I’ll check the valves? This is the stuff I’m bad at. I can do valves, pistons, rods, whatever just the nitty gritty I’m terrible at

 

[charliemeyer007]

I'm a fan of straight mechanical advance for the dizzy, let engine RPM determine the advance way simpler than vac advance/retard IMHO its a tractor. In the summer I run the timing out the window, in winter I do it by the book. I set my valves on the loose side - better to hear them than to burn them. I make sure my plugs have a square edge on the electrode.

Go out on a dark night, open the hood and have someone start it while you look at the wires and such for dancing lightning. Scrape the contacts in side the cap - aluminium oxide is an insulator. While you are there how loose is the rotor shaft in the dizzy.

Try some higher grade gas or a different station.

 

[Pighead]

That's the wrong port to go to a dizzy. What dizzy do you have? If it's a vacuum retard and it's getting that much vacuum at idle things will get weird. Does your carb have this port?

 

[fyton2v]

I have already replaced it and it’s working perfect. I’m about to pull the valve cover to re seal it so I’ll check the valves? This is the stuff I’m bad at. I can do valves, pistons, rods, whatever just the nitty gritty I’m terrible at

Cool. Listen to pighead. Get your timing and vacuum tubing correct, then report back. The problem you’re chasing is probably not caused by maladjusted valves.

 

[DeskJock]

That's the wrong port to go to a dizzy. What dizzy do you have? If it's a vacuum retard and it's getting that much vacuum at idle things will get weird. Does your carb have this port?

View attachment 3805932

I was going to post the back side of my carb, pig head beat me to it.

That’s the vacuum port I use ( next to the idle mixture adjustment)

Speaking of idle / fuel mixture - I followed the factory service manual as well as I could - but - with the factory air box in place - I can’t really get to that screw.

So - I’ve tuned the mixture with out the air box, the. Fine tuned it with the air box on.

My rig is messing ALL of the smog equipment, which in my opinion that idle needs to be higher than factory spec - reason being it that the engine no longer has the load of the smog system and if I run the engine at factory spec - I believe that I have less than optimum air / gas in the cylinder head.

As Charlie stated, I run my timing advance right at the window, more the motor pings ( running high octane fuel). At factory spec, bottom end torque goes bye bye ( or raise the idle - which helps.

If you still have points, gets a petronix replacement - which takes a variable out of the equation. You can never set the factory point to spec and - the runner on the distributor cam wears - continuously changing your timing.

 

[Grayscale]

Absolute newbie here. I have a stutter as soon as I hit the throttle. I did a vacuum test from the little hose in the carb. These are my readings. It actually seems like it runs better without it connected to the vacuum advance. Less of a stutter but still not gone. What would y’all do next?

Check the PCV valve for movement, remove and clean with solvent, if needed.

How do the spark plugs look? Fouled?

Early ignition systems are just fine. Points can be serviced just like spark plugs can be gapped. If their service is too inconvenient, it might be time to rethink time, space, and other resources going into the project; after 50-years it is considered an antique, not exactly equipment for luxury or convenience. Also, carbs without 'ported' vacuum are not necessarily going to be responsible for engine stutter.

Other service would be rebuilding the carb for corrosion and varnish. Fuel injector cleaner might be good preventative maintenance, keeping varnish from forming in the carb and restricting fuel flow - it is like the solvent soak part of the carburetor rebuild, but without disassembly.

I think that the lack of throttle response with a cold carb and intake manifold is kinda normal on these engines. Fuel-atomization issues such as cold temps, and a non-laminar intake charge flow from the carb to the intake head ports can 'paint' the internal surfaces with the bigger fuel droplets, leaning-out the Air/Fuel ratio in the intake, temporarily. Also, if you look at the top of typical intake valves, they are all caked-up with polymerized, and / or ashed oil deposits - again fuel injector cleaner can help the engine breathe easier, and, leaky valve stem seals can be replaced. If the stumble is present on a warm motor, it is probably something you can address.

 

[Pighead]

It actually seems like it runs better without it connected to the vacuum advance.

That could very well be if your dizzy is a vacuum retard model. If it is, cap off that vacuum line both ends.