Sorry, kind of long...
Okay, here's the history. This is a 73 Aisin carb on a 71 F motor. The carb was rebuilt about 3 years ago by Mark A and ported for vacuum advance. It has been running fine since then. Vacuum at idle was always around 20-21" Hg. However, recently, the idle was a little off and it just didn't seem to be running as good as it had. So, I went through the carb adjustments in the FSM and detailed here on Mud in the FAQs (I'll post it below for those wondering). Once done, I decided to adjust the valves.
So, I went out for a test drive and wow, it runs like crap. I went back through the carb adjustments, and now I can't get my vacuum above about 17-18 at idle. I can set the idle, but it is running much rougher. The biggest problem is now I have a "hiccup" (best way to describe it) at cruising speed. Every minute or so, I get this hiccup. It is almost like it is running too lean or the main jet is partially plugged. This is only when at a constant throttle position. If I stomp on the accelerator, it runs fine.
So, what I have done is:
1. Rechecked carb settings - followed Mud directions below
2. Rechecked valve adjustments
3. Checked timing - 11* as always - I've tried adjusting it down to 7*, but no change
4. Checked for vacuum leaks with cardboard over the intake and with carb cleaner, and I can't find any leaks
5. I adjusted the throttle postioner to factory specs
So, what did I do? Why did my vacuum drop all of a sudden? Why is my idle okay (although a little rough), but I get a hiccup at a constant speed. Since it doesn't hiccup at full throttle, I'm assuming the secondary is working properly as well as the fuel pump.
Any ideas? I just put in new plugs and wires. The cap and rotor look fine, but I did sand them down a little to clean them up. This is a vaccum advance dizzy with pertronics.
One quick question regarding carb adjustments - do you need to adjust the idle mixture again after you set your final idle speed? Below directions say just to set idle down to 450 or so and then set fuel mixture then finally set idle back to spec. I didn't readjust idle mixture after setting my idle back to 600 rpms.
Any help would be appreciated!
Thanks!
Carb adjustment from FAQs -
Vacuum leaks need to be fixed first. Pull the air cleaner and with the engine running, block off all airflow to the carb. Don't use your hand because it could backfire out the carb. I use a piece of cardboard that is big enough it won't get sucked in. As the engine dies, listen to the idle. If it goes up before it dies, you have a vacuum leak/leaks. If it just dies, proceed to carb adjustments.
If you have a vacuum leak/leaks, best way I know to find them is with water. Carb cleaner/Ether works but burning down you, your house, or your truck isn't fun. Just buy a cheap 1.00 spray bottle. Engine is running and warm so I highly doubt a cracked block is even possible. Mist around the engine on all the hoses and fittings. Anywhere you spray and the engine rpms drop, you have a vacuum leak. Somewhat messy but this is another one of WD-40's many uses as well.
The idle mixture screw is only for idle. If you look at a carb out of the truck, you can see a small hole below the throttle plate. It is very important you have the idle low enough that the engine is running off that hole. There is another above this one and as you increase your idle, you expose the second hole.
Your engine needs to be at operating temp.
Set your idle down all the way. Most Cruisers will allow you to lower it until the engine dies. Turn it up around 450-500 so the engine is slow but running by itself.
Make sure the choke is open completely.
Air cleaner needs to be installed. Even though it makes it harder to adjust, you need it on because it changes the air/fuel ratio.
Hook up a vacuum gage to direct vacuum. Your brake booster hose will work. (be careful not to break the plastic taking it off. Been there )
Adjust for highest vacuum at idle. 15+ inches of mercury at your altitude.
There is a plateau for max vacuum as you open up the idle mixture, and once there, I would then adjust down (tighten) to the point where the vacuum begins to fall. There is some delay in response after an adjustment, so beware. - IDave
Set idle back to spec.
Check timing.
Go through carb adjustments one more time.
Set idle
Grab a beer and let the truck cool down.
Adjust choke cold or if manual, pull until truck runs and go buy more beer. Make sure on both that the choke closed doen't completely block off air to the engine. The stops usually keep it slightly open even when fully closed or choked.
The only reason for a choke is to increase fuel until the intake manifold can warm up. At colder temps, fuel doesn't atomize. Instead it tends to condense on the sides of the intake. Once your truck warms up, this is no longer an issue. Assuming everything else is ok, it should run fine. Keep in mind how well and efficient a Cruiser's intake is designed.......2" vs. 8" to the 1 and 6 cyl doesn't help in the cold.
Okay, here's the history. This is a 73 Aisin carb on a 71 F motor. The carb was rebuilt about 3 years ago by Mark A and ported for vacuum advance. It has been running fine since then. Vacuum at idle was always around 20-21" Hg. However, recently, the idle was a little off and it just didn't seem to be running as good as it had. So, I went through the carb adjustments in the FSM and detailed here on Mud in the FAQs (I'll post it below for those wondering). Once done, I decided to adjust the valves.
So, I went out for a test drive and wow, it runs like crap. I went back through the carb adjustments, and now I can't get my vacuum above about 17-18 at idle. I can set the idle, but it is running much rougher. The biggest problem is now I have a "hiccup" (best way to describe it) at cruising speed. Every minute or so, I get this hiccup. It is almost like it is running too lean or the main jet is partially plugged. This is only when at a constant throttle position. If I stomp on the accelerator, it runs fine.
So, what I have done is:
1. Rechecked carb settings - followed Mud directions below
2. Rechecked valve adjustments
3. Checked timing - 11* as always - I've tried adjusting it down to 7*, but no change
4. Checked for vacuum leaks with cardboard over the intake and with carb cleaner, and I can't find any leaks
5. I adjusted the throttle postioner to factory specs
So, what did I do? Why did my vacuum drop all of a sudden? Why is my idle okay (although a little rough), but I get a hiccup at a constant speed. Since it doesn't hiccup at full throttle, I'm assuming the secondary is working properly as well as the fuel pump.
Any ideas? I just put in new plugs and wires. The cap and rotor look fine, but I did sand them down a little to clean them up. This is a vaccum advance dizzy with pertronics.
One quick question regarding carb adjustments - do you need to adjust the idle mixture again after you set your final idle speed? Below directions say just to set idle down to 450 or so and then set fuel mixture then finally set idle back to spec. I didn't readjust idle mixture after setting my idle back to 600 rpms.
Any help would be appreciated!
Thanks!
Carb adjustment from FAQs -
Vacuum leaks need to be fixed first. Pull the air cleaner and with the engine running, block off all airflow to the carb. Don't use your hand because it could backfire out the carb. I use a piece of cardboard that is big enough it won't get sucked in. As the engine dies, listen to the idle. If it goes up before it dies, you have a vacuum leak/leaks. If it just dies, proceed to carb adjustments.
If you have a vacuum leak/leaks, best way I know to find them is with water. Carb cleaner/Ether works but burning down you, your house, or your truck isn't fun. Just buy a cheap 1.00 spray bottle. Engine is running and warm so I highly doubt a cracked block is even possible. Mist around the engine on all the hoses and fittings. Anywhere you spray and the engine rpms drop, you have a vacuum leak. Somewhat messy but this is another one of WD-40's many uses as well.
The idle mixture screw is only for idle. If you look at a carb out of the truck, you can see a small hole below the throttle plate. It is very important you have the idle low enough that the engine is running off that hole. There is another above this one and as you increase your idle, you expose the second hole.
Your engine needs to be at operating temp.
Set your idle down all the way. Most Cruisers will allow you to lower it until the engine dies. Turn it up around 450-500 so the engine is slow but running by itself.
Make sure the choke is open completely.
Air cleaner needs to be installed. Even though it makes it harder to adjust, you need it on because it changes the air/fuel ratio.
Hook up a vacuum gage to direct vacuum. Your brake booster hose will work. (be careful not to break the plastic taking it off. Been there )
Adjust for highest vacuum at idle. 15+ inches of mercury at your altitude.
There is a plateau for max vacuum as you open up the idle mixture, and once there, I would then adjust down (tighten) to the point where the vacuum begins to fall. There is some delay in response after an adjustment, so beware. - IDave
Set idle back to spec.
Check timing.
Go through carb adjustments one more time.
Set idle
Grab a beer and let the truck cool down.
Adjust choke cold or if manual, pull until truck runs and go buy more beer. Make sure on both that the choke closed doen't completely block off air to the engine. The stops usually keep it slightly open even when fully closed or choked.
The only reason for a choke is to increase fuel until the intake manifold can warm up. At colder temps, fuel doesn't atomize. Instead it tends to condense on the sides of the intake. Once your truck warms up, this is no longer an issue. Assuming everything else is ok, it should run fine. Keep in mind how well and efficient a Cruiser's intake is designed.......2" vs. 8" to the 1 and 6 cyl doesn't help in the cold.
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How does that saying go, if it ain't broke...
