82 720 pickup bogs down after engine is warm

[Dono]

Hello, everyone. I recently aquired a 1982 Datsun 720 which is in great shape, but has some issues. I found this forum immediately and it is already proving to be an invaluable resource. Thank you all for the wealth of information available at my finger tips!

 

One of the problems that needs resolving is that the engine is bogging down after it warms up and the choke is opened up. It runs great when cold. After a few minutes it is seriously lacking power.

 

It is bogging down in a way that is especially noticeable during slow acceleration at low rpms. With WOT or at 3000+ rpms it feels weak but much smoother.

 

I found an older thread which I now lost that mentioned the jets might be clogged up. Today I took off the float bowl cover and sprayed carb cleaner down the two openings at the bottom in hopes that might help. No noticeable difference. I also replaced the fuel filter to no avail.

 

There are a bunch of openings on the carb and one on a hose from the brake booster that were plugged by a previous owner.I'm guessing the plugs are for emissions control stuff only, but I'm having a hard time figuring out exactly what they do. I took some pics and will try to get them uploaded ASAP.

 

I also noticed that the gasket between the carb and air filter housing is cracked. I'm not convinced that would cause the problem, but figured it's worth mentioning.

 

Also worth mentioning: The previous owner installed a new timing chain and distributor cap, it backfires under deceleration, and the spark plugs were all pretty fouled up. I cleaned and reinstalled them, but it didn't seem to help.

 

Any input as to what could be causing this problem? Any words of wisdom would be appreciated at this point. Will keep you updated as I try to get this sorted out. Thanks!

[datzenmike]

A choke forces a rich condition during warm up that can easily mask a lean condition caused by the primary jet being partly blocked. I assume it idles properly, and it should, as the idle circuit gets it's gas separately.
 
 
If you were to drive for ten minutes or so below 3,000 in this bogged down area and then were to pull over and remove a spark plug, I would expect it to look very white in color. Lean mixture burns very hot around the plug and this will scorch any deposits off the electrode and the porcelain insulator. Here is a plug from an extremely lean running engine. You might think it's brand new but it isn't.
 

 
This is dangerously lean. Try this and see if your plugs are light or white rather than tan or light brown.
 
 
 
The plugged hose connected to the brake booster is for use if you had an automatic.
 
Some backfiring in the exhaust during deceleration is normal.
 
 
 If your plugs are white I think the carb should come off and be cleaned. The primary jet is under the front of the carb and easy to get at...
 


You'll need a wide blade screwdriver probably ground down enough to just fit the hole. The jet(s) screw out. Don't poke wire through them use carb cleaner with a red straw to direct spray through them and the other air bleeds and passages in the carb body. If you find debris in the jet, clean it out and put back together. I wouldn't bother tearing it apart any further

[Roadster-ka]

I believe 720's were prone to secondary throttle plate corrosion causing them to stick shut. This might cause your lack of power.

[datzenmike]

It would still run well from idle up to when the secondary would come in. It would lack top speed and passing power but run smoothly.

 

I've driven a car with a 4bbl and one of the two front barrels was blocked. To drive you had to add gas slowly as the speed increases. If you pushed down on the gas too fast any power you had would drop off and it would bog. Once it struggled up over a certain speed you could get the back two barrels open and it would drive out on the highway, but poorly. The plugs were white like the picture I posted. 

[Dono]

Thanks for the replies. It's been raining all day, but when the weather clears up I will take it for a ride and inspect the plugs. I think I remember reading somewhere that when the carburetor is removed there is a gasket at the base which needs to be replaced. Is that something I should definitely do?

 

I figured out how to link the pictures so you can see the plugs on the carb. Also those two lines to the u-shaped metal tube on the air cleaner box were broken off when I bought the truck. I cut and reattached them, but that didn't seem to help at all, so I disconnected them again. Any idea what they are for?

 

Could the cracked gasket between the air filter box and carburetor be contributing to a lean running condition? I'm going to look for a replacement either way because it certainly won't hurt.

 

Let me know if anything in those pictures sends up a red flag for you guys. Thanks.

 

 

 

 

 

 

[Dono]

I just went for a slow cruise around the island then pulled the cylinder 3 and 4 exhaust side plugs for inspection. There was a little bit of white ash, but what's more concerning is the amount of crud built up already and the oil visible on the threads. I just cleaned them a few weeks ago. Looks like there is a significant oil leak into the cylinders.

 

I did a dry compression test while the engine was cold and all cylinders had ~175psi which would rule out faulty piston rings, right? There also isn't a lot of oil around the valve cover. Any ideas what I should check next?

 

Also these plugs say EGP on them instead of the recommended ES, but I already ordered new ones.

 

 

 

 

 

[Dono]

Finally got some time to work on the truck this weekend. After checking the rest of the plugs I noticed a lot of white buildup on some of the intake side plugs, so I pulled the carb as suggested and attempted to clean the jets. 

 

They actually looked clean to me, but I scuffed them up with a screwdriver while removing them. Do you think that will effect the fuel flow?

 

While spraying it out with carb cleaner I might have botched something else. In the first picture you can see where the jet on the left goes there is a metal tube visible. I don't see one on the right side. Is that normal? 

 

I also noticed a few old beat up vacuum lines and it appears a couple are missing. If you look at the last picture there are a couple metal tubes with no hose attached. Does anyone know where those should connect? I'm starting to think the problem might have been from the vacuum hoses.

 

I'm about to reinstall the carb and see what happens. I'll post an update when it's done.

 

 

 

 

[Dono]

So I figured out the one went to the carburetor, but I'm still not sure about this one that I'm pointing to.

 

[datzenmike]

Plug that one for now.

 

It looks like a vacuum leak which will force a lean mixture condition and poor or no idle. When the engine is cold the choke is on which forces a rich condition which masks this problem. Once the engine warms and the choke turns off it runs lean.

[Charlie69]

I would put the old plugs back in start it up and get it to operating temp, and then run it up to a steady 2000 rpm take the air cleaner off and trickle water into the carb while runinng at 2000 rpm warm.  Pour slowly so you do not kill the engine.  us about 8 ounces of water.

 

This will de carbon the combustion chamber and the head.  after this pull the plugs and see how much carbon is left.  If plugs still show a good amount of carbon put them back in and do it again.

 

Hard to do this ever 5000 miles on a new harley low rider I bought in 78, and I had to do this to onan generators in motor homes in the mid to late 70s.

[Stoffregen Motorsports]

We used to call that an engine douche. But we used substantially more water. A small stream from a garden hose at about 4000 rpms is how we did it back then.

 

There has to be a vacuum hose diagram somewhere. I just googled Datsun 720 vacuum diagram and came up with some promising results. Note that there are always a few iterations of vacuum hose routing on a given vehicle, which usually correspond the which part of the world the vehicle was originally sold.

[Dono]

Thanks for the advice guys. I'll try running some water through it to clean out the cylinders. 

 

Through process of elimination I figured the missing hose goes to the air cleaner. The diagram I found on Google seems to agree.

 

However when I got everything hooked up it ran like garbage. Lots of backfiring and the engine would just die out. Also, I reassembled everything without connecting the throttle cable. *headsmack*

 

Back to the drawing board...

 

So, I'm still concerned I botched up the carburetor internally while cleaning it. If you look at this picture, notice the jet on the left has a little brass tube behind it. I don't see that on the right side. Is it possible I knocked a piece loose inside? I did at one point jam the plastic spray tube in there while cleaning it. Not sure how fragile things are inside there.

 

I'm wondering if I could just plug all or the vacuum ports on the carb and try to run it. Would that work as a way to determine if my problem is a vacuum leak?

 

Also, I noticed that the recommended timing for this engine is 2btdc. I tried that before and the engine barely ran. I ended up setting it about 12-14btdc. If I had a bad vacuum leak before and now it's better, could changing the static timing  back towards factory spec help it run better? Just a thought...

 

Been super busy with work, but Iight have some time to work on it tonight. Any input would be welcome.