Old newbie need help. 82 720 4x4 2.2 5spd No power

[blownalcohol]

I have an old 1982 Datsun 720 King cab 4x4 5spd that I have ben working on. I've got way to much invested in this pile to give up now thanks to my pride and an ability to take a beating. I've been scouring the internet and this forum seems by far to be the best, you are my last hope. I am not new to cars. I am a professional mechanic 40 years in the business. I worked on trucks like this when they were new and I am ashamed to say that this one is playin me for a rookie. This truck can't pull itself out of a wet paper sack. Foot on the floor, slight grade 4th gear and maybe you can maintain 55 MPH. I've seen other posts with this complaint but none I've found seem to follow my particular problem and if they do there is no clear answer for a fix. So, I'm not looking to buy expensive carburetor mods or performance equipment. I used to work on these back in the day and the trucks that I remember would run circles around this dog with all of the standard equipment in place. So here's a call out to all of the Guru's on this forum, I NEED HELP :) Here's what we got,

 

ATK 2.2 engine low miles, compression good, valves adjusted, mechanical advance fixed, vacuum advance working, timing 5 degrees BTDC, new plugs and wires, cap and rotor good, both coils firing, valve timing correct, vacuum 15 inches at idle ( I'm at 6,000 feet) , Vacuum tests show healthy engine no exhaust restriction, dropped exhaust anyway in desperation and it made no difference. Rebuilt OEM carburetor, secondary's working and I'm getting full throttle. It passed emissions, CO is close to 0, hydrocarbons 140 ppm. Engine sounds awesome until you try to cruise up a slight grade, throw in a head wind and your back to 3rd gear trying to maintain 40 mph, not good on the freeway in heavy traffic. :no: Drivetrain seems ok, I replaced some u-joints and it has a new clutch. Diffs, transfer case and trans have been serviced and don't make any real abnormal noise or drag. Brakes are not dragging and the truck rolls very easily ?? So there you have it, there's nothing wrong with it, right? except shouldn't I be able to commute back and forth to work at  65MPH without my foot on the floor or going downhill ? If this is the nature of the beast I am going to be very disappointed.

 

[Lockleaf]

That is definately not the nature of these when in good shape. They do well enough even at reasonable freeway speeds.

 

You are possibly starving for fuel at high load. Partial fuel system clogs have created situations like yours in my trucks

[datsunaholic]

When you say mechanical advance is fixed, do you mean repaired or locked in place?   If it's locked in place and set at 5deg at idle, there's your problem. Vacuum advance alone doesn't do anything at heavy throttle+high RPMs because there's not enough vacuum to pull it, so at high RPMs you need mechanical advance or it falls flat on its face.  At "cruise" the advance on a dual-plug system is probably pretty close to 20 degrees (mechanical).

[blownalcohol]

I mean that it was frozen and I pulled the distributor apart cleaned and lubricated the fly weight mechanism, put it back in the truck and reset the timing.  I gained a little but not much its still a real dog. The thing is blowing my mind. It seems like a no brainer but I've tried everything I know to do. I have considered the idea that the secondary's are leaning out. I did replace the venturi O-rings when I rebuilt it. When your driving it feels like the timing is retarded big time or the brakes are on, it really feels like its dragging but its not, at least not that I have been able to determine. I thought maybe the timing pointer was off, I got a piston stop and found top dead but the timing indicator is right on the money. I checked the timing chain thinking that possible the valve timing was to blame but its OK. . :no: Its probably something stupid that I have missed, at idle or no load the engine sounds awesome.

[datzenmike]

Take for a drive at high speed for 10 min. Then pull over and remove a plug.

 

Black powdery is too rich... like choke on rich.

Black oily is oil fouled.

Light tan  or brown... about right

Light cream... lean-ish

Bright WHITE... extremely lean. Porcelain is so hot no deposits. 

 

Secondary jet is plugged or part plugged. Secondary opens but just air added which leans out the gas and air from the primary.  

[wayno]

What are you gauging/basing your power or lack of it on, other trucks you have drove, the 4X4 is a heavy truck(3400lbs?), 

[blownalcohol]

OK,, :thumbup: reading the plugs, That's my next move. I've looked at them of course but not after a hard run which is what needs to happen for an accurate account of what the secondary's are doing. Thanks for the suggestion, it knocked away some cobwebs in my old brain. Regardless the reading  I think I'll give the secondary circuit another hard look and report back.

 

Its been many years since I actually drove one of these things during the course of a work day so my memory serves me poorly and I don't have anything to gauge the power against but that. I got the truck in a basket and brought it back with the help of a couple donor trucks. I used to have a side job customer back in the day that had one of these and I worked on it a lot. I don't remember it being this doggy. I know it isn't going to be my turbo diesel, I just would like to cruise the freeway at 65 without having my foot on the floor with every slight grade or little bit of head wind. Is that expecting too much?

[Cheetahking]

I mean that it was frozen and I pulled the distributor apart cleaned and lubricated the fly weight mechanism, put it back in the truck and reset the timing.  I gained a little but not much its still a real dog. The thing is blowing my mind. It seems like a no brainer but I've tried everything I know to do. I have considered the idea that the secondary's are leaning out. I did replace the venturi O-rings when I rebuilt it. When your driving it feels like the timing is retarded big time or the brakes are on, it really feels like its dragging but its not, at least not that I have been able to determine. I thought maybe the timing pointer was off, I got a piston stop and found top dead but the timing indicator is right on the money. I checked the timing chain thinking that possible the valve timing was to blame but its OK. . :no: Its probably something stupid that I have missed, at idle or no load the engine sounds awesome.

Have you seen the timing advance as you rev up the engine - you can do this with a timing light.  It should advance quite a bit as you head towards 3000 rpm.  If not, your power will be compromised. Also - have you checked the vacuum advance?

 

I did have my secondary frozen on my first 510, but it actually still ran OK - better with it fixed.  Keep working through this methodically, you will figure it out.  

[datzenmike]

Vacuum advance is strictly load dependent. Heavy load produces vary low vacuum, light load very high vacuum. There is basically zero vacuum advance at full throttle.

 

At or before 3,000 RPM you should read about 32-34 total advance degrees.

[blownalcohol]

She was leaning out when the secondary opened. Plugs were white. Took it back to the shop and had a friend goose it while I looked down the carburetor throat, there was very little fuel entering the venturi when the secondary would start to open. I opened the secondary manually a few times a suddenly out of nowhere I had all kinds of fuel coming out of the venturi. The engine was revving great on the secondary alone. I took it out and drove it and now it cruises 65 4th gear on slight grade. I no longer have to keep it floored, it will even accelerate up the grade. Must have been a piece of crap in the secondary circuit that didn't come clean in the carb dip. I blew the hell out of it with compressed air too, go figure? This truck has been sitting a long time though. I just licensed it for the first time since 2002.  Thanks for the input guys helped me out a lot. :thumbup:

[wayno]

Another happy ending  :thumbup: