85 Nissan 720 no idle

[Tspite]

Hey Ratsun, my names Tommy and I have a 85 720 with a 32/36 weber conversion. Before the conversion the truck would start, but it had a problem of dieing and keeping idle, the carb was bad on it so I replaced it. With this conversion however I have unable to get it to idle at all! It will start when giving it some throttle but let off and it will just die. 

 

Things I've done: 

Weber 32/36

Base settings

Replaced spark plugs on exhaust side with copper NGKs, new sparkplug wires. 

New Fuel Filter

capped most vacuum. left essential vacuum to carb hooked up

 

I am not running a regulator, after hooking up a gauge I measured 3 psi to the carb. 

But when I hooked up a vacuum gauge. while turning the motor over and trying to start it I got no vacuum, so I'm assuming a vacuum leak somewhere, but I've looked it over, I've gotten rid of alot of vacuum lines, kept the one that hooks up to the carb to vac advance., and a couple others that T off behind the carb. 

 

I'm just having a hard time finding the leak, if that is the problem. My worse fear is that there is a leak coming from the carb base or intake manifold gasket, I've sprayed it with starter fluid to see if that helped but nothing.

 

I have plugged off where the inake manifold provided a vacuum source at the front, and if it is the base I assume the flat head allen bolts that you use for the weber adapter plate may not be letting it seal all the way (even though I've sprayed, I've been iffy about that, just didn't seam flush when you add the gasket and carb on top) 

 

I have no breather or hose hooked up to valve cover (left open), can a vacuum leak come from the valve cover?

 

So any help I can get would be GREATLY appreciated. I've tried searching everywhere, I've been doing a lot of reading before and since my conversion and I just wanted to detail most I can on how things are hooked up and what I've done. 

 

[Lockleaf]

First off, welcome to ratsun.

 

Second, pics pics pics pics pics pics. Words help but with pics the level of help and attention you get will go way up.

 

If you pour some gas down the carb, or some starter fluid, will it fire?

[datzenmike]

So the old carb and the new carb have the same problem? This would indicate that maybe the carb swap wasn't needed.

 

1/ First I would check the valve clearances. An over tight intake (or all intakes) would leak intake vacuum as would a burnt valve.

 

2/ Did you clamp the brake booster hose or remove and plug the fitting???

 

3/ Did you remove and clean the EGR valve?? If crusty with exhaust deposits it won't close and will leak air into the intake destroying vacuum and idle.

 

Most of the hoses on a 720 have nothing to do with intake vacuum at all.

 

 

The adapter under the webers are problematic. The bolts shake loose and if over tightened the adapter can split. If this is a used carb and adapter this is almost certain to be the problem. Pull them off and check that the adapter is flat. Rub on a sheet of glass with sand paper taped to it. Use good quality gaskets or material and make your own. Do NOT use silicon rtv sealer crap on this. Mount dry. Use Lock-Tite on the adapter and carb studs.

 

3 PSI fuel pressure is fine.

 

No the valve cover hose has zero effect on vacuum, but get a hose on it and connect to the under side of the weber air filter. There is a connection for this.

[MicroMachinery]

(^^^I think he means EGR valve)

[datzenmike]

Thank's mirco, finger(s) got ahead of me.

[Tspite]

With the old carb I could at least get it to idle, but it was shitty and had weak power. Old carb was definitely bad and it was leaking fuel from the float bowl cover. But I'll definitely check valve clearance, I'll take pictures and post them asap and I'll check on brake booster hose Mike. Thanks for the replies!

 

And thanks for the welcome Lockleaf, Really glad there is a community for these old Datsun/Nissan's. I've loved them all,  I currently own a 720 and a 240z.I had 2 hardbodies, and 2 240sxs. Eventally I would like to swap a ka into the 720.

 

Thanks again guys and I'll update with information and pictures soon! 

[datzenmike]

Don't forget the EGR valve. It's on the rear of the intake and has a metal hose to the exhaust manifold. That valve HAS to close at idle or exhaust will be let in diluting the intake gas and air resulting in poor idling and lower intake vacuum. Just remove and look at the underside. If it's stuck open or part open wire brush and get it working.

[Rhino13]

Is it a brand new Weber? I was having this issue with the throttle shafts creating a vacuum leak to the point where it wouldn't run. I removed all the vacuum lines and capped all of them other than the distributor advance going to the only vacuum port on a 32/36. Make sure you cap them as you take them off so you don't forget a spot, the one going to the brake booster is small as datzenmike mentioned. It might be a good idea to check for other vacuum leaks as well like it was mentioned on the adapter plate. Use a flammable aerosol to check for leaks, spray in potentially leaky spots and if the rpms increase, bingo.

[MicroMachinery]

Thank's mirco, finger(s) got ahead of me.

Now you're just toying with me.

[Tspite]

I figured it out! As suspected it was indeed the allen bolts that were holding the adapter down, I removed them and put the carb back on, tightened it down and tried firing it up and it started with no problem, nice smooth idle! Man I'm pretty happy I finally figured it out. Thanks for the help guys, 

 

I did check on the egr valve and it is freely moving, so everything seems to be working fine. The carb doesn't move around a whole lot, but I probably should find something I can use for the open adapter plate holes soon.