Isuzu SUV Forum banner
1 - 12 of 12 Posts

·
Registered
Joined
·
216 Posts
Discussion Starter · #1 ·
I built a 2.6 with 2.3 head for my 86 Trooper. The short block was decked and I'm assuming the head was also shaved, because it was a rebuilt head. When I set timing, the cam mark was off by 1 tooth. I set the timing with the cam advanced by 1 tooth instead of retarding it by 1 tooth.

When I line up the marks for the cam, the crank is about at about 15*BTDC.
If thats the case what should my timing be set at for BTDC? I have a high idle at the moment is it because the timing is too advanced?
Automotive tire Motor vehicle Alloy wheel Tread Automotive design
 

·
Registered
Joined
·
10,134 Posts
Your picture shows that cam a whole tooth off. If the crank is at zero, you should back the cam off to align the mark. Dennis
 

·
Registered
Joined
·
783 Posts
Agreed. Pull the belt off, set the crank to zero (it is best to pull the balancer off and align it by the inner marks), set the cam to zero and then but the belt back on. Your timing adjustment should be done by twisting the distributor.
 

·
Registered
Joined
·
216 Posts
Discussion Starter · #4 ·
Agreed. Pull the belt off, set the crank to zero (it is best to pull the balancer off and align it by the inner marks), set the cam to zero and then but the belt back on. Your timing adjustment should be done by twisting the distributor.
Your right, I tried it and it almost right on it. I got the timing set a 8*BTDC however the idle is still high. When I pull the vacuum advance hose idle goes down, is that hose supposed to have vacuum at idle?
 

·
Registered
Joined
·
783 Posts
Your right, I tried it and it almost right on it. I got the timing set a 8*BTDC however the idle is still high. When I pull the vacuum advance hose idle goes down, is that hose supposed to have vacuum at idle?
I don't know enough about the vac advance on the 2.3 dizzy, but hopefully someone else here does.
Two other things you can check though are the idle mixture screw on the carb and that the motor idles up (not down) when you turn the A/C on if you have it.
 

·
Registered
Joined
·
10,134 Posts
Okay, it's getting more clear now. You're running carb and mechanical dizzy. You set timing with dizzy vacuum UNPLUGGED (and hose plugged). I don't recall if the 2.3 vacuum chart shows dizzy hooked to straight vacuum (below the throttle plates - direct to manifold) or ported vacuum (plates barely cracked). I kind of lean towards straight vacuum. After setting initial timing, hook up vacuum hose and THEN adjust idle. Make sure there are no vacuum leaks. Dennis
 

·
Registered
Joined
·
4,701 Posts
A "normal" vacuum advance for a carbureted engine shouldn't have manifold vacuum connected, rather it should be fed from a source of ported vacuum. Such that vacuum rises with engine speed.

I'm assuming the ZuZu vacuum-advance distributor works in that manner, so that you get more spark advance when the throttle valve is opened??

Otherwise, with manifold vacuum on the advance you're gonna get full spark advance at idle and least advance at speed. Maybe good for a supercharged engine to pull back vacuum under boost, but that's not the way a normally-aspirated, carbureted engine would operate.

Easy way to check that is to put a timing light on the engine and pull the vacuum hose, the timing shouldn't change. If spark retards without the vacuum line connected, that's probably not gonna work well. And I expect could be the source of your high idle. Also if you pull the vacuum hose there should be no vacuum on it at idle, with the throttle closed.

Put a vacuum gauge on the hose and see what it does. That'll tell you right away how it's working.
 

·
Registered
Joined
·
216 Posts
Discussion Starter · #8 ·
I found out the source of the high idle, its that AC solenoid that is by the throttle linkage, it was being actuated and causing the high idle and in turn a vacuum in the ported vacuum line. When I disconnected it, idle was down to 900 and there was no vacuum on the line and I set timing at around 8*BTDC.

It ran good, but I now have a noise coming from the transmission. It goes away when clutch pedal is depressed. I installed a new clutch, new pilot bearing and a new throwout bearing, so it must be one of the bearings in the transmission, unfortunately.
 

·
Registered
Joined
·
783 Posts
Ah ha, the dreaded A/C solenoid. Glad to hear it's been straightened out. Since you are reasonably sure it is internal, you could just throw a couple hundred bucks at another MUA5. They are cheap, plentiful, and hard to break so this might be a good option if you don't want to tear apart the one you have.
 

·
Registered
1989 Trooper R/S
Joined
·
4,386 Posts
I installed a new clutch, new pilot bearing and a new throwout bearing, so it must be one of the bearings in the transmission, unfortunately.

Does it make the noise just sitting there idling or when your driving. Seems if the noise comes and goes with clutch pedal operation then that's where the problem is. But my issues with noisy throw-out bearings is they were noisy when in use. ( pedal depressed)
 

·
Registered
Joined
·
694 Posts
Mine is off by half a tooth on the cam sprocket. It will not line up perfectly I'm guessing due to the metal taken off when the block was decked. If its off by a tooth I would think it could be adjusted like Dennis mentioned.
 

·
Registered
Joined
·
32 Posts
A "normal" vacuum advance for a carbureted engine shouldn't have manifold vacuum connected, rather it should be fed from a source of ported vacuum. Such that vacuum rises with engine speed.
The difference between ported and manifold vacuum is that ported has no vacuum at idle when the throttle blade is closed. As soon as the throttle blade is opened the ported vacuum port has manifold vacuum exposed. So the difference between connecting the vacuum advance to a ported vs manifold source is that ported will have no additional advance at idle, and a manifold source will have the additional advance at idle. The engine actually prefers more advance at idle except for hot starting, as noted by the higher RPM when given more advance at idle. The engine is more efficient with say 32 degrees of advance at idle vs 12 degrees of advance at idle. Since there is basically no vacuum when starting the vacuum advance is not added at startup.

When under load at higher throttle positions the vacuum decreases in the manifold and the additional vacuum advance goes away. So a sudden wide open throttle from idle means manifold vacuum goes to zero, and the additional advance from the vacuum advance goes away. As the RPM increases the mechanical advance adds additional advance. While cruising at higher RPM, say 3,000 RPM the mechanical advance is added, and the throttle is low and manifold vacuum is high, so the additional advance of the vacuum advance can is added, again, the engine likes more advance at low throttle low load higher RPM situations.

So for a typical situation at idle, the engine would be spark timed for say 10 BTDC, with no mechanical advance added at idle. Adding 20 degrees advance at idle with a manifold sourced vacuum advance means the engine is idling at 30 BTDC, which generally is just about in the optimum range for idle. Open the throttle quickly and the additional 20 goes away and the mechanical is added to the static 10 BTDC as the RPM increases. With the vacuum advance can connected to a PORTED source there is no additional advance added at idle, so the engine will be idling at say 10 BTDC. At cruise at 3,000 RPM the manifold vacuum is high, the ported source is exposed to the manifold vacuum and the additional advance from the vacuum advance is added.

The bottom line is that the ONLY difference between a manifold connected vacuum advance can and a ported source connected vacuum advance can is that the ported source does not add the additional advance at idle. MOST engines like to idle with roughly 30 BTDC so the ported source is not preferred. If your engine does not run better with 30 BTDC than 10 BTDC at idle it is probably a CARB problem that can't tolerate higher advance. Lean mixtures burn slower than richer mixtures, and an out of adjustment carb changes the advance requirements in order for the peak cylinder pressure to occur at the appropriate time AFTER TDC. Too little advance makes the peak pressure occur too late, having a negative effect on engine performance.
 
1 - 12 of 12 Posts
Top