How to check if cam is timed right?

[280zwitha383]

so I figured I would just mess around and try a side gapping spark plug trick a buddy showed me.

 

LMAO, oh so you're one of them.....

Seriously though, when did the car stop pulling to 5500?

The thing that gets me is that you say the car will run all the way up to whatever rpm you want it if you do it gradually and not open up the secondaries. To me that rules out all ignition and timing and still brings you back to fuel delivery. [Edit: I don't think it's something as minor as jetting and I don't think the carb is too big (atleast not to the extent of cause this problem)]. Lastly, with the whole collapsing fuel line bit, it is possible because in the beginning you were pushing fuel and now you're pulling it.

With that said I don't think I can be of any more help (if I was any in the first place). Plus I would probably get into trouble if my fiance found out I was talking to a side-gapper this late at night on the internet.

[Alf]

Plus I would probably get into trouble if my fiance found out I was talking to a side-gapper this late at night on the internet.

 

 

 

 

Ohh man you're too much, haha. The guy that told me about it was from Texas, be carefull down there.

 

The carb stoped pulling when I changed the timing slightly, then I put in a fresh set of plugs and it ran way smoother down low but then it hit "the wall" again. The only reason I don't agree that its fuel delivery is because it ran right when I had side gapped plugs in it, so I know its capable of running right.

 

If the cam timing is advanced then the exhaust gasses wouldn't get fully pushed out of the chamber because of the valve closing while the piston is on its way up. This also means that the intake valve would open too soon, and it would be open while the piston is in a dwell period at TDC, therefor not sucking any mixture into the chamber for 10-20 degress of crankshaft revolution. With this scenario I know the engine would not run to its full potential, so it esentially makes itself smaller due to its ineficiency, therefor the carb seems huge to this engine.

[280zwitha383]

Wouldn't you have some funky backfiring problems if your cam was off that much? When are you going to tear it down? I'm curious to see what you find out because in what you just said your logic is sound. Also, I can't figure out how the side gapping plugs would make it run right. Did you take them out?

[Guest tony78_280z]

WWGD?

 

What Would Gruppy Do?

 

Start with the basics. Recheck everything (even if you have done this at some point in the past) And I'd do them in this order.

1. Return the carb to its "off the shelf" settings in jets, springs, cams, pumps, and nozzles.
   
2. Check for Vacuum leaks
   
3. Hook up a Vac guage and be sure there is a steady vacuum through the rev range. (Or at least until you hit your problem area)
   
4. Timing be sure the ENTIRE curve is timed correct according to your estimated Cam Specs. If you had a missing spring in your mechanical advance what would that do?
   
5. Constant and consistant fuel pressure. For a Carb that is 4 to 7psi no more, no less. And be sure the pump is rated for enough fuel flow. Flow and Pressure are not the same thing. I forget what Vizard says was optimal fuel pressure for the HP of a given motor.
   
6. Take a closer look at your DIZZY and COIL, is the spark dropping off as you get into higher revs? This could cause a stumble spot or a rich condition. This may be why you saw mild improvements from your "band aid fix" spark plug side gapped idear.
   
7. Compression test. (that might explain the black smoke)
   
8. Then look at the carb. The RPM Brick wall effect is normaly secondaries NOT opening or not enough fuel supplied to the bowl. Refresh your mind on proper float adjustment. (I've screwed this up before) Swap in another carb that you know runs decent

If all this fails, then look into the CAM timing or floating valves or something. The rest of these tests can be performed without getting too involved and tearing apart the motor, and will rule out the basic problems.

 

Perhaps the real grumpy will jump back in and lend a complaining hand.

 

"Would the real Grumpyvette please stand up,.. please stand up." Oh, and by the way... Grumpy, that is some great info on cam timing. Very good reading. How do you do it?

[jt1]

A few thoughts:

 

ATDC: After Top Dead Center

ABDC: After Bottom Dead Center

BTDC: Before Top Dead Center

 

These figures are referring to the amount in crankshaft degrees that the cam event in question occurs. It does sound like the cam is advanced, but it's really hard to just eyeball it.

 

In general, I would have guessed the cam was retarded, and the problems were caused by a combination of cam timing and low compression. With a 307 and 882 heads, 76cc IIRC, your compression ratio would be pretty low, like 7.5 or 8 to 1, or maybe less depending. The low compression and a retarded cam could cause your problem. The cam timing could be off because of several things: Crank or timing gear keys cut wrong, dots improperly marked, cam dowel wrong, or an accumulation of the above. A new timing set would eliminate everything except the crank key or cam itself. Degreeing the cam would eliminate all these, but it's not possible without knowing the cam specs, and I know you don't want to buy a new cam. Maybe check with the previous owner and see if he knows which cam it is, or if you're lucky he may have the cam card.

 

Before I tore the front off the motor, cracked the pan, and dealt with the inevitable oil leaks, I would:

 

1)Block off the return on the pump and see what happens. 3.5 psi should be enough, but it would be a simple and easy check.

 

2) Borrow a known good carb and try on it, just to eliminate the carb. A 650 is a little big for your combo, but not grossly so, it should run decently with the 650.

 

3) Try another coil on it. The fact that side gapping the plugs helped might mean the mixture isn't lighting well, and it's harder to light the mixture at higher rpm because it has to happen so much quicker. You might just have a weak spark. Take out a plug, put the wire on, lay it on the motor where it will ground, crank the engine, and look at the spark. It should be bluish-white. If it's reddish-orange, it's definitely weak. It's best to do this in a dimly lit area so you can see the spark better. Try another coil just to be sure.

 

If none of this helped I would try the cam timing, first the timing set, then new cam and degree if neccesary. Remember that most really good problems are not one single issue, but usually a combination of smaller issues.

 

In the long run, if you're gonna keep running this motor, I would look for a set of 461 or 462 heads w/64 cc chambers. Upping the compression ratio will give better power, gas mileage, and throttle response.

 

jt

[John Scott]

Hmmmm. This is so much like the problem I was having. I disconnected the secondary side and it would run to redline. If I just ran the primary side up to the point where secondary linkage starts, same thing, it would rev right past the "wall". Try it, If it runs fine on the two barrels, then forget the cam timing problem. Your pressure is low, but black smoke indicates too rich. Something is wacky with the metering. Right after it occurs, can you shut it down and look down the barrels? Are the boosters wet, dripping, or dry? Pull the sight plugs. Still just level with the bottom or running out of one side or the other? Do your run a plug or PV? PV check out OK? Someone have a carb you can borrow? Even if it carb is too big, you might have a bog or hesitation, then should pick up.

[Alf]

Okay, I did some further tests today. I checked the spark on the number one and three cylinders and they both had a nice big bluish purple spark way past the three grand mark. I also checked both cylinders with a normal plug and one of the side gaped ones that made it run right, there was no aparent difference in spark quality. For some reason this engine also burns the number three and number two plugs black, like soot, all others are tan. The only thing I can think of is that every intake lobe but those two are worn down, but this is so unlikelly its almost not really posible. The engine has never backfired through the carburetor. I tried an old coil I had laying around and there was no difference. I returned the carb to stock with no improvement. I didn't mess with timing, but I have messed with that so much in the past to no avail, plus it is set at 11 initial and 36 total with one medium weight spring and one light weight spring.

 

The fact that the car did, for a short time, run all the way up to redline with all four barrels open leads me to beleive that the problem is not the fuel system. The engine also has good spark and good timing so I think we can eliminate that as the problem. This only leaves some internal engine problem like worn cam, wrong cam timing, weak valvesprings, etc.

 

With that being said, I think I'm gonna take it apart this weekend and find out for sure what the problem is. Anyone have any last minute suggestions before I disassemble this thing?

[jt1]

The best advice I can give is to take your time on the disassembly and pay close attention to the orientation of all components, so that you can catch the problem. Compare the keyways and dots on the current gears to the new ones. If you pull the cam and lifters, the lifters must go back on the same lobes, and use cam lube on the cam and lifters. Check the pushrods for and bent ones. Check the lifters to see that they are all pumped up, it's at least possible the lifters are collapsing at higher rpm. See if the cam has any numbers on it you can trace down and get the cam specs.

 

The main thing is be methodical and eliminate each component as a possible problem.

 

John

[jt1]

Alf, what was the verdict on this?

 

John

[Alf]

I was ready to tear it apart and check the cam timing, but I just don't think it runs bad enough for it to be a tooth off. So I decided to wait untill I get home since the motor is coming out anyway. I'm thinking now it may be that I either have the wrong balancer or the ring has spun. I will check into that and let you guys know.