What else can I check-engine won't rev past 5k

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brian c

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Here's my scenario: My new to me '74 bucks a little at low rpm but runs smoothly to 3k, little rougher to 4.5k and then falls flat almost like it hitting a rev limiter. The had been sitting for a bit before I bought it and had two carb rebuild kits in the trunk so I figured fuel delivery was the problem and the carbs were to blame.

Here's the work I have done which has not improved anything significantly:
New spark plug wires
New fuel filter
New bosch blue coil
Points swapped out for Pertronix
Fresh fuel, new fuel lines, checked tank and pickup
Set timing on ball
New cap and rotor
No valve noise, compression check good and even between cylinders. 68k on engine, very clean looking under valve cover. Plugs all look good and even in color. Engine will rev to 6k plus in neutral.

Currently have at very reputable carb shop who has rebuilt the zenith carbs, dyno'd and is stymied too that the problem still exists. It feels like an ignition problem to both them and me but I'm not sure what else I can try to replace or fix in the ignition system. Any ideas on what to try next?
 
So it is doing this under load/driving conditions?

What is the condition of your fuel pump? Idle revving requires little fuel, load requires more. Hoses all tight?

Are you running a ballast resistor with that Blue coil?

Are you running any vacuum advance/retard?

No valve noise...have you measured the valve clearances to confirm they are set to spec?
 
brian c said:
Here's my scenario: My new to me '74 bucks a little at low rpm but runs smoothly to 3k, little rougher to 4.5k and then falls flat almost like it hitting a rev limiter. The had been sitting for a bit before I bought it and had two carb rebuild kits in the trunk so I figured fuel delivery was the problem and the carbs were to blame.

Here's the work I have done which has not improved anything significantly:
New spark plug wires
New fuel filter
New bosch blue coil
Points swapped out for Pertronix
Fresh fuel, new fuel lines, checked tank and pickup
Set timing on ball
New cap and rotor
No valve noise, compression check good and even between cylinders. 68k on engine, very clean looking under valve cover. Plugs all look good and even in color. Engine will rev to 6k plus in neutral.

Currently have at very reputable carb shop who has rebuilt the zenith carbs, dyno'd and is stymied too that the problem still exists. It feels like an ignition problem to both them and me but I'm not sure what else I can try to replace or fix in the ignition system. Any ideas on what to try next?
Click to expand...

do not reve the engine in neutral !
is it an automatic car ?
i had a simllar issue and it was points gap the first time and incorrect timing the second time
when you say timing on ball, can you be more specific ? revs, and vaccuum ports ?
no valve noise, ...that is not an M30 !
you say things are new, but are those new things checked by you ? i mean resistance, volts,...
 
Problems happen under load. May seem worse in 3rd gear than 2nd. Fuel pump seems to pump good quantity of fuel into a bottle while cranking. No ballast on coil. Original coil was black bosch with no ballast either. Vacuum advance connected, retard left open. Have not checked valve clearances.
 
Did you replace the fuel screen in the gas tank?

Did you replace all fuel lines from the trunk to the engine?
Could be sucking air.

Distributor may need to be sent out to Advance Distributors for a restoration and recurve. Do you have access to another distributor?

I had a problem with a Pertronix & Crane so I'm back with points, solved my problem.

Are you running Bosch plugs?
May want to switch to NGK, that's what I use in my old BMWs.

Is the fuel filter on correctly for the flow?

I've gotten bad new spark plug wires, go with Beru.

Do a search for Pertronix and corect resistance, may need a resistor. This has been dicussed several times on this forum, so there's plenty of material to read.
 
Manual tranny. Timed at 1700 with vacuum advance disconnected. Not sure if new parts are good. Cap rotor and wires I would think are pretty foolproof. I know there is some debate over bosch coil quality but thought the problem was early failure not starting necessarily starting out faulty. I definitely could check the resistance though. Did check that 12v was going to coil. Didn't know there was a problem revving an unloaded engine (within reason, below redline).
 
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Andrew,
Thanks for the response,
All rubber fuel lines replaced to tank. Checked screen in tank. Tank is very clean inside also, not gunky or rusty. All lines securely clamped. Blew air back to tank to check for blockages. Seems to flow a good amount of fuel. The car seems to run exactly the same with points and pertronix, I swaped back and forth once just to check so I think the pertronix is OK. I wish I had another distributor to try out since seems like the only ignition component I haven't replaced. I don't know much about distributor failure but just assumed the bearings got sloppy and didn't think about advance curves etc. I'm sure it wouldn't hurt to change plugs but they all looked OK and consistently colored.
 
Also ask your carb rebuilder if the floats are adusted correctly with the correct shims and that they are not getting hung up on a gasket.

Do you have access to another fuel pump?
 
About a year ago my coupe would shut down at high rpm. It seemed to be a little worse in taller gears. It ended up being the coil. Apparently, it was reaching saturation and not able to fire fast enough. A new coil solved the problem.

I see you replaced the coil. Are you sure the fuel pump is pumping enough fuel?

I run Pertronix and have not had a problem with it.
 
Brian, the good news is that it's a machine and it can be fixed :-). In your initial post you mentioned "new-to-me", so my question is; when you very first took delivery was this problem present? Many of us start right off the bat replacing those common components that make sense- cap, rotor, points... and on down the line. Nothing that you mentioned seems over-the-top, so what I might try is to back trace everything you touched since you took delivery. I had a problem, not like yours, but with the carbs because there seemed to be a spring missing on the linkage which I replaced... well I put the spring in the wrong place and I was not the one who discovered my mistake, in fact it cost me a little more than I would like to admit. So, I would just start at the last point that the car was running well- if it was at all before you bought it. Or ask the PO if possible. If you have no luck, start with the basics and move forward. All the suggestions below are correct possibilities made by smarter folks than I so if you need to go through "everything" that may be the best route but I would try to use your memory first if at all applicable. Good luck and please let us know how you solve the problem- which you will :-)
 
timing

basics means checking the damper and the pointer on the front case, then seeing which mark is in the timing window at TDC. If that's good, then pop the cap and check for the relationship of the rotor to number one wire/ post. If that's off, then reset it.

A common mistake for new owners is to not know there are two timing balls, one TDC and one advanced. The latter is the one you want to time to.

M30s have an awful lot of play at TDC built in. Then there's chain slack, cam off a tooth, distributer off a tooth, and the list goes on. It's not uncommon to have to manually set TDC and then slow rotate 11 teeth of the flywheel and mark it.

If the carbs were the issue, you probably would have to pull over and crank a bit to fill the bowls enough to start. If it's timing and you just slow down a bit and it catches that seems like timing.
 
Peter,
The car ran this way when I bought it. I just figured it wasn't a major problem since the health of the engine seemed to check out OK. All of the stuff I've done to it, I probably would have done anyway to a new purchase so not a big deal but it hasn't improved things either.

61porsche,
The timing is a real consideration. I definitely changed the timing from where it was when I bought it. I put it on the ball (didn't realize there were two balls) and it idled better so I figured I was on the money. I may need to establish TDC manually and go from there. I didn't see any marks or pointer on the damper, just the flywheel. Is this on the same side as the bellhousing window? This is my first car where the timing marks are on the flywheel and on the opposite side of the car from the distributor. I love the added element of danger in reaching over that spinning fan to rotate the distributor.
 
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brian c said:
Currently have at very reputable carb shop who has rebuilt the zenith carbs, dyno'd and is stymied too that the problem still exists. It feels like an ignition problem to both them and me but I'm not sure what else I can try to replace or fix in the ignition system. Any ideas on what to try next?
Click to expand...



I'd bet you are overlooking something simple.

Your description of the car might benefit from more detail. If it is a North American model, does it still have its emissions equipment? (see below) When properly functioning, emissions equipment can hamper engine performance. When malfunctioning or improperly connected, the results can be more problematic, especially in different gears. If applicable, trace those vacuum lines. If the egr is intact, ensure that it is at least closed and not leaking.

You described the carbs being rebuilt. This task may not be brain surgery yet it is not uncommon for gaskets to be inadvertently flipped and primary jets to find their way into secondary orifices and vice versa. Steve astutely mentioned rechecking float levels.

Revving the engine to 6000 rpm in neutral is not advisable, but since you brought it up, have you checked to see that the throttle plates on both carbs are opening fully? Is linkage correctly adjusted so that both carb throttles are synchronized? Could your exhaust be plugged or severely restricted?

Caps, rotors, spark plugs are all fool proof except when they are not. Despite great strides in brand name manufacturing quality, mistakes happen. Caps, rotors and spark plugs can be mishandled, rendering them unreliable. (I recently saw someone return a cap that had a broken center carbon contact.)

Rotors occasionally have too much resistance and may not be a perfect match (size wise) with the cap. I read something about points and pertronix in this thread. What's going on with the condenser, if and when it is installed with points? Hint: they don't often go bad, but when they do, some often describe the symptom as similar to a dying fuel pump.

Spark plugs? Ensure that the gaps are neither too big nor too small. .028" is a good starting point. Search spark plugs on this site and you will read that conventional wisdom favors copper or silver core plugs.

If your coil is unintentionally wired backwards, reversing polarity, the engine will run, but not optimally. Same with a sloppy distributor. Have you examined the centrifugal advance? Can you twist it by hand and is it's operation smooth? Does the vacuum advance plate move freely?

HTH, best of luck.


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old parts

My guess would be coil or coil wire. ive had this happen on two occasions. First time was when the i first bought the car. Same symptoms you described. Replaced the coil and cleaned up all of the connections and problem solved.

Second time I had been working on timing and replaced points. got everything running right for a day or so and then it started acting up... same problem again. Turned out the copper strand in one of the coil wire ends had corroded and backed off of the wire end cap - the old caps have tiny screws in the seat of the sockets that screw into the end of the wire. With all the jostling around that was going on when i was working on the ignition, the wire finally gave way. I trimmed 1/2 inch or so off of each end , cleaned the screws and re-seated the wire. Problem solved.

you might not have the same issue, but its worth a look.

good luck!
 
There are marks

on the damper and a pointer cast in the front lower cover. You can see them from the top once you know what to look for. Shine a light from under and find the one on the damper marked "O". With a 36mm socket on the crank nut you can turn the crank. Look up from below the distributer to the front case. It's there. Line those two up from up top once you know where. Careful on the knuckles- where a glove or stick a piece of cardboard against the radiator. You will bleed.

Check the rotor and make sure you're close to number one and not 180 out. Oil your advance weights.


Then go find the ball in the timing window on the flywheel. The other ball should just be visible and that's the one you want. Clean it, mark it, etc.

Don't use platinum plugs on an old car. They're made for a hot spark and have very high resistance. That's what the guys are telling you.

Or go old school- hook up a vacuum gauge and twist the distributer until you get the highest reading and back off just a bit. Bet you'll be close.
 
Mmercury, all good points. Thanks for the diagram. Emissions stuff has all been eliminated. Exhaust is in seemingly good condition so probably not plugged.

I'm guessing the carbs are good as the carb guys really seem to know Zenith carbs and said they really didn't look bad at all when they took them apart. For them to run exactly the same after the rebuilds makes me think they were probably fine from the start.

Thanks for all the good ideas. The more suggestions I hear, the more I am inclined to think it is a timing or advance issue. I think I'll get the car from the carb shop and concentrate on the timing and the distributor. I really think this makes the most sense with all of the things I have replaced. I will check out the timing marks per 61Porsche and make sure I am on the right ball. Is there a way to tell if the advance weights are moving correctly in the distributor. I sucked on the advance hose (wow, that sounded weird) and could see the plate moving slightly so I think that works. I would guess that even if the vacuum advance was not working that would not be the culprit as the symptoms really happen when the accelerator is pressed down and there probably isn't much vacuum anyway to advance the dist. Do I have to pull the distributor to check the weights?
 
Back to fuel pump.

Disconnect fuel line and see how much fuel it pumps in 1 minute.
If it's not pumping around 1/2 gallon then your motor is running out of fuel at the high rpm.
The reason I suspect this is my built motor would do the same thing with an electric fuel pump. This is running side draft Webers.

I had to get a bigger capacity pump. Not more pressure.

The point here is the same symptom but different hardware.

Worth a check because it seems like you have checked everything else.
BTW there were a bunch of bad manual pumps on the market a year or so ago.
Gary
 
Had the same problem with my CSI a few years ago! Problem was throttle linkage was not adjusted properly and was not opening fully!!
 
wazza, I think the linkage is OK and carb guys I'm sure checked that. It also starts to run crappy before hitting the 4500-5k ceiling so it definitely has problems before its limit.

Fuel pump still an option. Do people replace the whole fuel pump or just specific parts like the diaphragm?

Also remembered that if you keep your foot in it while it's struggling at its 5k rev limit that it will eventually backfire out the tailpipe and lay down dark smoke cloud. If you ease up on it, no cloud, no smoke, exhaust normally looks fine while running, startup, decel, etc. This also makes me think the carb is dumping enough fuel in but the ignition is dropping the ball for some reason.
 
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