High RPM flat spot from hell.

Chris Taunton

Daily Driver
My worthless experience is that major problems such as cam timing, ignition timing and even bad compression would show up more as an engine that's hard to start or with a terrible idle. If everything seems fine except for a dead high rpm issue then I wouldn't dig too deep for the source of the problem. I'm hanging my hat on an ignition that does not advance properly or a blocked fuel issue on the secondary.
Chris, I suggested swapping changing jet stacks because it's real easy to do and will instantly indicate if there is a blockage...which could be in the emulsion tube or even air corrector. Just helps in the process of elimination. If you had an O2 sensor and gauge you could watch that go super lean or rich when you hit the "dead zone" but I'm sure you don't have one of those hooked up. Do you have a timing light to check the advance of the ignition?
Hey Carl - I'm with you. That's why I pulled the jets out the first time. It seems like a lean secondary issue. I removed the jets and air correctors from the emulsion tubes and inspected all parts with a magnifying glass. There was no visible blockage .Next, I tried bigger jets with no improvement. Yes. I have a timing like and I have confirmed that the mechanical and vacuum advance function on the new distributor and the timing is set now with 15 degrees initial advance at idle.

I will try swapping the primary and secondary main "stack" as you suggest, just to eliminate that. I'll let you know how it goes. I have also been toying with getting a wide band O2 sensor and gauge, but will wait until I've checked the other suggestions in this thread.
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Chris Taunton

Daily Driver
Any time you're troubleshooting problems like this it's good to check the "easy stuff" first. Pulling out the jets and checking for blockage certainly falls into that category. But, if there was a lack of fuel flow at higher revs it's likely that the mixture would go lean, which could lead to popping, snorting, bucking, and all manner of complaining. You could expect the same symptoms if you had a problem with fuel delivery.
If on the other hand you had a problem with too much fuel at higher rpms, the engine would stumble, and blubber, and miss. That might be caused by a leaking needle valve, or even a float that's set too high (not very likely, that would affect the engine at all rpms). But checking the needle valve and float level is also pretty easy, so give it a look. Too much fuel at speed, blubbering, stumbling, etc could also be caused by something as simple as a clogged air filter. It happens.
A problem with ignition could be a bit more difficult to diagnose. Checking ignition timing is fairly easy if you have a dial back timing light. If you don't, not so much. A weak spark will cause an engine to loose power as the rpms climb, but that's not as easy to troubleshoot unless you just swap out old components for known good ones (like the coil?).
However, cam timing being off a tooth or so isn't always as easy to check. Even if the timing marks line up, you have no way to confirm that the cam is timed "right". If you have all the right pulleys and the correct indicators are installed (it's not impossible to install the incorrect ones), you are probably in pretty good shape. But if it's possible that someone shaved the head, and or, cam carrier, to "improve performance", then even if the factory marks lineup, you might have cam timing that is sub optimal.
An engine will run just fine with the cam timing off by quite a bit. In fact with computer controlled engine management systems it possible to have variable cam timing. Different cam timing for different conditions.
A good rule of thumb is that retarded cam timing will increase low rpm power at the expense of higher rpm performance. Kinda like what your experiencing. Hummmm... Did you say someone else "built" the engine? Taking material off the block, head, and cam carrier tends to cause cam timing to be retarded. Hummmm....
If you have an engine that's been "built" it may be difficult to get the cam timing "optimal". That's because there is no one setting that's best, and, until you degree the cam, you don't know where it is at in the first place. To get the engine to perform as you want it to perform (high rpm performance, low end torque, or whatever) you really need to do some dyno tuning. Definitely not in the "easy" category. A good compromise is to set the cam to "straight up" or no advance, or retard. From there you can experiment with advancing or retarding timing to optimizing the timing. But getting the cam timing straight up, isn't in the easy category either. It's possible, but it's something that many folks are not experienced in.
Just bolting "performance parts" on to an engine can lead to disappointment unless things are tuned properly.
Good luck in your troubleshooting efforts, confirm that the basics are right before you move to the more difficult stuff. You'll be surprised just how well a properly tuned SOHC engine will perform. 😀
It does not feel rich and the spark plugs looked lean. The air filter is a K&N and it has been cleaned and re-oiled.

I also thought about a weak coil, especially since the spark plug gap was large and that it ran better at less than full load. Less dwell time will cause a reduction in coil output. New and correctly gapped plugs did not improve things. If the coil was weak, it should have.

Since these problems predate my purchase of Gus, I'm in the dark as to how long it has been running so poorly. I can degree the cam if I want to remove the engine, but honestly, if I'm going to go to that much trouble I'd rather build my own engine with my choices of parts. I am planning to do that anyway, I just was not planning to do it right now! I was planning to enjoy it as is for a while.

My first car was a 1972 128 that I installed a stock 1300 into. Both the 1300 and the original 1100 are stronger than this motor. I love this engine when it is running correct;y.

Chris Taunton

Daily Driver
I would assume one of two things. It is running out of fuel flow, or not enough ignition advance. Does the distributor spark advance work? I would try a electric fuel pump. Also is there any type of in tank filter, is that basically clogged?
Hey Ted,

Good questions! Mechanical and vacuum advance are functioning and the distributor in new. I have no idea what kind of filter screen the fuel pickup might have. I've never seen the inside of a 128 fuel tank. There is fuel in the fuel filter. I will check the float bowl after running it to cutout rpm in second. I don't think it is a fuel supply problem, but I will try to eliminate that as a possibility.


True Classic
I just typed in "fiat 128 gas tank" on the web, got a bunch of pictures. Looks simple enough to remove the fuel pick up / fuel level assembly, similar to the X 1/9 bet the sock is clogged up, bet it runs just fine until you hit the limit of the fuel flow.


True Classic
Fuel? Timing? Fuel? Timing? Both are still possible. I like the electrical fuel pump idea. Cheap, simple. If it is running better, you found your problem. If not, well you may keep it as a spare or use it instead of the mechanical one. I don't remember all your steps. Did the fuel filter was changed recently?

Is the tensionner bearing is rolling easily? I have seen old tensioner bearing rolling slowly, creating heat on the timing belt which extend and provoque a bad timing... until you slowed down and it cooled down. and suddenly, everything is fine.


Old enough to know better
Before adding an electrical fuel pump I would just observe the output just from cranking the engine over. A 128 is not gulping down huge amounts of fuel even at full throttle. A properly working mechanical fuel pump is more than up to the job.

An electrical pump is just a different way to suffer a hair pulling failure.


True Classic
Ah, one more thing just occurred to me. Make sure the aux venturi on the secondary is in right. You can install it 180 degrees out and that blocks the path from the jet to the opening in the venturi. If it is in right, make sure the carb body internal passage between the jet and the aux venturi is not blocked. As you can tell, I still have my money on a fuel issue.

Chris Taunton

Daily Driver
Thanks to all. It is always helpful to bounce ideas off other people. I just got home from a quick drive. I'm looking forward to taking Gus to work tomorrow.

Chris Taunton

Daily Driver
So the cam had jumped a tooth?
Yes. The cam timing was advanced by one tooth.

More info: It appears that the engine was built by Courtney Waters (128kid). It has stock 1500 pistons (86.4mm) in a 1300 block. Crankshaft and rods are stock. The head has been milled and the ports have been smoothed. The valves are stock size, back cut, and the seats got a 3 angle valve job. Combustion chamber volume is 27 cc's (compression ratio about 9:1). The cam is a SX1 PBS regrind.

Since resetting the cam timing the fuel economy had improved from 22 mpg to 27 mpg (the last 2 fill ups). I have not re-installed the 115 secondary main jet (it still has the 125 installed). I drove it about 500 miles over the weekend. It seems a little weak as you approach redline, but otherwise strong. I might try a slightly bigger carburetor if I can find one.

If anyone knows who built the transaxle and what gearing was used, that is also a mystery.
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True Classic
chris, if you are still here..how did you checked the timing (i mean i step by step instructions)? i have a 1300 stock in my 128 but i feel it should go better than it does, and want have a look..

Chris Taunton

Daily Driver
chris, if you are still here..how did you checked the timing (i mean i step by step instructions)? i have a 1300 stock in my 128 but i feel it should go better than it does, and want have a look..
Hey Ghost,

I lined up the crankshaft pulley to the timing mark, then removed the timing belt cover to check the alignment of the cam timing marks. If you have a shop manual, there will be instructions. The mark on the cam pulley should be lined up to the mark on the engine mount (see picture in thread).

I have since replace the engine mount and the new mount lacks the timing mark, but the casting line in the center of the mount will also work.

If you don't own a shop manual, I will look up more specific instructions. Good luck tracking down your problem.




True Classic
I had a Lancia Beta. Cam timing, distributor and rebuilt carb were all optimized stock. It started and ran fine. On the freeway at wide open throttle, it would accelerate strong for a few seconds and then go flat. The O2 sensor said it was going lean. Revision of the pinched fuel hose by the tank and a new, larger fuel filter solved the problem.


True Classic
yes, i have manuals, i just had a look this morining..easy to line up the cam sprocket mark, checked also the dot in the clutch window allines to the zero mark... not so easy to check on the crankshaft side.. there is a mark on the big alternator belt pulley, it's less or more at 1 o'clock..but cannot see the mark on the casting of the block.. do i have to remove the alternator belt pulley to see the mark on the block?