High RPM flat spot from hell.

Chris Taunton

True Classic
Some of you know Gus (asparaGus) as Split Pea. I picked him up from George in Portland last November and have been fixing small things while using him as my daily driver. I understand that a previous owner and Xweb member built the warm 1300 engine with a PBS cam (of some sort), custom made 4 into 1 headers, Bosch electronic ignition from a '79 X1/9 ,and a 32 DMTRA carburetor.

I have been chasing a horrible flat spot the whole time. It revs to about 4300 rpm and hits a wall, almost like a rev-limiter. If I release the throttle, the engine picks up speed a little. Best power comes at about 90% throttle. I have replaced the distributor with a Marelli SE100 from MWB (the centrifugal advance was stuck in the Bosch distributor, and the installation of the Marelli is cleaner) and experimented with ignition timing and carburetor jetting. The engine runs much better than when I picked up the car in the low and mid rpm ranges, but will not rev.

Has anyone else experienced this problem? What was the eventual solution?
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I had a similar sounding problem with my X years ago where I could not go any faster on the freeway even with my foot to the floor. Over the period of a few weeks, my top speed further declined. It turned out to be the return spring on the mechanical fuel pump was slowly rusting away until the stroke got too short to supply enough fuel. The replacement pump did the same thing years later.
 
I guess this is an "archive" picture, or we don't have the same weather. Here we have heat waves from over a month, with just a week with "normal" temp.

Is your car acting always like that? I mean, if you start the car and rev it to 4300rpm without being in gear, will it do the same?
 
Yes, the picture was taken on the drive from Portland to Athens, GA last November. That was a a hotel in Wyoming. See my profile picture for how Gus looks post trip and detail.

It will rev to over 4300 rpm with no load. Under load, power flattens out and then cuts hard. If I ease off the throttle a little, it will accelerate a bit more. This engine should pull hard from 4000 to 6500, but it will not go there and I am a bit perplexed.
 
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I had a similar sounding problem with my X years ago where I could not go any faster on the freeway even with my foot to the floor. Over the period of a few weeks, my top speed further declined. It turned out to be the return spring on the mechanical fuel pump was slowly rusting away until the stroke got too short to supply enough fuel. The replacement pump did the same thing years later.

Thanks, Don! That is something I can check. If I run it to 4300 RPM in second with my foot on the floor, I can switch off the ignition, push in the clutch, coast to a stop, and check the float level in the carburetor.
 
Hi Chris,
I never had this problem myself. Sometimes, you just have to go to the basics. I like some of the suggestions in that article:

Basics like: how are your plugs, fuel pump and fuel filter, do you still have a catalytic converter, etc.
 
Also sounds like the secondary on the carb may have a plugged main jet or some other blockage in the fuel system of the secondary.
 
Hi Chris,
I never had this problem myself. Sometimes, you just have to go to the basics. I like some of the suggestions in that article:

Basics like: how are your plugs, fuel pump and fuel filter, do you still have a catalytic converter, etc.
Plugs are new (old ones looked a little lean and gapped at .040"). Fuel filter is new. No catalyst ('73 is pre-catalyst, header and stock exhaust system). This does not feel like a blocked exhaust system. It is more sudden than that. It goes from strong mid range to a fast roll off of power, then a complete cut like a rev limiter.

Fuel pump is mechanical and relatively new (installed by the previous owner). It is something I can check.
 
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Also sounds like the secondary on the carb may have a plugged main jet or some other blockage in the fuel system of the secondary.
I agree. I checked the secondary main jet and it was clear. I also tried the next two larger jet sizes (stock is 110, carburetor had 115, I tried 120 and 125 without any effect on the problem). There may be some blockage in the body downstream on the jets, but I've never seen that happen before. It is something I can check.

My current plan is to check fuel delivery volume, float level, and then rebuild the carburetor if the first two check out okay. I'm interested in any other suggestions anyone else might have.
 
If every time you get off full throttle and back to light throttle the motor acts OK then I don't think it's a float bowl filling problem. Which check on the jets is to reverse the primary and secondary jet stacks (main/emulsion tube/air corrector) and see if that makes a difference. Very easy to do.
 
Plugs are new (old ones looked a little lean and gapped at .040"). Fuel filter is new. No catalyst ('73 is pre-catalyst, header and stock exhaust system). This does not feel like a blocked exhaust system. It is more sudden than that. It goes from strong mid range to a fast roll off of power than complete cut like a rev limiter.

Fuel pump is mechanical and relatively new (installed by the previous owner). It is something I can check.

When you replace the mechanical fuel pump, you are supposed to shim it out (using a gasket of the right thickness) so the rod protrudes the right amount. Haynes says 15.0 - 15.5 mm. If the pump is shimmed out too far (so the rod does not stick out far enough) it limits the capacity of the pump. Any chance this could be your problem?
 
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How your car acts when it "hits a wall" is the first clue to finding the problem, especially if it's fuel related. For example, if the car just bogs down and "blubbers", there's a good chance it's getting too much fuel. If the car starts coughing and backfiring there's a good chance it's a lean condition.
If it just loses power, doesn't blubber or backfire, it could be ignition. Maybe timing, maybe inadequate spark.
Could also be cam timing, a retarded cam will act like you describe. May have skipped a tooth?
 
If every time you get off full throttle and back to light throttle the motor acts OK then I don't think it's a float bowl filling problem. Which check on the jets is to reverse the primary and secondary jet stacks (main/emulsion tube/air corrector) and see if that makes a difference. Very easy to do.
The primary has a 105 main jet (one step leaner than stock), F30 emulsion tube (stock) and 200 air correction (stock). The secondary has a 125 main jet (110 is stock, it had a 115), F30 emulsion tube, and a 195 air correction. Since I've had 3 different and clean jets in the secondary, what will I learn from swapping them?
 
When you replace the mechanical fuel pump, you are supposed to shim it out (using a gasket of the right thickness) so the rod protrudes the right amount. Haynes says 15.0 - 15.5 mm. If the pump is shimmed out too far (so the rod does not stick out far enough) it limits the capacity of the pump. Any chance this could be your problem?
This is something I can check, but Carl is right - it does not feel like a fuel starvation problem. But, I've been wrong before and my knowledge has not been the key to this problem so far. I will check it. Thanks, Bjorn!
 
How your car acts when it "hits a wall" is the first clue to finding the problem, especially if it's fuel related. For example, if the car just bogs down and "blubbers", there's a good chance it's getting too much fuel. If the car starts coughing and backfiring there's a good chance it's a lean condition.
If it just loses power, doesn't blubber or backfire, it could be ignition. Maybe timing, maybe inadequate spark.
Could also be cam timing, a retarded cam will act like you describe. May have skipped a tooth?
Mike, the timing belt was very loose when I picked up the car. I re-tensioned the belt on the drive from Portland and it made a big difference. Honestly, it feels like ignition timing more than fuel, but I have checked that. I have never experience retarded cam timing. I will check that tomorrow. There is a good chance it has skipped a tooth.
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So, a PBS cam, a loose timing belt, maybe a modified shaved head with an oversize tensioner bearing... or maybe not How is the belt tension now. You said it was better, but is it tight& It is possible the PO didn't finished the job correctly (no acccusation, maybe he could chime in and let us know what's in there). Some investigation on this side could lead to the problem. Do you have an adjustable cam sprocket? It could be a bad timing because of a skipped belt, but also because some components aren't working well together. On my car, with a cam, a modified shaved head, I had to test fit 2 length of timing belts and 3 sizes of tensioner bearing before I could get the timing belt with enough tension. Then I had to play with an adjustable cam to find the right timing.

That may be something completely different, but at least it is a possiblity to check. Good luck!
 
So, a PBS cam, a loose timing belt, maybe a modified shaved head with an oversize tensioner bearing... or maybe not How is the belt tension now. You said it was better, but is it tight& It is possible the PO didn't finished the job correctly (no acccusation, maybe he could chime in and let us know what's in there). Some investigation on this side could lead to the problem. Do you have an adjustable cam sprocket? It could be a bad timing because of a skipped belt, but also because some components aren't working well together. On my car, with a cam, a modified shaved head, I had to test fit 2 length of timing belts and 3 sizes of tensioner bearing before I could get the timing belt with enough tension. Then I had to play with an adjustable cam to find the right timing.

That may be something completely different, but at least it is a possiblity to check. Good luck!
All good points. The previous owner, George (a good guy), did not build the motor. It was build by someone a few owners ago who is an Xweb guy, but I don't know his name or how to find him. I think Mike Mittelstead owned Gus before George. I would love to talk to any previous owner and establish a history for Gus and get the specifics of the engine and transaxle builds. I understand he was rescued from Chris Oberg's place in Santa Cruz and brought back to life.

The belt tension feels good now (not sloppy), but I have not checked the cam timing or looked at the cam pulley. I hope this engine ran great when new, and something has happened to it since then. It burns no oil and feels crisp at low RPM since the distributor replacement. It would suck if it ran like this through multiple owners. I have not checked cylinder compression or valve clearances either, but it is on my list. My other cars have been hogging my time!
 
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?
 
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. 😀
 
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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?
 
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