Abrupt power loss at 5.5k on flat and uphill. 1969 srl311

[1969311]

Hello everyone,

I have been preparing for the shasta meet next month, and have been experiencing an abrupt power loss at 5500 rpm.

When it occurs the exhaust seems to be louder and the car will not continue to accelerate. As soon as it is below 5000 it will accelerate normally. This issue does not occur on the down hill, strictly when under load either on the flats or going up hill. Also it will free rev to upnto redline with no issues when stopped and in neutral.

Timing is set at 16 degrees(with vaccum line disconnected and around 800rpm idle) and seems to advance normally with increase in rpm. The car has an east coast electronic dizzy.

Most recent work includes; replaced and re-ringe the pistons, new oil pump install, tuned the carbs(su) and set valve lash.

Feels like fuel delivery, though the float levers measured within specs. I have not tested fuel pressure under load. Have not had enough miles on the piston job to retorque the head bolts.

I am curious if any of you have experienced something similar, and if so what cured your ails!

I have looked at the techwiki and only saw one post that seemed to describe the same issue, but it was a 1600 and improper float levels were the cause.

At any rate, if someone has thoughts or a suggested next step, I would greatly appreciate the advice.

Until next time, thank you all and Happy roadstering!

Chris
1969 srl311
Stock u20 with g boon electronic ignition.

[jrusso07]

Agree that it sounds like fuel starvation. If floats are right and needle valves are set right (with no debris in the valve), check the fuel filter screens in the carb banjo bolts and fuel pump rate of delivery. Also make sure you have oil in the carb piston dampers as they can go tool ean in hurry .

Assuming you have a fuel filter that is not opague. When parked and running at 6000 rpm (use the high speed balance screw on the carb linkage to set). Does the fuel level in the filter get really low? That wouls suggest a fuel pump issue or some type of fuel feed blockage towards the tank.

[1969311]

Joe,

Thank you for the reply, I checked the banjo bolt screens when I was looking at the floats, they appeared unobstructed and when held to the light could not see any visible debris.

I will try setting the high speed linkage to test the pump and feed lines next, thank you for your insight, it is appreciated. Will check in after the test with results.

Thanks!

[1969311]

Ok, so I locked the throttle at 5500rpm and did not notice the fuel level in the filter dropping abnormally low. Is it possible that the valves are slightly out of adjustment, and causing valve float?

Thanks for the insights!

[GeoffM]

You mention the fuel filter...Have you changed it recently? There should be surprisingly little fuel in the filter body in a clean filter.

[Gregs672000]

This may be a difficult one. It does sound fuel related, as spark issues would not likely be at a particular rpm and suddenly recover 500 rpm lower.

We are assuming that it's running lean, but we really don't know. What you describe sounds like it goes flat, not surging, sort of a fluttering "blaaaaaa" sort of sound without accelerating more, correct? Does it ping or make any other Sounds? Can you have someone drive behind you in some safe place to observe your exhaust while you are at 5000 and 5500rpm?

Without a wideband air/fuel set up on it, you're stuck reading plugs. I suggest you find a safe testing road where you can pull over easily and work on the car, go there and then install a new set of plugs (even if the current ones are new, start clean, you'll use them eventually anyway), gapped appropriately, then drive the car, getting to the failure point as soon as you can and holding it there (be nice to her) for some distance (as long as you can, a few minutes if possible), then while its failing TURN OFF THE IGNITION while you shut the throttle and coast to the side. Pull a plug from #1 and #4 and see what color the insulator is... white and it's lean, black and it's rich. Hopefully that will at least tell us what direction we need to go.

Also, you could go to Harbor Freight or the like and purchase a cheap DIAL BACK timing light to make sure your ignition is advancing and holding that advance, and it will tell you how much advance it's giving. Not likely again, as full advance is in by 3500 rpm and shouldn't change.

Other reasons why it won't rev higher could be a cam timing issue. You mentioned re-ringing the motor, which means you have had it apart and pulled the timing chains? If so, you could be a tooth off? Seems unlikely, but it's hard to say. Stretched timing chains? Were they replaced?

First thing I would do is just pull the plugs and look at their color as it is right now, or try the new plugs while holding it at 5500 or so in the driveway for a bit and read them... if it's that lean or rich it will probably show on the plugs even in the driveway. You can always do the actual test as above later... totally clean plugs just makes it easier.

A dyno pull with an exhaust analyzer will tell you a lot, costs about $120 bucks around here with the sniffer (they add $50 bucks to the pull!). That may tell you a lot more info about the health of the motor as well by showing what rpm your max torque is occurring, and if that's unusual then it could indicate a cam timing issue or something mechanical.

What's the history of the Engine? Did it rev to 6000+ before? Will it redline in ANY gear? If the valve springs are weak, could be valve FLOAT?

Keep testing and observing... these things can take time.

[fj20spl311]

Fill the gas tank and see if that make a difference. If the fuel pump is have a hard time keeping up, this should help.

The fuel requirement of no load at 5500 are vastly different from loaded, so test without a load is useless.
I would suggest a wide band Air/fuel meter. A good investment for everyone.
I have seen cars cut out when the polarity of the coil was reversed, but IIRC, its usually more like 4000 rpm.

Good luck

[SLOroadster]

Where are you located? If you are in the north bay area I can swing over and have a look this weekend. You can eyeball the cam timing, if the timing mark is not at exactly 3 or 9 o'clock at TDC, that is your starting point.

What cam are you running, and what is your valve clearance?

A plugged fuel filter will fill with fuel, but not drain out. A happy one will sit with some fuel, but not more than half while running correctly.

Will

[1969311]

First of all, thank you all for the input, this community really is phenominal in that regard.


Geoffm:
The fuel filter was replaced when I did the Pistons, it appear to be 25-33% full in the filter body, the filter is not in the OEM bracket, as I never changed the fuel hoses from the p.o. Currently it is routed almost level with the fuel pump, not sure if the small change in elevation has any noticable effect on the flow.

Gregs672000:

" What you describe sounds like it goes flat, not surging, sort of a fluttering "blaaaaaa" sort of sound without accelerating more, correct?"

Exactly this.

Does it ping or make any other Sounds?

No pinging unless I lug the engine, and even then it is mild

Can you have someone drive behind you in some safe place to observe your exhaust while you are at 5000 and 5500rpm.

Yes, this is a good idea, perhaps I will do this in accordance with the new pluggs, and pulling over after symptoms, though I admit I don't like the idea of keeping her at that rpm for any period of time behaving this way. Haha.

I pulled the plugs and both 1 and 4 seem to have an even light brown deposit, insulator in 4 is slightly more white still, but they are newish (150miles) b6 ngks. This was after holding under nonload in the driveway.

The cam gear in my car is slotted, so it is possible it is slightly off, though I did use the alignment pin when reconnecting the cam.

Wideband is on the list, and has been for a while, . Dyno is good suggestion, not sure where the closest is, I live in Grass valley.

Engine history is my uncle rebuilt it in the ninties with all stock components and it sat, I got her runningy again, and blew the piston, he had installed dished positions, too much compression ect. Before she blew she would red line in all gears, except fifth, and that's because I did not trust the car at thoes speeds.

Since the rebuild I was seating the rings and working a few other bugs out, and encountered this issue.

I will look to see if he did the valve springs, thank you for taking the time to help.

Will: the problem seems to be consistent at 5-5.5k in all gears, unless going down hill.

I live in Grass valley, close to colfax off hwy 80. A few hours north of the bay, I really appreciate the offer and wish I lived closer.

I will look at the can hear timing mark, thank you!


Fj20spl311: The tank is full, and has been for the last few tests. I appreciate the suggestion for the wideband, I would like to install one soon. Filter looks good to my untrained eye.


Thank you all!

[Gregs672000]

So it sounds like you did not disturb the timing chains or gears, correct? I just looked at the image I took of the timing gears etc all aligned correctly, and it appears that the crank keyway is straight up. You can use the large (0) crank timing mark on the pulley as a very close approximation of TDC. To check cam timing, put the engine at TDC on #1, remove the cam cover and observe your cam gear. Per the manual, the pin should be at the top. There's an arrow on the #1 cam bearing cap, and that should align fairly closely with the hole for the cam gear locating pin, more or less. A tooth off should be very obvious... and unless the gears were messed up from the get go or you changed something there it's probably fine given it reved before. This is more of an FYI if nothing else works.

At this point I'm agreeing that it sounds like a fuel issue. Some factors have changed since you worked on the engine. Before it was actually LOWER compression with the dished pistons, so fuel demands were probably lower too and may not have been as noticeable. SU carbs are not my specialty so how to richen them beyond trying a different or new needles is all I can suggest, other than to make sure they are with proper oil etc. However, I believe the oil is there mainly for sudden acceleration to keep the piston from flying up too fast and leaning out the engine. Yes, do be careful about running it hard while lean.

SU experts: Will pulling the choke further richen the mix at higher rpm since it causes the seat to drop lower, unshrouding the needle that much more? If so, that may help you figure out if it's lean or rich. Given what your plugs are saying off load, I'd guess it's lean, as otherwise they would be black if it's that far off. Also, you may not need the dyno pull to get some air/fuel info. Call around and see if you can find someone who has an exhaust gas analyzer, describe your problem and see if they can give you a reading off the tail pipe at high rpm. If it's that lean I think it will show up even when off load with numbers in the high 15s or above.

[1969311]

Greg,

Sorry I had mistyped, I mean domed pistions. So the compression was higher, darn words! Good catch though:)

I checked the cam gear, the pin is at 12 and the timing mark at 3 when cyl 1 is tdc, so that appears to be sound, and to confirm I did not mess with the chain or gears aside from putting the pin in, removing the cam bolts and taking the head off, also ziptying the tensioner, so hope fully no changes there.

Will had me curious about the issue across gears, I pulled to 6.5 k in first with no power loss, then 5-5.5 k had loss in second and third.

Good call on the lean condition, I will make a few calls and ask about that. May buy some new line and a filter, and double check the neadle and seat for debris. Do you know off hand, the normal psi the fuel pump is supposed to produce?

Thanks Greg!

[nismou20]

Does your engine do the herky-jerky or run without a miss when running out of steam at 5500? In my experience when tuning SUs they tend to do the jerky.

[1969311]

Nismou20,

Not completely sure to be honest, it sounds regularly intervaled, no new vibrations other than the loss of power and increase in exhaust noise.

[1969311]

So I went into the garage to ponder, haha.

Now the fuel filter is almost full of gas, go figure, there's just a small bubble of air in the housing.

I'm going to pull the floats out again and clean the seats.

Anyroadster nuts in Grass valley? I pay diagnostics in beer and food:) cheers!

[GeoffM]

Replace the filter and try it.