Engine rough at high rpms

[webbslinger]

I'm stumped. My engine craps out at about 5800 - 6000, starts sounding like detonation and has less power. It will go past that point to the limiter, but the way it feels and sounds you don't want to. I've been working on this for a couple of months since having the engine rebuilt. It was run in on a dino and performed well up to 7800. To get the engine in I took the carbs off but left the distributor on without touching it. The carbs are connected and came off and went back on as a unit. Here's what I've tried:

1. First I confirmed that the carbs are mounted loose like they're supposed to be and drained the old gas. I tried new 92 gas, then got some Trick racing gas and tried that.
2. New coil (Pertronix FT2), distributor cap, rotor, and plugs (NGK BP6ES).
3. Checked the timing and advance. 10 degrees at 1000 and 30 at 4000. It doesn't go past 30. (Pertronix ignition and distributor) The voltage is good.
4. Disabled the Pertronix rev limiter to see if it was the problem.
5. Tried switching from the F11 e tubes and 25 mains it used on the dino to F16 and 30's. It's a little more tractable on the 16's but the high speed problem remains.
6. Ditched the fuel filter and checked the fuel lines.
7. Installed a new fuel pump (QH mechanical) and checked it for pressure. It had 6.5 psi so I put on a pressure regulator and set it down to about 3 psi.
8. I just got an A/F gauge but haven't got a bung welded in so haven't got that info yet.

The engine idles smooth at 900 and runs well up to 5800. Does anyone have any ideas on what to try next? I'm trying to think of what could be different from the dino room and my engine bay?

[Elanconvert]

not an expert, but.......carb mounting too loose? [I did have an issue with conflicting info on thackeray washers versus gap between mounting flanges.......]
partially blocked air filter?
I'm sure someone on here will have the knowledge!!!

fred

[david.g.chapman]

Have you checked the side to side play in the distributor cam? I am no expert either but at high speed the distributor cam shaft might start vibrating and causing dwell angle instability.

A new bush should eliminate it if there is a lot of play there.

Dave Chapman

[rgh0]

You say the engine ran well to 7800 rpm on dyno so you clearly do not have standard engine and you have steel crank and non standard rods and pistons. ( or you destroyed the engine on the dyno at 7800 rpm which is your problem)

You must also have a non standard set of cams and valves springs to run at that rpm and probably a bunch of other modifications to engine and carbs and exhaust. I

So without a more complete description of engine and problem hard to tell whats going on. So lets start with description of your engine and its mods and then we can start to help with why it runs rough, otherwise I would just be guessing.

If its really a standard engine and you really ran it at 7800 rpm on dyno then you problem is you really need to rebuild it again

cheers
Rohan

[Craven]

Hi,
High RPM misfire, if this is what?s happening, is often caused by a weak mixture.
An air leak somewhere will cause this, your first check, the mixture strength at high RPM is controlled by the main jet AIR Corrector. Typical value on a road Twincam, 150 to 170, go back to F11 and try a smaller Air corrector.
In my experience a trial and error approach can be an expensive and time consuming, best sorted on a rolling road.
Ron.

[PeterK]

If the engine ran fine on the dyno and the ONLY change since then is to remove and replace the engine, and the ONLY disassembly of the engine was the removal of the carbs as a unit, and the engine has not rev'd cleanly ONLY since then and no other changes / adjustments have been made, then .....

Before you start changing items in and out, the first thing I would do is check the carbs attachment and balance

Peter
(call me simple if you like, but my experience is that if something doesn't work as well after I have touched it, then it is probably what I touched that has changed )

[webbslinger]

Fred, Dave, Rohan, Ron, Peter, thanks to you all for your help. I've been seeking advice from these forums for quite a few years as I built this car, my 3rd Elan, but this is my first post to the site. I hope to be able to help someone else someday.
Fred;, the first thing I checked was that the carbs were too tight and vibrating. I loosened them up and they aren't leaking and the filters are new.
Rohan; the engine is using the original crank and rods but with Bean's 0114 cams, .420 lift with their valves and springs to match in a big-valve head. It has 9.7/1 compression which was reached with custom pistons. It was intended to be a just a strong street motor and the outfit that built it is highly regarded. They were recommended by Bean's and they've built many of these engines. A guy with 20 years experience at a local vintage race shop advised the builder and when I spoke with him he said that he'd been involved with the rebuild of at least 25 twin cams over the years with this builder and in his opinion, they built a very nice street motor. It has headers and 2" exhaust but at this time, all that goes through a standard S-4 muffler. Hmm, over-revved on the dino? It doesn't smoke or make ticking sounds, but I will check the compression this afternoon - something I should have done just for the record at least.
Ron; I did go back to the F11's yesterday to test it with the proper fuel pressure. 150 air correctors were used with the F11's on the dino and on the builders advice I switched to 140's for the road. It had the same problem with the 150's though. Yes, time consuming already and if it continues much longer I'll take it someplace with a rolling-road to tune.
Peter; simple is best and often overlooked and you know what they say about assuming. So good advice, I will check the carb balance which I've assumed remained as it was on the dino.
Thanks guys, I'll get back to you in a few days after checking these new things out. Victor

[Chancer]

Carb balance has a massive effect at tickover and partial throttle opening and practically no effect on wide open throttle, at 6K rpm one pair of carbs will be flowing minutely less than the other pair but the mixtures will be correct and wouldnt IMO cause a misfire.

I agree with the thought process that whatever was the last thing touched, changed or modified is the first thing to check, often we may forget or not even think something was significant until we finally trace the fault, its then we realise the clues were there all the time.

I had a misfire at high revs once, the carbs were 48's and rather too big for the engine, as a consequence the main circuit didnt fully come in till above 5K when the misfire occured, 1st clue it was a main circuit starvation, it turned out that one venturi securing grub screw had Fallen out, the aux vent had rotated in the bareel thus cutting of the main jet supply to the central web doo dah thingy whatever it is called, similar symptoms, ran fine up to 5K even on wide open throttle then a misfire on (as it turned out) one cylinder only.

Worth a look.

[vincereynard]

Sorry if I am being simplistic, but would it not be easier to simply book a rolling road dyno. session.

[rgh0]

Engine sounds like a good road engine build. But it is certainly not capable of sustained operation at 7800 rpm so i hope its OK, not sure why anyone would dyno it at those speeds.

You most likely have a fuel or ignition problem and need to do some basic checks to determine what it is. A rolling road session would be the quickest and easiest way to determine whats going on. In the absence of doing that you are very much down to trial and error.


I presume you are doing acceleration tests on the road to get the rough running symptom at 5000 rpm but still reach the 6500 rpm redline slowly . Do you see any other symptoms apart from slowing acceleration and rough running, Does it misfire, backfire, does the tach jump around , any other strange noises or vibrations?

Starting with the carbs
It sounds like fuel supply should be OK but the fuel tank is different to when it was run on the dyno so potentially you have a partial blockage in the suction line somewhere. Can you stick a fuel pressure gauge out where you can see it when running at high revs and full load ?

Is the throttle opening fully when you push the pedal to the floor?

What size chokes do you have fitted I can perhaps comment on whether your jetting is reasonable.

On the ignition side
The advance seems correct and you have replaced many of the components.
I would replace the high voltage coil and spark plug leads if you did not do that when you changed the distributor cap. I would also try a set of points and condenser instead of the electronic ignition to see if that is the issue.

Cam timing could be an issue (though unlikely if the engine was built by an experienced builder) but I would check that also

If I got to this point and still no solution I would certainly be looking at a rolling road session to try to identify the problem

cheers
Rohan

[webbslinger]

Unfortunately, life prevented me from getting anything done on the car today Other than picking up on the fact that I have been misspelling dyno, all I accomplished was talking to the guy who conducted the dyno test. The engines problems are definitely something that have occurred in my garage because the test went perfectly and he ran me through what all the numbers meant including the running time intervals. It zipped to the cut-off point building hp very linearly time-wise and there was no lag anywhere although interestingly, at 5800 where the missing starts now is where the torque peaks and starts to fall off. And Rohan, you will feel a little better when you hear that I was mistaken, they only took it to 7600, not 7800 and I have no plans of running it nearly that hard. Also, Mr. Dyno put me in touch with a guy with a rolling road who also happens to be regarded as one of the most experienced twin cam tuners in the area. I agree with all of you that taking it to someone like him is probably the way to go. It was very interesting talking to the dyno operator and I am really sorry that I wasn't able to attend the session.
In the meantime, hopefully tomorrow I'll be able to confirm the carbs are synced and check the compression and I'll post those results when I get them.
As to a couple of questions, the chokes are 35's, which surprised me but they felt they would be good for this engine. The chokes being a little big might support Chancer's theory about an obstructed main do-hickey, I'll check that and thanks. Rohan, it doesn't reach 6500 slowly, it zips there. It's just that it gets ratty at 5800 or so and requires an extra push to keep the acceleration going, which it will then do quickly if you can grit your teeth at the way it sounds. Yes, we ran the fuel pressure gauge in through the window and watched what it did all the way to redline. All it did was drop from 6.5 psi to 6. That was before putting in the pressure regulator. I didn't mention it before, but I pressurized the tank to check for obstructions and everything is free flowing. I've really enjoyed working on this car and would like to get it working myself, but I will probably take it to the shop mentioned above if I don't solve this soon. What ever happens, I'll let you all know what the problem was. All for now, and thanks, Victor

[rgh0]

With 35mm chokes, which are a good selection for an engine to your specification i would expect main jets in the 130 +/- range and air correctors in the 160 to 180 range. I built and dyno tested an engine with very similar cam and ignition and carb specs as yours a few years ago but i cant find the jets specification I ended up using unfortunately.

You should not run a standard twin cam above 7000 rpm and go above 6500 rpm only briefly- I have seen to many fail when people think you can rev them to 7500 rpm

The fact that it is a rattle and vibration and you need to grit your teeth but it keeps accelerating rapidly to the red line and does not appear to be a miss-fire or loss of power may indicate a mechanical problem that was not apparent on the dyno rather than a carb or ignition problem.

Have you checked to ensure nothing is hitting the chassis such as the exhaust and that all the mounts are in good condition. What sort of balancing did the engine receive? Is the noise and rattle the same when revved under no load as when accelerating under load.

cheers
Rohan

[Chancer]

Do you have dual coil valve springs?

You mention gritting teeth, sounds like you can feel as well as hear the noise.

If so one of them may be broken, the coils that is, not your teeth but if you carry on revving it...................

[toomspj]

This sounds one of those horrible hard to chase problems.

I've had some similar issues in the past - i suggest two lines of attack:

1) If it happens under light load but high revs, then I would suggest you look for an ignition problem.

2) if it is under high load then look for a fuel problem. I don't say this definitively, but more as a way to approach it.

For the ignition, just because you replaced the cap, rotor, plugs and coil doesn't mean it isn't one of these. I've had all of them fail as brand new items. Does it do it from cold or only once it gets hot? Electronic ignition units can fail in this way too.

On the fuel side, have you checked all the connections upstream of the pump? I have had one instance of a leaking joint there that allowed air in and kept the motor from running flat out - mind you most fuel problems don't give the same response at high load as they do at light load.

What fuel pump are you running - also, sounds like you have a pressure regulator if you're able to adjust the pressure? I've also found that a Facet fuel pump can't keep flow rates up if the voltage at the pump is anything under 12v (check for wiring/earthing issues).

If you can get a bit analytical and measure fuel/air that would help.

Paul

[webbslinger]

Update: I've tried a couple of things and the problem persists. The compression is good and near exactly the same on all four and the carbs are balanced. I have an A/F gauge but haven't yet welded a bung in the exhaust. Planning on doing that in the next few days. When I get the A/F info I'll talk to a recomended tuner and arrange a rolling-road session if that doesn't shed any light. Will keep you posted. Victor