3,500 + rpm stumble

[jeremybarnes]

Hello all. Had a great drive yesterday -- beautiful weather here in Southern California, so went for a 70+ mile drive, the furthest i've driven the car since purchase. All was well in a combination of high-speed (70+ mph freeway) and stop-and-go traffic, with a rock-solid temp in traffic and none of the stalling issues I'd discussed in a previous post.

I did have one issue raise its head though, and want to bounce it off the experts: accelerating hard through 3500 rpm, I noticed a missfire accompanied with a backfire through the carbs once. the car runs cool -- right on 70 degrees, no pinging, idles perfectly, revs cleanly with no load, but just this issue. It's a shame, because the noise it makes at 3500 rpm means you want to rev it past there all the time!

I have a feeling it must be related to something electrical rather than fuel, so i'm thinking perhaps the distributor isn't advancing properly as it should. Timing at idle is dead-on. Plugs are fresh, as are wires, cap, points and condensor, and points are dead-on too. Am i going in the right direction? Is the simple solution to pull the dizzy and have it rebuilt (our equivalent of Distributor Doctor is Advanced Distributors in the US)? Or am I missing something else?

Thanks, as always.

Jeremy.

[Moeregaard]

It almost sounds like a fuel-delivery problem that's causing a weak mixture during acceleration Once you sort out and eliminate anything ignition-related, I'd have a look at the fuel filters and the fuel pump. Check the sediment-bowl filter on the firewall and then the filters located inside the individual carburator banjo fittings. It's also possible that the carbs' float levels are too low, so I'd have a look while you're in there. The only other fuel-related culprits are a weak pump, or debris in the sump that has restricted fuel flow at the pickup.

[chrisfell]

I'd check the fuel levels in the float chambers. They are best measured at the jet. I recall advice on this forum so it'll be in the archives somewhere.

[mark10337]

I had something very similar. Turned out the battery connection -ve wasn't tight and vibrations at that rpm caused less connectivity. Tightened it up and all was fine. Took 2 months to work it out. Could of course be a variety of things - but the lesson here was to check that everything is tight on the electrics.

Also check the fly weights in the distributor - everything should move freely and not stick.

[Hugo]

Why would a loose battery connection cause such a symptom? I can't see that it would make any difference - you can disconnect the battery altogether and the only problem you will have is that it will burn out the alternator. Unless perhaps the fluctuating voltage would interfere with the spark, but coils aren't normally that voltage-sensitive.
Going back to the original post, you do have the correct carb needles I take it? Is the misfire confined to that rev range, or is it possibly throttle-dependent? If the latter I would double-check to needles.
SU carbs aren't particularly sensitive to float levels, so that would be a little way down my list.

[Bfastr]

I just went thru this same problem, and it turned out to be my carbs were too Rich..
my car is a S2 with the Strombergs so maybe its a different issue, but I had installed adjustable jets from Curto and set them way too rich thinking the problem was not enough fuel. so I kept opening them and the problem got worse. so I started turning them back in a half turn at a time and road testing, under load in all gears, as I leaned things out the misfires and backfire would move up the rev scale to where now I rev to redline with no problems. as I leaned the jets the misfire moved to over 4000 then over 4500, now if it stumbles above 5000 I will never know, I really dont even like going that far up !!

This was after replacing the coil, cap rotor, points ( I am old school) plugs and plug wires. and triple checking my timing, rebuilding my diizzy and scratching my head.. a lot.

not saying this is your answer, but it ended up being mine.

and by the way, the problem only surfaced when the car warmed up, which makes sense as it likes to run rich when cold.

Bob F

69 S2 OTS

[caveman]

Hi Jeremy,

It sounds electrical. I had a very similar problem a few years ago now. It turned out to be my rotor arm breaking down when hot. New rotor arm sorted the problem with no repeat since. I carry a spare in my boot along with a coil as these are easy replacements/fault diagnosis out on the road.

Here's earlier links ref rotor arms etc.

viewtopic.php?t=7619&highlight=

viewtopic.php?t=8783&highlight=

[64etype]

Don't know if your distributer has vacuum advance, but I encountered an elusive issue ('64 3.8) during initial start up (haven't had the car on the road yet). Corrosion and dirt on the bearing surface between the stationary and vacuum actuated moving plate in the distributer would not allow the vacuum advance to release. So timing was unpredictable. Not sure if this could manifest in your symptoms... Cleaned and lubed...works fine. Long shot.

[44DHR]

I'm with Mark here on a potential fuel delivery issue, particularly on a long maintained high speed run.
Don't just look at the fuel filter in the glass bowl on the engine bulkhead as this won't tell you if the filter is blocked. The glass bowl may be perfectly clear, but as the fuel passes from inside the filter outwards, the inside of the filter may be clogged up.
Only by removing the filter - a few seconds job - and trying to blow through it will see if the filter is fully clear. If the car has been purchased from a previous owner and the history of the filter is unknown, as they are so cheap and easy to replace, I would make this a must do job.
Regards,
Dave