Intermittent Stumbling

After a re-haul of my ignition items and a new carb I am STILL experiencing an "engine-missing" type problem but its intermittent. Since the upgrades, the car starts instantly and runs very sweetly 95% of the time.
I have fitted new points, condensor, plugs (N9YC), cap and rotor arm. All brand new. The ONLY thing I havent changed out are the plug-leads and coil. Its got a recent fuel filter and the new weber carb so that should be good. Its weird as it runs perfecly but now and again if feels like the cars "stumbling" but then it goes away.
I was also suspicious of the fuel pump but its not that type of a "stumble" - its as if a plug (or all plugs) very briefly miss for a split-second.

Any thoughts appreciated.

Thanks!

I had something similar in a 71. Never did trace it satisfactorily and sold on the car before I had chance to finally nail it. I would defo change the leads and coil though, just to eliminate them as the source.

I have had similar issues with my GT. The first time it was a clogged fuel filter, the second it was a loose connection where the electrical plug goes into the back of the alternator (brown hot wires) and the third (I think) may have been the venting fuel cap not working - when I jiggled it I heard air being sucked in and the fuel tank popped back into shape. Only small things but they all gave the same results as you describe, particularly the intermittent nature of the "stumbling"

Tony

Thanks all for the interesting feedback. Im leaning towards a loose electrical connection as the issue comes and goes. Its either there or its not. When its not, the car runs perfectly. Good suggestion on the fuel cap. I did fill up on Sunday with gas and Sunday Monday the car was perfect. Tuesday I noticed the stumble and then this AM on the way to work (20 mins into the commute). Im at work right now but I will remove/replace the gas gap before the ride home and see if it stumbles at all.

I will purchase a set of leads this weekend as the ones installed look OK but theyve definately seen better days as far as beign dirty/oily etc. I think another area I could concentrate on is securing loose wires that are in the bay that used to connect to the gulp valve etc. I guess its possible that a wire could be shorting.

Please keep the suggestions coming and thanks for the PMs Ive received also.

John,

How many miles are on your distributor? Worn bushes at the spindle, hammered advance stop and dirt and grime can create stiction and intermittent (temperature induced and otherwise) running issues. Check out Advanced Distributors website, http://www.advanceddistributors.com/ and consider a refurbishment. No financial interest... etc.


warmly,
dave

I had similar intermittent stumble but was much more pronounced (took longer to magically clear). Ended up being a constricted fuel line (rust) from the tank. A constricted vent line could do similar. Try a fuel volume test with a glass jar, disconnecting the fuel line a the carb (search here for how).

A flakey ignition relay is a (remote) possibility too, though I would expect a more catestrophic on/off effect from that.

Intermittency is a PITA!

Bob

the downdraft Weber has a reputation for stumbling a bit when going from idle to full (or near full) throttle. There are many reputed "fixes" for this.

even properly set up and tuned SUs can have a slight lean surge or stumble when cracking the throttle open from idle. Caused by the vacuum building in the venturi (and the piston lifting) a little quicker than the fuel can escape from the jet. A fix for this is going slightly heavier on the dashpot oil.

Under what conditions do you see this stumbling?

Hi all,

Thanks again for the suggestions. I fitted a 25D distributor last year and its seems in good,clean condition. Ive not tried the fuel-delivery test but will this weekend.

As for the weber - I dont "think" its a flat spot issue. The car runs great the stumble is erratic as it happens when not accelerating (cruising) or when accelerating. The car just seems to miss a beat. Sometimes singularly and other times as a series.

I left from work tonight after taking off/putting back on the gas cap. The car ran great and then I hit the freeway and got up to 65mph and again a quick stumble and then it ran fine all the way home.

I really wish whatever is playing up would quit all together so that I could solve the mystery ;)

Thanks all.

Car has run great for 3 days without a hiccup. I hit the freeway this AM, powered on the gas to merge into traffic and the car "stumbled" pretty drastically. At one point I thought the engine was about to quit -t lasted a good 10 seconds of stumbling. I drifted over to the slow lane and in the process hit a pothole in the road. The car rattled as it hit the pothole but the engine IMMEDIATELY started running fine again and was perfect all the way into work.

So very weird - its got to be a loose connection of some kind.

I had a issue like this a few years ago and a refusal to go above 3500 RPM under load. A real tough dog. So I replaced the capacitor (checked on capacitor tester), points, distributor cap, ignition wire (bumblebee), rotor(red) and spark plug (BP6ES). I then reset the points to 0.015 nom. Problem went away. I followed up and had Joe Curto re-bush/throttle shafts on my HS4's to improve the idle.

Issue came back with a vengeance. At around 3000rpm Im getting an almost constant misfire. Im suspect that the newly fitted points (300 miles ago) have settled in and partially worn the new heel down as the issue has definitely got worse in the last few days. I tried a new fuel filter and full set of spark plug leads but still a misfire.

Tomorrow, I will reset the points, test the timing and if needed do a fuel delivery test.

Isnt this fun :)

I recently experienced some hesitation/lack of power under throttle and it turned out to be a bad ground wire in the distributor. I was looking at the condensor when I noticed the ground was ganging be a strand or two.

I was able to repair it. Took off the connectoron the end - actually it just fell off, and was able to reuse it. The actual wire appeared to be snagged under the plate, so that when the dist. advanced it tugged on the ground wire eventually causing to the sever.

John, I had the exact same problem last year, mine was a simple fix, it was the ignition relay, sometimes when they go bad, when you hit a bump they reconnect, it is a cheap fix $5.00 for radio shack. The relay when it is bad cuts out the ignition system cutting off the power, you have enough spark and heat to keep the engine running really rough similar to run on, once the relay reconnects due to a bump etc, you get the power back.
Craig

I've had 2 different instances of 'stumbling' and there were 2 different causes.

1. Is there any visual clue to the stumbling? i.e. does the Tach drop off for a second, then come back?
If so, it will be one of the ignition wires (feeds through the Tach). Mine turned out to be loose connector(s)
at the starter bundle (positive side). Once the connectors were cleaned and re-soldered, it was fine.

2. Last year after a carb re&re, I noticed a stumble when just 'cruising'. No visual clues.
Turns out that when I reinstalled the carbs, after some cleaning etc. the mixture was off just a bit.
A flat's worth of adjustment solved the stumble in this case.

Good luck with it. They're sometimes hard to track down, but usually simple fixes.

Thanks all - the ignition relay is an interesting thought. Ive pretty much swapped out everything else (apart from the fuel pump).

Last week I took out the distributor and reset the points and timing and this week it has run fairly decently. The dizzy earths looked to be in good shape after I tightened them up. I can now hold 3k RPM on my commute BUT last night on the drive home it stumbled again - just for a split second but a definite stumble. Im thinking it has to be electrical as the car runs very well apart from these intermittent "stumbles".

My tach/ Oil Pressure etc all look stable during the stumbles. The car has always had a strange issue where the Voltmeter will suddenly drop to 11v charging status for a short time. Then it will return to "normal" at 13v+. Its as if it shorts someplace for a brief amount of time. However, even when this happens the car does not stumble.

Its a mystery. I will play around with cleaning connections this weekend :)

Thanks all!

Take a look at the braided wire ground strap inside the distributor. If too many strands are broken it will act like that.

Thanks - I looked at that last weekend. It was a little frayed so I tweaked the connector and it seems a decent connection now.