Intermittent Stumble

[alpinems]

Hi all, my car has developed an intermittent stumble after run fine for a while. I went to the local Dyno days for charity the Porsche club puts on every year and it was real apparent on the dyno runs. I only made 75 hp on the best run and it sounded flat to my ears.

The car has dual Weber 45DCOE's with a mechanical dizzy. Car has been running fine up to now. I've checked fuel flow to the carbs - looks good; checked the idle and main jets in the carbs, all open. Carb fuel filters nice and clean. I replaced in the last 60 days, plug wires, rotor, dist cap, coil and put in a pertronix. Up until a week ago the car had been idleing fine. I tweeked the timing by doing a bunch of hill runs to remove the hint of pinging. Like I say, it sounded nice a crisp and ran strong. Now, with no changes, it sounds a bit flat, and there's definite stumbles when full throttle running to redline.

Things to check?

[c.d.iesel]

seat-of-the-pants diagnostics is difficult and impossible

for inexperienced - did the 'dyno' place place an exhaust probe

in the tail pipe while testiing? full throttle reading like Co and HC ?

Air/Fuel mixture ratio readings ?? well then untill you have some starting

point data you might as wall put it up on jack-stands for the winter now.

And your Seat-o-Pant ignition timing method causes more harm to your motor than you realise.

Invest in a timing light whan you own a car that has a distributor that

is not fixed like 'todays' electronic appliance vehicles.

Untill you have a ignition timing image and true firing point - your dead in the water.

SEE ? probe up the tail pipe. connections to the motor.

Rpm, intake manifold vacuum, repeatability

Partial Throttle readings

Full Throttle readings

simple : partial throttle 1.5 to 2.0 %Co Full throttle 3 to 5% Co reading

ignition timing DOES MATTER before anything else.

WATCH CLOSELY ! Timing Light --=>

[alpinems]

No the dyno guys didn't take a tailpipe sample, though I wish they had. We were lucky to even get the dyno guys to show up. They were Phillies fans and had been partying all the day and night before. They had to be rousted out of bed to show up.

I didn't go with the expectation of the dyno being a tuning session, it was something I take one of my cars (usually the racecar) to every year to support the charity. It's much more a fun event than a serious one. But it did give me some data about the performance as opposed to just wailing down the road and thinking it's not running right.

I hear you C.D. about the timing light. I borrowed one a while back but I wasn't able to ever get the light on the timing ball. Either I put the light on it or my head in place to see it, never the two together. That said, even with my sub-optimal hill tuning method, the car had been running great. No audible pinging, crisp throttle response, smooth sustained acceleration to redline, motor sounded crisp. It doesn't get run everyday, maybe every 4-5 days. And it went from fine to not fine with no changes - other than sitting for a week or two. I've marked the dist position and it hasn't changed. I'll go borrow the light again and take another wack at it.

[Guest Anonymous]

You know enough about engines to know that the 02's engine is not much different than other piston driven engines - especially with side drafts. I would go over the basics, including cleaning out the cars with gumout or chemtool to insure everything is free flowing.

Dirt can easily become an inadvertent "tweak" to the settings of any carburetor. Next, check your ignition system - and not just the settings. I am suggesting you check all connections to make sure you don't have any corrosion or carbon tracking around coil, cap, wires etc. It wouldn't be a bad idea to reexamine plugs too. A cracked insulator or loose plug wire can ruin your whole day!

Lastly, if you are going to the trouble of reexamining the timing, how about double checking the valve lash too? Others may disagree, but I have an M10 engine that will purr with the valves set to a fat .008". But if I use the tighter .006, the idle is not as smooth. I can't tell you why this works with the one engine even though my other M10 engines seem far more tolerant but it may be subtle differences in intake or exhaust systems.

hth