Stumble on acceleration

Gents, I’ve been wrestling with this for weeks. With engine warm my Mk2 hesitates on take off and when I get “on the pedal”. Timing is correct, vacuum advance is operating properly, carbs are balanced (Unisyn), mixture correct (Colourtune, plus a little), throttles working in synch, plugs swapped out, coil swapped with other car, rotor changed, cap swapped, leads are near new, no apparent vacuum leaks, set float level last night, piston drops straight to the bridge. The telling symptom that implicates the carbs is that: at each event if I back off a little and go again there is no further hesitation for that stroke of the pedal. Suspect too rapid piston movement? Have tried SAE 30 (spec for Mk2), SAE 20, ATF, and 15W60 Synthetic. Help. Paul

Stab at the dark, could you have some linkages not tight. Once you let off and press again they are in a better position? Have someone else watch the pedal travel and all of the linkages on several tests with the engine off.

Gerard

I had the lovely Rosanne work the pedal - no Joy Gerard. All work well. Paul

Paul, is the problem a recent development and the car has previously run properly? Any changes in the interim like a low restriction air filter? The symptom suggests a lean condition. I had similar symptoms which I solved with a slightly richer needle profile. But if your current set up has worked prior, it shouldn’t stop working if nothing changed - says Cap’n Obvious.

Just a FYI, when I first purchased my 57’ Mk1 I had a similiar issue…Spent weeks trying to figure it out. After going through timing, points, carbs, etc… the problem was I was setting the timing using the tach in the car…it was way off… Hooked up seperate tach, set timing, all good. JS Atlanta, GA

Hi Paul, I know you said your vac advance was good but are you sure your mechanical advance weights are free and move easily? Sticking weights are a possibility.

~Mike

Thanks for all the thoughts Gents. Finally worked out today. Two issues it seems. Front carb was lean - I must have done some roadside fiddling and lost track of my quarter and half turns on the mixture screw - should write them down. Today I zeroed the jet at the bridge and backed down 3 1/2 turns and got most of the hesitation out. The rest was down to a split in the rubber elbow at the dizzy. I should have tested vacuum back to the carb, not just at the dizzy. I made up a temporary elbow out of some PVC tube and heat shrink tube. Pulls like a steam train now! So in part, self inflicted. Paul

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Nice feeling when you find the problem.