Wedge Need help diagnosing a misfire / stutter / stalling when stepping on gas

[justin_mercier]

I picked up a TR7 just over a week ago and it has one problem which I have not been able to figure out myself. When steppping on the gas the engine hesitates / stalls / misfires. Stepping on the gas slowly to rev it up the engine purrs happily, but step down on the gas quickly or with any pressure and it breaks up.

Here's a pair of audio clips that I've uploaded to youtube.

First vid is with a cold engine, having just started it up (water choke has it idling around 1500 when first starting up). when you hear it rev up smoothly is when I'm stepping very slowly on the gas, i then stomp on the gas a couple times and stall the engine out completely.

https://www.youtube.com/watch?v=XbPv4AwnobU


Second video is after letting the engine warm up enough that the auto choke has slowed the idle down (still idles fast, around 1000) just stepping on the gass and letting off, and then stepping on the gas and holding it for a minute, you can hear the engine BUP BUP BUP

https://www.youtube.com/watch?v=TCN770Uy-qY



Today after dealing with the repair shop and picking up my pickup truck after they fixed collision damage, I went and bought a compression tester and figured that I would check the plugs and compression first just to make sure I couldn't determine anything mechanically wrong. Compression looks pretty close on all cylinders, a hair lower on 1 and 2 than 3 and 4, but all within range of eachother.
Cyl 1

Cyl2

Cyl3

Cyl4

The plugs look fine to, the 4th plug looks like the other 3, but the angle of the photo isnt showing the 'clean' side so it looks dirtier, but they're all about the same.

The carbs are brand new from Jeff at Paltech last summer, as are the points and rotor, ignition wires, and plugs.

 

[TR3driver]

I couldn't hear anything at all with the first video, and not much with the second. But my first thoughts are:
1) Is the enough oil in both dampers?
2) Is the fuel fresh?

Second video almost sounds like an old VW bug ... could there be an exhaust leak at the manifold?

 

[Healey_Z]

I would start with the basics; making sure the ignition side is in good working order, test the coil. Check timing and advance. Check for vacuum leaks. If all that is good, then look on the fuel side. Carbs balanced, 20w in the pots. If all that is good, I'd start looking at the Strombergs. Pull the top off and check the diaphragms.

 

[tdskip]

Agree that it sounds like the carbs.

 

[George_H]

Sounds like carbs but I would check timing and the diaphram on the carb. Just pull the vac line off of it at idle, the idle should increase. If not you either have no vacuum or a bad diaphram.

 

[justin_mercier]

So I checked and the dashpots have oil, and the diaphragms in the carbs look just fine. Need to get a timing light so that i can check the timing (i've never checked the timing on an engine before so i'll have to look up some vids) I misted some WD40 over the top of the engine and it didnt speed up any so it doesnt seem to be sucking in any air from where it's not supposed to. Want to get this problem licked ASAP so I can start to do some driving real soon =) Have owned the car for almost 2 weeks now, and I've only put 15 miles on it !

 

[justin_mercier]

So, I think that I've narrowed it down to just the rear carburetor. In this video you can see that if I reach down and hit the throttle on just the front carb, the engine revs up smoothly, but when I hit the rear carburetor it goes PUT PUT PUT PUT.

https://www.youtube.com/watch?v=rWoSwOJdjgA

 

[justin_mercier]

Pulled the tops off the carburetors and inspected the diaphragms and they look perfect and new, even stretching them and looking around with a light I could find no defects. Refilled the dampers with 30wt non detergent after that was recommended to me elsewhere. The fuel pump is a new electric one mounted back by the gas tank and is working fine (so no leaky diaphragm in the mech fuel pump either)

 

[justin_mercier]

Problem has been solved ! So, it took a bit of investigation, but I found the issue... The spring inside the adjustable needle that's supposed to keep the needle pulled up somehow was broken, resulting in the needle falling down a lot lower than it should be, and starving the engine for gas. I took the needle out of another ZS carb I had in the basement, and replaced it, and everything is running great now. Took it for a half hour drive around the back roads while it was warm and sunny this afternoon =)