missing, running rough etc etc

[Woodie]

Ok let me describe this again, cause I am getting no where fast (literaly) I have a 1275. 1970. It was recently running fine, with no airfilters on it. Drove it about 1/2 mile two or three times all seemed fine, oil pressure is good, temp is 1/3 up the scale (half way between the C and N). The car starts fine. I have cleaned the carbs and bowls filled the dashpot. The timing is set by ear( sorry cannot find the timing marks on the pully wheel) and am not skilled enough to set static timing. The timing cannot be to far off, as it ran fine. Now, I put on air filters, and she runs like do do. No power, miss fires, runs rough during acceleration and then sort of coughs(slight mis fires )while cruising. I can solve the poor running by pulling the choke about 3/4 of the way out and leaving it there. ANy suggestions....

 

[jlaird]

Oh just a wild shot in the dark hear. What airfilters. Do they have the hole covered that allows air to go to the piston. I don't know what it is called but on the carb is an extra whole the same size as the filter morunting bolts. It must be open and not sealed by the gasket or filter its self.

If I was not clear enough someone help me out pls.

Air filter end of carb has two bolt holes, one large center hole with butterfly in it and another hole. That's the one that must be free so the piston can breathe.

 

[Woodie]

They are stock crossland filters, I took the baseplates from the housings, and 4" diameter ABS with holes drilled for the tops. There is no surrounding housing, allowing more free air to carbs. It is possible that I turned 1 or both base plates 180 degrees. I will check at noon. But in my mind, I cannot see the extra hole you mention in the base plate, but hey I'm allowed 1 mistake a week !!

 

[jlaird]

Ahhh, look at chap 3 Haynes, lots of pics and drawings showing the hole in question. Now what it is for I've no clue. But it does go in then 90 degrees up into piston chamber.

 

[Bugeye58]

Woodie, from Tuesday morning.
https://www.britishcarforum.com/ubbthread...true#Post229365
Jeff

 

[Woodie]

I completely forgot I asked this last week.. DOH.
Too much going on..SORRY Guys,,I will still check this out at noon today ( drove SPARKY to work today )

 

[jlaird]

Does anyone know what that hole really does?

 

[Bugeye58]

Jack, it's the atmospheric vent hole for the piston. If it's blocked the piston won't rise or fall properly.
I'm playing with the Midget carbs right now. There's a couple of things I want to experiment with, to try and track down Woodies' problem. Will report back later.
Jeff

 

[jlaird]

"atmospheric vent hole"

See that, I knew it was important. Gota love the name.

 

[Woodie]

wow, with a name like that...wish I had a name that sounded big and important......just Woodie....

anyway, I look forward to your report on your investigation, and thanks

 

[Bugeye58]

[ QUOTE ]
"atmospheric vent hole"

[/ QUOTE ]

Now that I think of it, I get called a name that sounds like <u>part</u> of that every so often!
Jeff /ubbthreads/images/graemlins/devilgrin.gif /ubbthreads/images/graemlins/grin.gif

 

[Woodie]

UPDATE POST LUNCH

ok went out at lunch, well the base for the airfilter has 4 holes, 2 mounting holes for the bolts, and two atmospheric vent holes so the base is interchangeable with either carb. Both my baseplates were upside down ! Doh!!.. as soon as I loosened the bolts I could hear the engine change tone. Anyway I flipped them and took the car for a short spin,, ran even worse (cause i played with the carbs trying to get it to run with the filters on). OK its sunny and lunch time, and I'm dressed in office clothes (In Canada and today is a work day) I've done this before, so I pulled the air filters back off, screwed the mixture nuts in all the way on both, and HEY found that the front carb jet nut was loose, so tightened that up, adjusted the idle,played with it for 5 more minutes, took her for a short spin and it seems to be running fine again. Needs to be fine tuned I'm sure, but the hesitation seems to be gone nad the power seems to be back. Ill find out in 2 hrs on the ride home. Once again this forum has come to the rescue !!! ALl done in about 1 hr and no dirt on my clothes !!

 

[Bugeye58]

Way to go, Woodie! I guess there's no reason to publish the results of my experiment today, then.
Jeff

 

[Woodie]

Jeff I would still like to know what you found,

 

[jlaird]

Good deal Woodie. Always nice to make a wild stab and be right.

 

[Bugeye58]

Okay, you asked for it.
I started by adjusting the valves this morning, and doing all the basic tune up stuff. Did a wet and dry compression test, and the dry values were between 138 and 155, wet pressures were about 12 psi higher across the board. About right for an original, 65,000 mile engine.
Plugs were new yesterday, gapped the points, etc. Went through the entire carb adjustment / syncronization routine, and took it for a drive. Performance was exactly as expected. Then, I taped the two vent holes closed, and went for the same drive. Big surprise! The thing ran like doggy do-do! It chugged, sputtered, and had the acceleration of a snail on Valium. Once I coaxed it up to speed, it smoothed out, but you could tell it wasn't a happy camper. Came home, pulled the tape off, and it runs fine.
Interestingly enough, pulling the choke out only made things worse, as I suspected it should.
Well, that's how I spent my day. Over to you, Woodie.
Jeff

 

[Woodie]

WEll jeff, when you pulled the choke it made it worse ,but I played with the carbs while the filter basplates were upside down, and then it would run ok with the choke pulled. That is why I had too reset the carbs after I put the baseplates on correctly. The vent holes are for dust free carbs according to the SU manuals

 

[jlaird]

Where did you find this SU manual. I seemed to have missed something maybe?

 

[Woodie]

on this page
https://www.1978mgmidget.com/manuals_and_booklets.html
plus a few other goodies, all downloadable

 

[Bugeye58]

Woodie, did you get the carbs readjusted yet?
Jeff