BN2 still missing, think it's carbs not ignition ?

[bighealeysource]

Hey again,
I didn't continue the post from a week or so ago where I posted about my 100M missing upon harder than normal acceleration and kind of"burbling" at low speeds. I followed the advice and before I started to fool- and that is the operative word -with the carbs, check the ignition. Went through everything this weekend, plugs look great- nice gray color, Petronix ignitor is working great as had been for several years, plug leads do not appear to be damaged in any way, and have not touched the timing or done anything with the dizzy other than put on the new advance small copper tubing. As far as I can tell, advance is working properly. All of this came up it seems after I replaced the advance tubing which required that I take the carbs off as impossible to attach to bottom of carb without doing so. I feel that when I reconnected the choke cable to the linkage on the carbs I have somehow messed it up. Here is what I have - will choke to start and starts up pretty quickly, once at temp, will idle but roughly at 800-1000 rpm. When driving it, have determined that if choke is released, I get the burbling at low speed and missing at higher speed. Again, not terrible missing, but definitely not right. BUT, if I leave choke full on, runs smoothly at speed and on acceleration but obviously now running too fast at idle or low speeds. SO, somehow have I altered the jet levers so that adjustment is off ? I have not adjusted the jets at all but the choke linkage will of course move them up and down so at choke full off, must be fully seated but on acceleration not raising enough ? Seems other than adjusting the jets the only other adjustments to make on the 100M carbs is the slow running screw and throttle screw. BTW, when pushing in the infamous piston lifting pin, engine slows down which should mean I am running too lean. As you can probably surmise, carbs are not a strong point so struggling to figure this one out ! Help !

Thanks,
Mike

 

[Keith_M]

Mike,
I can't remember much about the previous posts, but I think this could be caused by a vacuum leak. When you put the carbs back on did you get all the gaskets back in place and tighten everything securely? Try spraying some WD40 around the junction between the carbs and intake manifold. If the engine starts missing, that's your problem.

Keith

 

[steveg]

You might want to check the choke function: Remove the domes, springs & pistons (be sure to not get them mixed up). Pull the choke all the way out. Measure the jet drop with a caliper or dial caliper on both carbs. Then push the choke all the way in and measure the jet drop. Both carbs should be fairly close. If one is staying down when you push the choke in, that could cause the problem. It happened to me. The jet on my rear carb was hanging up in the down position so I was driving with one carb normal and the other one fully rich. The car ran pretty rough.

Did you change either the coil, points or wires? Did you check the timing? Did you check all the wires for looseness in the cap? Are the wires copper? If they're the fabric wires, the cap loses connection with them after a while. Did you run the engine in the dark and look for sparks along the wires? Sparks along the outside of the wires can show bad wires.

 

[Johnny]

Before getting to involved with the carbs you could try some easy logical testing. You will have to take off the air cleaners and make sure you clean them well before replacing back on. While the air cleaners are off, with the car at idle place your hand gently over one carb and listen to the motor, does it stall (as it should) or does it just slow down a bit indicating you have an air leak. Now try the same to the other carb, they both should stall the engine considerabley. You haven't explained why you had to replace the choke tube, if it wasn't working, bent or clogged, previously someone had to adjust the carbs to compensate some way. The SU carbs aren't all that complicated get your hands on the manual and read carefully, boring I know, but you'll get an idea how things work properly.

 

[bighealeysource]

Thanks y'all for the advice and ideas ! Was speaking with another Healey owner today and he said same thing about the jets, make sure they are in same position when choke on and off and sounded like to him it was running too lean to run smoothly unless I had it fully choked - in essence lowering the jets to make it run richer. Also going to check the manifold gasket question as to leaks since I had to take carbs and manifold off when replacing the dizzy advance tubing.
Regards,
Mike

 

[Keith_M]

Also going to check the manifold gasket question as to leaks since I had to take carbs and manifold off when replacing the dizzy advance tubing.

If the car was running well before you replaced the advance tube, but now it doesn't run well, it makes sense to assume that something you did made it run poorly. I would look for something simple like a vacuum leak before starting to readjust the carburetors (unless you monkeyed with the linkages etc while removing and replacing them).

 

[bighealeysource]

Hey y'all,
UPDATE - turned out to be as simple as the right carb - as you look at them from carb side of engine - was too lean. Adjusted the jet to match the left carb and now it runs just fine ! Really don't know how it got out of adjustment but maybe the spring on the right carb is not as strong and allowed it to move over time ? If it starts to miss again will know where to look and get a new spring. Thanks again for all the advice, etc.
Regards,
Mike

 

[Keith_M]

What?? I was wrong?! Well... that's a first. :highly_amused: Glad you got it fixed, but I still might check for a vacuum leak around that carb. Unless you bumped the jet adjusting nut during disassembly and re-assembly, it's hard to see how the adjustment would have changed.

Keith

 

[bighealeysource]

Hey Keith,
I checked like you suggested and did not indicate a leak but definitely made sure all was tight with the carb attachment. I know, just not sure how it got out of adjustment. Who knows in my lust to have all things shinny and bright I might have turned it ! Oh well, runs great now.
Thanks again,
Mike