HELP! 1970 SU Trouble!

[JonnyRock]

So I'm having some real trouble with my SU's.

Ever since I got my headers and exhaust put on, my car hasn't been running super smoth. It ran like a top previously. I attributed the poorer running to the exhaust guys likely messing up the carb settings when they removed it all to put on my headers and not properly tuning them when they were put back on.

So in anticipation of tuning the carbs myself, I replaced the spark plugs. The car had been running very rich (noticable exhaust visible, soot on tailpipes and garage wall, and gasoline/water on the ground behind the pipes when warming the car up) and I figured the plugs could stand to be replaced. I was correct, as all but 2 had a good deal of carbon build up (#4 an 6 looked good I believe). Well two days later I came out to a dead battery, my glove box light was the culprit (damn pin!).

In the process of jumping the car, I fouled 2 plugs. It needed a lot of cranking to get started which probably did it. When it fired it was noticably stumbling. So I replaced those two plugs. That's 8 new plugs if you're counting. Well I didn't drive the car much for the next 2 weeks. I'm taking 19 credits, work 30+ hrs per week and have extra curriculars so there wasn't time for the Z. The little that I drove it though, it quickly deteriorated in terms of driving performance. Got to the point where it was stumbling, bogging, and backfiring almost constantly.

So yesterday I have the time to do a proper tune job. Or at least attempt it, as I've never touched carbs before let alone SU's. So I follow the steps in my 3 auto manuals. First I replace the plugs because I know that I need to be 100% sure that they're good. Once again, 4 of the 6 have a good deal of carbon (they were probably fine, but oh well). That's 14 plugs!

I heat up the engine. Remove the airbox. Loosen the throttle screws. Tighten the mixture nuts, then loosen them. Here's my first problem. The books say I should get 2 1/2 turns before the screws hit their stops. But mine spin like 5 times. I attach my test tach and start the engine. Second problem. My test tach reads 300rpm lower than my dash tach. I get the engine down to 600-700 rpm on the test tach, then use the uni-syn to match the carbs. Third problem. My MSA uni-syn doesn't read at that low a flow. I have to increase rpm just to get the float to move. But I get them matched. Then I start adjusting the throttle screws again. Then mixture nuts. Going step by step. Trying to follow the book and get things leveled out.

When all is said and it's running better. No more backfiring. But it definitley ain't driving great. It idles at 1000 on my dash (the test gauge said 700 I don't know which is correct). It's not particularly smooth. I seem to have gone over-center with the throttle. Meaning it's near dead pedal for the first bit of travel and then suddenly gives, revving the engine real quick.

I don't know what to do. I don't have experience with carbs, but I understand their function pretty well. It was a real pain in the arse getting these two synced. The pistons in both move well, and have proper damper fluid. But the rear one seems to increase flow more than the front. The rear linkage was sticking, but I lubed it. The line from the front float to the nozzle was wet, so I may have a leak there. I still don't know what to do about the mixture, as the smell is still very strong. There's still visible exhaust under idle and acceleration. My balance tube has a servo hanging off it which I need to plug.

Do I need to fully dissasemble and rebuild them? Should I take it to a shop? The only shop I can think of that would really know what they're doing is 'European Motorworks' as they deal with a lot of the older little British cars and their SU's. Should I spring for the rebuild kit from ZTherapy? If I do, what do I do in the mean time?

:stupid::stupid::stupid::stupid::stupid::stupid:

Edited April 26, 2009 by JonnyRock

[ChrisZ]

Jon, you are saying it all started when you had a headder installed, i think you should look for vacuum leak.

Make sure your valves are adjusted correct.

The mixture can be checked by raising one piston about ½", then the car will run on the other carb, it shall have a rough idle, if it dies you are to lean and if it continues idle at high RPM, you are to rich, the mixture nuts shall be turned all the way up (counter clockwise) then 2.5 turn down (clock wise) looking from top of carbs, tread adjustment screws until idle, balance, check mixture, adjust mixture if needed, balance, balance at high idle, you want to have your pistons raise the same, do this at different RPM's, turn back the high idle screw, check balance and you are there.

Every time you make a adjustment, you have to rev the engine, did you check and adjust the spark gap to 0.9 millimeter ?

If you have a vacuum gauge you can use that to adjust your ignition timing, on your front carb, there is two vacuum studs, hook up your vacuum meter to one of them, turn your dizzy to best vacuum, back it off just until the vacuum drops, back it off a little more if your car ping's

I have made a photo showing where it's placed

Green - mixture nut

Yellow - idle/balance screws

Blue - pistons

Red - high idle balance screw

I hope this helps you on the way

Chris

[ajmcforester]

OK the books out their are not that great for tuning the carbs. You also picked a very hard type of carb to work on.

I agree on checking for vacum leaks and with the backfiring when was the valves adjusted, that needs to be done anualy on these Z cars. I hoped you checked vacume before messing with the carbs it sounds like they where close. If you didn't set the carbs as close as you can to factory setting you may even need to adjust the neetles and check vacum pressure.

Do you have a smog pump and equipment hooked up, a portion, or none of it working? I picked up a car that had the emmtions hooked up but blocked off since the smog pump seazed, well I had a rebuilt pump that I conected and put all but one new hose (forgot to get that size of hose). Well I spent two days just to find a ball berring in that hose blocking the system from working. The point is that it was causing the car to run like you are explaining. You can block the smog system, and disconect the pump and see if that fixes it. It would only take one emisions part to go bad to cause your problem or they may not have blocked something off corectly causing problems if they disabled it.

[Bruce Palmer]

"You picked a very hard type of carb to work on"???? Hitachi SUs?? That is too funny.

Tech tip: At five turns downs on the nozzles, those carbs will almost assuradely be fat.

[JonnyRock]

Tech tip: At five turns downs on the nozzles, those carbs will almost assuradely be fat.

What?

[JonnyRock]

Do you have a smog pump and equipment hooked up, a portion, or none of it working?

I chucked my smog pump. It was seized and no longer hooked up because the manifold was removed.

[Bruce Palmer]

What?

A good base line setting for fresh SU's is 2 1/2 turns down on the nozzle. If your float levels are correct, at 5 turns down you should have gas pooled up on top of the nozzles yielding a rich condition. If your float levels are correct and the nozzles are set in the proper range, the static fuel level should be within a 1/16" below the top on the nozzle. Higher than that, things will be rich, lower than that things will be lean because the airflow through the carbs wont pull enough gas from way down the nozzle.

Our JUST SUs DVD will walk you through all this.

[Walter Moore]

If he person who installed the headers wasn't familiar with SU's they might have the choke cables too tight. When I installed the SU's in my car I tried three times to adjust the carbs but the mixture was always too rich and the plugs would foul after about 40 miles of driving.

The trick is to make sure that when the choke is RELEASED, the jets are pulled FIRMLY against the mixture adjusting ring, or stop, or whatever it is called. On my first two tries one or both of the jets would hang ever so slightly below the adjustment ring and turning the knob had no effect at some point. I could make it RICHER, but couldn't get it to ever lean out.

It is hard to see because it is under the carb.

[Bruce Palmer]

Walter has the choke cable thing right. We talk to a lot of people who ultimately find out that they have the choke cables too tight. With the nozzles in the full up position, leave a little slack in the cables before tightening them down.

[biddljj]

Buy the SU DVD from Bruce and Z Therapy ! This expense is the best investment you can do to the SU's on your car....

Jay

Not just a Z -zdisease

[JonnyRock]

Well I went and ordered the 'Just SUs' DVD from ZTherapy.

In the mean time, I hope to be able to do a valve adjustment tomorrow. I'll update as I can.

Thanks guys.

[ChrisZ]

Have you checked for air leaks around your intake manifold, look at #2

Chris