carb problem... searched no joy.

Mack · Post by **Mack** » Tue Mar 14, 2006 10:39 am

As many of you may know, I have a 68 1600 that was sitting for 24yrs. done a lot to get it running and now having problems with the carbs, need help!

alright, so it runs like carp on 2 cylinders, back carb is doing most of the work. then when I go to the front carb, I notice that the slide is higher on the front carb, so I push it down with my finger and it magically starts to smooth out and run proper. I check the back slide and it sits a lot lower. I re-adjusted the linkage and the throttle stops to no avail, fiddled around with the mixture screws and that didnt change. carbs have oil in them and move freely, albeit a little stiff.

what would make the slide on the front carb rise up faster/more than the one on the back? when I held it down, it ran fine. so, this is a air velocity thing or what? I know just enough about SU's to be dangerous.....

keith0alan · Post by **keith0alan** » Tue Mar 14, 2006 11:04 am

The slide being high is making that carb lean. First, the normal mantra. Valves, ignition dwell, ignition timing. Then pull off the domes and clean the pistons and inside of the domes. The book reccomends gasoline but I suggest a good spray carb cleaner. The auto zone house brand is good. While the domes are off check that each has the spring inside and they look the same. Some folks try to make things better by streaching the springs, bad idea. It really sounds like a sticky slide. There is about .003" clearance and it doesn't take much carbon and old gas goo to gum them up. Once the pistons are clean and free the piston height should be the same. If not, it pretty much has to be air flow balance. Let us know what you find.

keith

Mack · Post by **Mack** » Tue Mar 14, 2006 11:59 am

well, the slide isnt stuck. I can push it down with my finger, but when I take my finger off of it, it automatically moves back up. it sits higher than the rear carb only while the engine is running. when it isnt, they are level with eachother..... I will take the domes off and poke around, looks at the springs, maybe its MISSING the spring or something stupid..... who knows.

keith0alan · Post by **keith0alan** » Tue Mar 14, 2006 1:59 pm

Could be the spring is missing. Also check that the two little vent holes by the air cleaner mounting holes are open and the gasket has the appropriate holes for them. I've seen a number of carbs with dirt dauber nests in those holes. Having them stopped will cause the slide to sit too low and richen the mixture. If you are missing a spring I've got a few around and can send you one.

keith

SLOroadster · Post by **SLOroadster** » Tue Mar 14, 2006 3:27 pm

Have you had your carbs balanced? Also, check your dashpot oil. You need 20W, or ATF.

Will

MH69Dats · Post by **MH69Dats** » Tue Mar 14, 2006 4:56 pm

I'll throw a line out there too.

I just recently had similar problems with one of my carbs. For me. one of the choke linkages was stuck open, which opens the throttle plate on that one side. I freed it up and it worked just fine.

Mine sounded like 2 cyls too.

---Marty

Mack · Post by **Mack** » Wed Mar 15, 2006 12:35 am

well, I fixed it. the choke slides were one with the sleeve they are supposed to slide in and out of. and to top it all off, the carbs were adjusted and stuck all the way lean.

so, I completely tore down the carbs to their itty bitties, and blasted some parts and soaked other parts in carb cleaner. Also, it felt like the dashpots were filled with 40w oil. cleaned, emptied and filled with brake fluid. my freind has been using them on his 240Z SU's foir a while. seems to work. adjusted them about 2 1/4 turns out from full lean turned the throttle stop screws about 1/4 a turn, bolted the suckers on and it fired right up and dropped to a steady and smooth 800rpm idle.

tomorrow, time to take it on the road and see whats up. should be fun!

keith0alan · Post by **keith0alan** » Wed Mar 15, 2006 11:45 am

Good news!

One thing to watch out for is bad gas in the tank from sitting. On the car I am currently rebuilding the engine for the tank is so full of varnish that it just kept gumming the carbs up. She didn't drive it often and I spent way too much time unsticking carbs and changing fuel filters. Before I put the fresh engine into it, the tank is coming out and getting professionaly cleaned. Her previous mechanic had charged her and claimed to have done it but it's obvious that he had not.

As to dash pot oil. It's function is to slow the piston rise and perform and "accelerator pump" type of function. If you run no oil you will notice stumbling on acceleration. You want the thinnest oil that will stop the stumbling. Motorcycle shops carry fork oil in lots of different weights. On a couple of Zs I've converted from the flat top carbs back to the old style it took 50w to get them to run right. Roadsters should use 10 to 20 somewhere. Too thick and you loose the crispness in the acceleration.

keith