Help with SUs on L28 with E31 head

[240zip]

I have L28 with .5mm overbore, flat top pistons and an E31 (non-shaved) head. With a Felpro gasket I calculate 10.32:1 as the CR. I used a 260 duration and .450 lift (intake and exhaust) for the cam.

 

I'm very concerned about the system running too lean with the SUs. Can I used the Stock SU carbs with stock needles (I'm running at 5,000 feet elevation)? I'm considering getting SM needles, but given the altitude, I'm not sure if it's necessary. I have K&N filters for the SUs. I also have some extra SU needles (one for an automatic and one for the manual). I'm fairly certain that the code on the SUs on the car are from a manual transmission. I'm not sure if the automatic fuel needle profile would be better for this particular set-up.

 

I also have two sets of triple 42DCOE Webers. I didn't want to bother with the rebuild and setting up of the Webers if the SUs would work.

 

Not that it's critical to the question, but the ignition is a MSD 6a with Pertronix set-up. We have an O2 wideband to help tune the car and will likely pour some race gas into the tank for the first start-up.

[JMortensen]

I ran mine with an early set of SU needles for a 70 (not as rich as SM needles, but I think they were supposed to be richer than the 72 needles) at sea level and never had a problem. You should have less trouble at altitude, as you don't need as much fuel with your thinner air. Carbs deliver fuel based on the amount of air going through them, so you'll get more fuel out of them with a 2.8. I think the problem with SU's on an L28 is that they don't allow enough air through. When I switched to Mikunis I gained a LOT of top end power. Mikunis will work a lot better with a later 5 speed and 3.90 or 4.11 rear end, so if you can't do all of it you're probably better off with the SU's and better part throttle response IMO. Only use the race gas if you need it. You'll make more power without it if you can run without it. Dan Baldwin did back to back dyno runs with regular gas and with race gas and lost power with the race gas.

[blue72]

I don't think the problem is the carbs. Plenty of Z cars are running around with stock SU carbs on L28 engines. In fact, I had the exact opposite problem in my Z @5800 ft elevation, I was running way too rich. All I had to do was turn the fuel adjustment knobs up a bit and it cured that.

 

You didn't mention where you "feel" is is running lean. At idle? At the top end? Are you experiencing any sort of symptoms that lead you to believe this? Has the car even run yet, or is this a hypothetical question?

If you think it is / will be running lean at high rpms we just had a member trying to diagnose this and suggestions ranged from the fuel pump to the floats to the needles.

Edited August 23, 2010 by blue72

[240zip]

Interesting ... we'll start off slow. I didn't know race gas resulted in less power. We have an MSA exhaust and the 6 into 2 into 1 headers. We took the MSA exhaust and then added a Dynomax resonnator. It's all TIG welded (we just like using our TIG at the shop and it looks nice ... ). On the engine itself, it uses ARP bolts pretty much all around. The head to the header uses a copper gasket.

 

Dan runs a turbo motor (know him) with MegaSquirt. Great set-up. On the transmission / differential. We have a 5 speed and R180 LSD with a 3.9 ratio. I used JohnC's adaptors and the Subaru STi rear differential.

 

We'll do an initial tune with the wideband and then see how it goes on the dyno.

[madkaw]

Please post up your finding with your wideband. I wish I had that tool to diagnose a similiar issue. My 2.4(40 over), higher compression with cam and dual exhaust seems to be running lean at top end with SM needles installed. The SU's run great and MPG is super, but I am like Jon and believe they run out of breath at top end. I also have some Mikuni's of the shelf just begging to be put on, but the SU's will have their day first.

[burninator]

I have an L28 running with SU carbs from a '72 and it runs rich all over. I think it probly does run out of flow at high rpms the torque curve is fairly flat until about 5000 rpm and then it falls off rapidly. On the dyno with it as lean as I could get it with the adjustment knobs and still have it idle right it runs about 11:1 afr, but it wouldn't start with it set like that (choke cable wasn't hooked up) so I had to richen it up the next day. I'm at 4250 ft, so pretty high altitude compared to most people, but that's my experience.

[john turner]

240 ZIP

 

Have 72 240 with L28.

Compression approximately 9.7 cr.

Ztherapy carb rebuild.......SM needles.

DIS......maximum advance 35 degrees.

6 into 1 MSA headers......2.5 inch exhaust system.

 

Always thought S.U. 1.75 inch carbs too small to pass enough air to produce reasonable hp. from L28 engine..........until I remembered that my 1953 Jaguar XK120 had a 3.4

liter engine with 1.75 SU carbs.....and it performed pretty well.

 

Most of my driving is done near sea level....so SM needles may not be appropriate for your setup.....would suggest needles from earliest 240s as they are slightly richer...than those for the 72 model.

 

Regarding my car performance.... it turns right up past 7000 revs very easily......my thought that the two factors that contributed most are the Electromotive DIS. and the Ztherapy

work on the throttle shafts to prevent leaks.

 

Regards. John Turner