RPM Issue

[smg1404]

I think I have narrowed a performance issue with my 240 not revving as high as it should to carburator jetting. Although I could still be chasing the wrong cause. I have a stock motor except for carbs and headers. The carbs are tripple 40mm webers with the following setup.

Choke = 26mm

Main = 130

air comp = 170

Emulsion tube = f11

aux = 4.5

needle = 175

idle = 40/45 f9

Does this look correct for a 240 motor?

I am suspecting the choke size and emulsion tubes are incorrect.

[HS30-H]

Hi smg1404,

What's the history on those carburettors? Were they on the car when you bought it, or did you fit them yourself? Also, what "series" are they? ( you can see this stamped on the top cover, after the "type" stamp ). It will read "40 DCOE xx" - just tell me what the double x bit says.

Those chokes are slightly small ( I would expect 30 or 32, even 34mm chokes to be OK on an otherwise fairly stock L24 ) but I can't imagine them "strangling" a mild L24. Its probably more to do with other factors.

Your Emulsion Tubes would be better if they were F2, and you don't mention anything about Accelerator Pump Jets or Accelerator Pump Rod stroke length. Have you got any data on those? Your Main jets sound OK.

I would try and eliminate the linkage causing any problems first. Are you sure that you are opening the carbs fully with the pedal on its stop? You certainly want to get full opening ( but not to the point where you are trying to push the butterflies further than they want to go - as that will bend the spindles ). Surprising how many aftermarket linkages are no good in this respect.

Another possible cause may be lack of fuel pressure / volume at high RPM / load. Are you sure that you are getting enough?

If you are not too confident about trying to fettle the Webers, then I suggest you try to find a local specialist with a rolling-road dyno to help you with them. Too many people write off triple sidedraught conversions because of experience with badly-installed or badly "rebuilt" carbs that are not doing what they are supposed to.

Check out the previous Weber / triple carb. threads on this site.

Good luck.

Alan T.

[240znz]

You've probably already checked this but....how many miles has that engine gone? The valve springs might be had it.....old springs often cause the same fault.

[smg1404]

I did not check the accelerator pump specs, because I did not see how they would come into play at the upper rpm range. Perhaps a mistake. I had a set of tripple webers on a 280z that I had a few years ago. They were bought new, and setup for the car. They performed great, no problems at allafter learning to synchronize them.

I bought these on ebay, and they had set quite a while. I did however rebuild them and clean them. I did not however recalibrate them, and hoped they wouldn't need it since they came off of a zcar.

I became convinved that this may be my problem when I noticed that if I do not depress the throttle all of the way the rpm gets to 6500 .. no problem, although a bit slower than one would hope for. I decided this was a clue that the mixture was incorrect. I also am getting a bit lower mpg than I thought I should.. about 14 mpg. I dont know how much since this makes added together.

[smg1404]

forgot to mention... I did add a carter fuel pump. It made no difference. About the springs, wouldn't they cause me to be unable to reach the upper range at all times.

Before I added the carbs and headers (Motorsport 2.5" 6-2-1) the car would rev easily to arround 7000, now it won't . It does seem to pull harder in the mid range, I just want my full range back. I'm probably missing something... but ????

[240znz]

The springs would appear to be OK then if you didn't have problems revving upto 7000 before.

HMsports wrote in a thread a short time ago that he felt that the performance of the 6-2-1 header could be further improved by running twin pipes to the diff then 2-1 into a muffler and out. You may want to drop him a PM as I can't remember the exact details...but he is racing so it might just be your carbs as Alan suggests.

Keep going and keep us all informed to the outcome.

[HS30-H]

The reason I asked about the "series" number of the carbs is because this will tell us what Accelerator Pump Rod stroke you have ( presuming that nobody has changed them - which I doubt ). This could be a factor in the problem that you describe, and it sounds as though you pretty much went with the specs./ jets that were in the carbs "as bought".

You really need around 10mm of Accelerator Pump Rod stroke on an L24 using 40's. You need to be using 0.55 Accelerator Pump Inlet / Discharge Valves too ( three of them - one in the bottom of the well in each carb ).

If you don't mind spending some money, why not get a set of 32 or 34mm chokes and some F2 Emulsion Tubes while you are at it?

If you tell me what series the carbs are, I can tell you what they used to be on, and what the Accelerator Pump Rod stroke is.

Alan T.

[smg1404]

The carbs are 40DCOE -18

[smg1404]

I spoke to Mike at Pierce manifolds, and he recomended 30mm chokes. I was thinking more like 32mm, but I defaulted to their greater experience and ordered the 30's. I will let you know how this works out when I get them.

[HS30-H]

The 18 series is an excellent choice for the L24 engine, and Weber themselves recommended this model for it.

They have the 10mm Accelerator Pump Rod stroke as standard. That's very important for making sure you get good "driveability" from the conversion, and I notice that you said you had a nice mid-range performance - which is probably down to the Pump Rod stroke.

Once you get those bigger chokes installed and sort out any other details, you should be able to get to the bottom of the problem pretty quickly.

Good luck,

Alan T.