Dellorto Tuning Problem (not another one?)

bjinright · 06-14-2006, 09:32 AM

Ok, the carbs (DRLA 40's) were pre tuned with the CB PERF manual. Tuned to idle at 950. Balanced with an air-flow meter (at 5kg mark) at 950 rpm. But now adjusting the Idle mixture doesn't seem to do much. I'm going slow.

I've got to close it all the way down to see a difference. I opened it up to where the cylinder fires and then another 1/2 turn. That puts all four mix screws at about .6 to 3/4 open. That has to be lean, has anyone had this happen?

Details: Carbs are euro (extra large accel pump). Carbs are jetted to specs accept pump jet is 33 not 35. and receantly put on a Mallory unilight from Raby's AC tech at 28* btdc at 3500rpm.

Acox914 · 06-14-2006, 10:09 AM

idle jets too big?

bjinright · 12:49 PM

They are at 60. Do you suggest 55 or 50 . . . maybe lower? Do you know a good place to get them.

Kevin Powers · 06-15-2006, 06:57 AM

i have found it can be difficult to balance the carbs at a low idle. the unisyn needle can jump around a bit, giving you a inconsistant reading. using just ONE of the idle speed adj screws, turn the idle up to around 2500rpm or to the point when the needle is somewhat steady. recheck the bal on the 2 front throats. what is your engine displacement? sounds as tho your idles could be too large. the idle air mixture screws regulate the fuel flow, not air. opening them up that small amount would lead me to belive you are running rich (fat) while on the idle ciruit. are you using a facet pump (if so, you MUST use a GOOD fuel pressure regulator and a gauge to make sure what kind of pressure you're developing) or a self regulated rotary style?

k

bjinright · 06-15-2006, 07:14 AM

engine is a 1.8 and CB Perf manual say not to go below 60.
I do have some 55's from CB on the way though. ;-)

Fuel pump, I assume facet, but I'm not sure and would rather move to a rotary. Where is it on a '74. Never mind I'll just follow the fuel line from the tank.

I last . . . thing.
Is it normal for the right (passenger) side head to run 20*-30* hotter than the right? I assume the oil cooler on the right pre heats the air.

Kevin Powers · 06-15-2006, 08:05 AM

if you go to the "classic" messages are at www.914club.com, there is a thread detailing a very simple method to power your pump. you can use the stock pump harness with a little modification. you also get to use the the proper relay and fuse on the relay board in the engine compartment. the lower volume version of the rotary pump for carbs, avail from cb, is a nice unit. quiet and self regulated. i use one and have performed the jumper wire install.

k

Dave at Pelican Parts · Dave at Pelican Parts's Garage

You'll know if the pump is a Facet clacker. Just listen...

The pump on a carb'ed US-spec 914 is located wherever the heck the person who did the carb conversion felt like putting it. Check the stock pump location (bottom right front corner of the engine bay, visible from under the car) and check below the fuel tank. Or just follow the lines as you said.

--DD

bjinright · 06:52 PM

Well Great. Now I have popping back through the carbs under acceleration and at cruising under 3000 rpms. This may acctually be lean after all. I'll turn them up to 5 turns tommorrow and see if that makes any difference.

I'm sorry guys, this may be a dead issue until I fully check the fuel delivery system.

Also Kevin, What do you mean by turning up one Idle speed screw? If I did that wouldn't one carb be way out of sync to the other?

HarveyH · 06-16-2006, 03:31 AM

Don't know if Del's are quite the same as Weber's, but for W's the idle mixture screw only controls the mixture at static idle. As the throttle comes off the stops, the fuel flows through the transition ports, not just the mixture screw ports. If your throttle plates are open too far at idle, the transition ports will operate and the mixture screw will have very little efect on the 'idle' mixture. The idle jet meters fuel for the mixture screw port AND the transition ports, so it controls the mixture from idle up through about 2200 - 2500 rpm. It's possible to have the static idle mixture set just right, but still have the jet too large or too small to operate properly through the rest of the range it controls. You have to size the idle jet by low speed (1000 - 2500 RPM) operation, not just at static idle.
Harvey

Kevin Powers · 06-16-2006, 05:20 AM

using just 1 idle speed screw to increase the idle is an easy way to check for equal opening of the throttle plates. if the 2 front throats are flowing air unequally, matching the flow with the "free" (off stop) threaded rod (turning it in or out), you avoid any potential linkage push against the idle stop if you should have to lengthen the rod. once you have them flowing at the same rate, you turn the the idle back down and run the "free" idle speed screw back in to the point where it's just on the stop. flowing equally?

the idle/air mixture screws.... start with them all at a common setting. try 2 1/2 or 3 turns out from GENTLY seated. with the carbs flowing equally, timing and valves set properly, and the idle turned back down, turn the i/a mix screws in one at a time. you will sense a distinct stumble/poping. slowy turn each one back out till the idle smooths out.

k

Kevin Powers · 05:30 AM

oh, the cb book's trouble shooting guide will spell things out. redo the setup proceedure if you need to. setting the air by-pass screws to a common setting, prior to start up, is key also. p22 of tomlinson's book.

k

bjinright · 07:34 AM

Thanks Harvey, I'll check the butterflys close to the throtle side and see what the clearance is. Does it matter what side . . . of the butterfly not the carb.

So I'm thinking reset butterflies to .003

Open all air bypasses to 1 turn open baseline, preset idle mixture to 3 1/2 turns (being safe).

Balance front carb throughts with idle speed screws to 950rpms. At this point should there be an amount on the air flow to shoot for? 5 . . . 7 . . . 10?

Balance rears to fronts with air bypass. Rechecking idle and adjust both sides with equal small turns to speed screws.

Then begin back down of mixture screws to misfire and back off, slow and one at a time.

What do I do if during mixture screw back down, the idle goes up around 1 1/2 turn out? That is what got me into this. It seemed leaning the mix made it better, until I got in to drive.

See if that procedure is,
Ok, I'll try that during lunch and see what I get.

HarveyH · 06-16-2006, 07:16 PM

See Carbs 102 and Carbs 103 for good info.
http://www.aircooled.net/gnrlsite/resource/articles.htm
Harvey