I recently answered some questions for someone that found my email. This is the conversation.
On Sat, Nov 14, 2009 at 1:31 AM, BN720
hello, i hope you dont mind but if you would could you answer a couple of questions about swapping my z24 tbi to a weber 36 synchro? i read your post on NICO when you did it on your hardbody. i have an 86 720( hard to find i know), but as far as i can tell the engine is exactly the same. anyway, im using the intake off of a carbed 720 so that isnt an issue. my question is about the ingition and timing, and the ecu. on your swap you said you didnt change the distributor or the ecu and it ran fine? that is what i was thinking because the comp shouldnt know the difference. also i already had an in-line electric pump im using that should be perfect, what fuel pressure did you find worked the best? and as far as jetting goes did you run it rich? and did you keep any of the smog bulls**t or did you cap everything off( thats what im shottin for) thank you so much for your help. i just want to try to get a feel or any of the kinks i might run into thanks
Well yes basically the ECU doesn't know the difference. (I'm pretty sure about this next part) When it gets no feedback from the sensors on the tbi, it goes into "safe mode" which bases timing advance simply on rpms. The effect is like having a centrifugal distributor. If you've ever driven a car with a centrifugal-type distributor (like many old vw's are set up), then you will be aware that there is usually a "flat spot". I don't really mind it, but it does take some getting used to in order to drive smoothly. My 72 Karmann ghia has the same issue.
So then you can set and adjust the baseline timing by loosening the distributor and turning it. Mine likes about 5-7 degrees at 700 rpms. Also you will want to adjust the automatic choke so that it barely kicks on, even if you have to rest a toe on the gas to start it. Otherwise it idles too high before the oil pressure comes up. Fuel pressure is about 3 psi. I've put down almost 10k miles and love it.
Also note that weber technical support recommended the larger 38, and that they were the cheapest price, $338 for the kit, including shipping.
hey man thanks for all the help, the install was pretty straight forward, but now that ive got the carb on it idles really fast (3000-3200 rpm) ive backed the idle screw all the way and it didnt seem to make much of a differnce if any, all the diagrams and instructions that came with it are very vauge, any advice on what could be causing this? id thought of maybe a vacum leak, but there isnt any vac line running off the intake or carb? thanks for the help man
FP:That's the electric choke, I'd wager. Look for a plastic round thing, mine is white, about 1 inch in diameter, with a single wire going to it, held down but a strange brass clamp with three screws holding it in place. Adjusting the choke:
1. Remove air filter.2. loosen the three screws but do not remove them3. now you should be able to turn the white plastic thing and watch the big butterflies at the top of the carb open and close.4. find a good adjustment, with the butterflies half-way open or more (they were probably completely closed when you started.5. If you do not feel like dealing with finely adjusting the carb, adjust it all the way open and pull the wire off. That'd be disabling it completely. This means you may have to give it a little gas when you start it, but is much better for your engine.
FP:OK so I'm reading what I just wrote to you and I'm hoping it makes sense. I'm going to attempt to elaborate and clarify a few things....
How the electric choke works:there is a thermosensitive coil spring under the white plastic cap. It winds up or unwinds, depending on the ambient temperature. It is attached the end of a shaft which goes through the top of both venturis of the carb. Affixed to this shaft are 2 flat pieces of brass (butterflies) which pivot with the shaft to control how much air is getting into the venturis, which determines how rich the air/fuel mixture is. When it is shut- more fuel and less air(rich) for cold starts. When it is open- it isn't doing anything. that's your basic choke. This choke also has a "throttle step down" feature which is probably causing your problem. This feature is adjusted automatically as you turn the white plastic thing. So you don't need to worry about this part, just understanding it helps with diagnosis.
So why is no electric choke better for the engine and what are the drawbacks if any?
Cold starting is when most engine wear takes place, due to a couple of factors. Oil pressure is 0.0 when you first crank the engine. It takes a second to bring the pressure up. So the only thing that is lubing the internal components on that first crank is whatever oil is still on them from the last time it was run. So the walls of each cylinder have a piston ring riding on a film of oil left over from yesterday. Well gasoline is a powerful solvent... and it takes a much richer mixture to start a cold engine. The added gasoline strips the oil off the cylinder walls. Period. Expect a shorter life on the rings and more wear on the cylinder walls.
Not to mention that with the "throttle step down" feature wants to rev the crap out of engine right when all this other terrible stuff is happening. Good way to spin a main bearing.
Also, electric chokes are famous for flooding the engine so badly that you'll have gas coming out the tailpipe.
Drawbacks of disabling the choke:
You will now be in total control of how rich the fuel mixture is with the gas pedal. Pumping the gas pedal works the "accelerator pump" in the carb which is squirting gas into the manifold like a squirt gun. So pump it a little as you first start to richen it up and get it running. You may have to rest your foot on the gas to keep it running at first on a cold day. Set the idle when the car is fully warmed up. It will idle slowly when cold, and will come up to where you set it as the engine warms up. The car will have less power and just generally run pretty rough until it warms up in about three minutes. Then it will drive perfectly normally. That's it.
BN:i was eager to get my truck running so i messed around with it a little tonight, i adjusted the choke and found that it helped considerably, also the addition of a return spring along with that sloved the problem, now what im curious about is the timing, it seems to run pretty good with a pretty drastic increase in power so im not sure it even needs advanced but, in your first email you mentioned it, would that improve the performance further? or possibly better fuel economy? it was getting 23MPG with the TBI, im guessing thats out the window now, the power increase is well worth it though. just a thought. and thanks again for all the help
FP:Actually most people see an improvement in gas mileage, when they keep they're foot out of it. The technology on the new weber is better than the technology on that 1st gen tbi unit!!!
I'm very glad to know I've helped. I guess not too many people have ever done this. That is why I felt that it was important to document and post it on nico forums. If you don't mind, I would like to cut and paste these emails into that thread in case others would find it useful.
You can drive around with the weber pretty well just on the idle circuit, when you press harder on the gas, it opens up the main jets. So gas mileage really depends on how you're driving.
As for the timing, if it feels good, runs at the proper temperature and gets decent mileage then don't bother. Unless you just want to know where it is set for future reference.
Amazing how much better power these make. The response is way better. The torque is way better. Too bad the EPA would probably have something to say about it... Anyway have fun surprising ricers;-)
Modified by Frankie Pintado at 6:19 AM 1/13/2010