Thread: Carb to TBI Conversion - Major Hesitation

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I recently completed putting a stock TBI setup into my carb'ed '87 350. I have all new sensors, but have junkyard harness, computer, distributor, and throttle body. Everything seems to be correct, but the computer will not advance the timing on accelleration, and I get a LOT of hesitation under load. If I gradually accelerate while parked, it will run smooth as silk at 3500rpm. I went back to the standard HEI distributor setup but still experienced the major hesitation under acceleration and load, so I'm not sure it's related to the timing. Since the HEI didn't work much better, I went back to the computer controlled distributor.

I've tried another computer(1227747) including another EPEOM and the symptoms are exactly the same. I've also tried other, used sensors with the same symptoms. The voltage on the MAP responds properly to vacuum.

As you can imagine, this is really puzzling when a TBI is such a simple system that should be almost 'plug and play."


ANY ideas or direction would be GREATLY appreciated!!

bill.morrison@cox.net
Louisiana

Assuming you have all the required sensors in place and the computer controlled distributor, be sure that you have set your base timing correctly with the distributor advance wire disconnected.

Factory spec is 0deg base timing but many set it up to 8deg advanced. I set mine at 4-6deg advanced.

Once you have set the base timing you must reconnect the distributor wire and clear the error code that this process has generated by disconnecting the negative battery lead for about 30 seconds.

As an aside, when I got my truck someone had set the base timing at 10deg advanced and although it had better accelleration it would "ping" under load climbing hills etc even while using high octane pump gas.

Note that the anti-knock senser will pull the advance back when it senses detonation but may not be able to overcome a high advance base timing.

I tried to drive my truck after timing without clearing the error code once and it would run but with very poor power.

Try this and see if it helps.

I set the base at plus 6 with the wire disconnected. However, I didn't clear the code by disconnecting the battery cable. When I reconnected the advance wire, I kept the timing light on and pushed the throttle. The timing mark stayed totally steady under quick acceleration, sitting right on plus 6.

I'll try clearning the code. I don't have a knock sensor connected.

Thanks!!!

(I'm about ready to go back to the carb!!)

Could the lack of a knock sensor cause the timing not to advance? I don't have the documentation to know if the lack of a knock sensor feeds the computer erroneous information to where it thinks a knock is occuring so it holds back on the timing.

I don't know for sure if the lack of a knock sensor would effect the computer controlled advance but I would think this would generate an error code and may therefore effect the advance.

It is probably worth having the knock sensor to prevent detonation anyway. I'd hook it up to eliminate this potential source of the problem.

Do you have a code reader?

I have access to a reader, but my wiring harness didn't have an ALDL. I think I'm going to have to get one to see what is happening in order to make the thing work. It seems like a TBI setup is pretty basic, but I have something (probably simple) going on that doesn't allow the timing to advance.

Please let me know what you find out.

My 350 is in my 92 Jeep Wrangler and I have an toff-road rip planned for next week, so I've temporarily put a carb on it (Holley Truck Avenger 670cfm). As soon as that trip is over, I'll get back on the TBI. I'm very suspect my wiring harness is bad. I've checked quite a bit of it, but nothing shows up so far. When I get back on it in a couple of weeks, I'll have a new harness (with an ALDL) and a Holley throttle body. I've basically replaced everything except the throttle body and the harness. Then engine runs great with the Holley carb, so it is fundamentally sound. I really can't see any reason I can't make the FI work.

I'm really trying to get the max performance and usability out of the basically stock engine before I take it any further. After I get max performance out of the stock setup, I'd like to begin building the motor a bit. Though my Jeep is very capable off-road, I enjoy driving it on the street and I really enjoy the power over the stock 6cyl. This forum has such a wealth of engine knowledge - far beyond what I've found in off-road forums.

I'll stay in touch to let you know how it goes.

bm

TBI or MPFI is great for off road - no float bowls to flood you out at high angles or rough riding conditions.

Hope you find the problem.

This site may be of interest to you: http://www.xtremefi.com/

Please keep me posted on your progress!


PS: Used to have an FJ40 Land Cruiser that I swapped an SBC into. Great fun!!

The knock sensor dont have to be hooked up for it run good. It only works when the motor knocks, it grounds the computor, which retard the timing. wouldnt hurt to have it though. what yr. veh. did the computor come off of? you really need to get it fixed so you can read the codes.