Ok, I wouldn't even ask this, but I am just about at my wits end with this damn thing. My dad and I built a 351W for his Bronco. Nothing special, just a stockish rebuild. We decided to go with the GM TBI swap, because its cheap and fairly easy to do. I read on here and couple of other forums and had already done the junkyard swap on my 350 in my jeep.

So, on to the problem. We got everything going and it runs really good, except for a back fire. It seems like a timing issue, but everything I've checked seems like it is working exactly the way it should be working. I've set the base timing with the bypass unplugged at 0 3 and 6 and everything inbetween, and it seems to be the best at 3 degrees. The advance is smooth and advances as the throttle is depressed, just like it should. But when you are cruising down the highway at about 2300 rpm or so it wants to continually backfire and it also seems to have a hesitation. We put a new pickup in the ford disty, and have double checked all the wires and firing order.

Any ideas, if more info is needed just ask. Thanks.

Clay

TPS, MAP sensor, grounds, or vacuum leak to the MAP from the TBI?

Check your fuel pressure the Holly projection did the same thing, the pump would run but not produce enough PSI...idles fine, would run up the road a ways but then would start to pop and backfire and hesitate...Just a thought....L

Maybe your Ford motor is rejecting the GM parts.. lol i too would check the fuel pressure.. or maybe one of the injectors are screwed up..

Had a similar issue when I had Projection. I placed the fuel pump as near the tank as possible, but when I did I was using the fuel pump wire from the computer and it was not dellivering enough electricity to power the pump under load. I ran a large guage wire to a relay near the pump and use the fuel pump wire from the compter to only energize the relay. Problem fixed. Hope this helps.

It could also be a sticky valve.....

Had it happen on a newly rebuilt 302 years ago. The guides were too tight and it would hold the valve open slightly.

Email Holley's tech support. They are the ones who built that system.

2300 RPM sounds a little high for a cruise RPM, so maybe the computer is looking for the engine to shift...assuming your Bronco is a three speed automatic whereas the TBI was likely in a truck with a 4L60E/700R4... electronic overdrive transmissions.

Maybe your Ford motor is rejecting the GM parts.. lol



:laughing: a bad organ donation! :laughing:

:laughing: a bad organ donation! :laughing:


lol...I was gonna say it was organ rejection from donated parts, but I didn't want to add to the guy's misery. But I'm sure everyone who's read this thread has thought the same thing! :D

Thanks guys, I will try and get to the fuel pressure thing this weekend. Any and all remarks are accepted. I've been on this board long enough to know to wear my firesuit.....:flipoff2:

Backfiring sounds like the timing might be off.

I think the factory calls for 8 or 10 degrees of initial timing, so 3 degrees doesn't sound right, unless you are getting a shitload of advance.

One thing you might investigate is the timing chain.. Ford has retarded the cam timing 4 degrees, and many aftermarket replacements have the factory setting (-4), standard 0 (better power, more emissions), and +4 (hypo, I guess). If you are set to +4, that could have some bad effects.

are you using the vacuum advance dizzy or the tfi one?

I'm using the vaccum advance dizzy with the mechanical and vaccum advance locked out. I'm gonna try and put a pressure guage on the fuel today, se we'll see what the shoes.

I'm using the vaccum advance dizzy with the mechanical and vaccum advance locked out. I'm gonna try and put a pressure guage on the fuel today, se we'll see what the shoes.

If you have the timing set at 3 degrees and have the dizzy locked out, then it sounds like you have something really wrong with the timing.. most people that have their timing locked out have the timing set to 35-40 degrees and they are running drag cars.

If your timing is set to 3 degrees when you are cruising at 60mph, that is what your problem is.

unless I am misunderstanding what you are saying.

Mechanical advance is locked out too? That does not sound right to me. 3* at 2300 RPM, the plug is firing when the valves are open.