what's wrong with my engine?? weak,backfires,hard starting....

[01maroonz71]

well, i swapped in a 5.7GM crate motor in my truck a little over a month ago, and ive only put about 350miles on it so far(i only drive 2 miles to work everyday).

well, after it sits all night, i hit the gas once and it fires right up no problem(when it's cold) but if i were to park at work and go get in my truck less than an hour later the starter turns over and over and over and it takes like 10seconds to catch and finally start.

another symptom is that when the motor is cold in the morning, it seems to run like crap. it backfired twice on me under medium acceleration and sometimes the idle seems to jump a tad bit(when it's cold)
once it heats up, it does neither of the above....just starts hard.


the engine also seems weak in the low RPM range, but really really pics up after about 2k or so. could this be something with my idle mixture? or is it possible the advance is coming in too fast(mechanical)


i have an edelbrock 1406 carb on my crate motor and the advance is hooked to full vac. i set the timing at 8 BTDC, and at first i had the vac advance hooked to ported vac, but it ran horrible...so now it's hooked to full manifold vac. it runs way better, but im still having the above issues.

im afraid my battery is taking the toll also.

thanks guys
~Dave

 

[dremu]

Does sound like timing should be tweaked. Is the dizzy's advance good? Last time I had issues, I found the advance weights were basically rusted in place, making them give a pretty consistent advance regardless of RPM

The cold-vs-warm thing also makes me think about choke; is yours electric, hot-air, or manual or what?

-- A

 

[01maroonz71]

i have an elctric choke...i don't think that's the problem at all......

what should it be idling at, timing wise, with the advance hooked up at idle??? how far advanced should it be...i can hit it with a timing light later this afternoon....that would be a sure sign that the springs may be shot.

 

[K5dreamer]

what it sounds like is the engine you swapped in has different specs than the one you took out, it might be pulling more vacuum, have different cam specs, yadda yadda. carbs dont like that, what you need to do is actually tune the engine and carb combination you now have. setting the base timing and running the settings the carb came with out of the box will never net good results. youll need to set the idle mixture, set the idle speed, possibly adjust the fuel mixture on the carb. for the ignition timing, remember you have to unhook the vacuum advance to set base timing, if you have a vacuum guage, i would actually set the base timing by that. hook the guage up to the manifold vacuum, and adjust the timing until you get the highest vacuum reading. then lock it down, and hook up the vacuum advance. i think you said you had an edelbrock carb? using a timing light with a built in tach made my life easier working on my brothers bronco. its a little time consuming but not that hard, the directions are on edelbrocks website.

 

[broncoman6524]

dumb question but mine is also like this...the hard to start wehn its hot...the way you hooked it to constant vacuum...how did you do that? off of the thing on the intake runner, or on the carb.

 

[4X4HIGH]

You need to first start by hooking the vacuum advance up correctly. It needs to have ported vacuum. The way you have it now you have full vacuum advance when idling or cruising but the minute you nail the throttle you lose all advance and it should run like crap. unhook and plug the vacuum line to the distributor and set the base timing around 6-8 degrees. Now hook the vacuum line to a ported vacuum source. The idle should be 550 rpm with the trans in drive.

 

[01maroonz71]

when i had it like that, right after the swap...it ran like crap...it runs WAAAYYY better with full vac. but ill give that a go again i geuss.

 

[JEBSR]

What you are doing by hooking the the vacuum advance up to a full time vacuum source is masking your problem by advancing the timing at idle. You need to put it back to a ported vacuum and find the actual problem.

The way it sounds to me, you have two problems. One being getting the timing dialed in and Two being carb adjustments. You need to get your timing set the way 4X4HIGH was saying and then adjust the carb. Doing it in reverse will not work. Set timing, set air/fuel mixture, let engine get completely cold and set choke. Also check all of your vacuum lines to insure that you don't have anything crossed as well.

 

[01maroonz71]

will do guys...ill get it done right after work tomm. would the advanced timing at idle make my truck run a tad hot when it's idling????? that problem is prolly my fan clutch, but who knows. thanks alot ya'll!!!!!
~Dave

 

[4X4HIGH]

Definately could account for extra heat at idle.