well i rebuilt my 305 with cfi and put in comp cam 12-364-4 and am having nothing but problems. its a 1.6 ratio cam. i think that may be my problem. it backfires through the throttle bodies and the tach is all over the place. im going to pull the cam out and put a new one in. im not doing stock. i am however going back to 1.5 ratio. any suggestions on what cam to use? i would like to go cheap but not to cheap because you get what you pay for. i have stock heads and intake. thanks

 

I would think a cam close to "L69" specs should be ok with CFI 305-i don't have those specs handy,but the same cam was used on the 'vette CFI 350s.Will have to see if any CFI 305 users have any ideas on a good upgrade cam that isn't too fussy with a CFI...One thing nice with a CFI is you don't have to disturb the TBIs or pull the intake to swap cam/lifters-there is enough clearance under the intake to pull the lifters up with magnetic tools,drag them back and out the dizzy hole

 

That's a very small cam with a wide lsa, can't see it causing any computer problems. The difference 1.6 vs 1.5 ratio is valve lift; the only problem this might cause is interference with guide boss or spring bind, if you didn't do your homework and take some measurements before selecting the cam. However since we are talking under .480 lift even with 1.6 rr, this is also unlikely.

Backfiring thru the tb sounds more like a timing problem - do some diagnosis before throwing random parts at it.

 

Here are the LU5 cam specs, as mentioned, same as the cfi L83 Corvette 350: 403int/415exh @ .50. 20int2/206exh duration. As mentioned above, it sounds like a timing problem. Once your done diagnosing, if you do find that the problem does turn out to be the cam, a good upgradeable choice is the Comp Cams xe256.

 

i tried timing. pulling the distributor, advancing and retarding the timing, nothing seems to work. it always seems to backfire out of the tb's and die out. for all i know i may even have a rolled lobe. because now it sounds like i may have a dead cylinder. i was looking at comp cams 12-388-4 any input on how that cam works?

 

No correctly installed and set up cam is going to blow back thru the intake. You've got another problem - installing 'another cam' all day long wont fix that. If it's not ign timing, you've got a valve problem - like an intake hanging open, or an exhaust not opening etc.

Flat lobe could cause this, bent pushrods, bent/burnt valves etc etc - have you looked for damage like that? What was your cam break-in procedure? Do you have plenty of zddp in your oil?

Edit: Got that timing set on correctly?

 

im using comp cams 10 30 break in oil and my break in procedure was 2000-2500 rpm for 20 mins. ive done compression tests on all of the cylinders via plug holes and passed them all so i know its not a valve problem. i am not sure what else to do with the timing since i have already spent days on it. i have 12lbs of fuel pressure between the throttle bodies, so i dont think its a fuel issue. i carefully put everything back together on the engine and i had help along the way, so i dont think its anything ive done. but the main reason i think its the cam is because the only drastic change was from 1.5 to 1.6 ratio. not sure if i stated it on this thread or not but i also have an issue with my tach. once i get to about 30mph or so the tach is either all over the map or it drops to nothing. so im wondering if the computer doesnt like the reading off the new cam. its the stock distributor just with msd rotor cap coil.

 

[quote=82_Camaro;5403850]i it drops to nothing.

Have you tried testing for fuel pressure under load while cruising at that speed? I had a similar problem with my CFI a couple of years ago. I would run 12ish lbs of fuel at idle all day but when I would start driving my tach would pretty much do the same thing yours is doing. I did a fuel pressure test while I was driving it during that condition and sure enough I wasn't getting adequete pressure. This may not necessarily pertain to you but food for thought I guess. Are you throwing any codes?

 

[QUOTE=ChrisC;5403855]i do not have a check engine light on nor did i have any codes but another issue i have is that the diagnostic machine loses connection and i cant do data display. its a snap on solus. it works with any other vehicle so i know its not the machine. ive asked around and many other people have this issue so i think it may be common. i have not checked my fuel pressure while moving just at idle.

 

Try pinning the ALDL and testing for a code like that..I think it's pin 5 and 6 connected. If the check engine light blinks "1" stop "1-2" stop and keeps repeating then the system is working correctly and it is clear.

 

None of this is caused by your camshaft unless you've killed a lobe.

The issue is somewhere else. Like the others have said I'd check your timing again and make sure it doesn't walk when you hit the gas. Backfiring through the intake usually means it's too far retarded.

I'm not sure about diagnosing a CFI setup, but this could also be a computer issue if it has anything at all to do with your timing.

Does the car idle properly? Can you drive it? When exactly does it backfire and kill itself?

 

Another way to look at it. Fire's getting into the intake. So:

* Intake valve(s) are open when the spark fires. Ign / valve timing problem, rocker(s) too tight, bent valve etc.

Or

* Exhaust valve(s) are not opening fully or at all, forcing burning charge back thru the subsequently opening intake valve. Rocker(s) too loose, stud pulling out, bent pushrod, wiped lobe etc.

Or

* Gross EGR leak into intake (just a complete guess).

 

it idles fine i can drive it but as soon as i hit about 40 mph or if i get on it at all it backfires doesnt always die sometimes stalls the goes again. i tried to see if it walks while driving and it doesn't. if your wondering i had the hood off and someone sitting in the engine compartment with a timing light. yes very safe. he insisted we did it that way. but had the intake off once already and the rods are all fine when i cranked the engine over all the rockers dropped and came back up. not sure now but ill be pulling it apart soon if i cant fix it externally

 

Something is allowing that exploding charge back into your intake and that usually only happens if your intake valves are open when the spark plugs go off. Typically this involves very retarded timing. Are you SURE your timing gun is accurate? What method did you use to check timing? It's easy to do on a new motor and i've had it happen to me. You'll likely need around 12-16 degrees initial based on your camshaft.

Check for bent intake valves or push rods. Something is off with your valve train. It COULD be a wiped lobe but if nothing is in your oil I doubt that's it. If you've got the timing down then you've got something bent or an intake valve not seating properly. Did you install comp's recommended springs or better?

 

Hang on - so it's not firing thru the intake all the time? Only when you hit about 40 mph?

Are you sure that's what it's doing - how would you know? Sure hope your mate wasn't spread-eagled over the engine at that speed

Are you sure this isn't a surge or misfire, rather than detonation in the intake? This is now sounding more like ign module, vac leak, ecm, something along those lines, or timing still.

Also - with the timing - I know nothing about cfi, but assume that since there's an ecm involved, I'd think a timing connector should be disconnected, and the base timing set to some low value. Similar to 0 or 6 like tpi, tbi. Just my guess - do some research and find out.