RSA350

Gday All,

Ive just registered but been a visitor for ages! Ive got a problem with my 253 backfiring out of the carby.

Basically its a second hand engine, from what i can tell and have been told it was a mild build. Its running flat top pistons and redline torquer manifold with a 600 holley. It feels like its cammed, not sure how radical but it feels best between 3500 and 7000 rpm.

The engine was in a vk commodore that i brought but i have since swapped it into my '84 CJ-10 jeep. In the commo it was mint, sat all day on revs, plenty of power etc etc. In the jeep it back fires thru the carb before it picks up revs.

This is what i have checked so far in regards to the backfiring.
- Carby, definitely not running lean, fuelly smell when running. Accel pump works fine.
- Dizzy, was mallory twin point but have since changed it out for standard HZ v8 points dizzy, vac advance works, weights not siezed, new cap, points, condenser, rotor button, leads and NGK BP5FS plugs.
- Removed the rocker covers and ran the engine, all valves seem to have about the same 'lift' compared to each other. I was told that holden v8s sometimes wear cam lobes away to nothing. No excessive clacking by any of the lifters either.

Has anyone got any suggestions about other avenues i can try before i start tearing my hair out!?

-Cheers, Adam.

coopers308

You sound like you know what your looking for, but you didn't mention re-doing the timing after you've replaced the dizzy and all, did you just forget to mention that? If not i'd be looking there first

RSA350

Gday mate, nah the timing is set about 14 degrees all sweet, not sure about total, ive only got an el cheapo light. Most of what i listed was done in a process of elimination trying to stop it backfiring thru the carby.

Is it possible that if its a pretty serious cam i should be running more initial advance??? Like i said it pulls hard from 3500 to 7000, well it used to in the commo anyway!

Just had another thought... this engines had the bejeezuz revved out of it, is it possible a stretched timing chain could cause these dramas?!?

-Ads

coopers308

I would've thought if it was chain wear it would've been noticed when you had the engine in the commy. From what your saying it's started doing this since the engine swap and dizzy swap is that right? Thats why im thinking dizzy/timing etc. Also single points is probably a bit risky at 7000rpm, have you had it up that far with that dizzy? Its very high revs for a 253! Also i thought a 600 carby was pretty big aswell but if it all worked previously it must be good.

ahk068

Try fitting a ballast resister to the coil. Years ago I ran a points dizzy in a vc commy and had same problem. Something to do with the wiring being different for an factory electronic dizzy. Ampage or something. Cant really remember but it fixed the problem.

stroker355

RSA, you should get rid of the points dizzy asap, specially if your reving past 5000 rpm, points just cant cut it at high rpm, and it will then rev a lot cleaner.

also wit the back firing,.. turn the dizzy to more advance and try, then turn more retard and try, just to see if it makes any changes at all.

Oldman

Nothing to do with the timing,it's in the carb.When does it back fire.Under exceloration,cruising,deceloration.As much info as possible please.

RSA350

Hi guys,

I understand that a single point dizzy will be no good for high rpm. It pulled 7000 back in the commo with the twin point lol, the engine is in a 4x4 now and could really use a torquier cam. I converted the current dizzy with to an optical pickup with a crane xr700 conversion kit. In an effort to find the cause of carb backfire I threw points back in, ill get around to re-instating the xr700 shortly.

As for the back fire, when free revving the engine I hold the revs up at about 1000 to 1500 then crack the throttle wide open, CHOOF, back fire, from what I can tell there is no flame (riding valve on compression stroke?) the revs then pick up.

When driving it constantly back-fires during acceleration under load, once the revs get up it seems to be ok. I can sometimes avoid the backfiring by gently feathering the throttle up to speed.

Point to note, i did put a new power valve in the holley a while back....

Your thoughts?

-Ads

richo

if it's CHOOF back threw the carbi, pick ya dizzy up and move back a tooth, if that don't work go the other way two teeth, and if thatdon't work move back one tooth and your back where you started. putthe electric dizzy backin it. if it doing the poping all the way as your trying to drive after doing the above, change the power valve back to the one you had. jetsand powervalves can be easily missmatched and give big problems.

400HPONLPG

Methinks the Oldman is on the money , its a carbi problem , sound like parts of the enrichment circuit are dead , power valve,accelerator pump etc etc

Tinman

I put a third vote on the carbie. Was this carbie always on it?

Need more info.
engine idle vacume?
power valve size & condition?

Accelerater pump, cam, nozzle, sizes, condition & adjustment?

__________________
Cheers Tinman

Jack of all trades master of none

Oldman

Rsa 350 you're enrichment from idle is to lean,bigger squiter or cam needed,change the cam size first.If that doesn't fix it go the next size squiter up.common problem.Do not change the timing it will hide the problem.Also popping in the exaust is not caused by timing,the fuel is being sucked through the squiter,get an antipullover squiter,& put it in the front position.

VL_Turbo30

sounds a bit like over fueling to me. Definately think there is a carbi issue. Not the dizzy.

RSA350

Gday Guys,

Thanks for the support so far, Tinman, Oldman ill have a play with the carby and see how it runs after work if its not too late. Ill get back to you with my findings. The exaust fell off when i was out 4x4ing and im already in the sh!t with the neighbours, sounds like a race car, loud and obnoxious! As for the carby, its the same one that came with the engine, nothing has changed since it ran good except for the power valve and new metering block/bowl gaskets!


Would moving the dizzy really matter? By moving the dizzy a tooth here or there your only changing the position of your rotor. This change would then be counterracted when you then twist the dizzy to time it correctly??? If my light shows 14 degrees, then its 14 degrees no matter where when dizzy is located, as long as the number one spark lead is in the right spot... can anyone clear this up for me... doin my head in LOL!

You're sorta correct the purpose of the ballast resistor is to limit the current in the ignition circuit. It increases the life of the points. Depending on the age of the vehicle this was achieved by a ballast resistor or a resistive cable (my old HZ has the resistive cable type). The resistor must be bypassed when using an electronic dizzy as they require a full 12 volts to operate.

Im not an expert but im pretty sure carby backfire is due to a lean condition, backfiring thru the exhaust would be caused by over fuelling ?(correct me if im wrong)

Sorry for the long winded reply, got no jobs on here at the workshop!

Cheers, Ads
DISCLAIMER: Self taught mechanic, go easy im just a sparky!

stroker355

Unless cam was not degreed in and is retarded quite a bit, (camtiming)

And was hidden in its previous guise?,..may explain why it loves to rev.

just a thort.

but of corse, check carby first