carby re-jetting questions

[lcgtr1970]

Hi guys
Ive got a mild 253 in my hq van mated to a trimatic.
I have a Street Demon 625 carby on it but have found that it pumps black exhaust when i give it a boot full...obviously overfuelling it.
Now what Id like to know is, can the 625 be rejetted to suit a 253 or am I going to have dramas with atomisation etc?
I have (since buying this carby) been told a 465 or 500 would be ideal....
Anyway id like to be able to fix this one if possible considering I already have it and getting a new carb again aint cheap....but let me know if its possible or if im chasing my tail.
Thanks

[axistr]

I haven't played around much with Demon carbys, jetted plenty of Holley's & a few Rochester's, the principals are all the same for tuning. Is the Street Demon a vacuum secondary and have a square bore or spread bore, or is it a double pumper carby. ? If its a vacuum secondary carby you should be able to make it work or at least reasonably drivable.

Tuning or a good tune depends on many factors, compression ratio, cylinder balance, timing curve, manifold vacuum (particularly at lower rpm), cam type and overlap, exhaust system and engine condition. I was running a Rochester on my van with 253 manual mild cam and 9.8-1 compression. The spread bore Rodchesters are actually a 770cfm carby from memory.

However hear are a few things to try first.

If it's blowing lots of black smoke under acceleration at low to medium RPM then I would start by reducing the size of the power valve circuit, it will have a valve or jets. If its smoking under quick acceleration then clears up slightly then look at reducing the accelerator pump circuit. This might be as easy as adjusting the linkages to reduce stroke travel on the arm that pushes on the plunger. If its smokey under mid to high rpm and doesn't quickly clear when you accelerate then reduce the size of the main jets. Check your timing, you might be able to increase the timing advance to allow more time to burn more of the fuel and increase power and economy. Also check that the disy is in good nick and the advance mechanism isn't flogged out. Check the vacuum advance is working. Worn shaft bushes can also effect spark timing between cylinders. And lastly check you don't have any vacuum leaks. Holden V8s commonly have leaks around the inlet manifold and hoses.

Cheers Lenny.

[lcgtr1970]

Hi axistr, thanks for the reply....pretty sure we're on another forum together too!
The carb is a vac secondary type.
I had a look at the linkage on the accelerator pump and its on the lowest setting already. I did have it one hole up higher when i first installed it but moved the linkage which made it slightly better under load.
The dissy is new and is an electronic dissy....so hopefully its all good there.
Its rich at idle, but according to mates doesnt really blow smoke until i "get up it"....eg overtaking on the highway etc.

[axistr]

If its rich at idle and you have correctly set the idle mixtures then double check the float levels. Incorrectly set float levels will effect the entire RPM range. Its also possible there could be a problem with the power valve circuit. I'm not sure how the Demon carbys control the power valve circuit. Its somewhat common for Holley's to have a split in the power valve diaphram or incorrect size valve and cause similar issues. Leaking emulsion block and base gaskets can also give the same symptoms.

It might be as easy as dropping down a couple of jet sizes in the secondary's to clear up the top end. I sometimes muck around with the vacuum secondary spring tension on Holleys to get them to cut in a bit later in the rev range to stop early rich under load situations. I would start with the float level checks first, then set the idle mixtures and timing and see if that clears up the rich mixture from idle to mid-range. If not then investigate the carby internals more closely, example power valve, gaskets and correct external plumbing connections . You can also check with the engine idling around 750 RPM and look down the carby venturis and see if you can see any fuel entering above the butterflies primary's and secondary's, if so you have a problem.

The above checklist is only a couple of common areas that can cause the problems your experiencing but you have to start somewhere.