TUNING A ROCHESTER

I gots a little problem i fink.

My engine has a flat spot between about 1000rpm and 1800rpm. I had my quaddy rebuilt and bigger jets put it in whilst building my engine, if i have it idling after the car is warm then stab the throttle, it hesitates then picks up at about 1800Rpm.

It has electronic ignition, slightly colder plugs than recommended as standard.
If i pull up to a corner, turn then stand on the throttle it hesitates and feels like its flooding but after its reaches about 1500 to 1800Rpm im outta there.

I got a performer manifold, L34 valves, rollers and 1 3/4 primaries, 2500 stall and 3.55 diff.

Any ideas.

cheers
JNR_ATE

What size engine? 308? Just check the butterfly on the secondaries. Does it open smoothly, i.e. no hesistation when you push down with your finger? Quite common for the rear two bolts on the air horn to be overtightened and deform the air horn, which makes the secondaries stick. When they do open (with lots of vacuum), they open too fast, flooding the engine. Just push the secondary flap down a bit and see if there is an even gap around the top butterflies. If not, your going to have to die grind it back round.

That would be my first step anyways.

Oh, just read the other thread about young mechanics. Air horn is the term used for the top cover on the carbie, not the snout on the standard air-cleaner.

Unfortunately this is not an easy one to answer, as it could be numerous things. It could as Yella has suggested be the air horn deformed or just the secondaries air valve adjustment, could be opening too early or too late, need to experiment with that one. Could also be float adjustment as it sounds like it is mainly when it is hot or cornering. which could be fuel flooding into the carby, so check float level, and fuel pressure. too high, and you will get same result. There are baffles that can be fitted to Rochies to help prevent this from occuring. Bigger jets may not have been needed as the standard ones can supply most engines.There is a great book on Rochesters by Doug Roe, maybe get it from the Library, or buy it $45 I think, its well worth a read and has lots of mods you can do to get these carbies to work really well. Good Luck Jim.

Too true Jim Jet. For float too high, pop top off the charchoal cannister and have a sniff, as it will generally overflow down the vapour line to the charcoal cannister.

Have that book, yet to read it. Simple when you know how they work, just complex when they put the four different systems all in one inmeshed little package.


Where are you located JNR? Somebody ought to know a carbie guru up your way. We have a dude down here who is excellent. My concern is that these guys are getting thinner and thinner on the ground, time to teach ourselves I think.

If you are in or near Sydney, get Louie on the case.

Can be things as simple as gummed up needles as well, a rod fallen off the hanger, faulty accelerator pump, etc.

Unless you know what your doing, best to call in the experts. Don't think my carbie has ever had the same fault.

Problem could be all of above,
as said, check the secondary air valve is not set too loose,
also check timing not retarded.
What model Quad is it?


Louie

Im in sunbury, not far from YiriSS. The guy who rebuilt my quaddy for me and changed the jets because of the info done to the 308 is a older bloke who has been round for years. spose even pro's can make mistakes.

Might have to take it back to standard or even just get it checked out, its just really flat to pull, even with a 3.55 diff ratio.

Thanks for your ideas guys.

Hey yella, thanks for the info on the "AIR HORN" now i gotta cram it in me head with all the playstaion cheats i have memorised, will pop some useless info out the other side now.

Cheers
JNR_ATE