Inliners International Forums General Discussion Engines Rochester Tune-Up Tips

I sat down at the bench last night and carefully went through Pat Smith's "Improving Driveability With: Multiple Rochesters". I just installed two BC Rochesters on an Offenhauser intake and will be doing the final tuning this weekend.

The article is great and by carefully going through Pat's instructions I learned a lot and besides that it was great fun.

I have a few questions

In step # 10 I worked out a small aluminum plug to get access to a tube inside the throat of the carb.
My question is once I've done the road tests and made sure passage size is optimal for my car "should I re-install the aluminum plug?"

Now that the lead plug has been removed should I close the hole in the exterior of the carb body with a short screw, or?

I've been told another approach to deal with the reduced vacumm caused by using two carbs is to shorten the power valve piston spring (Fig B, part f) by 1 1/2 coils after assuring that both springs are of the same length. Is this an alternative to optimizing perfromance and dealing with the hesitation at acceleration?

Thanks so much for your comments and special thanks to Pat Smith for sharing his expertise!

Ted

I do not know of the article you speak, but the bit about the power valve piston springs is something in addition to the other changes, other wise you will be probably into the power stages of your carbs way too soon during accelleration and possibly during idle too, it will cause poor gas milage and stuffy performance from being too rich (sometimes it can make your eyes water- well maybe not that far with little model B's).

When you have multiple carbs, you have multiple places for vacuum leaks (sloppy carb shafts, internal carb vacuum leaks ,etc). As a exercise in learning, put a vacuum gage on you stock motor, then check it after the new setup- most likely there will be a slightly lower reading with the multiple setup. You will try to keep as close to original as possible. I'm talking about at idle conditions (vacuum numbers somewhere around 17, 18 inches are good- higher the better). Vacuum leaks will affect your vacuum advance, power brakes (if any) AND your wipers.

And on the plug- try to reinstall- epoxy on cleaned surfaces works pretty good.

On dual single bbl. setups, dont go cheap on the linkage, this is often the biggest headache to idling and nice street manners. good luck.

Thanks panic, I have seen that before and forgot about it-its good stuff... and did y'all notice that Pat Smith was running a (wait for it, wait for it) a 3.55 rear gear!

Every time I read Pat's article there is another statement that makes more sense than it did the prior time. I'm setting up my second dual Rochester BC intake (Fenton) on a 235 with street cam, vacuum 15-16, in a 51 pickup, stock drive line but with 3.55 ring and pinon.

My concerns/questions are:

1- When in gear and moving off idle gently there is still some hesitation but not as much as before. What happens if I continue enlarging the off-idle passage and make it too large? How do I tell if I've gone too far?
2- whenever engaging the clutch there is a significant pause, drop off (hesitation) then acceleration, that has not changed. It happens with every gear change.
3- Should I, and if so when, trim the power valve springs?

This is where I am now. Referring to the numbered paragraphs in Pat's article:

1. I have not compared passages in jet stands but will do so tonight.
2. My power valve springs are identical and have not been trimmed.
3. The power valve pistions are identical
4. the check ball for the power valve is correctly installed
5. The accelerator linkage is identical
6. I've installed new # 53 jets
7. floats and neoprene seat assembles are correct
8. easing into the off idle circuit is improving but still there
11. Increasing drill bit size one at a time but still experiencing hesitation

I have extra carb bodies and can swap them out if I make the off idle passage too large. Am I still going in the right direction?

I'm also learning about the importance of good linkage and will be upgrading it when I go to Reno in August.

Thanks for all your help!

Ted