Could I get some help tuning my Rochester MV1 1BBL

[Custom/Ten]

Been looking for info specific to tuning these carbs and haven't found everything I needed so thought I'd start this thread. I don't know much about carbs so here goes.

I want to tune it using a tach and vacuum gauge. Is the mixture screw adding fuel, or air and which direction does it turn to add more?

Should I go for highest vacuum or go highest then do a lean drop lowering vacuum/rpms? If dropping vacuum/rpms how far should I drop them and what direction do I turn it?

The sticker says 750 rpm idle for my 250 3ott. I do not have ac or air injection reactor system. It's a 72 btw.

I think that's everything I need to know and everything you need to know to help me. Please feel free to add any extra info or ask any questions.

I'm sure I sound ignorant or stupid but hopefully with a little help from you guys I'll be a little less of one or both. Thanks!

[Custom/Ten]

Why not throw in some pics

[Custom/Ten]

[SS Tim]

Factory service manual and overhaul manual have a lot of information and specs. If you don't have them I'd suggest at least doing a download and read through of both, Section 6M.

[Custom/Ten]

Thanks Tim

[geezer#99]

Originally Posted by Custom/Ten

Been looking for info specific to tuning these carbs and haven't found everything I needed so thought I'd start this thread. I don't know much about carbs so here goes.

I want to tune it using a tach and vacuum gauge. Is the mixture screw adding fuel, or air and which direction does it turn to add more?

Should I go for highest vacuum or go highest then do a lean drop lowering vacuum/rpms? If dropping vacuum/rpms how far should I drop them and what direction do I turn it?

The sticker says 750 rpm idle for my 250 3ott. I do not have ac or air injection reactor system. It's a 72 btw.

I think that's everything I need to know and everything you need to know to help me. Please feel free to add any extra info or ask any questions.

I'm sure I sound ignorant or stupid but hopefully with a little help from you guys I'll be a little less of one or both. Thanks!

For rookie carb guys, the mix screw adds enough fuel just for idle purposes.
That's the simple explanation.
We could get all tech about how it works, but it just adds more fuel.
You want your vacuum as high as possible at the lowest rpm.
Your sticker says 750 rpm but you can achieve a nice smooth idle at 600 rpm. Or less.

Start at 1 1/2 turns out on the mix screw. Motor warmed up.
Adjust screw in 1/4 turn at a time until the motor stumbles (drops rpm).
Go slowly.
Then at that point adjust screw out a bit to get your highest vacuum.
If you can adjust the screw all the way in (be gentle-easily damaged) then that tells you you need to find a way to close the throttle more to cover the transition slot.
That can be done by increasing your timing, which increases your idle and then you can turn the idle back down and get the mix screw working again.
Hopefully not to complicated for you!
A link that has a section on timing and transfer slots for you. About halfway down the page refers to a holley carb but your carb works the same.
http://www.crankshaftcoalition.com/w...EI_distributor

Question for you.
What's your initial timing?

[Custom/Ten]

Thanks geezer, that's a great explanation, better than any I've read. Sounds like you know what your talking about. I'm running an hei off a 80 250 and I timed it to 10 initial advance because that's what the Hanes said for the 80 250 but I'm wondering if I shouldn't try for a little more. I read people on here convinced it's 10, 12, 14, or even 16. Ive read a ping test is the best way to know for sure but i haven't tried it yet. If I remember correctly the motor will die around 3/4 or 1/2 turn from the idle screw being all the way in. I defenitly can't turn it all the way in.

[geezer#99]

Originally Posted by Custom/Ten

Thanks geezer, that's a great explanation, better than any I've read. Sounds like you know what your talking about. I'm running an hei off a 80 250 and I timed it to 10 initial advance because that's what the Hanes said for the 80 250 but I'm wondering if I shouldn't try for a little more. I read people on here convinced it's 10, 12, 14, or even 16. Ive read a ping test is the best way to know for sure but i haven't tried it yet. If I remember correctly the motor will die around 3/4 or 1/2 turn from the idle screw being all the way in. I defenitly can't turn it all the way in.

Sounds like you're close on your transfer slot with that amount of mix screw adjustment. You could try to fine tune a bit.
Let's 'splain the ping test!!
My method with all motors.
Advance your timing (motor hot) until you hear pinging with the motor under load (under load means 3rd gear at 25 mph, squeeze the gas pedal down about half way). If you hear what sounds like a rock rattling in a big metal can, then adjust your timing back until you hear no pinging under load. That initial timing will be close. Even at that setting if it cranks slowly you might need to pull the initial back a couple degrees more.
Your idle will increase because of more timing.
Then adjust your carb.
If you don't understand the concept of advancing timing, just ask!!

[indymachinist]

Thank you for this thread with these answers in it. I am about to try to get my carb going as well and will need these same things.

[Custom/Ten]

The thing that had me most confused about the idle screw is that when I go to far in/clockwise the tailpipe starts to get a little smoke out of it. You can hardly see it but it looks to be a little bit black. Maybe it's grey, does that make sense/sound normal, is that what happens when you idle to lean? I was thinking that meant it was rich and that's most of the reason other threads were confusing me and why I wanted to start a thread for the Rochester but sounds like it works about the same as any other carb.

[geezer#99]

Originally Posted by Custom/Ten

The thing that had me most confused about the idle screw is that when I go to far in/clockwise the tailpipe starts to get a little smoke out of it. You can hardly see it but it looks to be a little bit black. Maybe it's grey, does that make sense/sound normal, is that what happens when you idle to lean? I was thinking that meant it was rich and that's most of the reason other threads were confusing me and why I wanted to start a thread for the Rochester but sounds like it works about the same as any other carb.

My uneducated guess would be that your running lean. Super lean makes the combustion hotter. Hotter combustion could then start to clean some of the carbon residue out of the chambers. Could look sooty or black out the pipe.
I've seen that myself!

[Custom/Ten]

Well that got a little more complicated than I had planned but after a half bottle of tranny fluid.

[Custom/Ten]

Got a nice steady idle at about 600 rpm and over 20in of vacuum
I think it's good to go.thanks for the help!

[geezer#99]

Tranny fluid??
What tranny fluid??

[geezer#99]

Go easy with that stuff!!
It's a detergent and will clean out a lot more than you might like. It loosens stuff but won't make it finer. Little chunks of carbon can get in your oiling passages.
Use water instead.
It's free and it turns to steam in the chamber which scours the carbon out the pipe.

[Swar]

I second that, geezer. Water works great, it will steam clean and remove carbon off the head and pistons.

[Custom/Ten]

Ok thanks geezer, won't use it again! Just went out for a drive and it runs great! So smooth and quiet I almost thought it died at the first light till I looked at my tach and it was just sitting at 600. No more rattle in the shifter. This thing has been getting better and better since I got it but this was like a huge leap forward. I was misunderstanding how to tune the carb before but it runs so good right now. This is great, thanks a bunch man, your explanation really cleared some things up for me!

[geezer#99]

You guys should google 'how a carburetor works'.
It's good stuff to know.

Now get out there and waste some tires!!