Running rich, small jets

[Cheapsnake]

351w with a mild build, Holley 650 DP, runs great, plenty of power, idle mixture set with vacuum gauge. Everything sounds great (except for the lousy mileage, 15 mpg), but A/F reads consistently in the mid 15's, black spark plugs and sooty sidepipes tell me I'm rich.

I'm running the smallest jets Holley offers for this carb, 64 primary, 65 secondary so I'm at a loss as to how to lean this thing out. Any thoughts?

[edhunter]

Did you buy the carb new, or is is secondhand?? Reason I ask is I bought a used 650 carb, and found the PV channels had been drilled out. So when the power valve opened the car ran extremely rich, and I couldn't tune it out with the jets... I think my 351 with mighty demon 650 carb runs quite well on 68/76 jets - at least I think thats whats in it...

[Greg_M]

Originally Posted by Cheapsnake

Everything sounds great (except for the lousy mileage, 15 mpg), but A/F reads consistently in the mid 15's, black spark plugs and sooty sidepipes tell me I'm rich.

Are you saying your AFR is a super lean 15:1 and other evidence indicates you are rich? Is the AFR 15:1 at cruise and WOT?

Does it ever drop lower? What are you using to log the AFR? Something doesn't add up.

Greg

[Tennessee Tim]

I agree with Greg, something isn't adding up. An AFR in the 15s is lean, not rich.

Under what conditions are you reading the plugs? At best, plug reading gives you an indication of engine conditions for the 20 seconds or so prior to engine cut. If you were idling before engine cut, reading the plugs will only give you an indication of idle mixture.

Tim

[shearwave]

Check your power valves. The power valve setting should be 1/2 of the intake vacuum. If the vacuum is 13 at idle, you should have a 65 power valve. My engine at idle pulls 6.5 inches and had 65 power valves. It ran very rich. Changed to 25 power valves and saw immediate improvement.

James

[Cheapsnake]

My mistake, hit a 5 instead of a 3 for the A/F ratio, should be ~13:1. I guess one of the factors I ignored is the power valve. I'm going to pull the bowls tonight and check the size, along with vacuum reading. Thanks for the ideas guys and yes ed, this was a "rebuilt" carb so who knows what the "rebuilder" did to it.

Tom

[Greg_M]

Even 13:1 is not considered very rich. It's not even rich enough to make max power at WOT. If you have a rich PVRC or premature PV opening issue, it should show up somewhere in the AFR reading at some mode of operation. If you said my AFR is running 12:1 or less a lot of the time, that would defiantly cause sooty pipes and plugs.

Tom, are you using an LM1 or LM2 wideband logger or a narrowband AFR gauge.

Greg

[Cheapsnake]

Greg, it's an LM-1 wideband and when I say 13, that's a mean value, with WOT tending higher and cruise tending lower.

Tom

[emac]

FWIW, I had a carb on a 351W in a ski boat that started running rich, but for the most part performed ok. I rebuilt it three times. Once myself and twice by two different experienced engine builders. Set it up properly according to Holley. Never could get it to do right. Finally bought a new carb and all was well. Someone told me it could have been the metering block was clogged. That isnt replaced in a typical kit. Who knows. I was done tinkering with it.

On my roadster (351W) the electric choke was causing rich/fast idle problems. Put the fuse in the circuit (duh!) and it cleared up.

[CraigS]

...when I say 13, that's a mean value, with WOT tending higher and cruise tending lower...
Do you numerically? That would be a WOT of say 14 (leaner) and a cruise of say 12 (richer). That would be a really weird situation compared to mine.

[Cheapsnake]

I checked vacuum at idle and it's a bit over 15. The only numbers I could find on the power block are J, 7, 5, 6 - spread around the circumference. Translation?

[Greg_M]

Let's break this down into something that makes sense before you get into the carb. Idle vacuum at > 15" is good assuming idle speed is under 1000 rpm.

Can you give us the AFR readings at:

1. Idle (13 - 13.5)
2. Steady cruise 2500 to 3000 rpm (13.5 - 14.7)
3. Slight acceleration from 2500 to 4000 (13 - 14)
4. Hard accelleration from 2500 to 4000 (12.8 - 13.3)
5. WOT from 2000 to 5500 (12 - 12.8)

What is the lowest/highest AFR numbers you see and when do they occur. Can you post some plots.

If your power valve size or operation is an issue, it will show up some place in modes 1-5.

Greg

EDIT: Added some AFR targets for each of the 5 modes)

[Cheapsnake]

Dammit Greg, I knew you were going to get technical on me.

Unfortunately, I never plotted the ratio and with the carb apart it'll be tomorrow before I can reassemble and drive. In the meantime, are you saying an A/F ration in the 13's is not overly rich?

[Greg_M]

No it's not. It's a little lean for WOT and about right for light accelleration and idle. Anything under 12 is rich and under 10 is blowing black smoke. Anything in the 13.5 - 14.7 range is good for cruise. Above 15 and the car will likely sputter from being too lean.

That's not technical . The LM1 is a great tool and will reveal all the carbs dirty little secrets.

Greg

[Cheapsnake]

Originally Posted by Greg_M

No it's not. It's a little lean for WOT and about right for light accelleration and idle.

Greg

OK, so now I feel better about my A/F ratio, but still have crappy mpg.

BTW, great looking TR6 Greg. I had number 239 off the assembly line.

Tom

[Greg_M]

Thanks Tom. WOW 239 must have been a 1969. Do you remember the commission number? If not I might be able to do the math and find out if your old car is still around.

15 mpg is not bad if you are ripping it around town. What engine do you have? You could be at 19 - 23 on the hiway in a 302 or 347.

EDIT:
Tom - assuming your TR6 was a carb car not a PI car, your comm number would be CC25242-L. The only car known around that build number is a Damson CC25246-L in KC Kansas.

Greg

[Cheapsnake]

Yes, it was a '69. In fact, I ordered a TR250 from Viet Nam before I even knew there was such a thing as a TR6 in the works. When they informed me that I was getting a TR6 I didn't even know what it looked like. In the end, I'm glad it worked out that way cuz it was a pretty decent car and you can thank us early guys for working out some of the bugs. It's doubtful that it's still around since I used it as my DD year round and Wisconsin winters (salt) took their toll. Last I saw of it, it was some kid's hot rod. RIP.

Now, back to the subject at hand. Holley ID'd the PV as a 6.5. With 15+ vacuum at idle it appears I'm running a bit too much PV - enough to matter?. The engine is a 351w.

Tom

[Greg_M]

Actually a 6.5 PV is OK for your vacuum rating. It delays enrichment until you get into the throttle good and has no effect at idle or cruise in your case.

My TR6 was purchased by a USAF captain and he kept it until he was a colonel.

Greg

[SwiftDB4]

Maybe just a side issue, but do you have an MSD ignition? They're notorious for burning out the center electrode on the distributor. My Roush 427 ran really rich until I discovered this. Rotor needs to be bent up to contact the electrode fully.

[Cheapsnake]

Originally Posted by SwiftDB4

Maybe just a side issue, but do you have an MSD ignition? They're notorious for burning out the center electrode on the distributor. My Roush 427 ran really rich until I discovered this. Rotor needs to be bent up to contact the electrode fully.

Funny you should mention because my center electrode on the cap was burned up, but I replaced the cap, rotor and wires this spring so that should take them out of suspicion.

Tom

[belkbmw]

With same size, can't hurt. I know Holley claims they have a blow-out proof circuit for the PV now, but I think they also go bad sometimes. Dunno why. I had a bad one once, just like having the wrong size one. When they go bad, they dump LOTS of fuel, all the time. Could that be why it has small jets in it? DPs pump more fuel everytime you move the pedal too.

If its been rebuilt, has someone goosed up the acc. pump cams or set them up tight for 'extra' fuel on acceleration? They may have doen that to overcome the smallish jets, which may be to compensate for a bad power valve? Don't remember all the adj. on accel. pumps, but its in the Holley manual. jus' sumpin' ellis to look at.

It's all connected, and changing one thing can mess up another thing, leading someone to try to 'compensate' with a third change. I guess just try to make sure everything is close to factory settings and working correctly. That should get you pretty close most times.

JB

[kevin12973]

I had the same rich problem. Went down to 65 primary and 69 secondary an still 12.5 afr.at cruise. Then decided to swap the stock FRP single plane intake and install a performer rpm dual plane and went back to 72 primary and 75 secondary and voala.....14.5 afr at cruise and 12.5 at wot.
I was told that single plane intake is not producing a good signal at lower rpm's (under 3,500) therefore not allowing the carbs metering to function well.
I love the new intake, idles much better too.