Horrendous Exhaust Fumes

[George Snyder]

My car's exhaust fumes are bad enough at idle to bring tears to your eyes.

Idle is at 800. Timing is 12 intial + 21. Intake vacuum is 15".

Carb is a Holley 650 DP with 64 & 67 jetting. Seems good.

I changed the Power valve from 6.5 to 10.5 and that helped some.

Increase initial timing?

Is this condition being caused by the short headers?

Any other suggestions or is this the nature of the animal.

Thanks,
Bud

[Tom Wells]

Hello Bud,

When my car went to the dyno it was doing the same thing. It was caused by the idle mixture being too lean!

I used to think the "tears" were caused by too rich an idle mixture. Not so.

When we richened the idle, the tears went away

Tom

[George Snyder]

Hi Tom,

I will try your suggestion. I too would have thought that a too rich idle would be causing the problem. Vacuum gauge was reading as high a steady reading as I could get.

I am really considering that the initial timing may be the problem. When they installed the engine, I was told that they set the initial at 12. I think it may need to be higher to assure that all the fuel is being burnt at idle.

Unfortunately my timing light does seem to be compatiable with the MSD 6A box.
Until I find time to get a new one, I may just try the old SBC method and just advance the timing until the engine won't start and back it off a little.
Thanks,

Bud

[Excaliber]

I don't think 12 base timing is to much myself, sounds about right.

Ernie

[Back in Black]

And I thought my idle mixture was too rich, although the screws seem to be barely at the standard "turns" out. So I am going to try turning them out a bit.

I have a question also, if anyone recognizes the symptom.. After my car sits for several hours or more, (esp-ecially overnight) then when started, some water comes out of the sidepipes at first.. not much, and it is moisture. I initially thought it was just condensation in the pipes and the metal NASCAR baffles, but now I am not so sure... could it be a head gasket ?

It comes out of both pipes equally... ?

[blykins]

Hey George....does it just do it idling? It may just have a lean idle circuit. Try changing the idle air bleeds. If I can remember correctly, if it's idling lean, you need to go with a smaller orifice.

[G.R.]

BnB
Mine does the same thing--my mechanic said it is very typical on most cars up here in the NW--just moisture condensation build up, heat and cold ,moist air--much more noticible with the bigger exhausts on the Cobra where your really notice it because it's right where you really see it--bet your other cars do the same, mine all do--just never really noticed it because I rarely pay attention to the exhaust after I start or shut down one of the other cars, noticed the moisture puddles right off with the Cobra and the steam from the residule moisture being burned off---
Our sidepipes probably won't last as long due to this, even S/S will start to rust eventually

But I'm not a mechanic so this FWIW

[turnpike boy]

Bud, FRPP told me use AT LEAST 14* - 16* at idle, and as much as 36* total. I've got mine at 16* and it still starts easy...total is as yet around 30* so I gots to get into the distrib and redo stop bushings. And, yes, mine stinks at idle too; and I do get some black sooty smoke out of the pipes when I blip the throttle. I got to believe mine, at least, is still rich at idle - haven't gone thru the carb fully as yet.

[George Snyder]

Well so much for the SBC method. It didn't work on a Ford! I guess that is why it's called the Chevy method. As much timing as you can as fast as you can just doesn't seem to apply to the new technology roller cams and heads.
The car started like there were no pistons in the block, but ran really bad. I backed it somewhere CLOSE to where it was originally. Probally should have marked that location!

I need to get a timing light that will work with the MSD box.

If it is truely the carb, maybe I should just let this Holley die a graceful death and buy a new Demon! A Speed Demon replaced this Holley on my other car. Sometimes you just have to let go.

Thanks to all that have posted so far, and if you think of anything else, post this is not a one shot and done topic.

Bud

[Jack21]

Any good timing light should work with MSD box.

That being said, Demon carb replacing Holley carb makes a world of difference! Replaced Holley 700DP (4778) w/ Demon 650DP, and couldn't believe it was the same engine (351W, TFS/TW heads, CompCams hydraulic roller cam & rockers, Ede Perf RPM, MSD dist, 6A box, & plug wires.) Idles smooth, runs smooth, & accelerates like mad!

[George Snyder]

While making a nuisance of myself at two of the local speed shops here is what I was told.
The base of the problem is the amount of overlap in the 303 cams.
The cams were designed to be used with EFI. They work well in this case because of two major conditions.
#1 the idle fuel is regulated by the computer thru readings from
the 02 sensor.
#2 The EFI has a much larger air plenum than the Edelbrock
intake I am using. The smaller plenumis causing unaccurate
signals to be sent to the idle circuit in the carb.
Solutions suggested.
#1 Retard cam by 4 degrees would help.
#2 Cut small pieces of a large paper clip and insert them into the
idle orfices? This will restrict their flow. Ok sounds logical?
I tried what the suggested. First a large and then a smaller one. No luck the engine refused to idle both times. I remove the pins and I am back where I started.
I have done everything suggested with the exception of changing the idle air bleeds? I have no idea what or where they are? Are they where I inserted the paper clip?
Thanks, but I need more help!!
Bud

[Bugger]

Check the position of the primary and secondary throttle blades. If they are too far open then the transition holes/slots are exposed and you are into the main vice idle circuit. If your cam is extremely large then drilling holes in the plates may be required to get the idle up while avoiding this condition.

[Mr.Fixit]

Look down the top of the carb, where the "boosters" hang out into the 4 bbls of the carb. Where they attach to the carb body, there are little holes that you can see from the top of the carb. Cover the outer two with your fingertips while the carb is idling, that will drastically richen the idle circuit. those are the idle air bleeds.

[George Snyder]

Thanks to all you Guys for staying with this post and helping me along. Update.
I am told by other guys that I should have lowered my Power Valve,not raised it. I started at 6.5, then changed to 10.5.(it was in the rebuild kit). They are suggesting that I should go with 4.5 or even 3.5? My concern going that low is what will it do to my throttle response under normal driving conditions?

The throttle plates on the secondaries are fully closed.
I adjusted the idle mixture screws to the highest vacuum.(14.5).
I started the engine and followed Mr fixit's suggestion. I had to use two pencils erasers, but achieved the same results. I have big meat hooks!
The engine began to indle EXTREMELY rough and the fumes were over whelming. It took starting the engine 3 or 4 times until it returned back to normal not closing off the air bleeds.

What now? Questions?

Does this mean that my idle circuit is lean or rich? If that richen it, I assume that i need to lean it?
What power valve do you guys suggest?
When I restricted the idle feed restrictor holes with the paper clips, I only did the primery ones. Should I have done the secondary ones also, or should I just bag the paper clip fix completely since the car refused to even attempt to idle?
I have been reading my Demon instruction manual also.
It states that the idle feed restrictors can be adjusted, but it doesn't explain how it can be done? Holleys the same?
It also states that the air bleeds can be adjusted also, but ONLY BY SOMEONE WITH A DEEP UNDERSTANDING OF CARBURATION!
Is that someone like me?
Take care and thanks again to all. I'll wait for your suggestions.
Bud

[DaveR]

I don't know how to solve your problem, but you might want to add an air/fuel mixture gauge to your car. That's what I plan to do because I expect to have similar issues to the ones you're having, and I don't want to be guessing. Gauges are $50 from Jegs, plus O2 sensor. Good Luck.

[SuperHart]

You know, I had almost the identical problem with my car, 351W and Holley #4777 650DP carb. Every time I drove the car I ended up smelling like rich exhaust fumes. Nothing I tried helped. Finally I swapped out the 650DP for a #1850 600 vacuum secondary carb and the problem seemed to markedly improve with no change in timing although I felt that performance suffered a little bit. A supposedly knowledgeable engine builder in my area told me that Holley double pumpers always seem to run on the rich side. I have also heard that Demon carbs give noticeably better performance then box stock Holleys too.

I am currently preparing a pair of Holley #8007 390 cfm carbs for use on a home made dual quad manifold for my 351W. I figured even they could be made better so I sent them out to a carb shop for tweaking. I have two friends who both have dual quad setups using the #8007 Holleys and both report that they are getting better performance, better mileage , and cleaner exhaust than with a single Holley of comparable cfm rating. Makes me wonder if the idle circuitry of the Holley DP carbs is the culprit.

[Hotfingrs]

You definately don't want a 10.5 PV.....I use only 2.5 in all my Holley applications. You only want the PV open at WOT...and 2.5 inches will take you well past the speed limit... rule of thumb if at end of quarter mile vacuum is over 1 inch carb is to small, if reading is zero, carb is to big....you want to shoot for a .5-1 inch reading at end