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Hi guys. I have a tunnel ram with a single 4781 Holley 850 DP on my 355ci. I can't get the idle mix screws to do anything. The transfer slots are NOT over exposed. I used the secondaries to adjust the idle so the transfer slots had the right amount expossed. I have about 5"-6"HG at idle in gear. so I have 2.5powervalve in both sides(4corner idle mix). The cam is [email protected] duration. single pattern. Here is the cam card. The drivers side does a little when turned in all the way, but wont kill the motor, the passenger side does nothing at all. I checked for a vacuum leak around the carb base and the intake runners, that's all good. I know there was a recent thread on this, that's where I got most of the info on setting the carb up.

If there was a vacuum leak under the intake in the lifter valley would it still run like crap? And, how would I be able to tell if this is happening?

Thanks for any help.
 

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years ago i had a problem like that, (but not on a tunnelram) with a 750 holley with the same vaccum reading and pv as yours, i even adjusted my secondaries but had to finally drill a small hole in the down side of the primary throttle blades to get idle and mixture adjustability back and had close the secondaries. sorry cant forsure remember the size of the hole, i think it was 3/32.
 

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I used the secondaries to adjust the idle so the transfer slots had the right amount exposed.
Do you have all 4 slots at .020? If the secondary slots are overexposed the mixture will bypass the mixture screws and give the symptoms you describe. If a 4 corner idle carb is set up right you can close the front screws and open the rear screws and get it to idle, and then close the rear screws and open the front screws and get it to idle. The front and back circuits are identical. DP idle circuits are usually too rich so you may need to restrict the IFRs.
 

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Discussion Starter #4
Do you have all 4 slots at .020? If the secondary slots are overexposed the mixture will bypass the mixture screws and give the symptoms you describe. If a 4 corner idle carb is set up right you can close the front screws and open the rear screws and get it to idle, and then close the rear screws and open the front screws and get it to idle. The front and back circuits are identical. DP idle circuits are usually too rich so you may need to restrict the IFRs.
The transfer slots are no more than .020 on either side. I'm not sure I know what IFR stands for. I've seen it alot, but cant figure out what it stands for.:confused:
 

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Idle Feed Restriction. #25 in this pic. http://i18.tinypic.com/61klx5s.jpg It is like the main jet for the idle circuit. To reduce the size of it you can insert a 'v' shaped piece of wire into the hole marked #25. The other leg of the 'v' goes in hole #24 to hold the wire in place. Try a wire about .018 dia. This works good if your mixture screws can only be out 1/4 to 1/2 turn without going too rich. You restrict the IFR and then you'll need 3/4 to 1 turn out to get the same mixture. This also leans down the transfer slot fuel. You may have other issues though. If the slot exposure is good you should be able to kill the motor by turning in all 4 mixture screws. If you have a blown power valve (hole in the diaphragm) it will suck fuel thru the vacuum feed passage (hole in bottom of cavity #18 in picture) and into the manifold. It's getting fuel from somewhere else if mixture screws won't kill it. Transfer slots, blown power valve, float level way to high. Is fuel coming out of boosters?
 

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From what I understand the single 4 tunnel rams don't work very well.

With that said your vacuum is way to low. That cam is not that radical to have vacuum that low. The voodoo 60104 in our 355 has slightly less duration than your cam and pulls 10 to 11" of vacuum.

I honestly wouldnt think you would need to mess with the idle feed restricion on a standard single 850. People tend to get into that when running 2 carbs but not when running a single carb.


http://www.youtube.com/watch?v=W9AmbTx1W0k
 

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Discussion Starter #8
Thanks for the answers guys.

I was out in the garage and decided to put my old 3310-4 Holley 750 vac. sec; on and I had drilled holes in the front throttle plates a long time ago, got the motor up to temp and I was able to kill the motor on either side of the barb with the mix screws. Does this help at all?

I will try the wire thing. I would run the 3310 but I think I should use a carb with Mech, sec????

I should mention the Holley 4781 I would like to use, is an Ebay item... Who knows what could be wrong with it.

What do you think about locking my timing at 36-38* I've read that Idle and throttle response is killer with the timing locked out?
 

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Discussion Starter #9
From what I understand the single 4 tunnel rams don't work very well.

With that said your vacuum is way to low. That cam is not that radical to have vacuum that low.

I honestly wouldnt think you would need to mess with the idle feed restricion on a standard single 850. People tend to get into that when running 2 carbs but not when running a single carb.
i met a Nova owner from Minot ND who runs a single t-ram on his 350 and his runs awesome. I've had so may different intakes on that motor and the vacuum is always between 5 and 6 inches?:confused::confused:
 

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I would run the 3310 but I think I should use a carb with Mech, sec????
Why? If it works better, use it.
I run locked out timing on mine, no problem.
I don't see why a cam that size would have such low vacuum.
 

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Discussion Starter #11
Why? If it works better, use it.
I run locked out timing on mine, no problem.
I don't see why a cam that size would have such low vacuum.
That's what everyone says, but that's what it runs at. I thought my guage might have been bad so I picked up a quality one, and it reads the same numbers......IDK???
 

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Good idea trying another carb. Ebay carb is a crapshoot. Hope nobody drilled anything. Have you opened it up to check jetting, power valve and gasket condition? Could just need a freshening up.
 

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i've never

heard of too many rave reviews of single 4 barrel tunnel rams either , but i've never tried one myself so i can't speak from first hand experience. I've " heard " you get terrible fuel distribution , but if your plugs are all looking the same , ya don't know till u try it.

Did you use a timing chain set that advances the cam timing 4 degrees ?? Most comp cams need this, IF you didn't degree the cam. This might be why your a little low on vacume at idle . ( what RPM are you getting this vacume reading ? ) What timing chain did you use ???

Give us some more info on your combination. 850 4781 will work with alot of compression and converter/gear set up with a cam thats gonna see the high side of 8000 rpm IMO. I think i'd stay with the 750 VS with the cam you have. They do better than alot of people give them credit for i think.

The idle air/fuel ratio is more rich with the 850 than the 750 VS. Could be why your experiencing this.

Did you degree the cam in ??
 

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Discussion Starter #14
Good idea trying another carb. Ebay carb is a crapshoot. Hope nobody drilled anything. Have you opened it up to check jetting, power valve and gasket condition? Could just need a freshening up.
Everything looks ok. I've had the carb apart sevral times and checked and cleaned all th passeges. I asked the guy if it had been drilled anywhere and he said he didn't do any drilling. He said his friend bought it new and then decided to go EFI. Who knows???

heard of too many rave reviews of single 4 barrel tunnel rams either , but i've never tried one myself so i can't speak from first hand experience. I've " heard " you get terrible fuel distribution , but if your plugs are all looking the same , ya don't know till u try it.

Did you use a timing chain set that advances the cam timing 4 degrees ?? Most comp cams need this, IF you didn't degree the cam. This might be why your a little low on vacume at idle . ( what RPM are you getting this vacume reading ? ) What timing chain did you use ???

Give us some more info on your combination. 850 4781 will work with alot of compression and converter/gear set up with a cam thats gonna see the high side of 8000 rpm IMO. I think i'd stay with the 750 VS with the cam you have. They do better than alot of people give them credit for i think.

The idle air/fuel ratio is more rich with the 850 than the 750 VS. Could be why your experiencing this.

Did you degree the cam in ??
I did degree the cam in, but the way the numbers worked out, it ended up being installed 4* retarded.

355ci TFS aluminum heads, compression is 11.47:1 th350 with a 5000 stall converter. 4.56 gear and 28" tall slick. I shift at 7500rpm.
 

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hmm

well having to retard one of those cams to hit the 106 ICL seems a little odd , but it looks like its running very well considering the weight of a 73 and it being a 355 with only a street/strip cam. Kudos :yes:

i'd take the 850 apart , make sure its got the right metering blocks , things like that.

your showing a 900 ft elevation for Fargo , do you know the correction to sea level for your track ???
 

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Discussion Starter #16
well having to retard one of those cams to hit the 106 ICL seems a little odd , but it looks like its running very well considering the weight of a 73 and it being a 355 with only a street/strip cam. Kudos :yes:

i'd take the 850 apart , make sure its got the right metering blocks , things like that.

your showing a 900 ft elevation for Fargo , do you know the correction to sea level for your track ???
I don't know the corrected alt. I was going to check the metering blocks but I gapped it out with everything that's happening.

Thanks for the help.
 

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Regarding poor idle and low vacuum at idle, what is your initial ignition timing? How much mechanical timing advance is built in to the distributor?

A 248 @ 0.050 cam is not too terribly big on a 355, but it could use more initial timing and a shorter advance curve, especially with a tunnel ram. On something like an MSD distributor, you can install a larger bushing to limit the mechanical advance curve. On a stock GM HEI or points distributor, you can shorten the advance slots by filling in with weld.

If you have something like, say 10 degrees initial and 36 total timing; and you went to 15 or 20 degrees intial (keeping total the same) it would really clean up the idle and help with throttle response.

Locked out timing is tough on the starter in a street- type car -unless you use a separate ignition switch like a race car would have. The earlier timing advance builds more cylinder pressure the starter has to overcome.
 

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Discussion Starter #18
Regarding poor idle and low vacuum at idle, what is your initial ignition timing? How much mechanical timing advance is built in to the distributor?

A 248 @ 0.050 cam is not too terribly big on a 355, but it could use more initial timing and a shorter advance curve, especially with a tunnel ram. On something like an MSD distributor, you can install a larger bushing to limit the mechanical advance curve. On a stock GM HEI or points distributor, you can shorten the advance slots by filling in with weld.

If you have something like, say 10 degrees initial and 36 total timing; and you went to 15 or 20 degrees intial (keeping total the same) it would really clean up the idle and help with throttle response.

Locked out timing is tough on the starter in a street- type car -unless you use a separate ignition switch like a race car would have. The earlier timing advance builds more cylinder pressure the starter has to overcome.
I'm running 16* with a total of 37* all in by 3000rpm. I have the MSD Pro-Billet Distributor Part No. 85551 and the MSD Digital 6 Plus Part number: 6520.It has a start retard built into it.

As far as the metering blocks go, I have found the Holley numerical listing guide, so I write the numbers down and go out to the garage only to find one four digit number on the metering blocks and NONE of the numbers match anything on the list for any of the 4781's. This is the kinda crap that makes me want to sell the whole works.
 
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