Demon too rich at idle?

[GTO Geoff]

Using vacuum on it's own as an indicator as to whether more idle timing is needed is not a good idea & leads to false conclusions with low vacuum cams.
Two reasons for this. When you get to low vac situations such as 7", the vac is never steady, nature of the beast. More timing is more than likely to help here, but the increase in vac will be incremental & hard to see on a fluctuating gauge.
You think 25* of timing at idle is a lot? Pontiacs were driven from showroom floors, low duration smooth idling cam, with a steady 18" of idle vacuum....& 26* idle timing. yes, 26*, not a misprint. Chevs were not far behind....
The LS engines......with very good, compact tight combustion chambers that only need about 28* @ WOT. They idle at 22*. Production engines.
See a trend here?

The best way to test for how much timing the engine wants is to advance the timing [ turn the dist ] with the engine idling....until you get the highest rpm....which will coincide with the smoothest idle & highest vac obtainable. You can use a digital tach to determine max rpm, but I find sight/feel is better because with low vac you get a rough, rolling idle & tachs are not much help.

[Right hand drive]

[bigblockmark]

Thanks again for all the ideas. I have been absent for a day going to a Nostalgia race at Byron Dragway. Great day of seeing all the old gassers.

I put the carb back together with all new parts after sonic cleaning. did not get to try the .033" IFR due to my drill bits breaking. I use a block of aluminum drilled and tapped for 10-32, 8-32, 6-32 set screws.never had any trouble making 10-32 air bleeds or transfer slot restrictors in larger sizes.

Put the carb back on just before going to the race. Only had time to start it briefly. The original issue it was brought to me for was it would not stay running cold and had to constantly work the throttle pedal to keep it running.Adding fuel with the accelerator pump or it would die. Until I took the carb off and rebuilt it, I had not seen that at all. Just the low 7" vacuum at idle and the normal cold start not very good response until it warmed up. But it drove fine once warm including full throttle and part throttle. Now it is totally different. It is like what was described. Will not stay running at all unless constantly working the throttle pedal. So now I am thinking big vacuum leak. It will be tomorrow before I get to retrace my work to see how I accidentally reintroduced this symptom. this engine does have a 1" open spacer and a nitrous plate on it. I did replace. the original 3 gaskets between them that were really stuck tightly together. I am going to temporarily eliminate those to start, maybe warped.

[bigblockmark]

Took out the 1" open spacer, NOS plate and gaskets. Bolted carb back on and now it good at 1000 RPM after warm. Not so much when cold.

Still only 7" vacuum at idle. Increasing initial timing does raises RPM but barely effects vacuum reading. I crutched it at 30* initial timing and then hooked distributor up to manifold vacuum. Better but sure not correct. A/F screws are not consistent on adjustment. E Z Idle does have some effect. Going to look at valve adjustment tomorrow, just to be sure it is correct. Then I don't know what else to change from the outside.

Lowered compression and still wild cam may be not going to let this end as good as it should?

[GTO Geoff]

If a vacuum leak has been eliminated as the cause of the low 7" of vacuum, then it sounds like cam duration/overlap is the cause.

The reason your idle rpm increased going to 30* initial is very simple: the engine is making more HP!!! It likes the extra timing, as I knew it would.

Connecting vac adv to manifold vac should increase timing at idle. I say should. Because with only 7" of vac, only adj VA units adjusted to the softest setting will work with vac this low. You should check with a timing light that the timing is actually advancing when you connect MVA. It might not be, or the timing might be fluctuating. Also, most adj vac units add about 30* of timing. A total of 60* at idle would probably be too much. 53* is the most I have used.

Doubt that a 0.033" IFR is going to work with 7" of vac, will need a lot of more, closer to 0.040".

[Right hand drive]

[bigblockmark]

I came to a point where nothing I tried really made any difference to the better side. The 7" of vacuum at idle seems to be the stumbling block. I did a compression test and the first compression stroke is 90# climbing to 125# maximum. This is too low by my standards. I found that there were lower compression pistons installed preparing for using a Procharger. The grumpy cam was removed at that time and a Procharger friendly cam installed. Shortly there after the new cam had a lifter bar link fail and wiped out the cam. For some reason the original grumpy cam was reinstalled. The owner dies in a high speed crash and his Mom has had the truck ever since, not driving it. She does not know details. Friends or the original owner have contributed what I know to this point. Other friends have tried to work on it for the mom until it found its way to me. At this point it is difficult to start and keep running for less than 5 minutes and then pretty tolerable after that. Will start warm with just the turn of the key. She wants to sell it and not invest any more money into it. Short of going inside the engine, I think it is what it is.

Thank you again for offering the good suggestions and info.

[rgalajda]

Wouldn't 170 lbs compression be considered low?

[bigblockmark]

My BBC 582 pump gas 10.5 static compression drag race engine only has 160# compression. My BBC 565 14.7 static compression has 245# compression. Combination of compression, camshaft overlap, and ring seal can be all over the place, I guess. I would be happy with 110# first compression stroke and 150# highest. I think that alone would get this engine to be happy, be more efficient, and faster. Just guessing because I do not know the compression ratio or the cam events.

[RFSPHPbb]

intake closing point rules here. Static CR comes in second.