[DeTomaso] Carb question

[Mikael]

OK, I'm late to this party, but here goes: Buy my book :-) Sorry, couldn't refuse...

And now to the useful part of this mail:
The problem you have seem to indicate a very weak/incorrect idle situation. Engines with 200+ HP should not bog down because of A/C use. You should either feel a very slight decrease in rpm or none at all. The solenoid solution is compensating for the problem instead of correcting it, I suggest to not spend time on that. Solenoids were introduced back when idle for environmental reasons had to be set too lean, for non-pollution controlled engines they're not needed.

Back to your engine:
1. Maybe I've missed it, is your 750 vacuum or double pumper? If it's the latter, you'll need a good 4 corner idle, harder to get to, but a lot better
2. What's your vacuum at idle? And is it steady (*)? If you don't have a vacuum gauge, buy one. Must be connected to manifold vacuum, not ported vacuum
3. After idling a while, how do your plugs look? Are all 8 the same? If not, probably a vacuum leak near the lean looking plugs. If yes, do they look lean or rich? 

If you answer 1-3, I'll try and help diagnose per mail? Up to you... :-)

(My engine is a 408 stroker with a wild cam, only 8inHg at vacuum. But a nice 4 corner idle @ 1000 rpm. When A/C is in use, there's no decrease in rpm that I can feel. So it can be done, if your engine is working properly and adjusted correctly)

(*):
>From my book:
Diagnosing/troubleshooting
As mentioned, a lot of different faults can be found with a vacuum gauge, because the behavior of the needle is different from one fault to another. Here are the seven most common scenarios (engine idling and warm):
 
1.	Steady needle, at 15-22 inHg: Engine in good condition. (An engine with long-duration camshaft will have a lower vacuum and a less steady needle, maybe 10-15 inHg with a few “jerks”. That’s still an engine in good condition)

2.	Steady needle, lower than normal: Ignition timing too late or vacuum leak

3.	Steady needle like in 1. but occasionally fluctuates at idle: Ignition miss or sticking valve 

4.	Steady needle like in 1. but drops regularly: Valves need adjustment or burnt valve 

5.	Floating needle, maybe in the range of 10-20 inHg: Carb out of adjustment or vacuum leakage 

6.	Shaking needle, becomes worse as rpm increases: Ignition miss, blown head gasket 

7.	Shaking needle, becomes steady as rpm increases: Worn valve guides

--

Hope this helps

Mikael
Tuning Made Easy, available at amazon.com

-----Oprindelig meddelelse-----
Fra: shawkins777 at comcast.net [mailto:shawkins777 at comcast.net] 
Sendt: 5. august 2012 11:48
Til: Christopher Kimball
Cc: detomaso at realbig.com
Emne: Re: [DeTomaso] Carb question



There are solenoids available that will raise the rpm when the compressor turns on. 



Steve 



----- Original Message -----


From: "Christopher Kimball" <chrisvkimball at msn.com>
To: detomaso at realbig.com
Sent: Saturday, August 4, 2012 2:55:37 PM
Subject: [DeTomaso] Carb question 

I have a 408 stroker with a Holley 750.  When I try to set the idle, I seem to have only two choices.  1) set the idle too fast (1800 rpms) so that when I turn on the air conditioner, the car won't die (it goes to about 1100 rpms with the a/c on), or 2) set the idle for 1100 without the a/c, but when I turn on the a/c it drops down to about 500 rpms and is at risk of dying when I let out the clutch. 

I've tried adjusting the idle screw on the carb in minuscule increments, but no luck getting better results. 

Suggestions? 

Thanks, 

Chris
3846
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