Vintage Mustang Forums banner

1 - 16 of 16 Posts

·
Registered
Joined
·
495 Posts
Discussion Starter #1
So I just got my new heads and cam installed. Trick flow, 11R 170s, with stage one cam. I am driving around and when I come to a stop the idle stays up around 1200 rpms for about 10 seconds and then slowly goes down to about 750 where I have it set. Any ideas why it's doing this. Its a Holley 600cfm carburetor, weiend street warrior dual plane, with a 1inch four hole spacer. I kind of think it might be a vacuum leak but wanted to see if any one else has any thoughts. With the new cam I get about 12 in of vacuum. Old stock cam was around 19.
 

·
Premium Member
Joined
·
9,522 Posts
You don't have a dashpot on the carb by any chance, do you?
Maybe the throttle linkage is binding up as it returns to idle.
Is your throttle return spring connected correctly? The throttle lever on the side of the carb moves in an arc as the throttle opens and closes. I have seen return springs connected in such a way that the spring moves "over center" as the lever travels in it's arc and the spring is no longer pulling on the lever at the end of the lever's travel.
 

·
Registered
Joined
·
495 Posts
Discussion Starter #3
No dashpot and I have dual return springs (little inside a big) connected to the correct return spring hole on the linkage. I didn't have the issue before. Same carb I have been running for months now.
 

·
Registered
Joined
·
1,714 Posts
Maybe something to do with this extreme heat we've been having here in the South? Mine does the same thing sometimes.
 

·
Registered
Joined
·
495 Posts
Discussion Starter #5
I have been running this carb all summer. Temps well over 100 degrees here in San Antonio. This is the first time I have had an issue with it. I sprayed carb cleaner around the carb and seems like driver's side base and vacuum secondary has leaks.
 

·
Registered
Joined
·
2 Posts
Texas Heat Idle Issue

Really interesting thread. I’m somewhat new to the forum, don’t know jack about engines, and purchased my 69 4 speed R code Mach 1 428CJ w/shaker back in January. I have run into a similar problem, but I think a bit worse. Parked in a Houston stadium lot one day with very high heat, and a football game later came out to head home. Car started right up, but then would idle very low and shut down. Using a combination of choke, gas pedal, and gliding/starting up again I made my way home. I figured it was the extreme heat so put it in the garage for a week or so, and then tried to drive around again - same story. Have to keep it revving high to continue running, otherwise it idles low and shuts off. Would appreciate any insights and/or advice! Thanks!
 

·
Registered
Joined
·
244 Posts
No single answer for everyone’s problems, but it could be that the extreme heat that a lot of us have been in for the past couple months, could be causing fuel percolation in the carburetor. Another idea might be that fuel pressure is too high, or slightly oversized jets , that only dump too much fuels when you have the engine very hot. Sometimes the flyweight springs (mechanical advance) in the distributor are not strong enough to pull the timing in quickly. Of course, vacuum leaks and that sort of stuff, are always a likely culprit, and some of the stuff that I mentioned would not apply if the engine ran fine before the heat , and if you haven’t changed anything as far as jetting or springs in the distributor- Just a few things to be considered.
 

·
Registered
Joined
·
495 Posts
Discussion Starter #9
So I was thinking about what @indygta was saying and I did some research online. I think that I have it figured out. My distributor was set up for my old heads and carb, despite what many said before I installed this cam, I didn't lose bottom end power. I feel like I actually gained some low end. So now at idle my engine has a more torque than before and the mechanical advance is staying engaged for a little bit. I verified this with my timing light tonight. I have base timing set at 18 degrees. When I rev the engine and then let off it settles at 20 for a little bit before going down to 18. That is enough to bump the idle speed 200-300 rpms. I will adjust the springs on the advance tomorrow to give a little more tension and see how that works. I briefly considered the power valve as the culprit. But from my understanding the power valve doesn't affect idle speed unless it's blown.
 

·
Registered
Joined
·
244 Posts
Correct! Most (If not all) resources pertaining to tuning a Holley say that when the corner idle screws are adjusted, and seem to make no difference, the next step is to look for a blown power valve. However, the newer style power valves are not near as likely to be blown as the old power valves were. There are several informative and complete Holley tuning videos on YouTube.
 

·
Registered
Joined
·
495 Posts
Discussion Starter #11
So I am trying to diagnose the idle in my car. Switched carburetor back to my demon. That seems to help some. But when I spray carb cleaner or starting fluid along the intake gasket on the passenger side of the engine the idle stumbles. I thought that if it was a vacuum leak that the idle should surge a little because it sucks in the fluid and burns it. Only vacuum connections on the carb are the PCV and vacuum advance. Those don't seem to be the culprit the lines are practically new and not cracked.
 

·
Registered
Joined
·
495 Posts
Discussion Starter #12
I think I am past any vacuum leaks now. However I had to switch out my Holley 4160. It wasn't sealing well along the base plate. I went back to my street demon 625. For the most part it's ok but I am really trying to dial the idle in. I have read that the transfer slots need just about . 020 showing. Which usually equates to a small square. When I have it set like that the idle does want to come down. It will idle at around 950. I can back the idle set screw out but then the transfer slots get covered up by the throttle blades. Or I can screw the idle mixture screws in but I lose vacuum and that defeats the purpose of the screws. Is it imperative that the transfer slots are exposed? What is the exact reason they are exposed? I have read a lot of conflicting information about them.
 

·
Registered
Joined
·
495 Posts
Discussion Starter #14
Try fine tuning your secondary idle setting.
I don't think that this carb has a secondary idle. I went ahead and turned the idle set screw back a little and am idling at 750 now. Seems to be idling fine. Will run it for a few days and see how it performs.
 

·
Registered
Joined
·
113 Posts
I have a stage 1 TFS cam for a gt40 explorer block I am building. Can you comment on gears you're running and low speed behaviour of the cam? Like parking lot speeds? Do you have an automatic or manual? I couldnt find many accounts online of people running this cam carbed and the couple I did find were asking for help with low speed bucking issues. Thanks
 

·
Registered
Joined
·
495 Posts
Discussion Starter #16
I have a stage 1 TFS cam for a gt40 explorer block I am building. Can you comment on gears you're running and low speed behaviour of the cam? Like parking lot speeds? Do you have an automatic or manual? I couldnt find many accounts online of people running this cam carbed and the couple I did find were asking for help with low speed bucking issues. Thanks
It actually handles very well at low speeds. I don't notice any bucking at all. I am running a T5 with the stock 2.80 gear ratio.
 
1 - 16 of 16 Posts
Top