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XW GS Fairmont
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47 Posts
Discussion Starter #1
Just got a recoed 750DP Holley last week which replaced a 750VS Holley. However, it is running shite!! Any help with tuning would be appreciated.

Prior to installing I swapped the 3.5" power valve out of the VS into the DP. I didn't touch the jets (70's in the primaries of the VS and 75's in the DP). I can't get the idle down any lower than 1100rpm and even then it gets a stumble and stalls (with the VS it idled nicely at 850-900rpm). THe vacuum at idle (1100rpm) is 10inHg. The timing at idle(1100rpm) is 18 BTDC (I have bumped it up from 16 BTDC). Adjusting the idle mixture screws to maximise the vacuum and then repeating to lower the idle and it still won't play ball!!

When taking off it also has a massive bog from idle. It's lucky I have a manual and can slip the clutch somewhat. I moved the accelerator pump cam from the 1 position to 2 because the idle is so high ,but hasn't made much difference.

I think I have gone so far along I don't know what to do next?
Is it a vacuum leak? If so, how to find it?
Does the bigger primary jets (75 vs. 70) effect the idle ?(I don't think so)
 

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330 Posts
Sounds like vacuum leak. But have you checked float levels just to be sure. Float not stuck?
Needle and seat could have shite in it.
Vacuum hose left off, or split is good start point.
 

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XW GS Fairmont
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47 Posts
Discussion Starter #3
float levels are ok.
I've pulled the primary needle and seat out and cleaned.
There are no fuel drips into the carby at idle (the VS used to do this occaisionally).
 

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Registered
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397 Posts
it does sound like a vacumn leak, however check the throttle is closed and the idle transfer slots are not uncovered. Also if it has a choke check the fast idle screw isn't wound in. The mains are seperate from the idle circut, why did you richen up the primarys so much? The 3.5 p/v would only be used if you have a very radical camshaft with very low vac at idle
 

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V8 man
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73 Posts
shard351 said:
Just got a recoed 750DP Holley last week which replaced a 750VS Holley. However, it is running shite!! Any help with tuning would be appreciated.

Prior to installing I swapped the 3.5" power valve out of the VS into the DP. I didn't touch the jets (70's in the primaries of the VS and 75's in the DP). I can't get the idle down any lower than 1100rpm and even then it gets a stumble and stalls (with the VS it idled nicely at 850-900rpm). THe vacuum at idle (1100rpm) is 10inHg. The timing at idle(1100rpm) is 18 BTDC (I have bumped it up from 16 BTDC). Adjusting the idle mixture screws to maximise the vacuum and then repeating to lower the idle and it still won't play ball!!

When taking off it also has a massive bog from idle. It's lucky I have a manual and can slip the clutch somewhat. I moved the accelerator pump cam from the 1 position to 2 because the idle is so high ,but hasn't made much difference.

I think I have gone so far along I don't know what to do next?
Is it a vacuum leak? If so, how to find it?
Does the bigger primary jets (75 vs. 70) effect the idle ?(I don't think so)
Could be a vacuum leak but could be a block in one of the idle circuits...Take your idle mixture screws out and blow the passages out with an air compressor.I would use a 6.5 power valve with that much idle vacuum...if you go to low with your power valve you can get off idle stumbles as well.If its one of the off idle circuits blocked and it does happen,you will have to pull the metering block off and remove the little welsch plugs from the top and blow the circuits out with compressed air...Believe me if there is any junk in there no matter how small it will give you hours of grief...Try all easy options first though,like vacuum leaks! :hy: :driver:
 

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XW GS Fairmont
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47 Posts
Discussion Starter #7
Well I've replaced the carby gasket and nipped it up nice and tight. I also replaced the rubber vacuum blanks, and the cracked hose to the brake booster. Still no joy with the stalling at idle :nothpy: .

I have adjusted the accelerator pump better - no bog on take off now.

The carby came with the 75 jets already installed - just too lazy to change them. Swapped them back for the 70's from the VS today.

One thing I noticed today was when I tried to hook up the vacuum gauge to the vacuum port the PCV hooks to at the bottom rear of the carby. The idle immediately dropped and stalled. Hooking back to the PCV made it idle better.
Could it be a faulty PCV valve?
 

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V8 man
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73 Posts
shard351 said:
Well I've replaced the carby gasket and nipped it up nice and tight. I also replaced the rubber vacuum blanks, and the cracked hose to the brake booster. Still no joy with the stalling at idle :nothpy: .

I have adjusted the accelerator pump better - no bog on take off now.

The carby came with the 75 jets already installed - just too lazy to change them. Swapped them back for the 70's from the VS today.

One thing I noticed today was when I tried to hook up the vacuum gauge to the vacuum port the PCV hooks to at the bottom rear of the carby. The idle immediately dropped and stalled. Hooking back to the PCV made it idle better.
Could it be a faulty PCV valve?
No thats common cause the engine runs as lean as hell without the pcv hooked up and they will usually run rough or stall.Incidentally no matter what you read in books,setting your idle mixtures to the highest vacuum reading at idle never works....Find the best idle vacuum setting and then turn the mixture screws at least another quarter turn each..most common problem with idle and off idle problems are lean mixtures. :priest:
 

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XW GS Fairmont
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47 Posts
Discussion Starter #9
found the culprit!!

The carby and vacuum issues are ok. The Distributor is knackered - too much play causing the spark to die and hence stall at idle.

Time to go shopping. Whats the best dissy these days??
 

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under thumb
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286 Posts
Depends on your budget. $500+for msd/crane ready to run or $200ish for a hei type! Just check out just parts mag and you will find what you need
 
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