Read 'em and weep. Carb tuning issue - Chevelle Tech
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post #1 of 26 (permalink) Old Jul 14th, 20, 4:34 PM Thread Starter
Steve
 
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Read 'em and weep. Carb tuning issue

Thanks in advance for reading

Recently purchased '65 crew cab malibu. Has 383 stroker with a QF 750 vacuum secondaries. PO told me little about the engine other than it had a Skip White rotating assembly with ProMaxx heads. I do not know the cam in this. The car ran great when I test drove it with the execption of a bad shifting Th350 tranny. Price was right so I took the chance and bought it. 2-weeks later I blew the tranny to pieces at 6500rpm fooling around. Had a local tranny shop build me a new tranny set up for daily driver use. The old trans apparently had a 3400 stall converter and my new trans is 2500 stall.

The issues I am having with the engine is getting it to idle in gear without having to have the curb idle at 1200rpm. the RPM drop is almost 500-600rpm when put into gear. As you can see below, I am also having an issue with plugs fouling like crazy. Plugs are in order with my finger at the 1 and 2 cylinder. Fouling is bad up front and good toward the back.

Current specs

- I have triple verified that the carb transition slots are little squares in both primary and secondaries.
- I have confirmed the the idle mixture screws have some authority and will stall the engine if turn all the way in.
- Initial advance is 22 degrees, mechanical advance is 10 degrees, 32 degrees total, vacuum advance is not used.
- Carb pulls 11-12 in/hg of vacuum at 1000rpm and when I put it into gear it drops to 5 in/hg at 500-550ish . Idle is quite choppy.
- I currently have a 2.5 power valve installed.
- 6 psi to carb at idle, 5 psi at 3000+rpm on mechanical fuel pump. (converting it now to electric pump)
- Carb has been rebuilt and every orfice chased clear.
- I do not have an AFR gauge.

Other observations.
- For what its worth, jets are 74 in primaries and 82 in secondaries. Even with the above mentioned issues, once the car is driving in part throttle, there is zero bog or lag from part throttle to WOT-- this thing really rips!
- In chasing this problem for a few weeks the issue of drilling the butterflies has surfaced. I will admit to drilling out the primaries and secondary butterflies 1/16" which has improved the fouling problem with the rear cylinders. The drilling also is what helped the idle mixture screws achieve some control over idle vacuum. The drilling has not helped the big drop in rpm when put into gear. I am tempted yet hesitant to continue drilling larger holes.
- I am beginning to suspect that my cam may be too hairy for the converter.

So with that said, I think that I may have multiple issues at once here but I lack any further skill in figuring this out. Your input would be helpful.
Steve
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post #2 of 26 (permalink) Old Jul 14th, 20, 6:05 PM
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Sam
 
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First thing I would do is add vacuum advance. I believe you need more idle timing which the vacuum advance would supply and might help bring up the idle vacuum. I'd also check for vacuum leaks.
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post #3 of 26 (permalink) Old Jul 14th, 20, 6:16 PM
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Sam
 
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I should've said you may need more idle timing it depends on the engine really and what cam you have. I have 383 and my cam duration is in the 230 range. I have my initial at 20* with the vacuum advance adding 10* for 30* at idle. My engine doesn't like 22* at idle.
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post #4 of 26 (permalink) Old Jul 14th, 20, 6:27 PM
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Sam
 
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I'd get rid of the 2.5 power valve its to low in my opinion. I'm sure that's part of your rich condition. You want a pv rated for half your idle vacuum in gear. Once you get everything sorted out the vacuum shouldn't drop so much in gear. My engine idles at 12" of vacuum and drops to 10" in gear.
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post #5 of 26 (permalink) Old Jul 14th, 20, 6:42 PM
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pete
 
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Re: Read 'em and weep. Carb tuning issue

Quote:
Originally Posted by '65newb View Post
Thanks in advance for reading

Recently purchased '65 crew cab malibu. Has 383 stroker with a QF 750 vacuum secondaries. PO told me little about the engine other than it had a Skip White rotating assembly with ProMaxx heads. I do not know the cam in this. The car ran great when I test drove it with the execption of a bad shifting Th350 tranny. Price was right so I took the chance and bought it. 2-weeks later I blew the tranny to pieces at 6500rpm fooling around. Had a local tranny shop build me a new tranny set up for daily driver use. The old trans apparently had a 3400 stall converter and my new trans is 2500 stall.

The issues I am having with the engine is getting it to idle in gear without having to have the curb idle at 1200rpm. the RPM drop is almost 500-600rpm when put into gear. As you can see below, I am also having an issue with plugs fouling like crazy. Plugs are in order with my finger at the 1 and 2 cylinder. Fouling is bad up front and good toward the back.

Current specs

- I have triple verified that the carb transition slots are little squares in both primary and secondaries.
- I have confirmed the the idle mixture screws have some authority and will stall the engine if turn all the way in.
- Initial advance is 22 degrees, mechanical advance is 10 degrees, 32 degrees total, vacuum advance is not used.
- Carb pulls 11-12 in/hg of vacuum at 1000rpm and when I put it into gear it drops to 5 in/hg at 500-550ish . Idle is quite choppy.
- I currently have a 2.5 power valve installed.
- 6 psi to carb at idle, 5 psi at 3000+rpm on mechanical fuel pump. (converting it now to electric pump)
- Carb has been rebuilt and every orfice chased clear.
- I do not have an AFR gauge.

Other observations.
- For what its worth, jets are 74 in primaries and 82 in secondaries. Even with the above mentioned issues, once the car is driving in part throttle, there is zero bog or lag from part throttle to WOT-- this thing really rips!
- In chasing this problem for a few weeks the issue of drilling the butterflies has surfaced. I will admit to drilling out the primaries and secondary butterflies 1/16" which has improved the fouling problem with the rear cylinders. The drilling also is what helped the idle mixture screws achieve some control over idle vacuum. The drilling has not helped the big drop in rpm when put into gear. I am tempted yet hesitant to continue drilling larger holes.
- I am beginning to suspect that my cam may be too hairy for the converter.

So with that said, I think that I may have multiple issues at once here but I lack any further skill in figuring this out. Your input would be helpful.
Steve

This is more 'process' than specific 'fix'.

Recommend reading:

https://www.chevelles.com/forums/374...ion-101-a.html

Please pay particular attention to the 'initial', 'vacuum' and 'mechanical', advance discussions. They all need to correct individually and in combination.

Highly recommend getting ignition CORRECT (and KNOW it is CORRECT) before looking at carb or anything else.

Also recommend spending time reading posts in the 'ignition' section. You likely will find posts discussing issues similar to yours. Post by Dave Ray are recommended reading.

Just my initial opinion based on your data, your timing is off. Perhaps initial, vacuum, and mechanical.

Pete
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post #6 of 26 (permalink) Old Jul 14th, 20, 8:35 PM
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Jim
 
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Re: Read 'em and weep. Carb tuning issue

PV has no bearing on idle. Give it about 10 more deg. at idle just to see how it responds, don't drive it like that unless you limit the curve. If you add vac. advance it will have to stay fully activated when in gear.
Set the idle mixture in gear to get the highest vac.

Jim

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post #7 of 26 (permalink) Old Jul 14th, 20, 11:28 PM
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Re: Read 'em and weep. Carb tuning issue

If the QF is one of those Slayer 750's get rid of it, put a Brawler or an Avenger 750 on the engine. That QF needs to be completely re-calibrated. We had one and it was the the worst thing I have worked on in a long time. Burned your eyes and fouled plugs. Put an Avenger 770 on the car, ran perfect after adjusting the idle screws.
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post #8 of 26 (permalink) Old Jul 15th, 20, 12:39 AM
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Eric
 
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Re: Read 'em and weep. Carb tuning issue

Nobody noticed he went from good behavior with a 3400 stall, put a 2500 stall behind it and now complains of too much rpm drop at idle in gear??

It's a no-brainer in my opinion, classic parts mismatch now, you now don't have enough stall converter to go with the cam that is in the engine.....it has nothing to do with any carb or ignition adjustments. More initial advance or a locked at 32-36° setting may help it out, but the problem is trying to use too low of stall.

You could look into restricting the transfer slot feed at the main body to clear up the dirty plugs. Common issue with a lot of current QuickFuel and Holley carbs lately.

https://www.tapatalk.com/groups/raci...ure-t1392.html

Also learn how the open or closed power valve has zero effect on idle richness.
https://www.tapatalk.com/groups/raci...dle-t1314.html
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post #9 of 26 (permalink) Old Jul 15th, 20, 7:09 AM
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Re: Read 'em and weep. Carb tuning issue

as stated above your stall is not big enough
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post #10 of 26 (permalink) Old Jul 15th, 20, 8:20 AM Thread Starter
Steve
 
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Re: Read 'em and weep. Carb tuning issue

Quote:
Originally Posted by Ericnova72 View Post
Nobody noticed he went from good behavior with a 3400 stall, put a 2500 stall behind it and now complains of too much rpm drop at idle in gear??

It's a no-brainer in my opinion, classic parts mismatch now, you now don't have enough stall converter to go with the cam that is in the engine.....it has nothing to do with any carb or ignition adjustments. More initial advance or a locked at 32-36° setting may help it out, but the problem is trying to use too low of stall.

You could look into restricting the transfer slot feed at the main body to clear up the dirty plugs. Common issue with a lot of current QuickFuel and Holley carbs lately.

https://www.tapatalk.com/groups/raci...ure-t1392.html

Also learn how the open or closed power valve has zero effect on idle richness.
https://www.tapatalk.com/groups/raci...dle-t1314.html

I should have added that I was having the same idle problem even before switching to the new converter.
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post #11 of 26 (permalink) Old Jul 15th, 20, 8:21 AM Thread Starter
Steve
 
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Re: Read 'em and weep. Carb tuning issue

Quote:
Originally Posted by 68Chevele View Post
If the QF is one of those Slayer 750's get rid of it, put a Brawler or an Avenger 750 on the engine. That QF needs to be completely re-calibrated. We had one and it was the the worst thing I have worked on in a long time. Burned your eyes and fouled plugs. Put an Avenger 770 on the car, ran perfect after adjusting the idle screws.
It is a QF Slayer. what is it specifically about these carbs that would cause my problem?
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post #12 of 26 (permalink) Old Jul 15th, 20, 8:37 AM
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Re: Read 'em and weep. Carb tuning issue

Quote:
Originally Posted by '65newb View Post
It is a QF Slayer. what is it specifically about these carbs that would cause my problem?
The t-slot circuit in the main body will need to be restricted as it's way too fat out of the box. The other thing I noticed was the t-slot in the pri. side was open over .060 and that tends to make a carb real stinky at idle. I closed it down to .025 then started the car up and it barley ran at 500 RPM. I open the sec side to get the idle were I needed it to be at 800 RPM but it was still super rich on the wideband so rather than having to rework the new carburetor over I sent it back to Summit as you should never have to spend that much time on a new carb to get it to be clean at idle. They exchanged it for the 770 Holley and the car runs perfect. I did call QF on the issue and they were horrible to deal with as a company, absolutely no help. I also have a 750 QF Hot Rod DP that I have used and while it a pretty good carb at the track it also runs pig rich at idle. I may put some restrictors in either the metering blocks or the main body someday but since I have a bunch of nice carburetors to use the QF just sits. Never had any rich idle problems when I used my 3310's years ago either, they are good street carburetors.
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post #13 of 26 (permalink) Old Jul 15th, 20, 9:15 AM Thread Starter
Steve
 
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Re: Read 'em and weep. Carb tuning issue

Quote:
Originally Posted by japete92 View Post
This is more 'process' than specific 'fix'.

Recommend reading:

https://www.chevelles.com/forums/374...ion-101-a.html

Please pay particular attention to the 'initial', 'vacuum' and 'mechanical', advance discussions. They all need to correct individually and in combination.

Highly recommend getting ignition CORRECT (and KNOW it is CORRECT) before looking at carb or anything else.

Also recommend spending time reading posts in the 'ignition' section. You likely will find posts discussing issues similar to yours. Post by Dave Ray are recommended reading.

Just my initial opinion based on your data, your timing is off. Perhaps initial, vacuum, and mechanical.

Pete

Astounding! Simply an eye opening bit of info!!!! Thanks for including the link in your post. I wasn't using the vacuum advance because it made zero difference to my initial timing whether it was plugged in or not. Now I can see that the advance can could be the wrong spec for the vacuum i'm pulling at idle. I am going to check this out and try to get my ignition dialed in first and put the drill bits away
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post #14 of 26 (permalink) Old Jul 15th, 20, 9:21 AM
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Gene
 
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Re: Read 'em and weep. Carb tuning issue

Quote:
Originally Posted by '65newb View Post
I should have added that I was having the same idle problem even before switching to the new converter.
Key piece of data right there. We all ASSUMED it went bad after the tighter stall was fitted. You may want vac adv to smooth out the idle and make the jump into gear less, as said. PS this is why folks "lock out" their timing to full advance all the time. For street duty, as Sam said, it'll be a tuning process to make it behave AND drive well (other than WOT)

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post #15 of 26 (permalink) Old Jul 15th, 20, 9:33 AM
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Jeff
 
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Re: Read 'em and weep. Carb tuning issue

The vacuum advance would have to be ported with 7-8" of vacuum at idle, I'd leave it off..

I think the whole issue is the cam, it has too much duration for a street car... You would have much more fun with that car if you would get a lower duration cam.. Make sure the compression is also low, around 8.5 to 9ish...
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