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Discussion Starter #1
Should I try a colder heat range spark plug to help get rid of my engine dieseling issue?

Was going to install these ATL-45 with heat range of 5: https://www.summitracing.com/parts/atl-45

But now I'm considering these ATL-124 with heat range of 4: https://www.summitracing.com/parts/atl-124

Do I bother?

Engine specs:
'69 351W
Manifold (not ported) vacuum advance
Idle is 700-750 (no AC in the car)
Timing (idle) 10* (If I advance timing much further, I get detonation issues while driving - heavy advance springs already installed)
Always run premium fuel (91 octane in California)
Summitt 600CFM carb
MSD dizzy (no box)
Worked stock heads
Eddy RPM cam
 

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What cam shaft are you using ?I ask because when I put my 5.0 in using a B303 a lot of people said to use manifold vacuum instead of ported So I tried it for a while and didn't like it there was a fine line between a good idle and a dieseling engine when I shut it off. I did some research and found the cure for this "back in the day" from GM anyway was a throttle up solenoid https://www.summitracing.com/parts/smp-es110?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-standard-motor-products&gclid=CjwKCAiA5qTfBRAoEiwAwQy-6eAMQqYuw4Hcw5dCA_UyXh4I06Wr1XwE7ivfCba4U1QaFrZZLHDh6RoChsMQAvD_BwE
https://www.google.com/search?rlz=1C1GGRV_enUS751US751&ei=myPqW_zqKe2zgges7a7wCQ&q=throttle+up+solenoid+dieseling&oq=throttle+up+solenoid+dieseling&gs_l=psy-ab.3...138649.152606..153197...2.0..0.127.1140.8j4......0....1..gws-wiz.......0i71j0i22i30j33i22i29i30j33i160j33i299.OM0ons-IgU0
 

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+1 The primary throttle blades are too far open. I realize you have to raise the idle rpm with a bigger cam but you "may" have to open the secondary throttle blades ( limit stop adjustment) so you can close the primary some and stop the fuel flow there. The other possibility is to drill a hole in the primary throttle blades and again allow less throttle blade opening.
Randy
 

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I also have the Summit carb with an aggressive cam. It does have the ability to open the secondary throttle blades. It is hard to to get to with the carb installed....but not impossible. I use a small flat head screw drive that I cut some of the handle off of.

There are a couple of videos on Youtube that talk about how much of the idle transfer slots should/shouldn't be exposed.
 

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Is it a higher mileage engine? I would do some de-carboning, either some hellish WOT highway runs or some Marvels/trans fluid or water down the intake while running:) That was our fix in the '80s anyway.
 

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The Summit carbs are notorious for being limited in getting a rich enough idle a/f in stock form. Short centered cams and/or carbon, as mentioned, make it worse. Idling as low as you say it is, and still running on, my guess is you have a severely lean idle a/f mix. Correct fix is remove the idle a/f screws and using number drills find a snug fit. Then go 2 sizes up and drill the idle discharge ports. That's assuming you've backed them out to no avail already. Or, remove the idle screws, measure the largest of the taper, (just before it meets the flat) and use a drill size a few thou smaller. You'll find idle a/f adjustability is within 1 full turn from seated vs almost falling out of the carb to make it idle good. Side note. I know I know, Fords run ported vac to dist...……..Gospel. Changing that to manifold vac WILL lower combustion temps and in itself might cure your problem without getting into carb mods. Band aid but still..
 

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Discussion Starter #10
Thanks everyone for the excellent tips. I will try the above suggestions and report back.

A few more "difficulties" with this seriously fun engine:
- It is high compression (guessing it's close to 11:1), but I don't have the previous owner's build specs on the short block so I don't know for sure (stock/worked '69 351W heads).
- If I advance timing any further, I get detonation/engine knock issues while driving. My dizzy curve is already as slow as you can get, which hurts some performance and MPGs. I'll be playing with this soon as I'm always trying to improve MPGs (10 MPG currently...) since the '67 is my daily driver.
- I'd rather spend the $$$ to go EFI rather than start drilling holes in carbs - but then EFI can have it's own set of issues as we've all observed here.
- As a stick-shift car, I've become accustomed to turning the engine off while in gear, and letting out the clutch to stop the dieseling when it happens (it only happens about once every ten times I turn off the car - but it's annoying (and embarrassing) when it does...)
- Yes, high mileage, probably about 100,000 miles since last rebuilt 12+ years ago

Regardless of these issues, the '67 is still darn awesome to drive as a daily driver in L.A.! ;-)

Cheers!
 

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I think you have all the ingredients for carbon build up. A bit rich and incomplete fuel burn, I dont know how your CA gas might add to it. It can even be the carbon that is preventing you from advancing the ignition very far.
Some of the new DFI engines even suggest de-carbon treatment every 30K miles so there are several products meant to address the issue as simple as pouring a bottle into the tank or an intake spray that might be better than my old shade tree fixes.
 

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Discussion Starter #12
I think you have all the ingredients for carbon build up. A bit rich and incomplete fuel burn, I dont know how your CA gas might add to it. It can even be the carbon that is preventing you from advancing the ignition very far.
Some of the new DFI engines even suggest de-carbon treatment every 30K miles so there are several products meant to address the issue as simple as pouring a bottle into the tank or an intake spray that might be better than my old shade tree fixes.
I sense that Marvel Mystery Oil is in my gas tank's future...

 

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Discussion Starter #13
FYI - I ended up just using the regular Autolite 45 spark plugs, and just put 8oz of Marvel Mystery Oil into the gas tank at fill-up. No changes or adjustments to the carb - yet. I'll report back after a few gas tanks with the Marvel Mystery Oil. Thanks for the tips everyone!
 
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