detonation / knock issue on 340

Mattax

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That's a model 4150 with 1 9/16 primary and secondary throttles
From an older numberical list, the Throttle body and shaft assembly is (was) p/n 12R11086A

Don't think its available new. Could check for new old stock on ebay or look for a cheap used one, or carb with one.

I don't recommend paying stupid money for a new carb unless you're racing. Find an old one at reasonable price. If you can find another baseplate for your 600 double pumper, it should be easy to get a nice crisp throttle response. Yes it may be down a few mph in the quarter mile compared to a well tuned larger carb, especially with headers and gearing. Doesn't seem like thats your focus right now.

It may be easier to find another 4776, or a 4777 to get you going for now.
 
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Mattax

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74 with a 72 340. electronic ignition, will throw a timing light on it tonight and get back to you on that but ive gone up and down. Sound gets worse when i advance it. there is a vacuume advance and it is operating. 91 octane fuel.

This is an excellent idea.
Measure the timing at the lowest rpm it will run, and then write down the rpm for every 2* it advances. Or the other way, every 200 or 250 rpm note the timing. Cap or plug the vacuum advance line when doing this. Write it down. Later on you can graph it.
This will provide critical pieces of information needed for tuning this unknown build.

i was thinking that too, vacuume gauge reads good on manifold and ported. but since spraying the base of the carb and getting the reaction I did ill try the gasket next.
No idea what you mean by 'reads good'.
If you mean vacuum is stable, and that suggests the engine is OK, that's certainly a good sign.
At slow idle, ported should be zero, or at most a few inches of mercury.
In contrast, manifold vacuum can as bad as 4 to 5" Hg on a radical street/strip build, to 16" Hg or more on a stock build.

If the vacuum is really strong at slow idle (650-750 rpm), then engine is less radical and less initial timing will be needed. 12 - 14* BTDC might be a reasonable starting point.
If the vacuum at say 700 rpm is around 11 - 15"hg, then it will need more initial timing. 15 to 18* might be a starting point.

With a Holley 4150/60 set the initial throttle position as shown in the post above here:
Large RPM and Vacuum drop when shifting from park to gear

Tuning for performance procedure outlined here Setup for 750DP on this 408

Excellent books for 4150/60 carbs is the one by Urich, or the one by Urich and Fisher. You find them used for cheap since they've been in print since the 70s.
 
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mgoblue9798

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People have run much more than that and there are other factors involved than just cylinder pressure.

240 psi on pump is possible when you know what you're doing.

I am certain you are correct, but we are trying to help figure out an unknown build. 180 psi and up on an unknown combo is at least enough to consider as a possible issue.
 

Hysteric

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I am certain you are correct, but we are trying to help figure out an unknown build. 180 psi and up on an unknown combo is at least enough to consider as a possible issue.

I agree but again there is more to detonation than just cylinder pressure. Are you suggesting he pull the engine apart and change the compression so as he can now safely say that he is what other people consider safe territory for cylinder pressure?

I haven't seen any mention of initial timing or what his advance curve is like from the OP? Where is it detonating? is it detonating in the lower rpms or higher rpms?
 

shimsham1

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I agree but again there is more to detonation than just cylinder pressure. Are you suggesting he pull the engine apart and change the compression so as he can now safely say that he is what other people consider safe territory for cylinder pressure?

I haven't seen any mention of initial timing or what his advance curve is like from the OP? Where is it detonating? is it detonating in the lower rpms or higher rpms?


It was in the lower rpm. Thinking once I replace the cracked carb base it will be all good
 

RustyRatRod

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It’s not lean.
Maybe, maybe not. I can tell you about mine. "All of a sudden" it started spark knocking again a while back. Not "as bad" as before I tuned it out. Guess what it ended up being? The carburetor had loosened up on the mounting gasket. To my stupidity, it was an old one I reused, so I got a new thick one and tightened it down good. Problem gone. So that tells you how "close" the tune is on mine. I just bet I could remove one of the plugs from an unused port on the base plate of the carburetor and it would start rattling again. It doesn't take much sometimes.
 

RustyRatRod

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You can do it. You just have to have a purpose going in.

And that’s one reason why cranking compression isn’t a very good predictor of detonation.
There's a guy over on The Hamb running an old school 392 Hemi. He has a measured 13.5:1 with iron heads and a mild solid lifter cam. He runs on pump gas. No spark knock. So yes, it can be done.

And you're right. Cylinder pressure certainly isn't a good indicator for detonation, since we have so many different chamber and piston designs, so many different camshaft profiles, so many different assembly procedures, including losing all the sharp edges. Cylinder pressure is only one SMALL part of the equation.
 

RustyRatRod

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That's a model 4150 with 1 9/16 primary and secondary throttles
From an older numberical list, the Throttle body and shaft assembly is (was) p/n 12R11086A

Don't think its available new. Could check for new old stock on ebay or look for a cheap used one, or carb with one.

I don't recommend paying stupid money for a new carb unless you're racing. Find an old one at reasonable price. If you can find another baseplate for your 600 double pumper, it should be easy to get a nice crisp throttle response. Yes it may be down a few mph in the quarter mile compared to a well tuned larger carb, especially with headers and gearing. Doesn't seem like thats your focus right now.

It may be easier to find another 4776, or a 4777 to get you going for now.
Just get a quick fuel 450. That's the base plate they have.
 

mgoblue9798

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I agree but again there is more to detonation than just cylinder pressure. Are you suggesting he pull the engine apart and change the compression so as he can now safely say that he is what other people consider safe territory for cylinder pressure?

I haven't seen any mention of initial timing or what his advance curve is like from the OP? Where is it detonating? is it detonating in the lower rpms or higher rpms?

You haven't read through all the posts as they address exactly your questions including what my suggestions at this point are.

Question is do you care to add anything helpful to the discussion?
 

Hysteric

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You haven't read through all the posts as they address exactly your questions including what my suggestions at this point are.

Question is do you care to add anything helpful to the discussion?

What's the initial and total timing again?
 

G70464

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Here is remidal plug reading for what a lean plug looks like.
Since it appears we have people
image.jpeg.27de9119a93a0e0f9ca23cac61654f7e.jpg
 

shimsham1

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So having no luck with a replacement bottom plate. Looking for suggestions on a replacement carb. Found a rebuilt thermoquad 850 cfm locally but on the fence if I should just get something new. Opinions?
 

Mattax

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If the intake manifold doesn't have the bolt pattern and openings for the Thermoquad, then it becomes a hassle and IMO not is not usually a good match for the intake.
So assuming the intake is for a Holley squarebore with not adapters, then find a used Holley and switch over the throttle extension from the old one. New is not always better, its just new. That said, if you want something new more like an T-quad but fits a square bore intake, then the "Street Demon' is worth looking at.
 

shimsham1

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If the intake manifold doesn't have the bolt pattern and openings for the Thermoquad, then it becomes a hassle and IMO not is not usually a good match for the intake.
So assuming the intake is for a Holley squarebore with not adapters, then find a used Holley and switch over the throttle extension from the old one. New is not always better, its just new. That said, if you want something new more like an T-quad but fits a square bore intake, then the "Street Demon' is worth looking at.

27CF80AB-C769-4F65-A1D6-7584EC142462.jpeg


my intake
 

RustyRatRod

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I do not mean this insulting in any way shape form or fashion, so please take it in the intent it is given. If you do not have good and I mean GOOD carburetor tuning skills, don't get a thermoquad. They are great carburetors, but the learning curve is pretty steep if they are new to you. Also, if you run a square bore carburetor on that intake, you will need the thin plate from Edelbrock to assure you have no vacuum leak around the secondaries. This is all you need. Most O'Reilly stores stock it.

Edelbrock 2732 Edelbrock Carburetor Adapters | Summit Racing
 

shimsham1

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I do not mean this insulting in any way shape form or fashion, so please take it in the intent it is given. If you do not have good and I mean GOOD carburetor tuning skills, don't get a thermoquad. They are great carburetors, but the learning curve is pretty steep if they are new to you. Also, if you run a square bore carburetor on that intake, you will need the thin plate from Edelbrock to assure you have no vacuum leak around the secondaries. This is all you need. Most O'Reilly stores stock it.

Edelbrock 2732 Edelbrock Carburetor Adapters | Summit Racing
I don’t get offended, I am asking for help and appreciate the advice. I had a ton of Experience way back in the late 80’s early 90’s with thermoquads ( I used to call them thermo bogs ) but damn when those secondary’s kick in. It’s just been a while. I have zero experience with holleys but have learned a lot over the last few weeks. Just really want something that will be an easy use. Not looking to run 1/4 mile times or get every horse out of the motor.
 

RustyRatRod

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I don’t get offended, I am asking for help and appreciate the advice. I had a ton of Experience way back in the late 80’s early 90’s with thermoquads ( I used to call them thermo bogs ) but damn when those secondary’s kick in. It’s just been a while. I have zero experience with holleys but have learned a lot over the last few weeks. Just really want something that will be an easy use. Not looking to run 1/4 mile times or get every horse out of the motor.
If you want something easy peasy, I suggest looking at the Quick Fuel carburetors. They have so much MORE adjustability than the old Holleys, it's pitiful. I have a 450 on a slant six and it runs fantastic.
 

ir3333

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180 psi is too high for 91 octane with LA iron heads.
Get another carb and go back to post #10
 

512Stroker

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180 psi is too high for 91 octane with LA iron heads.
Get another carb and go back to post #10
I have 180-190 CCP in my street 340 with ported X heads. Run 91 octane no ethanol fuel all day crz'n - no issues
Custom ignition 34* of timing all in at 2700 rpm plus 10 * of added vacuum advance.
 


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