377 small block chevy, dual 425s, many issues?

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broflow
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377 small block chevy, dual 425s, many issues?

Postby broflow » Tue Jul 25, 2017 12:14 am

So I am new to propane and new to this forum. Many other forums seem to lead to here and I've read many,many informative posts here and elsewhere. The more I read, the more I learn but the more confused I am on some aspects of propane. Anyways let me tell you what I have and what Im experiencing and I'm sure I've made a few mistakes but someone ought to be able to point me in the right direction.
I have a dedicated off-road K-5 blazer. It's been about 7 years since it was first torn apart and now it's finally ready for use again. The motor ran very strong before. Its primary use before was mud drags. I ran 92 pump gas blended 50/50 with race gas and didn't have any detonation issues. I decided to switch to propane based on the higher price of race fuel and the ability of propane to run in off camber/ off-road/ better trailability. I admittedly didn't research the specifics of propane fuel systems and I made an impulse buy and bought a brand new complete kit from an internet supplier. It is a dual impco 425 mixers with model E vaporizers. The mixers are on an adaptor mounted to an 850 Holley baseplate. It came with an electric shutoff solenoid and all of the proper hoses and fittings. Everything is brand new.

Here's my motor specifics (Bear with me I'm not an engine builder and it was 15-20 years ago that I had this motor built) It's a 377 Chevy , dart cast iron 64 cc heads with extensive port work and high quality valves. It has domed pistions, steel crank, forged rods. From what my engine builder could remember it's about 12.2 to 1 C.R. It's a solid lifter cam (non roller) the duration @ .050 was about 230ish with about 500 lift. Not a large cam I'm told. This motor would pull 15 " of vacuum at Idle. It has a high rise single plane intake. It's running a MSD billet distributor with mechanical advance only and a 6AL box.
I'm running NGK xxx9 plugs with a .030 gap. These are one range cooler than stock. Auto trans. I'm sure I'm missing something else here just ask me what?

I installed the lightest springs in the distributor and the largest stop. It should be allowing 18* of mechanical advance and it coming in fully at 2600 according to my advance timing light. I set it at 30*@ 2500RPMs like I read in some forum. I have also installed a wide band O2 sensor recently but haven't gotten any real world data yet. It's at mid 15s at idle.

With all of this I have some terrible detonation under load, and very hard starting when warmed up. I did verify my timing marks on the balancer by finding TDC on #1. It was running very hot (like headers starting to glow) when I was setting the timing at 2500 rpms. I found after my last start up and timing session that I had a large vacuum leak. (rubber cap came off the large vacuum port on the carb base plate) .
I also discovered what I think is another problem. 1 of the Brand new model E vaporizers when I press the primer button on it reliefs propane out the brass colored vent screen on it and I don't hear any gas flowing into the mixer like I can when I press the button on the other model E?

I know this is a really long, wordy post. I hope someone can make sense of it and help me out.

My top 3 questions are.
1- Is my compression too high for propane?
2- Could the large vacuum leak have been my main problem?
3- Could the 1 model E not be working and essentially rendering It's mixer useless?

Thanks for any advice in advance.

Brody


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storm
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Re: 377 small block chevy, dual 425s, many issues?

Postby storm » Tue Jul 25, 2017 1:46 am

Hi Brody, you don't say where you are from so my replies are based on the assumption you are using LPG that has approximately 105-110 Octane rating.
broflow wrote:1- Is my compression too high for propane?
This depends on many factors. Iron heads, compared to alloy heads, can require 1 full number less. e.g. same engine with alloy heads can run 12.5-1 iron heads require 11.5-1. I personally wouldn't push it far above 11 but that's me. Having said that the 230 degree cam should bleed of cylinder pressure at low to mid range rpms (idle to 2500-3000 rpm)
broflow wrote:2- Could the large vacuum leak have been my main problem?
I don't think it would make the engine detonate, others who have more experience than I may say yes.
broflow wrote:3- Could the 1 model E not be working and essentially rendering It's mixer useless?
It sounds like the regulator has a tear in diaphragm.

To work out the detonation issue more information will be required.
At what load is the detonation occurring? (this is best explained using vacuum or MAP readings)
At what rpm is the detonation occurring?
What is your initial (idle) timing spec? (I've read your post and done the math but this is something we need to be sure about).
Do you have vacuum advance installed and connected? If yes what is the timing specs for it?

So far I think your compression is 1 full point to high and your advance is to much to quick. 1 or the other will probably need to change.
Fuel flow requirements calculations viewtopic.php?f=5&t=1638

broflow
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Re: 377 small block chevy, dual 425s, many issues?

Postby broflow » Tue Jul 25, 2017 7:10 am

I am located in the southern Wisconsin, U.S.
I do not have a vacuum advance style distributor. Its mechanical advance only.
I set the timing at 30* full advance at 2500-2600 rpm. That put my idle timing at 10* or so from what I remember.
I don't have any audible detonation at Idle or when revving the engine under no load. The detonation occurs as soon as a good heavy load is applied when driving in any gear. I would say about 2500-3000 rpm? pretty much as soon as throttle is applied and the converter starts to lock up and pull.

I stopped my test drive it when I heard it so I don't have any good info on what the Wide band O2 sensor was reading while driving?

Not sure what you were after with vacuum or map readings? Please explain.

Thanks-Brody

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storm
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Re: 377 small block chevy, dual 425s, many issues?

Postby storm » Tue Jul 25, 2017 4:09 pm

Vacuum or MAP (Manifold Absolute Pressure) is an indication of how much load the engine is seeing at the time. If the car has EFI, I knows yours isn't, the ECU will read a MAP sensor and adjust timing accordingly. There are aftermarket kits that can log many different engine parameters including but not limited to MAP. If your car has a vacuum gauge you can read what the vacuum is at cruise and then what it goes to when you apply load and it starts to detonate.

Do you have the original springs for the dizzy? If I were you, as a short term fix until a better solution can be found, I'd be fitting heavier springs and slowing down the advance weights.
Fuel flow requirements calculations viewtopic.php?f=5&t=1638

C3H8
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Re: 377 small block chevy, dual 425s, many issues?

Postby C3H8 » Wed Jul 26, 2017 12:13 am

Sounds like a nice engine package.
The thing to remember is that the timing specs you will typically find on our site refer to factory produced engines. The SBC in the early 80's only had about 8.5:1 compression. The late 80's and 90's upped the number to a high of 9.5:1. These engines required a more aggressive timing curve to produce power and also were very tolerant of detonation.

You state your engine is 12.2:1. At that compression ratio your going to have to modify your numbers. I would suggest backing off your maximum to around 22 or 24 degrees as a starting point. Also reduce your initial to between 8 to 10 degrees. The higher the initial timing the more difficult it is to start an engine. You could also do as storm says and slow down the rate of advance.

The hot exhaust could be related to improper mixtures. To repeat storm's diagnosis you almost certainly have a tear in the diaphragm of the model E. This would really screw up your mixtures. They would likely be seriously lean with the torn diaphragm plus the vacuum plug missing. Once fixed you will have to check your mixtures to be sure you aren't over fueling at full load. That setup can provide a lot of fuel and air.

What model of 425 do you have? Check the number on the label and let us know.

Your mixtures may be very lean during a no load 2500 RPM with dual 425's for your engine size. Depending on what the maximum RPM you run dual 425's may be too much. Reaction of the mixers gas valves may be quite slow especially at lower RPM's. A dual 425 will supply 920 CFM and support about 600 HP. A single does 460 CFM and just shy of 300 HP. A 377 CI NA engine only uses 481 CFM at 5500 RPM. Reaction of dual mixers to airflow would be sluggish until you reach much higher RPM's. Dual 425's may also make the starting slower. You would need a pretty aggressive starter to wind the engine over fast enough to lift the mixer valves properly for starting. If you don't have a primer you would definitely need one to provide proper enrichment of starting mixtures.

broflow
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Re: 377 small block chevy, dual 425s, many issues?

Postby broflow » Wed Jul 26, 2017 9:30 pm

C3H8 wrote:Sounds like a nice engine package.
The thing to remember is that the timing specs you will typically find on our site refer to factory produced engines. The SBC in the early 80's only had about 8.5:1 compression. The late 80's and 90's upped the number to a high of 9.5:1. These engines required a more aggressive timing curve to produce power and also were very tolerant of detonation.

You state your engine is 12.2:1. At that compression ratio your going to have to modify your numbers. I would suggest backing off your maximum to around 22 or 24 degrees as a starting point. Also reduce your initial to between 8 to 10 degrees. The higher the initial timing the more difficult it is to start an engine. You could also do as storm says and slow down the rate of advance.

The hot exhaust could be related to improper mixtures. To repeat storm's diagnosis you almost certainly have a tear in the diaphragm of the model E. This would really screw up your mixtures. They would likely be seriously lean with the torn diaphragm plus the vacuum plug missing. Once fixed you will have to check your mixtures to be sure you aren't over fueling at full load. That setup can provide a lot of fuel and air.

What model of 425 do you have? Check the number on the label and let us know.

Your mixtures may be very lean during a no load 2500 RPM with dual 425's for your engine size. Depending on what the maximum RPM you run dual 425's may be too much. Reaction of the mixers gas valves may be quite slow especially at lower RPM's. A dual 425 will supply 920 CFM and support about 600 HP. A single does 460 CFM and just shy of 300 HP. A 377 CI NA engine only uses 481 CFM at 5500 RPM. Reaction of dual mixers to airflow would be sluggish until you reach much higher RPM's. Dual 425's may also make the starting slower. You would need a pretty aggressive starter to wind the engine over fast enough to lift the mixer valves properly for starting. If you don't have a primer you would definitely need one to provide proper enrichment of starting mixtures.


So how hard is it to take apart the Model E and find this diaphragm? is it just removing the screws and pulling that cap with the primer button in it? What would cause it to tear? These were brand new Model Es or so I was told? They do look new and they were bought from a propane parts vendor.

I can't figure out how too attach a picture of the mixers. It says my file size is too large. On the top of the mixer it has the # C1-30. Is that the model #?

This motor used to run up to 7000rpm pretty well. It was a completely balanced and bluprinted build. after the original build we added the heads that are on it now and my engine builder really wanted to put a larger camshaft in it to complement the bigger heads but we left it alone. He thought it would fall off after 6000rpms but it ended up pulling really strong up to 7000? He figured it to be 450 to 500 go if I remember correctly? That was why I felt I needed the dual 425s.

Thanks again
-Brody

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storm
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Re: 377 small block chevy, dual 425s, many issues?

Postby storm » Wed Jul 26, 2017 11:29 pm

Take the lid (top) of the Model E and the diaphragm will either be sitting on top of the reg or stuck to the underside of the lid.
Fuel flow requirements calculations viewtopic.php?f=5&t=1638

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BigBlockMopar
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Re: 377 small block chevy, dual 425s, many issues?

Postby BigBlockMopar » Thu Jul 27, 2017 5:57 am

Aside from the issues already mentioned, a cold air intake system and blocked off heatriser ports would be mandatory in keeping everything as cool as possible inside the engine's intake manifold.
https://www.bigblockmopar.nl
'73 Dodge Dart - 318ci - IMPCO E vaporizer / 425 mixer - A518-OD trans - 3.55 gears - 225/50/17" tires.

broflow
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Re: 377 small block chevy, dual 425s, many issues?

Postby broflow » Thu Jul 27, 2017 5:06 pm

Heat riser ports are blocked off. and currently I do not have a hood on it so that should be helping to keep cooler air coming in. I could fabricate some sort of an air intake in the future. I'm a sheet metal worker by trade.

gottago
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Re: 377 small block chevy, dual 425s, many issues?

Postby gottago » Fri Jul 28, 2017 3:11 pm

As noted broflow, you may be pushing the line with 12:1 + compression. Your cam can tame it enough to get past the detonation range but it is close imo. I run a 377 at 12.5:1 compression lately. Iron heads. The cam specs are quite important, the lsa makes a big difference. You have only given duration but not the lsa. As example, I could not run a hydraulic cam with 234 duration on a 112 lsa in mine without severe pinging when installed at manufactures specs. I had to retard it in order to drop the compression just that last little bit even running vacuum advance now. The cams open close points need to be looked at closely.Then the timing could be maximised for power. Strangely contrary to standard practice, It runs best with way more initial timing and then a mild curve that brings it all in around 30 degrees at 3000 rpm. It has always wanted very high initial timing in order to make power no matter what other changes. I run the rhs iron vortec heads with 64 cc chambers. They are somewhat similar to the darts.

broflow
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Re: 377 small block chevy, dual 425s, many issues?

Postby broflow » Mon Aug 07, 2017 8:27 pm

gottago wrote:As noted broflow, you may be pushing the line with 12:1 + compression. Your cam can tame it enough to get past the detonation range but it is close imo. I run a 377 at 12.5:1 compression lately. Iron heads. The cam specs are quite important, the lsa makes a big difference. You have only given duration but not the lsa. As example, I could not run a hydraulic cam with 234 duration on a 112 lsa in mine without severe pinging when installed at manufactures specs. I had to retard it in order to drop the compression just that last little bit even running vacuum advance now. The cams open close points need to be looked at closely.Then the timing could be maximised for power. Strangely contrary to standard practice, It runs best with way more initial timing and then a mild curve that brings it all in around 30 degrees at 3000 rpm. It has always wanted very high initial timing in order to make power no matter what other changes. I run the rhs iron vortec heads with 64 cc chambers. They are somewhat similar to the darts.

Great info Gottago. I wish I still had the cam specs. It's just been too long and I've lost any of the paperwork I once had in my multiple moves over the years. I'm hoping that with fixing the bad diaphragm and getting rid of the vacuum leak that maybe things will get better? If not I'll have to look at re-degreeing the cam.

gottago
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Re: 377 small block chevy, dual 425s, many issues?

Postby gottago » Tue Aug 08, 2017 1:00 pm

Definitely need to address all the minor issues like vacuum leaks etc before you will get anywhere. On the bright side, your cam was a fairly standard street performance stick back in the day and with 12.2 compression meant to run on gasoline it was likely a wider spread. Meaning you should be able to tune it in for propane. Valve lash etc needs to be set well. MSD without vacuum advance is a good way to start out. I'm pretty sure I'm running near 20 degrees initial ignition timing on mine and you may need to go that high to get it to start and run properly. I start on first revolution. It gets finicky the higher the compression. Anywhere near a 30 degree curve at 3000 rpm works fine for starters. Fix the leaks and check back.

Helmet
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Re: 377 small block chevy, dual 425s, many issues?

Postby Helmet » Wed Aug 16, 2017 5:01 am

See and try if you can contact this guy, he has a similar setup to you:

http://www.pirate4x4.com/forum/chevy/83 ... ild-2.html

(scroll a bit down, to near the end of the page for the propane stuff)

I might be able to give you some advice
'72 Cutlass Supreme, 455 BBO (out of order)
'85 CUCV, HO454


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